Oxytocin (OT) is a nonapeptide synthesized by hypothalamic neurons mainly located in the paraventricular (PVN) and supraoptic nuclei.48 Oxytocin acts both as a neurohormone when released into the blood through the posterior pituitary and as a neurotransmitter when synaptically released into the spinal cord (SC) and supraspinal regions. Recently, our laboratory identified a small population of 30 parvocellular neurons in the PVN, which coordinate the peripheral and spinal release of OT to limit inflammatory pain symptoms.16 Indeed, parvocellular OT neurons of the PVN trigger OT secretion in superficial and autonomic layers of the SC where OT receptors are found.48 Oxytocin displays antinociceptive properties by increasing GABAergic inhibition in the SC thus decreasing excitability in superficial and deep SC neurons.48 In addition to these central-related effects, a peripheral component of OT-mediated analgesia through a direct effect at the dorsal root ganglion level has been documented.42
Activation of the OT analgesic system occurs under different conditions such as inflammatory sensitization,16 neuropathic pain,36 or forced swim stress (FSS).62 In the carrageenan model of inflammatory sensitization, spinal OT levels are increased for at least 24 hours and OT receptor–mediated local production of the neurosteroid allopregnanolone (AP) leads to a sustained GABAergic activity to maintain spinal antinociceptive control.25 After a non-noxious FSS, OT is also responsible for significant and transient stress-induced analgesia (SIA).62 In this context, a deficit in OT signaling might be related to neurodevelopmental brain pathologies such as autism spectrum disorders.39,64
The development of the nociceptive system starts before birth and follows a similar pattern in the rat and in the human fetus.55,57 It is well accepted that the developmental stage of the rat just after birth is similar to that of the human fetus at the beginning of the third trimester of gestation. Descending controls of pain mature during the first 3 weeks after birth in the rat19 and begin to mature during the end of the third trimester in humans.21 At the SC level, spinal inputs and chloride homeostasis undergo reorganization steps after birth to be fully matured,4,5 making them possibly vulnerable to early adverse events. Neonatal maternal separation (NMS) is a well-established model of early life stress (ELS) that induces long-term neuroendocrine and behavioral changes in rodents, with impact on the hypothalamic-pituitary-adrenal (HPA) axis, social and cognitive behaviors, and sensory and nociceptive systems.27,46 Long-term consequences of NMS are indeed visceral hypersensitivity,56 epigenetic changes, and associated sensory dysfunctions.41,59 Recently, a chronic adult treatment with histone deacetylase (HDAC) inhibitor was shown to be efficient to counteract NMS-induced visceral hypersensitivity.41
In this study, we used behavioral measures and in vivo electrophysiological recordings from SC dorsal horn neurons to characterize the long-term consequences of NMS on the oxytocinergic descending control of pain. Then, we aimed to prevent the development of NMS pain phenotype using pharmacological tools, targeting the OT system or epigenetic factors early during the postnatal period.
2. Materials and methods
2.1. Animal care and neonatal maternal separation procedure
Female Wistar rats with litters were housed in a temperature-controlled room (22°C) under a 12-hour light–dark cycle (lights on at 07:00 AM), with ad libitum access to food and water. All experiments were conducted in conformity with the recommendations of the European Committee Council Direction of September 22, 2010 (2010/63/EU), with an authorization for animal experimentation from the French Department of Agriculture (License 67-116 to P.P.) and with a procedure agreement evaluated by the regional ethical committee (CREMEAS authorization number APAFIS#7678-20 16112223027528 v2).
At birth, litters were randomly assigned to 2 groups: NMS and nonseparated control litters. The litters allocated to the NMS group were removed from the nest cages 3 hours per day from postnatal days 2 (P2) to 12 (P12) and placed on a heating pad (37°C) in a separate cage. The litters allocated to the control group remained with their mother in their home cages during the entire NMS interval and received no special handling other than that necessary to change the bedding of their cage. Pups were weaned at P21 and housed 4 rats per cage before being randomly chosen at 8 weeks or more for behavioral analysis or electrophysiological recordings. In each experiment, animals were chosen randomly in at least 2 different litters. Both males and females were used in this study, as no sex-specific differences could be seen for the different neurophysiological and behavioral measures.
2.2. Measure of basal mechanical and thermal nociception
Mechanical nociception was measured on adult rats with a calibrated forceps (Bioseb, Vitrolles, France) as previously described.34 Briefly, the habituated rat was loosely restrained with a towel masking the eyes to limit stress by environmental stimulations. The tips of the forceps were placed at each side of the paw, and a gradually increasing force was applied. The pressure producing withdrawal of the paw corresponded to the nociceptive threshold value. Thermal nociception was measured by a blind experimenter with the Hargreaves method, as previously described.20 Briefly, rats were placed in Plexiglas boxes (20 × 25 × 45 cm) for a 15- to 20-minute habituation period and then hind paws were exposed to a radiant heat, and the latency time required to induce paw withdrawal was recorded and considered as the nociceptive heat threshold. For both tests, measurements were performed 3 times for each hind paw, and values were averaged.
2.3. Forced swim stress
At adulthood, rats were exposed to a FSS procedure in a Plexiglas cylinder (diameter: 30 cm; wall height: 60 cm), filled with water (height: 40 cm) at a temperature of 20°C for 10 minutes. After FSS exposure, animals were gently dried. Mechanical thresholds were measured before (baseline) and 15 minutes after FSS in both NMS and control animals. This protocol has been described to be associated with a plasmatic and spinal release of OT.62 The oxytocinergic contribution of FSS to SIA was confirmed in a subset of rats receiving an intrathecal (i.t.) injection of 10 µL of OT receptor (OTR) antagonist dOVT ((d(CH2)5 1,Tyr(Me)2,Thr4,Orn8,des-Gly-NH2 9)-vasotocin, Bachem, Germany) at 200 µM or 10 µL of saline 30 minutes before FSS.
2.4. Carrageenan inflammatory pain model
To induce a painful inflammatory sensitization in the rat, 150 µL of carrageenan (3% in NaCl 0.9%; Sigma Aldrich, Darmstadt, Germany) was injected in the plantar surface of the right hind paw for behavioral experiments, whereas injections in both hind paws were performed for electrophysiological experiments. Control animals received either similar injections of NaCl 0.9% or no injection.
2.5. In vivo recording of spinal cord neurons and analysis
Male and female rats were anaesthetized with isoflurane (2%; pushed by pressurized air at a flow rate of 0.5 L·min−1), and body temperature was regulated using a thermostatically controlled heating blanket (Harvard Apparatus Ltd, Les Ulis, France) maintaining the body temperature at 37°C. Animals were setup in a stereotaxic frame (La Précision Cinématographique, Eaubonne, France), with the cervical and sacral vertebrae firmly held. The lumbar SC (L4 and L5) was exposed by laminectomy. After removal of the meninges, the SC surface was covered with a thin isolating layer of mineral oil. This opening allowed us to record SC neurons and apply several drugs directly on the SC. At the end of the experiment, animals were killed with an overdose of isoflurane.
Single-unit extracellular in vivo recordings were made from wide-dynamic-range neurons, located in lamina V and receiving peripheral sensory messages mediated by Aβ-, Aδ-, and C-type primary afferent fibers. In the lumbar enlargement of the SC, we monitored voltage changes through stainless microelectrodes (FHC, Bowdoin, ME) connected to an extracellular differential amplifier (DAM-80; WPI, Aston, United Kingdom). Electrical signals were then digitized (Power 1401; CED, Cambridge, United Kingdom), processed with the Spike 2 software (CED). Neurons were characterized by their responses to transcutaneous electrical stimulation of the hind paw receptive field, as previously described.26 Electrical stimulation was applied through 2 thin cutaneous pin electrodes placed in the center of the receptive field on the hind paw. Increasing intensities were applied from 1 V to C-fiber activation threshold (duration of 1 ms). Trials consisted of a train of 90 electrical stimuli (3 × C-type fiber threshold) applied repetitively (1 Hz) to the excitatory receptive field. This method has been usually used to obtain a stable and reproducible “wind-up,”37 a property of C-type nociceptive fibers. The electrically evoked response of the neuron was captured and displayed as a poststimulus histogram, for A-type and C-type discharge. Action potentials appearing early after the stimulus artifact were classified as A type (0-50 ms), whereas the delayed response corresponded to C type (50-800 ms).
2.6. Spinal cord sampling and PCR analysis
Lumbar SC samples were collected, reconstituted in a guanidine thiocyanate/β-mercaptoethanol preparation using ultraturax and stored at −80°C. Total RNA was extracted according to a protocol derived from the original procedure of Chomczynski and Sacchi,13 consisting of 2 independent total RNA extractions separated by a DNAseI treatment (TURBO DNaseTM; Ambion, Life technologies, Saint Aubin, France), as previously described in detail.28 Eight hundred nanogram RNA was used for reverse transcription with the RT iScript kit (Bio-Rad, Marnes-la-Coquette, France). Quantitative PCR was performed using SYBR Green Supermix (Bio-Rad), on the iQ5 Real Time PCR System (Bio-Rad). Amplifications were performed in 42 cycles (20 seconds at 95°C, 20 seconds at 60°C, and 20 seconds at 72°C). Standardization was made possible using standard curves made from serial dilutions of samples, and amplification data are shown as relative gene expression, calculated as the ratio between cDNA concentration of the gene of interest and that of the housekeeping gene. Preliminary experiments showed that among the different classic housekeeping genes routinely analyzed in the laboratory, and compared with 18S ribosomal RNA, hypoxanthine-guanine phosphoribosyltransferase (HPRT) transcripts remained highly stable among the different samples. Therefore, hypoxanthine-guanine phosphoribosyltransferase was selected as the housekeeping gene to analyze and evaluate potential changes in gene expression in all experiments presented here. To specifically amplify mRNA encoding various rat proteins, we designed specific primer sets (sense and antisense, respectively) using Oligo6.0 and M-fold softwares. Primer sequences for all genes of interest can be found in supplementary Table 1 (available at http://links.lww.com/PAIN/A649). Samples were accurately dispensed in duplicates using a robotic workstation (Freedom EVO100; Tecan, Lyon, France), and amplification efficacy given by standard curves was always close to 100% (±2%), whereas amplification specificity was assessed by a melting curve study.
2.7. Neonatal rescue procedure
To reverse NMS long-term consequences on pain behavior, a pharmacological neonatal treatment was performed in NMS pups. During the NMS protocol, all pups in the litter were injected with the same molecule and then tested at adulthood to investigate the long-term efficiency of neonatal treatment. Oxytocin (Bachem, Weil am Rhein, Germany), the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA, Vorinostat; ApexBio Technology, Houston, TX), or vehicle (NaCl, oil, or 10% dimethyl sulfoxide, respectively) was injected intraperitoneally (i.p. 10 µL) at 1 mg/kg and 5 mg/kg, respectively, once daily (o.d.) during the NMS procedure (ie, from P2 to P12). In another subset of animals, AP (Santa Cruz biotechnology, Dallas, TX) was injected subcutaneously (s.c.) at 10 mg/kg once every 2 days during the NMS procedure. To avoid unnecessary supplementary handling of the pups, injections were performed at the beginning of the separation process. Pups were then normally weaned at P21, and no special handling was performed until P45, when the efficiency of neonatal treatment was assessed for each neonatal group using behavioral testing. First, mechanical thresholds were measured before (baseline) and after FSS (SIA measurement). Then, 1 week later, the efficacy of the OT descending control of pain after carrageenan inflammation of the hind paw was measured with the same protocol as the one previously described (using spinal dOVT injection 24 hours after inflammation).
2.8. Drugs and treatments
Oxytocin receptor agonists and antagonist, as well as V1aR antagonist were prepared as a concentrated solution in water (1 mM) and diluted to the needed concentration in NaCl (0.9%) before experiment. The selective OTR antagonist dOVT (d(CH2)5-Tyr(Me)-[Orn8]-vasotocine) was used at a final concentration of 1 µM in saline (NaCl 0.9%). dOVT was injected i.t. (volume 20 µL) or directly on the SC in electrophysiological recordings (volume 500 µL). V1aR antagonist (Phenylacetyl1,O-Me-D-Tyr2,Arg6,8,Lys9)-vasopressinamide (AV1AR; Sigma Aldrich, Saint-Louis, France) was injected i.t. at a concentration of 1 µM (volume 20 µL). Oxytocin and TGOT ((Thr4,Gly7)-Oxytocin) (Bachem, Weil am Rhein, Germany) were used as an agonists for OTRs. Oxytocin was dissolved in saline and injected i.t. or i.p. at a concentration of 10 µM or 1 mg/kg, respectively. TGOT was also dissolved in NaCl 0.9% and injected at a concentration of 1 µM (i.t.). Allopregnanolone and SAHA were freshly prepared before the experiment. Allopregnanolone was used at 10 mg/kg and dissolved in mineral oil by sonication at 4°C. The nonselective HDAC inhibitor SAHA was dissolved in 10% dimethyl sulfoxide and injected i.p. at a concentration of 5 mg/kg.
2.9. Statistical analysis
All data are expressed as mean ± SEM. Statistical analysis was performed using GraphPad Prism 6 software (LaJolla, CA). For each experiment, parametric analysis was used only after verifying the normal distribution of the values. The unpaired Student t test was used to compare baseline nociceptive sensitivity and OTR and oxytocinase (OTase) expression in NMS and CTRL rats. Repeated-measures 2-way analysis of variance (ANOVA) followed by the Sidak post hoc test was used to analyze inflammatory symptoms and to analyze wind-up efficiency. Repeated-measures 2-way ANOVA followed by the Tukey post hoc test was used to evaluate the effect of OT and TGOT spinal application in control and NMS rats. One-way ANOVA and the Tukey post hoc tests were used to compare the effects of spinal application of dOVT in control or NMS rats, to evaluate the efficiency of SIA in both groups, to analyze the success of the rescue experiments on baseline nociceptive sensitivities, and to evaluate spinal mRNA expression of 3α-hydroxysteroid dehydrogenase (3α-HSD), 5α-reductase type II (5α-red II), P450scc, and translocator protein (TSPO). The Mann–Whitney test was used to compare the effects of spinal application of the V1aR antagonist in control and NMS rats. The Kruskal–Wallis (KW) followed by the Dunn post hoc test was used to compare wind-up frequencies and spinal expression of HDACs' transcripts. Differences were considered to be statistically significant for P < 0.05.
3.1. Consequences of neonatal maternal separation on the nociceptive sensitivity and on the efficacy of oxytocinergic stress-induced analgesia
We first characterized the long-term effects of NMS on baseline mechanical and thermal nociceptive thresholds (Fig. 1A), as well as the intensity of stress-induced mechanical analgesia (SIA) measured after FSS (Fig. 1B). Compared to the control group, NMS animals exhibited hypersensitivity to both mechanical and thermal stimulation at adulthood (P > 100). Neonatal maternal separation rats displayed significantly lower mean pressure thresholds (control: 445.5 ± 6.1 g, N = 12; NMS: 268.5 ± 7.1 g, N = 12; P < 0.001, the unpaired Student t test, t = 18.83, df = 22) and paw withdrawal latencies to noxious heat (control: 21.1 ± 0.8 seconds, N = 12; NMS: 12.6 ± 0.4 seconds, N = 12; P < 0.001, the unpaired Student t test, t = 9, df = 22).
Previous studies showed that both central and peripheral release of OT contributed to SIA after FSS.26,50 Because NMS is known to alter the normal reactivity to stress and lead to a dysfunction of the HPA axis, we investigated the efficacy of the oxytocinergic component of SIA in NMS animals (Fig. 1B). As expected, the FSS paradigm induced a significant analgesia 20 minutes after swim in the control group (Fig. 1B). Mean baseline-corrected mechanical thresholds were increased by 239 ± 12.3 g in the control group (N = 8) but only by 10.7 ± 23.4 g in the NMS group (N = 8). An injection of dOVT before FSS decreased SIA intensity to 56.9 ± 20.1 g in the control group (N = 8) and had no effect in the NMS group (N = 8) where it varied of 20.75 ± 20.7 g (P < 0.001, one-way ANOVA, F = 28.9). These results suggest a blunted adaptation to stressful conditions in NMS rats associated with possible dysfunction of the oxytocinergic descending control of pain.
3.2. Behavioral response to a painful inflammatory sensitization by carrageenan
We next compared the intensity and time course of inflammatory pain symptoms triggered by an intraplantar hind paw injection of carrageenan to possibly highlight differences in the expression of mechanical/thermal hyperalgesia in control and NMS rats (Fig. 2). In both groups, carrageenan injection induced a sustained mechanical and thermal hyperalgesia that peaked 7 hours after injection, as previously described.25 Although basal mechanical nociceptive thresholds (ie, before carrageenan injection) were significantly lower in NMS rats, the time course of mechanical hyperalgesia was not significantly different between both groups (Figs. 2A1 and B1). In the control group, mean pressure threshold dropped from 445.8 ± 18.7 to 30.1 ± 5.1 g, 7 hours after injection (N = 6; repeated-measures 2-way ANOVA, treatment × time, F(12,120) = 63.81, P < 0.001) and mechanical hyperalgesia lasted for at least 6 days for controls. As mentioned above, at baseline, NMS rats exhibited a lower mean pressure threshold of 295.5 ± 4.8 g (N = 6), which further decreased to 70.8 ± 12.6 g, 7 hours after carrageenan injection. As illustrated in Figure 2B1, hyperalgesia was detected at a significant level for 6 days (N = 6; repeated-measures 2-way ANOVA, treatment × time, F(12,120) = 30.78, P < 0.001). Concerning thermal hot sensitivity, 7 hours after carrageenan injection, mean latencies decreased to 3.3 ± 0.6 and 4.2 ± 0.8 seconds in control and NMS rats (N = 6 per group), respectively. Interestingly, thermal hot hyperalgesia persisted for at least 1 day for control rats and up to 4 days in NMS rats (Fig. 2B2) (repeated-measures 2-way ANOVA, treatment × time, F(8,80) = 13.67, P < 0.001 for CTRL and F(10,100) = 10.69, P < 0.001 for NMS).
3.3. Oxytocinergic contribution to the intensity of carrageenan-induced pain symptoms and to the spinal cord processing of nociceptive messages
In the carrageenan model of inflammatory sensitization, spinal concentrations of OT are elevated during the early phase of inflammation and have been shown to limit pain sensitivity.26 To investigate whether NMS affects OTergic antihyperalgesia, i.t. application of the selective OTR antagonist dOVT was performed 24 hours after carrageenan injection. As OT may act through the activation of vasopressin V1a receptors (V1aRs) at high concentrations, we also examined a possible contribution of these receptors using i.t. application of V1aR selective antagonist.
In the control group and as previously shown,25 a single i.t. injection of dOVT (1 µM, 20 µL) induced a rebound of mechanical and thermal hyperalgesia. Antihyperalgesia mediated by OTR could be seen in the CTRL group as a decrease in the mean baseline-corrected mechanical threshold 4 hours (ie, 28 hours after carrageenan) after dOVT injection by −75.6 ± 8.4 g (N = 10 Fig. 3A). This decrease was absent in the saline-treated group (−2 ± 8.4 g, N = 10) (one-way ANOVA F = 17.5, P < 0.001). For thermal withdrawal latency (Fig. 3B), OTR antihyperalgesia could also be seen as a significant decrease in the mean values after dOVT compared with saline (N = 10; one-way ANOVA, F = 9.3, P < 0.001). This corresponded to a change of −3.9 ± 0.6 seconds (N = 10) and of 1.1 ± 0.8 seconds (N = 10), for the dOVT and saline groups, respectively. Intrathecal injection of the V1aR antagonist was without effect on mechanical (before: 155.25 ± 3.2 g; after i.t. injection: 177.25 ± 18.2 g, N = 4; the Mann–Whitney test: U = 4.0; P = 0.3429) or thermal nociceptive threshold (before: 10.0 ± 0.9 seconds; after i.t. injection: 11.0 ± 0.9 seconds, N = 4; the Mann–Whitney test: U = 5.0; P = 0.4571) in the control group of rats.
Both results confirmed OTR activation to be responsible for a limitation of pain symptoms early after carrageenan injection in control animals. Interestingly, there were no changes in the mean mechanical (dOVT: delta of 5.8 ± 10.8 g, N = 8; saline: delta of −27.2 ± 9 g, N = 7; one-way ANOVA F = 17.5 P < 0.001) and thermal hyperalgesia values (dOVT: delta of −1 ± 0.7 seconds, N = 8; saline: delta of 0.1 ± 0.7 seconds, N = 7; one-way ANOVA F = 9.3 P < 0.001) in NMS animals. As for control rats, a single i.t. injection of the V1aR antagonist did not change mechanical (before: 164.65 ± 9.9 g; after i.t. injection: 145.75 ± 10.3 g, N = 8, the Mann–Whitney test: U = 20.0; P = 0.2241) nor thermal nociceptive thresholds (before: 6.13 ± 0.4 seconds; after i.t. injection: 5.5 ± 0.5 seconds, N = 8; the Mann–Whitney test: U = 22.0; P = 0.3514) in NMS rats.
Together, this suggests that NMS has long-term consequences on the efficacy of OTR-mediated spinal analgesia, at least in short-term inflammatory painful sensitization.
To further characterize this NMS-altered OTR-mediated analgesia in NMS adult rats, we recorded wide-dynamic-range neurons in the SC of anesthetized rats and analyzed the spinal processing of nociceptive messages through the expression of wind-up, a nociceptive-specific short-term plasticity.23 As illustrated in Figure 4, wind-up recorded 24 hours after carrageenan injection was significantly amplified after the spinal application of dOVT (1 µM). In control animals (N = 6, Fig. 4A), the wind-up protocol led to an average maximum frequency of 23.6 ± 1.5 spikes/seconds. This value significantly increased to a maximum frequency of 44.1 ± 4.8 spikes/seconds after dOVT application (N = 6; 2-way repeated-measures ANOVA, treatment × time, F(14,70) = 7.811; P < 0.0001). As shown in Figure 4B, we did not see any change in wind-up frequencies in NMS animals (N = 6) after dOVT application (before application: 26.8 ± 4.3 spikes/seconds; after application: 25 ± 4.1 spikes/seconds; 2-way repeated-measures ANOVA, treatment × time, F(14,70) = 1.366; P = 0.1936). Summary of results can be found in Figure 4C (the KW test: KW = 8.9; P = 0.03).
Altogether, electrophysiological and behavioral experiments suggested that the OT descending inhibitory control of pain is inefficient in NMS rats.
3.4. Oxytocin receptor expression and oxytocin receptor–induced analgesia
To better understand the possible molecular and cellular alterations induced by NMS, we first analyzed the expression of OTR and a possible dysfunction of its downstream signaling, focusing on the SC because it is the first relay for nociceptive messages. There was no difference in the spinal expression of OTR gene transcripts between control (N = 7; 3.1 ± 0.9 spinal relative expression) and NMS (N = 8; 2.4 ± 0.2 spinal relative expression) rats (the unpaired Student t test, P = 0.44, t = 0.78, df = 13), as illustrated in Figure 5A.
To further confirm that OTRs were not only expressed at similar levels in both groups but were equally functional, we measured the analgesic behavioral response to i.t. injection of OT (1 µM 20 µL) or to the selective agonist TGOT (1 µM, 20 µL) in CTRL and NMS rats (Fig. 5B). In CTRL animals, OT (N = 8) or TGOT (N = 8) significantly increased mechanical thresholds 20 minutes after the injection (OT: from 467.4 ± 19.4 to 741.6 ± 67.1 g; TGOT: from 471.8 ± 14.6 to 813.2 ± 26.3 g; 2-way repeated-measures ANOVA, treatment × time, F(4,40) = 14.67; P < 0.0001). Both OT (N = 7) and TGOT (N = 8) significantly increased mechanical thresholds 20 minutes after the injection in a reversible manner in NMS animals (OT: from 288.4 ± 5.0 to 533.9 ± 13.1 g; TGOT: from 246.7 ± 11.2 to 460.1 ± 17.3 g; 2-way repeated-measures ANOVA, treatment × time, F(11,38) = 66.94; P < 0.001). This result confirmed that spinal OTRs are functional in NMS rats and capable of producing efficient analgesia. We thus checked whether NMS could lead to an increase in the spinal degradation of OT. As illustrated in supplementary Figure 1A (http://links.lww.com/PAIN/A649), we found no difference in the spinal expression of oxytocinase (OTase) mRNA between control and NMS rats (N = 8; the unpaired Student t test, t = 0.1317, df = 14; P = 0.8971).
3.5. Neonatal treatments with oxytocin and allopregnanolone
Many studies provided evidence that OT in the neonatal period could be protective against early life adverse events or lesions.3,38 Moreover, NMS is known to reduce maternal behavior towards the pups and a subsequent altered development of OT systems, possibly leading to blunted OT levels.32 This includes molecular mechanisms affecting chloride homeostasis and the production of neuroprotective neurosteroids such as AP.25
In a first set of experiments, we treated NMS rat pups o.d. from P2 to P12 with OT (1 mg/kg in 0.9% NaCl, i.p., Fig. 6). As shown in Figure 6A, neonatal OT treatment fully restored normal mechanical nociceptive thresholds when measured at P50 (control: 320.7 ± 10.9 g, N = 12; NMS: 235.1 ± 6.9 g, N = 8; OT-treated NMS: 320.4 ± 6.3 g, N = 10; one-way ANOVA, F = 23.39, P < 0.001). Oxytocin neonatal treatment also restored an efficient analgesia after FSS (Fig. 6B; delta for the control group: 261.8 ± 18.6 g, N = 9; NMS: 17.8 ± 11.6 g, N = 8; OT-treated NMS: 168.9 ± 10.4 g, N = 9; one-way ANOVA, F = 30.08, P < 0.001). In contrast to SIA after FSS, neonatal OT treatment failed to restore OT antihyperalgesia 24 hours after carrageenan-induced inflammation, measured as before using i.t. dOVT injection (Fig. 6C). The dOVT injection did not significantly amplify mechanical hyperalgesia (delta for NMS: 5.8 ± 10.9 g, N = 8; OT-pretreated NMS: 45.6 ± 19.7 g, N = 8; one-way ANOVA, F = 14.28, P < 0.001).
Recently, an original long-lasting molecular mode of action of OT in SC neurons has been proposed. It leads to the stimulation of AP synthesis, a neurosteroid well-known for its analgesic and neuroprotective roles.49 We thus tried to restore OT function in NMS rats by submitting newborn pups to AP during the NMS period. As illustrated in the different panels of Figure 6, a neonatal treatment with s.c. AP (10 mg/kg) between P2 and P12 was associated with a mean mechanical threshold at adulthood that was higher (ie, hyposensitivity) than those measured in control animals (Fig. 6A; control: 320.7 ± 10.9 g, N = 12; AP-treated NMS: 394.1 ± 21.9 g, N = 10; one-way ANOVA, F = 23.39, P < 0.001). As for neonatal OT treatment, SIA could be restored by AP treatment (Fig. 6B; baseline-corrected mean mechanical threshold in AP-treated NMS: 351 ± 47 g, N = 8, one-way ANOVA, F = 30.08, P < 0.001). As previously reported, endogenous OT antihyperalgesia could not be revealed after dOVT i.t. injection (Fig. 6C; AP-treated NMS: −23 ± 16 g, N = 8; one-way ANOVA, F = 14.28, P < 0.001). Taken together, our results likely suggested NMS to be associated with a deficit in OT signaling. However, we found no significant difference in the spinal transcripts levels of 3α-hydroxysteroid dehydrogenase (3α-HSD), 5α-reductase type II (5α-red II), nor cholesterol side-chain cleavage enzyme (P450scc), enzymes involved in AP synthesis (supplementary Figure 1B, available at http://links.lww.com/PAIN/A649; N = 8; one-way ANOVA, F = 0.7409, P = 0.6384). Similarly, spinal translocator protein (TSPO) mRNA levels remained unchanged in NMS rats compared with controls.
3.6. Neonatal maternal separation–induced epigenetic alterations and rescue of maternal separation pain phenotype by targeting histone deacetylases
The long-term consequences of NMS on pain sensitivity and on the rat ability to cope with stress/pain potentially rely on epigenetic mechanisms. Indeed, several reports highlight the effects of NMS on the HPA axis and on maternal behavior.8,10 We thus measured the spinal expression of a set of transcripts coding for several epigenetic factors at different time points (from P7 to P100) in NMS and control animals (Fig. 7A). We found that mRNA levels for some class I (HDACs 1 and 8) and class II HDAC (HDAC 7) were significantly increased in NMS animals at P24 (HDACs 1 and 7), P45 (HDACs 1, 7, and 8), and until P100 for HDAC 7 (KW test; HDAC 1: KW = 60.33, P < 0.0001, N = 7; HDAC 7: KW = 61.41, P < 0.0001, N = 7; and HDAC 8: KW = 49.28, P < 0.0001, N = 6).
In particular, HDAC 7 mRNA spinal expression was significantly increased in NMS animals compared with controls at P24 (0.49 ± 0.04 relative expression in CTRL and 1.22 ± 0.11 in NMS), as illustrated in Figure 7B, and levels remained significantly higher until adulthood at P100 (0.36 ± 0.03 relative expression in CTRL and 1.01 ± 0.04 in NMS). These results likely suggest that NMS triggers long-lasting changes in gene expression, involving at least HDAC functions.
We hence performed a neonatal chronic treatment from P2 to P12 with SAHA (10 mg/kg i.p.), a nonselective HDAC I and II inhibitor. Neonatal treatment of newborn pups with SAHA during NMS was associated with mean mechanical nociceptive thresholds at adulthood (P50), which were similar to the control group (Fig. 7C; control: 320.7 ± 10.9 g, N = 8; SAHA-treated NMS: 303.8 ± 9.4 g, N = 10; one-way ANOVA, F = 14.28, P < 0.001). This treatment also partially restored SIA after FSS (Fig. 7D; SAHA-treated: 130 ± 26.7 g, n = 7; one-way ANOVA, F = 34.73, P < 0.001). Interestingly, SAHA treatment partially restored OT antihyperalgesia after carrageenan inflammation (Fig. 7E; control: 75.6 ± 8.4 g; SAHA-treated: −41.7 ± 12.3 g; one-way ANOVA, F = 15.75, P < 0.0001).
In summary, we provide functional evidence indicating that NMS strongly affects adult nociception and the efficacy of the oxytocinergic descending inhibitory control. Neonatal MS is associated with (1) hypersensitivity to mechanical and thermal hot stimulation at adulthood, (2) increased vulnerability to thermal inflammatory–induced hyperalgesia, and (3) dysfunction of the OT descending control of pain after nonpainful FSS or painful inflammatory sensitization. We further succeeded in rescuing these abnormalities in the nociceptive phenotype by repeated neonatal injections of OT, AP, or SAHA. Altogether, our data suggest that NMS has a strong impact on nociceptive integration, which can blunt the efficacy of descending controls of pain such as the oxytocinergic system that targets the spinal GABAergic inhibitory system. It also suggests that the OT or epigenetic systems can be good targets to develop early treatments to counteract the negative effects of ELS, at least in this model.
4.1. Early life events promote nociceptive hypersensitivity and alter oxytocin receptor–mediated analgesia
Early life stress includes a wide range of adverse events that have long-term negative effects on brain function. In humans, ELS has been associated with an increased risk of developing chronic pain states at adulthood,15 including irritable bowel syndrome.51,52 In animal studies, the visceral hypersensitivity after NMS has been widely demonstrated and correlated with differential activation of supraspinal regions, including the periaqueductal gray, which is a key area for descending pain controls. Here, we confirmed that nociceptive hypersensitivity extends to somatic mechanical and thermal hot stimuli, as previously published by our laboratory27 and in other ELS models.1,47,58,63 Our observation of an increased thermal pain symptom after carrageenan is consistent with studies showing an increased response after formalin injection60 or after partial sciatic ligation45 in NMS animals. Moreover, OT analgesic action on thermal sensitivity also relies on an action on TRPV1 receptors because OT seems able to directly activate and promote the rapid desensitization of TRPV1 receptors expressed by C-type sensory fibers.44 Although not tested in this study, we cannot exclude such an effect in NMS animals, as an increased duration of thermal hot hyperalgesia could be due to the loss of OT action on TRPV1 receptors.
Nociceptive hypersensitivity could be easily explained by a long-lasting upregulation of voltage-gated channels Nav1.8 and Nav1.9 in dorsal root ganglion neurons as recently published.27
In this study, impairment of the oxytocinergic pain control has been revealed after recruitment by a nonpainful or painful stress. The hypothalamus, where OT producing neurons are located, seems particularly vulnerable to ELS and a dysfunction of the oxytocinergic system may contribute to these alterations, as OT plays a key role in the modulation of stress responses, anxiety, social interactions, and pain responses. Despite several trials, we have not been able to demonstrate that endogenous OT concentrations are different between groups. Recently, NMS in rodents was shown to be associated with changes in OTR- and V1aR-binding sites in specific brain regions,35 suggesting a vulnerability of this system to the early environment. In humans, ELS has been associated with low OT concentrations in cerebrospinal fluid.22 Here, we show that OT analgesia is impaired after NMS, both in painful and nonpainful conditions. To exert its analgesic effects at the spinal level, OT relays on 3 main mechanisms: (1) activation of a spinal microcircuit leading to an increased GABAergic inhibitory tone,7 (2) decrease of the excitability of SC neurons in the superficial layers,6 and (3) stimulation of AP synthesis, which further amplifies GABAergic inhibition by allosteric modulation.25 Here, we showed that spinal OTRs are functional in NMS rats and that OTR activation can induce an efficient and transient analgesia, suggesting that the lack of OT analgesia could hence result from supraspinal alterations. In agreement with our previously published results,16,26 we failed to reveal any spinal contribution of the vasopressin V1A receptor in the fine tuning of basal nociceptive threshold or to induce oxytocinergic analgesia after painful or nonpainful stress. This control was important because some discrepancies exist in the literature, often pointing out a specie-specific issue and a concentration-dependent action difficult to compare because of different administration procedures.54 Exploring hypothalamic OT neurons, pharmacological and electrophysiological characteristics might help us to have a deeper understanding of the mechanisms leading to pain-related ELS consequences. A lack of AP synthesis after OTR activation could also explain the lack of OT antinociception following carrageenan inflammation in NMS rats, although we found no change in the spinal expression of transcripts coding for proteins responsible for AP synthesis in NMS rats, nor in the spinal expression of OTase mRNA.
4.2. Rescuing neonatal maternal separation long-term alterations of the nociceptive system
In our working hypothesis, we asked whether the lack of physical interaction between the mother and the pups could be a critical determinant to explain the deleterious plasticity of the nociceptive system and the deficit in oxytocinergic analgesia. Indeed, mother presence has already been shown to be protective against pain. Kangaroo mother care, in particular, is more frequently recommended and has been shown to have immediate analgesic effect on pain responses in the newborn24,33 and to be protective against long-term alterations usually after preterm birth.11,18 As OT has a key role in the modulation of parental behavior and attachment,9,17 we performed a rescue experiment by repeated neonatal OT treatment during NMS, which reversed some consequences of NMS on pain behavior. In the literature, OT has been proposed as a therapeutic agent for different neurodevelopmental pathologies such as autism spectrum disorders because it decreases anxiety and optimizes social behavior.39,64 In animal models, postnatal injections of OT have been shown to prevent feeding, and social and memory deficit in the Prader–Willi model.38,53 Here, we only studied the beneficial consequences of an early treatment with OT on pain behaviors, but further experiments would be helpful to confirm whether this treatment could also counteract the detrimental effects of NMS on pain-related comorbidities such as stress and anxiety. These observations might have great clinical relevance in neonatal intensive care units because it confirms, once again, the crucial role of physical interactions between the parents and the newborn to optimize newborn development and install proper homeostatic controls to cope with stress (including pain).
A close reciprocal relationship exists between the oxytocinergic system and neurosteroids. Allopregnanolone is not only a downstream second messenger of OTR activation,25 it has well-known anxiolytic, analgesic, and neuroprotective properties.49 In this study, a rescue experiment with neonatal AP treatment appeared successful to restore normal pain sensitivity and a functional OT analgesia after FSS. Neonatal AP level alterations can indeed lead to changes in behavior at adulthood.14 Our result is also in good agreement with other studies using AP treatment to prevent neonatal anxiety after a short-term NMS (24 hours).31 The mechanisms by which AP could be protective against NMS consequences on pain controls are still unknown. However, neonatal AP level alterations could interact with chloride homeostasis development because AP has also been shown to change KCC2 expression in the hippocampus.40
Eventually, it is not surprising that HDAC inhibitor and epigenetic changes are involved in the long-term consequences of NMS on nociception and OT analgesia. It has long been known that neonatal stress has long-lasting effects through epigenetic mechanisms.8,10,43 Epigenetic changes in chromatin compaction through histone modification such as histone acetylation have been demonstrated after NMS in different brain areas.29,41,43 Here, we also demonstrate that the spinal expression of different pain-related epigenetic factors is altered after NMS, in particular HDACs 1, 7, and 8, which are overexpressed in a long-lasting manner compatible with the observed pain phenotypes. In the pain context, histone acetylation can modulate nociceptive response in inflammatory neuropathic and visceral pain models.61 Spinal class II HDACs are involved in inflammatory hyperalgesia,2 which could explain the beneficial effects of SAHA treatment observed in this study after carrageenan inflammation and the critical role of spinal HDAC 7 overexpression in NMS animals. In line, it has been suggested that suppressing HDAC 1 expression in the SC attenuated neuropathic pain symptoms.12 The OTR gene promoter is also subjected to increased association of acH3K14 after NMS in critical structures for pain control.30 However, we did not see any difference in the spinal expression of OTR in our study, providing further evidence that histone modification may be necessary but not sufficient to exert significant and long-lasting changes in specific gene expression and that epigenetic alterations targeting the OTR gene may actually be region-dependent.
Corroborating our results, Moloney et al.41 showed that NMS is associated with an alteration in histone acetylation at the spinal level and were the first to show that adult SAHA treatment can reverse NMS-induced visceral hypersensitivity. Here, we show that a preventive neonatal treatment with the same agent is also effective to prevent mechanical and thermal hypersensitivity at adulthood. At this stage, the underlying mechanisms are still to be identified as well as the molecular targets by which SAHA exerts its protective effects against NMS.
In conclusion, NMS induced both a basal hypersensitivity to pain and a dysfunction in OT descending control of pain in painful and nonpainful conditions. Some of these alterations can be rescued by neonatal SAHA or OT/AP treatment, but other alternatives need to be explored to be able to recover completely the OT antinociceptive action under inflammatory conditions. By studying the mechanisms of action of these pharmacological treatments, we might be able to identify new therapeutic targets to avoid NMS-induced reprogramming of pain circuits.
Conflict of interest statement
The authors have no conflict of interest to declare.
This project has been supported by public funding from University of Strasbourg and CNRS. M. Melchior and G. Gazzo are fellows of the French Ministry of research. P.-E. Juif was awarded a Gisele Guilbaud/Sfetd research price in 2013. We thank the following research programs of excellence for their support: FHU Neurogenycs, French National Research Agency (ANR) through the Programme d'Investissement d'Avenir (contract ANR-17-EURE-0022, EURIDOL Graduate School of Pain).
Supplemental digital content
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Supplemental video content
Video content associated with this article can be found online at http://links.lww.com/PAIN/A650.
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