Direct application of cannabinoids to the medullary dorsal horn (MDH) inhibits lamina V nociceptive neurons. The present study compared the effect of the cannabinoid receptor agonist, WIN 55,212-2 (WIN-2) on the activity of lamina I and lamina V MDH neurons using extracellular single unit recording in anesthetized rats. Activity evoked by a contact thermode was measured before and after local application of WIN-2 (0.5–2.0 μg/μl) to the brainstem. Fast and slow heat ramps were used to differentiate between activity evoked primarily by A-delta and C primary afferent fibers, respectively. In lamina V neurons, WIN-2 produced a concentration dependent decrease in activity evoked by both fast and slow heat, reaching significance at 1.0 μg/μl. In lamina I neurons, WIN-2 administration inhibited slow heat-evoked activity beginning at 1.0 μg/μl, but had no significant effect on fast heat-evoked activity, even at the highest concentration (2.0 μg/μl). In separate experiments, the effect of intrathecal administration of WIN-2 to the MDH on head withdrawal latencies elicited by fast and slow heat ramps applied to the whisker pad was assessed in lightly anesthetized rats. Head withdrawal latencies elicited by slow but not by fast heat stimulation were increased by WIN-2. Taken together, these results emphasize the importance of lamina I neurons in the control of a nociceptive heat-evoked reflex.
aDepartment of Oral Diagnosis, School of Dentistry, Nihon University, Kanda-Surugadi, Chiyoda-ku, Tokyo 101-8310, Japan
bDepartment of Physiology, College of Osteopathic Medicine, University of New England, 11 Hills Beach Road, Biddeford, ME 04005, USA
*Corresponding author. Tel.: +1 207 602 2195; fax: +1 207 294 5931.
Received May 8, 2008
Received in revised form October 20, 2008
Accepted November 24, 2008.