Blockage of Ventrolateral Periaqueductal Gray Matter Cannabinoid 1 Receptor Increases Dental Pulp Pain and Pain-Related Subsequent Learning and Memory Deficits in Rats

Cannabinoid 1 receptor (CB1R) signaling has a pivotal role in the modulation of both pain and cognitive responses. This study aims at investigating the role of CB1R in the ventrolateral periaqueductal gray matter (vlPAG) on both pulpal pain and pain-related subsequent changes in learning and memory performances in rats. The adult male Wistar rats were cannulated in the vlPAG. The rats were pretreated by intra-vlPAG administration of selective CB1R antagonist AM-251 (2, 4 and 8 µg/rat) and vehicle dimethylsulfoxide. The drugs were microinjected 20 min before the induction of capsaicin-induced pulpalgia. The nociceptive behaviors were recorded for 40 min. Then, passive avoidance and spatial learning and memory were assessed using the shuttle box and Morris water maze tests, respectively. Following the administration of intradental capsaicin, there was a significant nociceptive response that increased after an induced blockage of CB1R by AM-251 at 4 and 8 µg. In addition, capsaicin impaired passive avoidance and spatial memory performance of rats. Microinjection of AM-251, prior to capsaicin, could dose-dependently exaggerate capsaicin-related learning and memory deficits in both tests. The present data indicated that the vlPAG endocannabinoid system is involved in the modulation of pain signals from dental pulp. It was also accompanied by learning and memory impairments.