Figure 4.

Schematic showing brain regions with elevated levels of nAChRs, within which microinjection of nicotine elicited analgesia. The ventrolateral PAG and Pptg project to RVM, which gives rise to bidirectional modulation of spinal nociceptive transmission. Nicotine acting at supraspinal sites is postulated to engage RVM-spinal descending pathways that exert a predominantly inhibitory effect on spinal nociceptive transmission. Blue shading: areas with neurons expressing nAChRS. Abbreviations: RVM, rostral ventromedial medulla. +, excitatory synapse; −, inhibitory synapse.