The objectives of this research proposal are twofold. First, it will be determined if a cholinergic neuronal pathway exists between the pedunculopontine tegmental nucleus (PPTn) and the nucleus raphe magnus (NRM), and if the antinociceptive effects of microinjections of nicotine (NIC) and a muscarinic agonist ((+)-cis-dioxolane, CD) into the PPTn arise from activation of this pathway. It is hypothesized that this pathway has cell bodies in the PPTn with nerve terminals synapsing on serotonin-containing cells of the NRM. The second objective of this research is to determine the functional and practical significance of the PPTn-to-NRM cholinergic pathway in the production of antinociception. It is hypothesized that this cholinergic path is activated by nicotinic and muscarinic agonists in the PPTn resulting in the stimulation of the known descending serotonergic pain inhibitory system originating from raphe magnus. It is further hypothesized that antinociceptive tachyphylaxis does not develop to repetitive activation of the PPTn-NRM cholinergic path, and that antinociceptive cross-tolerance between PPTn stimulation-induced antinociception and morphine does not develop.
The specific aims are: to determine if ibotenic acid lesions of the PPTn, and if local anesthesia, lesions and M1 receptor antagonism of the NRM, disrupt nicotinic and muscarinic-induced antinociception arising from PPTn. The degree of destruction of the putative PPTn to NRM cholinergic pathway induced by AF64A will be examined immunohistochemically and correlated with the loss of NIC and CD antinociception originating from PPTn. The roles of descending serotonergic and noradrenergic pathways in the antinociception arising from the PPTn will be determined. Finally, it will be determined if antinociceptive tachyphylaxis develops to repeated electrical or chemical stimulation of the PPTn, and if cross-tolerance develops to morphine. The methods to be used in this study include subcortical drug microinjection and electrical stimulation, hotplate and tailflick antinociception, subcortical electrolytic and chemical lesions, intrathecal drug administration and perfusion, and visualization of cholinergic and serotonergic neurons using retrograde tracing, immunohistochemistry, and histofluorescence. This research is of clinical importance as knowledge about the brain's nonopioid pain suppression systems will increase the probability of developing new analgesics with little or no dependence liability, and may improve the pharmacological intervention of pain suffered by narcotic-tolerant individuals and of pain insensitive to the narcotic analgesics.
