Systemic administration of
nicotinic receptor (nAChR) agonists is antinociceptive in models of
acute pain whereas their intrathecal (i. t.) administration has been reported to be antinociceptive, nociceptive or without effect. It has been hypothesized that the action induced is dependent upon the subtype and location of the nAChR activated. In addition, there is considerable evidence that nAChR
ligand-induced antinociception is mediated by other
neurotransmitter systems via descending pathways from the brainstem to the spinal cord. The present study investigated the effects of i. t. and systemic administration of
A-85380, a novel nAChR agonist, in the paw withdrawal model of acute thermal
pain in the rat. Given i.t. ,
A-85380 (1 and 10 nmol/rat) decreased the latency to paw withdrawal by 2-4 s. This pronociception was accompanied by a spontaneous flinching behavior. Both of these effects were differentially blocked by i.t. pretreatment with the nAChR antagonists
mecamylamine (10 nmol)>MLA (100 nmol)>DHbetaE (50% with 1000 nmol) but not by
alpha-bungarotoxin (0% at 0.63 nmol). Given systemically,
A-85380 (0.56 micromol/kg, i.p.) induced antinociception as indicated by an increased latency to paw withdrawal, an effect differentially altered by i.t. pretreatment with monoaminergic antagonists (100 nmol/rat). While
mecamylamine and
prazosin had no effect,
scopolamine,
methysergide and
MDL 72222 partially antagonized and
idazoxan completely antagonized A-85380-induced antinociception. Finally, as measured by in vivo microdialysis, levels of
5-HT, but not NE, in the i.t. space of the lumber region of the spinal cord were significantly increased following the systemic administration of
A-85380. Together these data suggest that the nociceptive properties of spinally administered nAChR agents are not mediated by either an alpha(4)beta(2) or an alpha(7) subtype nAChR, whereas the antinociceptive properties of systemically-administered nAChR agents are mediated by descending noradrenergic, serotonergic and
muscarinic inhibitory pathways.