The role of biogenic agents in the actions of centrally-acting analgesics

Effects of naloxone on the secretion of prolactin and corticosterone induced by 5-hydroxytryptophan and a serotonergic agonist, mCPP

The effects of naloxone, an opiate "pure" receptor antagonist, on the release of prolactin and corticosterone in the rat were studied following the administration of the serotonin precursor 5-hydroxytryptophan or the serotonin receptor agonist (-) -m-chlorophenylpiperazine. Naloxone clearly antagonizes the release of prolactin induced by 5-hydroxytryptophan administered alone at a dosage of 50 mg/Kg/b.wt. or at dosage of 30 mg/Kg/b.wt. preceded 60 minutes before injection by the administration of the serotonin uptake blocker fluoxetine. The opiate antagonist does not modify the increase in blood level of prolactin induced by (-) -m-chlorophenylpiperazine. Naloxone itself does not reduce the increase in plasma level of corticosterone induced by 5-hydroxytryptophan, 5-hydroxytryptophan+fluoxetine or (-)-m-chlorophenylpiperazine. The results suggest that endogenous opioids may be involved in the increase in serum level of prolactin induced by 5-hydroxytryptophan and also indicate the existence of different serotonergic neurotransmitter circuits capable of modulating the release of prolactin and corticosterone. A mutual interplay between serotonergic and opiate neurons may be involved in controlling the release of prolactin, but such an interplay does not seem to occur in the secretion of corticotrophin-releasing hormone.

Comparison of the antinociceptive activity of intraventricularly administered acetylcholine to narcotic antinociception

Intraventricularly administered acetylcholine inhibits mouse tail-flick latency in a dose-dependent manner with an ED50 of about 20 microgram. This antinociceptive activity is not mimicked by biogenic amine neurotransmitters (i.e. norepinephrine, epinephrine, dopamine, serotonin or histamine) and is not markedly affected by selective depletors of brain catecholamines or serotonin. However, pretreatment with reserpine or tetrabenazine dramatically reduces acetylcholine-induced antinociception. Tolerance develops rapidly to the antinociceptive effects of acetylcholine. Cross-tolerance to morphine in acetylcholine-tolerance mice is minimal, but the antinociceptive activity of acetylcholine is markedly reduced in mice chronically pretreated with morphine. Acetylcholine-induced antinociception differs from narcotic antinociception in the reversed stereoselectivity of several narcotic antagonists and in the in vivo pA2 values for inhibition by naloxone. Therefore, the antinociceptive activity of intraventricularly administered acetylcholine cannot be described as a specific narcotic action.