Reduced nicotinic receptor-mediated antinociception following in vivo antisense knock-down in rat

Pharmacological activation of neuronal nicotinic acetylcholine receptors ca n produce non-opioid antinociception in rodents. However, multiple nAChR su btypes exist, the most abundant of which contain alpha 4 and beta 2 subunit s. The purpose of the present study was to investigate the role of alpha 4- containing nAChRs in mediating nicotinic antinociception using an in vivo a ntisense strategy. Both i.c.v. infusion and repeated bolus injections into the cerebral aqueduct of an antisense oligonucleotide against the alpha 4 s ubunit significantly attenuated the antinociceptive effects of the nAChR ag onist A-85380 in the paw withdrawal test of acute thermal pain. Rats treate d with a scrambled oligonucleotide displayed a full antinociceptive respons e to A-85380, while discontinuing antisense treatment restored the antinoci ceptive effects of the nicotinic agonist. Double immunohistochemical labeli ng revealed near-complete overlap of expression of the serotonin marker try ptophan hydroxylase and the alpha 4 nAChR subunit in the dorsal raphe nucle us. The expression of alpha 4-containing nAChRs by serotonergic neurons in the dorsal raphe offered a means to address nonspecific alpha 4 knock-down, i.e., oligonucleotide-induced neurotoxicity. Immunohistochemical detection of 014 expression was reduced by nearly 50% in the dorsal raphe of antisen se-treated rats as compared to either saline or missense-treated controls. In contrast, the expression of tryptophan hydroxylase, as well as, the alph a 7 nAChR subunit in antisense-infused rats was similar to that observed in saline- and missense-treated controls. The results of these studies sugges t that alpha 4-containing nAChRs, possibly expressed by serotonergic neuron s, are involved in nicotinic-mediated analgesia. However, these data do not eliminate the possibility that other nicotinic subunit combinations may al so play a role in antinociception produced by nAChR activation. (C) 2000 El sevier Science B.V. All rights reserved.