Nicotinic acetylcholine receptors in neonatal motoneurons are regulated byaxotomy: An electrophysiological and immunohistochemical study in human bcl-2 transgenic mice

Motoneuron axotomy was exploited as a model system for studying functional and morphological changes caused in motoneuron cell bodies by peripheral ax on injury. Rodent facial motoneurons express functional nicotinic acetylcho line receptors. We have determined the effect of neonatal unilateral facial nerve transection on these receptors by using electrophysiological and imm unohistochemical techniques. To avoid rapid apoptotic cell death of axotomi zed motoneurons, the study was done in mice overexpressing the human bcl-2 transgene. Intact motoneurons responded to acetylcholine by generating a ra pidly rising inward current, which was insensitive to methyllycaconitine, a selective antagonist of alpha7-containing nicotinic receptors, bur was sup pressed by dihydro-beta -erythroidine, a broad-spectrum antagonist. This in dicates that mouse facial motoneurons possess nicotinic receptors which art : probably devoid of the alpha7 subunit. In striking contrast, axotomized m otoneurons displayed little or no sensitivity to acetylcholine. Axotomy did not affect the sensitivity of facial motoneurons to the selective glutamat e receptor agonist alpha -arnino-3-hydroxy-5-methyl-5-isoxaxolepropionic ac id. Immunohistochemical studies revealed that the alpha4 nicotinic receptor subunit was present in intact motoneurons but was undetectable in axotomiz ed motoneurons. By contrast, the beta2 subunit was comparable in intact and axotomized motoneurons. alpha3 immunoreactivity was undetectable, both in intact and in axotomized motoneurons. Thus, mouse facial nicotinic receptors are possibly of the alpha4 beta2 typ e and axotomy interferes negatively with the expression of the alpha4 subun it. By down-regulating nicotinic receptors, peripheral nerve injury may fac ilitate motoneuron degeneration. Alternatively, nicotinic receptor downregu lation and motoneuron degeneration may be independent consequences of perip heral axotomy. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.