AXOTOMY-INDUCED LOSS OF M2 MUSCARINIC RECEPTOR MESSENGER-RNA IN THE RAT FACIAL MOTOR NUCLEUS PRECEDES A DECREASE IN CONCENTRATION OF MUSCARINIC RECEPTORS

The abundance of muscarinic receptors and m2 muscarinic receptor mRNA in the facial nuclei of rats was evaluated by autoradiographic procedu res at various times up to 14 days after transection of the right faci al nerve. Receptors were labelled by in nitro incubation of brain sect ions with L-[H-3]quinuclidinyl benzilate, while in situ hybridization with a S-35-labelled oligonucleotide was used to identify m2 muscarini c receptor mRNA in neighbouring sections. The right and left facial nu clei of non-operated control rats appeared equivalent in abundance of muscarinic receptors (359 +/- 8 versus 376 +/- 9 fmol per mg tissue, n = 5) and the presence of m2 mRNA. Axotomy had no effect on the concen tration of receptors in the contralateral facial nucleus but caused a gradual loss of receptors from the ipsilateral side. No change was det ected at 1 day after nerve transection, but a 23% decrease relative to the contralateral facial nucleus had occurred by 3 days. A maximum de crease of 51% was achieved by 1 week after nerve transection. By compa rison, m2 mRNA was nearly eliminated from the ipsilateral facial nucle us at 1 day post-taxonomy and remained depleted for the duration of st udy. Previous work has established that no significant loss of motoneu rons occurs within this period. Accordingly, it is postulated that axo nal injury inhibits transcription of the m2 muscarinic receptor gene, resulting in a later decrease in muscarinic receptor protein expressio n.