MUSCARINIC ACETYLCHOLINE-RECEPTOR IMMUNOREACTIVITY IN THE AMYGDALA .2. FEAR-INDUCED PLASTICITY

Changes in the distribution of muscarinic acetylcholine receptor-immun oreactive neurons were examined in the amygdaloid complex al different time-intervals following a single training session of active shock av oidance in a two-way shuttle-box. Muscarinic acetylcholine receptors w ere visualized using M35, a monoclonal antibody raised against purifie d muscarinic acetylcholine receptor protein. Both in naive animals and 2 h after active shock avoidance training, muscarinic acetylcholine r eceptor immunoreactivity was high in the central nucleus, and only low to moderate in other amygdaloid regions. Twenty-four hours after trai ning, however, the muscarinic acetylcholine receptor immunoreactivity distribution pattern was reversed, showing a dramatic increase in the corticomedial nucleus, while in contrast, in other amygdaloid regions including the central nucleus, muscarinic acetylcholine receptor immun oreactivity was reduced to only a few scattered neurons. Additional st udies with a modified experimental design indicated that fear conditio ning mechanisms in association with the severity of the aversive stimu li, and not the learning of the avoidance response, may account for th e changes in muscarinic acetylcholine receptor immunoreactivity in the amygdala. These results are consistent with the prominent role of the central nucleus in the conditioning and expression of the fear respon se. A closer examination revealed that 8 h after training the changes in both the central and corticomedial nuclei became significant. The d ifferences still existed after 25 days, but three months after the tra ining session the receptor distribution was returned to normal. The lo ng-lasting, but reversible nature of these changes indicates that fear conditioning is accompanied by a dynamic plasticity of muscarinic ace tylcholine receptor immunoreactivity in the amygdaloid complex. Copyri ght (C) 1996 IBRO.