Induction of Fos-like immunoreactivity in the basal ganglia by ether anaesthesia: effects of injections of muscimol in rostral versus caudal substantia nigra pars reticulata.

Exposing rats to ether anaesthesia for 20 min induced a massive expression of c- fos like (FL) immunoreactivity in specific regions of the brain, including the basal ganglia. This expression of FL immunoreactivity in the basal ganglia was successfully blocked by local injections of muscimol (GABA(A) agonist) in the substantia nigra (SN). This model was used to investigate our recent finding that inhibition of caudal SN pars reticulata (SNpr) with muscimol is more potent in suppressing the motor component of tonic seizures than comparable inhibition of the rostral part. Two experiments were carried out. In experiment I, the effects of ether anaesthesia on c- fos expression in the brain were investigated. Brain sections taken from untreated control rats and stained immunocytochemically with c- fos antibodies. The results showed no FL immunoreactivity in the SNpr, subthalamic nucleus (STN), entopeduncular nucleus (EPN) and globus pallidus (GP). However, a few scattered FL immunoreactive nuclei were observed in the caudate-putamen (CPu). In rats exposed to ether, numerous FL immunoreactive nuclei were detected in the SNpr, STN, EPN and GP. In the CPu, a slight increase in number of labelled nuclei was noted. In experiment II, rats were prepared with intracranial chronic cannulae through which the effects of muscimol (60 ng) injected unilaterally in either the rostral or caudal SNpr on c- fosexpression induced by ether were investigated. Injections in both sites caused remarkable suppression of the ether-induced c- fos throughout the ipsilateral SNpr; however, the rostral injection also inhibited the expression of c- fos in the STN, EPN and GP. The implications of these results for microinjection mapping studies of the SN are discussed.