REGIONAL DISTRIBUTION OF THE ANTICONVULSANT AND BEHAVIORAL-EFFECTS OFMUSCIMOL INJECTED INTO THE SUBSTANTIA-NIGRA OF RATS

Previous anatomical investigations have reported a direct projection f rom substantia nigra pars lateralis to the dorsal midbrain anticonvuls ant zone. The present study tested the hypothesis that the anticonvuls ant properties of nigral inhibition previously attributed to substanti a nigra pars reticulata were, in fact, due to the suppression of neura l activity in the adjacent pars lateralis. Using the electroshock mode l of epilepsy, a systematic map of the anticonvulsant effects of bilat eral injections of muscimol (60 ng/0.5 mu l per side) into different p arts of substantia nigra was constructed. Electroshock (1 s of 40 mA 5 0 Hz AC) was administered via ear-clip electrodes 5 or 60 min followin g injections of muscimol, or 60 min after control injections of saline . To provide insight into the functional mechanisms whereby nigral inh ibition might suppress tonic seizures the behavioural effects elicited by muscimol were also noted. No evidence supporting the experimental prediction was found. The most sensitive region of substantia nigra fo r suppressing tonic hindlimb extension was caudal pars reticulata. The se data indicate a serious mismatch between the results of microinject ion mapping studies and underlying patterns of anatomical connectivity . The behavioural reaction most closely associated with tonic seizure suppression was stereotyped locomotion; both were obtained maximally f rom caudal pars reticulata. Rostral substantia nigra was associated mo re with oral stereotypy, while a raised head position was observed at lateral injection sites and a lowered positioning of the head at media l locations. These data suggest that the rat substantia nigra may cont ain a functional organization based on a form of somatomotor topograph y. This organization may influence which part of the substantia nigra is most effective in suppressing seizures expressed by different muscl e groups of the body.