Role of the substantia nigra in GABA-mediated anticonvulsant actions

The relationship between cerebral GABA content and susceptibility to seizures is addressed from the point of view of specific brain loci at which GABA synapses may control convulsive activity. The substantia nigra (SN) has been identified as a critical site at which GABA-agonist drugs act to reduce susceptibility to a number of types of experimentally induced generalized seizures. Moreover, the ability of GABA-elevating agents to protect against seizures in the maximal electroshock model is directly correlated with increases in GABA specifically in the nerve-terminal compartment of SN. Studies with 2-deoxyglucose indicate that a marked increase in metabolic activity in SN is a common feature of several types of generalized seizures; it is possible that some of this increased activity is associated with GABAergic nerve terminals that become activated in an attempt to suppress seizure spread. Because GABA has been shown to inhibit nigral efferents, it is likely that GABA terminals inhibit nigral projections that are permissive or facilitative to seizure propagation. In support of this, bilateral destruction of SN attenuated clonic and tonic chemoconvulsant and electroshock seizures. Other treatments capable of reducing nigral output, namely opiate agonists (morphine and D-Ala-Met-enkephalin), and substance P antagonist analogs, were also found to have anticonvulsant effects when applied bilaterally into SN. Thus, the seizure-facilitating nigral efferents may be subject to inhibition by both GABA and opiates and may normally be driven by substance P. Of the various outputs from SN, the GABAergic projections to thalamus, reticular formation and/or superior colliculus are most likely responsible for influencing seizure propagation. Experimental evidence does not indicate a significant role of pars compacta nigrostriatal dopamine neurons for controlling the various types of seizures subject to nigral influence. We propose that the inhibition of the GABAergic outputs from SN pars reticulata can suppress the progression of seizure discharge through circuits involving the target areas of these outputs. Because chemical or electrical stimulation of SN does not initiate convulsions, it appears that seizure activity generated elsewhere in the brain may be amplified or sustained by activity in these nigral outputs.