Activation of substantia nigra neurons: role in the propagation of seizures in kindled rats.

The substantia nigra (SN) is crucial to the propagation of seizures in kindled rats and in other experimental seizure models. However, the mechanisms by which the SN acts to facilitate the propagation of seizures are unknown. To investigate these mechanisms we quantified the activity of SN neurons during seizures in kindled and naive rats in a paralyzed, ventilated state and examined the relationship between activity of neurons in the SN and the seizure-facilitating action of this structure. Our principal findings were that the majority of both SN dopamine and SN pars reticulata neurons, in kindled rats, fired in bursts temporally correlated to EEG waveforms recorded outside the SN during seizures; this response was only rarely found in SN neurons of naive rats during seizures elicited by stimulation of the amygdala; unlike kindled rats, lesions of the SN in naive rats did not suppress seizures. The finding that SN neurons fired in bursts during seizures in kindled, but not naive, rats indicates that seizure activity propagated into SN only in kindled rats. The correlation between seizure-suppressant effects of lesions and SN activation during seizures leads us to propose that one mechanism by which the SN promotes seizure propagation involves SN activation and transmission of seizure activity to targets of SN.