Role of the substantia nigra in audiogenic seizures: a neuroethological analysis in the rat.

The audiogenic crisis (AC) includes an initial phase of bursts of circular, stereotyped running. Since it has been shown that the nigrostriatal system is important in the control of lesion- and drug-induced stereotyped rotational behavior, the effects of unilateral electrolytic lesions of the substantia nigra (SN) on the AC were investigated. Behavior was recorded using a detailed ethogram by means of systematic observational techniques and analyzed quantitatively in terms of sequential structure by multivariate statistical methods. An index of susceptibility was computed on the basis of the frequency and intensity of the convulsive behavioral patterns observed during 4 successive pre- and post-operative tests. The lesion caused an increase in the susceptibility to AC of previously insusceptible rats or of low-susceptibility rats (LI), in relation to rats with sham lesions. No other differences of this type were detected for the groups of susceptible lesioned (LS) or non-lesioned animals. The sequential analysis disclosed a degradation in the tonic-clonic convulsive pattern of LS animals, with decreased frequency and more random transitions between their constituent units, as well as in the appearance of characteristic running patterns in LI rats. It is concluded that the SN plays a general inhibitory role in the central neural mechanisms of AC, and is involved in the integration and control of stereotyped local movements and of tonic-clonic convulsions. However, the SN might play only a secondary role in the control of the laterality and elaboration of the rotational phase.