GABAergic nigro-collicular pathways modulate the defensive behaviour elicited by midbrain tectum stimulation.

Midbrain tectum (MT) structures such as the dorsal periaqueductal gray matter and deep layers of superior colliculus are well-known for the organization and generation of defensive behaviour. Electrical stimulation or microinjection of GABA antagonists into these structures produce aversive behaviour. In order to determine whether the nigrocollicular GABAergic fibers exert some control over this behaviour, rats bearing neurochemical lesions with kainic acid in the substantia nigra, pars reticulata (SNpr) and compacta (SNpc), were submitted to MT microinjections of bicuculline or electrical stimulation at aversive thresholds. The same procedure was carried out after enhancement or inhibition of GABAergic transmission in SNpr through microinjections of muscimol or bicuculline, respectively. Animals with SNpr neurochemical lesion exhibited a significant decrease in the aversive thresholds and an increase in the responsiveness to bicuculline microinjections. An opposite effect was observed following microinjections of bicuculline into the SNpr. The enhancement of the GABAergic transmission into the SNpr following microinjection of muscimol mimicked the effects produced by the lesion with kainic acid. These results suggest an inhibitory control of GABAergic fibers from the substantia nigra, pars reticulata, on aversive behaviour induced by midbrain stimulation.