Blockade of
GABA receptors in the rat superior colliculus (SC) has been shown to protect against maximal electroshock-induced tonic convulsions and spontaneous generalized non-convulsive
seizures. In the present study, we determined that blockade of
GABA receptors in SC could also protect against focally evoked limbic
motor seizures. Limbic
motor seizures were induced by the unilateral focal application of
bicuculline methiodide into area tempestas (AT), an epileptogenic site in the deep prepiriform cortex. Control rats (receiving bilateral infusions of saline into SC) all exhibited convulsive
seizures following
bicuculline in AT. Rats pretreated (5 min before) with
bicuculline (50 pmol) bilaterally in the deep layers of the SC, were protected against the AT-evoked convulsive
seizures. Unilateral application of
bicuculline in the deep SC or bilateral application in the superficial layers of the SC did not alter the convulsive response to
bicuculline in AT. These results indicate that the
anticonvulsant action of
GABA blockade in SC is not limited to tonic convulsive
seizures but extends to the clonic manifestations evoked by
seizures originating in forebrain limbic circuits. Given that the deep layers of SC receive inputs from GABA neurons in substantia nigra and that suppression of the activity of nigral neurons is
anticonvulsant against a variety of
seizures (including those evoked from AT), it is likely that the
anticonvulsant action of
bicuculline in SC is due to interference with the influence of a nigrotectal GABAergic projection.