THE DORSAL MIDBRAIN ANTICONVULSANT ZONE .3. EFFECTS OF EFFERENT PATHWAY TRANSECTIONS ON SUPPRESSION OF ELECTROSHOCK SEIZURES AND DEFENSE-LIKE REACTIONS PRODUCED BY LOCAL INJECTIONS OF BICUCULLINE

Having provided an anatomical description of the efferent projections of the dorsal midbrain anticonvulsant zone [Shehab S. et at. (1995) Ne uroscience 65, 681-695], our purpose in the present study was to estab lish which outputs from this region are responsible for mediating the anticonvulsant and behavioural properties of dorsal midbrain activatio n. The ability of unilateral injections of bicuculline into the dorsal midbrain anticonvulsant zone to suppress tonic hindlimb extension in the electroshock model of epilepsy was tested before and after three d ifferent knife cuts: (i) a transection of ipsilateral descending proje ctions on the same side as the injection of bicuculline; (ii) an ident ical cut except it was placed contralateral to the injection; (iii) a cut which transected rostral projecting fibres from the dorsal midbrai n anticonvulsant zone including most ipsilateral ascending and crossed descending projections. A fourth group of operated control animals wa s included to establish a baseline for the schedule of repeated testin g. Qualitative observations of behaviour were taken immediately prior to the administration of the electroshocks. Unilateral transection of ipsilateral descending efferents prevented the suppression of electros hock-induced hindlimb extension by injections of bicuculline into the dorsal midbrain anticonvulsant zone on the same side of the brain. Bot h the control cuts on the opposite side of the brain and the rostral c uts were ineffective. Transection of the ipsilateral descending projec tion on the same side as the injection of bicuculline also reduced the incidence of defensive reactions induced by the GABA antagonist, incl uding explosive motor behaviour, oral attack and vocalization. Damage to this projection on the opposite side had little effect on the expre ssion of behavioural reactions, neither did transection of the ascendi ng efferents. These data suggest that ipsilateral descending efferents are critical for the suppression of electroshock-induced extension of the hindlimbs and the expression of defensive reactions elicited by a ctivation of the dorsal midbrain.