THE DORSAL MIDBRAIN ANTICONVULSANT ZONE .2. EFFERENT CONNECTIONS REVEALED BY THE ANTEROGRADE TRANSPORT OF WHEAT-GERM-AGGLUTININ HORSERADISH-PEROXIDASE FROM INJECTIONS CENTERED ON THE INTERCOLLICULAR AREA IN THE RAT

Activation of the dorsal midbrain has a powerful anticonvulsant effect in the maximal electroshock model of epilepsy. The suppression of ton ic seizures can be obtained most reliably from an area centred on the intercollicular nucleus overlapping into the deep layers of the superi or colliculus and adjacent mesencephalic reticular formation. As part of a series of investigations to identify neural mechanisms responsibl e for mediating the anticonvulsant properties of the dorsal midbrain, the present study provides an anatomical description of the efferent p rojections of this region. Small amounts of wheatgerm agglutinin-horse radish peroxidase (10-30 nl of a 1% solution) were injected into the i ntercollicular nucleus and surrounding tissue. The resulting anterogra de transport of the tracer was plotted on a set of standard atlas sect ions. Four major output pathways were identified: (i) an ipsilateral d escending projection which had terminations in the microcellular tegme ntal nucleus, lateral and ventral pontine reticular nucleus pars orali s, ventrolateral tegmental nucleus, ventral and caudal pontine reticul ar nucleus pars caudalis, raphe magnus nucleus and the gigantocellular nucleus; (ii) a contralateral descending projection which for the mos t part targeted the same brainstem structures but with weaker terminal labelling; (iii) a projection to the contralateral dorsal midbrain wi th comparatively weak terminal label in the contralateral superior col liculus, intercollicular nucleus, periaqueductal gray, mesencephalic r eticular formation and cuneiform area; (iv) ipsilateral ascending path way with terminations in the red nucleus, zona incerta, peripeduncular area, parafascicular nucleus, lateral hypothalamus, parts of the pret ectum and caudal thalamus. At a general level the dorsal midbrain anti convulsant zone shares its major output projections and efferent targe ts with at least one of its near neighbours, including the superior co lliculus, periaqueductal gray, the cuneiform nucleus and pedunculopont ine nucleus. The possibility that anticonvulsant properties of the int ercollicular area can simply be attributed to a unique set of efferent projections is therefore not supported by the anatomy.