Activation of the dorsal midbrain has a powerful anticonvulsant effect in the maximal electroshock model of epilepsy. The suppression of tonic seizures can be obtained most reliably from an area centred on the intercollicular nucleus overlapping into the deep layers of the superior colliculus and adjacent mesencephalic reticular formation. As part of a series of investigations to identify neural mechanisms responsible for mediating the anticonvulsant properties of the dorsal midbrain, the present study provides an anatomical description of the efferent projections of this region. Small amounts of wheatgerm agglutinin-horseradish peroxidase (10-30 nl of a 1% solution) were injected into the intercollicular nucleus and surrounding tissue. The resulting anterograde transport of the tracer was plotted on a set of standard atlas sections. Four major output pathways were identified: (i) an ipsilateral descending projection which had terminations in the microcellular tegmental nucleus, lateral and ventral pontine reticular nucleus pars oralis, ventrolateral tegmental nucleus, ventral and caudal pontine reticular nucleus pars caudalis, raphe magnus nucleus and the gigantocellular nucleus; (ii) a contralateral descending projection which for the most part targeted the same brainstem structures but with weaker terminal labelling; (iii) a projection to the contralateral dorsal midbrain with comparatively weak terminal label in the contralateral superior colliculus, intercollicular nucleus, periaqueductal gray, mesencephalic reticular formation and cuneiform area; (iv) ipsilateral ascending pathway with terminations in the red nucleus, zona incerta, peripeduncular area, parafascicular nucleus, lateral hypothalamus, parts of the pretectum and caudal thalamus. At a general level the dorsal midbrain anticonvulsant zone shares its major output projections and efferent targets with at least one of its near neighbours, including the superior colliculus, periaqueductal gray, the cuneiform nucleus and pedunculopontine nucleus. The possibility that anticonvulsant properties of the intercollicular area can simply be attributed to a unique set of efferent projections is therefore not supported by the anatomy.
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