Enkephalinergic projection from the ventromedial hypothalamic nucleus to the midbrain central gray matter in the rat: an immunocytochemical analysis.

The projection of enkephalin-like immunoreactive (EnkI) neurons in the ventromedial hypothalamic nucleus (VMH) in the rat was investigated using immunocytochemistry. These neurons project to the dorsal portion of the midbrain central gray matter (DCG), because the destruction of the VMH resulted in a marked ipsilateral reduction of EnkI fibers there. Destruction of the dorsomedial portion of the VMH caused the appearance of fibers accumulating EnkI ventrolateral to the lesion and these fibers could be traced to the ventrolateral portion of the VMH where many EnkI neurons were concentrated. Transection of the brain at the level of the premammillary nucleus, mammillary body and superior colliculus caused the appearance of fibers accumulating EnkI in the posterior hypothalamic nucleus (PH), central gray matter medial to the fasciculus retroflexus Mynert and in the DCG, respectively. We concluded that the axons from these neurons first project dorsomedially to situate in the PH. These fibers ran caudodorsally and entered into the dorsal longitudinal fasciculus in the DCG at the midbrain.