Afferent and efferent connections of the cholinoceptive medial pontine reticular formation (region of the ventral tegmental nucleus) in the cat.

Following minor concussive brain injury when there is an otherwise general suppression of CNS activity, the ventral tegmental nucleus of Gudden (VTN) demonstrates increased functional activity (32). Electrical or pharmacological activation of a cholinoceptive region in this same general area of the medial pontine tegmentum contributes to certain components of reversible traumatic unconsciousness, including postural atonia (31, 32, 45). Therefore, in an effort to examine the neuroanatomical basis of the behavioral suppression associated with a reversible traumatic unconsciousness, the afferent and efferent connections of the VTN and putative cholinoceptive medial pontine reticular formation (cmPRF) were studied in the cat using the retrograde horseradish peroxidase (HRP), HRP/choline acetyltransferase (ChAT) double-labeling immunohistochemistry, and anterograde HRP and autoradiographic techniques. Based upon retrograde HRP labeling, the principal afferents to the VTN region of the cmPRF originated from the medial and lateral mammillary nuclei, and lateral habenular nucleus, and to a lesser extent from the interpeduncular nucleus, lateral hypothalamus, dorsal tegmental nucleus, superior central nucleus, and contralateral nucleus reticularis pontis caudalis. Other afferents, which were thought to have been labeled through spread of HRP into the medial longitudinal fasciculus (MLF), adjacent paramedian pontine reticular formation, or uptake by transected fibers descending to the inferior olive, included the nucleus of Darkschewitsch, interstitial nucleus of Cajal, zona incerta, prerubral fields of Forel, deep superior colliculus, nucleus of the posterior commissure, nucleus cuneiformis, ventral periaqueductal gray, vestibular complex, perihypoglossal complex, and deep cerebellar nuclei. In HRP/ChAT double labeling studies, only a very small number of cholinergic VTN afferent neurons were found in the medial parabrachial region of the dorsolateral pontine tegmentum, although the pedunculopontine and laterodorsal tegmental nuclei contained numerous single-labeled ChAT-positive cells. Anterograde HRP and autoradiographic findings demonstrated that the VTN gave rise almost exclusively to ascending projections, which largely followed the course of the mammillary peduncle (16,21) and medial forebrain bundle, or the tegmentopeduncular tract (4). The majority of fibers ascended to terminate in the medial and lateral mammillary nuclei, interpeduncular complex (especially paramedian subnucleus), ventral tegmental area, lateral hypothalamus, and the medial septum in the basal forebrain. Labeling that joined the mammillothalamic tract to terminate in the anterior nuclear complex of the thalamus was thought to occur transneuronally. Some projections were also observed to nucleus reticularis pontis oralis and caudalis, superior central nucleus, and dorsal tegmental nucleus adjacent to the VTN...