Brain stem afferents to the anterior dorsal ventricular ridge in a lizard (Varanus exanthematicus)

The anterior dorsal ventricular ridge (ADVR), a large intraventricular protrusion in the reptilian forebrain, receives information from many different sensory modalities and in turn, projects massively onto the striatum. The ADVR possesses functional similarities to the mammalian isocortex and may perform complex sensory integrations. The ADVR in lizards is composed of three longitudinal zones which receive visual, somatosensory and acustic information, respectively. These projections are relayed via thalamic nuclei. Previous retrograde tracer studies also suggested brain stem projections to the ADVR arising in the midbrain reticular formation and in certain monoaminergic brain stem nuclei (substantia nigra, locus coeruleus and nucleus raphes superior). In the present study the powerful retrograde fluorescent tracer 'Fast Blue' was applied as a slow-release gel to the ADVR of the savanna monitor lizard, Varanus exanthematicus. Thalamic projections were confirmed and various direct brain stem projections to the ADVR were demonstrated. Brain stem afferents to the ADVR were found from the laminar nucleus of the torus semicircularis (possibly comparable to the mammalian periaqueductal gray), from the midbrain reticular formation, from the substantia nigra (pars compacta and reticulata) and the adjacent ventral tegmental area, from the nucleus raphes superior, from the locus coeruleus, from the parabrachial region, from the nucleus of the lateral lemniscus and even from the most caudal part of the brain stem (a few neurons in the nucleus of the solitary tract and lateral reticular formation, possibly comparable to the mammalian A2 and A1 groups, respectively). These data strongly suggest direct ADVR projections from the parabrachial region (related to visceral and taste information) as well as distinct catecholaminergic (presumably dopaminergic: substantia nigra, ventral tegmental area and, noradrenergic: locus coeruleus, respectively) and serotonergic projections (nucleus raphes superior).