The deep mesencephalic nucleus as an output center of basal ganglia: Morphological and electrophysiological similarities with the substantia nigra

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region b etween the superior colliculus, the substantia nigra compacta, the periaque ductal gray, and the medial geniculate body. Although some data suggest tha t it is involved in nociception and visceral control, its functions remain unclear. In the present study, by using morphological (combination of anter ograde and retrograde tracers with immunocytochemistry and in situ hibrydiz ation) and electrophysiological (firing activity and transynaptic response to striatal stimulation) methods, we show that a subpopulation of DMN cells shares many morphological and electrophysiological characteristics with th ose of the substantia nigra reticulata (SNR). These similarities include th e following: 1) firing rate; firing pattern, and conduction velocity; 2) ex pression of GAD65, GAD67, and PV; 3) excitatory and inhibitory inputs from the striatum; and 4) projections to the ventral thalamus, superior collicul us, and pedunculopontine tegmental nucleus. Some differences were also foun d. In comparison with SN, DMN cells and striatal afferents are more sparsel y distributed and they show conspicuous contralateral projections to the th alamus and superior colliculus. This suggests that, similarly to the SNR, t he DMN acts as an output center of basal ganglia and probably facilitates t he inter-hemispheric regulation of these centers.