Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

Abstract

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of ³HD‐aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA‐HRP). Cells retrogradely labeled by WGA‐HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with ³H‐D‐aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with ³H‐D‐aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with ³H‐D‐aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using ³H‐GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of ³H‐GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests ³H‐GABA has limited usefulness as a transmitter‐specific retrograde tracer.