Distribution of VTA glutamate and dopamine terminals, and their significance in CA1 neural network activity.

Abstract Reciprocal connection between the ventral tegmental area (VTA) and the hippocampus forms a loop that controls information entry into long-term memory. Compared with the widely studied VTA dopamine system, VTA glutamate terminals are anatomically dominant in the hippocampus and less understood. The current study employs anterograde and retrograde labeling of VTA dopamine and glutamate neurons to map the distribution of their terminals within hippocampal layers. Also, functional tracing of VTA dopamine and glutamate projections to the hippocampus involved photostimulation of VTA cell bodies, combined with CA1 extracellular voltage sampling in vivo. VTA dopamine terminals predominantly innervate the CA1 basal dendrite layer and modulate the firing rate of active putative neurons. In contrast, anatomical dominance of VTA glutamate terminals in the CA1 pyramidal cell body and apical dendrite layers suggests the possible involvement of these terminals in CA1 excitability regulation. In support of these outcomes, the photostimulation of VTA dopamine neurons increased the firing rate and did not alter intrinsic excitability parameters for putative CA1 units. Conversely, activation of VTA glutamate neurons increased CA1 network firing rate and burst rate. Furthermore, VTA glutamate inputs reduced the interspike and interburst intervals for putative CA1 neurons. Taken together, we deduced that layer-specific distribution of VTA dopamine and glutamate terminals in the hippocampus is likely to be the determinant of the different CA1 functional properties recorded during the photostimulation epochs.