Elsevier

Neuroscience

Volume 146, Issue 3, 25 May 2007, Pages 1259-1274
Neuroscience

Neuropharmacology
Glutamate synaptic inputs to ventral tegmental area neurons in the rat derive primarily from subcortical sources

https://doi.org/10.1016/j.neuroscience.2007.02.016Get rights and content

Abstract

Dopamine and GABA neurons in the ventral tegmental area project to the nucleus accumbens and prefrontal cortex and modulate locomotor and reward behaviors as well as cognitive and affective processes. Both midbrain cell types receive synapses from glutamate afferents that provide an essential control of behaviorally-linked activity patterns, although the sources of glutamate inputs have not yet been completely characterized. We used antibodies against the vesicular glutamate transporter subtypes 1 and 2 (VGlut1 and VGlut2) to investigate the morphology and synaptic organization of axons containing these proteins as putative markers of glutamate afferents from cortical versus subcortical sites, respectively, in rats. We also characterized the ventral tegmental area cell populations receiving VGlut1+ or VGlut2+ synapses according to their transmitter phenotype (dopamine or GABA) and major projection target (nucleus accumbens or prefrontal cortex). By light and electron microscopic examination, VGlut2+ as opposed to VGlut1+ axon terminals were more numerous, had a larger average size, synapsed more proximally, and were more likely to form convergent synapses onto the same target. Both axon types formed predominantly asymmetric synapses, although VGlut2+ terminals more often formed synapses with symmetric morphology. No absolute selectivity was observed for VGlut1+ or VGlut2+ axons to target any particular cell population. However, the synapses onto mesoaccumbens neurons more often involved VGlut2+ terminals, whereas mesoprefrontal neurons received relatively equal synaptic inputs from VGlut1+ and VGlut2+ profiles. The distinct morphological features of VGlut1 and VGlut2 positive axons suggest that glutamate inputs from presumed cortical and subcortical sources, respectively, differ in the nature and intensity of their physiological actions on midbrain neurons. More specifically, our findings imply that subcortical glutamate inputs to the ventral tegmental area expressing VGlut2 predominate over cortical sources of excitation expressing VGlut1 and are more likely to drive the behaviorally-linked bursts in dopamine cells that signal future expectancy or attentional shifting.

Section snippets

Subjects and surgeries

As approved by the Institutional Animal Care and Use Committee at the University of Pittsburgh and in accordance with the NIH Guide for the Care and Use of Laboratory Animals, tract-tracing experiments were performed on six adult male Sprague–Dawley rats (Hilltop Laboratory Animals Inc., Scottsdale, PA, USA, 330–360 g). Three additional naive rats were used to determine the optimal dilution of antibodies directed against VGlut1 or VGlut2 and to examine single labeling for these proteins within

Light microscopy

The injections of the retrograde tracer FG into the NAc and PFC were conducted in a manner similar to those used in our previous studies (Carr and Sesack 2000, Omelchenko and Sesack 2005, Omelchenko and Sesack 2006a) and produced comparable results. Visualized by immunoperoxidase, FG filled both the core and shell subdivisions of the NAc at mid rostro-caudal levels (Fig. 1A), whereas FG injections into the PFC were placed within prelimbic and infralimbic areas (terminology according to Krettek

Discussion

This study represents the first ultrastructural comparison of VTA inputs from glutamate axons expressing VGlut1 versus VGlut2 as putative markers of cortical versus subcortical origins, respectively. The larger size, higher density, more frequent convergence, and more proximal contacts observed for VGlut2+ axons suggest that subcortical glutamate inputs to the VTA predominate over cortical sources of excitation in driving behaviorally-linked burst firing in DA cells. Other morphological

Acknowledgments

Support was provided by USPHS grant NIMH 067937. The authors gratefully acknowledge the technical contributions of Maureen Walsh, the statistical consulting assistance of Dr. Allan Sampson at the University of Pittsburgh, and the generous supply of antibodies provided by Dr. Robert Edwards at the University of California at San Francisco.

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