Interactions between stress and the mesocorticolimbic dopamine system have been suggested from behavioral and electrophysiological studies. Since stress responses are largely mediated by corticotropin releasing factor (CRF), we investigated possible interactions between neurons containing CRF and those producing dopamine (DA) in the ventral tegmental area (VTA). We first investigated subcellular distribution of CRF in VTA by immunolabeling VTA sections with anti-CRF antibodies and analyzing these sections by electron microscopy. We found that CRF-immunoreactivity (CR-IR) is present mainly in axon terminals establishing either symmetric or asymmetric synapses with VTA dendrites. Of functional importance, we established that the CRF asymmetric synapses are glutamatergic, as the CRF-IR terminals in these synapses co-expressed the vesicular glutamate transporter 2, and that CRF symmetric synapses are GABAergic, as the CRF-IR terminals in these synapses co-expressed glutamic acid decarboxilase. We then looked for synaptic interactions between CRF- and DA-containing neurons, by using antibodies against CRF and tyrosine hydroxylase (TH, a marker for DA neurons). We found that the majority of synapses between CRF-IR terminals and TH neurons are asymmetric (glutamatergic), suggesting that glutamatergic neurons containing CRF are part of the neuronal circuitry that mediates stress responses involving the mesocorticolimbic dopamine system.? The presence of CRF synapses in the VTA offers a mechanism for interactions between the stress-associated neurohormone CRF and the mesocorticolimbic dopamine system.
