Two isoforms of the vesicular glutamate transporter, VGLUT1 and VGLUT2, were recently cloned and biophysically characterized. Both VGLUT1 and VGLUT2 specifically transport glutamate into synaptic vesicles, making them definitive markers for neurons using glutamate as a neurotransmitter. The present study takes advantage of the specificity of the vesicular transporters to afford the first detailed map of putative glutamatergic neurons in the rat hypothalamus. In situ hybridization analysis was used to map hypothalamic distributions of VGLUT1 and VGLUT2 mRNAs. VGLUT2 is clearly the predominant vesicular transporter mRNA found in the hypothalamus; rich expression can be documented in regions regulating energy balance (ventromedial hypothalamus), neuroendocrine function (preoptic nuclei), autonomic tone (posterior hypothalamus), and behavioral/homeostatic integration (lateral hypothalamus, mammillary nuclei). Expression of VGLUT1 is decidedly more circumspect and is confined to relatively weak labeling in lateral hypothalamic regions, neuroendocrine nuclei, and the suprachiasmatic nucleus. Importantly, dual-label analysis revealed no incidence of colocalization of VGLUT1 or VGLUT2 mRNAs in glutamic acid decarboxylase (GAD) 65-positive neurons, indicating that GABA neurons do not express either transporter. Our data support a major role for hypothalamic glutamatergic neurons in regulation of all aspects of hypothalamic function.

Neuronatomical and pharmacological studies have established GABA-mediated inhibition of the HPA axis at the level of the PVN. The origin of this innervation is a series of local hypothalamic and adjacent forebrain regions that project to stress-integrative hypophysiotropic CRH neurons. While a role in tonic inhibition of the stress axis is likely, this system of inhibitory loci is also capable of producing a dynamic braking capacity in the context of the neuroendocrine stress response. The latter function is mediated in large part by glutamatergic forebrain afferents that increase GABA release at the level of the PVN. In addition, this local GABA system can be inhibited by upstream GABAergic projection neurons, producing activation of the HPA axis via removal of GABAergic tone. This PVN projecting GABA network interfaces with a wide range of homeostatic mechanisms, and is capable of biochemical plasticity in response to chronic stress. Collectively, the elements of this system provide for exquisite control of neuroendocrine activation in the face of stressful stimuli, and loss of this regulatory capacity may underlie many stress-related disorders.

No abstract