Embedded synaptic feedback in the neuroendocrine stress axis

Glutamate Spillover Dynamically Strengthens Gabaergic Synaptic Inhibition of the Hypothalamic Paraventricular Nucleus

The hypothalamic paraventricular nucleus (PVN) is strongly inhibited by γ-aminobutyric acid (GABA) from the surrounding peri-nuclear zone (PNZ). Because glutamate mediates fast excitatory transmission and is substrate for GABA synthesis, we tested its capacity to dynamically strengthen GABA inhibition. In PVN slices from male mice, bath glutamate applied during ionotropic glutamate receptor blockade increased PNZ-evoked inhibitory postsynaptic currents (eIPSCs) without affecting GABA-A receptor agonist currents or single-channel conductance, implicating a presynaptic mechanism(s). Consistent with this interpretation, bath glutamate failed to strengthen IPSCs during pharmacological saturation of GABA-A receptors. Presynaptic analyses revealed that glutamate did not affect paired-pulse ratio, peak eIPSC variability, GABA vesicle recycling speed, or readily releasable pool (RRP) size. Notably, glutamate-GABA strengthening (GGS) was unaffected by metabotropic glutamate receptor blockade and graded external Ca2+ when normalized to baseline amplitude. GGS was prevented by pan- but not glial-specific inhibition of glutamate uptake and by inhibition of glutamic acid decarboxylase (GAD), indicating reliance on glutamate uptake by neuronal excitatory amino acid transporter 3 (EAAT3) and enzymatic conversion of glutamate to GABA. EAAT3 immunoreactivity was strongly localized to presumptive PVN GABA terminals. High bath K+ also induced GGS, which was prevented by glutamate vesicle depletion, indicating that synaptic glutamate release strengthens PVN GABA inhibition. GGS suppressed PVN cell firing, indicating its functional significance. In sum, PVN GGS buffers neuronal excitation by apparent "over-filling" of vesicles with GABA synthesized from synaptically released glutamate. We posit that GGS protects against sustained PVN excitation and excitotoxicity while potentially aiding stress adaptation and habituation.

Ascending mechanisms of stress integration: Implications for brainstem regulation of neuroendocrine and behavioral stress responses

In response to stress, defined as a real or perceived threat to homeostasis or well-being, brain systems initiate divergent physiological and behavioral processes that mobilize energy and promote adaptation. The brainstem contains multiple nuclei that engage in autonomic control and reflexive responses to systemic stressors. However, brainstem nuclei also play an important role in neuroendocrine responses to psychogenic stressors mediated by the hypothalamic-pituitary-adrenocortical axis. Further, these nuclei integrate neuroendocrine responses with stress-related behaviors, significantly impacting mood and anxiety. The current review focuses on the prominent brainstem monosynaptic inputs to the endocrine paraventricular hypothalamic nucleus (PVN), including the periaqueductal gray, raphe nuclei, parabrachial nuclei, locus coeruleus, and nucleus of the solitary tract (NTS). The NTS is a particularly intriguing area, as the region contains multiple cell groups that provide neurochemically-distinct inputs to the PVN. Furthermore, the NTS, under regulatory control by glucocorticoid-mediated feedback, integrates affective processes with physiological status to regulate stress responding. Collectively, these brainstem circuits represent an important avenue for delineating interactions between stress and health.