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Norepinephrine (NE), glutamate (Glu), and GABA have been identified as important neurotransmitters governing neuroendocrine mechanisms represented in the paraventricular nucleus of the hypothalamus (PVH). Microinjection studies were used to compare the efficacy of these transmitter mechanisms in stimulating PVH output neurons. Local administration of NE provoked an increase in plasma corticosterone levels and Fos induction in the both the parvocellular and magnocellular divisions of the nucleus. This treatment also stimulated a robust increase in corticotropin-releasing factor (CRF) heteronuclear (hn) RNA in the parvocellular PVH and a more subtle, although reliable, increase in arginine vasopressin (AVP) hnRNA in this same compartment. Local administration of the GABA(A) receptor antagonist bicuculline methiodide (BMI) resulted in increased plasma corticosterone and, in contrast to NE treatment, Fos induction limited primarily to the parvocellular PVH. BMI elicited marked increases in both CRH and AVP hnRNAs within the parvocellular division of the nucleus. Over a wide range of concentrations, Glu failed to produce reliable increases in corticosterone secretion and induced only weak activational responses limited primarily to non-neurosecretory regions of the PVH. Local Glu administration did, however, provoke Fos induction in identified GABAergic neurons immediately adjoining the PVH, suggesting that the muted response to Glu may be a consequence of concurrent activation of local inhibitory interneurons. These results support a differential involvement of adrenergic, glutamatergic and GABAergic mechanisms in regulating neurosecretory populations of the PVH and suggest that involvement of local circuit neurons must be carefully considered in the interpretation of microinjection studies.
Cole et al. (Fri,) studied this question.