Vasopressin secretion as a possible target of the gamma-aminobutyric acid-mediated component of the corticosteroid feedback effect

Copeptin Levels Before and After Transsphenoidal Surgery for Cushing Disease: A Potential Early Marker of Remission

Context

Arginine-vasopressin and CRH act synergistically to stimulate secretion of ACTH. There is evidence that glucocorticoids act via negative feedback to suppress arginine-vasopressin secretion.

Objective

Our hypothesis was that a postoperative increase in plasma copeptin may serve as a marker of remission of Cushing disease (CD).

Design

Plasma copeptin was obtained in patients with CD before and daily on postoperative days 1 through 8 after transsphenoidal surgery. Peak postoperative copeptin levels and Δcopeptin values were compared among those in remission vs no remission.

Results

Forty-four patients (64% female, aged 7-55 years) were included, and 19 developed neither diabetes insipidus (DI) or syndrome of inappropriate anti-diuresis (SIADH). Thirty-three had follow-up at least 3 months postoperatively. There was no difference in peak postoperative copeptin in remission (6.1 pmol/L [4.3-12.1]) vs no remission (7.3 pmol/L [5.4-8.4], P = 0.88). Excluding those who developed DI or SIADH, there was no difference in peak postoperative copeptin in remission (10.2 pmol/L [6.9-21.0]) vs no remission (5.4 pmol/L [4.6-7.3], P = 0.20). However, a higher peak postoperative copeptin level was found in those in remission (14.6 pmol/L [±10.9] vs 5.8 (±1.4), P = 0.03]) with parametric testing. There was no difference in the Δcopeptin by remission status.

Conclusions

A difference in peak postoperative plasma copeptin as an early marker to predict remission of CD was not consistently present, although the data point to the need for a larger sample size to further evaluate this. However, the utility of this test may be limited to those who develop neither DI nor SIADH postoperatively.

GABAergic control of neuropeptide gene expression in parvocellular neurons of the hypothalamic paraventricular nucleus

To assess the functional impact of local inhibitory gamma-aminobutyric acid (GABA)ergic interneuron population on the cellular and transcriptional activity of parvocellular neurosecretory neurons in the hypothalamic paraventricular nucleus (PVH), we followed the expression of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) genes along with the activation marker c-fos in response to the blockade of GABA-A receptors. First, we analysed the effect of the GABA-A receptor antagonist bicuculline methiodide (BMI) in organotypic cultures of hypothalamic slices. These preparations preserve the cytoarchitecture of CRH-synthesizing cell populations and elements of local interneuronal networks, while remote connections originating from limbic- and brainstem areas are missing. In vitro, BMI resulted in a selective induction of c-Fos immunoreactivity that was localized exclusively to the PVH and upregulated both CRH mRNA and AVP hnRNA levels. Local microinjection of BMI into the paraventricular region of freely moving rats increased the adrenocorticotropin secretion and activated PVH neurons ipsilateral to the injection. c-Fos immunoreactivity was distributed within the PVH and in the perinuclear region, where it appeared in GABAergic and also in non-GABAergic profiles. This treatment induced AVP hnRNA expression in the parvocellular compartment without any reliable stimulation of CRH transcription in the parvocellular- and AVP hnRNA levels in the magnocellular neurons. These results reveal an intrinsic GABAergic mechanism in the PVH microenvironment that by itself, without limbic contribution, impinges a tonic inhibitory influence on the parvocellular CRH neurons in vitro. In vivo, remote inputs are superimposed on the local circuit, allowing differential transcriptional regulation of CRH and AVP genes in the hypophysiotropic neurons.

Neurotransmitter regulation of cellular activation and neuropeptide gene expression in the paraventricular nucleus of the hypothalamus

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.