Corticotropin-releasing hormone and dexamethasone do not alter secretion of immunoreactive beta-endorphin from dissociated fetal hypothalamic cell cultures

Vasoactive intestinal peptide and corticotropin-releasing hormone increase beta-endorphin release and proopiomelanocortin messenger RNA levels in primary cultures of hypothalamic cells: effects of acute and chronic ethanol treatment

BACKGROUND

beta-Endorphin (beta-EP) neurons are involved in ethanol's action on a variety of brain functions, including positive reinforcement. These neurons are innervated by vasoactive intestinal peptide (VIP)-containing and corticotropin-releasing hormone (CRH)-containing neurons in the hypothalamus. Whether these neuropeptides affect beta-EP neuronal function in the presence or absence of ethanol has not previously been determined.

METHODS

The authors determined the effects of VIP and CRH on gene expression and peptide release from beta-EP neurons in primary cultures of mediobasal hypothalamic cells. The effects of receptor antagonists on VIP- and CRH-induced beta-EP release was determined. Furthermore, the authors studied the effects of acute and chronic treatment with ethanol on the response of beta-EP neurons to VIP and CRH. Real-time reverse-transcription polymerase chain reaction was used for messenger RNA (mRNA) detection, and radioimmunoassay was used for hormone measurements.

RESULTS

We show that beta-EP neurons responded concentration dependently to VIP and CRH treatments by increasing both beta-EP release and proopiomelanocortin mRNA expression. Simultaneous treatment with a nonspecific receptor antagonist reduced the ability of CRH or VIP to induce beta-EP release from mediobasal hypothalamic cells. Acute treatment with ethanol increased beta-EP neuronal gene expression and the secretory response to CRH and VIP. However, previous exposure to chronic ethanol reduced the CRH and VIP responses of these neurons.

CONCLUSIONS

These results indicate that VIP and CRH stimulate beta-EP release from hypothalamic cells in primary cultures and that the stimulatory and adaptive responses of beta-EP neurons to ethanol may involve alteration in the responsiveness of beta-EP-secreting neurons to CRH and VIP.

Glucocorticoid regulation of hypothalamic proopiomelanocortin

Although glucocorticoids clearly inhibit proopiomelanocortin (POMC) gene transcription and peptide synthesis in the anterior pituitary, the effects of glucocorticoids on POMC in the hypothalamus are still unclear, even though most POMC neurons in the arcuate nucleus are known to have glucocorticoid receptors. In this study, we have therefore examined the effect of adrenalectomy (ADX) and glucocorticoid replacement on POMC mRNA and peptide (beta-EP and alpha-MSH) levels in the medial basal hypothalamus (MBH) of the rat. POMC mRNA was measured by a sensitive solution hybridization S1 nuclease protection assay, and beta-EP and alpha-MSH were measured by radioimmunoassay. In a first experiment, animals were studied 7 days after ADX or sham surgery. The mean POMC mRNA concentration was 1.01+/-0.14 pg/microg RNA (means+/-SE) in the intact animals and decreased to 0.55+/-0.07 pg/microg RNA in the MBH of the ADX animals (p < 0.005). Beta-EP levels decreased in parallel from 4.30+/-0.18 to 3.36+/-0.11 ng/mg protein (p < 0.001); alpha-MSH levels decreased from 3.25+/-0.21 to 2.41+/-0.16 ng/mg protein (p < 0.005). In a second experiment, animals were studied 2 weeks after ADX. POMC mRNA levels again fell significantly from 1.15+/-0.19 pg/microg RNA in the intact animals to 0.51+/-0.06 pg/microg in the ADX animals (p < 0.01). Beta-EP levels fell also, but this was not significant. In a third experiment, all animals underwent ADX, and half of them received daily subcutaneous injections of dexamethasone (20 microg). Nine days after ADX, the mean POMC mRNA level was 0.66+/-0.04 pg/microg RNA in the saline-treated animals and increased to 0.98+/-0.08 pg/microg RNA in the dexamethasone-treated animals (p < 0.005). A parallel increase in beta-EP levels from 5.03+/-0.41 to 6.01+/-0.53 ng/mg protein was also noted, but this was not statistically significant. We conclude that POMC gene expression is significantly inhibited in the MBH at 1 and 2 weeks after ADX. This effect was reversed by glucocorticoid replacement with doses close to the physiological range. The parallel changes in POMC mRNA and peptide levels strongly suggest that, in contrast to the anterior pituitary, low doses of glucocorticoids stimulate the biosynthesis of POMC in the MBH of ADX rats.

Protein kinase C activators stimulate beta-endorphin secretion from hypothalamic cells

Relatively little is known about the regulation of secretion of hypothalamic beta-endorphin, the potent opioid that is believed to play a variety of physiological roles in brain. Previous work has shown that arginine vasopressin (AVP), which acts in brain primarily via activation of the phosphoinositol (PI) second messenger system, stimulates secretion of hypothalamic beta-endorphin. To test the hypothesis that activators of protein kinase C (PKC), which is activated following PI hydrolysis, stimulates secretion of beta-endorphins from hypothalamus, we studied the separate effects of stimulators of PKC including phorbol ester 12-myristate-13-acetate (PMA) and 1-oleolyl-2-acetyl glycerol (OAG- a diacyl glycerol analogue) on secretion of immunoreactive (IR-) beta-endorphin (measured by RIA) from dissociated fetal rat hypothalamic cell cultures. We also studied AVP and angiotensin II (Ang II), hypothalamic peptides which activate the PI second messenger pathway, and interactions of PMA and forskolin (FSK), an activator of the cyclic AMP/protein kinase A (PKA) pathway. PMA, OAG, AVP, and Ang II stimulated IR-beta-endorphin secretion. The stimulatory effect of both PMA and FSK on IR-beta-endorphin secretion was greater than that of PMA or FSK alone and was essentially additive.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro regulation of immunoreactive beta-endorphin secretion from adult and fetal hypothalamus by sequential stimulation with corticotropin-releasing hormone

Previous work has shown corticotropin-releasing hormone (CRH) stimulation of beta-endorphin (END) secretion from hypothalamus. We tested the hypothesis that CRH stimulation of beta-END (measured by radioimmunoassay) from hypothalamic explants is dependent on: (1) ovine CRH dose, (2) pattern and sequence of CRH stimulation, (3) androgen status, and (4) hypothalamic age. Hypothalami from adult male rats and day 17 fetal rats were studied. In adult hypothalami, CRH-stimulated immunoreactive (IR)-beta-END secretion with 10(-7) M was greater than that with 10(-8) M CRH and showed dose-dependent stimulation. Serial stimulation for 20 min by 10(-8) M CRH followed by a 40 min interval without CRH stimulation resulted in a brief stimulation of secretion of IR-beta-END and also secretion of IR-alpha-melanocyte-stimulating hormone (MSH), another peptide derived from pro-opiomelanocortin, the precursor of beta-END. Subsequent stimulation with 10(-6) M CRH showed a desensitization to stimulation despite readily releasable pools of IR-beta-END shown by potassium-induced depolarization. In addition, prolonged stimulation for 1 h with 10(-7) M CRH or increasing concentrations of CRH produced a sustained increase in IR-beta-END release as long as CRH was present. Dihydrotestosterone treatment had no effect on basal nor CRH-stimulated IR-beta-END release in orchiectomized rats. The pattern of IR-beta-END secretion from fetal hypothalamic explants exposed briefly (20 min) or for a prolonged period (1 h) to CRH was similar to that from adult explants. These results demonstrate that: (1) CRH-stimulated IR-beta-END secretion from hypothalamus is dose-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat hypothalamic proopiomelanocortin messenger RNA is unaffected by adrenalectomy

The negative feedback control of hypothalamic cortocotrophin releasing factor (CRF) and anterior pituitary proopiomelanocortin (POMC) by corticosteroids is well understood. However, less is known about the mechanisms that regulate POMC gene expression in the arcuate nuclei in the medial basal hypothalamus (MBH). Using a sensitive and specific S1 endonuclease protection assay, we have examined the effect of adrenalectomy on POMC mRNA in the rat MBH and pituitary. Our results show that adrenalectomy does not change POMC mRNA levels in the MBH at 7 or 14 days post surgery. The neurointermediate lobe of the pituitary was similarly unaffected by adrenalectomy, while in the anterior lobe, POMC mRNA increased 7-10 fold at both time points, effects that were prevented by dexamethasone treatment. We conclude that while POMC mRNA in the anterior lobe of the pituitary is regulated by plasma glucocorticoids, in the MBH and neurointermediate lobe, it is not.

Activation of cyclic AMP second messenger system stimulates secretion of beta-endorphin from fetal hypothalamic cells

Relatively little is known about physiological regulators of hypothalamic beta-endorphin (END) secretion and mechanisms by which they stimulate secretion. We sought to determine whether activation of the cyclic AMP (cAMP) second messenger pathway was involved in stimulating hypothalamic beta-END secretion from dissociated fetal hypothalamic cells in culture. Forskolin (FSK), a direct activator of adenylate cyclase which stimulates cAMP formation, stimulated immunoreactive (IR)-beta-END secretion. Because FSK can also stimulate independent of increased cAMP formation, we studied dibutyryl cAMP and 8-bromo-cAMP, analogues of cAMP, which also stimulated IR-beta-END secretion. From these studies we conclude: (1) activation of the cAMP second messenger system stimulates IR-beta-END secretion from hypothalamic cells and supports the rationale that endogenous regulators which stimulate this pathway could be involved in the physiological regulation of hypothalamic beta-END secretion; (2) coupling between the cAMP second messenger pathway and stimulation of hypothalamic beta-END secretion which is presumably present at maturity (adulthood) originates at early stages of development (fetal life).