Interaction of corticotropin-releasing factor (CRF) and alpha-helical CRF on rat neurointermediate lobes: in vitro studies

The role of beta-endorphin in the pathophysiology of major depression

A role for beta-endorphin (beta-END) in the pathophysiology of major depressive disorder (MDD) is suggested by both animal research and studies examining clinical populations. The major etiological theories of depression include brain regions and neural systems that interact with opioid systems and beta-END. Recent preclinical data have demonstrated multiple roles for beta-END in the regulation of complex homeostatic and behavioural processes that are affected during a depressive episode. Additionally, beta-END inputs to regulatory pathways involving feeding behaviours, motivation, and specific types of motor activity have important implications in defining the biological foundations for specific depressive symptoms. Early research linking beta-END to MDD did so in the context of the hypothalamic-pituitary-adrenal (HPA) axis activity, where it was suggested that HPA axis dysregulation may account for depressive symptoms in some individuals. The primary aims of this paper are to use both preclinical and clinical research (a) to critically review data that explores potential roles for beta-END in the pathophysiology of MDD and (b) to highlight gaps in the literature that limit further development of etiological theories of depression and testable hypotheses. In addition to examining methodological and theoretical challenges of past clinical studies, we summarize studies that have investigated basal beta-END levels in MDD and that have used challenge tests to examine beta-END responses to a variety of experimental paradigms. A brief description of the synthesis, location in the CNS and behavioural pharmacology of this neuropeptide is also provided to frame this discussion. Given the lack of clinical improvement observed with currently available antidepressants in a significant proportion of depressed individuals, it is imperative that novel mechanisms be investigated for antidepressant potential. We conclude that the renewed interest in elucidating the role of beta-END in the pathophysiology of MDD must be paralleled by consensus building within the research community around the heterogeneity inherent in mood disorders, standardization of experimental protocols, improved discrimination of POMC products in analytical techniques and consistent attention paid to important confounds like age and gender.

The mammalian pituitary intermediate lobe: an update on innervation and regulation

The pituitary intermediate lobe (IL) in mammals is an area of uniform endocrine cells which synthesize and release specific peptide products of the proopiomelanocortin gene. The lobe receives direct synaptic connections onto the endocrine cells from hypothalamic dopaminergic neurons. This review updates information on the dopaminergic as well as the gamma-aminobutyric acid inhibitory neuroregulation for the IL. It also provides a discussion of stimulatory molecules which are likely to affect peptide release, particularly the neurotransmitter serotonin, which may be present via uptake into the dopaminergic nerve terminals. Other stimulatory molecules discussed which are likely to significantly affect peptide secretion are norepinephrine, corticotropin-releasing factor, and several opiate peptides. A new direction of study involves the potential interaction of neurotrophic factors, which are present in all areas of the pituitary, and may be suggested to have a supportive role for the neural elements of the IL. The endocrine cells of the IL and their direct hypothalamic innervation are considered to be an easily accessed peripheral model for study of both neural-endocrine and neurotrophic-target cell interactions.

Involvement of central, but not placental corticotropin releasing hormone (CRH) in heat stress induced immunosuppression during pregnancy

To clarify whether corticotropin releasing hormone (CRH) and beta-endorphin (betaEP) system mediate maternal immunosuppression in pregnant rats exposed to heat through central or placental pathway, we examined the effects of intravenous (iv) (100 or 500 microg) or intracerebroventricular (icv) (5 microg) administration of CRH receptor antagonist alpha-helical CRH (9-41) on splenic natural killer cell activity (NKCA) as well as betaEP in blood, pituitary lobes, and placenta in pregnant rats at 15 to 16 days gestation. Two-way ANOVA revealed that heat reduced NKCA and elevated blood and pituitary betaEP but did not change placental betaEP. Iv administered 500 microg and icv administered alpha-helical CRH reversed the reduced NKCA and the elevated pituitary betaEP, while iv administration of 100 microg alpha-helical CRH did not. The increased blood betaEP was reversed by iv 100 and 500 microg alpha-helical CRH and icv administration. Both iv and icv administrations reduced placental betaEP independent of heat exposure. Thus, the response of placental betaEP to iv administration of alpha-helical CRH seemed to be stronger than that of pituitary betaEP. These results indicate that alpha-helical CRH which acts on pituitary betaEP antagonizes heat-induced immunosuppression during pregnancy, suggesting that immunosuppression produced by heat stress during pregnancy is mediated by the central CRH system. The placental CRH-betaEP system seems unlikely to be involved in the immunosuppression. Physiologic roles of placental CRH and opioid system should be clarified by future in vitro experiments using placenta specimen including placental immunocyte.

Mu and delta opioid receptor regulation of pro-opiomelanocortin peptide secretion from the rat neurointermediate pituitary in vitro

We investigated the ability of selective opioid agonists and antagonists to influence pro-opiomelanocortin peptide secretion from the rat neurointermediate lobe in vitro. The mu-opioid agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]enkephalin) significantly stimulated beta-endorphin and alpha-melanocyte-stimulating hormone release relative to controls early (30 min) in the incubation period. Similar effects on beta-endorphin secretion were observed with the selective mu-opioid agonist dermorphin. The delta-opioid receptor agonist DPDPE ([D-Pen(2,5)]enkephalin) weakly inhibited beta-endorphin secretion relative to controls while the kappa-opioid receptor agonist U50488 had no effect. The mu-opioid selective antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)) inhibited basal beta-endorphin secretion while kappa- and delta-opioid receptor antagonists had no effect. Our data support a role for local mu-opioid receptor control of intermediate lobe pro-opiomelanocortin peptide secretion. Peptide secretion from melanotropes appears to be tonically stimulated by activation of mu-opioid receptors in the absence of intact neuronal innervation to the intermediate lobe.

Alterations in brain and pituitary beta-endorphin content in genetically epilepsy-prone rats

We measured beta-endorphin concentrations in the anterior and neurointermediate lobes of the pituitary gland and in microdissected brain regions of moderate-seizure genetically epilepsy-prone rats (GEPR-3), severe-seizure GEPR-9s and Sprague-Dawley non-epileptic control rats. Plasma concentrations of beta-endorphin and beta-melanocyte-stimulating hormone (alpha-MSH) were also measured as indicators of pituitary POMC-peptide secretion. Concentrations of beta-endorphin in the anterior lobe of GEPR-3s were 53% higher compared to controls and 57% higher compared to GEPR-9s. There were no differences in neurointermediate lobe beta-endorphin concentrations between control and either GEPR strain. Plasma beta-endorphin concentrations were significantly lower in GEPR-9s than controls. Plasma levels of alpha-MSH did not differ between control and GEPRs. In the hypothalamus of GEPR-9s beta-endorphin concentrations in the arcuate nucleus were significantly greater than in GEPR-3s. Concentrations of beta-endorphin in the central amygdala of GEPR-9s were two- to threefold greater than in control or GEPR-3s. Beta-Endorphin concentrations in the central gray of GEPR-3s were 58% lower than control or GEPR-9s. These data suggest that anterior lobe beta-endorphin secretion is reduced in GEPR-9s. Furthermore, brain endorphinergic pathways appear to be differentially altered in GEPR-3s and GEPR-9s. Alterations in pituitary beta-endorphin secretion and brain endorphinergic systems may contribute to seizure susceptibility in GEPRs and to differences in seizure severity between GEPR-3s and GEPR-9s.

Regulation of pituitary beta-endorphin secretion in aging rats: in vitro responsiveness to dopamine

The intermediate lobe of the mammalian pituitary is highly responsive to dopamine inhibition of beta-endorphin secretion. In this study, the ability of aged (12 months) intermediate lobes to respond to dopamine was compared to that of young (6 weeks) tissue, using a short-term in vitro incubation of isolated rat neurointermediate lobes, with measurement of peptide release by radioimmunoassay. Tissue from the aged rats released greater amounts of beta-endorphin peptide than amounts measured from young tissue at all time periods studied. The aged lobes were also found to be significantly more sensitive to dopamine than young glands, as measured by percent change of each group compared to respective baseline release. In comparison, incubation of tissue from young animals in which the intermediate lobe had been acutely denervated by treating rats with injections of the catecholamine neurotoxin, 6-hydroxydopamine, did not differ in responsiveness to dopamine as compared to tissue from control rats. The observations suggest that aging intermediate lobe, while being hypersecretory, is supersensitive to dopamine, perhaps as the result of gradually reduced innervation.

Effects of the enkephalin analog (D-Met2,Pro5)-enkephalinamide on alpha-melanocyte-stimulating hormone secretion

We used the met-enkephalin analog (D-Met2,Pro5)-enkephalinamide (DMPEA) to investigate enkephalinergic control of alpha-melanocyte-stimulating hormone (alpha-MSH) secretion. Systemic (s.c.) administration of DMPEA elevated plasma titers of alpha-MSH in a dose- and time-related manner. Pretreatment with the opiate antagonist naltrexone had no effect on basal plasma levels of alpha-MSH but blocked DMPEA-induced alpha-MSH release. Treatment with a dose of naltrexone sufficient to block DMPEA-induced secretion of alpha-MSH had no effect on stress-induced secretion of alpha-MSH. Although pretreatment with the dopamine receptor agonist apomorphine prevented DMPEA-induced alpha-MSH secretion, DMPEA had no effect on the synthetic activity of tuberohypophysial dopamine neurons as gauged by measuring the accumulation of 3,4-dihydroxyphenylalanine in the neurointermediate lobe (NIL) following administration of NSD-1015. In vitro treatment of isolated NILs with DMPEA resulted in a significant increase in alpha-MSH release. Naltrexone completely blocked the stimulatory effects of DMPEA on alpha-MSH release in vitro. Our results indicate that DMPEA stimulates alpha-MSH secretion by acting directly through opiate receptors at the level of the NIL.

Benzodiazepine suppression of corticotropin-releasing factor (CRF)-induced beta-endorphin release from rat neurointermediate pituitary

Dopamine and gamma-aminobutyric acid (GABA) inhibit POMC peptide release from the pituitary intermediate lobe, via interaction with D2 or GABA-A/benzodiazepine receptors. Here, we examined the effects of an antianxiety triazolobenzodiazepine, adinazolam, on corticotropin-releasing factor (CRF)-stimulated POMC peptide secretion from the rat neurointermediate pituitary. Neurointermediate lobes (NILS) were incubated with CRF (10(-7) M), then adinazolam (10(-8) or (10(-9) M) was added, with CRF remaining in the medium. Aliquots were removed at 15-min intervals and frozen for radioimmunoassay of beta-endorphin. Adinazolam alone did not significantly affect secretion as compared to controls or CRF alone. Adinazolam incubated with CRF led to significant inhibition of beta-endorphin secretion, as compared to CRF alone. In addition, adinazolam was as effective as dopamine or the CRF antagonist, alpha-helical CRF, in preventing CRF-induced beta-endorphin release. Adinazolam appears to act directly on the pituitary to suppress hormone release induced by a stress-related hypothalamic peptide.