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

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.

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.

alpha-melanocyte-stimulating hormone and habituation of prey-catching behavior in the Texas toad, Bufo speciosus

We investigated dose-dependent effects of alpha-melanocyte-stimulating hormone (alpha-MSH) on habituation in the Texas toad, Bufo speciosus. Additionally, we determined changes in plasma and brain levels of alpha-MSH following peripheral administration of the peptide or following exposure to an ether stressor. The ability of alpha-MSH to facilitate acquisition of habituation was dose dependent. Plasma alpha-MSH concentrations were elevated within 5 min of dorsal lymph sac injection and remained elevated up to 600% over controls after 30 min. Administration of 50 microgram alpha-MSH had no effect on plasma corticosterone levels. Radiolabeled alpha-MSH was detected in cerebrospinal fluid microdialysates within minutes of peripheral injection. Concentrations of alpha-MSH in the telencephalon and preoptic area were significantly lowered after ether exposure, whereas levels in the optic tectum, thalamus/hypothalamus, brainstem, and plasma were unchanged. We conclude that alpha-MSH administered peripherally facilitates habituation in a dose-dependent fashion. Our results confirm that the effects of alpha-MSH are independent of corticosterone secretion. The peptide is cleared rapidly into the bloodstream and enters the cerebrospinal fluid after dorsal lymph sac injection. Neuronal alpha-MSH may help toads gather information about their environment when exposed to certain stressors.

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.

Endogenous opioids in frontal cortex of patients with Down syndrome

The main purpose of this study was to investigate differences regarding endogenous opioids in post-mortem frontal cortex of adult patients with Down syndrome (DS), patients with Alzheimer disease (AD) and neurologically healthy persons, respectively, using specific radioimmunoassays. The results of this study show that there is an increase in the levels of leu-enkephalin and dynorphin A in the frontal cortex of patients with DS as compared to the control group. An almost identical increase was also observed when comparing patients with AD to controls. In conclusion, the results of this study suggest a relationship between elevated tissue levels of leuenkephalin and dynorphin A in cerebral cortex and cognitive impairments in patients with DS and AD.