Endogenous gamma-aminobutyric acid tonically inhibits release of alpha-melanocyte-stimulating hormone from rat hypothalamic slices

Altered levels of α-melanocyte stimulating hormone in cerebrospinal fluid and plasma of patients with traumatic brain injury

Traumatic brain injury (TBI) is closely associated with marked inflammation. Although alpha-Melanocyte-Stimulating Hormone (α-MSH) exerts powerful anti-inflammatory effects, changes in endogenous α-MSH levels following TBI remain poorly understood. We investigated the changes of α-MSH levels in the cerebrospinal fluid (CSF) and plasma of post-TBI patients and the association of these changes with the severity of TBI and inflammation. TBI severity was assessed by the GCS coma scale from which, patients were separated into three groups. Clinical data were collected on days 1, 3, 5, and 7 including levels of α-MSH, tumor necrosis factor (TNF-α), and intracranial pressure (ICP). α-MSH levels in CSF steadily increased for one week (peak at day 5) but plasma α-MSH decreased and remained low. These changes were more substantial in the Severe Group of TBI with lower GCS. TNF-α levels were similarly increased in both CSF and plasma (peak at day 3). In the early phase of TBI elevated TNF-α and ICP dominated, and CSF α-MSH displayed a slow and insufficient increase. In later phases of TBI, TNF-α and ICP levels were alleviated concordantly with sustained increases in central α-MSH, wherein an anti-inflammatory environment might predominate. The relationship between plasma α-MSH and TNF-α showed significant negative correlation, and the relationship between CSF α-MSH and TNF-α showed significant positive correlation with a two-day lag. In conclusion, plasma α-MSH levels decreased, but CSF levels increased slowly following TBI. These changes were more substantial in severe patients with a lower GCS. Increases in central α-MSH paralleled alleviation of inflammation.

GABAergic agents prevent alpha-melanocyte stimulating hormone induced anxiety and anorexia in rats

Alpha-melanocyte stimulating hormone (alpha-MSH) is a hypothalamic peptide believed to play a tonic inhibitory role in feeding and energy homeostasis. Systemic administration of alpha-MSH is known to produce anorexia and anxiety. Since synaptic contacts between gamma-aminobutyric acid (GABA)ergic terminals and alpha-MSH neurons in the hypothalamus have been reported, the present work was undertaken to refine our knowledge on the role of GABAergic systems in anxiety and anorexia induced by intracerebroventricular (icv) administration of alpha-MSH in rats. The anxiety was assessed by elevated plus maze, and spontaneous food consumption was monitored during dark cycle. Prior administration of diazepam and muscimol that promote the function of GABA(A) receptors reversed the anxiogenic response and decreased food intake elicited by alpha-MSH. In contrast, bicuculline, the GABA(A) receptor antagonist, not only enhanced the effects of alpha-MSH but also prevented the influence of GABAergic drugs on alpha-MSH-induced anorexia and anxiety. These findings suggest that alpha-MSH-induced anxiety and anorexia are due to its negative influence on GABAergic system.

Alpha-melanocyte-stimulating hormone is decreased in plasma of patients with acute brain injury

The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is a proopiomelanocortin derivative that has potent anti-inflammatory influences within the brain. The aim of the present research was to determine if there are changes in blood concentrations of this peptide in patients with acute traumatic brain injury (TBI) or subarachnoid hemorrhage (SAH). Concomitantly, we recorded clinical parameters and measured blood concentrations of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). Twenty-three patients were enrolled in this study--18 had TBI and five SAH. Blood samples for determination of alpha-MSH and TNF-alpha were collected daily from day 1 to day 4 after injury. Baseline concentration of plasma alpha-MSH in patients with acute brain injury of either traumatic or vascular origin was significantly lower than in controls. Patients with TBI or SAH had similar alpha-MSH concentrations and the peptide remained consistently low over four post-injury days. Circulating TNF-alpha on day one was measurable in all patients and there was a negative correlation between plasma TNF-alpha and alpha-MSH. Alpha-MSH was measured again after the acute phase in eight patients. The peptide was substantially increased in all subjects except for two who had an unfavorable outcome. From the well-known protective anti-inflammatory influences of alpha-MSH in the host, reduction in this circulating peptide may have detrimental consequences in brain injury. The data raise the possibility that restoration of normal circulating alpha-MSH through administration of the peptide could be beneficial in patients with brain injury.

The gamma(2)-MSH peptide mediates a central analgesic effect via a GABA-ergic mechanism that is independent from activation of melanocortin receptors

Using the latency for tail-flick after thermal stimulation we have assessed the effects of alpha-, gamma(1)- and gamma(2)-MSH on nociceptive threshold in the mice. Intracisternal injections of gamma(2)-MSH induced a distinct analgesia, while gamma(1)-MSH in the same doses gave only a minor analgesia. Intracisternal alpha-MSH instead gave a short-term hyperalgesia. The effect of gamma(2)-MSH was not blocked by any of the MC(4)/MC(3)receptor antagonist HS014, naloxone or by the prior intracisternal administrations of gamma(1)-MSH. However, the gamma(2)-MSH analgesic response was completely attenuated by treating animals with the GABA(A)antagonist bicuculline. The gamma(2)-MSH analgesic effect was moreover additive to the analgesia afforded by muscimol and ethanol, but not to that afforded by diazepam. In addition both gamma(1)- and gamma(2)-MSH induced moderate catalepsy, but could at the same time attenuate haloperidol induced catalepsia. We conclude that gamma(2)-MSH mediates a central analgesic effect via GABA-receptor dependent pathway that is distinct from melanocortic- and opioid-receptors. Moreover, the mechanism for gamma(2)-MSH's analgesic effect appears to be distinct from that causing moderate catalepsia by gamma-MSH's.

GABAergic inhibition of the pituitary release of adrenocorticotropin and alpha-melanotropin is impaired in dogs with hepatic encephalopathy

gamma-Aminobutyric acid (GABA) is the principal depressant neurotransmitter system, but its possible role in the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis has not yet been investigated in the dog. Moreover, GABA is one of the factors underlying the syndrome of hepatic encephalopathy (HE), and in dogs with HE, the regulation of the HPA axis is deranged. We have therefore investigated the role of the GABA system in the regulation of the HPC system in 10 healthy dogs and 10 dogs with HE due to congenital portosystemic shunts. The effect of an intravenous injection of the GABA antagonist bicuculline on the release of adrenocorticotropin (ACTH), alpha-melanotropin (MSH), and cortisol was measured in plasma. In healthy dogs, a dose of 1.0 mg/kg caused a marked release of ACTH, MSH, and cortisol, but doses of 0.001 to 0.5 mg/kg produced an inconsistent or no response. The high release of MSH after bicuculline administration indicated that the effect of GABA was predominantly in the neurointermediate lobe of the pituitary. In order to investigate whether the effect of GABA was exerted in the pituitary or at a suprapituitary level, the effect of incubation with GABA on basal and corticotropin-releasing hormone-induced ACTH release was measured in primary cultures of anterior and neurointermediate lobe cells from healthy dogs, and no response was observed. We conclude that in healthy dogs, GABA inhibits the release of ACTH and MSH from the neurointermediate lobe of the pituitary at a suprapituitary level. In dogs with HE, 1.0 mg/kg of bicuculline caused virtually no stimulation of the secretion of ACTH, MSH, or cortisol, indicating deranged GABAergic neurotransmission in HE. This may be explained by an increased GABA tone that prevents the effect of the antagonist. Such a high GABA tone associated with HE has been documented in several other species. Dogs with HE had significantly increased basal levels of ACTH, MSH, and cortisol in plasma, and their cortisol:creatinine ratios in 24-hr urine samples (63 +/- 14.10(-6)) were higher than those of healthy dogs (9 +/- 2.10(-6)). An increased basal HPA activity in dogs with HE is not in agreement with augmented GABAergic inhibition, but this contradiction may be explained by the predominance of effects of dopaminergic disinhibition that has been reported in such dogs.

Effect of centrally administered neuropeptide Y on hypothalamic and hypophyseal proopiomelanocortin-derived peptides in the rat

In a previous study, we have shown that neuropeptide Y inhibits the release of alpha-melanocyte-stimulating hormone from the rat hypothalamus in vitro. The aim of the present study was to investigate the possible effect of neuropeptide Y on the regulation of proopiomelanocortin-derived peptides in vivo. Rats received acute or chronic administration of neuropeptide Y in the lateral ventricle and the amount of alpha-melanocyte-stimulating hormone was measured in the hypothalamus and in the neurointermediate lobe of the pituitary. In the same experiments, the amounts of corticotropin-releasing factor and corticotropin were quantified in the hypothalamus and anterior pituitary, respectively. Acute treatment with synthetic neuropeptide Y (0.1 to 10 micrograms/rat) did not modify the amount of alpha-melanocyte-stimulating hormone in the hypothalamus. In contrast, chronic infusion of neuropeptide Y (1.25 micrograms/h) over a seven day period significantly decreased the hypothalamic content of alpha-melanocyte-stimulating hormone, suggesting that neuropeptide Y regulates the synthesis and/or the processing of proopiomelanocortin. Concurrently, we found that both acute and chronic infusion of neuropeptide Y induced a significant reduction in corticotropin-releasing factor in the hypothalamus as well as a significant decrease in alpha-melanocyte-stimulating hormone and corticotropin in the neurointermediate and anterior lobes, respectively. Quantitative in situ hybridization histochemistry showed that chronic administration of neuropeptide Y also caused a reduction of proopiomelanocortin messenger RNA levels both in the intermediate and anterior lobes of the pituitary. Administration of neuropeptide Y (10(-6) M) on perifused rat hypothalamic slices caused a significant increase in corticotropin-releasing factor release.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous glutamate stimulates release of alpha-melanocyte-stimulating hormone from the rat hypothalamus

Release of alpha-melanocyte-stimulating hormone (alpha-MSH) and glutamic acid was quantified from superfused slices of rat hypothalamus. Application of L-glutamic acid 10(-4) M failed to evoke release of alpha-MSH but, in the presence of 10(-4) M dihydrokainic acid (DHK) an inhibitor of glutamate uptake systems, caused significant stimulation of release. DHK caused gradual and sustained increases in both alpha-MSH and glutamate release. That in alpha-MSH was blocked by 10(-4) M DL-2-amino-5-phosphopentanoic acid, a competitive N-methyl-D-aspartic acid (NMDA)-type glutamate receptor antagonist. We conclude that hypothalamic glutamate is subject to rapid uptake through mechanisms blocked by DHK and that alpha-MSH release is stimulated by endogenous and exogenous glutamate through NMDA-type receptors.