ACTH-(1-24) and alpha-MSH antagonize dopamine receptor-mediated inhibition of striatal dopamine and acetylcholine release

Semax, an ACTH(4-10) analogue with nootropic properties, activates dopaminergic and serotoninergic brain systems in rodents

Corticotrophin (ACTH) and its analogues, particularly Semax (Met-Glu-His-Phe-Pro-Gly-Pro), demonstrate nootropic activity. Close functional and anatomical links have been established between melanocortinergic and monoaminergic brain systems. The aim of present work was to investigate the effects of Semax on neurochemical parameters of dopaminergic- and serotonergic systems in rodents. The tissue content of 5-hydroxyindoleacetic acid (5-HIAA) in the striatum was significantly increased (+25%) 2 h after Semax administration. The extracellular striatal level of 5-HIAA gradually increased up to 180% within 1-4 h after Semax (0.15 mg/kg, ip) administration. This peptide alone failed to alter the tissue and extracellular concentrations of dopamine and its metabolites. Semax injected 20 min prior D: -amphetamine dramatically enhanced the effects of the latter on the extracellular level of dopamine and on the locomotor activity of animals. Our results reveal the positive modulatory effect of Semax on the striatal serotonergic system and the ability of Semax to enhance both the striatal release of dopamine and locomotor behavior elicited by D-amphetamine.

Alpha-melanotropin hormone inhibits the binding of [3H]SCH 23390 to the dopamine D1 receptor in vitro

We have previously demonstrated that the simultaneous presence of alpha-melanocyte stimulating hormone (alpha-MSH) and dopamine resulted in a reduction in cyclic AMP (cAMP) levels in slices containing caudate putamen and accumbens nuclei as compared to those treated only with dopamine or alpha-MSH. This study was carried out to explore if the interaction between alpha-MSH and dopamine could be explained on the basis of a direct interaction between alpha-MSH and the dopamine D1 receptor. Saturation curves for [n-methyl-3 H](R)-(+)-8 chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1 H-3-benzazepin-7-o] hemimaleate ([3H]SCH 23390) binding in the presence of increasing concentrations of alpha-MSH were performed. Nonlinear regression in the presence of alpha-MSH revealed an increased dissociation constant (Kd). The binding capacity (Bmax) was not affected by the peptide. These data suggest an apparent competitive interaction between alpha-MSH and [3H]SCH 23390 in striatal membranes on the dopamine D1 receptor; (Ki = 1.2 X 10(-7) M). The present data show that alpha-MSH could interact with the dopamine D1 receptor modulating allosterically the affinity of [3H]SCH 23390 for the receptor or by causing a change in the lipid environment of the dopamine receptor, resulting in an inhibition of the ligand binding to it.

Interaction between alpha-MSH and gabaergic agents upon striatal cAMP levels: an in vitro model

We have tried to investigate the possible interaction between the gabaergic system and alpha-MSH at a cellular level in an in vitro model of male albino rats tissue slices containing accumbens and caudate-putamen nuclei. Alpha-MSH alone increases cAMP levels, as does diazepam and phaclofen; however, these effects were blocked by SCH-23390. Both flumazenil and baclofen induced a decrease in the cAMP content. When both alpha-MSH and gabaergic agents were incubated together, cAMP levels were modified. It can be assumed that cAMP production by the neuropeptide and the gabaergic agents could be linked to the activation of dopaminergic D1 receptors. The latter receptors had no prominent effect on the interaction between alpha-MSH and the GABA agonists and antagonists. In summary, our results suggested that alpha-MSH and GABA system could be biochemically linked to produce a cellular effect.

Effect of alpha-MSH upon cyclic AMP levels induced by the glutamatergic agonists NMDA, quisqualic acid, and kainic acid

This study was carried out to investigate possible interactions between some glutamatergic agonists and the peptide alpha-MSH upon the cyclic AMP levels. We used an in vitro tissue slice preparation incubated in the presence of different glutamatergic agonists such as N-methyl-D-aspartic acid (NMDA), quisqualic acid (QUIS), kainic acid (KA), and the peptide alpha-MSH together with each agonist. Slices containing caudate putamen and accumbens were chosen according to neurochemical data indicating that the striatum contains a moderate amount of MSH binding sites and also receives glutamatergic innervation. Exposure of these slices to either MSH or to the agonists NMDA or QUIS resulted in an increase in the cAMP levels in relation to controls. Nevertheless, incubation with KA resulted in no changes in the nucleotide levels. The combination of MSH/NMDA induced a reduction of cAMP levels in relation to those obtained with NMDA alone. The combinations of QUIS/MSH or KA/MSH also induced variations in the values of nucleotide in relation to the those obtained with the peptide alone or with the corresponding agonist; these changes were related to the dose of agonist used in each case. The results obtained in these experiments suggest the existence of some interaction between the peptide and the agonist used.

alpha-MSH changes cyclic AMP levels in rat brain slices by an interaction with the D1 dopamine receptor

The exposure of rat brain slices containing caudate putamen and accumbens nuclei to alpha-MSH or dopamine (DA) results in an increase in cyclic AMP (cAMP) levels. When tissues are compared with those containing both alpha-MSH and DA, a reduction in the cyclic nucleotide is observable. This study was carried out to determine whether variations in tissular cAMP levels induced by alpha-MSH might be explained by an interaction between the peptide and some dopaminergic receptors. Therefore, we measured cAMP in tissues and medium in response to alpha-MSH in the presence of haloperidol, the selective D1 (SCH 23390) or D2 (sulpiride) antagonists, or the selective D1 (SKF 38393) or D2 (bromocriptine) agonists. Haloperidol by itself induced no changes either in the cAMP content or in the cAMP efflux to the medium. When slices were exposed to alpha-MSH and haloperidol, the latter blocked the alpha-MSH effect of inducing an increase in the content of cAMP. None of the specific antagonists (at the administered doses) induced changes in the content of cAMP when compared with the control group. The presence of SCH 23390 in the incubation medium together with alpha-MSH yielded a reduction in cAMP levels compared with those incubated with alpha-MSH. A slight stimulatory effect on cAMP formation was observed when the dopaminergic agonists (SKF 38393 10 microM) were used. We conclude that alpha-MSH interacts with the D1 dopamine receptor, changing the cAMP levels in striatum and accumbens nuclei.

Melanocortins as factors in somatic neuromuscular growth and regrowth

Melanocortins, non-corticotropic fragments of adrenocorticotropic hormone, accelerate growth of the developing neuromuscular system and regrowth of damaged neurons, both in the adult and neonatal rat. Morphological, electrophysiological and behavioral characteristics are all improved by melanocortins, which, however, vary in potency, with alpha-MSH being the most effective. Tissue substrate, dosage, critical time periods and pattern of neuropeptide administration are all important variables. Melanocortins protect central neurons affecting motor behavior during development or following neuronal damage in the adult brain. Possible mechanisms of melanocortin action are discussed.

Independent and simultaneous effects of alpha-MSH and dopamine on cyclic AMP levels in rat brain slices

This study was carried out to explore whether the postulated interaction between alpha-MSH and dopamine (DA) could be explained on the basis of variations in tissular cyclic AMP (cAMP) levels. We used slices containing caudate putamen and accumbens nuclei incubated in the presence of alpha-MSH, DA, or both simultaneously. Exposure of slices to alpha-MSH or DA resulted in an increase in cAMP levels. The simultaneous presence of alpha-MSH and DA resulted in a reduction in the accumulation of the cyclic nucleotide in the tissues as compared with those treated only with DA or alpha-MSH. The effect of alpha-MSH and DA on cAMP efflux was also tested. Incubation of slices with DA led to a marked increase in cAMP efflux; this efflux could be prevented if alpha-MSH was present in the medium. The results suggest that the inhibition by alpha-MSH of DA-induced striatal cAMP accumulation and efflux are a consequence of an interaction between the peptide and the neurotransmitter; DA and alpha-MSH transducing mechanisms could be biochemically linked.

Adrenocorticotropic hormone influences the development of adaptive changes in dopamine autoreceptors induced by chronic administration of desipramine

The influence of adrenocorticotropic hormone (ACTH) in the adaptive changes on central dopamine (DA) autoreceptors following chronic administration of desipramine (DMI) has been examined in rats. Dopamine had an inhibitory effect on basal and K(+)-induced release of [3H]DA from slices of striatum and n. accumbens of rats treated chronically (10 days) with ACTH (50 IU/kg, s.c.), DMI (10 mg/kg, i.p.) or the combination of ACTH and DMI. In slices of n. accumbens, but not in slices of striatum of rats exposed to the combined treatment of ACTH and DMI, a significant decrease in the inhibitory effect of exogenous DA on stimulated release of [3H]DA was observed. Chronic administration of ACTH or DMI alone had no effect. The effect of the combined treatment with both agents, on the reactivity of these DA receptors was evaluated by means of apomorphine-induced hypoactivity. The administration of ACTH and DMI (5 mg/kg, i.p.) reduced the hypoactivity induced by apomorphine, as compared to hypoactivity in rats treated with ACTH or DMI alone. Experiments with ACTH4-10 revealed that the peptide modified biochemical and behavioural parameters of dopaminergic function, which may implicate a direct action of the peptide on the brain, rather than on the release of adrenal hormones. These findings suggest that ACTH accelerates the onset of DMI-induced adaptive changes on dopamine in the mesolimbic area. However, because the effect of ACTH4-10 on release of adrenocortical hormone was not investigated, the possibility cannot be disregarded that the effect of the peptide was secondary to an enhancement of release of adrenal hormone.

ACTH/MSH-like peptides inhibit the binding of dopaminergic ligands to the dopamine D2 receptor in vitro

ACTH-(1-24) decreased the binding of the dopamine D2 receptor agonist, [3H]N-propylnorapomorphine ([3H]NPA), to rat striatal membranes in a concentration-dependent manner, with a Ki of 5 x 10(-7) M. Saturation curves for [3H]NPA binding in the presence of increasing concentrations of ACTH-(1-24) were performed. Scatchard analysis in the presence of ACTH-(1-24) revealed an increased dissociation constant (Kd), while the binding capacity (Bmax) was not affected by the peptide, suggesting an apparent competitive interaction between ACTH-(1-24) and [3H]NPA. ACTH-(1-24) also reduced the binding of the dopamine D2 receptor antagonist [3H]spiperone to striatal membranes, with a Ki of 10(-6) M. Much higher concentrations of ACTH-(1-24), up to 10(-4) M, were needed for the displacement of appropriate radiolabelled ligands from dopamine D1 receptors, serotonin 5-HT1A, serotonin 5-HT1B, muscarinic M1 acetylcholine and histamine H1 receptors. ACTH-(1-24) also inhibited the binding of [3H]spiperone to dopamine D2 receptors in membranes of the pituitary gland, the septum and the substantia nigra. ACTH-(1-39) and most ACTH fragments and analogs were less potent than ACTH-(1-24) in displacing [3H]NPA from the dopamine D2 receptor in striatal membranes. In general there was a relationship between displacing potency and chain length. ACTH-(7-16)-NH2 and benzyloxycarbonyl-ACTH-(8-16)-NH2, however, were more potent than ACTH-(1-24) in reducing the binding of [3H]NPA to dopamine D2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Melanotropin receptors in the brain are differentially distributed and recognize both corticotropin and alpha-melanocyte stimulating hormone

Melanotropinergic neurons in the brain may mediate the known modulatory effects of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH) on brain functions including thermoregulation, pituitary regulation, learning and behavior. To determine the distribution of brain melanotropin receptors, we used biologically active [125I]Nle4, D-Phe7-alpha-MSH ([125I]NDP-MSH) for in situ binding and autoradiography in frozen rat brain sections. Specific (alpha-MSH-inhibitable) [125I]NDP-MSH binding sites were distributed in a region-specific pattern, and were present in numerous structures within the septal area, hypothalamus, thalamus, epithalamus, olfactostriatal complex, and midbrain. Each brain structure studied showed a characteristic, reproducible distribution and relative intensity of binding. Receptor peptide selectivity was assessed by comparing the dose-response relationships for inhibition of binding by alpha-MSH, NDP-MSH and ACTH. In all brain structures studied, the 3 peptides gave comparable maximal inhibition of tracer binding, indicating that all detectable binding sites recognized all 3 melanotropins. The respective relative potencies were: NDP-MSH (EC50 = 1.7 +/- 0.6 nM) greater than alpha-MSH (EC50 = 46.9 +/- 11.7 nM) = ACTH. These results provide a preliminary neuroanatomic map of potential target sites for melanotropin actions, and indicate that these sites are capable of recognizing multiple products of the intrinsic melanotropinergic system of the brain.

Seborrhea and persistent tardive dyskinesia

The following communication concerns two schizophrenic patients with Tardive dyskinesia (TD) in whom fluctuations in the severity of the dyskinesias were accompanied by changes in the severity of the seborrheic skin lesions. Since seborrheic dermatitis may be associated with increased plasma melanocyte-stimulating hormone (MSH) level, these observations suggest an association between the severity of TD and increased pituitary MSH release. In addition, TD may be associated with hypothalamic-pituitary dysfunction of MSH autoregulation.

ACTH-(1-24) enhances the electrically stimulated release of [3H]dopamine from rat septal slices via a dopamine D2 receptor-independent mechanism

ACTH-(1-24) enhanced the basal as well as the electrically stimulated release of [3H]dopamine from rat septal slices in vitro. In the absence of Ca2+ from the superfusion medium the effect of ACTH-(1-24) on the electrically stimulated release of [3H]dopamine was abolished. The stimulus-evoked release of [3H]dopamine from septal slices appeared to be modulated through dopamine receptors of the D2 subtype: the dopamine D2 receptor agonists 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) and quinpirole reduced, whereas the dopamine D2 receptor antagonist sulpiride enhanced the electrically stimulated release of [3H]dopamine. The magnitude of the effect of ACTH-(1-24) on [3H]dopamine release was the same in the presence or absence of N-0437, quinpirole and sulpiride. ACTH-(1-24) had no effect on either the basal or the electrically stimulated release of [3H]noradrenaline. Also when the electrically stimulated release of [3H]noradrenaline was reduced by the alpha 2-adrenoceptor agonist clonidine, the peptide was without effect. These results show that ACTH-(1-24) selectively enhances the release of [3H]dopamine from septal slices. The effect of the peptide is independent of the degree of activation of dopamine D2 receptors which modulate the stimulus-evoked release of [3H]dopamine. These results suggest that ACTH-(1-24) enhances the stimulus-evoked release of dopamine in the septum via a mechanism not associated with dopamine D2 autoreceptors.

Hypothalamic compensatory mechanisms in Parkinson's disease

Parkinson's disease (PD) is associated with loss of dopaminergic neurons of the nigrostriatal bundle and to a lesser extent of the mesolimbic and hypothalamic dopaminergic systems. Frank symptoms of the disease usually emerge when at least 70-80% of striatal dopamine (DA) content have been reduced, raising the possibility that the preclinical phase of the disease might be due to compensatory changes that permit residual dopaminergic neurons to subserve functions previously carried out by the entire projection. These neurochemical compensatory mechanisms are known to occur at a striatal level of the rate limiting enzymes of catecholamine synthesis as well as alterations in the sensitivity of the dopaminergic postsynaptic receptors (Zigmond et al., 1984; Bokobza et al., 1984). Compensatory mechanisms, although generally less well recognized, also occur in the hypothalamus and involve endocrine regulation, abnormal neuropeptide release and the emergence of several autonomic and sensory symptoms manifesting prior to or during the course of the disease. Such neurochemical and clinical adaptation mechanisms of the hypothalamus may help to explain the neurobiological events underlying the preclinical phase of Parkinsonism. Moreover, progressive failure of these hypothalamic adaptive mechanisms may be critical in the transition of the disease into the "malignant phase" (Danielczyk et al., 1980).

Influence of adrenocorticotrophic hormone on the behaviour in the swim test of rats treated chronically with desipramine

Chronic desipramine (DMI) administration induced a dose-dependent reduction in the immobility time of the swim test in rats. A combined treatment of ACTH (50 iu kg-1 s.c.) and DMI (5 or 10 mg kg-1 i.p.) for 7 days potentiated the anti-immobility effect of DMI. ACTH 4-10, a fragment peptide with little corticotrophic activity, mimicked ACTH-induced potentiation. No stimulating effect on locomotor activity was observed following seven daily co-administrations of ACTH or ACTH 4-10 and DMI (10 mg kg-1). This behavioural evidence indicates that ACTH potentiation involves a central mechanism and demonstrates a functional interaction between ACTH and DMI at the behavioural level.

In vitro interaction of ACTH with rat brain muscarinic receptors

ACTH-(1-24) inhibits the in vitro binding of the muscarinic antagonist [3H]QNB to membranes from rat brain. The magnitude of inhibition is dependent on the concentration of ACTH-(1-24). Kinetic analysis indicates a pure competitive inhibition which is suggestive of a reversible interaction of ACTH with muscarinic receptors. A mechanism involving an interaction of ACTH-(1-24) with the phospholipid core of the receptors is suggested. Structure activity studies point to a relation with reported effects of intracerebroventricularly administered ACTH on the turnover rate of acetylcholine and the ACTH-induced stretching and yawning syndrome.