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

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.

Adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides modulate adenylate cyclase activity in rat brain slices: evidence for an ACTH/MSH receptor-coupled mechanism

The regulation of adenylate cyclase activity by adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides was investigated in rat brain slices using a superfusion method. Adenylate cyclase activity was concentration-dependently increased by ACTH-(1-24), alpha-MSH (EC50 values 16 and 6 nM, respectively), and [Nle4,D-Phe7]alpha-MSH (EC50 value 1.6 nM), in the presence of forskolin (1 microM, optimal concentration). 1-9-Dideoxyforskolin did not augment the response of adenylate cyclase to ACTH-(1-24). Various peptide fragments were tested for their ability to enhance [3H]cyclic AMP production. [Nle4,D-Phe7]alpha-MSH increased [3H]cyclic AMP formation with a maximal effect of 30% and was more potent than ACTH-(1-24), ACTH-(1-16)-NH2, alpha-MSH, ACTH-(1-13)-NH2, [MetO4]alpha-MSH, [MetO2(4),D-Lys8,Phe9]ACTH-(4-9), ACTH-(7-16)-NH2, ACTH-(1-10), and ACTH-(11-24), in order of potency. This structure-activity relationship resembles that found for the previously described peptide-induced display of excessive grooming. ACTH-(1-24) stimulated adenylate cyclase activity in both striatal (maximal effect, approximately 20%) and septal slices (maximal effect, approximately 40%), but not in hippocampal or cortical slices. Lesioning of the dopaminergic projections to the striatum did not result in a diminished effect of [Nle4,D-Phe7]alpha-MSH on [3H]cyclic AMP accumulation, which indicates that the ACTH/MSH receptor-stimulated adenylate cyclase is not located on striatal dopaminergic terminals. ACTH-(1-24) did not affect the dopamine D1 or D2 receptor-mediated modulation of adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ACTH-(1-24) on dopamine and noradrenaline release, B-50 phosphorylation and calmodulin binding to B-50 in vitro

ACTH-(1-24), 1 microM, enhanced the Ca(2+)-dependent release of [3H]dopamine ([3H]DA) from intact septal synaptosomes by approximately 30%, but had no effect on the release of [3H]noradrenaline ([3H]NA) from intact cortical synaptosomes. Since a strong correlation has been reported between B-50 (phosphorylation) and [3H]NA release from intact or streptolysin-O- (SL-O-) permeated cortical synaptosomes, we investigated whether the effects of ACTH-(1-24) on the release of radiolabelled transmitters are mediated by B-50. We observed that the increment in the release of [3H]DA from SL-O-permeated septal synaptosomes as a result of exposure to a high Ca2+ concentration was much less pronounced than that of the release of [3H]NA from SL-O permeated septal and cortical synaptosomes. ACTH-(1-24) concentration-dependently inhibited [3H]NA release from SL-O-permeated cortical synaptosomes (IC50 value of approximately 10 microM) when ACTH-(1-24) was added 150 s prior to the Ca2+ trigger. Simultaneous addition of ACTH-(1-24), SL-O and Ca(2+)-buffers to cortical synaptosomes did not lead to a change in [3H]NA release at any of the ACTH-(1-24) concentrations tested. ACTH-(1-24) had no effect on B-50 phosphorylation in intact synaptosomes, whereas it concentration-dependently inhibited B-50 phosphorylation in permeated cortical synaptosomes (IC50 value of 100 microM). ACTH-(1-24) inhibited (IC50 value of 10 microM) B-50/calmodulin binding in vitro. We conclude that the effects of high concentrations of ACTH-(1-24) on various biochemical B-50 related parameters are not likely to represent the mechanisms underlying the action of ACTH-(1-24) on neurotransmitter release.

Ultrastructural localization of adrenocorticotrophic hormone and the phosphoprotein B-50/growth-associated protein 43 in freeze-substituted, Lowicryl HM20-embedded mesencephalic central gray substance of the rat

Previous studies have shown that the endogenous phosphorylation of the neuron-specific protein B-50 in isolated synaptic plasma membranes is inhibited by adrenocorticotrophic hormone(1-24). The aim of this study is to examine if there is a specific neuroanatomical interaction of adrenocorticotrophic hormone and B-50 in the mesencephalic central gray substance of the rat. With light microscopy, high B-50 immunoreactivity was detected throughout the mesencephalic central gray substance, overlapping with those areas where adrenocorticotrophic hormone-immunoreactive fibres were present. To study the ultrastructural localization of B-50 and adrenocorticotrophic hormone, we employed a method of immunogold labelling on ultrathin sections of freeze-substituted and Lowicryl HM20-embedded fixed brain tissue. This offered optimal morphological preservation together with high retention of antigenicity. At the electron microscopic level, adrenocorticotrophic hormone immunoreactivity was detected in dense-core secretory granules present in non-junctional regions of axoinal varicosities. This suggests a non-synaptic release of adrenocorticotrophic hormone from the axons. Using double immunolabelling techniques we showed that in adrenocorticotrophic hormone-innervated areas of the mesencephalic central gray substance B-50 immunoreactivity was present at plasma membranes of all unmyelinated axons and axonal varicosities and virtually absent in dendrites. The result on B-50 localization agrees well with previous studies in the hippocampus [Van Lookeren Campagne et al. 1990 J. Neurocytol. 19, 948-961] and in the pyramidal tract [Gorgels et al. 1989 J. Neurosci. 9, 3861-3869] of the rat and suggests that in the mature rat central nervous system, B-50 expression in axons is a general phenomenon. For the adrenocorticotrophic hormone-innervated areas, we discuss the proposal that non-synaptically released adrenocorticotrophic hormone modulates B-50 phosphorylation in axons and axon terminals.

Catecholaminergic, GABAergic, and hippocamposeptal innervation of GABAergic "somatospiny" neurons in the rat lateral septal area

This study deals with the neurochemical characterization of the rat lateral septal area (LSA) somatospiny neurons and their innervation by hippocamposeptal, catecholaminergic, and GABAergic fibers. Electron microscopic single and double immunostaining methods were used to label catecholaminergic fibers and GABAergic cells and boutons. Axon terminals originating in the hippocampus were labeled by acute anterograde axon degeneration induced by fimbria-fornix transection 36 hours before sacrifice. Three types of experiments were performed. The convergent catecholaminergic and hippocamposeptal innervation of LSA somatospiny neurons was studied by combining immunostaining for tyrosine hydroxylase (TH) with fimbria-fornix transection. GABAergic neurons and their hippocamposeptal afferents were identified and characterized in colchicine pretreated animals immunostained for glutamic acid decarboxylase (GAD) combined with fimbria-fornix transection. The third experiment aimed at simultaneously visualizing the relationships between catecholaminergic boutons, hippocamposeptal excitatory amino acid containing axon terminals and GABAergic profiles by double immunostaining for TH (the PAP technique) and GAD (the immunogold method) combined with fimbria-fornix transection. The results are summarized as follows: 1) The same LSA somatospiny neurons receive synaptic inputs from the hippocampus and TH immunoreactive fibers which form pericellular baskets around these cells. 2) LSA somatospiny neurons are GABAergic and are postsynaptic targets of GABAergic boutons with unknown origin and hippocamposeptal axon terminals. 3) The double immunostaining experiment, finally, provided direct evidence that the same GABAergic somatospiny neurons are postsynaptic targets of both catecholaminergic and hippocamposeptal afferents. The synaptic interconnections described in this study provide anatomical basis for a better understanding of the action of catecholamines, excitatory amino acids, and GABA on the activity of LSA neurons.