Prostaglandins and the synergism between VIP and noradrenaline in the cerebral cortex

We have previously shown that vasoactive intestinal peptide (VIP) and noradrenaline (NA) interact synergistically to increase cyclic AMP levels in mouse cerebral cortical slices. The pharmacological mechanism of this synergism is the potentiation by NA, through alpha 1 adrenergic receptors, of the stimulatory effect of VIP on cAMP formation. A similar interaction has been confirmed in guinea pig cerebral cortex and in discrete nuclei of the rat hypothalamus. Furthermore VIP and NA interact synergistically to depress the spontaneous activity of identified neurons in rat neocortex. At the cellular level, this synergistic interaction suggests that VIP- and NA-containing neuronal systems may converge, at least in part, on the same target cells to increase cAMP levels in the cerebral cortex. At the molecular level, the interaction may occur at various steps in signal transduction, between receptors, intramembrane transduction processes or intracellular effector mechanisms. Here we report that the alpha 1-adrenergic potentiation of the increases in cAMP elicited by VIP involves the formation of arachidonic acid metabolites and is mimicked by prostglandins F2 alpha and E2.

VIP receptor subtypes in mouse cerebral cortex: evidence for a differential localization in astrocytes, microvessels and synaptosomal membranes

Pierre Magistretti, Jean-Luc Martin

The binding characteristics of a monoiodinated form of vasoactive intestinal peptide (M-[125I]VIP) to the membranes of astrocytes, intraparenchymal microvessels and synaptosomes were analyzed in mouse cerebral cortex. Binding to astrocytes, studied in primary cultures, indicates the presence of a single class of high affinity binding sites with a Kd of 3.3 nM and a Bmax of 565 fmol/mg protein. The structurally related peptide secretin does not compete for sites labeled by M-[125I]VIP. In cultured astrocytes, VIP has been previously shown to promote glycogenolysis. Secretin, despite its lack of interaction with sites labeled by M-[125I]VIP, stimulates glycogenolysis with an EC50 of 0.5 nM, thus demonstrating the presence in astrocytes of functional secretin receptors independent from those for VIP. Trypsinization of the primary astrocyte cultures followed by replating as secondary cultures, reveals a second class of low affinity binding sites, with a Kd of 41.3 nM and a Bmax of 881 fmol/mg protein. Secretin does not compete for this class of low affinity binding sites either. Binding of M-[125I]VIP to intraparenchymal microvessels reveals the presence of two classes of binding sites with Kd of 1.4 and 30.3 nM, and Bmax of 7.1 and 73.8 pmol/mg protein, respectively. Similar to what is observed in primary or secondary astrocyte cultures, secretin does not interact with these sites. In this cell type VIP stimulates cAMP formation with an EC50 of 18 nM, while secretin is ineffective. Finally, in agreement with previous reports in rat and guinea pig cerebral cortex, two classes of binding sites are observed in synaptosomal membranes: a high affinity class with a Kd of 4.9 nM and a Bmax of 316 fmol/mg protein, and a low affinity class with a Kd of 42.8 nM and a Bmax of 1578 fmol/mg protein. In contrast to what is observed in non-neuronal membranes, in synaptosomal membranes, secretin effectively competes for sites labeled by M-[125I]VIP with an EC50 of approximately 150 nM. These results indicate that secretin may represent a useful tool to discriminate between neuronal and non-neuronal VIP binding sites, since it competes with M-[125I]VIP exclusively for the neuronal class of binding sites.

1992

Release of vasoactive intestinal peptide in mouse cerebral cortex: evidence for a role of arachidonic acid metabolites

Pierre Magistretti, Jean-Luc Martin

In rodent cerebral cortex, vasoactive intestinal peptide (VIP) is contained in a homogeneous population of radially oriented bipolar interneurons. We have observed that 4-aminopyridine (4-AP), a K+-channel blocker, promotes a concentration- and Ca2+-dependent release of VIP from mouse cerebral cortical slices, with a significant effect already observed at 50 microM. Over 70% of VIP release elicited by 4-AP is blocked by 2 microM tetrodotoxin (TTX). Mepacrine, an inhibitor of phospholipase A2 (PLA2) activity and hence of arachidonic acid (AA) formation from membrane phospholipids, markedly inhibits (IC50 of approximately 15 microM) the release of VIP evoked by 4-AP. The inhibitory effect of mepacrine is not additive to that of TTX, thus indicating an involvement of PLA2 activation in the TTX-sensitive component of the 4-AP-evoked release. As a corollary, melittin (0.1-10 micrograms/ml), a PLA2 activator, promotes VIP release. Inhibition of AA metabolites of the lipoxygenase pathway by nordihydroguaiaretic acid, 5,8,11,14-eicosatetranoic acid, and caffeic acid results in a concentration-dependent inhibition of VIP release evoked by 4-AP. This set of observations indicates for the first time that the formation of AA metabolites of the lipoxygenase pathway plays a role in the release of a peptide in the mammalian CNS. Furthermore, these observations together with the previously reported potentiation by prostaglandins of the increase in cyclic AMP elicited by VIP in mouse cerebral cortex (Schaad et al., 1987) indicate that AA metabolites may act at both the presynaptic (lipoxygenase metabolites) and the postsynaptic (cyclooxygenase metabolites) levels to increase the "throughput" or "strength" of VIP-containing circuits in the rodent neocortex.

1989

VIP and noradrenaline act synergistically to increase cyclic AMP in cerebral cortex

Pierre Magistretti

There is growing evidence that two, or possibly more, neurotransmitters can coexist within the same neurone. In particular, the presence of a peptide and a biogenic amine has been demonstrated in the same terminals of central and peripheral neurones. These findings have led to the hypothesis that neurotransmitters, coexisting within the same neurones, can interact at pre- or postsynaptic sites in a functionally coordinated manner. However, interactions between neurotransmitters contained in distinct neuronal systems terminating within the same region of the central nervous system (CNS) can be envisaged. We have examined this last possibility in the cerebral cortex, an area of the CNS where the two neurotransmitters vasoactive intestinal polypeptide and noradrenaline are contained in separate neuronal systems and where they both stimulate the formation of cyclic AMP. We report here that vasoactive intestinal polypeptide and noradrenaline act synergistically to stimulate the formation of cyclic AMP and that this synergistic interaction is antagonized by the specific alpha-adrenergic antagonist phentolamine.

1984

VIP receptor subtypes in mouse cerebral cortex: evidence for a differential localization in astrocytes, microvessels and synaptosomal membranes

Pierre Magistretti, Jean-Luc Martin

1992