L. Fiore

Estrogen modulates stimulation of inositol phospholipid hydrolysis by norepinephrine in rat brain slices

The influence of estrogen on stimulation of inositol phospholipid hydrolysis by norepinephrine and carbamylcholine has been studied by measuring the accumulation of [3H]inositol-monophosphate ([3H]InsP) in cortical, hippocampal and striatal slices from ovariectomized rats. Repeated (but not a single) subcutaneous injections of estradiol benzoate (EB) (2 micrograms/animal once every 2 days for 10 days) markedly reduced stimulation of inositol phospholipid hydrolysis by norepinephrine in hippocampus and corpus striatum. Conversely, the efficacy of norepinephrine was increased in cortical slices. Estrogen treatment did not affect basal or carbamylcholine-stimulated [3H]InsP formation. In vitro addition of 17 beta-estradiol (1-100 nM) failed to modify norepinephrine- or carbamylcholine-induced [3H]InsP production in all regions examined. An increased density of alpha 1-adrenergic binding sites in cortical membranes paralleled the enhanced responsiveness of inositol phospholipid hydrolysis to norepinephrine induced by EB treatment in this area, whereas no significant changes in [3H]prazosin binding were found in membranes from hippocampus and corpus striatum. These results indicate that estrogen may affect inositol phospholipid hydrolysis in discrete brain areas, suggesting a complex role for estradiol in modulating noradrenergic receptor activity in the central nervous system.

Inositol Hexakisphosphate (Phytic Acid) Enhances Ca2+Influx and D‐[3H]Aspartate Release in Cultured Cerebellar Neurons

Abstract: Inositol hexakisphosphate (InsP6) increased 45Ca2+uptake in cultured cerebellar granule cells. This increase was concentration dependent (EC50= 20 μM), exhibited slow kinetics, and was present after 5 days of cell maturation in culture. InsP6 also enhanced D‐[3H]aspartate release in cerebellar granule cells at 11‐12 days in vitro. Stimulation of 45Ca2+ uptake was also produced by inositol pentakisphos‐phate but not by inositol 1,3,4,5‐tetrakisphosphate. The increase in 45Ca2+ influx induced by InsP6 was independent of extracellular Na+ and was only partially reduced by the organic calcium channel blocker nifedipine. The intrinsic action of InsP6 was not affected by competitive or noncompetitive glutamate receptor antagonists. In addition, stimulations of 45Ca2+ uptake by InsP6 and glutamate were additive. These data provide evidence that InsP6 directly activates a specific population of neurons in the CNS.

Effect of Dihydroergocryptine and Dihydroergocristine on Cyclic AMP Accumulation and Prolactin Release in vitro: Evidence for a Dopaminomimetic Action

Dihydroergocryptine and dihydroergocristine, two C-9, 10-hydrogenated ergot alkaloids, inhibited in a concentration-dependent manner prolactin release and cyclic AMP accumulation in cultured anterior pituitary cells. The inhibitory effect of dihydroergocryptine was more potent and started at lower concentrations than that of dihydroergocristine. Haloperidol and pimozide, two dopamine receptor antagonists, completely abolished the inhibitory activity of the ergot alkaloids. The involvement of the adenylate cyclase-cyclic AMP system in the inhibitory action of the two compounds was demonstrated by the antagonism by pertussis toxin of the reduction of both prolactin release and cyclic AMP accumulation produced by dihydroergocryptine and dihydroergocristine.

Effect of calcitonin on ACTH secretion

We have investigated the effects of salmon calcitonin injected intracerebroventricularly on adrenocorticotropic (ACTH) secretion in rats. Salmon calcitonin was able to increase significantly (p less than 0.05) ACTH secretion at the dose of 5 ng/rat and (p less than 0.01) at the doses of 12.5, 25 and 50 ng/rat; the effect (50 ng/rat) was maximal at 30 min and still present after 60 and 120 min. The maximal dose of calcitonin produced a significant (p less than 0.05) increase in serum ACTH concentration also in stressed animals. It is suggested that calcitonin may increase serum ACTH secretion by central norepinephrine and/or 5-hydroxytryptamine pathways that regulate corticotropic releasing factor activity.

TRH potentiates excitatory amino acid-stimulated phosphoinositide (PI) hydrolysis in neuronal cultures

TRH is well-recognized for its regulatory action on the anterior pituitary function. It has also been localized throughout the central nervous system (CNS) of humans and of several mammalian and nonmammalian species, where it is known to exert various neurochernical and behavioral effects that appear to be independent of its neuroendocrine function.’ The mechanism(s) by which TRH produces these effects at the CNS level has not been completely elucidated yet. At the pituitary level, TRH enhances PI hydrolysis, and this effect is related to stimulation of prolactin and thyroid-stimulating hormone (TSH) release.2 We reported recently that TRH produced a significant increase in inositol phosphate accumulation in rat brain slices and that the entity of the response in the diRerent brain areas well paralleled the distribution of TRH receptors in the CNS.3 In this study, we investigated the action of TRH on primary cultures of cerebellar neurons. In these cells, TRH (500 nM-SO pM) slightly (25-30%) enhanced the accumulation of [3H]inositol-l -monophosphate, but exhibited a clear synergistic action with L-glutamate, L-aspartate, and N-methy1-D-aspartate (NMDA), a selective agonist of a specific class of Mg2+-sensitive glutamate receptors coupled with PI hydrolysis (GPI receptors), but not with the muscarinic receptor agonist carbamylcholine (FIG. 1) . When PI hydrolysis was measured in Mg2+-free conditions, stimulation by NMDA was potentiated by more than 80% in the presence of TRH. The facilitative action of TRH appears to be specific for PI as a signaltransducing mechanism. In fact, TRH failed to affect the stimulation of 4’Ca2+ influx induced by glutamate, NMDA, or kainate in cultured cerebellar granule cells (TABLE 1). We conclude that TRH potentiates signal transduction at a specific “Mg*+sensitive” glutamate receptor (GP, receptor), acting, presumably, as a positive allosteric modulator. This action of TRH may be of physiological relevance and may be responsible, at least in part, for the therapeutic effects of the peptide in neurologic disorders.