Coronaridine congeners potentiate GABAA receptors and induce sedative activity in mice in a benzodiazepine-insensitive manner

To determine whether (+)-catharanthine induces sedative- or anxiolytic/anxiogenic-like activity in male mice, proper animal paradigms were used. The results showed that (+)-catharanthine induces sedative-like activity in the 63-72 mg/Kg dose range in a flumazenil-insensitive manner, but neither this effect nor anxiolytic/anxiogenic-like activity was observed at lower doses. To determine the underlying molecular mechanism of the sedative-like activity, electrophysiological and radioligand binding experiments were performed with (+)-catharanthine and (±)-18-methoxycoronaridine [(±)-18-MC] on GABA_A (GABA_ARs) and glycine receptors (GlyRs). Coronaridine congeners both activated and potentiated a variety of human (h) GABA_ARs, except hρ1. (+)-Catharanthine-induced potentiation followed this receptor selectivity (EC₅₀'s in μM): hα1β2 (4.6 ± 0.8) > hα2β2γ2 (12.6 ± 3.8) ~ hα1β2γ2 (14.4 ± 4.6) indicating that both α1 and α2 are equally important, whereas γ2 is not necessary. (+)-Catharanthine was >2-fold more potent and efficient than (±)-18-MC at hα1β2γ2. (+)-Catharanthine also potentiated, whereas (±)-18-MC inhibited, hα1 GlyRs with very low potency. Additional [³H]-flunitrazepam competition binding experiments using rat cerebellum membranes clearly demonstrated that these ligands do not bind to the benzodiazepine site. This is supported by the observed activity at hα1β2 (lacking the BDZ site) and similar effects between α1- and α2-containing GABA_ARs. Our study shows, for the first time, that (+)-catharanthine induced sedative-like effects in mice, and coronaridine congeners potentiated human α1β2γ2, α1β2, and hα2β2γ2, but not ρ1, GABA_ARs, both in a benzodiazepine-insensitive fashion, whereas only (+)-catharanthine slightly potentiated GlyRs.

Regulation of neurosteroid biosynthesis by neurotransmitters and neuropeptides

The enzymatic pathways leading to the synthesis of bioactive steroids in the brain are now almost completely elucidated in various groups of vertebrates and, during the last decade, the neuronal mechanisms involved in the regulation of neurosteroid production have received increasing attention. This report reviews the current knowledge concerning the effects of neurotransmitters, peptide hormones, and neuropeptides on the biosynthesis of neurosteroids. Anatomical studies have been carried out to visualize the neurotransmitter- or neuropeptide-containing fibers contacting steroid-synthesizing neurons as well as the neurotransmitter, peptide hormones, or neuropeptide receptors expressed in these neurons. Biochemical experiments have been conducted to investigate the effects of neurotransmitters, peptide hormones, or neuropeptides on neurosteroid biosynthesis, and to characterize the type of receptors involved. Thus, it has been found that glutamate, acting through kainate and/or AMPA receptors, rapidly inactivates P450arom, and that melatonin produced by the pineal gland and eye inhibits the biosynthesis of 7α-hydroxypregnenolone (7α-OH-Δ(5)P), while prolactin produced by the adenohypophysis enhances the formation of 7α-OH-Δ(5)P. It has also been demonstrated that the biosynthesis of neurosteroids is inhibited by GABA, acting through GABA(A) receptors, and neuropeptide Y, acting through Y1 receptors. In contrast, it has been shown that the octadecaneuropetide ODN, acting through central-type benzodiazepine receptors, the triakontatetraneuropeptide TTN, acting though peripheral-type benzodiazepine receptors, and vasotocin, acting through V1a-like receptors, stimulate the production of neurosteroids. Since neurosteroids are implicated in the control of various neurophysiological and behavioral processes, these data suggest that some of the neurophysiological effects exerted by neurotransmitters and neuropeptides may be mediated via the regulation of neurosteroid production.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Structure−Activity Relationships of a Series of Analogs of the Endozepine Octadecaneuropeptide (ODN11-18) on Neurosteroid Biosynthesis by Hypothalamic Explants

We have previously shown that the endozepine octadecaneuropeptide (ODN) stimulates the biosynthesis of neurosteroids from frog hypothalamic explants. In the present study, we have investigated the structure-activity relationships of a series of analogs of the C-terminal octapeptide of ODN (OP) on neurosteroid formation. We found that OP and its cyclic analog cyclo1-8OP stimulate in a concentration-dependent manner the synthesis of various steroids including 17-hydroxypregnenolone, progesterone, 17-hydroxyprogesterone and dehydroepiandrosterone. Deletion or Ala-substitution of the Arg1 or Pro2 residues of OP did not affect the activity of the peptide. In contrast, deletion or replacement of any of the amino acids of the C-terminal hexapeptide fragment totally abolished the effect of OP on neurosteroid biosynthesis. The present study indicates that the C-terminal hexapeptide of ODN/OP is the minimal sequence retaining full biological activity on steroid-producing neurons.

Immunohistochemical localization and biological activity of the steroidogenic enzyme cytochrome P450 17α-hydroxylase/C17, 20-lyase (P450C17) in the frog brain and pituitary

It is now clearly established that the brain has the capability of synthesizing various biologically active steroids including 17-hydroxypregnenolone (17OH-Delta(5)P), 17-hydroxyprogesterone (17OH-P), dehydroepiandrosterone (DHEA) and androstenedione (Delta(4)). However, the presence, distribution and activity of cytochrome P450 17alpha-hydroxylase/C17, 20-lyase (P450(C17)), a key enzyme required for the conversion of pregnenolone (Delta(5)P) and progesterone (P) into these steroids, are poorly documented. Here, we show that P450(C17)-like immunoreactivity is widely distributed in the frog brain and pituitary. Prominent populations of P450(C17)-containing cells were observed in a number nuclei of the telencephalon, diencephalon, mesencephalon and metencephalon, as well as in the pars distalis and pars intermedia of the pituitary. In the brain, P450(C17)-like immunoreactivity was almost exclusively located in neurons. In several hypothalamic nuclei, P450(C17)-positive cell bodies also contained 3beta-hydroxysteroid dehydrogenase-like immunoreactivity. Incubation of telencephalon, diencephalon, mesencephalon, metencephalon or pituitary explants with [(3)H]Delta(5)P resulted in the formation of several tritiated steroids including 17OH-Delta(5)P, 17OH-P, DHEA and Delta(4). De novo synthesis of C(21) 17-hydroxysteroids and C(19) ketosteroids was reduced in a concentration-dependent manner by ketoconazole, a P450(C17) inhibitor. This is the first detailed immunohistochemical mapping of P450(C17) in the brain and pituitary of any vertebrate. Altogether, the present data provide evidence that CNS neurons and pituitary cells can synthesize androgens.