Modulation of Neurosteroid Synthesis/Accumulation by l‐Ascorbic Acid in Rat Brain Tissue: Inhibition by Selected Serotonin Antagonists

Abstract

We have investigated the possibility that the synthesis/accumulation of neurosteroids, i.e., brain‐produced steroids putatively endowed with modulatory actions in the CNS, is regulated by monoaminergic receptor‐mediated mechanisms. In minces of rat brain cortex, l‐ascorbic acid concentration‐dependently (0.07–1.0 mM) increases the levels of pregnenolone, allotetrahydrodeoxycorticosterone, and dehydroepiandrosterone. This effect of l‐ascorbic acid is region‐dependent: in hippocampus, progesterone and allopregnanolone are also increased, whereas dehydroepiandrosterone is unchanged, and in corpus striatum only progesterone is increased significantly. 5‐Hydroxytryptamine (10 µM), 1‐(3‐chlorophenyl)piperazine (1.0 µM), and 5‐methoxytryptamine (0.4 µM) mimic the effect of l‐ascorbic acid, whereas a pretreatment with p‐chlorophenylalanine (400 mg/kg i.p., 2 days) reduces the amplitude of the l‐ascorbic acid effect on brain cortical neurosteroids. The effect of l‐ascorbic acid is blocked by the nonselective serotonin antagonists methiothepin, clozapine, methysergide, and pizotifen, but not mesulergine, spiperone, MDL 72222, and dl‐propranolol, nor by the catecholaminergic receptor antagonists prazosin and S(−)‐sulpiride. l‐Ascorbic acid is not additive with dibutyryl‐cyclic AMP and, furthermore, the inhibition of adenylate cyclase by MDL 12330A, but not of phospholipase C by U‐73122, markedly attenuates the l‐ascorbic acid‐induced increase of pregnenolone in rat brain cortical minces. Together these data suggest that l‐ascorbic acid plays a role in the modulation of neurosteroidogenesis, presumably by favoring the activation of the purported serotonin type 6 receptor by endogenous serotonin.

Clozapine, but not Haloperidol, Increases Brain Concentrations of Neuroactive Steroids in the Rat

“…Our present data showing that clozapine increases the brain concentrations of neuroactive steroids may seem inconsistent with those of our previous study showing that clozapine inhibits, through its antagonist action at 5HT 6 receptors, the stimulation of neurosteroid production by L-ascorbic acid in brain cortical minces in vitro (Roscetti et al 1998). However, this latter effect was apparent at nanomolar concentrations of clozapine, and it is likely that the concentrations achieved in the brain after systemic administration of a dose of 5 to 20 mg/kg allow clozapine to interact also with GABA A receptors, possibly masking other effects of the drug.…”

2‐Deoxy‐d‐glucose enhances tonic inhibition through the neurosteroid‐mediated activation of extrasynaptic GABAA receptors

“…To further confirm the involvement of neurosteroids in the 2‐DG–induced activation of tonic inhibition, we tested the effect of DHA, the oxidized form of vitamin C. Ascorbic acid, an essential nutrient for animals and plants, is the most potent antioxidant agent in the brain and was reported to exhibit antiseizure and neuroprotective activities . DHA, similarly to 2‐DG, is known to enhance PPP, decrease glucose oxidation through glycolysis, and dramatically increase neurosteroid levels . The observations that (1) DHA was able to increase the GABAergic tonic inhibition; (2) its effects were totally abolished by finasteride and the enhancement of PPP; and (3) NADPH production is shared by DHA and 2‐DG treatments; all indicate that the inhibition of glycolysis by either 2‐DG or DHA and the compensatory PPP activation stimulate the GABAergic tonic inhibition via neurosteroids.…”

“…37 DHA, similarly to 2-DG, is known to enhance PPP, decrease glucose oxidation through glycolysis, 38 and dramatically increase neurosteroid levels. 29,39 The observations that (1) DHA was able to increase the GABAergic tonic inhibition;…”

Brain Steroidogenesis Mediates Ethanol Modulation of GABAAReceptor Activity in Rat Hippocampus

“…Preparation of rat hippocampal minces. Rat hippocampal minces were prepared as described (Roscetti et al, 1998). In brief, the brain was excised rapidly (within 1 min) from the skull, and the hippocampus was dissected on ice and cut along two orthogonal planes into blocks (300 ϫ 300 m) with a McIlwain tissue chopper.…”