Depot testosterone attenuates the anticonvulsant effect of flurazepam in mice.

We examined whether pretreatment with depot testosterone produces a functional alteration in benzodiazepine receptor sensitivity. This was accomplished by testing the ability of flurazepam to increase the threshold voltage for seizure production in groups of mice given vehicle or testosterone cypionate (1 or 5 mg/kg) 21 days prior to testing in an electroconvulsive shock paradigm. Depot testosterone treatment reduced flurazepam's antiseizure potency in this paradigm. That testosterone altered the ability of flurazepam to protect mice from seizures is consistent with previous reports suggesting a complex interaction between steroids and the BDZ/GABAA system.

Profound stress-induced alterations in flurazepam's antiseizure efficacy can be attenuated.

A new procedure is described for assessing the sensitivity of the benzodiazepine receptor in intact animals. This procedure measures the ability of benzodiazepines to antagonize electrically-induced seizures precipitated by incremental increases in voltage. This functional measure detected stress-induced alterations in benzodiazepine receptor sensitivity, an alteration shown previously with in vitro and in vivo receptor binding techniques. In contrast, mouse rotorod performance, which has been proposed as a behavioral measure of benzodiazepine receptor sensitivity did not show stress-induced alterations. The mechanism of these stress-induced alterations was explored using this incremental electroconvulsive shock procedure (IECS). A 'GABA-negative' benzodiazepine mimicked the effects of swim stress, whereas treatment of animals prior to swim stress with Ro15-1788, a 'GABA-neutral' benzodiazepine, attenuated these stress-induced alterations. These data suggest both that these stress effects may be mediated by an endogenous ligand, and that Ro15-1788 may have a novel indication as a prophylactic intervention for individuals at risk for exposure to severe and unusual stress.

Neurotensin inhibits the activation of midbrain serotonergic neurons produced by random inescapable sound.

Previous studies indicate that exposure of rats to randomly presented, inescapable loud sound, referred to as sound stress, increases central serotonin turnover as well as the ex vivo activity of tryptophan hydroxylase (EC 1.14.16.4), the rate-limiting enzyme in serotonin biosynthesis. The purpose of this investigation was to determine whether intracerebroventricular (i.c.v.) administration of neurotensin (NT), a tridecapeptide found within the midbrain raphe, influences the activation of the midbrain serotonergic neurons by sound stress. Accumulation of 5-hydroxytryptophan (5-HTP) in vivo, in the presence of the aromatic amino acid decarboxylase inhibitor, NSD 1015 (m-hydroxybenzylhydrazine, 100 mg/kg i.p.) given immediately before a 30 min sound stress, was used as an index of in vivo tryptophan hydroxylase activity. Sound-stressed rats had significantly higher levels of 5-HTP in cortex and midbrain compared to sham-stressed controls. NT (0.01-3.3 nmol total), given i.c.v., 5 min prior to 30 min sound stress, completely blocked the enhanced accumulation of 5-HTP, but had no effect on basal accumulation of 5-HTP, except at the highest doses of 1.0 or 3.3 nmol, which others have previously shown to inhibit basal serotonergic metabolism. NT (0.3 and 3.3 nmol) blocked the increase in cortical tryptophan hydroxylase activity, ex vivo, in response to 30 min sound stress, without affecting basal enzyme activity. These and other recent data suggest a possible role for endogenous NT in the regulation of serotonergic neuronal activity within the midbrain raphe.

Interaction of cholinergic and glutamatergic transmission in the hippocampus: an in vitro autoradiographic receptor analysis.

Quantitative in vitro receptor autoradiography was used to examine the effect of acute scopolamine administration on specific binding to components of the N-methyl-D-aspartate (NMDA) receptor complex in four regions of mouse hippocampus. The binding of [3H]glycine to the strychnine-insensitive site was increased 1 h after administration of scopolamine hydrobromide (10 mg/kg) in the ventral dentate gyrus. The study suggests that rapid alterations in strychnine-insensitive glycine binding can occur in response to cholinergic perturbations. Moreover, these data suggest a delicate interaction between cholinergic and glutamatergic projections in the hippocampus.

Ethanol's antiseizure efficacy is reduced by stress.

The ability of ethanol to antagonize the electrical precipitation of seizures in an incremental electroconvulsive shock paradigm was examined in groups of stressed and control mice. In stressed mice, the dose-response curve for ethanol's antiseizure efficacy was down-shifted and right-shifted relative to controls. These data may have clinical implications with respect to the interaction between stress, ethanol, and proneness to seizures.

Glycinergic interventions potentiate the ability of MK 801 to raise the threshold voltage for tonic hindlimb extension in mice.

Milacemide, an acylated prodrug of glycine, was able to increase the efficacy with which [+]-5-methyl-10,11-dihydro-5h-dibenzo[a,d]cyclohepten-5,10-imine meleate (MK 801) antagonized the electrical precipitation of seizures in mice. The mechanism of milacemide's potentiation of MK 801's antiseizure efficacy in intact mice is unclear; however, a glycine agonist selective for the strychnine-insensitive site on the NMDA receptor complex was also able to potentiate MK 801. The exciting possibility exists that an exogenous glycinergic intervention can potentiate NMDA-mediated neural transmission in intact animals.

Potentiation of ethanol via interference with calcium channels.

The relevance of ethanol's ability to inhibit voltage-gated and receptor-gated calcium ion conductance in vitro to its acute effects in intact animals was examined. Specifically, nimodipine (a voltage-sensitive calcium antagonist of the dihydropyridine class), indole-2-carboxylic acid (a competitive antagonist of the strychnine-insensitive glycine binding site), MK-801 (a noncompetitive allosteric antagonist of the NMDA receptor complex), and d-cycloserine (a partial agonist of the strychnine-insensitive glycine binding site) were examined for their ability to alter ethanol's antiseizure efficacy in an incremental electroconvulsive shock paradigm. The results showed that drugs known to interfere with voltage-gated and receptor-gated calcium ion conductance potentiated ethanol's antiseizure efficacy. These results implicate voltage-gated and receptor-gated calcium ion conductance in ethanol's acute pharmacologic effects in intact animals.

Effects of milacemide, a glycine prodrug, on ethanol's antiseizure efficacy.

A variety of in vitro data suggest that ethanol interferes with N-methyl-D-aspartate (NMDA)-stimulated calcium ion conductance. This effect occurs at ethanol concentrations in blood associated with acute intoxication in the nontolerant human (less than 50 mM) and may involve its selective action at the strychnine-insensitive glycine binding site on the NMDA receptor complex. Moreover, there are in vitro data showing that glycinergic interventions can attenuate ethanol's inhibitory actions on NMDA-mediated transmission. The relevance of these in vitro findings to the intact animal was tested in an incremental electroconvulsive shock (IECS) paradigm using milacemide, a lipophilic prodrug of glycine. In this paradigm, the influence of milacemide on ethanol's ability to antagonize the electrical precipitation of seizures was tested. Doses of 3.2 and 32.0 mg/kg did not change ethanol's antiseizure efficacy, whereas 320.0 mg/kg potentiated ethanol's antiseizure efficacy. The mechanism of potentiation of ethanol's antiseizure efficacy by milacemide is unknown. Potentiation could result from stimulation of chloride ion conductance in the brainstem by glycine liberated from the lipophilic prodrug and acting at the strychnine-sensitive site. Alternatively, unmetabolized milacemide, which accumulates at the highest administered dose, may antagonize NMDA-mediated neural transmission. The latter explanation would be consistent with a role for receptor-gated calcium ion conductance in the mediation of ethanol's actions.