Pregnenolone sulfate potentiates the effects of NMDA on hippocampal alanine and dopamine

The aim of the present study was to analyze biochemical effects of a neurosteroid, pregnenolone sulfate (PS), which accompany changes in the threshold of seizures, and to establish the contribution of local, hippocampal monoaminergic and amino acid systems, to the control of convulsive activity. Pretreatment of mice with PS (intracerebroventricularly) selectively enhanced the potency of peripherally (intraperitoneally) administered NMDA at the LD16 (88.0 mg/kg) to induce clonic-tonic convulsions (PS, LD84 = 184.7 nM; 95% CL = 181.4-188.1). The proconvulsive actions of picrotoxin and bicuculline, the GABA-A receptor antagonists, were not modified by pretreatment of mice with PS. Administration of PS alone (up to 240 nM icv) did not show any seizure-like activity. PS given at LD84, together with NMDA (at the LD16), increased the hippocampal concentration of alanine, and enhanced local metabolism of dopamine in a period immediately preceding the onset of seizures significantly stronger than did NMDA alone. These and other data indicate that the enhancement by PS of hippocampal levels of alanine may contribute to the seizures development as this amino acid is a precursor of glutamate, and a co-agonist of the NMDA receptors. On the other hand, simultaneously occurring stimulation of hippocampal dopaminergic system may be considered a compensatory phenomenon, limiting seizures propagation through the limbic forebrain. Summarizing, our results show that PS-induced potentiation of NMDA seizures is accompanied by selective changes in hippocampal dopamine turnover and alanine concentration.

Opposite effects of olanzapine and haloperidol in rat ultrasonic vocalization test

The opposite effects of the classical antipsychotic, haloperidol, and atypical neuroleptic, olanzapine, in the rat ultrasonic vocalization test of anxiety were observed. The present data are discussed in relation to growing body of evidence of specific brain biochemical changes after pretreatment with different antipsychotics.

The effects of physostigmine and cholinergic receptor ligands on novelty-induced neophobia

The aim of the study was to analyse in a well-established model of neophobia the effects of peripheral and central (ICV) administration of a prototypical and easily penetrating to the brain acetylcholinesterase inhibitor (AChE-I)--physostigmine, hemicholinium, a selective blocker of the high affinity choline uptake sites, as well as muscarinic and nicotinic receptor ligands. Thus, an attempt was made to address the question whether anxiolytic-like effects of AChE-I, reported in the clinic, are directly related to the anti-emotional action. The effects of peripherally and centrally administrated cholinergic ligands on novelty-induced decrease in exploratory behaviour were examined in rats. It was found that in a limited dose-range physostigmine and nicotine given peripherally or ICV selectively disinhibited rat exploration in the open field, whereas scopolamine stimulated animal motor activity and increased thigmotaxis. Locomotor effects of physostigmine and nicotine appeared at the higher doses and could be easily separated from their anti-neophobic action. The rat's exploratory behaviour tended to be attenuated by central administration of hemicholinium (a choline uptake blocker), and it was significantly inhibited by mecamylamine (a nicotinic receptor antagonist), and pirenzepine (a selective M1 receptor antagonist). Gallamine, a selective M2 receptor antagonist, did not influence on animal novelty-induced anxiety-related behaviour. It is concluded that AChE-I can selectively affect brain emotional processes evoked by neophobia-related stimuli. Probably both nicotinic and M1 cholinergic receptors mediate such an action of AChE-I.

Tolerance to the anticonvulsant activity of midazolam and allopregnanolone in a model of picrotoxin seizures

The effects of an intracerebroventricular (i.c.v.) administration of a non-selective full benzodiazepine receptor agonist, midazolam, and a neuroactive steroid, allopregnanolone, on picrotoxin-induced seizures and striatal dopamine metabolism, were studied in mice. It was found that acute i.c.v. injections of midazolam (ED50=38.25 nmol) and allopregnanolone (ED50=26.34 nmol) blocked picrotoxin-induced seizures to a similar extent. After repeated administration at the ED(85) doses (midazolam-56.6 nmol, allopregnanolone-94.2 nmol; once or twice daily for 5 days) tolerance developed to the anticonvulsant activity of midazolam (ED50=94.14 nmol) and allopregnanolone (ED50=186.70 nmol). Acute i.c.v. injections of midazolam and allopregnanolone (at the ED50 doses established in the model of picrotoxin seizures: 38.25 and 26.34 nmol, respectively), significantly decreased the concentration of dopamine metabolites: 3-methoxytyramine and 3,4-dihydroxyphenylacetic acid, as well as the dopamine turnover rate (homovanilic acid/dopamine ratio; by about 20%), in the mouse striatum. These findings together with the recently published data on the potentiation by midazolam and allopregnanolone of ethanol-induced sleep [Pharmacol. Biochem. Behav. 67 (2000) 345] indicate a very similar central effect profile of benzodiazepines and neurosteroids. Moreover, similar efficacy of allopregnanolone and midazolam at the GABA(A) receptors has been found. Overall, the results of the present study, along with the possibility of neurosteroid conversion in the brain into other steroid hormones (testosterone, estradiol, aldosterone), add to the accumulating evidence suggesting a less favorable pharmacological profile for this class of drugs than was previously thought.

The anxiolytic-like effect of nicotine undergoes rapid tolerance in a model of contextual fear conditioning in rats

The effects of repeated administration of nicotine on contextual fear conditioning, locomotor activity, and pain threshold, were examined in rats. It was found that a single injection of nicotine prior to the training session (three 0.7-mA footshocks, each 0.5 s long), decreased the freezing reaction during the retest 24 h later. The locomotor activity was moderately enhanced, and the pain threshold remained unchanged. The baseline freezing measured immediately after administration of a single dose of nicotine was not significantly different from the saline-treated group. The anxiolytic-like effect of nicotine was as potent as that of midazolam, a benzodiazepine derivative. After five day-by-day injections, the anxiolytic-like effect of nicotine (0.6 mg/kg, sc) was no longer present, independently whether the last drug injection was given 24 h or 5 min (i.e., the sixth, additional, nicotine injection), prior to the training session. Thus, it appeared that the expression of tolerance to the nicotine-induced anxiolytic-like action did not require a direct stimulation of nicotinic receptors. Simultaneously, in this group of animals, nicotine caused a potent stimulation of locomotor activity in the open field test. The applied dosage and regimen of nicotine administration did not change rat pain threshold (flinch-jump test). Collectively, the present data showed for the first time, that short-term, intermittent, administration of nicotine was sufficient to induce tolerance to the anxiolytic-like effect of this drug, in the model of fear conditioning to context. Importantly, a clear dissociation between the locomotor and anxiolytic-like effects of nicotine was present. This effect appeared independent also of changes in rat pain threshold. The possible mechanisms of this phenomenon are discussed.

Rapid down-regulation of GABA-A receptors after pretreatment of mice with progesterone

The effect of a single administration of a high dose of progesterone on brain [3H]muscimol binding, was examined in mice using quantitative autoradiography. It was found that progesterone given at the dose of 150 mg/kg ip (the ED85 dose established previously in the model of picrotoxin seizures, Członkowska et al., Pharmacol. Biochem. Behav., 2000, 67, 345-353), significantly decreased the specific binding of [3H]muscimol to the nucleus caudatus and nucleus accumbens, as early as 1 h after injection. A similar tendency, close to the statistically significant level, was also present in the dentate gyrus of the hippocampus (p = 0.07). It is suggested that a high dose of progesterone and ensuing excessive stimulation of GABAA receptors by its metabolites, neurosteroids, could bring about rapid changes in the GABAA receptor number and/or affinity.

Locomotor activity and a conditioned fear response: correlation with cortical and subcortical binding of the dopamine D1 receptor antagonist

Rat behavior in the open field and conditioned fear response test was correlated with specific binding of dopamine D1 receptor antagonist [3H]SCH 23390 within different brain structures assayed with autoradiography. A significant positive correlation was found between the ligand binding in the substantia nigra pars reticulata and both animal motor activity (r = 0.67, p < 0.05) and the number of entries into the central sector of the open field (r = 0.59, p < 0.05). On the other hand, rat motility and the central entries were negatively correlated with [3H]SCH 23390 binding within the caudate putamen (r = -0.64, p < 0.05 and r = -0.61 p <0.05, respectively). No correlation was revealed between the ligand binding in the examined brain areas and freezing reaction in the contextual fear conditioning test. The present data indicate for the first time a significant, structure-dependent correlation between rat motor behavior and the dopamine D1 receptor ligand binding within the nigrostriatal system.

The effects of neurosteroids on picrotoxin-, bicuculline- and NMDA-induced seizures, and a hypnotic effect of ethanol

The effects of intraperitoneally (IP) or intracerebroventricularly (ICV) administered neurosteroids [allopregnanolone (AP); 5beta-tetrahydrodeoxycorticosterone (5beta-THDOC); dehydroepiandrosterone sulfate (DHEAS); pregnenolone sulfate (PS)] and their precursors [progesterone (PROG), pregnanedione (PREG)] on N-methyl-D-aspartic acid (NMDA)-, picrotoxin (PTX)- and bicuculline (BIC)-induced seizures and ethanol-induced sleep were studied in mice. It was found that IP injections of (+)MK-801 most potently antagonized NMDA-, PTX- and BIC-induced seizures, as compared to diazepam (DZP), PROG and PREG. Both precursors of neurosteroids appeared only marginally active in the applied models of convulsions. ICV injections of AP selectively blocked PTX- and BIC-induced seizures, whereas 5beta-THDOC and (+)MK-801 also antagonized NMDA-induced convulsions. ICV administered DHEAS induced seizures in a dose-dependent way. ICV injections of AP and midazolam shortened the latency and prolonged the duration of sleep induced by IP injections of ethanol (5.0 g/kg). On the contrary, DHEAS and PS significantly reduced the hypnotic-like effect of ethanol. The obtained results suggest that neurosteroids may modulate in an agonistic (AP, 5beta-THDOC), or antagonistic way (PS, DHEAS), the GABA(A) receptor complex functions. Some of them (5beta-THDOC) also interact with NMDA receptors. AP appeared to be the most selectively acting compound, with its profile of action fully comparable to that of midazolam. AP also enhanced the hypnotic effect of ethanol, pointing out to the propensity to interact with centrally depressant agents. These findings, together with the possibility of conversion of some neurosteroids in the brain to other steroid hormones (testosterone, estradiol and aldosterone), indicate the limitations of their use for the treatment of neurological and psychiatric disorders.

Effects of buspirone, diazepam, and zolpidem on open field behavior, and brain [3H]muscimol binding after buspirone pretreatment

The effects of 5-HT(1A) receptor agonist buspirone, a nonselective (diazepam), and a selective (zolpidem) GABA(A) receptor agonist were compared in the open field test of neophobia. Unhabituated rats were pretreated with the drugs once, prior to a first exposure to the open field, and their behavior was recorded both during this test and during a second trial 24 h later. It has been hypothesized that the decrease in exploratory activity observed during the second test session may be considered an adaptive reaction to the first day aversive experience (neophobia). If so, a selective modulation of 5-HT and GABA systems activity during the test could bring about significant changes in animal behavior on the retest. Buspirone at the lowest dose of 0.3 mg/kg revealed anxiolytic-like properties on the first day, whereas the action of diazepam and zolpidem was modulated by the dose-related sedative effect. At the dose of 2.4 mg/kg buspirone elicited delayed in time anxiolytic-like action, i.e., produced the antithigmotactic effect during the retrial 24 h later. Diazepam and zolpidem failed to exhibit similar profile of action. Autoradiography of [3H]muscimol binding after pretreatment of rats with buspirone showed a significant increase in the selective radioligand binding within the frontal cortex and a similar, near-significant tendency in the dentate gyrus of the hippocampus. The behavioral data validate buspirone as important drug for the treatment of anxiety disorders, devoid of disruptive influence on motor and cognitive processes. The open field test, as modified by us, appeared sensitive in distinguishing the behavioral profiles of action of different anxiolytic compounds, including 5-HT(1A) receptor agonist. The present results support the assumption that reduced turnover of 5-HT due to stimulation of 5-HT(1A) autoreceptors, may bring about changes in GABA(A) receptor system activity, in some brain structures, leading to the anxiolytic effect.

The effects of neurosteroids on rat behavior and 3H-muscimol binding in the brain

The effects of ICV administration of metabolites of progesterone and deoxycorticosterone [i.e., neurosteroids: AP (3alpha-hydroxy-5alpha-pregnan-20-one, allopregnanolone), 5alpha(-THDOC (3alphat-21-dihydroxy-5alpha-pregnan-20-one, 5alpha-tetrahydrodeoxycorticosterone), 5beta-THDOC (3alpha-21-dihydroxy-5beta-pregnan-20-one, 5beta-tetrahydrodeoxycorticosterone), and PS (3beta-hydroxy-5-pregnen-20-one sulfate, pregnenolone sulfate] were studied in the open-field test of neophobia and Vogel's test of conflict behavior in rats. The influence of in vivo administered 5beta-THDOC, a positive allosteric modulator of the GABA(A) receptor complex, on 3H-muscimol binding in different brain structures, was also studied with the help of quantitative autoradiography. The presented data did not reveal any anxioselective effects for a range of centrally active neurosteroids, in the ethologically orientated and conflict models of anxiety, after intracerebral drug administration. Their central effects appeared secondary to changes in rat gross behavior. It is possible that high local concentration of neurosteroids after ICV injection and production of a narrower range of behavioral effects than that of benzodiazepines, precluded manifestation of the antianxiety effects of AP, 5alpha-THDOC and 5beta-THDOC. Autoradiography did not reveal any significant changes in the specific binding of 3H-muscimol in brain structures after in vivo ICV administration of 5beta-THDOC at the behaviorally active dose. Thus, the possibility that neuroactive neurosteroids may provide a novel potential site for therapeutic interventions in anxiety disorders is not supported. The part of the experiment with 5beta-THDOC is interpreted as contributing to other results, suggesting the existence of a new category of neurosteroids acting as partial agonists of the GABA(A) receptor.