The effects of electrical hippocampal kindling of seizures on amino acids and kynurenic acid concentrations in brain structures

Our study demonstrated that the development of seizures during the electrically induced kindling of seizures is associated with significant changes in the concentration of kynurenic acid (KYNA) and its precursor, tryptophan (TRP). The primary finding of our study was an increase in KYNA levels and the KYNA/TRP ratio (a theoretical index of activity of the kynurenine pathway) in the amygdala and hippocampus of kindled animals. We also found decreases in the concentration of tryptophan in the hippocampus and prefrontal cortex. Changes in the concentration of KYNA and TRP in the amygdala were accompanied by a significant decrease in γ-Aminobutryic Acid (GABA) levels and an increase in the glutamate/GABA ratio. Moreover, we found a significant negative correlation between the local concentrations of KYNA and glutamate in the amygdala of kindled rats. However, there were no changes in the local concentrations of the following amino acids: glutamate, aspartate, glutamine, glycine, taurine and alanine. In conclusion, these new results suggest a modulatory influence of KYNA on the process of epileptogenesis, characterized by a negative relationship between the KYNA and glutamate systems in the amygdala.

The effect of CRF2 receptor antagonists on rat conditioned fear responses and c-Fos and CRF expression in the brain limbic structures

The influence of intracerebroventricular-administered selective corticotropin-releasing factor receptor 2 (CRF(2)) antagonists (antisauvagine-30, astressin-2B), on rat anxiety-like behavior, expression levels of c-Fos and CRF, and plasma corticosterone levels were examined in the present study. In fear-conditioned animals, both CRF receptor antagonists enhanced a conditioned freezing fear response and increased the conditioned fear-elevated concentration of serum corticosterone. Exogenously administered antisauvagine-30 increased the aversive context-induced expression of c-Fos in the 1 and 2 areas of the cingulate cortex (Cg1, Cg2), the central amygdala (CeA) and parvocellular neurons of the paraventricular hypothalamic nucleus (pPVN), and it enhanced the effect of conditioned fear in the secondary motor cortex (M2) and medial amygdala (MeA). Immunocytochemistry demonstrated an increase in CRF expression in the Cg1, M2 areas of the cortex, and pPVN, and it revealed the effect of conditioned fear in the CeA 35 min after antisauvagine-30 administration and 10 min after the conditioned fear test. Furthermore, astressin-2B, another CRF(2) receptor antagonist, enhanced expression of c-Fos and CRF in the CeA and pPVN, and revealed the effect of conditioned fear in the Cg1. These data support a model in which an excess in CRF(1) receptor activation, combined with reduced CRF(2) receptor signaling, may contribute to stronger expression of anxiety-like responses.

Midazolam inhibits neophobia-induced Fos expression in the rat hippocampus

The effect of midazolam on expression of c-Fos protein was examined in the rat hippocampus, following the open field test of neophobia. It was found that pretreatment of rats with midazolam, at the dose of 0.5 mg/kg, enhanced rat exploratory behavior, and inhibited neophobia related stimulation of c-Fos in the CA-1 and CA-3 areas of the hippocampus. The presented results provide new immunocytochemical data on the involvement of hippocampus in emotional processes related to neophobia, and indicate a possible site of action of benzodiazepines.

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.

Role of serotonergic and noradrenergic systems in a model of visceral pain

The effects of selective manipulations of activity of the serotonergic and noradrenergic systems were examined in the rat model of visceral pain. It was found that neither p-chlorophenylalanine(p-CPA)- nor N-chloro-ethyl-2,2--bromo-benzylamine(DSP-4)-induced strong and selective depletion of the brain and spinal cord serotonin and noradrenaline, respectively, changed in a significant way rat visceral pain perception. On the other hand, 8-OH-DPAT, a full selective 5-HT1A receptor agonist, prazosin, an alpha1-adrenoceptor antagonist, clonidine, an alpha2-adrenoceptor agonist, and two beta-adrenoceptor antagonists: propranolol and metoprolol, dose-dependently reduced the number of body writhes induced by intraperitoneally administered 2% solution of acetic acid (the writhing test). The results obtained with selective receptor ligands, DSP-4 and p-CPA, indicate that the noradrenergic and serotonergic innervation of the central nervous system contribute in a complex way to the animal behavior in the writhing test. The 5-HT1A receptors and alpha2-adrenoceptors play an inhibitory role in the expression of rat behavior in this model of visceral pain. On the other hand, adrenergic alpha1 and beta1 receptors facilitate the behavioral effects of the irritant agent.

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.

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.

Supraspinally mediated analgesic effect of antidepressant drugs

Imipramine and amitriptyline, nonselective monoamine reuptake inhibitors, citalopram, selective serotonin reuptake inhibitor, and maprotiline, selective noradrenaline reuptake inhibitor, were tested after intracerebroventricular (icv) and intrathecal (it) administration in the rat writhing test to establish the role of the spinal and/or supraspinal structures in their effects. All drugs evoked dose-dependent analgesia after icv but not after it microinjections. The changes in the nociceptive behavior of rats pre-treated with antidepressants were not due to the non-specific influences of the drugs on the animals' gross behavior, as revealed by the open field test. The obtained results suggest that an antidepressant-induced analgesia mainly depends on the supraspinal effect with minor, if any, contribution from the spinal mechanisms.

The influence of serotonin depletion on rat behavior in the Vogel test and brain 3H-zolpidem binding

The influence of p-chlorophenylalanine (p-CPA) and 5,7-dihydroxytryptamine (5,7-DHT)-induced serotonin depletion on rat behavior as well as on zolpidem's the behavioral effects and binding to some brain areas of zolpidem, was examined with the help of Vogel's punished drinking test and autoradiography, respectively. Moreover, changes in the serotonin levels and turnover rate were studied in the forebrain and brainstem of rats pretreated with various ligands at the benzodiazepine (BDZ) receptors (midazolam, bretazenil, abecarnil, zolpidem). These drugs were given at doses shown previously to significantly disinhibit animal behavior suppressed by punishment in the Vogel test (Nazar et al., 1997). It was found that serotonin decrease in the frontal cortex and hippocampus after p-CPA significantly and inversely correlated with rat behavior controlled by fear in the VT. p-CPA produced an anticonflict activity in the absence of effect on spontaneous drinking, pain threshold and motility of animals. All applied benzodiazepine receptor ligands decreased the 5-HT turnover rate in the frontal cortex and hippocampus, whereas in the brainstem only abecarnil and zolpidem diminished 5-hydroxyindoleacetic acid levels. This part of the study replicated earlier data with neurotoxins and indicated that the anxiolytic-like effect of 5-HT depletion in some models of anxiety did not depend on changes in animal appetitive behavior or stimulus control. Moreover, the fact that all nonselective and selective (zolpidem) agonists of the type 1 benzodiazepine receptors seemed to produce the same anticonflict effect and decreasing 5-HT turnover indicates that this subtype of benzodiazepine receptor may be important for the interaction between brain 5-HT and GABA/BDZ systems. Accordingly, it was found that serotonin decrease enhanced the anticonflict effect of zolpidem in the Vogel test and increased 3H-zolpidem binding to the occipital cortex and substantia nigra. Altogether, the present study provides more arguments for the role of changes in the activity of brain 5-HT innervation in the control of emotional processes. Moreover, it points to the BDZ1 receptor subtype as a possible target of interaction between brain 5-HT and GABA(A)/BDZ systems.

The role of the hippocampus and 5-HT/GABA interaction in the central effects of benzodiazepine receptor ligands

The effects of an intrahippocampal administering of a nonselective full (midazolam), a partial benzodiazepine (BDZ) receptor agonist (bretazenil), and a BDZ1 selective (zolpidem) receptor ligand were examined in the open field test (OFT) of neophobia and Vogel's test (VT) of conflict behavior in rats. Moreover, the influence of local injections of a noncompetitive GABA(A) receptor antagonist, picrotoxin, on the anxiolytic-like effect of serotonin (5-HT) depletion (p-chlorophenylalanine, p-CPA) in the Vogel test was studied. It was found that in the OFT only midazolam (0.1 microg/site) given to the hippocampus (HP) disinhibited rat exploratory behavior, whereas all the examined compounds inhibited animal motor activity when injected locally at 10.0 microg/site, the highest dose used in the tests. In the VT, again, only midazolam disinhibited rat conflict behavior on a dose-dependent basis. Picrotoxin administered to the HP produced a tendency to increase locomotor activity in rats, and significantly attenuated the anti-conflict action of serotonin depletion without changing the pain threshold and spontaneous drinking of the animals. p-CPA induced potent, dose-dependent and selective 5-HT and 5-hydroxyindoleacetic acid decrease in the HP after administering the dose used in the behavioral experiment. Thus, the present data provide evidence for the lack of selective anxiolytic activity of a partial non-selective agonist and a full selective agonist at the BDZ1 receptor after their administration to the HP. The model of intra-HP drug injections appeared effective in discriminating the anxiolytic spectrum of activity of new psychotropic compounds. Moreover, the obtained results indicate that the dorsal HP is one of the central sites important for GABA/5-HT interaction that modulates rat emotional behavior.

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.

Blood pressure and hypothalamic NA-GABA interaction in spontaneously hypertensive rats (SHR): effect of administration DSP-4

The effect of DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] on the content of catecholamines (NA and DA) and gamma-aminobutyric acid (GABA) in the hypothalamus and on the blood pressure were studied in rats. Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were injected with DSP-4 (50 mg/kg i.p. twice) and tested for 15 days thereafter. Fifteen days after DSP-4 lesioning, a significant reduction of NA levels without changes in DA and GABA concentration in the hypothalamus of both strains was found. However the blood pressure appeared unaffected by the DSP-4 pretreatment in WKY and SHR rats. In line with previous data the amounts of catecholamines and GABA in the hypothalamus were significantly lower in SHR control animals than in WKY control rats. The results suggest that NA/GABA interaction in the hypothalamus do not play an important role in blood pressure regulation. It may be further supposed that the local NA transmission does not play an important role in the phenomenon discussed. Moreover, a contribution of hypothalamic GABA and DA to the control of blood pressure is confirmed.

Alcohol intake and brain [3H]muscimol binding sites in alcohol-preferring and non-preferring rats

The present study has employed in vitro autoradiography to determine the distribution and density of [3H]muscimol binding sites in the brains of alcohol high-preferring line of rats (WHP) and alcohol low-preferring line of rats (WLP). While the density of [3H]muscimol binding was found to be similar in the frontal cortex, caudate-putamen, nucleus accumbens, lateral and medial septum, the density of [3H]muscimol binding was lower in cingulate cortex of alcohol low-preferring rats as compared to alcohol high preferring rats. Moreover, the density of muscimol binding sites within this area was positively correlated with the intensity of ethanol consumption.

The effect of serotonin depletion on motor activity habituation, and [3H]muscimol binding in the rat hippocampus

The effect of serotonin depletion (p-chlorophenylalanine pretreatment) on habituation of exploratory motor activity, and on cortical and hippocampal [3H]muscimol binding in vitro, was examined in rats. It appeared that the very strong decrease in serotonin concentration abolished motor habituation in the open field and decreased [3H]muscimol binding to cortical and hippocampal brain slices. The GABA(A) receptor down-regulation was due to a decrease in the apparent affinity of the radioligand for the receptors. p-Chlorophenylalanine-induced biochemical changes were selective and most probably secondary to serotonin depletion, as the serotonin synthesis inhibitor did not displace [3H]muscimol from its binding sites in neural membranes taken from the occipital cortex. It is concluded that there is a functional interaction between brain serotonin and GABA (gamma-aminobutyric acid) systems, both at behavioral and biochemical levels, that is involved in the motor activity habituation process.

Analgesic effect of antidepressant drugs

Imipramine, amitriptyline, citalopram, and maprotiline were examined in different models of a nociceptive reaction after single-dose, and 21-day long administration, in rats. Animals' behavior in the Porsolt and open-field tests was also studied to compare analgesic and antidepressant-like action of drugs and to check the contribution of changes in the rats' gross behavior to animals' reactions to the nociceptive stimuli. The time- and dose-dependent fluctuations in the blood and brain concentrations of imipramine were evaluated in another group of animals. Imipramine, amitriptyline, citalopram, and maprotiline were shown to exert analgesic activity in some tests only. The most unequivocal analgesic effects were observed in the writhing test (2% acetic acid solution I.P.). The antinociceptive action of antidepressants in this test was probably not due to their local anaesthetic activity, because it was also present after intragastric drugs administration. Alterations in the open-field behavior of rats subjected to the treatment with antidepressant drugs did not correlate with animals' behavior in the writhing test. In the Porsolt test, the antidespair effects of antidepressants were not observed after acute drugs administration at the doses effective in the writhing test. Moreover, in contrary to the writhing reaction, the antiimmobility effect was potently enhanced after repeated administration of tricyclic drugs. Additionally, no association was found between the blood and brain concentrations of chronically administered imipramine and its effects in the writhing test. The obtained results indicate: (a) disparate sensitivity to antidepressant treatment of differently evoked behavioral reactions to the nociceptive stimuli; (b) the most potent effects of administered antidepressants in the model of visceral pain; (c) a better correlation of the brain concentration of imipramine with its antiimmobility than analgesic effect: (d) the lack of relationship between the analgesic and antidepressant-like effects of examined antidepressants compounds.

Benzodiazepine-GABAA receptor complex ligands in two models of anxiety

In the present study, the actions of several compounds with different intrinsic activities and BDZ receptor selectivity were examined in two well established animal models of anxiety: the open field test (OFT) and Vogel's punished drinking text (VT). Full agonists at the BDZ GABAA receptor (midazolam and diazepam) showed anxiolytic-like effects in both tests; however, the doses necessary to disinhibit animal behavior controlled by fear were higher in the VT than in the OFT. None of the partial BDZ receptor agonists studied (bretazenil, Ro 19-8022 and abecarnil) diminished neophobia-like behavior of rats in the OFT, and their sedative influence on gross behavior prevailed. On the other hand, all three drugs produced a clear-cut anxiolytic effect in the VT. A selective BDZ, receptor subtype full agonist (zolpidem) had a similar profile of action to that of partial agonists with an even stronger sedative effect in the OFT. Alpidem (a selective BDZ1 receptor partial agonist) did not reveal any anxiolytic action in either test. Flumazenil (an antagonist at the BDZ-GABAA receptors) also produced no effect in the OFT, or the VT. An inverse BDZ receptor agonist, beta-carboline-3-carboxylate methyl ester (beta-CCM), evoked an anxiogenic-like response in the OFT, but not in the VT. In summary, it appeared that partial agonists and selective ligands at BDZ1 receptors revealed less advantageous anxiolytic-like action than did full allosteric GABAA receptor modulators. This study also indicates the test dependent profiles of action of BDZ-GABAA receptor ligands. It also indirectly suggests a different neurobiological background underlying the applied tests.

The behavioral effects of NMDA antagonists in serotonin depleted rats

The influence of serotonin (5-HT) depletion (5,7-dihydroxytryptamine, 5,7-DHT, 250.0 micrograms, ICV), on behavioral effects of non-competitive (MK-801) and competitive (CGP 37849) NMDA antagonists, was examined in rats. 5,7-DHT induced very potent and long lasting decrease in the 5-HT concentration in the brainstem and limbic forebrain. One week after 5,7-DHT administration, dopamine metabolism was found enhanced in the brainstem. The lesion did not change rat baseline motor and exploratory activity, but it significantly disinhibited animals' behavior suppressed by shock, in the Vogel test. Serotonin depletion revealed locomotor stimulating effect of MK-801, administered IP at the doses of 0.05 and 0.2 mg/kg. However, no change in striatal dopamine metabolism was detected in rats injected with the same dose of MK-801 (0.2 mg/kg), and examined one week after serotonergic denervation. Serotonergic lesions antagonized both enhancements of exploratory behavior, and motor suppression produced by the dose of 1.0 and 10.0 mg/kg of CGP 37849, respectively. Thus, 5,7-DHT-induced lesions influenced in a complex way the effects of NMDA antagonists. It is reasoned, that enhancement of motor stimulating effects of MK-801 in neurotoxin pretreated animals, reflects synergistic disinhibition of activity of dopaminergic neurons by MK-801 and serotonin depletion. On the other hand, antagonism of CGP 37849-caused motor depression can be explained by the lowering influence of 5,7-DHT on serotonin content. It is known that the release of serotonin is strongly stimulated by higher doses of CGP 37849, and takes part in the expression of some symptoms of the serotonin-like syndrome, including motor disturbances.

Some behavioral effects of AMPA/kainate receptor agonist and antagonists

The effects of an agonist (S-AMPA, i.c.v.), as well as competitive (CNQX, NBQX, DNQX, i.p.), and noncompetitive antagonists (GYKI 52466, i.p.) at the AMPA/kainate receptors were examined in the open field and the Vogel tests of anxiety. It was found that both kinds of antagonists inhibited rat exploratory behavior in a dose-dependent manner, at the dose range exhibiting a clear-cut tendency to decrease rat locomotor activity. They appeared inactive in the Vogel test over an examined dose-range. S-AMPA, whereas not changing in a significant way rat behavior in the open field, significantly enhanced the suppressive influence of a shock on drinking in the Vogel test. The drug administered at the dose of 2 micrograms/5 microliters, i.c.v., revealed also a tendency to decrease the motor activity followed by prodromal symptoms of epileptic-like activity in some subjects. It is concluded that AMPA/kainate receptors probably are not directly involved in the control of rat emotional behavior. Thus, their primary role as putative neuroprotective and anticonvulsant agents is indirectly confirmed.

Pharmacology of tolerance to benzodiazepine receptor ligands

State of the art of pharmacology of tolerance to and dependence on benzodiazepine receptor ligands is shortly summarized. Firstly, the inadequacies of animal models of drug-dependency are underlined. Then, some elements of molecular biology of the GABA-A receptor complex are described to give a theoretical background for discussing pharmacological profiles of action of the selective type 1 (e.g. zolpidem), and partial benzodiazepine receptor agonists (e.g. bretazenil). These groups of benzodiazepine receptor ligands are considered to exhibit low tolerance and dependence liability. The evidence from behavioral and electrophysiological experiments so far favors the concept that the gradually developing hypofunction of the brain GABA system is the basis for tolerance to benzodiazepines, and the cause of abstinence symptoms. Receptor binding studies on adaptive changes in the GABA-A and benzodiazepine receptors after chronic benzodiazepine administration, insofar gave negative or contradictory results. The available up to now data on the long-term processes occurring in the subunits of the GABA-A receptor complex are also not homogenous, and it is difficult to discuss them. Likewise, the contribution of neurosteroids and specific ligands of the peripheral type of benzodiazepine receptor to the discussed phenomenona, is a matter of controversy. In conclusion, it should be stressed that the selective type 1 benzodiazepine receptor agonists, and partial benzodiazepine receptor agonists, either due to excitation of a smaller fraction of receptors, or lower efficacy, do not entirely readjust the function of the GABA-A receptor complex, thus leaving the place for action of the endogenous neurotransmitter. Such profile of central activity should prevent the development of strong tolerance and abstinence symptoms.

Influence of serotonergic drugs on restraint stress induced analgesia

The effects of drugs selectively effecting central serotonergic systems on immobilization-induced analgesia (SIA) were tested in the rat. The drugs were used in dose ranges previously shown to effect emotional processes. SIA was tested using the tail withdrawal method. It was found that pretreatment of rats with para-chlorophenylalanine (p-CPA), an inhibitor of serotonin synthesis, significantly attenuated SIA, measured immediately after stress session. Ritanserin, a 5-HT2A/2C receptor antagonist, ondansetron, a 5-HT3 receptor antagonist and citalopram, a selective serotonin re-uptake blocker increased the baseline pain threshold, whereas 8-OH-DPAT, a full 5-HT1A receptor agonist and buspirone, a partial 5-HT1A receptor agonist expressing also high affinity towards dopaminergic D2 receptors, were without effect on pain perception and stress induced analgesia. It has been concluded, that modification of SIA by serotonergic drugs probably merely reflects changes in the activity of the 5-HT system on the spinal cord level, with minor, if any, contribution of supraspinal emotional centers.

Isradipine suppresses amphetamine-induced conditioned place preference and locomotor stimulation in the rat

The locomotor activating and the reinforcing effects of psychomotor stimulants are considered to be correlated with and responsible for the development and maintenance of stimulant addiction. Experiments were conducted to examine the effects of isradipine, the L-type calcium channel inhibitor, on the d-amphetamine-induced (1 mg/kg IP) reinforcement (conditioned place preference) and locomotor stimulation. Isradipine dose-dependently (0.6, 1.2, 2.5 mg/kg IP) attenuated the reinforcing effect of amphetamine. Two higher doses completely blocked the induction of place preference. At these doses isradipine also prevented the increase in the number of intercompartment crosses that was observed in both amphetamine- and vehicle-treated controls. In an acute experiment, isradipine failed to affect locomotor activity on its own either in the place preference boxes or in the open field. Amphetamine increased the open field activity but did not change the number of crosses in the place preference boxes. Only the highest (2.5 mg/kg) dose of isradipine significantly suppressed amphetamine-induced hyperactivity in the open field. The present results suggest that isradipine interferes with amphetamine-derived reinforcement at doses lower than those needed to block the acute locomotor effects of amphetamine. Given the qualitatively similar, previously reported results with verapamil, we conclude that the antireinforcing effects of the L-type calcium channel blockers cannot be exclusively explained by the suppression of psychomotor stimulation. The present results further support the notion that the L-type calcium channel blockers may be effective against stimulant addiction.

Antagonism of behavioral effects of electroconvulsive shock but not those of desipramine by the selective 5-HT-3 receptor antagonist, ondansetron

The influence of ondansetron (ON) on desipramine (DMI) and electroconvulsive shock (ECS) effects in the open field (OFT) and forced swim test (FST) were studied in Wistar male rats. ON failed to influence the action of DMI but abolished stimulatory effect of ECS in the OFT and, in the lower dose (0.01 mg/kg) reduced ECS action in the FST. This result suggests that 5-HT-3 receptors are involved in behavioral effects of ECS but not DMI.

Synthesis and anxiolytic activity of N-substituted cyclic imides N-[4-[(4-aryl)-1-piperazinyl]alkyl]-5,7-dioxabicyclo[2.2.2]octane-2, 3-dicarboximide

The preparation of N-substituted cyclic imides N-[4-[(4-aryl)-1-piperazinyl]alkyl]-5,7-dioxabicyclo[2.2.2]octane- 2, 3-dicarboximides by condensation of N-(3-chloropropyl)- or N-(4-chlorobutyl)imides with appropriate amine has been described. One of compounds was tested in the Vogel's test and displayed an expected activity on CNS.

Anxiolytic-like action of intra-hippocampally administered NMDA antagonists in rats

The effects of intra-hippocampally administered NMDA receptor antagonists MK-801, and AP-7 on rat behavior in the Vogel's and open field tests of anxiety were examined. MK-801 and AP-7 given locally stimulated at the same time rat exploratory and motor activities, in the open field. In the Vogel's test, both compounds attenuated the suppressive effect of shock on drinking, in a dose-range not affecting rat motility. It is concluded that hippocampal NMDA innervation may play a role in the control of motor activity. Furthermore, the Vogel's test data indicate more selective involvement of hippocampal NMDA innervation in processing of fear-related signals.

Limbic mechanisms of anxiolytics acting on 5-HT receptors

The role of brain serotonergic system innervating hippocampus and nucleus accumbens in the anxiolytic-like action of the 5-HT1A receptor agonists and 5-HT3 receptor antagonists, is discussed. The data on the effects of intrastructural microinjections of selective serotonergic agonists and antagonists in the Vogel and open field (neophobic reaction) tests are described and critically reviewed. It is concluded that both postsynaptic inhibition of the temporal lobe function (the hippocampus), and attenuation of the cell body firing of the raphe neurons appears to be important elements of anti-anxiety action of benzodiazepines and 5-HT1A receptor antagonists. Thus, it is hypothesized that this dual mechanism of the 5-HT1A receptor agonists and the 5-HT3 receptor antagonists action cooperates synergistically in the processing of emotional functions.

[A new concept of antidepressant drugs action: clinical and preclinical data]

The authors propose a new concept of antidepressant drug action based on adaptive receptor changes occurring in the course of chronic antidepressant treatment. Some clinical data and more equivocally preclinical studies indicate an enhancement of activity of those neurotransmitter systems which exert an activating impact on diverse cns functions (dopaminergic and alpha-1 adrenergic receptors). On the other hand the same treatment is found to inhibit the activity of central inhibitory systems (e.g. serotonergic and GABAergic receptors), thus resulting in an increase of positive reinforcement and psychomotor processes in the cns.

Studies on antidepressant action of a new oxazolidinone derivative AS-8

On the basis of previous laboratory studies AS-8 was suggested to possess antidepressant-like activity. Forced swim test, learned helplessness and conflict Vogel's test were performed after three prior administrations of AS-8 (24, 5 and 1 h before the test). The data have shown that AS-8 produces moderate antidepressant effect but did not induce anxiolytic-like action. Biochemical data revealed increased brain 5-HT and 5-HIAA levels following AS-8 administration. The combined treatment of rats with AS-8 (100 mg/kg) and amitriptyline (5 mg/kg) or desipramine (1.25 mg/kg) significantly stimulated active behavior in the forced swim test above the level obtained with each of the drug given separately. The present data suggest the potential antidepressant efficacy of AS-8 in conjunction with small doses of tricyclic antidepressants.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of midazolam and 5-HT1A receptor agonists

An involvement of serotonergic innervation of the hippocampus (HP) and the nucleus accumbens septi (NAS) in anxiolytic activity of benzodiazepine midazolam and 5-HT1A receptor agonists was studied in two different animal models of anxiety. Injection of midazolam (10.0 and 20.0 micrograms) or 8-OH-DPAT (0.5 and 1.0 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. Buspirone given at 0.1, 0.5 and 1.0 microgram was ineffective in the Vogel test, while at 5.0 micrograms it enhanced shock-induced suppression of drinking. In the open-field test midazolam (0.01 and 0.1 microgram), 8-OH-DPAT (0.1, 0.5 and 1.0 microgram) and buspirone (2.5 and 5.0 micrograms) increased the number of entries into the central part of the open-field and the time spent in the central sector. Depletion of 5-HT had no influence on the anxiolytic-like effect in the open-field test of intrahippocampally-administered 8-OH-DPAT (0.5 microgram), but the drug tended to increase motor activity in lesioned animals. Midazolam and buspirone injected into the NAS did not have an anxiolytic effect in the Vogel test. A small increase in punished drinking was observed after 8-OH-DPAT (1.0 and 2.5 micrograms). Following intra-NAS injection, midazolam, 8-OH-DPAT and buspirone all failed to produce any marked anxiolytic-like effect in the open-field test. It appears that the hippocampus, rather than the NAS, is involved in mediating anxiolytic-like effects of 5-HT1A receptor agonists. Hippocampal postsynaptic 5-HT1A receptors may account for the anti-emotional influence of this group of drugs. The results indicate some similarities in the psychotropic profile of 5-HT1A receptor agonists and midazolam.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of the 5-HT3 receptor antagonists

The roles of hippocampus (HP) and the nucleus accumbens septi (NAS) in the anxiolytic activity of two 5-HT3 receptor antagonists were studied in two animal models of anxiety, in rats. Injection of tropisetron (0.005 and 0.01 microgram) or ondansetron (1.0 and 2.5 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. In the open field test neither 5-HT3 receptor antagonists had anxiolytic-like effects. Tropisetron (0.01 and 0.025 microgram) injected into the NAS caused a marked increase in punished drinking, while ondansetron (0.01-15.0 micrograms) had no effect. In the open field test, tropisetron (0.001, 0.005 and 0.01 microgram) and ondansetron (1.0 and 2.5 micrograms) given to the NAS increased the number of entries into the central part of the open-field, and the time spent in the central sector of the arena. Depletion of 5-HT significantly enhanced the anxiolytic-like effect of intra-NAS-injected tropisetron in the open field, at the dose of 0.005 microgram. Moreover, 5,7-DHT lesions produced a tendency to increase motor activity in tropisetron-treated rats. Both hippocampal and accumbens 5-HT3 receptors seem to contribute to the anxiolytic-like effects of 5-HT3 antagonists in the Vogel test. It also appears that this effect of 5-HT3 receptor antagonists is related to their action on postsynaptic 5-HT3 receptors within the NAS, and depends on the functional state of the 5-HT innervation ascending from the raphe nuclei. Thus, the present data add more arguments for the more specific involvement of this limbic nucleus in emotional control.

Open field behavior of rats reared in different social conditions: the effects of stress and imipramine

Behavioral changes in rats divided on the weaning day into two groups (crowded and isolated) were studied in the computerized open field. At the end of the experiment the effects of an acute stress (1 h long immobilization) on open field behavior, body weight and body temperature, were examined. In a separate experiment both groups of rats were chronically pretreated with tricyclic antidepressant imipramine (5.0 mg/kg, po), and subsequently exposed to the same stress procedure. Social deprivation significantly enhanced spontaneous locomotion and exploratory activity. Restraint stress significantly decreased exploration, body weight and temperature, in the isolated animals only. Chronic pretreatment of rats with imipramine significantly attenuated the effect of stress on motor and exploratory behavior, as well as stress-induced changes in body weight and temperature, in the group of isolated animals. The present data indicate sensitization to stress of behavioral and vegetative processes in socially deprived animals. The relationship between rearing conditions and behavioral and vegetative reactivity to the environmental challenges is confirmed.

The comparison of benzodiazepine derivatives and serotonergic agonists and antagonists in two animal models of anxiety

The present paper compares the effects of different serotonergic agonists and antagonists with benzodiazepine derivatives in two animal models of anxiety; the Vogel's and the open-field tests. In the Vogel's conflict test, both diazepam and midazolam produced an anti-punishment action. The drugs 8-OH-DPAT (0.025 and 0.05 mg/kg), buspirone (0.62 mg/kg), gepirone and ipsapirone (0.3 and 0.62 mg/kg, respectively) increased punished intake of water. Ritanserin disinhibited the behaviour of rats at the doses of 2.5 and 5.0 mg/kg and ICS 205-930 (0.001 and 0.01 mg/kg) exerted a marked increase in punished drinking, while ondansetron was active only after the largest dose (1.5 mg/kg). In the open-field test, all drugs increased the number of entries into the central area, as well as the time spent in the central sector of the open-field. The present data indicate similar but not identical spectra of pharmacological sensitivity of both ethologically-oriented and conflict tests, for various classes of anxiolytic drugs. The 5-HT1A receptor agonists and 5-HT2 receptor antagonist have been shown to have similar anxiolytic-like profile to the benzodiazepines but in a narrower dose-range. The 5-HT3 receptor antagonists appeared to be unique in respect to their very strong anti-emotional activity (ICS 205-930), devoid of any clear-cut general inhibitory properties upon locomotion.

Accumbens GABA-ergic innervation contributes to the stressor-induced locomotor depression in rats

The effect of intra-accumbens injections of drugs changing the function of GABA-A and GABA-B receptor systems on stressor-induced motor depression, was studied in rats. Local injections of picrotoxin and baclofen, but not of midazolam and muscimol, attenuated the inhibitory effect of inescapable footshock on locomotor activity in the open field test, examined 24 h after a single exposure of rats to the stressful event. The results obtained with picrotoxin may be related to the general disinhibitory properties of the convulsant on brain neuronal activity, in a period of time important for consolidation of central processes evoked by inescapable shock. The lack of effects of muscimol and midazolam, further underlines the minor and/or indirect role of accumbens GABA-A receptor-related innervation in the neural processes generated by stressful event. On the other hand, the results obtained with baclofen confirm the reports indicating an inverse relationship between the number of GABA-B receptors in the frontal cortex and the development of helpless behavior in rats. It is also noteworthy that most antidepressant drugs which have been shown to prevent or reverse behavioral deficits after inescapable shock, upregulate GABA-B receptors in the frontal cortex. Hence, it appears that GABA-B receptor-related systems within the nucleus accumbens, may contribute to the footshock-induced behavioral depression, including locomotor inhibition. The reduction of stress effect by baclofen does not seem to reflect changes in fear and anxiety, since the drug was given after the stress session, and the anxiolytic midazolam appeared to be ineffective in this test.

MIF-1 potentiates the action of tricyclic antidepressants in an animal model of depression

In the present paper, the effect of simultaneous treatment of rats with low doses of MIF-1 and tricyclic antidepressants on rat behavior in the forced swim test was studied. It was found that MIF-1 stimulated in a dose-dependent manner "active" behavior of animals in this paradigm. The effect of MIF-1 appeared to be independent of changes in rats' locomotion in the open field test. The combined treatment of rats with MIF-1 (0.01 mg/kg IP) and amitriptyline (5 mg/kg IP) or desipramine (1.25 mg/kg) IP) significantly stimulated active behavior in the forced swim test above the level obtained with each of the drugs given separately. The present data suggest the potential clinical efficacy of a combined therapy of depressive patients with MIF-1 and small doses of tricyclic antidepressants.

Serotonergic mechanisms in the nucleus accumbens affected by chronic desipramine treatment

The effects of repeated treatment of rats with desipramine on 5-HT mechanisms within the nucleus accumbens (NAS) have been studied in a functional model. Local microinjections of 5-HT, quipazine as well as 5-HT1A receptor agonist buspirone, 8-OH-DPAT and NDO-008, inhibited rat locomotor activity in the open-field test. The effect of 5-HT and buspirone was blocked by serotonergic receptor antagonists methysergide and cyanopindolol, respectively. Chronic, but not acute treatment of rats with desipramine (10 mg/kg, PO, twice a day for 21 days, tests were performed 24 h after the last dose) significantly attenuated behavioral depression after 5-HT and quipazine microinjections, while the effect of buspirone was left unchanged. On the basis of present data, it may be concluded that whereas both accumbens 5-HT1A and 5-HT2 receptors appear to be important to regulation of animals' motility, only 5-HT2 receptors seem to be the most likely targets of antidepressive treatment. These data, along with previously reported changes in limbic noradrenergic and dopaminergic activity after antidepressive treatment, may explain the energizing influence of drugs and electroconvulsive shocks on psychomotor retardation, a part of endogenous depression.

Effects of intra-accumbens administration of dopamine agonists on stress-induced behavioural deficit

The effect of post-footshock injections of (+)-amphetamine, the selective D2-receptor agonist quinpirole (LY 171555), and the D2-receptor antagonist metoclopramide, into the nucleus accumbens, on the formation of the open field deficit, has been studied in rats. Microinjections of (+)-amphetamine (10 micrograms) stimulated rat locomotor activity tested 5 min later, while quinpirole (10 micrograms) significantly inhibited animal motility in the test. The open field behaviour was not changed 24 h after injection of either drug. Amphetamine applied immediately after inescapable footshock did not modify stress-induced locomotor depression, when the rats' behaviour was examined 24 h later. On the other hand, post-shock injections of quinpirole significantly attenuated the long-term effects of the stressor, in the open field. Metoclopramide (10 micrograms) inhibited rat locomotor activity 5 min, but not 24 h, after local injection. Administration of a solution containing both quinpirole (10 micrograms) and metoclopramide (1 microgram) decreased motor activity of unstressed rats to a smaller degree than did quinpirole (10 micrograms) alone. Post-footshock injection of metoclopramide did not affect stress-induced hypomotility. It is concluded that the present data support the hypothesis that local depletion of brain dopaminergic stores causes some behavioural effects of stressors.

Intra-hippocampal buspirone in animal models of anxiety

The effect of intra-hippocampal injections of the serotonergic 5-HT1A receptor agonist, buspirone, on rat exploratory activity was evaluated in the 'open field' and 'elevated plus maze' tests. The dose of 2.5 micrograms, but not of 1 microgram, of buspirone administered to the dentate gyrus of the hippocampus increased the time spent on exploration of open arms in the elevated plus maze, as well as it increased the number of central entries in the open field. The results indicate an anti-emotional influence of local stimulation of 5-HT1A receptors by buspirone.

Restraint stress-induced changes in saccharin preference: the effect of antidepressive treatment and diazepam

The effect of antidepressive treatment and of diazepam on saccharin preference has been studied in a model of acute restraint stress-induced saccharin preference deficit. It has been shown that 1-hr stressor produces short-term, and significant decrease of saccharin preference in a two-bottle test, measured at 24-hr periods of time. Single doses of desipramine and citalopram (10 mg/kg, IP) given prior to stress session significantly attenuated the deficit in saccharin preference. Less strong, but similar effects appeared after postshock antidepressant administration. On the other hand, electroconvulsive shock treatment rather enhanced the depressive influence of the stressor, while diazepam (1 mg/kg, IP) antagonized the suppression of saccharin preference, especially when the drug was given immediately after restraint stress. It is concluded that the acute immobilization-induced decrease in saccharin preference most probably reflects changes in brain emotional processes. The role of disturbances in central motivational mechanisms and its contribution to the effects of antidepressive drugs remains to be established.