Adam Płaźnik

Hippocampal mGluR1 and consolidation of contextual fear conditioning

The effects of post-training intra-hippocampal injections of group I mGluR agonists and antagonists, were examined in the contextual fear test, in rats. It was found that (S)-3,5-dihydroxyphenylglycine (DHPG) (a mGluR1-5 agonist) decreased, and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (a mGluR1 antagonist) increased fear conditioning (a freezing reaction), examined 24h after conditioning session. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) (a mGluR5 agonist), and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (a mGluR5 antagonist) did not cause any effect. In the immunocytochemical study, the post-conditioning administration of AIDA decreased the c-Fos induction in the dentate gyrus and CA1 layer of the hippocampus proper, 2h after exposure of animals to the aversive context, and 24h after conditioning session. It is suggested that overactivation of glutamatergic transmission in the critical for memory trace formation structure and period of time, may result in an attenuation of memory consolidation. On the other hand, reduction of an exaggerated glutamatergic tone can facilitate learning and memory processes. The immunocytochemical study and factor analysis of experimental data revealed that hippocampal mGlu1 receptors significantly influence the memory consolidation in a way dependant on the level of glutamatergic activity. Furthermore, they indicate that changes of glutamatergic activity within brain limbic structures can affect the threshold for the induction of the long-term neuronal plasticity, involved in some forms of learning and memory.

Aggressive behavior inhibition by serotonin and quipazine injected into the amygdala in the rat.

The effects on aggressive behavior, open-field activity, and pain threshold of bilateral microinjections of serotonin (20 micrograms) and quipazine (20 micrograms), the direct serotonergic receptor agonist, into the cortico-medial amygdala were investigated in Wistar rats. Both drugs significantly prolonged the attack latency in isolated killer rats (predatory aggression model), and suppressed the incidence of aggressive postures/attacks in shock-induced fighting test (affective aggression). The only difference in the open-field behavior was the lower number of central square entries in drug-treated compared to saline-injected rats. None of the substances produced any significant change in jump threshold. It is concluded that stimulation of serotonin receptors within the amygdala produces inhibition of affective and muricidal behavior in isolated rats. The effect does not seem to be dependent on changes in general activity and pain sensitivity.

The effects of acute and chronic administration of corticosterone on rat behavior in two models of fear responses, plasma corticosterone concentration, and c-Fos expression in the brain structures

The aim of this paper was to examine changes in rat emotional behavior, and to find the brain structures, which are involved in the mediation of behavioral effects, related to the repeated administration of glucocorticoids. The effects of acute and chronic pretreatment of rats with two doses of corticosterone (5 and 20 mg/kg) were analyzed in two models of fear responses: neophobia-like behavior in the open field test, and freezing reaction in the conditioned fear test. Behavioral effects of repeated glucocorticoid administration were compared to changes in blood total corticosterone concentration, and expression of immediate early gene (c-Fos) in brain structures. It was found that acute administration of corticosterone (90 min before tests) enhanced rat exploratory behavior, and decreased freezing reaction. On the other hand, repeated administration of corticosterone (for 25 days, the final injection 90 min before contextual fear conditioning training) decreased plasma corticosterone concentration, inhibited exploratory behavior, enhanced freezing responses on retest and produced a complex pattern of changes in c-Fos expression, stimulated by exposure of rats to the aversively conditioned context. Aversive context induced c-Fos in the magnocellular neurons of the hypothalamic paraventricular nucleus (mPVN), dentate gyrus (DG), cingulate cortex area 1 (Cg1), and primary motor cortex (M1). In rats chronically treated with corticosterone this effect was attenuated in the mPVN and DG, enhanced in the M1, and additionally observed in the CA1, CA2 layers of the hippocampus, and in the central nucleus of amygdala (CeA), in comparison to control animals not subjected to contextual fear test. In sum, the present data suggest that chronic corticosterone treatment enhances the activity of primary motor cortex and CeA with subsequent improvement of memory of aversive events, and simultaneously stimulates a negative feedback mechanism operating in PVN with ensuing decrease in blood corticosterone concentration.

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.

A stimulatory effect of intraaccumbens injections of noradrenaline on the behavior of rats in the forced swim test.

Intraaccumbens injections of catecholamines noradrenaline and dopamine, though not of serotonin, stimulated locomotion by rats in an open field, 10–15 min later. Similar effects were observed 5 min after microinjection of apomorphine whereas clonidine only attenuated locomotor activity. On the other hand, intraaccumbens administration of phenylephrine, isoproterenol and quipazine, in doses similar to an effective dose of noradrenaline, did not alter rat open field behavior. The escape-directed activity of rats in the forced swim test (FST) was stimulated 5 min after local administration of noradrenaline, phenylephrine, isoproterenol or apomorphine only. No effects in the FST were observed 15 min after noradrenaline injection or after intracaudate noradrenaline administration. The stimulatory effects of intraaccumbens noradrenaline injection in the FST were antagonized by the local pretreatment of rats with phentolamine, though not with propranolol. Accordingly, it is possible to conclude that both catecholamines, but not serotonin, play complex and probably distinct roles within the nucleus accumbens in the stimulation of activity by rats in the FST and the open field test.

Changes in the concentration of amino acids in the hippocampus of pentylenetetrazole-kindled rats

It is not well recognized how disturbances in the local metabolism of some amino acids, especially glutamate and GABA, may lead to seizures. In the presented study, we have examined changes in the hippocampal steady state concentrations of amino acids involved in pentylenetetrazole-kindled and freely moving rats. It was found that in the kindled animals, the concentration of alanine, arginine, glutamate, aspartate and taurine was increased in the interictal period of seizures compared to the control group, whereas kindling reduced the extracellular levels of GABA. No differences between kindled and not-kindled animals in the glycine, histidine and glutamine levels were present. There also appeared an over fourfold increase of the Glu/GABA ratio, a theoretical marker of the neuronal excitation level, in the kindled animals. A multivariate classification tree analysis showed that the hippocampal concentration of taurine, together with GABA and Glu, had the relatively largest prediction accuracy in discriminating between kindled and non-kindled animals, suggesting a specific role of these amino acids in the shaping of a new equilibrium between excitatory and inhibitory processes in the hippocampus of kindled animals.

Expression of c-Fos and CRF in the brains of rats differing in the strength of a fear response.

The aim of the study was to examine the neurochemical background of differences in the individual responses to conditioned aversive stimuli, using the strength of a rat conditioned freezing response (the contextual fear test), as a discriminating variable. It was shown that low responders (LR), i.e. rats with duration of a freezing response one standard error, or more, below the mean value, had a higher activity of the M2 cortical area, and the median raphe nucleus (c-Fox expression), in comparison to the high responders (HR), i.e. rats with the duration of a freezing response one standard error, or more, above the mean value. These animals had also stronger 5-HT- and CRF-related immunostaining in the M2 area, and increased concentration of GABA in the basolateral nucleus of amygdala (in vivo microdialysis). The LR group vocalized more during test session in the aversive band, and had higher serum levels of corticosterone, examined 10 min after test session. It was shown that different natural patterns of responding to conditioned aversive stimuli are associated with different involvement of brain structures and with dissimilar neurochemical mechanisms.

Behavioral studies in spontaneously hypertensive rats

In the present study we examined some behavioral patterns in spontaneously hypertensive (SHR) male rats as compared with Wistar (W) rats. The following methods were employed: open field test, two-way active avoidance, passive avoidance, shock-induced fighting, shock-induced suppression of drinking (conflict test), and dominant-subordinate behavior in rats competing for water. Spontaneously hypertensive rats showed higher level of locomotor activity, lower emotionality, lower anxiety level, increased acquisition of avoidance tasks, and enhanced dominance behavior. In addition, hypertensive rats were less aggressive in shock-induced fighting test. It is concluded, that spontaneously hypertensive rats seem to be hyperactive in terms of locomotor activity while their emotionality appears to be paradoxically reduced.

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.

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.