Dopamine D1-like receptors agonist SKF 38393 increases cFOS expression in the paraventricular nucleus of the hypothalamus--impact of acute and chronic cocaine.

The present study indicates that activation of dopamine D1-like receptors by administration of SKF 38393 leads to dose-dependent (doses: 5, 10 and 20 mg/kg) increases in the expression of cFos proteins in the rat paraventricular nucleus of the hypothalamus (PVN). This effect was abolished by administration of SCH 23390, a dopamine D1-like receptor antagonist (0.5 and 1 mg/kg, given 30 min before SKF 38393--10 mg/kg), suggesting that the apparent effect is specific for activation of dopamine D1-like receptors. Expression of cFos after SKF 38393 (10 mg/kg) was observed in some, but not all, CRF-immunoreactive neurons, as well as in small portion of oxytocin- but not vasopressin-immunoreactive neurons (double-immunofluorescence experiments). There were also certain populations of nuclei that showed expression of cFos but did not co-localize with the above markers. We also found that both acute and repeated (once daily for 5 consecutive days) exposure to cocaine (25 mg/kg) attenuated the induction of cFos expression triggered by SKF 38393 when administered 24 hours after single or the last dose of cocaine (25 mg/kg). Attenuation was observed at the same level after single and chronic exposure to cocaine, indicating a rapid functional down-regulation of dopamine D1-like receptors that are resistant to subsequent doses of cocaine. These data provide evidence for the functional role of dopamine D1-like receptors in the PVN and indicate a functional adaptation of dopamine D1-like receptors following a single dose of cocaine without further progression of adaptation or resistance of D1-like receptor-mediated genomic function in the course of repeated cocaine intake.

DOI, an agonist of 5-HT2A/2C serotonin receptor, alters the expression of cyclooxygenase-2 in the rat parietal cortex.

The hallucinogenic effect of DOI, serotonin 5-HT2A/2C receptor agonist, is known to be associated with the activation of cortical 5-HT2 receptors. However, the effect of DOI on excitability of cortical neurons and their subsequent function is still not quite understood. Previous immunohistochemical studies using Fos proteins expression as a marker of neuronal activity showed the involvement of arachidonic acid cascade, particularly cyclooxygenase metabolic pathway, in DOI-induced Fos proteins expression in the rat parietal cortex. DOI increases arachidonic acid release which is transformed itself via acceleration of cyclooxygenase metabolic pathway to biologically active metabolites, such as prostaglandins and thromboxanes. Since cyclooxygenase-2 (COX-2) expression correlates with neuronal activity, it was of interest to investigate whether DOI is capable of influencing the level of COX-2 protein and mRNA expression in the rat parietal cortex. It was observed that neurons which were positive for 5-HT2A receptors showed constitutive COX-2 immunoreactivity. It was found further, that COX-2 protein level was increased at 1 h, and returned to the control level at 3 and 6 h after DOI (5 mg/kg) administration. In contrast, DOI decreased the COX-2 mRNA expression at all tested time points (1 h, 3h and 6h after DOI treatment). The obtained results further support the suggestion that COX-2 activation and possibly arachidonic acid metabolites generated by COX-2 may be considered as important mediators of functional responses generated by activation of cortical 5-HT2A/2C receptors.

Search for the presence of glucocorticoid receptors in dopaminergic neurons of rat ventral tegmental area and substantia nigra.

Using non-fluorescent immunocytochemical double-labelling procedure and specific antibodies visualizing GR (glucocorticoid receptors) and TH (tyrosine hydroxylase) we have been looking for the co-localization of both antigens in neurons of the rat ventral tegmental area and adjacent substantia nigra. This experimental direction has been inspired by the available data showing that alterations in the level of circulating glucocorticosteroids have distinct effects on the intensity of dopaminergic neurotransmission. Thus, it was of interest to find the anatomical background for the above interaction. It has been found that the rat ventral tegmental area and substantia nigra possess a relatively moderate number of cells with active GR, i.e. receptors which are condensed in the nuclei. Further, we found that dopaminergic neurons (TH-positive) of the ventral tegmental area and substantia nigra were not immunopositive for GR. This observation was in the sharp contrast to the results from the locus coeruleus, where the co-localization of GR with TH was a general rule. Above anatomical data indicate that glucocorticoid receptors influence the dopaminergic neurotransmission by an indirect mechanism, which possibly involves intermittent neurotransmitter.

[The involvement of 5-HT1a serotonin receptors in the pathophysiology and pharmacotherapy of schizophrenia]. / Udzial receptorów serotoninowych 5-HT1A w patofizjologii i farmakoterapii schizofrenii.

Many studies of schizophrenic brains indicate the dysfunction of dopamine and glutamate systems in the prefrontal and frontal cortex. It seems that better understanding of mechanisms regulating functions of these neuronal cortical systems could contribute to creation of new drugs acting in the cortex selectively. This might be profitable in cognition of dysfunction and negative symptoms in schizophrenia. This article presents preclinical data concerning the role of 5-HT1A serotonin receptors in the modulation of cortical dopamine system and in psychotomimetic effects of non-competitive NMDA receptor antagonists. Neurochemical studies have shown that 5-HT1A receptor agonists increase dopamine release in the rat prefrontal cortex on the one hand, and they inhibit the augmentation of dopamine release induced by stress or amphetamine, on the other. However, the increase of dopamine release induced by non-competitive NMDA receptor antagonists is blocked by 5-HT1A receptor antagonists. Blockade of 5-HT1A receptors seems to be important also in reduction of most psychotomimetic effects induced by non-competitive NMDA antagonists both involving (locomotor hyperactivity, working memory impair) and not involving (sensorimotor gating deficits) dopamine mechanism. Thus, binding with 5-HT1A receptors can be an important site for the regulation of cortical dopamine system, both in physiological conditions and in disregulation of the system induced by stress, psychostimulants or psychotomimetics. On the other hand, 5-HT1A receptors modulate most of psychotomimetic effects of non-competitive NMDA receptor antagonists. The above results of preclinical investigations indicate that 5-HT1A receptor can be involved in the pathology of schizophrenia, what is partly confirmed by clinical postmortem studies of schizophrenic brains. These studies showed the increase of 5-HT1A receptor density in prefrontal and frontal cortex in schizophrenic brains. It also seems that 5-HT1A receptors might be a good target for the antipsychotic drugs. Although the clinical studies have demonstrated controversial data, maybe further studies using substances with selectivity to 5-HT1A receptors would help to determine more precisely the role of these receptors in pathology and pharmacotherapy of schizophrenia.

Metyrapone, an inhibitor of corticosterone synthesis, blocks the kainic acid-induced expression of HSP 70.

It is shown in the present study that metyrapone (100 mg/kg), an inhibitor of corticosterone synthesis, given twice, 30 min before and 6 h after kainic acid (10 mg/kg) administration, blocks the kainic acid-evoked induction of heat shock proteins 72 kDa (HSP 70). Specifically, it was observed that metyrapone completely prevented kainic acid-induced appearance of HSP 70 in the rat amygdala, habenula, parietal cortex, and significantly decreased the number of HSP 70-positive neurons in the CA1, CA3, and CA4 subregions of hippocampus. The reduction in HSP 70 induction was paralleled by a complete prevention of the kainic acid-induced rise in the circulating corticosterone level by metyrapone; however, in applied doses metyrapone evoked slight enhancement of blood corticosterone. Despite the fact that metyrapone blocked/attenuated the kainic acid-evoked induction of HSP 70, its administration did not affect the behavioral effects of kainic acid, regarded as "limbic status epilepticus." It is concluded that the blockade of corticosterone synthesis might have neuroprotective effects in the pathological states associated with the overstimulation of glutamatergic receptors.

WAY 100135, an antagonist of 5-HT1A serotonin receptors, attenuates psychotomimetic effects of MK-801.

In the present study, we investigated whether the antagonist of 5-HT1A receptors, WAY 100135, was capable of modifying the psychostimulant and psychotomimetic effects of MK-801, a non-competitive antagonist of NMDA receptors. It was found that: 1) WAY 100135 (10 and 20 mg/kg, but not 1.25, 2.5, and 5 mg/kg) transiently, in a dose dependent manner, attenuated the locomotor stimulant effects of MK-801 (0.4 mg/kg). Given alone, WAY 100135 had no effect on the locomotor activity of rats; 2) WAY 100135 (1.25 and 2.5 mg/kg, but not 10 or 20 mg/kg), attenuated or abolished the disruptive effects of MK-801 on the sensorimotor gating measured in a prepulse-induced inhibition of the acoustic startle response paradigm. WAY 100135 in all tested doses had no effect on the sensorimotor gating or amplitude of the acoustic startle response; 3) WAY 100135 (1.25, 2.5 mg/kg, but not 5 mg/kg) attenuated the detrimental effects of MK-801 on working memory and selective attention, measured in a delayed alternation task. Again, given alone, WAY 100135 did not influence the behavior of rats in that experimental paradigm; and 4) MK-801 (0.4 mg/kg) had no effect on the 5-HT1A receptor mRNA level in rat hippocampus, measured 2 and 24 hours after MK-801 administration. These data indicate that 5-HT1A receptors might be involved in the psychotomimetic effects of non-competitive NMDA receptor antagonists. In addition, 5-HT1A serotonin receptor antagonists and partial agonists may have potential antipsychotic properties.

Cortical localization of dopamine D4 receptors in the rat brain--immunocytochemical study.

Using polyclonal antibody against dopamine D4 receptor we investigated cortical distribution of D4 receptors, with the special emphasis on regions of the prefrontal cortex. Prefrontal cortex is regarded as a target for neuroleptic drugs, and engaged in the regulation of the psychotic effects of various substances used in the experimental modeling of schizophrenia. Western blot analysis performed on samples from the rat cingulate, parietal, piriform cortices and also striatum revealed that antibody recognized one main band of approximately 40 kD, which corresponds to the predicted molecular weight of D4 receptor protein. In immunocytochemical studies we found D4 receptor-positive neurons in all regions of prefrontal cortex (cingulate, agranular/insular and orbital cortices) and all cortical regions adjacent to prefrontal cortex, such as frontal, parietal and piriform cortex. Substantial number of D4 receptor-positive neurons has also been observed within the striatum and nucleus accumbens. In general, a clear stratification of the D4 receptor-positive neurons was observed in the cortex with the highest density seen in layers II/III and V/VI. D4 immunopositive material was also found in the dendritic processes, particularly clearly visible in the layer II/III. At the cellular level D4 receptor immunoreactivity was seen predominantly on the periphery of the cell body, but a certain population of neurons with clear cytoplasmatic localization was also identified. In addition to cortical distribution of D4 receptor-positive neurons we tried also to define types of neurons expressing D4 receptor protein. In double-labeling experiments, D4 receptor protein was found in nonphosphorylated neurofilament H-positive, calbindin-D28k-positive, as well as parvalbumin-positive cells. Since, used proteins are markers of certain populations of pyramidal neurons and GABA-ergic interneurons, respectively, our data indicate that D4 receptors are located on cortical pyramidal output neurons and their dendritic processes as well as on interneurons. Above localization indicates that D4 receptors are not only directly influencing excitability of cortical inter- and output neurons but also might be engaged in dendritic spatial and temporal integration, required for the generation of axonal messages. Additionally, our data show that D4 receptors are widely distributed throughout the cortex of rat brain, and that their cortical localization exceeds the localization of dopaminergic terminals.

Distribution of dopamine D1 receptors in the nucleus paraventricularis of the hypothalamus in rats: an immunohistochemical study.

The present study investigated the distribution of dopamine D1 receptor protein in the nucleus paraventricularis of the hypothalamus. It was found that the nucleus paraventricularis of the hypothalamus contains a relatively large number of cells which are positive for presence of dopamine D1 receptor protein. The vast majority of dopamine D1 receptor-positive neurons was found in the magnocellular part, but they were also present in considerable quantity in the parvocellular part of this subregion of the hypothalamus. When measured by the Western blot technique, the quantity of D1 receptor protein found in the paraventricular nucleus of the hypothalamus was at the level found in the prefrontal cortex. It was also found that dopamine D1 receptor protein was present in neurons constitutively displaying phosphorylated CREB protein, i.e. neurons which are, as might be speculated, under the tonic influence of neurotransmitters whose receptors operate via cAMP and pCREB as second or third messengers. The presence of dopamine D1 receptors in the nucleus paraventricularis of the hypothalamus may suggest, at an anatomical level, that these receptors are involved in controlling the release of hormones, as well as their synthesis at the level of transcription, which is regulated by phosphorylation of CREB protein. Finally, the present immunocytochemical findings offer an anatomical substrate for the role of dopamine and its receptors of D1 subtype in the regulation of the activity of paraventricular neurons seen in the functional studies.

[Distribution of dopamine D1 receptors in the paraventricular nucleus of the rat hypothalamus and their colocalization with phosphorylated forms of CREB protein]. / Dystrybucja receptorów dopaminowych D1 w jadrze przykomorowym podwzgórza mózgu szczura oraz ich kolokalizacja z ufosforylowana forma bialka CREB.

The aim of the present immunohistochemical study was to investigate distribution of dopamine D1 receptors in the paraventricular nucleus of hypothalamus and to estimate whether D1 receptors colocalize with pCREB protein, a functional marker of stimulation generated via receptors positively linked to cAMP/PKA system. D1 receptors were found in numerous neurones of the paraventricular nucleus of hypothalamus, especially in its magnocellular part. In double-staining experiment, 74% of all stained neurones showed colocalization of D1 receptors and pCREB protein, 23% of neurones was pCREB-positive only and 3%--D1 receptor-positive only. The presence of D1 receptors in the paraventricular nucleus of hypothalamus indicates that these receptors may be possibly engaged in regulation of the endocrine system (release of oxytocin, vasopressin or CRH). Almost complete colocalization of D1 receptors with pCREB protein suggests that these receptors are likely functional (active). The presence of D1 receptors and pCREB protein in studied structures of rat brain, as well as the specificity of applied antibodies were confirmed by Western Blot method. It was demonstrated that antibodies against D1 receptor and pCREB protein recognized main bands with molecular weight approximately 40 and approximately 46 kDa, respectively, what correlates well with the literature data.

c-Fos proteins, induced by the serotonin receptor agonist DOI, are not expressed in 5-HT2A positive cortical neurons.

In the present study, we tried to find out whether the expression of c-Fos proteins induced by DOI, an agonist of 5-HT2A/2C receptor subtypes is colocalized with 5-HT2A receptor protein in cortical neurons. 5-HT2A receptor protein was found in two major neuronal elements: dendritic processes (seen in layers II/III-V) and less abundantly in cell bodies (layer V). In our experiment, DOI (8 mg/kg) induced a robust appearance of c-Fos proteins mainly in neuronal nuclei of the upper part of layer V/IV, and a moderate amount of sparsely distributed nuclei in deep cortical layers (V and VI). It was found that c-Fos proteins never occurred in cortical neurons, which were immunopositive for the presence of 5-HT2A receptor protein. It is concluded that the induction of c-Fos proteins expression by DOI though initiated by activation of 5-HT2A receptors, requires the involvement of intermediate neurotransmitter(s). Additionally, our study indicates that the appearance of DOI-induced c-Fos proteins cannot be used as a simple and direct marker of localization and site of activation of 5-HT2A receptors.

The distribution of the NMDA R1 subunit in the rat hippocampus--an immunocytohistochemical study.

In the present study we examined immunocytochemically the distribution of the R1 subunit of NMDA receptors in rat hippocampus. The applied antibody directed against the III-IV transmembrane region of the R1 subunit of NMDA receptors revealed a heterogeneous distribution of NMDA R1 protein which was highest in the CA1 pyramidal layer and lowest in the stratum lacunosum moleculare. The high immunoreactivity that corresponded to the presence of NMDA R1 receptor subunits was observed mainly in layers of cell bodies of hippocampal neurons, such as deep pyramidal layers of CA1, CA2 and CA3 regions and a granular cell layer of the dentate gyrus. The obtained data are discussed in terms of correlation between the receptor localization and the vulnerability of hippocampal neurons to overstimulation associated with activation of NMDA receptors.

The corticosterone synthesis inhibitor metyrapone decreases dopamine D1 receptors in the rat brain.

Experiments were performed to examine the effect of metyrapone, an inhibitor of corticosterone synthesis, on the level of dopamine D1 receptors and their transcripts in the caudate-putamen, nucleus accumbens and olfactory tubercle of the rat brain. The binding to dopamine D1 receptors was measured by receptor autoradiography using the specific D1 receptor antagonist [3H]SCH 23390. The level of dopamine D1 receptor messenger RNA was determined by in situ hybridization histochemistry. The results obtained have shown that metyrapone (two injections of 150 and 50 mg/kg, i.p., given 20 and 3 h before killing, respectively) induced a decrease in the D1 receptor-specific binding in the studied areas of the rat brain. In the caudate putamen, the decrease in [3H]SCH 23390 binding was stronger in the medial (31-39%) than in the lateral part (24-27%). Decreases similar to those in the caudate-putamen were observed in the nucleus accumbens (21%) and olfactory tubercle (32%). Furthermore, metyrapone decreased the level of dopamine D1 receptor messenger RNA in the caudate putamen (17-28%), nucleus accumbens (20%) and olfactory tubercle (18%). In conclusion, our study indicates that glucocorticoids might be involved in the regulation of dopamine D1 receptor level in the rat brain. since metyrapone (which inhibits the synthesis of these hormones) decreases the messenger RNA encoding D1 receptor synthesis, as well as the specific binding to this receptor.

Single doses of MK-801, a non-competitive antagonist of NMDA receptors, increase the number of 5-HT1A serotonin receptors in the rat brain.

In the present study, we investigated the impact of MK-801, a non-competitive NMDA receptor antagonist, on the density of serotonergic receptors of the 5-HT1A subtype and on the metabolism of serotonin in various regions of the rat brain containing terminals and cell bodies of serotonergic neurons. The binding of [3H]8-OH-DPAT to 5-HT1A serotonin receptors was increased after MK-801 (0.4 mg/kg) as was shown by autoradiographic studies in the frontal, cingulate and part of enthorinal cortex, subregions of the hippocampus and raphe nuclei. The above receptor changes were observed at 2 h and, in some brain regions, at 24 h after MK-801. In saturation binding studies, an increase in the Bmax value in the rat hippocampus was found after MK-801 (0.4 mg/kg) while no changes being noted in the Kd value. MK-801 (0.4 mg/kg) increased the concentration of the serotonin metabolite 5-HIAA in the prefrontal cortex and hippocampus, respectively, at 2 and 3 or 3 h after administration, being without effect on the level of serotonin. In the dorsal raphe nucleus, MK-801 (0.4 mg/kg) decreased the level of serotonin without affecting the level 5-HIAA (0.5 h after administration) or increased the level of 5-HIAA without altering the concentration of serotonin (3 h after administration). It is concluded that single administration of MK-801 may alter the density of serotonergic 5-HT1A receptors and in consequence influence the function of the central nervous system associated with activation of 5-HT1A receptors.

Presence and function of dopamine D1 receptors in the rat ventral tegmental area.

In the recent years there is a growing number of reports suggesting that dopamine D1 receptors are involved in the phenomenon of sensitization. In this article we are reviewing the data suggesting the crucial role of dopamine D1 receptors, localized in the rat ventral tegmental area in this phenomenon.

Impact of metyrapone on MK-801-induced alterations in the rat dopamine D1 receptors.

Our earlier studies have shown that changes in serum corticosterone levels played an important role in the acquisition of sensitization to MK-801, a non-competitive NMDA receptor antagonist. Dopaminergic mechanisms are found to be particularly important in the development of sensitization; hence in the present study we assessed the binding of [3H]SCH 23390 at brain dopaminergic D1 receptors, after administration of MK-801 (0.4 mg/kg), in rats in which corticosterone synthesis was inhibited by metyrapone (150 + 50 mg/kg). Such metyrapone pretreatment prevented the increases in serum corticosterone level induced by MK-801. The binding studies, using receptor autoradiography, were performed in the following brain structures: the striatum, nucleus accumbens, olfactory tubercle and substantia nigra. Metyrapone per se did not change or slightly increased D1 receptor binding in the substantia nigra, while in other brain structures tested it decreased the number of these receptors by about 30%. MK-801 increased the level of D1 receptors in the nucleus accumbens core and olfactory tubercle, being without effect in the remaining brain structures tested. In rats which were pretreated with metyrapone, the effect of MK-801 on D1 receptors was inhibited in the nucleus accumbens core only. In substantia nigra, metyrapone provoked the MK-801-induced decrease in D1 receptors whereas in all other structures MK-801 reversed the effects of metyrapone on D1 receptors. Additionally, the effect of metyrapone and MK-801 on grooming behavior induced by D1 receptor agonist SKF 38393 (10 mg/kg) was tested. Metyrapone did not influence grooming induced by SKF 38393, but significantly potentiated the inhibitory effect of MK-801 on this behavior. Finally, we found that metyrapone did not influence the expression of the sensitization induced by MK-801. Our results seem not to support hypothesis that MK-801 evokes enhancement of dopaminergic neurotransmission (at the level of D1 receptors) via corticosterone liberation, since in most brain regions studied inhibition of increases in corticosterone level did not prevent MK-801-induced effects on D1 receptors. The present study may suggest that NMDA receptors are involved in the corticosterone-dependent regulation of the density of the D1 receptors.

Modulation of the forskolin-induced cyclic AMP accumulation by corticosterone.

Experiments were undertaken to examine the influence of corticosterone on forskolin-stimulated cyclic AMP accumulation in rat cerebral cortical slices. Incubation in vitro of cerebral cortical slices with increasing concentrations of corticosterone (1 nM-100 microM) did not influence basal cyclic AMP response. Despite the lack of effect when used alone, corticosterone attenuated the effect of forskolin (10 microM) on cyclic AMP accumulation with IC50 = 24.1 +/- 5.1 microM. Corticosterone (10 microM) added to the incubation medium, for 10 min, reduced the cyclic AMP accumulation in response to increasing concentrations of forskolin (1 microM-100 microM), the concentration-response curve was shifted to the right by about one order of magnitude. In order to compare the in vitro effect of corticosterone with its effects in vivo in the next experiment the forskolin-induced cyclic AMP accumulation was measured in cerebral cortical slices from rats which were treated with corticosterone or vehicle. Similarly as in in vitro model, single dose of corticosterone (10 mg/kg sc) given to rats 2 h before sacrifice inhibited forskolin-stimulated cyclic AMP accumulation when compared with vehicle-treated control animals. In contrast, prolonged administration of corticosterone (10 mg/kg sc, twice daily for 4 and 7 days) increased forskolin-stimulated cyclic AMP accumulation in rat cerebral cortical slices when measured 2, 24 and 48 h after the administration of the last dose. These findings suggest that glucocorticoids exert multiple actions on the adenylate cyclase-coupled cyclic AMP generating system in the brain, with the ultimate effect being dependent upon amount and duration of exposure to these hormones.

Competitive and non-competitive NMDA receptor antagonists induce c-Fos expression in the rat anterior, cingulate cortex.

In the present study we tried to find out whether the competitive NMDA receptor antagonist CGP 40116 was capable of inducing c-Fos expression in the rat cingulate cortex in a manner similar to that described previously for the non-competitive NMDA receptor antagonist MK-801. Induction of fast early genes by MK-801, especially in the rat cortex, has recently been linked with the neurotoxic effects of non-competitive NMDA receptor antagonists on cortical neurones, hence it was of interest to extend those studies to another class of NMDA receptors antagonists i.e., competitive one. It was found that CGP 40116 (2.5 and 5 mg/kg) induced c-Fos expression in the rat anterior cingulate cortex. That effect was dose-dependent and was shown as an increase in the number of cells expressing the c-Fos immunoreactivity. A qualitatively similar, but quantitatively stronger, effect was observed after administration of MK-801 (0.2 and 0.4 mg/kg), which also caused a dose-dependent increase in the number of c-Fos positive neurones. The described dose-dependent effects of CGP 40116 and MK-801 are shown as an increase in the number of c-Fos-positive neurones, but not as an increase in the optical density of c-Fos immunostaining in c-Fos positive neurones. In control, vehicle-injected rats, the constitutive c-Fos immunoreactivity was not found in the rat anterior cingulate cortex. The obtained data indicate that both competitive and non-competitive NMDA receptor antagonists may induce similar effects on the c-Fos immunoreactivity in the rat anterior cingulate cortex, and that their administration may lead to similar functional consequences resulting form activation of fast, early genes.

Imipramine increases the 5-HT1A receptor-mediated inhibition of hippocampal neurons without changing the 5-HT1A receptor binding.

The effect of repeated treatment with imipramine on the 5-HT1A receptor-mediated inhibition of a population spike was studied in the rat CA1 hippocampal region ex vivo. Serotonin (5-hydroxytryptamine, 5-HT) and the selective 5-HT1A receptor agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) decreased dose-dependently the amplitude of population spikes; this effect was blocked by the selective 5-HT1A receptor antagonist (S)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenylpro panamide dihydrochloride [(S)-WAY 100135]. Repeated (14 days, twice daily), but not single, administration of imipramine (10 mg/kg) shifted the dose-response curves for serotonin and 8-OH-DPAT to the left. Repeated treatment with imipramine did not change the density of 5-HT1A receptors in the hippocampus as measured by autoradiography using [3H]8-OH-DPAT as a ligand. The latter findings indicate that the imipramine-induced increase in the responsiveness of hippocampal neurons to stimulation of 5-HT1A receptors may not involve an increase in the density of this receptor subtype. To find out whether the efficacy of the postreceptor transduction mechanism is changed by repeated treatment with imipramine, we examined the effect of baclofen. The baclofen-induced inhibition of the population spike was not changed by imipramine. Our results suggest that repeated treatment with imipramine induces sensitization to the inhibitory effects of 5-HT1A receptor agonists in the hippocampus.

The enhancement and the inhibition of noradrenaline-induced cyclic AMP accumulation in rat brain by stimulation of metabotropic glutamate receptors.

1. The actions of several metabotropic glutamate receptor and antagonists on noradrenaline (NA)-stimulated [3H]-cyclic AMP accumulation were investigated in rat cerebral cortical slices. 2. Quisqualate (QUIS), L-2-amino-3-phosphonopropionic acid (L-AP3) and glutamate (GLU) elicited concentration-dependent inhibition of (NA)-stimulated [3H]-cyclic AMP accumulation, with IC50 values of 105 +/- 29, 275 +/- 36 and 944 +/- 150 microM respectively. In contrast (Rs)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) (0.5 mM) and N-methyl-D-aspartic acid (NMDA) (0.5 mM) had no effect. 3. (2S,3S,4S)-alpha-(Carboxycyclopropyl)glycine (L-CCGI), 1-Aminocyclo-pentane-1S,3R-dicarbo-xylate (1S,3R-ACPD), ibotenate (IBO) and (RS)-4-carboxy-3-hydroxy-phenylglycine (CHPG)elicited a concentration-dependent enhancement of NA-stimulated [3H]-cyclic AMP accumulation, with EC50 values of 2.5 +/- 0.11, 42 +/- 1.3, 97.8 +/- 2.1 and 157 +/- 13.4 microM, respectively. 4. (S)-3-carboxy-4-hydroxyphenylglycine (3C4HPG) and (S)-4-carboxy-3-hydroxyphenyl-glycine (4C3HPG) produced a biphasic effect, at concentrations up to 100 and 500 microM, respectively, they significantly enhanced the action of NA (100 microM), at 1mM concentration both compounds as well as alpha-methyl-4-carboxyphenylglycine (MCPG) produced a significant inhibition of NA-stimulated cyclic AMP accumulation. 5. A putative mGluR antagonist-L-AP3, inhibited the 1S,3R-ACPD (100 microM) induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation in a biphasic manner with an IC50 of 4.5 microM for the high affinity site, which represented 65% of the total and an IC50 of 283 microM for the low affinity site. 6. beta-adrenoceptor antagonist propranolol inhibited the interaction between 1S,3R-ACPD (100 microM) and NA (100 microM) on [3H]-cyclic AMP accumulation by about 80%, with an IC50 of 0.52 +/- 0.011 microM, to the level observed after 1S,3R-ACPD alone. Prazosin, an alpha 1-adrenoceptor antagonist was more potent (IC50 of 0.091 +/- 0.012 microM) but less efficacious (60% inhibition) as an inhibitor of the interaction either between NA and 1S,3R-ACPD while yohimbine, na alpha 2-adrenoceptor antagonist (up to 1 microM) had no effect. 7. Neither the protein kinase C inhibitor - staurosporine (10 microM) nor thapsigargin (1 microM), which depletes IP3 sensitive calcium stores, inhibited significantly the 1S,3R-ACPD (100 microM)-induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation. 8. Adenosine deaminase (0.5 U/ml) abolished both the 1S,3R-ACPD (100 microM)-induced [3H]-cyclic AMP accumulation and the synergistic interaction of this compound with NA (100 microM). 9. These results indicate the existence of different subtypes of metabotropic glutamate receptors in rat brain which either inhibit or enhance the NA-stimulated [3H]-cyclic AMP accumulation. The enhancement in cerebral cortical slices is mediated via receptors which are blocked with high affinity by L-AP3 and occurs via interactions with endogenous adenosine; the inhibition is mediated by receptors sensitive to quisqualate, L-AP3 and glutamate and may represent a predominant interaction between NA and excitatory amino acids (EAA), which in cerebral cortical slices is masked by excitatory effects.

The impact of a competitive and a non-competitive NMDA receptor antagonist on dopaminergic neurotransmission in the rat ventral tegmental area and substantia nigra.

The study compares effects of the competitive and non-competitive NMDA receptor antagonists, CGP 40116 and MK-801 respectively, on the metabolism of dopamine and on the density of D-1 and D-2 dopaminergic receptors in the rat ventral tegmental area and substantia nigra. The effects of CGP 40116 were tested in a range of doses which either were devoid of or had locomotor- or stereotypy-stimulating effects. It was found that (1) CGP 40116 given in a dose of 5 mg/kg enhanced the locomotor activity of rats and evoked a stereotypy-like activity; doses of 1.25 and 2.5 mg/kg were devoid of such effects; (2) CGP 40116 (5 mg/kg) enhanced the concentrations of dopamine, DOPAC and HVA in the ventral tegmental area, whereas the lowest dose, 1.25 mg/kg was without effect; a dose of 2.5 mg/kg increased the concentration of dopamine only; the only effect of CGP 40116 (5 mg/kg) observed in substantia nigra, was an increase in dopamine concentration; its doses of 1.25 and 2.5 mg/kg were ineffective. (3) MK-801 (0.2 and 0.4 mg/kg) enhanced the concentrations of dopamine, DOPAC and HVA in both structures. A dose of 0.1 mg/kg increased the dopamine concentration only. The effects of MK-801 in substantia nigra were quantitatively weaker than those observed in ventral tegmental area. (4) Both CGP 40116 (5 mg/kg) and MK-801 (0.4 mg/kg) evoked alterations in the density of dopaminergic receptors. D-2 receptors, were up-regulated by MK-801 in ventral tegmental area and subregions of substantia nigra, i.e. pars compacta and pars reticulata, whereas CGP 40116 evoked similar effects in ventral tegmental area only. D-1 receptors in pars compacta and pars reticulata of substantia nigra were down-regulated after administration of either drug. It is concluded that competitive NMDA receptor antagonists in doses which evoke hyperlocomotion and stereotypy-like activity, may have a substantial impact on the dopaminergic neurotransmission in the rat ventral tegmental area and substantia nigra, similar to that described for MK-801, a non-competitive NMDA receptor antagonist. The obtained results may suggest that CGP 40116 and, possibly, other competitive NMDA antagonists may have dopaminomimetic properties, and that their clinical potentials may be limited by the risk of evoking dopamine-dependent psychotomimetic and abusing effects, similar to those described for MK-801.