The impact of maternal separation on the number of tyrosine hydroxylase-expressing midbrain neurons during different stages of ontogenesis.

Early life stressors have life-long functional and anatomical consequences. Though many neurotransmitters are involved in the functional impact of early life stress, dopamine seems to be important because of its roles in motor control, adaptation to stressful conditions, mood, cognition, attention and reward. Thus, in the present study, we investigated the way that early life stress, in the form of maternal separation (MS), affects the populations of tyrosine hydroxylase-immunoreactive (TH-IR) dopaminergic neurons in rat midbrain structures during ontogenesis. We included in the study the sub-regions of the substantia nigra (SN) and the ventral tegmental area (VTA). In both the control and MS rats, we found that the estimated total number of TH-expressing neurons fluctuated during ontogenesis. Moreover, MS influenced the number of TH-IR cells, especially in the SN pars reticulata (SNr) and VTA. Shortly after the termination of MS, on postnatal day (PND) 15, a decrease in the estimated total number of TH-IR neurons was observed in the SNr and VTA (in both males and females). On PND 35, MS caused a transient increase in the number of TH-IR cells only in the SNr of female rats. On PND 70, MS affected the number of TH-IR neurons in the VTA of females; specifically, an increase in the number of these cells was observed. Additionally, MS did not alter TH-IR cell sizes or the total levels of TH (measured by Western blot analysis) in the SN and VTA for all stages of ontogenesis in both males and females. The results from the study herein indicate that early life stress has enduring effects on the populations of midbrain TH-expressing dopaminergic neurons (especially in female rats), which are critically important for dopamine-regulated brain function throughout ontogenesis.

Maternal separation affects the number, proliferation and apoptosis of glia cells in the substantia nigra and ventral tegmental area of juvenile rats.

Adverse early life experiences can increase the risk of psychiatric and neurological disorders as the result of interference with brain development and maturation. In the present study, we tested whether early life stress, that is, maternal separation (MS), affects cell number, cell proliferation and constitutive apoptotic processes in the substantia nigra (SN) and the ventral tegmental area (VTA) of juvenile male and female rats. It was found that MS decreased the total number of glia but not neuronal cells in the SN pars compacta (SNc) and VTA of males. Moreover, MS reduced the number of S-100ß-immunoreactive (IR) glial cells in the SN of females and in the VTA of males. It was also observed that MS decreased the rate of proliferation (as measured by Ki-67-immunoreactivity) in the SN pars reticulata (SNr) and VTA of both males and females. Additionally, MS reduced the number of TUNEL-positive cells in the SNc of both males and females and in the SNr and VTA of males only. Moreover, MS decreased the number of cleaved caspase-9-IR cells in the SN and VTA of male rats. Cleaved caspase-9 was present in microglia cells, which exhibited the morphological hallmarks of apoptosis, but not in neuronal, astrocytic or oligodendrocytic cells. On the contrary, MS increased the number of cleaved caspase-3-IR cells in the SN and VTA of both male and female rats. Cleaved caspase-3-IR cells did not display signs of apoptosis and had an astrocytic phenotype (S-100ß-IR). In males exposed to MS, a decrease in caspase-3 enzymatic activity in the SN was also observed. In summary, the results of the present study revealed that early life stress affects the number, proliferation and naturally occurring apoptosis of glia cells in the SN and VTA in a sex-dependent manner and consequently may impair brain functions that are regulated by these structures.

Activation of CB1 cannabinoid receptors impairs memory consolidation and hippocampal polysialylated neural cell adhesion molecule expression in contextual fear conditioning.

We investigated the role of CB1 receptors in hippocampal-dependent memory consolidation mediated by polysialylated neural cell adhesion molecule (PSA-NCAM) during contextual fear conditioning (CFC). The CB1 receptor agonist 3-(1,1-dimethylheptyl)-(-)-11-hydroxy-Delta(8)-tetrahydrocannabinol (HU-210) (0.1 mg/kg) was given immediately after training during the memory consolidation phase, and freezing behavior was measured 24 h after conditioning. Administration of HU-210 attenuated freezing behavior measured in CFC. Western blot analysis showed that CFC induced a decrease in the expression of NCAM-180, but did not change the level of NCAM-140 and increased PSA-NCAM expression measured 24 h after training in the rat hippocampus. HU-210 (0.1 mg/kg) injection did not affect the reduction in NCAM-180 levels induced by CFC, but it blocked the increase in PSA-NCAM expression. Since the dentate gyrus (DG) of the hippocampus is known to be involved in memory consolidation and expresses a high level of PSA-NCAM protein, we measured the effects of CFC and HU-210 administration on PSA-NCAM-immunoreactive (IR) cells in the DG. CFC caused an increase in the number of PSA-NCAM-IR cells in the DG, but not K(i)-67- or doublecortin (DCX)-IR cells. This increase in PSA-NCAM-IR cells was abolished by HU-210 injection. Administration of the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM-251) (3 mg/kg immediately before HU-210) inhibited the effects of HU-210 on freezing behavior and PSA-NCAM expression in the DG. These results indicate that activation of CB1 receptors disturbs consolidation of fear memory in CFC, likely by affecting PSA-NCAM expression in the DG, which plays an important role in synaptic rearrangement during the formation of memory traces.

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.

A search for colocalization of serotonin 5-HT2A and 5-HT1A receptors in the rat medial prefrontal and entorhinal cortices--immunohistochemical studies.

Recently developed antipsychotic drugs ameliorating the negative symptoms of schizophrenia act not only on dopamine D2 receptors but also on serotonin 2A (5-HT2A) and 1A (5-HT1A) receptors in specific regions of the cerebral cortex. Since it is not yet known whether serotonin 5-HT1A and 5-HT2A receptors coexist in the same population of neurons in the cortex, the present study investigated their colocalization in the rat medial prefrontal (MPC) and entorhinal (EC) cortices. Using antibodies that recognize epitopes specific to the serotonin 5-HT2A or 5-HT1A receptors, studies employing confocal microscopy have shown that in the MPC 5-HT2A receptors are preferentially, if not exclusively, present on the pyramidal neurons and that 5-HT1A-immunopositive material is present in the axonal hillocks and, to lower extend, in cytoplasm of presumably pyramidal cell bodies. With the regard of labeling of active receptors (i.e. present in shafts and axonal hillocks) we found that about 38% of neurons positive for the presence of serotonin 5-HT2A receptors, are also positive for serotonin 5-HT1A receptors in the MPC. In the EC, only 22% of serotonin 5-HT2A-positive neurons were positive for serotonin 5-HT1A receptor-immunoreactivity. In the respect of cytoplasmatic serotonin 5-HT1A receptor-immunoreactivity (possibly inactive receptors), 65% and 73% of serotonin 5-HT2A receptor-positive neurons were colocalized with serotonin 5-HT1A receptors in the MPC and EC, respectively. Data obtained on serotonin 5-HT2A and 5-HT1A receptor localization provide anatomical grounds for at least three distinct populations of pyramidal neurons, one governed only by 5-HT2A, one only by 5-HT1A and one by both types of serotonin receptors.

Impact of postnatal blockade of N-methyl-D-aspartate receptors on rat behavior: a search for a new developmental model of schizophrenia.

The malfunction of glutamatergic neurotransmission in the neonatal or postnatal periods may be a risk factor for the appearance of neuroanatomical, neurochemical or functional changes that are characteristic of schizophrenia. Thus, the present study was undertaken to investigate whether blockade of N-methyl-d-aspartate (NMDA) receptors in the postnatal period influences rat behavior in tests characterizing schizophrenia-like deficits such as psychomotor agitation, impairments of sensorimotor gating, working memory, and intensity of social interactions. (E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116), a competitive antagonist of NMDA receptors, was given postnatally (1.25 mg/kg on days 1, 3, 6, 9; 2.5 mg/kg on days 12, 15, 18; and finally 5 mg/kg on day 21, all injections s.c.), and rats were tested at 60 days old. We found that blockade of NMDA receptors in the postnatal period led to an enhancement of exploration, mimicking psychomotor agitation, impairments in sensorimotor gating as measured by a prepulse-evoked inhibition of acoustic startle response, and an impaired working memory, as measured by an increase in the latency to achieve accurate rate of response in the delayed alternation task. Decreases in non-aggressive social interactions and increases in aggressive interactions were also observed. In addition to cognitive deficits typical of schizophrenia, rats treated postnatally with NMDA receptor antagonists also showed higher level of fear exhibited in the elevated plus maze. Thus, the blockade of NMDA receptors in the postnatal period may model deficits that are characteristic of schizophrenia.

Glutamatergic neurons of rat medial prefrontal cortex innervating the ventral tegmental area are positive for serotonin 5-HT1A receptor protein.

The present study was designed to investigate whether serotonin 5-HT1A receptor protein (5-HT1A receptor-immunoreactivity), is present on cortical pyramidal neurons of the rat medial prefrontal cortex (MPC) innervating the ventral tegmental area (VTA). Recent data stress the role of serotonin 5-HT1A receptors in the pathology of schizophrenia, and in the mechanism of action of novel antipsychotic drugs. It was found that approximately 52% of cells in layers II/III of the MPC whose axons initial segments were immunoreactive for serotonin 5HT1A receptor were also labeled with Fluoro-Gold (FG), a retrograde tracer injected into the VTA, indicating that certain portion of neurons forming glutamatergic innervations of the VTA may be controlled by serotonin 5-HT1A receptors. In deep cortical layers (V/VI) retrogradely labeled neurons never colocalized with serotonin 5-HT1A receptormmunoreactivity. These anatomical data indicate that serotonin 5-HT1A receptors might potentially control the excitability and propagation of information transmitted by the pyramidal cells to the VTA. Moreover, our results indicate that the drugs operating via serotonin 5-HT1A receptors in the MPC, might control from this level the release of glutamate in the VTA and restore function of glutamate neurotransmission, whose dysfunction is observed for example in schizophrenia.

Blockade of NMDA receptors in postnatal period decreased density of tyrosine hydroxylase immunoreactive axonal arbors in the medial prefrontal cortex of adult rats.

Malfunction of glutamatergic neurotransmission in postnatal period is considered to be a risk factor for development of schizophrenia. Thus, the present study investigates the impact of NMDA receptor blockade in the postnatal period on the density of tyrosine hydroxylase immunoreactive axonal arbors in the rat medial prefrontal cortex. Behavioral experiments revealed that adult rats (60 days old) treated in the postnatal period with a competitive antagonist of NMDA receptors, CGP 40116 (1.25 mg/kg on days 1, 3, 6, 9; 2.5 mg/kg on days 12, 15, 18; and finally 5 mg/kg on day 21, all injections s.c.), showed enhancement of the locomotor activity stimulated by quinpirole (0.3 mg/kg s.c.) and amphetamine (0.5 mg/kg s.c.), which suggests development of functional supersensitivity of dopaminergic systems. It has been found that CGP 40116, given in postnatal period decreased the density of tyrosine hydroxylase immunoreactive axonal arbors in the medial prefrontal cortex of adult animals. The decrease was observed in superficial (II/III) and deep (V/VI) layers of the medial prefrontal cortex, while the average length of tyrosine hydroxylase immunoreactive axonal arbors was increased in both superficial and deep cortical layers. Changes in the density of tyrosine hydroxylase immunoreactive axonal arbors have not been followed by a significant decrease in the content of tyrosine hydroxylase protein measured by Western blot. Thus, NMDA receptor blockade in the early period of life evokes changes in architecture of tyrosine hydroxylase immunoreactive axonal arbors and that malfunction of glutamatergic neurotransmission, in early period of life may produce anatomical changes which resemble those observed in the brains of schizophrenics.

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.

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.

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.