Bilateral lesions of the entorhinal cortex differentially modify haloperidol- and olanzapine-induced c-fos mRNA expression in the rat forebrain

c-Fos expression response to olanzapine, amisulpride, aripiprazole, and quetiapine single administration in the rat forebrain: Effect of a mild stress preconditioning

Antipsychotics have been shown to stimulate different forebrain areas, whereas some of them are sensitive to stress. In the present study, effect of a single administration of olanzapine (OLA), amisulpride (AMI), aripiprazole (ARI), and quetiapine (QUE) on the activity of cells in the striatal dorsolateral (stDL) area, the periventricular zone (peVZ), the septal ventrolateral (seVL) nucleus, and the accumbens nucleus shell (shACC) and core (coACC) was investigated in male rats preconditioned with a mild stress complex (CMS) for 20 days. The objective of the study was to extend the anatomical-functional knowledge on the mechanism of selected antipsychotics with the goals: 1) to analyze the ability of the selected antipsychotics to induce c-Fos protein expression in the above mentioned forebrain structures and to map the pattern of their topography and 2) to find out whether longer-lasting mild stress preconditioning may modify the impact of the selected antipsychotics on the activity of cells in the forebrain areas in adult rats. Ten groups of rats were used. CMS complex contained five stressors: cage crowding, air-puff noising, wet bedding, predator stress, and forced swimming. AMI (20 mg/kg), OLA (5 mg/kg), QUE (15 mg/kg), and ARI (10 mg/kg/b.w.) were administered intraperitoneally and 90 min later the animals transcardially perfused by fixative. c-Fos was visualized by ABC complex. In unstressed animals, OLA and ARI elevated c-Fos expression in all areas studied, AMI and QUE in all areas except stDL, seVL and coACC, shACC FL-2 (shACC posterior level), respectively. CMS potentiated the effect of AMI in coACC, and QUE in shACC FL-2 and suppressed the effect of AMI in peVZ, and ARI in peVZ and seVL. The present data provide new insights into activity of cells in response to CMS challenge, which might be helpful in understanding the diverse clinical effects of atypical antipsychotics.

Neuroanatomical substrates of the disruptive effect of olanzapine on rat maternal behavior as revealed by c-Fos immunoreactivity

Olanzapine is one of the most widely prescribed atypical antipsychotic drugs in the treatment of schizophrenia. Besides its well-known side effect on weight gain, it may also impair human parental behavior. In this study, we took a preclinical approach to examine the behavioral effects of olanzapine on rat maternal behavior and investigated the associated neural basis using the c-Fos immunohistochemistry. On postpartum days 6-8, Sprague-Dawley mother rats were given a single injection of sterile water or olanzapine (1.0, 3.0 or 5.0mg/kg, sc). Maternal behavior was tested 2h later, after which rats were sacrificed and brain tissues were collected. Ten brain regions that were either implicated in the action of antipsychotic drugs and/or in the regulation of maternal behavior were examined for c-Fos immunoreactivity. Acute olanzapine treatment dose-dependently disrupted various components of maternal behavior (e.g., pup retrieval, pup licking, nest building, crouching) and increased c-Fos immunoreactivity in the medial prefrontal cortex (mPFC), nucleus accumbens shell and core (NAs and NAc), dorsolateral striatum (DLSt), ventral lateral septum (LSv), central amygdala (CeA) and ventral tegmental area (VTA), important brain areas generally implicated in the incentive motivation and reward processing. In contrast, olanzapine treatment did not alter c-Fos in the medial preoptic nucleus (MPN), ventral bed nucleus of the stria terminalis (vBST) and medial amygdala (MeA), the core brain areas directly involved in the mediation of rat maternal behavior. These findings suggest that olanzapine disrupts rat maternal behavior primarily by suppressing incentive motivation and reward processing via its action on the mesocorticolimbic dopamine systems, other limbic and striatal areas, but not by disrupting the core processes involved in the mediation of maternal behavior in particular.

Effects of Aripiprazole and Haloperidol on Fos-like Immunoreactivity in the Prefrontal Cortex and Amygdala

Objective

Aripiprazole, a dopamine system stabilizer, shows efficacy against both negative symptoms and positive symptoms in patients with schizophrenia. The aim of this study was to investigate the effects of aripiprazole and haloperidol on c-FOS expression in rat brain.

Methods

Aripiprazole (1, 10 and 30 mg/kg, i.p.) and haloperidol (0.1 and 1 mg/kg, i.p.) were administered to adult Male Sprague-Dawley rats. After 2 h of drug or vehicle administration, the rats were killed and their brains were removed and perfused with fixative, then cut into 40 µm slices on a freezing microtome. Brain regions of interest were the medial prefrontal cortex (mPFC), the nucleus accumbens core and shell (NAC-C and NAC-S), the hippocampus (CA1, CA3 and DG), the central amygdala (Ce), the basolateral amygdala (BL) and the temporal cortex (Tc). Immunohistochemistry was performed to label cell bodies containing c-FOS.

Results

The administration of aripiprazole at all doses (1, 10 or 30 mg/kg) resulted in greater Fos-like immunoreactivity (FLI) in the investigated brain areas, as compared to the vehicle. Comparable increases in FLI were demonstrated in the NAC-C and NAC-S in response to both aripiprazole and haloperidol treatment. The administration of haloperidol (0.1 or 1 mg/kg) also resulted in greater FLI in the investigated brain areas, except the mPFC, where no changes were observed. In the Ce and BL, a significant increase in Fos-positive neurons was observed only with 0.1 mg/kg of haloperidol.

Conclusion

Both aripiprazole and haloperidol increased FLI in limbic areas, which are considered important targets of antipsychotic drugs. The differential action of aripiprazole on FLI in the amygdala and mPFC as compared to haloperidol may be a good way to differentiate atypical from typical antipsychotics.

Lesion of the ventral tegmental area amplifies stimulation-induced Fos expression in the rat brain

Unilateral lesions of the ventral tegmental area (VTA), the key structure of the mesolimbic system, facilitate behavioral responses induced by electrical stimulation of the VTA in the contralateral hemisphere. In search of the neuronal mechanism behind this phenomenon, Fos expression was used to measure neuronal activation of the target mesolimbic structures in rats subjected to unilateral electrocoagulation and simultaneously to contralateral electrical stimulation of the VTA (L/S group). These were compared to the level of mesolimbic activation after unilateral electrocoagulation of the VTA (L group), unilateral electrical stimulation of the VTA (S group) and bilateral electrode implantation into the VTA in the sham (Sh) group. We found that unilateral stimulation of the VTA alone increased the density of Fos containing neurons in the ipsilateral mesolimbic target structures: nucleus accumbens, lateral septum and amygdala in comparison with the sham group. However, unilateral lesion of the VTA was devoid of effect in non-stimulated (L) rats and it significantly amplified the stimulation-induced Fos-immunoreactivity (L/S vs S group). Stimulation of the VTA performed after contralateral lesion (L/S) evoked strong bilateral induction of Fos expression in the mesolimbic structures involved in motivation and reward (nucleus accumbens and lateral septum) and the processing of the reinforcing properties of olfactory stimuli (anterior cortical amygdaloid nucleus) in parallel with facilitation of behavioral function measured as shortened latency of eating or exploration. Our data suggest that VTA lesion sensitizes mesolimbic system to stimuli by suppressing an inhibitory influence of brain areas afferenting the VTA.

Entorhinal cortex and cognition

Understanding the function of the entorhinal cortex (EC) has been an important subject over the years, not least because of its cortical intermediary to and from the hippocampus proper, and because of electrophysiological advances which have started to reveal the physiology in behaving animals. Clearly, a lot more needs to be done but is clear to date that EC is not merely a throughput station providing all hippocampal subfields with sensory information, but that processing within EC contributes significantly to attention, conditioning, event and spatial cognition possibly by compressing representations that overlap in time. These are transmitted to the hippocampus, where they are differentiated again and returned to EC. Preliminary evidence for such a role, but also their possible pitfalls are summarised.

Evidence for a critical role of entorhinal cortex at pre-exposure for latent inhibition disruption in rats

Latent inhibition (LI), that is the decrease in conditioned response induced by the repeated nonreinforced pre-exposures to the to-be-conditioned stimulus, is disrupted by entorhinal cortex (EC) lesions. The mechanism involved in this disruption is unknown, and in particular the experimental stage (pre-exposure or conditioning) at which the integrity of EC is necessary has to be determined. The purpose of this study was to address this issue by using reversible inactivation of the EC by local micro-infusion of tetrodotoxin (TTX). TTX was infused either before the pre-exposure phase, before the conditioning phase, or before both phases. LI was unaffected in rats that received TTX before conditioning or before both pre-exposure and conditioning. In contrast, LI was disrupted in rats that received TTX before pre-exposure only. These results are discussed in the framework of LI models.

Neuropeptide Y mRNA expression levels following chronic olanzapine, clozapine and haloperidol administration in rats

Using quantitative in situ hybridization, this study examined regional changes in rat brain mRNA levels encoding neuropeptide Y (NPY) following olanzapine, clozapine and haloperidol administration (1.2, 1.5 and 2.0 mg/kg, oral) for 36 days. The NPY mRNA expression levels and patterns were examined after the last drug administration at both time points enabling the measurement of immediate effect at 2h and the effects after 48 h of drug administration. It was found that all these drugs had an immediate effect on NPY mRNA expression, while virtually all these changes normalized 48 h after the drug treatments. A similarity in altered NPY mRNA expression patterns was seen between the olanzapine and clozapine groups; however, haloperidol was very different. Olanzapine and clozapine administration decreased NPY mRNA levels in the nucleus accumbens, striatum and anterior cingulate cortex (from -60% to -77%, p<0.05). Haloperidol decreased NPY mRNA expression in the amygdala and hippocampus (-69%, -64%, p<0.05). In the lateral septal nucleus, NPY mRNA levels significantly decreased in the olanzapine group (-66%, p<0.05), a trend toward a decrease was observed in the clozapine group, and no change was found in the haloperidol treated group. These results suggest that the different effects of atypical and typical antipsychotics on NPY systems may reflect the neural chemical mechanisms responsible for the differences between these drugs in their effects in treating positive and negative symptoms of schizophrenia. The immediate decrease of NPY mRNA levels suggests an immediate reduction of NPY biosynthesis in response to these drugs.

Lesion of the lateral entorhinal cortex amplifies odor-induced expression of c-fos, junB, and zif 268 mRNA in rat brain

Paradoxical facilitation of olfactory learning following entorhinal cortex (EC) lesion has been described, which may result from widespread functional alterations taking place within the olfactory system. To test this hypothesis, expression of the immediate early genes c-fos, junB, and zif 268 was studied in response to an olfactory stimulation in several brain areas in control and in EC-lesioned rats. Olfactory stimulation in control rats induced the expression of the three genes in the granular/mitral and glomerular layers of the olfactory bulb, as well as c-fos and junB expression in the piriform cortex. However EC lesion was devoid of effects in nonstimulated animals; it significantly amplified the odor-induced expression of the three genes in these areas, as well as in the amygdala, hippocampus, and parietal-temporal cortices. The data suggest that EC lesion modifies the neural processing of odor by suppressing an inhibitory influence on brain areas connected to this cortex.

Entorhinal cortex stimulation modulates amygdala and piriform cortex responses to olfactory bulb inputs in the rat

The rodent olfactory bulb sends direct projections to the piriform cortex and to two structures intimately implicated in memory processes, the entorhinal cortex and the amygdala. The piriform cortex has monosynaptic projections with the amygdala and the piriform cortex and is therefore in a position to modulate olfactory input either directly in the piriform cortex, or via the amygdala. In order to investigate this hypothesis, field potential signals induced in anesthetized rats by electrical stimulation of the olfactory bulb or the entorhinal cortex were recorded simultaneously in the piriform cortex (anterior part and posterior part) and the amygdala (basolateral nucleus and cortical nucleus). Single-site paired-pulse stimulation was used to assess the time courses of short-term inhibition and facilitation in each recording site in response to electrical stimulation of the olfactory bulb and entorhinal cortex. Paired-pulse stimulation of the olfactory bulb induced homosynaptic inhibition for short interpulse interpulse intervals (20-30 ms) in all the recording sites, with a significantly lower degree of inhibition in the anterior piriform cortex than in the other structures. At longer intervals (40-80 ms), paired-pulse facilitation was observed in all the structures. Paired-pulse stimulation of the entorhinal cortex mainly resulted in inhibition for the shortest interval duration (20 ms) in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. Double-site paired-pulse stimulation was then applied to determine if stimulation of the entorhinal cortex can modulate responses to olfactory bulb stimulation. For short interpulse intervals (20 ms) heterosynaptic inhibition was observed in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. The level of inhibition was greater in the basolateral nucleus than in the other structures. Taken together these data suggest that the entorhinal cortex exerts a main inhibitory effect on the olfactory input via the amygdala basolateral nucleus and to a lesser extent the piriform cortex. The potential role of these effects on the processing of olfactory information is discussed.