Research reportDifferential patterns of induction of NGFI-B, Nor1 and c-fos mRNAs in striatal subregions by haloperidol and clozapine
Introduction
The NGFI-B/Nur77 family of nuclear orphan receptors consists of NGFI-B, Nor1 and Nurr1 [14]. They all bind to an extended half site termed the NGFI-B response element [28] as monomers [29] and can function as constitutively active transcription factors. NGFI-B and Nurr1 can also heterodimerize with retinoid x receptor (RXR) which binds and becomes activated by 9-cis retinoic acid [9], [19]. NGFI-B and Nor1 are widely expressed in the adult brain, including the target regions for antipsychotic drugs such as haloperidol and clozapine [26]. Both NGFI-B and Nor 1 are induced by psychoactive drugs such as cocaine and morphine in accumbens and caudate putamen [27]. Dysfunction of retinoid-mediated transcription has recently been suggested to be an important factor in the etiology of schizophrenia [11]. First, retinoic acid toxicity or deficits can result in symptoms that partly resemble those of schizophrenia, such as thought disorder, mental deficit, enlarged ventricles and more. Also the locus of RXRβ is 6p21.3 and several linkage studies have pointed to this region as a schizophrenia susceptibility locus [23], [24]. However, also negative reports on this locus and schizophrenia have been reported [10]. In addition, in a recent study, Nurr1 knock out mice were shown to lack mesencephalic dopamine neurons, thus suggesting that Nurr1 and possibly Nurr1-RXR heterodimers are essential in the development of dopamine neurons and of the etiology of schizophrenia [31]. All drugs used in treatment of schizophrenia have in common that they bind to the dopamine D2 receptor. Therefore, dysfunction of dopamine transmission has been suggested as a cause of schizophrenia in the dopamine hypothesis of the disease [25]. The dopamine D2 receptor [22] is transcriptionally regulated by retinoid activated transcription factors and several other important candidate genes for schizophrenia such as glutamate [30], nicotine [2] receptors as well as the rate limiting enzyme in dopamine synthesis tyrosine hydroxylase [17] and choline acetyl transferase [3] are also regulated by retinoids.
Antipsychotic drugs bind to the dopamine D2 receptor with different affinities and are more or less specific for the dopamine D2 receptor. In human studies using PET analysis it has been demonstrated that in clinical doses typical drugs such as haloperidol occupies more than 70–80% of all dopamine D2 receptors in the putamen and that there is a correlation between high occupancy of D2 receptors and extrapyramidal side effects. The atypical neuroleptic clozapine which does not induce extrapyramidal side effects, occupies approximately 40–60% of dopamine D2 receptors in the human putamen [7]. The induction of c-Fos immunoreactivity is proposed as a marker for neuronal activity in rodents and is used in histological sections, to map out brain regions which are activated after treatment with different drugs with antispychotic actions in animal models [6], [20], [21]. Haloperidol which is a typical neuroleptic, induces c-Fos immunoreactivity in both nucleus accumbens and caudate putamen, whereas the induction by clozapine is restricted to limbic parts such as nucleus accumbens after acute treatment [20].
In the light of the data concerning a suggested link between retinoids and schizophrenia it is important to study the regulation of molecules involved in retinoic signaling and related molecules (NGFI-B and Nor1) after acute and chronic treatment by the typical neuroleptics haloperidol and the atypical clozapine. In this study we performed acute and chronic treatments with haloperidol and clozapine and analyzed mRNA levels of NGFI-B and Nor1 mRNAs and compared these to the induction of c-fos mRNA. Acutely, we found that NGFI-B and Nor1 mRNAs were induced in a pattern similar to that of c-fos mRNA in accumbens and caudate putamen after haloperidol or clozapine. Chronic haloperidol increased NGFI-B mRNA in accumbens core.
Section snippets
Drug treatments
Male 250-g Sprague–Dawley rats (B&K Sollentuna, Sweden) were used. In acute experiments i.p. injections of 1.8 mg/kg haloperidol (n=6) and 35 mg/kg clozapine (n=6) or the acetic vehicle (n=6) required to solublilize the drugs was administered 1 h before sacrifice. For chronic experiments haloperidol or clozapine was added to the drinking water at doses planned to correspond to 1.8 mg/kg/day (n=6) and 35–40 mg/kg/day (n=6) respectively, for 3 weeks as previously described [1], [8]. This protocol
Localization of NGFI-B, Nor1 and c-fos mRNAs in nucleus accumbens, caudate putamen and cerebral cortex
NGFI-B, Nor1 and c-fos transcripts were detected in patterns similar to what previously has been described [26]. While the three probes had clearly different overall patterns of distributions, they were all detected in accumbens, although at different levels in shell and core as in medial and lateral caudate putamen and cingulate, motor and sensory cortex (Fig. 2a–c).
Regulation of NGFI-B, Nor1 and c-fos mRNAs after antipsychotic drugs or vehicle
Rats that were administered the acetic vehicle i.p. and dissected 1 h later had no increase in any of the analyzed markers in
Discussion
In the present study we have analyzed the effects of acute and chronic treatment with antipsychotic drugs with different clinical profiles on the regulation of the nuclear orphan receptors NGFI-B and Nor1 and compared these effects with the induction of the proto-oncogene c-fos which is a nuclear transcription factor binding to the AP-1 response element in promoter sequences of target genes. We compared the typical neuroleptic haloperidol to the atypical neuroleptic clozapine.
Conclusions
NGFI-B, Nor1 and c-fos mRNAs are increased in both motor and limbic regions of the basal ganglia acutely after the typical antipsychotic drug haloperidol, whereas the atypical antipsychotic drug clozapine only increases NGFI-B, Nor1 and c-fos in limbic parts of the basal ganglia. In addition to a general neuronal activation in these regions caused by the treatments, it is possible that the increase in NGFI-B also reflects a specific increase in sensitivity for retinoid signaling in discrete
Acknowledgements
Supported by the Swedish Medical Research Council (21x-11642-03A and 14x-03185), USPHS grants, Thurings and Kapten Erikssons stiftelse. SB was supported by a fellowship from the Swedish Brain Foundation.
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