Temporal changes in the messenger rna levels of cellular immediate early genes and neurotransmitter/receptor genes in the rat neostriatum and substantia nigra after acute treatment with eticlopride, a dopamine D 2 receptor antagonist

Abstract

The cellular immediate early genes are involved in the transcriptional events associated with the dopaminergic regulation of neurotransmitter expression within neurons of the neostriatum. To characterize these events in detail, quantitative in situ hybridization histochemistry was used to assess the temporal effects of acute dopamine receptor blockade with eticlopride, a dopamine D 2 receptor antagonist, on the messenger RNA expression of the immediate early genes and neurotransmitters/receptors in the caudate-putamen and ventral tegmental area/substantia nigra pars compacta of the rat. Groups of rats were injected with a single dose of either isotonic saline or eticlopride (0.5 mg/kg i.p.) and killed at various time intervals ranging from 5 min to 24 h and frozen brain sections processed by in situ hybridization histochemistry. Using computerized image analysis, the changes in messenger RNA expression for c-fos, c-jun, jun B, jun D, nerve growth factor I-A and nerve growth factor I-B and for neurotensin, glutamate decarboxylase, proenkephalin, the dopamine D 1 receptor and the short and long isofonns of the D 2 receptor were examined in the caudate-putamen. In the ventral tegmental area and substantia nigra pars compacta, the messenger RNA expression of the above early response genes and that for neurotensin, tyrosine hydroxylase, cholecystokinin and the D 2; receptor isoforms were also examined. In the neostriatum, eticlopride caused a rapid increase in c-fos messenger RNA with significantly increased levels at 10 min (P < 0.01). The levels peaked at 30 min and thereafter declined to control levels. A similar profile was observed for jun B messenger RNA, although levels were still significantly (P < 0.01) elevated at l h and declined to basal levels thereafter. No significant changes were observed for c-jun, jun D, nerve growth factor I-A and nerve growth factor I-B messenger RNAs. In the dorsolateral neostriatum, there was an increase in proneurotensin messenger RNA 10 min after eticlopride, this increase becoming significant (P <0.01) at 60 min. Levels were maximal at 2–6 h and decreased after 12 h to basal levels. There were small increases in proenkephalin messenger RNA, but these were not significant (P < 0.05) until 6 h after the injection. Eticlopride did not have any significant effects on the messenger RNA levels for glutamate decarboxylase, the d 1 receptor and the short and long isoforms of the D 2; receptor. In the nucleus accumbens, both c-fos and jun B messenger RNAs increased in a temporal manner (as seen in the caudate—putamen in response to eticlopride), but proneurotensin messenger RNA levels were observed to be higher than control levels at 4 and 6 h after eticlopride injection in only 50% of the animals. In the ventral tegmental area/substantia nigra pars compacta, there were no significant changes in messenger RNA levels for any of the early response genes nor for tyrosine hydroxylase, cholecystokinin, proneurotensin and D 2; receptor isoforms. These results suggest that acute dopamine receptor blockade may affect a restricted population of neurons in the neostriatum (but not those in the ventral tegmental area/substantia nigra pars compacta), leading to increased synthesis of neurotensin, and the transcriptional events involved a combinational interaction of c-fos and jun B but not of the other cellular immediate early genes studied.