Mapping of c‐fos gene expression in the brain during morphine dependence and precipitated withdrawal, and phenotypic identification of the striatal neurons involved

Abstract

The c-fos gene is expressed in the central nervous system in response to various neuronal stimuli. Using in situ hybridization, we examined the effects of chronic morphine treatment and withdrawal on c-fos mRNA in the rat brain, and particularly within identified striatal neurons. Morphine dependence was induced by subcutaneous implantation of two pellets of morphine for 6 days and withdrawal was precipitated by administration of naltrexone. Placebo animals and morphine-dependent rats showed a very weak c-fos mRNA expression in all the structures studied. Our study emphasized the spatial variations in c-fos mRNA expression, and also revealed a peak expression of c-fos mRNA at 1 h after naltrexone-precipitated withdrawal in the projection areas of dopaminergic neurons, noradrenergic neurons and in several regions expressing opiate receptors. Interestingly, morphine withdrawal induces c-fos mRNA expression in the two efferent populations of the striatum (i.e. striatonigral and striatopallidal neurons) both in the caudate putamen and nucleus accumbens. Moreover, the proportions of activated neurons during morphine withdrawal are different in the caudate putamen (mostly in striatopallidal neurons) and in the shell and core parts of the nucleus accumbens (mostly in striatonigral neurons). The activation of striatopallidal neurons suggests a predominant dopaminergic regulation on c-fos gene expression in the striatum during withdrawal. On the contrary, c-fos induction in striatonigral neurons during withdrawal seems to involve a more complex regulation like opioid-dopamine interactions via the mu opioid receptor and the D1 dopamine receptor coexpressed on this neuronal population or the implication of other neurotransmitter systems.

Nucleus accumbens D2- and D1-receptor expressing medium spiny neurons are selectively activated by morphine withdrawal and acute morphine, respectively

“…It is difficult to reconcile our findings with the double in situ hybridization study of Georges et al (2000) in morphine withdrawn rats. In their study, naltrexoneprecipitated withdrawal induced c-fos mRNA expression mostly in D1R mRNApositive and pre-pro-enkephalin A mRNA-negative neurons of the NAc, identified as striatonigral neurons (Georges et al, 2000).…”

“…In their study, naltrexoneprecipitated withdrawal induced c-fos mRNA expression mostly in D1R mRNApositive and pre-pro-enkephalin A mRNA-negative neurons of the NAc, identified as striatonigral neurons (Georges et al, 2000). Interestingly, however, they reported in the same animals a preferential distribution of c-fos mRNA in striatopallidal (presumably D2R positive) neurons of dorsal striatal regions.…”

“…As a matter of fact, opioid withdrawal enhances c-Fos expression in GABAergic VTA neurons (Bagley et al, 2011). An intriguing question from these studies is how an abrupt and virtually complete cessation of dopamine neuron firing induced by morphine withdrawal (Diana et al, 1995) can lead to such a strong neuronal activation in ventral striatal regions (Hayward et al, 1990;Rasmussen et al, 1995;Georges et al, 2000;Gracy et al, 2001). It is tempting to speculate that elimination of dopamine release in the NAc during withdrawal removes the sustained stimulation of high affinity, inhibitory D2Rs (Richfield et al, 1989).…”

“…In pharmacological models of morphine withdrawal, naloxone-precipitated morphine withdrawal induces high levels of c-Fos expression in striatal neurons, an effect that is particularly strong in the NAc (Hayward et al, 1990;Rasmussen et al, 1995;Georges et al, 2000;Gracy et al, 2001). We therefore studied the effects of pharmacological precipitation of morphine withdrawal syndrome in the NAc of drd2-eGFP mice (Fig.…”

“…It is well established that different components of the global withdrawal response arise from distinct structures in the brain Williams et al, 2001). In particular, several studies point to the important contribution of the VTA and the NAc to the aversive signs of opioid withdrawal (Stinus et al, 1990;Gracy et al, 2001), a view that is supported by the strong neuronal activation recorded in neurons of the NAc shortly after precipitation of withdrawal (Koob et al, 1989;Hayward et al, 1990;Nylander et al, 1995;Rasmussen et al, 1995;Georges et al, 2000;Gracy et al, 2001). …”