Serotonin_2C receptor stimulation inhibits cocaine-induced Fos expression and DARPP-32 phosphorylation in the rat striatum independently of dopamine outflow

Highlights

• The 5-HT_2CR is known to control dopamine neuron function.

• The 5-HT_2CR may control the effects of cocaine at postsynaptic level in the striatum.

• Cocaine-induced striatal DA outflow is unaltered by Ro 60-0175.

• Cocaine-induced striatal DARPP-32 phosphorylation is inhibited by Ro 60-0175.

• Cocaine behavioral effects may involve the 5-HT_2CR control of striatal DA signaling.

Abstract

The serotonin_2C receptor (5-HT_2CR) is known to control dopamine (DA) neuron function by modulating DA neuronal firing and DA exocytosis at terminals. Recent studies assessing the influence of 5-HT_2CRs on cocaine-induced neurochemical and behavioral responses have shown that 5-HT_2CRs can also modulate mesoaccumbens DA pathway activity at post-synaptic level, by controlling DA transmission in the nucleus accumbens (NAc), independently of DA release itself. A similar mechanism has been proposed to occur at the level of the nigrostriatal DA system. Here, using in vivo microdialysis in freely moving rats and molecular approaches, we assessed this hypothesis by studying the influence of the 5-HT_2CR agonist Ro 60-0175 on cocaine-induced responses in the striatum. The intraperitoneal (i.p.) administration of 1 mg/kg Ro 60-0175 had no effect on the increase in striatal DA outflow induced by cocaine (15 mg/kg, i.p.). Conversely, Ro 60-0175 inhibited cocaine-induced Fos immunoreactivity and phosphorylation of the DA and c-AMP regulated phosphoprotein of Mr 32 kDa (DARPP-32) at threonine 75 residue in the striatum. Finally, the suppressant effect of Ro 60-0175 on cocaine-induced DARPP-32 phosphorylation was reversed by the selective 5-HT_2CR antagonist SB 242084 (0.5 mg/kg, i.p.). In keeping with the key role of DARPP-32 in DA neurotransmission, our results demonstrate that 5-HT_2CRs are capable of modulating nigrostriatal DA pathway activity at post-synaptic level, by specifically controlling DA signaling in the striatum.

Introduction

Along with its dense localization in brain dopaminergic regions, the central serotonin_2C receptor (5-HT_2CR) is a well-established modulator of dopamine (DA) systems (Alex and Pehek, 2007, Berg et al., 2008), and is currently considered as a promising pharmacological target for improved treatments of DA-related disorders, such as Parkinson's disease, depression, schizophrenia and drug addiction (Millan, 2005, Di Giovanni et al., 2006, Müller and Huston, 2006, Filip et al., 2012, Meltzer et al., 2012, Cunningham et al., 2013, Higgins et al., 2013).

Compelling evidence indicates that 5-HT_2CRs afford an overall inhibitory control on the mesocorticolimbic and nigrostriatal DA pathways (Berg et al., 2008, Aloyo et al., 2009). In particular, electrophysiological and neurochemical studies have shown that 5-HT_2CRs modulate DA neuronal function by controlling both DA neuronal firing and DA exocytosis at terminals (Porras et al., 2002, Navailles et al., 2004, Berg et al., 2008). Studies assessing the role of 5-HT_2CRs in the neurochemical and behavioral effects of cocaine have shown that the 5-HT_2CR agonist Ro 60-0175 has no effect on striatal and accumbal DA release in halothane-anaesthetized rats (Navailles et al., 2004), but inhibits cocaine-induced DA-dependent behaviors, such as hyperlocomotive, discriminative stimulus and reinforcing properties (Filip et al., 2012, Devroye et al., 2013). These findings led to the proposal that 5-HT_2CRs could also regulate the activity of subcortical DA pathways at postsynaptic level, by controlling DA transmission in the nucleus accumbens (NAc) and the striatum independently of DA release (Navailles et al., 2004). We have recently confirmed this hypothesis at the level of the mesoaccumbens DA pathway, by showing that the peripheral administration of Ro 60-0175 is able to inhibit cocaine-induced phosphorylation of the DA and c-AMP regulated phosphoprotein Mr 32 kDa (DARPP-32) in the NAc (Cathala et al., 2014). Specifically, DARPP-32, in agreement with its location in dopaminoceptive neurons and its key role in DA signaling, provides a useful tool for evaluating DA neurotransmission (Nishi et al., 2000).

Thus, given the importance of the nigrostriatal DA system in the effects of cocaine (Koob and Volkow, 2010, Wise, 2009; Pierce and Vanderschuren, 2010), the present study was aimed at investigating whether, as for the mesoaccumbens DA pathway (Cathala et al., 2014), 5-HT_2CR stimulation modulates nigrostriatal DA pathway activity at postsynaptic level, by controlling DA transmission in the striatum independently of DA release itself.

To this purpose, we assessed the ability of Ro 60-0175 to modulate cocaine-induced changes of DA activity at both pre- and post-synaptic levels. Specifically, using intracerebral microdialysis in freely moving rats, we first studied the effect of Ro 60-0175 on cocaine-induced striatal DA outflow, as an index of pre-synaptic DA neuron activity (Panin et al., 2012). Second, to evaluate possible changes of post-synaptic neuronal activity, we assessed the effect of Ro 60-0175 on cocaine-induced changes of Fos immunoreactivity, and phosphorylation states at threonine 34 and 75 residues of DARPP-32. Finally, the specific involvement of 5-HT_2CRs in the effect of Ro 60-0175 on cocaine-induced DARPP-32 phosphorylation was studied in animals pretreated with the selective 5-HT_2CR antagonist SB 242084 (Kennett et al., 1997).