PDE10A and PDE10A-dependent cAMP catabolism are dysregulated oppositely in striatum and nucleus accumbens after lesion of midbrain dopamine neurons in rat: A key step in parkinsonism physiopathology

Abstract

Loss of dopamine neurons in experimental parkinsonism results in altered cyclic nucleotide cAMP and cGMP levels throughout the basal ganglia. Our objective was to examine whether expression of phosphodiesterase 10A (PDE10A), an isozyme presenting a unique distribution in basal ganglia, is altered after unilateral injection of 6-hydroxydopamine in the medial forebrain bundle, eliminating all midbrain dopaminergic neurons, such that cyclic nucleotide catabolism and steady state could be affected. Our study demonstrates that PDE10A mRNA levels were decreased in striatal neurons 10 weeks after 6-hydroxydopamine midbrain lesion. Such changes occurred in the striatum ipsilateral to lesion and were paralleled by decreased PDE10A protein levels and activity in striatal neurons and in striato-pallidal and striato-nigral projections. However, PDE10A protein and activity were increased while PDE10A mRNA was unchanged in the nucleus accumbens ipsilateral to the 6-hydroxydopamine midbrain lesion. Accordingly, cAMP levels were down-regulated in the nucleus accumbens, and up-regulated in the striatum ipsilateral to the lesion, but they were not significantly changed in substantia nigra and globus pallidus. Unlike cAMP, cGMP levels were decreased in all dopamine-deafferented regions. The opposite variations of cAMP steady state in striatum and nucleus accumbens are concordant and likely dependent, at least in part, on the down-regulation of PDE10A expression and activity in the former and its up-regulation in the latter. On the other hand, the down-regulation of cGMP steady state in the striato-nigral and striato-pallidal complex is not consistent with and is likely independent from the concomitant down-regulation of PDE10A. Therefore, dopamine loss inversely regulates PDE10A gene expression in the striatum and PDE10A post-transcription in the nucleus accumbens, therein differentially modulating PDE10A-dependent cAMP catabolism.

Introduction

The physiological actions of dopamine in nigro-striatal neurotransmission are generally well known, however, the adaptive responses of the striatal neurons secondary to a dopamine loss, such as in Parkinson's disease, are still partly understood.

According to the stimulatory interactions of dopamine with D1 receptors and the synthesis of adenosine 3′, 5′-cyclic monophosphate (cAMP) by adenylate cyclases (Garau et al., 1978, Kebabian, 1978), a deficit of dopamine in the striatum should imply a reduced stimulation of D1 receptor and hence a reduction in the synthesis of the cAMP. Unexpectedly, experimental lesions of the nigro-striatal dopaminergic projections with 6-hydroxydopamine (6-OHDA) induce an increase in cAMP levels after several weeks, as evidenced by increased basal adenylate cyclase activity in dopamine-denervated rat striata (Hossain and Weiner, 1993, Tenn and Niles, 1997). Unlike cAMP, guanosine 3′,5′-cyclic monophosphate (cGMP) levels decrease in response to dopamine loss. Such decrease in cGMP loss is associated with decreased nitric oxide synthase expression and activity, probably leading to a down-regulation of the nitric oxide–guanylate cyclase pathway (Sancesario et al., 2004).

The intracellular levels of cAMP and cGMP, are controlled by their rate of synthesis via adenylate and guanylate cyclase respectively (for a review see Cooper, 2003, Koesling et al., 1991, Wedel and Garbers, 2001), and by their rate of degradation via 3′,5′-cyclic nucleotide phosphodiesterases (PDE), a group of enzymes that hydrolyze cAMP and cGMP to their inactive 5′-derivatives (Beavo, 1995, Zhao et al., 1997). Indeed, dopamine loss has been associated with increased expression and activity of phodiesterase 1B (PDE1B) (Sancesario et al., 2004), a calcium/calmodulin dependent phodiesterase, highly localized in the caudate–putamen and nucleus accumbens, which hydrolyzes both cAMP and cGMP substrates, with higher specificity for the latter (K_m value of 24 versus 2.7 μM) (Polli and Kincaid, 1994, Zhao et al., 1997). These data suggest that the down-regulation of the cGMP steady state in dopamine-denervated rat striata may be affected by long-term changes in the processes of synthesis as well as catabolism (Sancesario et al., 2004). Furthermore, the data raise the question that a differentiated process of PDE-dependent catabolism may involve an increase in cAMP levels after dopamine loss.

PDE isozymes have been classified into 11 families according to structural characteristics, substrate and inhibitor profiles (Soderling and Beavo, 2000). Aside from PDE1B, a dual-substrate phosphodiesterase gene family PDE10A has been selectively identified in the basal ganglia, which is calcium/calmodulin independent with a higher hydrolyzing activity for cAMP (K_m 0.05 μM) than for cGMP (K_m 3 μM) (Soderling et al., 1998, Soderling et al., 1999, Fujishige et al., 1999a, Fujishige et al., 1999b, Siuciak et al., 2006a). In addition, the PDE10A isozyme has a unique distribution in the basal ganglia, with medium spiny neurons (MSNs) in the striatum, MSN axons/terminals in the substantia nigra reticulata and external globus pallidus, and with the neuronal bodies in the nucleus accumbens (Seeger et al., 2003, Xie et al., 2006). In addition, papaverine, which aggravates all the signs and symptoms of Parkinson's disease (Duvoisin, 1975), has recently been identified as a potent, specific inhibitor of PDE10A (Siuciak et al., 2006b).

In the present study we sought to determine whether unilateral lesions of dopaminergic neurons in the rat midbrain affect PDE10A expression and related cyclic nucleotide catabolism in the basal ganglia. Moreover, we used papaverine as an inhibitor to ascertain the contribution of PDE10A to total cAMP–PDE activity in lesioned and contralateral hemisphere.