Pivotal Role of Adenosine Neurotransmission in Restless Legs Syndrome

Abstract

The symptomatology of Restless Legs Syndrome (RLS) includes periodic leg movements during sleep (PLMS), dysesthesias, and hyperarousal. Alterations in the dopaminergic system, a presynaptic hyperdopaminergic state, seem to be involved in PLMS, while alterations in glutamatergic neurotransmission, a presynaptic hyperglutamatergic state, seem to be involved in hyperarousal and also PLMS. Brain iron deficiency (BID) is well-recognized as a main initial pathophysiological mechanism of RLS. BID in rodents have provided a pathogenetic model of RLS that recapitulates the biochemical alterations of the dopaminergic system of RLS, although without PLMS-like motor abnormalities. On the other hand, BID in rodents reproduces the circadian sleep architecture of RLS, indicating the model could provide clues for the hyperglutamatergic state in RLS. We recently showed that BID in rodents is associated with changes in adenosinergic transmission, with downregulation of adenosine A₁ receptors (A1R) as the most sensitive biochemical finding. It was hypothesized that A1R downregulation leads to hypersensitive striatal glutamatergic terminals and facilitation of striatal dopamine release. Hypersensitivity of striatal glutamatergic terminals was demonstrated by an optogenetic-microdialysis approach in the rodent with BID, indicating that it could represent a main pathogenetic factor that leads to PLMS in RLS. In fact, the dopaminergic agonists pramipexole and ropinirole and the α₂δ ligand gabapentin, used in the initial symptomatic treatment of RLS, completely counteracted optogenetically-induced glutamate release from both normal and BID-induced hypersensitive corticostriatal glutamatergic terminals. It is a main tenet of this essay that, in RLS, a single alteration in the adenosinergic system, downregulation of A1R, disrupts the adenosine-dopamine-glutamate balance uniquely controlled by adenosine and dopamine receptor heteromers in the striatum and also the A1R-mediated inhibitory control of glutamatergic neurotransmission in the cortex and other non-striatal brain areas, which altogether determine both PLMS and hyperarousal. Since A1R agonists would be associated with severe cardiovascular effects, it was hypothesized that inhibitors of nucleoside equilibrative transporters, such as dipyridamole, by increasing the tonic A1R activation mediated by endogenous adenosine, could represent a new alternative therapeutic strategy for RLS. In fact, preliminary clinical data indicate that dipyridamole can significantly improve the symptomatology of RLS.

Keywords: ENT1; Restless Legs Syndrome; adenosine; dopamine; glutamate; hyperarousal; periodic leg movements during sleep.

Figure 1

Adenosine and dopamine receptors in the striatal glutamatergic and dopaminergic terminals and in the dendritic spines of the direct and indirect medium spiny neurons (MSN). (A) With normal brain iron conditions, extracellular concentrations of adenosine keep an inhibitory presynaptic tone of adenosine on glutamate and dopamine transmission, mediated by A1R, which results in a relatively low activation of the direct and indirect MSN. (B) Downregulation of A1R induced by brain iron deficiency leads to hypersensitive glutamatergic terminals, to disinhibition of glutamate and dopamine release, to triatal hyperdopaminergic and hyperglutamatergic states, which leads to an increase and decrease in the activity of the direct MSN and indirect MSN, respectively. DA, dopamine; GLU, glutamate; ADE, adenosine. The equilibrative nucleoside transporter ENT1 (also localized in neurons) is only represented in the astroglial process.

Figure 2

Adenosine Gs-Gi-coupled heterotetramers. (A) The A2AR-D2R heterotetramer, constituted by homodimers of the Gs-coupled A2AR and the Gi-coupled D2R, enables two types of reciprocal antagonistic interactions: an allosteric interaction, by which A2AR ligands modulate the affinity and intrinsic efficacy of D2R ligands, and a canonical interaction at the adenylyl cyclase (AC) level, by which D2R agonists inhibit A2AR agonists-mediated AC activation. (B) The A1R-A2AR heterotetramer, constituted by homodimers of the Gs-coupled A2AR and the Gi-coupled A1R, enables an allosteric interaction, by which A2AR ligands modulate the affinity and intrinsic efficacy of A1R ligands, but does not enable a canonical interaction at the AC level; A1R signals independently through voltage-dependent Ca²⁺ channels. (C) The A1R-D1R heterotetramer, constituted by homodimers of the Gs-coupled D1R and the Gi-coupled A1R, enables unidirectional nonreciprocal allosteric and canonical antagonistic interactions, with A1R ligands modifying the ligand binding properties and adenylyl cyclase activation induced by D1R agonists.

Figure 3

Adenosine-dependent modulation by dipyridamole on the locomotor activation induced by dopamine receptor agonists in reserpinized mice. Locomotor activity in male C57BL/6J mice (20–30 g) 20 h after administration of reserpine (5 mg/kg, s.c.; method described in detail in Marcellino et al., 2008) induced by the D1R agonist SKF81297 (5 mg/kg, i.p.; SKF; A) or the D2R agonist quinpirole (5 mg/kg, i.p.; QUIN; B), with or without the previous administration of dipyridamole (10, 30 or 100 mg/kg, i.p., 15 min before SKF or QUIN; 10DIP, 30DIP, or 100DIP) or caffeine (30 mg/kg, i.p., 30 min before SKF or QUIN; CAFF). All animals received three i.p. injections, with either drugs or the corresponding vehicle. One reserpinized group also received caffeine without dipyridamole or dopamine agonists. The dashed line represents the average locomotor activity of reserpinized mice receiving only vehicle administrations. Statistical differences were analyzed by one-way ANOVA followed by Newman-Keuls post-hoc test; ^*, ^**, and ^***: p < 0.05, p < 0.01 and p < 0.001, respectively, as compared with either SKF or QUIN.