Up-regulation of striatal adenosine A_2A receptors with iron deficiency in rats. Effects on locomotion and cortico-striatal neurotransmission.

Brain iron deficiency leads to altered dopaminergic function in experimental animals, which can provide a mechanistic explanation for iron deficiency-related human sensory-motor disorders, such as Restless Legs Syndrome (RLS). However, mechanisms linking both conditions have not been determined. Considering the strong modulation exerted by adenosine on dopamine signaling, one connection could involve changes in adenosine receptor expression or function. In the striatum, presynaptic A_2A receptors are localized in glutamatergic terminals contacting GABAergic dynorphinergic neurons and their function can be analyzed by the ability of A_2A receptor antagonists to block the motor output induced by cortical electrical stimulation. Postsynaptic A_2A receptors are localized in the dendritic field of GABAergic enkephalinergic neurons and their function can be analyzed by studying the ability of A_2A receptor antagonists to produce locomotor activity and to counteract striatal ERK1/2 phosphorylation induced by cortical electrical stimulation. Increased density of striatal A_2A receptors was found in rats fed during 3weeks with an iron-deficient diet during the post-weaning period. In iron-deficient rats, the selective A_2A receptor antagonist MSX-3, at doses of 1 and 3mg/kg, was more effective at blocking motor output induced by cortical electrical stimulation (presynaptic A_2A receptor-mediated effect) and at enhancing locomotor activation and blocking striatal ERK phosphorylation induced by cortical electrical stimulation (postsynaptic A_2A receptor-mediated effects). These results indicate that brain iron deficiency induces a functional up-regulation of both striatal pre- and postsynaptic A_2A receptor, which could be involved in sensory-motor disorders associated with iron deficiency such as RLS.