Up-regulation of striatal adenosine A(2A) receptors with iron deficiency in rats: effects on locomotion and cortico-striatal neurotransmission

Abstract

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 3 weeks 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 3 mg/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.

Figure 1

Transferrin receptor density in three brain regions from rats with control or iron-deficient (Fe def) diet for two (a) or three weeks (b). Results are expressed as means ± S.E.M. in % of control values (n=6–8 per group). * and **: statistically different versus respective control (p<0.05 and p<0.01, respectively).

Figure 2

Adenosine A_2A receptor density in the lateral striatum from rats with control or iron-deficient (Fe def) diet for three weeks. Results are expressed as means ± S.E.M. in % of control values (n= 4 per group). *: statistically different versus control (p<0.05).

Figure 3

Locomotor activity during the exploratory period (first 90 min of measured locomotion), and after the administration of the A_2A receptor antagonist MSX-3 (1 and 3 mg/kg, i.p.) in rats fed with control or iron-deficient (Fe def) diet for three weeks. Results are expressed as means ± S.E.M. (n= 9 per group) of the accumulated ambulatory counts during both the exploratory activity period and after the injection of the respective dose of MSX-3. *,**: statistically different versus respective control group (*: p<0.05,**: p<0.01).

Figure 4

Striatal ERK1/2 phosphorylation induced by cortical electrical stimulation in rats fed with control or iron-deficient (Fe def) diet for three weeks. Results are expressed as means ± S.E.M. in % of control values (n= 5 per group) and representative Western blots. *: statistically different versus respective non-stimulated group (p<0.05).

Figure 5

A_2A receptor antagonist-induced blockade of the striatal ERK1/2 phosphorylation induced by cortical electrical stimulation in rats fed with control or iron-deficient (Fe def) diet for three weeks. (a) 3 mg/kg, i.p., of MSX-3 counteracted the effect of cortical electrical stimulation in control and iron-deficient rats. (b) 1 mg/kg, i.p., of MSX-3 did not counteracted the effect of cortical electrical stimulation in iron-deficient rats. Results are expressed as means ± S.E.M. in % of control values (n= 8 per group) and representative Western blots. **: statistically different versus respective non-stimulated group (p<0.01).

Figure 6

A_2A receptor antagonist-induced blockade of the motor output induced by cortical electrical stimulation in rats with control or iron-deficient (Fe def) diet for three weeks. (a) Representative recordings of the input signal (current pulses) delivered from the stimulator in the orofacial area of the motor cortex (upper traces) and the EMG output signal obtained from the jaw muscles (lower traces), after the administration of either saline (left traces) or the A_2A receptor antagonist MSX-3 (3 mg/kg, i.p.; right traces). (b) Representative input stimulator signal power (time constant 0.01 sec; upper traces) and output electromyographic (EMG) signal power (time constant 0.01 sec; lower traces) after the administration of either saline (left traces) or MSX-3 (right traces). (c) The systemic administration of MSX-3 (1 mg/kg, i.p.) significantly decreased the power correlation coefficient (PCC) in rats with control or iron-deficient (Fe def) diet. Results are expressed as means ± S.E.M. n= 7–8 per group); **,***: significantly different compared to respective vehicle (p<0.01 and p<0.001, respectively). MSX-3 was significantly most effective in iron-deficient rats than in controls (bifactorial ANOVA: significant drug-diet interaction effect; p<0.05).