Endogenous adenosine facilitates neurotransmission via A2A adenosine receptors in the rat superior colliculus in vivo.

The concentration of endogenous adenosine in the cerebrospinal fluid increased 2-3-fold of the original level in the area of rat superior colliculus after the intraperitoneal administration of an adenosine deaminase inhibitor, EHNA (erythro-9-(2-hydroxy-3-nonyl)adenosine, 10 mg/kg). Potentials evoked in the superior colliculus by optic tract stimulation were also facilitated by 120-160% of their initial amplitudes. A selective A1 adenosine receptor antagonist, DPCPX (8-cyclopentyl-1,3-dipropylxanthine), failed to reduce such EHNA-induced facilitation. However, a selective A2A adenosine receptor antagonist, KF17837 (8(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine) completely eliminated the facilitatory effects of EHNA. Northern blot analysis demonstrated abundant expression of A1 adenosine receptor mRNA in the superior colliculus. RT-PCR analysis was able to detect the concomitant expression of A2A adenosine receptor mRNA, but at levels lower than one-tenth of the striatal expression. In the superior colliculus, A2A adenosine receptors function predominantly on the facilitatory effects of adenosine, irrespective of the ubiquitous expression of A1 adenosine receptors.