Characterisation of central adenosine A 1 receptors and adenosine transporters in mice lacking the adenosine A 2a receptor

Abstract

The present study was designed to assess whether adenosine A 2a receptor knockout mice exhibit altered purine utilisation in brain nuclei. Specifically, the properties of adenosine transporters and adenosine A 1 receptors were characterised in brain membranes and on slide-mounted sections. The B MAX for [ 3H]nitrobenzylthioinosine ([ 3H]NBTI) binding (adenosine transporter density) was significantly reduced in brainstem membranes of homozygotes (560±52 fmol/mg protein, n=5, P<0.05, Kruskal–Wallis ANOVA) compared to wildtype (1239±213 fmol/mg protein) and heterozygous mice (1300±558 fmol/mg protein). Quantitative autoradiography data indicated that [ 3H]NBTI binding in the medulla oblongata of heterozygous mice was seen to decrease significantly ( P<0.05) in the subpostremal nucleus tractus solitarius (NTS), medial NTS, inferior olive and area postrema (AP). On the other hand, in the homozygous mice a decrease was seen in the medial NTS and AP. In the pons, [ 3H]1,3-dipropyl-8-cyclopentylxanthine ([ 3H]DPCPX) (adenosine A 1 receptor density) binding increased significantly ( P<0.05, Kruskal–Wallis ANOVA) in the lateral parabrachial nucleus, caudal pontine reticular nucleus and locus coeruleus of homozygotes compared to wildtype. In higher brain centres, [ 3H]NBTI binding was reduced in the paraventricular thalamic nucleus of both heterozygous and homozygous mice, whereas [ 3H]DPCPX binding was reduced in the hippocampus and lateral hypothalamus of heterozygotes. In homozygotes, [ 3H]DPCPX binding in the hippocampus increased compared to wildtype mice. The present study indicates that deletion of the A 2a receptor may have contributed to region-specific compensatory changes in purine utilisation in brain nuclei associated with autonomic, neuroendocrine and behavioural regulation.