Behavioral characteristics of centrally administered adenosine analogs

Involvement of A2 adenosine receptors in spinal mechanisms of antinociception

Preliminary investigations suggest that spinal adenosine induces significant behavioral effects and may interact with descending antinociceptive systems stimulated by morphine administered intracerebroventricularly (i.c.v.). In the present study, rank order potencies for antinociception induced by adenosine agonists administered intrathecally (i.t.) were determined. Interactions of adenosine agonists (i.t.) with morphine (i.c.v.)-induced antinociception were also examined. Dose-dependent antinociception, as measured in tail flick and hot plate assays, was observed in mice administered adenosine or adenosine agonists i.t. Rank order potencies were 5'-N6-ethylcarboxamidoadenosine (NECA) greater than N6-(R-phenylisopropyl)-adenosine (R-PIA) greater than 2-chloroadenosine (CADO) greater than N6-(S-phenylisopropyl)-adenosine (S-PIA) greater than adenosine. Rank order potencies were identical for adenosine agonist (i.t.) synergism with morphine (i.c.v.)-induced antinociception. Further, i.t. injections of NECA or nitrobenzylthioinosine (NBI), an adenosine reuptake inhibitor, were able to potentiate morphine (i.c.v.)-induced antinociception. Hind limb paralysis induced by high doses of adenosine agonists (i.t.) was dissociated from antinociceptive effects. Rank order potencies determined in our studies support involvement of A2 adenosine receptors in spinal mechanisms of antinociception. In addition, these results confirm spinal adenosine interactions with antinociceptive systems stimulated by i.c.v. injections of morphine.

[3H]dipyridamole binding to guinea pig brain membranes: possible heterogeneity of central adenosine uptake sites

The binding of [3H]dipyridamole ([3H]DPR) to guinea pig brain membranes is described and compared to that of [3H]nitrobenzylthioinosine ([3H]NBI). The binding of [3H]DPR is saturable, reversible, and specific with pharmacologic evidence indicating that this ligand is binding to the adenosine uptake site. Compared to [3H]NBI the binding of [3H]DPR is of higher capacity (Bmax = 208 +/- 16 fmol/mg protein for [3H]NBI and 530 +/- 40 fmol/mg protein for [3H]DPR) and lower affinity (KD = 0.35 +/- 0.02 nM for [3H]NBI and 7.6 +/- 0.7 nM for [3H]DPR). The adenosine uptake inhibitors are the most potent inhibitors of binding (Ki of 10(-8)-10(-7) M) whereas adenosine receptor ligands such as cyclohexyladenosine, 2-chloroadenosine, and various methylxanthines are several orders of magnitude less potent (Ki 10(-5)-10(-2). The inhibition of [3H]DPR binding by NBI is biphasic, with only 40% of binding being susceptible to inhibition of NBI concentrations less than 10(-5) M. The tissue distribution of [3H]DPR binding parallels that of [3H]NBI although in most cases significantly more sites are observed with [3H]DPR. Calcium channel blocking agents such as nifedipine, nimodipine, and verapamil are also inhibitors of [3H]DPR binding with potencies in the micromolar range. The data are consistent with [3H]DPR being a useful additional ligand for the adenosine uptake site and provide evidence that multiple uptake binding sites exist of which only about 40% are NBI-sensitive.

Dissimilarities between benzodiazepine-binding sites and adenosine uptake sites in astrocytes and neurons in primary cultures

The question whether the benzodiazepine receptor site in astrocytes or in neurons might be identical to the adenosine uptake site was studied by determining pharmacological profiles, inhibition types, and the effects of benzodiazepine antagonists in primary cultures of either astrocytes or neurons. Fourteen different benzodiazepines and five different adenosine uptake inhibitors displaced [3H] diazepam and inhibited adenosine uptake in both astrocytes and neurons. However, the rank orders (determined as IC50 values) with which these two parameters were affected were profoundly different, indicating dissimilarities between these two sites. For several of the compounds a difference in inhibition type (competitive vs. noncompetitive) was observed between the benzodiazepine-binding site and the adenosine uptake site in astrocytes and/or neurons, which further corroborated the conclusion of a difference between the benzodiazepine-binding site and the adenosine uptake site. Finally, the neuronal benzodiazepine antagonists RO 15-1788 and CGS-8216 and the astrocytic benzodiazepine antagonist PK 11195, which reverse the action of benzodiazepines, were not able to reverse inhibition of adenosine uptake by diazepam but exerted an inhibitory effect of their own.

Potentiation of the depression by adenosine of rat cerebral cortical neurones by progestational agents

The effects of four progestational agents pregnenolone sulphate, cyproterone acetate, norethindrone acetate and progesterone, on adenosine-evoked depression of the firing of rat cerebral cortical neurones have been studied. When applied iontophoretically, pregnenolone sulphate, cyproterone, and norethindrone enhanced the actions of iontophoretically applied adenosine and failed to potentiate the depressant effects of adenosine 5'-N-ethylcarboxamide and gamma-aminobutyric acid. Cyproterone acetate (50 micrograms kg-1) and progesterone (200 micrograms kg-1) administered intravenously enhanced the depressant actions of iontophoretically applied adenosine. When applied by large currents, cyproterone, and less frequently norethindrone, depressed the firing of cerebral cortical neurones. The depressant effects of cyproterone were antagonized by caffeine. Pregnenolone sulphate tended to excite cortical neurones but neither this action, nor its potentiation of adenosine were reproduced by application of sulphate ions. It is hypothesized that some of the psychotropic actions of progestational agents may involve an enhancement of 'purinergic' tone in the central nervous system.