Adenosine‐dependent regulation of cyclic AMP accumulation in primary cultures of rat astrocytes and neurons

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Abstract
The regulation of intracellular cyclic AMP (cAMP) formation by adenosine (Ado) and its analogues has been examined in primary cultures of rat‐brain astrocytes and neurons. In the presence of the phosphodiesterase inhibitor, Ro 20‐1724, basal levels of cAMP ranged from 40–120 pmol/mg protein in both cell types. Levels were not altered by treating the cells with Ado deaminase, which suggested that they did not produce appreciable amounts of endogenous Ado under standard culture conditions. In the astrocytes, μM quantities of agonists increased cAMP up to 30‐fold higher than basal values; the relative potencies were typical of an A2 Ado receptor (NECA > Ado > R‐PIA). Neuron‐enriched cultures exhibited a maximum fourfold increase in cAMP in response to NECA; this was decreased a further eightfold when the cultures had prolonged exposure to the antimitotic agent, c‐Ara, to eliminate >98% of the nonneuronal cells. Low (nM) amounts of the Ado agonists inhibited cAMP formation in both cell types. In the astrocytes, the order of potency of inhibition of isoproterenol‐stimulated cAMP formation was typical of an A1 receptor (R‐PIA > Ado > NECA); maximum inhibition was 55–65%. Isoproterenol did not increase cAMP in the neuronal cultures. However, forskolin‐stimulated formation was effectively (∼50%) inhibited by A1 Ado agonists; inhibition was not affected by prolonged treatment with c‐Ara. From this study we tentatively concluded that rat astrocytes and neurons both contain inhibitory A1 Ado receptors, but that the stimulatory “A2” subtype is localized mainly on astrocytes.