Biochemical Pharmacology, Vol. 37, No. 4, pp. 715-723. 1988. hinted in Great Britain. CME-2952/8@ $3.00 + 0.00 Q 1988. Pergamon Journals Ltd. INFLUENCE OF ROLIP~M ON THE CYCLIC 3’,5’- ADENOSINE MONOPH~SPHATE RESPONSE TO HISTAMINE AND ADENOSINE IN SLICES OF GUINEA-PIN CEREBRAL CORTEX JILL DONALDSON, ANTHONY M. BROWN and STEPHEN J. HILL Department of Physioiogy and Pharmacology, Medical School, Queen’s Medical Centre, Clifton Boulevard, Nottingham, NG7 ZUH, U.K. (Received 14 May 1987; accepted 21 August 1987) Abstract-The effect of the phosphodiesterase (PDE) inhibitor rolipram on the cyclic AMP responses to adenosine, histamine and combinations of these two agonists, was examined in l~H]adenine-labelled slices of guinea-pig cerebral cortex. Constant levels of [3H]-cyclic AMP were achieved within 10 min of agonist addition, both in the presence and absence of rolipram (0.1 m&l). Histamine (1 mM) produced an s-fold increase in f3H]-cyclic AMP (compared with basal) which was increased 7-fold by rofipram. The responses to adenosine (0.1 mM) and adenosine and histamine in combination were larger than that to histamine alone (46fold or more compared with basal) but the potentiation by rolipram was much smaller (25fold or less). With both agonists the effect of rolipram was dose-dependent, the steady state [3HJ-cyclic AMP leveb increasing I-2-fold for a LO-fold increase in rolipram concentration. Removal of the histamine or adenosine stimulus once steady state had been reached resulted in a rapid fall in [3H]-cyclic AMP levels with a half time of less than 5 min. Rolipram (0.1 mM) did not significantly alter the initial rates of fall in [‘HI-cyclic AMP levels but increased the time taken for them to return to basal levels. The findings of higher steady state levels of cyclic AMP in the presence of rolipram, together with an almost unaltered rate of cyclic AMP turnover, are consistent with an interaction of rolipram with PDE which is overcome by an increase in cyclic AMP concentration. However, the relatively smaller effects of rolipram on the higher steady levels of cyclic AMP produced by adenosine and the rather shallow dose-de~ndence of the PDE inhibitor on the responses to both agonists are inconsistent with a simple competitive inhibition of total PDE activity in responding cells. The results can be explained, however, by the involvement of different forms of PDE, with the rolipram-sensitive, calcium-independent form dominating at low cyclic AMP levels and the rolipram-insensitive, calcium-dependent form becoming more important when cyclic AMP levels are higher. It is now well established that a number of cellular responses (e.g. alterations in membrane potential, cell growth and secretion) elicited by neuro- transmitters and hormones are mediated by changes in intracellular cyclic AMP levels [l-3]. There are several ways in which these agents can act. Some alter the rate of cyclic AMP generation directly by interacting with receptors coupled to guanine nucleo- tide regulatory components of adenylate cyclase [4]. Receptors coupled to the Ns subunit (e.g. histamine Hz, adenosine AZ) stimulate the cyclase, while those coupled to the Ni subunit (e.g. opiate, muscarinic) inhibit it [4-T]. In addition, some neurotransmitte~ can modify cyclic AMP levels in brain tissues via receptors whose link to the enzymes involved in cyclic AMP turnover seems to be indirect [&Xl]. For example, in guinea-pig cerebral cortical slices, the accumulation of cyclic AMP produced by stimulation of adenosine AZ-receptors can be augmented by histamine H1-receptor stimulation {12-1.51. It remains to be established whether the potentiating effect on cyclic AMP levels is exerted by stimulation of the cyclase or by inhibition of the enzyme which * Abbreviations used: PDE, phosphodiesterase. breaks down cyclic AMP, phosphodiesterase (PDE).* Because of the important role of cyclic AMP in cell unction, there has been interest recently in modulating intracellular cyclic AMP concentration at the post-receptor level, with the use of inhibitors of PDE. We were interested in the potential use of these agents to study the mechanism by which histamine HI-receptor stimulation potentiates ade- nosine-induced cyclic AMP accumulation in guinea- pig brain slices. However, the choice of PDE inhibi- tor for this purpose was made difficult by the fact that many PDE inhibitors such as the alkyl xanthines (e.g. 3-isobutyl-1-methylxanthine) are also potent adenosine receptor antagonists [ 16-lS]. One PDE inhibitor which does not appear to antagonise adenosine receptors in brain slice prep arations is the antidepressant drug rolipram, which potentiates the cyclic AMP responses to both histamine and adenosine 119). This drug is a rather potent and competitive inhibitor of the cyclic AMP specific, calcium-independent isoform of PDE [ZO,21 J. This selective action on one particular form of PDE raised the possibiIity that its effect on the changes in cyclic AMP levels might throw some light on the form of PDE involved in cyclic AMP turnover 715