Norepinephrine stimulated cyclic AMP accumulation in rat limbic forebrain slices: partial mediation by a subpopulation of receptors with neither alpha nor beta characteristics.

Both isoproterenol and norepinephrine (NE) increase cyclic AMP in slices of the rat limbic forebrain and the responses are enhanced in the presence of the phosphodiesterase inhibitor RO 20-1724. However, even in the presence of RO 20-1724, no accumulation of cyclic AMP was observed after the addition of dopamine, serotonin or the alpha-agonists methoxamine and phenylephrine. This suggests that these agents do not activate adenylate cyclase in this preparation or that their respective receptors--unlike the beta-receptor--are not coupled to adenylate cyclase. Isoproterenol, which has a high affinity for this adenylate cyclase system but only 20-30% of the maximal activity of NE, does not interfere with the agonist activity of NE. Moreover, the effect of isoproterenol is not additive with that of NE thus suggesting that isoproterenol is acting on a subpopulation of NE receptors. The results indicate that two populations of NE receptors coupled to adenylate cyclase are present in slices of rat limbic forebrain: one which has beta-characteristics and the other with neither alpha- nor beta-characteristics based on agonist studies.