Neuron, Vol. 10, 83-88, January, 1993, Copyright 0 1993 by Cell Press Enhancement of p-Adrenergic Responses by Gi-Linked Receptors in Rat Hippocampus Rodrigo Andrade Department of Pharmacological and Physiological Science St. Louis University School of Medicine St. Louis, Missouri 63104 Summary Many excitable cells express a class of neurotransmitter receptors functionally defined by their ability to increase potassium conductance through C proteins of the Gil G, class that directly activate the potassium channels. Biochemical studies have shown that these same recep- tors can also inhibit forskolin-stimulated adenylyl cy- clase, although the functional significance of this effect remains unclear. In this study electrophysiological tech- niques were used to examine how activation of serotonin and y-aminobutyric acid receptors belonging to this class affect g-adrenergic responses signaled through adenylyl cyclase. Surprisingly, activation of these receptors not only failed to inhibit but actually enhanced g-adrenergic responses. These observations are consistent with evi- dence indicating that G-linked receptors can enhance the ability of C, to stimulate certain adenylyl cyclases. Introduction Electrophysiological studies havecharacterized afam- ily of neurotransmitter receptors that act through G proteins(Andradeetal.,1986)oftheGi/G,class(Simon et al., 1991) to elicit a membrane hyperpolarization and inhibit excitable cells (Nicoll et al., 1990; Birn- baumer et al., 1990). Previous work in several systems has shown that this effect is mediated by a class of inwardly rectifying potassium channels whose activa- tion is signaled by a direct interaction between the G protein and the ion channel (Pfaffinger et al., 1985; Andradeet al., 1986; Kurachi et al., 1986). Interestingly, parallel biochemical studies have shown that these receptors are also capable of inhibiting adenylyl cy- clase. This raises the question of the possible func- tional roleofthesechanges in cyclic nucleotide levels. ln particular, does the ability of these receptors to in- hibit adenylyl cyclase allow them to reduce signaling through G,-adenylyl cyclase-CAMP? To address this question I have determined the effects of 5-hydroxy- tryptamine 1 (5-HT,) and y-aminobutyric acid B (GABAtJ receptors, two receptor subtypes known to increase potassium conductance and inhibit adenylyl cyclase, on physiological responses signaled through the G,- adenylyl cyclase-CAMP signaling cascade in single py- ramidal neurons of the rat hippocampus. Results Intracellular recordings were obtained from pyrami- dal cells of the CA1 region in in vitro rat brain slices. Previous studies have demonstrated that administra- tion of norepinephrine (NE) to these cells results in a decrease in the calcium-activated afterhyperpolariza- tion (AHP) present in these cells (Madison and Nicoll, 1986a; Haas and Konnerth, 1983) (Figure IA). This re- sponse is mediated by the activation of 8-adrenergic receptors, which in turn stimulate adenylyl cyclase, increase intracellular CAMP, and signal the reduction of this afterpotential (Madison and Nicoll, 198613). This well-established 8-adrenergic response in the hippo- campus provides an avenue for testing the possible functional significance of the regulation of cyclic nu- cleotides by 5-HT1 and GABAe receptors at the level of a single neuron. Therefore we first examined whether stimulation of 5-HT, receptors modulated the ability of NE to signal a reduction in the AHP. As previously reported (Andrade and Chaput, 1991) administration of 5-carboxyamidotryptamine (5-CT, 300 nM) elicited a membrane hyperpolarization that was associated with an increase in membrane conductance (Figure IA). This hyperpolarization reflects the activation of 5-HTIA receptors by 5-CT in these cells (Andrade and Nicoll, 1987; Andrade and Chaput, 1991). As could be expected from an indirect effect of the increase in membrane conductance, the AHP amplitude was de- creased by the administration of 5-CT. Nevertheless, when NE (0.1-I PM) was administered in the presence of 5-CT, its ability to reduce the AHP was significantly enhanced (Figure 1). This enhancement in the ability of NE to reduce the AHP was observed in all 6 cells tested. Overall, in this group of cells 5-CT increased the effect of NE by approximately 50%, from a mean reduction in AHP amplitude of 3.5 mV, observed when NE was applied alone, to 5.5 mV, when NE was admin- istered in thepresenceof 5-CT(p<O.OOl, paired ttest). This enhancement in the effect of NE was observed in spite of the increase in conductance, which would work against observing such an increase. To compen- sateforthe5-CT-induceddecreasein membraneresis- tance, the effects of NE were normalized with respect to the AHP amplitude immediately preceding its ad- ministration (Figure IC). Under these conditions 5-CT was found to enhance the ability of NE to reduce the AHP, from 26% to 53% (p < 0.001; Figure 2A). Receptors of the ~-HTIA and GABAt, subtypes in the rat hippocampus hyperpolarize pyramidal cells of the CA1 region through a shared mechanism that results in the activation of acommon subpopulation of potas- sium channels (Andrade et al., 1986). Therefore it was of interest to determine whether activation of the GABAe receptors in these cells could also potentiate B-adrenergic responses. As previously reported (New- berry and Nicoll, 1984; Andrade et al., 1986), adminis- tration of the selective GABAe agonist baclofen (IO- 30 PM) resulted in a membrane hyperpolarization. In addition, as seen with 5-CT, concurrent with the hy- perpolarization there was a significant enhancement in the ability of NE to reduce the AHP. In this group