GABA(B) AND ADENOSINE RECEPTORS MEDIATE ENHANCEMENT OF THE K+ CURRENT, I-AHP BY REDUCING ADENYLYL-CYCLASE ACTIVITY IN RAT CA3 HIPPOCAMPAL-NEURONS

1. gamma-aminobuturic acid-B (GABA(B)) and adenosine A(1) receptors, which are expressed in hippocampal pyramidal cells, are linked to pertussis toxin-sensitive G-proteins known to be coupled negatively to the enzyme adenylyl cyclase. This study investigates the electrophysiological consequences of adenylyl cyclase inhibition in response to stimulation of these receptors. 2. Single-electrode voltage-clamp recordings were obtained from CA3 pyramidal cells in rat hippocampal slice cultures in presence of tetrodotoxin. The calcium-dependent potassium current (I-AHP), which is very sensitive to intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP), was used as an electrophysiological indicator of adenylyl cyclase activity. 3. Application of baclofen (10 mu M), a selective agonist at GABA(B) receptors, or adenosine (50 mu M) each resulted in a transient decrease followed by a significant enhancement in the amplitude of evoked I-AHP. The initial reduction in amplitude of I-AHP probably reflects inadequacies in voltage clamp of electrotonically distant dendritic sites, due to the shunting caused by concomitant activation of potassium conductance by baclofen/adenosine. Comparable increases in membrane conductance in response to the GABA(A) agonist, muscimol, caused a similar reduction in I-AHP. The enhancement of I-AHP is consistent with an inhibition of constitutively active adenylyl cyclase. 4. The receptor mediating the responses to adenosine was identified as belonging to the A(1) subtype on the basis of its sensitivity to the selective antagonist 8-cyclopentyl-1,3-dipropylxanthine. 5. The beta-adrenergic agonist, isoproterenol(0.1-1 mu M), which stimulates receptors linked with G-proteins of the G(s) subtype, reduced the evoked I-AHP to 50.4 +/- 3.6% (mean +/- SE) of the control amplitude. This action of isoproterenol was curtailed significantly in the presence of baclofen (reducing I-AHP to 69.9 +/- 4% of control amplitude) or adenosine (to 76.5 +/- 4.2% of control amplitude). In contrast, baclofen did not significantly alter the ability of 8-bromo-cAMP, a membrane permeant cAMP analog, to reduce I-AHP This provides evidence for an interaction between the signal transduction pathways associated with beta-adrenergic and GABA(B)/adenosine receptors at the level of adenylyl cyclase. 6. Inhibition of cAMP-dependent protein kinase with the competitive inhibitor Rp-cAMPS( 100 mu M) enhanced I-AHP amplitude by 30 +/- 6%. Under these conditions baclofen no longer potentiated I-AHP. 7. These results indicate that activation of GABA, or Al adenosine receptors inhibits adenylyl cyclase activity resulting in an enhancement of I-AHP. This will lead to a reduction in repetitive cell firing and thus represents a further mechanism by which activation of these receptors can decrease neuronal activity.