Isoproterenol potentiates synaptic transmission primarily by enhancing presynaptic calcium influx via P- and/or Q-type calcium channels in the rat amygdala

Abstract

The effects of selective beta-adrenergic receptor agonist isoproterenol (Iso) on neuronal excitability and synaptic transmission were investigated in brain slices of rat amygdala. Iso (15 microM) produced a long-lasting enhancement of the EPSP that was not blocked by pretreatment with 20 microM D-2-amino-5-phosphonovalerate (D-APV) alone or D-APV in combination with kynuretic acid (1 mM). The sensitivity of postsynaptic neurons to the glutamate receptor agonist AMPA was unchanged by Iso pretreatment. Superfusion of Iso reversibly blocked the after-hyperpolarization (AHP) that followed a depolarizing current pulse and caused more action potential firing. Intracellular application of a selective inhibitor of the catalytic subunit of cAMP-dependent protein kinase A blocked the effect of Iso on the AHP, whereas Iso-induced potentiation was entirely normal in the same neuron. In addition, Iso decreased the magnitude of paired-pulse facilitation, which is consistent with a presynaptic mode of action. Substituting the Mg2+ for Ca2+ in the medium completely abolished the Iso-induced enhancement of the EPSP. The effect of Iso also was blocked by low concentrations of omega-agatoxin-IVA, but not by nifedipine or omega-conotoxin-GVIA. These results suggest that Iso enhances synaptic transmission in the amygdala via a presynaptic site of action: the mechanism underlying the potentiating effect likely is attributable to an increased Ca2+ influx through P- and/or Q-type Ca2+ channels.