Selective enhancement of P-type calcium currents by isoproterenol in the rat amygdala

Abstract

We investigated activation of beta-adrenergic receptor-adenylyl cyclase-cAMP cascade on the whole-cell voltage-dependent Ca2+ currents (ICa) in acutely isolated rat basolateral amygdala neurons. Application of beta-receptor agonist isoproterenol (Iso) caused a long-term enhancement of ICa. The effect of Iso was blocked by concurrent application of beta-receptor antagonist propranolol. However, delayed application of propranolol after the ICa enhancement did not affect Iso-induced potentiation, suggesting that the sustained effect was not caused by a slow washout of Iso. Nimodipine and omega-conotoxin-GVIA reduced the ICa by approximately 35 and approximately 29%, respectively, without reducing enhancement of ICa by Iso significantly. The modulation appeared to involve P-type current, because the enhancement was abolished after pretreatment with omega-agatoxin-IVA. Forskolin, an adenylyl cyclase activator, mimicked the action of Iso in enhancing ICa, and this effect was blocked by an inhibitor of cAMP cascade, indicating a cAMP-dependent mechanism. Iso also induced a long-term potentiation (LTP) of synaptic transmission, which could be prevented by P-type Ca2+ channel blockers. These results suggest that P-type Ca2+ channels were selectively upregulated in the basolateral amygdala neurons, and enhancement of P-type currents could contribute to presynaptic form of LTP.

Fig. 1.

Inhibition of Iso-induced synaptic potentiation by P-type Ca²⁺ channel blockers. The EPSP amplitude was plotted against time. Bars denote periods of application of Iso (15 μm), ω-AgTX (10 nm), or nimodipine (1 μm). The slices were incubated in 10 nm ω-AgTX or 1 μm nimodipine for at least 30 min before being transferred to the recording chamber where the drug was present at the same concentration.

Fig. 2.

Iso does not affect the IPSP. Monosynaptic IPSP was evoked in the presence of 10 μm6-cyano-7-nitroquinoxaline-2,3-dione and 50 μmd-2-amino-5-phosphonovalerate. At the end of experiments, 20 μm bicuculline (Bic) was added to confirm that the IPSP was indeed mediated by GABA_Areceptors.

Fig. 3.

Pharmacological separation ofI_Ca in amygdalar neurons. Plot of peakI_Ca versus time is shown for an experiment in which 1 μm nimodipine, 1 μm nimodipine plus ω-CgTX, 10 nm nimodipine plus ω-CgTX plus ω-AgTX, and 100 μm Cd⁺⁺ were sequentially applied to an amygdala neuron.

Fig. 4.

Persistent enhancement ofI_Ca by Iso. A, The percent change of peak I_Ca was plotted as a function of time.Bar denotes the periods of delivery of Iso (15 μm) or Iso plus propranolol (1 μm). The Iso-induced potentiation was prevented by concurrent application of propranolol. B, Delayed application of propranolol afterI_Ca enhancement did not affect Iso-induced potentiation significantly.

Fig. 5.

Selective blockade of Iso-induced enhancement ofI_Ca by ω-AgTX. A, The percent change of I_Ca was plotted as a function of time. The isolated amygdalar neurons were incubated in 1 μm ω-CgTX or 10 nm ω-AgTX for at least 10 min before recordings were made. B, Thebars represent the enhancement ofI_Ca (mean ± SEM) by Iso in control and after application of various types of Ca²⁺ channel blockers.

Fig. 6.

Sustained enhancement ofI_Ca by forskolin and its inhibition by ω-AgTX and Rp-cAMPS. A, The percent change of peakI_Ca was plotted as a function of time.Bar denotes the period of application of 25 μm forskolin. In 10 nm ω-AgTX-treated neurons, the neurotoxin was present for at least 10 min before and during the period of recording. B, The effect of forskolin was abolished by Rp-cAMPS. The isolated neurons were incubated with 25 μm Rp-cAMPS for at least 20 min before and during the period of recording.

Fig. 7.

Iso does not affect the delayed rectifier K⁺ currents. Delayed rectifier K⁺currents were elicited by depolarizing pulses over the potential ranges of −60 to +60 mV from a holding potential of −70 mV in the presence of 100 μm Cd⁺⁺ and 5 mm4-aminopyridine.