Biophysical and pharmacological characterization of voltage-sensitive calcium currents in neonatal rat inferior colliculus neurons

Calcium conductances have been found in neonatal inferior colliculus neuron s, however the biophysical and pharmacological profiles of the underlying c alcium currents have not yet been characterized. In this study, we examined which types of voltage-activated calcium currents comprise the whole-cell inward current of neonatal inferior colliculus neurons (10-22 mu m in diame ter). On the basis of their voltage-dependence and pharmacological sensitiv ities, three major components of barium currents were identified. A low thr eshold voltage-activated current that activated around -70 mV, a mid thresh old voltage-activated current that activated near -50 mV, and a high thresh old voltage-activated current that activated around -40 mV. Low and mid thr eshold voltage-activated currents were present in 33% and 41% of the record ings, respectively, whereas high threshold voltage-activated currents were recorded in all inferior colliculus neurons tested. Nickel chloride (50 mu M) and U-92032 (1 mu M), which both block low threshold voltage-activated c urrents, reduced the amplitude of low threshold voltage activated peak curr ents at a test potential of -60 mV by 72% and 10%, respectively. In additio n, 50 mu M nickel chloride and 1 mu M U-92032 reduced the amplitude of mid threshold voltage-activated peak currents measured at -20 mV by 55% and 21% , respectively. Further pharmacological analysis indicated the presence of multiple types of high threshold voltage-activated currents in neonatal inf erior colliculus neurons. The dihydropyridine nimodipine (1 mu M), a select ive L-type current antagonist, reduced the amplitude of high threshold volt age-activated peak currents by 25%. In addition, FPL 64176 (1 mu M), a non- dihydropyridine L-type current agonist caused a dramatic 534% increase in t he amplitude of the slow sustained component of the tail current measured a t -40 mV. These data indicate that inferior colliculus neurons express L-ty pe channels. omega-Conotoxin GVIA (1 mu M), a selective blocker of N-type c urrent, inhibited high threshold voltage-activated peak currents by 28% ind icating the presence of N-type channels. omega-Agatoxin IVA (300 nM), a pot ent P/Q-type antagonist, reduced high threshold voltage-activated peak curr ents by 27%, suggesting that inferior colliculus neurons express P/Q-type c hannels. Concomitant application of nimodipine (1 mu M), omega-conotoxin GV IA. (1 mu M) and omega-agatoxin TVA (300 nM) onto inferior colliculus neuro ns decreased the control high threshold voltage-activated peak currents onl y by 62%. Thus, inferior colliculus neurons may express at least one more type of cal cium current in addition to low and mid threshold voltage-activated current s and L-type, N-type and P/Q-type high threshold currents. (C) 2000 IBRO. P ublished by Elsevier Science Ltd.