Pharmacological characterization of calcium currents and synaptic transmission between thalamic neurons in vitro

Abstract

We recorded from pairs of cultured, synaptically connected thalamic neurons. Evoked excitatory postsynaptic currents (EPSCs) reversed at +17 mV and were blocked reversibly by 1 mM kynurenic acid, a glutamate receptor antagonist. NMDA and non-NMDA receptors mediated excitatory post-synaptic responses, as shown by selective block of EPSC components with 50 μM (±)-2- amino-5-phosphonopentanoic acid and 10 μM 6,7-dinitroquinoxaline-2,3-dione, respectively. Inhibitory postsynaptic responses were evoked less frequently and were blocked by the GABA(A) receptor antagonist (-)-bicuculline methochloride. The pharmacological profiles of whole-cell calcium currents and evoked EPSCs were compared. With 50 μM cadmium chloride (Cd), whole-cell low voltage-activated (LVA) calcium currents were reduced in amplitude and high voltage-activated (HVA) calcium currents and excitatory synaptic transmission were completely blocked. This suggests that the residual calcium influx through LVA channels into the presynaptic terminal does not suffice to trigger transmitter release. A saturating concentration of ω-conotoxin GVIA (ω-CgTx) (2.5 μM) blocked one-third of whole-cell HVA calcium currents and evoked EPSCs. The dihydropyridine nifedipine (50 μM) reversibly reduced whole-cell HVA calcium currents in a voltage-dependent manner but not excitatory synaptic transmission. Cd and ω-CgTx did not alter amplitude distributions of miniature EPSCs, demonstrating that the inhibition of synaptic transmission was due to block of to presynaptic calcium channels. We conclude that excitatory glutamatergic transmission in thalamic neurons in vitro was mediated mainly by HVA calcium currents, which were insensitive to ω-CgTx and nifedipine.