The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2284-2293
Copyright ©1999 by the American Physiological Society

-AgaIVA-Sensitive (P/Q-type) and -Resistant (R-type) High-Voltage-Activated Ba²⁺ Currents in Embryonic Cockroach Brain Neurons

 ¹Groupe de Neurobiologie, Equipe Canaux et Récepteurs Membranaires, UPRES-A Centre National de la Recherche Scientifique, Université de Rennes1, 35042 Rennes Cedex; and  ²Centre National de la Recherche Scientifique-UPR, Biologie des Neurones Endocrines, Centre CNRS-INSERM de Pharmacologie-Endocrinologie, 34090 Montpellier Cedex 5, France

Benquet, Pascal, Janine Le Guen, Govindan Dayanithi, Yves Pichon, and François Tiaho. -AgaIVA-Sensitive (P/Q-type) and -Resistant (R-type) High-Voltage-Activated Ba²⁺ Currents in Embryonic Cockroach Brain Neurons. J. Neurophysiol. 82: 2284-2293, 1999. By means of the whole cell patch-clamp technique, the biophysical and pharmacological properties of voltage-dependent Ba²⁺ currents (I_Ba) were characterized in embryonic cockroach brain neurons in primary culture. I_Ba was characterized by a threshold of approximately 30 mV, a maximum at ~0 mV, and a reversal potential near +40 mV. Varying the holding potential from 100 to 40 mV did not modify these properties. The steady-state, voltage-dependent activation and inactivation properties of the current were determined by fitting the corresponding curves with the Boltzmann equation and yielded V_0.5 of 10 ± 2 (SE) mV and 30 ± 1 mV, respectively. I_Ba was insensitive to the dihydropyridine (DHP) agonist BayK8644 (1 µM) and antagonist isradipine (10 µM) but was efficiently and reversibly blocked by the phenylalkylamine verapamil in a dose-dependent manner (IC₅₀ = 170 µM). The toxin -CgTxGVIA (1 µM) had no significant effect on I_Ba. Micromolar doses of -CmTxMVIIC were needed to reduce the current amplitude significantly, and the effect was slow. At 1 µM, 38% of the peak current was blocked after 1 h. In contrast, I_Ba was potently and irreversibly blocked by nanomolar concentrations of -AgaTxIVA in ~81% of the neurons. Approximately 20% of the current was unaffected after treatment of the neurons with high concentrations of the toxin (0.4-1 µM). The steady-state dose-response relationship was fitted with a Hill equation and yielded an IC₅₀ of 17 nM and a Hill coefficient (n) of 0.6. A better fit was obtained with a combination of two Hill equations corresponding to specific (IC₅₀ = 9 nM; n = 1) and nonspecific (IC₅₀ = 900 nM; n = 1) -AgaTxIVA-sensitive components. In the remaining 19% of the neurons, concentrations 100 nM -AgaTxIVA had no visible effect on I_Ba. On the basis of these results, it is concluded that embryonic cockroach brain neurons in primary culture express at least two types of voltage-dependent, high-voltage-activated (HVA) calcium channels: a specific -AgaTxIVA-sensitive component and DHP-, -CgTxGVIA-, and -AgaTxIVA-resistant component related respectively to the P/Q- and R-type voltage-dependent calcium channels.