Neurotransmitters decrease the calcium conductance activated by depolarization of embryonic chick sensory neurones.

Several neurotransmitters including noradrenaline (NA), gamma‐aminobutyric acid (GABA) and serotonin (5‐HT), and also certain peptides, decrease the duration of the Na+‐Ca2+ action potential recorded in cell bodies of embryonic chick dorsal root ganglion neurones maintained in cell culture. To determine if these agents decreased action potential duration by affecting Ca2+ channels (inward current) or K+ channels (outward current) membrane currents were recorded in voltage‐clamped sensory neurone somata. 1. Depolarization produced a prominent inward Na+ current and a smaller and slower inward Ca2+ current (ICa). The inactivation of ICa was not simply dependent on membrane potential but apparently required prior entry of Ca2+. Two components of outward current, voltage‐activated and Ca2+‐activated, were evident in most cells. 2. The effect of NA, and also of GABA and 5‐HT, was shown to result from a direct effect on ICa because: NA decreased the TTX‐resistant tail current recorded at EK and also the inward current recorded in the presence of 125 mM‐TEA and TTX (in which Na+ and K+ currents were blocked). 3. The decrease in ICa is most likely due to an effect on the number of available Ca2+ channels and/or the single Ca2+ channel conductance rather than to a shift in either the kinetics of channel activation or the Ca2+ equilibrium potential. 4. No effect of the several transmitters on the voltage‐dependent Na+ and K+ currents was observed. 5. Implications of ICa modulation for the phenomenon of presynaptic inhibition are discussed.