Two distinct modulatory effects on calcium channels in adult rat sensory neurons.
D-ala2-D-leu5-enkephalin (100 to 1000 nM) reduces HVA Ca2+ currents of approximately 60% in 92% of the adult rat sensory neurons tested. In 80% of the cells sensitive to enkephalin, the reduction in Ca2+ current amplitude was associated with a prolongation of the current activation that was relieved by means of conditioning pulses in a potential range only about 10 mV positive to the current activation range in control conditions. The time course of the current activation was fitted to a single exponential in control, (tau = 2.23 msec +/- 0.14 n = 38) and double exponential with enkephalin, (tau 1 = 2.18 msec +/- 0.25 and tau 2 = 9.6 msec +/- 1, test pulse to -10 mV, 22 degrees C). A strong conditioning depolarizing prepulse speeded up the activation time course, completely eliminating the slow, voltage-sensitive exponential component, but it was only partial effective in restoring the current amplitude to control values. The voltage-independent inhibitory component that was not relieved could be recovered only by washing out enkephalin. In the remaining 20% of the cells affected, enkephalin decreased Ca2+ current amplitude without prolongation of Ca2+ channel activation. In these cases the conditioning voltage pulse was not effective in relieving the inhibition that persisted also at strong positive test potentials, on the outward currents. The voltage-dependent inhibition occurred slowly after enkephalin superfusion (tau congruent to 12 sec), whereas the voltage-independent one developed about ten times more rapidly. Dopamine (100 microM) could also induce both voltage-dependent and independent modulations. In some sensory neurons the two different effects were separately induced by the two substances. GTP-'y-S (100 ,uM) intracellularly perfused mimicked both the modulatory effects. The two modulations may have different functions in processing nociceptive inputs.
Institute of Human Physiology II, University of Milan, Italy.
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Arrigoni
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D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D004298:Dopamine / Q000494:pharmacology; D004594:Electrophysiology; D016308:Enkephalin, Leucine-2-Alanine / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D009475:Neurons, Afferent / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D015220:Calcium Channels; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D016308:Enkephalin, Leucine-2-Alanine; D004298:Dopamine