Calcium-dependent plateau potentials in a crab stomatogastric ganglion motor neuron. I. Calcium current and its modulation by serotonin

Zhang, B.; Harris-Warrick, R.M.

Journal of Neurophysiology 74(5): 1929-1937


ISSN/ISBN: 0022-3077
PMID: 8592186
DOI: 10.1152/jn.1995.74.5.1929
Accession: 008257316

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1. Using current- and voltage-clamp techniques, we examined the biophysical properties of a voltage-dependent Ca-2+ current and its physiological role in plateau potential generation in the dorsal gastric (DG) motor neuron of the stomatogastric ganglion in the crab, Cancer borealis. 2. Stimulation of one of a set of identified serotonergic/cholinergic mechanosensory cells, the gastropyloric receptor (GPR) cells, induced plateau potentials in DG. A brief pressure application of serotonin (5-HT) closely mimicked the effect of the GPR cells. The 5-HT-evoked plateau in DG was not blocked by the sodium channel blocker, tetrodotoxin (TTX), or a combination of TTX with potassium channel blockers, including tetraethylammonium (TEA) and 4-aminopyridine (4-AP), and the I-h blocker, CsCl. The 5-HT-evoked plateau was eliminated by the Ca-2+ channel blockers Co-2+ and Cd-2+, suggesting that Ca-2+ entry is essential for plateau potentials in DG. During the plateau, we observed a 30% decrease in input resistance. 3. When sodium and potassium currents were blocked pharmacologically, injection of suprathreshold depolarizing current evoked all-or-none plateau-like responses lasting several seconds, even in the absence of 5-HT. This response was blocked by Ca-2+ channel blockers, further supporting a role for Ca-2+ in plateau generation. 5-HT significantly prolonged the duration of this plateau. 4. We isolated a voltage-dependent Ca-2+ current in voltage-clamped DG neurons. This current was analyzed with the use of either Ca-2+ or Ba-2+ as the charge carrier after other currents had been maximally blocked with extracellular TTX. TEA, 4-AP, and CsCl and intracellular loading with Cs+ and ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The Ca-2+ current was detectable at -45 mV, peaked at - 15 mV, and was estimated to reverse at +45 mV, Co-2+ and Cd-2+ effectively blocked the Ca-2+ current. 5. The voltage dependence of activation of the Ca-2+ current was quantantitively analyzed by fitting the voltage-conductance relation with a third power Boltzmann relation. The maximum conductance ( hivin g-A), half-activation voltage (V-A) for individual gating steps, and the slope steepness (k) were 0.19 +- 0.02 (SE) mu-S, -36.5 - 2.0 mV. and 4.4 +- 1.4 mV/e-fold, respectively. 6. 5-HT significantly potentiated the hivin g-A by apprx 42% without affecting V-A and k. 7. We conclude from our current- and voltage-clamp results that a voltage-dependent Ca-2+ current plays an important role in generating plateau potentials in the DG neuron. Enhancement of the voltage-dependent Ca-2+ current by 5-HT is one of the mechanisms for 5-HT-evoked plateau potentials.