Abstract
I used sharp intracellular electrodes to record from parasol cells in the semi-isolated crayfish brain to investigate pacemaker currents. Evidence for the presence of the hyperpolarization-activated inward rectifier potassium current was obtained in about half of the parasol cells examined, where strong, prolonged hyperpolarizing currents generated a slowly-rising voltage sag, and a post-hyperpolarization rebound. The amplitudes of both the sag voltage and the depolarizing rebound were dependent upon the strength of the hyperpolarizing current. The voltage sag showed a definite threshold and was non-inactivating. The voltage sag and rebound depolarization evoked by hyperpolarization were blocked by the presence of 5–10 mM Cs2+ ions, 10 mM tetraethyl ammonium chloride, and 10 mM cobalt chloride in the bathing medium, but not by the drug ZD 7288. Cs+ ions in normal saline in some cells caused a slight increase in mean resting potential and a reduction in spontaneous burst frequency. Many of the neurons expressing the hyperpolarization-activated inward potassium current also provided evidence for the presence of the transient potassium current IA, which was inferred from experimental observations of an increased latency of post-hyperpolarization response to a depolarizing step, compared to the response latency to the depolarization alone. The latency increase was reduced in the presence of 4-aminopyridine (4-AP), a specific blocker of IA. The presence of 4-AP in normal saline also induced spontaneous bursting in parasol cells. It is conjectured that, under normal physiological conditions, these two potassium currents help to regulate burst generation in parasol cells, respectively, by helping to maintain the resting membrane potential near a threshold level for burst generation, and by regulating the rate of rise of membrane depolarizing events leading to burst generation. The presence of post-burst hyperpolarization may depend upon IA channels in parasol cells.
Highlights
Periodic electrical activity in neurons and other rhythmic cells largely depends for its expression on intrinsic membrane currents
I undertook the present study to determine the possible presence of the pacemaker currents Ih and inactivated potassium current (IA) within the parasol cell population
Data presented in this paper suggest that about half of parasol cells in neuropil II of the crayfish hemiellipsoid body exhibit a hyperpolarization-activated current
Summary
Periodic electrical activity in neurons and other rhythmic cells largely depends for its expression on intrinsic membrane currents. These currents are responsible for the oscillations in membrane potential, which act to generate the cellular output and are at least partially responsible for activation of the currents themselves. Motor neurons of the pyloric network in the lobster and crab stomatogastric ganglion (STG) are readily-studied model systems to investigate synaptic interactions among rhythmically active neurons, modulation of network and cellular properties by naturally occurring biogenic amines and peptides, and intrinsic properties of the motor neurons themselves [14, 15, 16] In the dorsal gastric motor neuron of the crab STG, for example, the currents believed to modulate potential changes leading up to plateau potentials that underlie a spike burst envelope are a transient, depolarization-activated and inactivated potassium current (IA) with relatively fast kinetics and a hyperpolarization-activated, noninactivating cation current (Ih) with very slow kinetics [17,11]
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