Two calcium‐activated potassium conductances in a subpopulation of coeliac neurones of guinea‐pig and rabbit.

Abstract

SUMMARY1. Some of the sympathetic neurones in coeliac ganglia isolated from young guinea-pigs and rabbits were found to generate action potentials followed by afterhyperpolarizations with durations of 3-8 s, much longer than those ( -300-500 ms) observed in the majority of other mammalian sympathetic neurones.2. This type of ganglion cell discharged only once at the onset of a depolarizing step unless a very high intensity current was applied. Passive and voltage-dependent membrane conductances studied in detail in guinea-pig ganglia differed from those in the two other classes of sympathetic ganglion cell described previously (Cassell, Clark & McLachlan, 1986).3. By using a single microelectrode to voltage clamp the soma, it was possible to demonstrate that both fast and slow components of the tail current following initiation of an uncontrolled 'action current' in neurones with long afterhyperpolarizations (l.a.h.) were carried by K+ ions, as was the fast tail current (time constant, T-130 ms) present in other coeliac neurones.4. The amplitude of both components of the tail current in l.a.h. neurones was markedly reduced by the replacement of Ca2+ by Mn2+, Co2+ or Ba2+ ions. These manoeuvres had similar effects on the fast tail current in other coeliac neurones.5. Both time course and amplitude of the fast tail current were increased when Ca2+ concentration was raised, or when several 'action currents' were initiated, whereas only the amplitude of the slow tail current was affected.6. The time course of the slow tail current could be described by the sum of two exponentials with Ton = 285 ms and Toff = 1-3 s at 35°C occurring after a delay of 60 ms. This current had a Qlo of about 4 between 35 and 25 'C. In contrast, the Qlo of the fast component was about 2.7. Morphine (10-6 M) and vasoactive intestinal polypeptide (106 M) had no effect on the outward tail current in l.a.h. neurones, but 5-hydroxytryptamine (106 M) was found to abolish the slow component without affecting the fast component.8. The slow tail current was activated in the subthreshold range of membrane

Postnatal Maturational Changes in Rat Pelvic Autonomic Ganglion Cells: A Mixture of Steroid-Dependent and -Independent Effects

“…A type of phasic neuron commonly found in many autonomic ganglia has a particularly long AHP carried by a second class of calcium-activated K channel, g KCa2 (Cassell and McLachlan 1987;Jobling et al 1993). Neurons of this type comprise a minority (ϳ5%) of mouse pelvic ganglion neurons (Rogers et al 1990) and were encountered only rarely in our studies on the rat.…”

Large-conductance calcium-activated potassium channels in neonatal rat intracardiac ganglion neurons

“…IK,Ca1 in sympathetic ganglion cells from rat and guinea-pig [4,19,23] is blocked by nanomolar concentrations of apamin. A second, slow Ca2+-dependent K+ current, IK,Ca2, is found in some sympathetic neurons [7,19] exhibiting a long afterhyperpolarization (LAH). This current is voltage insensitive and its activation is secondary to either Ca2+ influx or release from internal stores.…”

Characteristics of synaptic input to three classes of sympathetic neurone in the coeliac ganglion of the guinea‐pig.

“…Two firing patterns are observed, namely, short phasic bursts early after the onset of depolarization or tonic firing at lower frequency that persists for as long as depolarization is maintained. These patterns result from the presence of different voltage-and calcium-dependent potassium conductances activated at potentials less negative than resting membrane potential (RMP) and as a consequence of the initiation of action potentials Cassell & McLachlan, 1987). In some central neurones, phasic firing has been thought to result primarily from activation of K+ channels by Ca2+ influx during the action potential (Madison & Nicoll, 1984), but such a conductance change occurs in lumbar sympathetic neurones of both phasic and tonic types ).…”

“…In the coeliac ganglion of guinea-pig and rabbit (Cassell & McLachlan, 1987), some neurones discharge phasically because they also possess another Ca2+-activated K+ conductance which has slow kinetics; this conductance resembles those described in myenteric neurones (Hirst, Johnson & van Helden, 1985), nodose neurones (Fowler, Greene & Weinreich, 1985), preganglionic neurones (Yarom, Sugimori & Llina's, 1985;Yoshimura, Polosa & Nishi, 1986) and some other sympathetic neurones (Christian & Weinreich, 1988). These neurones (called LAH) have a prolonged afterhyperpolarization after the action potential that lasts several seconds; they normally discharge only once over a wide range of depolarization amplitudes (Cassell & McLachlan, 1987).…”