Egg-laying hormone of Aplysia induces a voltage-dependent slow inward current carried by Na+ in an identified motoneuron.

Abstract

This report presents studies on ionic currents in Aplysia motoneuron B16 that are modulated by the neuropeptide egg-laying hormone (ELH) of Aplysia. ELH induces an inward current that persists in the presence of the peptide and that decays slowly after ELH is removed from the bath. The effect is not due to a decrease in the delayed potassium current, the calcium-activated potassium current, or the transient potassium current. Current-voltage measurements indicate that ELH produces increased inward currents from -80 mV to =%0 mV. The effect is particularly enhan in the region from -40 mV to -25 mV where a negative slope conductance due to a voltage-dependent slow inward current is observed. The slow inward current and the response to ELH persist in saline solutions in which Ca2 is replaced with Co2+ but are eliminated when Na+ is replaced with equimolar concentrations of either Tris or N-methyl-D-glucamine. The response to ELH is unaffected by replacing chloride with equimolar acetate; by increasing the potassium concentration; or by adding tetraethylammonium chloride, CsCl, 4-aminopyridine, or tetrodotoxin to the saline bath. In addition, the reversal potentials for the ELH response (range, -28 to +46 mV), obtained from difference current-voltage relationships, are consistent with an increase in the Na+-dependent slow inward current. We conclude that at least one of the effects of ELH on B16 is to increase a slow inward current carried by Na+.Peptides are used as neurotransmitters in the central and peripheral nervous systems and are involved in the production or modulation ofvarious behaviors. While neuropeptides are ubiquitous features of both vertebrate and invertebrate nervous systems, only in relatively few cases have the cellular actions of a peptide been related to its behavioral effects (1-6).The neuropeptide egg-laying hormone (ELH) ofAplysia is synthesized in a cluster of neurosecretory neurons, the "bag cells," which are located on the rostral margin of the abdominal ganglion (7). The bag cells fire in a burst of activity termed afterdischarge (8), and they release ELH (and associated peptides) into the interstitial spaces of the ganglion. ELH diffuses into the hemolymph where, acting as a hormone, it induces egg-release from the ovotestis (9). Purified ELH induces egg-laying behavior when injected into a sexually mature Aplysia (10, 11). In addition, an inhibition of feeding also occurs, which begins before the appearance of eggs in the genital groove and persists for the duration of egg-laying behavior (11).Mayeri and colleagues (5) have documented several direct, neurotransmitter-like effects of ELH on identified neurons in the abdominal ganglion. ELH also acts on neurons in the buccal ganglion (11), including the motoneuron B16 (12, 13). B16 mediates contraction of the accessory radula closer muscle (ARC), which closes and retracts the radula (12), the organ used to grasp food.ELH, at nanomolar concentrations, induces a prolonged excitation of B16, an effect that has a slow onset (1-4 min)...

The voltage-dependent, slow inward current induced by the neuropeptide FMRFamide in Aplysia neuron R14

“…This is caused by a shift of the onset of the NSR region to a more negative membrane potential rather than a conductance decrease. Similar voltage dependencies of responses of buccal ganglion neuron B16 to Aplysia egg-laying hormone (Kirk and Scheller, 1986) and of Aplysia neurons L2-L6 to intracellular injections of CAMP (Connor and Hockberger, 1984;Hara et al, 1985) have been observed. Connor and Hockberger established by intracellular measurements with ion-sensitive electrodes that Na+ was the major charge-carrying ion in the responses of Archidoris and Aplysia neurons to intracellular injections of CAMP.…”

Calcium dependence of voltage sensitivity in adenosine 3',5'‐cyclic phosphate‐stimulated sodium current in Pleurobranchaea.

“…Green, unpublished observations). A similar current is activated by neuropeptide hormones in neurones of the buccal ganglion of Aplysia (Kirk & Scheller, 1986).…”

Dopamine‐induced depolarizing responses associated with negative slope conductance in LB‐cluster neurones of Aplysia.

“…In the latter case an involvement of K+ conductance changes cannot be excluded, since these authors neither studied the transmitter-induced response at potentials more negative than EK, nor altered external K+ concentration. However, like the DA-induced current described here, the responses studied by Kirk & Scheller (1986) disappear in Na+-free media and are enhanced in Ca2"-free media.…”

“…observed such an atypical voltage dependence in response to DA in Aplysia ganglion cells, and in view of its block by the calcium channel blocker, Cd2 , concluded that the response she observed was carried by 210 DOPAMINE-INDUCED SLOW DEPOLARIZATION Ca2". Inward currents showing a negative slope conductance were also observed in cell B16 of the buccal ganglion of Aplysia by Kirk & Scheller (1986) in response to 5-hydroxytrypatamine (5-HT) or egg-laying hormone (ELH). In the latter case an involvement of K+ conductance changes cannot be excluded, since these authors neither studied the transmitter-induced response at potentials more negative than EK, nor altered external K+ concentration.…”