Contraction, Serotonin-Elicited Modulation, and Membrane Currents of Dissociated Fibers of Aplysia Buccal Muscle

Abstract

Feeding muscles of the buccal mass of Aplysia are innervated by cholinergic and serotonergic neurons. Buccal muscle 15 contracts in response to acetylcholine (ACh). During feeding arousal, ACh-elicited contraction of muscle 15 is potentiated by serotonin (5-HT). This paper demonstrates a dissociated cell preparation of muscle 15 in which cellular mechanisms regulating contraction can be investigated. Dissociated muscle fibers contracted in response to both KCl and ACh. Serotonin (10-6 M) significantly potentiated the shortening caused by both KCl and ACh. Potentiation lasted at least 4 min, similar to potentiation in intact muscles. Four types of currents recorded by patch clamp methods are illustrated. With 540 mM KCl in the patch electrode, stretch-activated channels having a chord conductance of 150 pS are observed in on-cell patches. In whole cell configuration, ACh elicits inward current at a holding potential of -60 mV. With high potassium in the electrode, depolarization elicits an outward current. The voltage-dependent outward current is blocked with cesium in the electrode and 4-aminopyridine and tetraethylammonium outside the cells. The remaining voltage-dependent inward current is calcium dependent. The voltage-dependent inward and outward currents are activated within the range of depolarization produced by ACh and may therefore play roles in regulating contractile responses elicited by ACh.

Serotonin Injections Induce Metamorphosis in Larvae of the Gastropod Mollusc Ilyanassa obsoleta

“…5-HT plays numerous roles in molluscan nervous systems (Walker, 1986) and major groups of serotonergic neurons have been identified in several gastropod taxa (Koshtoyants et al, 1961;Walker 1986;Jahan-Parwar et al, 1987;Goldberg and Kater, 1989;Diefenbach and Goldberg, 1990;McPherson and Blankenship, 1991;Fitzgerald and Carew, 1991). For example, 5-HT regulates neurite extension and growth-cone motility in the freshwater snail Helisoma trivolvis (Price and Goldberg, 1993); stimulates motor neurons in Aplysia california (Ram et al, 1991); plays an important role in molluscan peripheral tissues, especially the heart (Walker, 1986); and heightens motor activity in embryonic gastropods in a developmentally sensitive manner (Koshtoyants et al, 1961;Diefenbach et al, 1991). In these young animals exogenous serotonin increases ciliary activity during early embryonic stages, then sensitivity to 5-HT decreases to an intermediate level.…”

Development of serotonin-like immunoreactivity in the embryos and larvae of nudibranch mollusks with emphasis on the structure and possible function of the apical sensory organ

“…There is precedence for this suggestion in that 5-HT has been shown to modulate muscle contractility in adult opisthobranchs (see, e.g., Ram et al, 1991;.…”

Two Types of Voltage-Gated K+Currents in Dissociated Heart Ventricular Muscle Cells of the SnailLymnaea stagnalis

“…Those that have been published have concentrated on the unstriated muscles of invertebrates, which are most similar to vertebrate smooth muscle. These include studies on the accessory radula closer (ARC) muscle of Aplysia (Brezina et al 1994a-c;Brezina and Weiss 1995;Ram et al 1990Ram et al , 1991 and its antagonist the accessory radula opener (ARO) (Scott et al 1997), the buccal mass retractor muscles of Philine (Dorsett and Evans 1991) and most recently the parapodial swim muscles of Aplysia (Laurienti and Blankenship 1996a,b). These muscles contained a mixture of Ca 2ϩ and K ϩ currents that was similar to those recorded in invertebrate neurons and are related to those present in vertebrate muscle.…”