David J. Prior

WATER-REGULATORY BEHAVIOUR IN TERRESTRIAL GASTROPODS

1. Terrestrial snails and slugs are exceedingly susceptible to dehydration due to evaporative water loss from the integument and lung surface, and the deposition of a dilute mucous trail. Active slugs can lose 30–40% of their initial body weight (IBW) within 2 h.

Autoactive molluscan neuron: Reflex function and synaptic modulation during feeding in the terrestrial slug,Limax maximus

SummaryThe salivary burster (SB) is an autoactive motoneuron to the salivary duct of the terrestrial slugLimax maximus. The SB is electrically coupled to protractor motoneurons and inhibited by the metacerebral giant cell. During feeding these synaptic inputs cause SB activity to be phase-locked to the protraction-retraction cycle. The SB can be used as a clear and reliable monitor of feeding motorprogram activation.

The effects of feeding on heart activity in the terrestrial slug, Limax maximus: central and peripheral control

SummaryThe role of the central nervous system (CNS) in the modulation of heart activity induced by feeding was investigated in the terrestrial slug,Limax maximus. Intact slugs and semi-intact preparations were used to examine the effects of food, non-nutritive bulk, digestive tract distension, and the concentration of hemolymph glucose on the control of heart activity. The heart rate of intact slugs increased following ingestion of food or nonnutritive bulk and in response to injections of glucose. The heart rate of semi-intact preparations increased in response to gradual crop inflation and to perfusion of the heart with a glucose solution for longer than 30 min. The present results indicate that the increase in heart rate observed in intact slugs following a meal is mediated in part by the CNS and in part is a direct response of the heart musculature. The CNS mediates an immediate response to proprioceptive input from stretch of the crop while the heart musculature responds directly to increased hemolymph glucose concentration following ingestion of food.

Paracellular water uptake and molecular sieving by the foot epithelium of terrestrial slugs.

SummaryA paracellular pathway in the foot epithelium ofLehmannia valentiana can be opened by dehydrating the slug. Movement of water from a wet pad through the opened pathway into the haemolymph of this terrestrial slug is rapid. The sieving properties of this paracellular pathway have been determined using the reference isotope3HOH and various14C-labelled solutes.1.Paracellular uptake of14C-inulin (Fig. 1) and3HOH (Fig. 2) is initial rate for at least 3 min. If the wet pad contains 1,000 cpm of14C per ml of3HOH, slugs absorb only about 400 cpm of14C with cach ml of3HOH absorbed representing a sieving ratio of 0.4 for inulin.2.The sieving ratio of14C-inulin does not change when the concentration is increased from 0.1 to 2.5 mmol/l. Moreover, the sieving ratio of14C-inulin was not affected significantly by the nature of the labelling, i.e.,14C-carboxyl vs14C-methoxy.3.Sieving ratios for14C-mannitol (182 Da),14C-polyethylene glycol (4,000 Da), and14C-inulin (5,250 Da) were 0.92, 0.63, and 0.39, respectively (Table 1), indicating that sieving is dependent on molecular size.14C-Dextran (70,000 Da) and blue dextran (200,000 Da) were excluded from the paracellular pathway (Fig. 4).4.The effective pore size of the paracellular pathway was estimated using the relationships between sieving ratio and molecular weight of3HOH and the various solutes that can pass through the pathway. The extrapolated pore size is equivalent to that of a sieve having a molecular weight cutoff of about 10,000 Da (Fig. 3).

Control of swimming in cercariae of Proterometra macrostoma (Digenea)

A specific acetylcholinesterase (AChE) was localized histochemically in Proterometra macrostoma cercariae. This esterase activity was inhibited by eserine, but was resistant to tetraisopropyl pyrophosphoramide. A transverse band of intense AChE activity was observed in the posterior region of the tail. Suction electrode recordings from the tail showed that the action potentials associated with swimming activity were conducted from the transverse band region toward the anterior end of the tail. Removal of the body of the cercaria had no effect on the cyclical electrical activity. 5-Hydroxytryptamine had a strong excitatory effect on the electrical activity whereas acetylcholine and eserine were inhibitory. These observations suggested that swimming behavior was initiated in the transverse band of the tail and that the neuromuscular activity underlying this behavior may be modulated by acetylcholine. Swimming behavior of Proterometra macrostoma cercariae is characterized by alternating periods of swimming and sinking (Prior and Uglem, 1979). Aided by a pair of flat furcae, alternating lateral contractions of the tail pull the cercaria up through the water. A single, biphasic, action potential precedes each contraction of the tail. Following a burst of swimming the cercaria drifts passively downward until touching some object. Once it contacts something another sequence of swimming is initiated. Although the duration of a swimming period is quite regular, a burst of swimming can be initiated at any time during a period of sinking. The body of this large furcocystocercous cercaria is attached to the anterior end of the tail, and is usually withdrawn into the tail at the time of emergence from the snail. One of the most striking aspects of the electrical pattern underlying swimming activity is that the pattern continues unaltered after detachment of the body from the tail. Furthermore, the potentials are conducted from the posterior to the anterior end of the tail. These observations suggested that swimming activity is generated in the posterior region of the tail. We have localized a band of intense, acetylcholinesterase activity in the posterior region of the tail using the histochemical method of Gomori (1952). Our report describes the transverse band and also the effects of various drugs that suggest that the neuromuscular activity underlying swimming behavior may be modulated by acetylcholine. Received 14 October 1982; revised 28 December 1982; accepted 6 January 1983. MATERIALS AND METHODS Snails of the genus Goniobasis Lea infected with Proterometra macrostoma were collected from local, springfed streams and placed in shallow pans of well-aerated natural spring water. Cercariae emerged from the snails after several hours in the laboratory. Some snails were cracked open and the cercariae carefully teased from the tissues. The cercariae were transferred to artificial spring water (ASW; 0.5 mM NaCl, 0.05 mM KC1, 0.4 mM CaCl2, and 0.025 mM MgCl) and held for 2 hr at 25 C prior to use in experiments. Acetylcholinesterase (AChE) was localized histochemically according to the method of Gomori (1952). Worms were incubated (37 C) in the reaction medium for 3 to 6 hr. The reaction medium contained as substrate acetylthiocholine iodide (2 mg/ml; Sigma, St. Louis, Missouri), and in some cases cholinesterase inhibitors, 10-3 M eserine sulfate or 10-4 M tetraisopropyl pyrophosphoramide (isoOMPA; Sigma). The stained cercariae were dehydrated, cleared and prepared as whole mounts for photomicrography. The electrophysiological recordings were made by applying thin plastic tubing as suction electrodes to the tails of cercariae that had been pinned to the bottom of a recording chamber filled with ASW (18-22 C). The signals were amplified and recorded with a Grass P79 polygraph or kymograph camera. Drugs were applied to cercariae by replacing the ASW in the recording chamber (approximate volume = 2.0 ml) using a perfusion system. The effect of each drug on the swimming rhythm was based on observations of at least 25 bursts of swimming. Results are expressed as the x + SD. Differences were analyzed using the Students t-test; a value of P < 0.05 was considered significant. Acetylcholine, 5-hydroxytryptamine (5-HT), carbamylcholine chloride (carbachol) and eserine sulfate were obtained from Sigma. All chemicals were reagent grade.

An ultrastructural study of peripheral neurons and associated non-neural structures in the bivalve mollusc, Spisula solidissima.

The ultrastructural morphology of peripheral neurons and associated structures in the bivalve mollusc. Spisula solidissima have been studied in an effort to describe the synaptic topography and to provide anatomical correlates of previous physiological observations. The somata of the peripheral neurons are located within the perineurium at branch points of the siphonal nerves. There are many fiber-fiber synaptic contacts which are characterized by isolated sites of contact with membrane specialization and unilateral accumulation of synaptic vesicles. There are also synaptic contacts involving the somata, both axo-somatic and somato-axonic, the two being distinguishable on the basis of the polarity of vesicle accumulation. All of the observed synaptic profiles were similar in appearance regardless of the neuron loci involved. Much of the non-synaptic soma surface is covered with processes of glial cells. Likewise, in many cases, individual fibers and groups of fibers are encases with glial processes. Unique clusters of membrane bound, pigment containing glial like cells occur throughout the nervous system of Spisula. The heterogenous appearance of the inclusions and the presence of lysosome-like bodies in the cytoplasm of these cells suggest a possible phagocytic role.

The pneumostome rhythm in slugs: a response to dehydration controlled by hemolymph osmolality and peptide hormones

1. One response of the terrestrial slug, Limax maximus to dehydration is the initiation and modulation of the pneumostome rhythm. When a slug has lost 15-20% of its initial body weight by evaporation, the frequency of pheumostome closures, which is less than 0.5 closures/min in fully hydrated slugs, begins to increase. 2. The frequency increases with further dehydration, but the average duration of each closure remains constant. Thus, the proportion of time during which the pneumostome is closed increases. Simultaneously, the area of the pneumostome opening decreases. 3. This behavior appears to be controlled in part by both the osmolality of the slugs hemolymph and by a peptide closely related to arginine vasotocin (AVT) and arginine vasopressin (AVP). Injecting intact slugs with mannitol, which increases the osmolality of the hemolymph, or with AVT or AVP, can initiate the pneumostome rhythm. 4. Mannitol injections, however, do not provoke the decrease in the area of the pneumostome opening which is induced by natural dehydration or by AVT or AVP injection. This suggests that at least two systems may be involved in the overall control of the pneumostome.

Hymenolepis diminuta: Chloride fluxes and membrane potentials associated with sodium-coupled glucose transport

Abstract Glucose transport by Hymenolepis diminuta was inhibited when Cl− in the bathing medium was replaced with acetate (C2H3O2Post−), but was unaffected when Cl− was replaced with SCN−. The relative effectiveness of the anions to inhibit influx of 7.4 mM Cl− in the presence of 1 mM glucose was SCN− > Cl− > C2H3O2Post−. Glucose stimulated the influxes of 120 mM Cl− and SCN−, but had little effect on 120 mM C2H3O2Post− influx. While the diffusion rates of the anions were C2H3O2Post− > SCN− = Cl−, the preference of the glucose transport system for the anions was SCN− > Cl− > C2H3O2Post−. Efflux of Cl− was not affected by the rate of glucose influx. Finally, microelectrode recordings of worms anesthetized with 2 mM arecoline revealed a transmembrane potential (TMP) of −45 ± 3.6 mV (inside negative). Three to four minutes after addition of glucose (5 mM) there was a progressive hyperpolarization of the TMP to −58 mV. A revised model of the glucose transport system that is consistent with previous observations on this organism is proposed.

Water-Orientation Behavior in the Terrestrial Gastropod Limax maximus: The Effects of Dehydration and Arginine Vasotocin

When specimens of the terrestrial slug Limax maximus are dehydrated to 70% of their initial body weight (IBW), they exhibit water-orientation behavior that is characterized by both an increase in locomotor activity and a preference for the moist arm of a Y maze. Water orientation was elicited in fully hydrated slugs by injection of 10⁻⁶ M arginine vasotocin (AVT, calculated final hemolymph concentration). Injection of 10⁻⁷ M AVT elicited water-orientation behavior in slugs dehydrated to only 80% IBW, demonstrating that the effects of AVT and dehydration are additive. Injection of slugs with the octapeptides angiotensin II (AII) and arginine vasopressin (AVP), at concentrations ranging from 10⁻⁶ to 10⁻⁸ M, failed to elicit water orientation. Water orientation does not seem to be mediated solely by the alterations in hemolymph volume or osmolality that occur during dehydration. Rather, it appears that a dehydration-induced increase in hemolymph ionic concentration is involved in initiating water orientation.

A study of the electrophysiological properties of the incurrent siphonal valve muscle of the surf clam, Spisula solidissima.

Abstract 1. The incurrent siphonal valve muscle of Spisula solidissima is polyneuronally innervated, the synaptic contacts being localized near the entrance of the associated nerve branch. 2. Tactile stimulation can elicit from the muscle twitch responses, as well as graded increases in tension. The graded responses seem to be due to an asynchrony of motorneuron activation. 3. Muscle action potentials arise from summation of EJPs generated in response to tactile stimulation of the siphons or mantle margin. 4. Action potentials recorded from single muscle fibers in a localized region of the muscle seem not to be actively conducted.