Mechnisms underlying different facilitation forms at the lobster neuromuscular synapse

SUMMARY In cold-blooded species the efficacy of neuromuscular function depends both on the thermal environmental of the animal's habitat and on the concentrations of modulatory hormones circulating within the animal's body. The goal of this study is to examine how temperature variation within an ecologically relevant range affects neuromuscular function and its modulation by the neurohormone serotonin (5-HT) in Homarus americanus, a lobster species that inhabits a broad thermal range in the wild. The synaptic strength of the excitatory and inhibitory motoneurons innervating the lobster dactyl opener muscle depends on temperature, with the strongest neurally evoked muscle movements being elicited at cold (<5°C) temperatures. However, whereas neurally evoked contractions can be elicited over the entire temperature range from 2 to >20°C, neurally evoked relaxations of resting muscle tension are effective only at colder temperatures at which the inhibitory junction potentials are hyperpolarizing in polarity. 5-HT has two effects on inhibitory synaptic signals: it potentiates their amplitude and also shifts the temperature at which they reverse polarity by approximately +7°C. Thus 5-HT both potentiates neurally evoked relaxations of the muscle and increases the temperature range over which neurally evoked muscle relaxations can be elicited. Neurally evoked contractions are maximally potentiated by 5-HT at warm (18°C) temperatures; however, 5-HT enhances excitatory junction potentials in a temperature-independent manner. Finally, 5-HT strongly increases resting muscle tension at the coldest extent of the temperature range tested (2°C) but is ineffective at 22°C. These data demonstrate that 5-HT elicits several temperature-dependent physiological changes in the passive and active responses of muscle to neural input. The overall effect of 5-HT is to increase the temperature range over which neurally evoked motor movements can be elicited in this neuromuscular system.)

Section: Discussionmentioning

confidence: 99%

Temperature dependent modulation of lobster neuromuscular properties by serotonin

Hamilton

Edwards

Holt

et al. 2007

“…The fast facilitation appears to be mainly induced by the accumulation of free Ca 2+ (Bykhovskaia et al, 2004) or by a rapid, high concentration of residual Ca 2+ near a fast site of exocytosis (Fisher et al, 1997;Zucker, 1999). Post-tetanic potentiation, however, seems to be due to an increase in the readily releasable vesicle pool caused by Ca 2+ effects with slower kinetics, such as the release of mitochondrial Ca 2+ (Bykhovskaia et al, 2004;Fisher et al, 1997;Zucker, 1999;Zucker and Regehr, 2002). Ca 2+ -independent mechanisms have been proposed (Nussinovitch and Rahamimoff, 1988).…”

Section: Discussionmentioning

confidence: 99%

“…Synaptic integration has been studied extensively in crustaceans, particularly at the neuromuscular junction (Atwood, 1976;Atwood and Wojtowicz, 1986;Bittner, 1968;Bykhovskaia et al, 2004;Dudel and Kuffler, 1961;JorgeRivera and Marder, 1997;Jorge-Rivera et al, 1998;Katz et al, 1993;Morris and Hooper, 1997;Morris and Hooper, 1998;Morris and Hooper, 2001;Msghina et al, 1998). STG motor neurons provide the large gastric mill muscle fibers with a number of spatially separated synaptic sites, which ensures that the entire muscle fiber receives depolarizing inputs and contracts as a whole.…”

Section: Consequences For Behaviormentioning

confidence: 99%

Functional consequences of activity-dependent synaptic enhancement at a crustacean neuromuscular junction

Stein

Smarandache

Nickmann

et al. 2006

intact animals and in the isolated nervous system showed a great variability in firing frequency and temporal distribution of motor neuron bursts. Train stimulations with various stimulus frequencies (5·Hz, 10·Hz, 20·Hz) and inter-train intervals (2·s, 4·s, 8·s, 16·s, 32·s) revealed that augmentation acted in addition to facilitation. Augmentation increased muscle EJPs during stimulations with inter-train intervals of 16·s or less. The effects of augmentation increased with shorter inter-train intervals, but were independent of stimulus frequency.Augmentation also contributed to the electrical response of the muscle during gastric mill rhythms, which were obtained in vitro and in vivo, and was also reflected by an increase of muscle force and the slope of force development during repetitive train stimulation. We conclude that the augmentation of EJPs at the neuromuscular junction tunes the muscle response to support force production during rhythmic motor patterns.

“…Bykhovskaia et al, 1999;Bykhovskaia et al, 2004;Golan et al, 1994;Golan et al, 1996;Goy and Kravitz, 1989;Grossman and Kendig, 1990;Kravitz et al, 1980;Vorob'eva et al, 1999;Worden et al, 1997;Worden and Camacho, 2006;Worden et al, 1995). The composition of this saline is based on a 'perfusing solution' that Cole developed more than 60 years ago by comparing a series of solutions with varying concentrations of sodium, potassium, calcium, magnesium and sulfate to determine which was the optimal mixture for maintaining the strength and frequency of the lobster heartbeat in vitro at 17°C (Cole, 1941).…”

Section: The Temperature Dependence Of Hemolymph Phmentioning

confidence: 99%

Temperature and acid–base balance in the American lobsterHomarus americanus

Qadri

Camacho

Wang

et al. 2007

However, thermal change can alter lobster acid-base status over a time course of minutes. Acute increases in temperature trigger a respiratory compensated metabolic acidosis of the hemolymph. Both the strength and frequency of the lobster heartbeat in vitro are modulated by changes in pH within the physiological range measured in vivo. These observations suggest that changes in acid-base status triggered by thermal variations in the environment might modulate lobster cardiac performance in vivo.