Quantal analysis of presynaptic inhibition, low [Ca²⁺]₀, and high pressure interactions at crustacean excitatory synapses

Abstract

The cellular mechanisms underlying the effects of high pressure, GABAergic presynaptic inhibition, and low [Ca2+]0 on glutamatergic excitatory synaptic transmission were studied in the opener muscle of the lobster walking leg. Excitatory postsynaptic currents (EPSCs) were recorded with or without prior stimulation of the inhibitor using a loose macropatch clamp technique at atmospheric pressure and at 6.9 MPA helium pressure. High pressure reduced the mean EPSC amplitude and variance, decreased the quantal content (m), but did not affect the quantum current (q). Pressure shifted the median of the amplitude histogram to the left by 1-2 q. Under normal pressure conditions, presynaptic inhibition and low [Ca2+]0 induced similar effects. However, quantal analysis using a binomial frequency distribution model revealed that high pressure and low [Ca2+]0 diminished n (available active zones) and slightly increased p (probability of release), but presynaptic inhibition reduced p and slightly increased n. At high pressure, presynaptic inhibition was reduced, at which time the major contributor to the inhibitory process appeared to be reduction in n and not p. The similarity of the alterations in quantal parameters of release at high pressure, low [Ca2+]0, and in some conditions of presynaptic inhibition is consistent with the hypothesis that pressure reduces Ca2+ inflow into the presynaptic nerve terminals to affect the Ca(2+)-dependent quantal release parameters n and p.

Temperature and acid–base balance in the American lobsterHomarus americanus

“…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).…”

Neuronal sensitivity to hyperoxia, hypercapnia, and inert gases at hyperbaric pressures

“…The threshold pressure for HPNS is variable and affected by the rate of compression (18). Details regarding the cellular mechanism(s) underlying HPNS are unclear (114), but it appears that both synaptic (74,76,85,228,229) and intrinsic membrane properties are involved (72,73,191,192,216). Several excellent reviews on HPNS exist elsewhere (18,100,104,114).…”

Selective modulation of cellular voltage-dependent calcium channels by hyperbaric pressure—a suggested HPNS partial mechanism

“…Indeed, it has previously been shown that synaptic transmission is suppressed by HP whether in vertebrate CNS synapse (Parmentier et al, 1981; Schleifstein-Attias et al, 1994), squid giant synapse (Parmentier et al, 1981), or crustacean neuromuscular junction (NMJ) (Campenot, 1975; Grossman and Kendig, 1990), either in excitatory (Grossman and Kendig, 1988; Golan et al, 1994) or inhibitory (Golan et al, 1994) synapses. Postsynaptic reduction in receptor sensitivity, observed in glycine receptors (Shelton et al, 1993), will exacerbate pressure-induced suppression in specific synapses (Daniels and Grossman, 2003).…”