Adaptive responses to temperature and photoperiod in neuromuscular transmissionin carcinus maenas (l)

Abstract

Neuromuscular synaptic transmission in the ectothermal crab Carcinus maenas was used to assess the extent to which photoperiod affected the attainment of temperature acclimation. A separate series of experiments were carried out to determine thermotolerance to high temperature (CTMax) and its dependence on prior heat shock and heat shock proteins. Dactylopodite closer muscle resting potentials and the characteristics of evoked excitatory junction potentials in response to stimulation of the excitatory tonic axon were recorded with intracellular microelectrodes and the extent of acclimation to temperature under different photoperiodic regimes was measured. Animals were acclimated for at least two week to either 8 C or 25 C with short day (8hr: 16hr, L/D) or long day (16hr: 8hr, L/D) photoperiods. Photoperiod differences had a marked effect on acclimation to temperature as measured by muscle resting potential and excitatory junction potential amplitude and facilitation. In short day photoperiods there was an acclimatory shift of muscle resting potential in crabs acclimated to 25 C compared with those acclimated to 8 C. Such an acclimatory shift of muscle resting potential was not seen in animals from long day photoperiods. By contrast facilitation showed acclimatory changes to temperature in long day but not in short day photoperiods. Excitatory junction potential amplitudes were increased in cold acclimation; this increase was larger from long day acclimated groups compared with short day groups. Furthermore photoperiod had a large effect on closer muscle tension in animals acclimated at low temperatures but little effect on animals acclimated at high temperatures. Heat shock reduced the amplitude of EJPs recorded in the closer muscle but enabled the EJP to be maintained for much higher temperatures. Heat shock also resulted in an increase in thermotolerance as measured by an elevation of CTMax from 33.04 C to 34.17 C.