Effects of thermal history on long term neuromuscular facilitation in intact crayfish and isolated claw preparations

The generation of long term facilitation (LTF), a form of synaptic facilitation in Crustacea that results from maintained synaptic activity, is established for the first time in an intact animal. Facilitation of stretcher muscle myogram amplitude could be monitored in crayfish (Procambarus clarkii) while stimulating the excitor motoneuron in both intact animals and isolated preparations (Figs. 2, 3).At an average stimulation frequency of 8 Hz, colder temperatures enhance the development of LTF, monitored either intracellularly or by myogram (Figs. 1, 2). Neuromuscular depression counteracts the effects of LTF on myogram amplitude at extremely cold temperatures (5 °C, Fig. 4), or after extended stimulation periods at less cold temperatures (10 °C). Less LTF and negligible depression are detected during stimulation at warmer temperatures (15 °–20 °C).LTF is produced most effectively at and below the acclimation temperature. The temperature range favouring effective LTF development shifts with a decrease in acclimation temperature. Animals acclimated at a low temperature (7 °C) and tested at 15 °C show less rapid onset of LTF than animals acclimated at 17 °C. In addition, cold-acclimated animals show reduced ability to generate LTF at warmer temperatures and delayed onset of neuromuscular depression at cold temperatures.Orconectes virilis, a northern crayfish, studied in vitro, accomplishes nearly complete compensation of LTF development during cold acclimation (Fig. 6). At warm test temperatures, cold-acclimated animals show much reduced LTF.Procambarus, a crayfish from temperate regions, exhibits partial compensation of LTF development during cold acclimation when tested in vitro (Fig. 5). There is little reduction of LTF at warmer temperatures after cold acclimation. Temperature compensation is more complete in intactProcambarus than in the isolated claw (Fig. 4).It is concluded that LTF can serve as a mechanism to enhance neuromuscular performance at low temperatures, and that the temperature range over which LTF appears can be modified by thermal acclimation.