Correlations between temperature effects on behavior inaplysiaand firing patterns of identified neurons

A Computational Model of the Temperature-dependent Changes in Firing Patterns in Aplysia Neurons

We performed experiments using Aplysia neurons to identify the mechanism underlying the changes in the firing patterns in response to temperature changes. When the temperature was gradually increased from 11℃ to 31℃ the firing patterns changed sequentially from the silent state to beating, doublets, beating-chaos, bursting-chaos, square-wave bursting, and bursting-oscillation patterns. When the temperature was decreased over the same temperature range, these sequential changes in the firing patterns reappeared in reverse order. To simulate this entire range of spiking patterns we modified nonlinear differential equations that Chay and Lee made using temperature-dependent scaling factors. To refine the equations, we also analyzed the spike pattern changes in the presence of potassium channel blockers. Based on the solutions of these equations and potassium channel blocker experiments, we found that, as temperature increases, the maximum value of the potassium channel relaxation time constant, τ(n)(t) increases, but the maximum value of the probabilities of openings for activation of the potassium channels, n(t) decreases. Accordingly, the voltage-dependent potassium current is likely to play a leading role in the temperature-dependent changes in the firing patterns in Aplysia neurons.

Temperature compensation in the escape response of a marine copepod, Calanus finmarchicus (Crustacea)

Calanus finmarchicus, the dominant mesozooplankter of the North Atlantic, is an important food source for many fishes and other planktivores. This species, which has limited diel vertical migration, depends on its fast-start escape response to evade predators. It has myelinated neuronal axons, which contribute to its rapid and powerful escape response. The thermal environment that C. finmarchicus inhabits ranges from below 0 degrees C to 16 degrees C. Previous studies have shown that respiration, growth, and reproductive rates are strongly dependent on temperature, with Q10 > 2.5. A comparable dependence of the escape response could place the animal at higher risk for cold-compensated predators. Our work focused on the temperature dependence of the behavioral response to stimuli that mimic predatory attacks. We found that in contrast to other biological processes, all aspects of the escape response showed a low dependence on temperature, with Q10 values below 2. This low temperature dependence was consistent for escape parameters that involved neural as well as muscle components of the behavioral response. These findings are discussed in the contexts of the predator-prey relations of copepods and the thermal dependence of behavior in other taxa.

The effect of cooling on the acetylcholine-induced current of identified Helix pomatia Br neuron

The Br neuron of the snail Helix pomatia, involved in neuronal regulation of various homeostatic and adaptive mechanisms, represents an interesting model for studying effects of temperature changes on neuronal activity of poikilotherms. The acetylcholine (ACh) induces a transient, inward dose-dependent current in the identified Br neuron. In the work presented, we analyses the effects of cooling on the ACh-induced inward current. The amplitude of ACh-induced inward current was markedly decreased after cooling and the speed of the decay of ACh response was decreased. Sensitivity to cooling of Ach-activated current on the Br neuron is mediated by a mechanism that does not involve change in the apparent receptor affinity or the cooperativity of binding.

Temperature dependence of egg laying in Aplysia brasiliana and A. californica

The temperature dependence of egg laying was examined in winter-caught Aplysia. Cold-water Aplysia californica and warm-water A. brasiliana were individually housed in the same large aquarium for 16 days at 15 degrees C, and then for 16 days at 20 degrees C. Initially, the majority of the A. californica were not reproductively mature (as determined by injections of atrial gland extracts) whereas all of the A. brasiliana were reproductively mature. When the temperature was increased from 15 to 20 degrees C, both species showed a marked increase in the frequency of egg laying. At both temperatures, A. brasiliana laid eggs more frequently but produced smaller egg masses than A. californica. We conclude that increased egg laying in A. californica was attributable both to facilitation of oogenesis in previously reproductively immature animals and to increased activity of the bag cells which release an egg-laying hormone. Increased egg laying in A. brasiliana was attributable primarily to increased bag cell activity.

Effects of sea water temperature on bursting pacemaker activity in cell R15 in the intact Aplysia

Thermosensitivity of bursting pacemaker activity (BPA) in neuron R15 of Aplysia was compared in an intact preparation and after excision of the ganglion containing R15. We noted no differences indicative of an in situ antagonism of cooling-induced suppression of BPA. Interruption of circulation hastened hemolymph cooling during exposure of the animal to cold sea water, suggesting mechanisms to protect R15 and other cells from transient cooling of the animal.