Temperature dependence of photoreceptor neuromodulation: Octopamine and Substance P modulate the duration and speed of sensory signals in the lateral eye of Limulus
An endogenous circadian clock that releases Octopamine at night and increases ERG amplitudes drives efferent fibers that run from the brain to the lateral eye of the horseshoe crab. At ambient temperatures, in vitro investigations have revealed that Octopamine induces response prolongations and temporal rescaling (increased latency) while in vivo investigations revealed that nighttime ERGs not only show response prolongation but also acceleration (decreased latency). Since temperature affects the physiology of visual responses in Limulus, the current experiments regulated chamber temperature during in vitro microelectrode recordings from Limulus lateral eye. The purpose of this study was to characterize in vitro effects of Octopamine on response duration and onset at cold temperatures and also to determine if another neuromodulator, Substance P might be involved with response acceleration at both ambient and cold temperatures. Receptor waveforms were recorded from a Limulus lateral eye while either Octopamine or Substance P was perfused. Complementary infusion experiments where a lateral eye was steeped in either Octopamine or Substance P were included in these investigations in order to characterize the interaction effects of each modulator and temperature. Octopamine induced response prolongations accompanied by temporal rescaling with similar probability in cold temperatures as well as ambient temperatures. Substance P was found to modulate the timing of photoreceptor responses by response acceleration. Of particular interest, is the finding that the accelerative effect of Substance P was augmented in cold temperatures, suggesting temperature dependent modulation of sensory signals in the lateral eye of Limulus .
Wasserman, Purdue University.
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