Nonlinear mechanisms for gain adaptation in locust photoreceptors

Abstract

Intracellular membrane potential responses were recorded from locust photoreceptors under two stimulus conditions: pairs of flashes to dark-adapted receptors, and white-noise modulated light at a range of background intensities from 500 to 15,000 effective photons per second. Nonlinear analysis of the input-output relationships were performed by estimating the Volterra and Wiener kernels of the system. The Volterra kernels obtained from the double-flash experiments were similar to the Wiener kernels obtained from the white-noise experiments, except for a change of time scale. The structure of the second-order kernels obtained with either method gave evidence for a gain control mechanism acting at an early stage of the cascade. Both feedforward and feedback nonlinearities could account for the observed system behavior at any one background level. The differences in amplitude between the kernels obtained at different background levels could be accounted for by an adaptation process which further decreased the gain of the system, acting on a slower time scale, also at some early stage of the cascade.

Sublinear summation of responses in locust photoreceptors

“…In a previous investigation (Pece et al 1990), we suggested that the nonlinear depression observed with a pseudo-random stimulus has the same mechanism as the depression observed with pairs of flashes. We also proposed a biochemical mechanism for both processes, in which the depression is due to enzyme inhibition within the phototransduction cascade.…”

“…The difference trace obtained in this way always had a single minimum, as shown in Fig. 1 and in French and Kuster (1985) and Pece et al (1990). The peak depression was taken as a measure of the interaction between the two stimuli, as in French and Kuster (1985).…”

“…The same cells still showed a normal depression when the decline in the step response had been blocked by TEA. Pece et al (1990) pointed out that the fast depression process must be distinguished from the slower adaptation, since the two processes alter the gain of phototransduction in very different time scales. We can now add that the slower changes of gain must be further distinguished into at least 2 components, one of which takes place in a time scale of 100 ms to 1 s and the other in a time scale of several seconds.…”

“…It has been proposed that the fast gain control process observed in insect photoreceptors is operating mostly at the level of the biochemical phototransduction cascade (French and Kuster 1985;Pece et al 1990). A reduction in the number of rhodopsin isomerizations was ruled out by the fast time scale and the small number ofisomerizations required for the process, while a shunting conductance mechanism was thought to be unlikely because the process is manifest at very low levels of light-induced depolarization.…”

Development of multifunctional materials exhibiting distributed sensing and actuation inspired by fish

“…Researchers have used Wiener/Volterra kernals for system identification to gain insight into the function and mechanisms underlying various biological systems. For example, orthogonalization techniques for estimating the kernel values have been developed and applied to various sensory systems, such as the catfish retina [15], the visual cortex in the cat [16], the locust and fly photoreceptor [17][18][19] and the cockroach tactile spine [20,21]. Recently, researchers have used neural networks in system identification and modeling of various biological systems, such as insect walking [22], renal autoregulation in rats [23], and locomotor oscillators in lamprey spinal cords [24,25].…”