The stimulus response relation of the epidermal lateral-line organ of Xenopus laevis was studied by recording activity of single afferent nerve fibres in isolated preparations. Linear frequency response analysis over a frequency range of 0.1--100Hz was performed under steady-state conditions, using small amplitude, sinusoidal water displacements produced by a glass sphere at a short distance from the skin. Period histograms of afferent nerve activity were computed, and amplitude, phase and mean activity of the response were determined by means of Fourier analysis. A standardization procedure at the start of each experiment made scaling of the frequency responses of different preparations unnecessary. The results show that for small stimulus amplitudes the response of the lateral-line organ over the whole range of frequencies studied can adequately be described as a modulation of the spontaneous activity. The amplitude of the response is proportional to the stimulus amplitude, and the phase of the response is independent of stimulus amplitude. The lateral-line organ of Xenopus laevis can thus be regarded as a linear system for stimuli which produce modulation of the spontaneous activity. The frequency response demonstrates unequivocally that the lateral-line organ of Xenopus laevis functions as a water velocity detector. For frequencies of stimulation from 0.1--20Hz the gain increases with a slope of 7.5 dB/oct, and up to 5Hz the response is almost in phase with the water velocity. The extent to which the different transmission steps between stimulus and response will contribute to the frequency response is discussed.

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The effects of 4 different pyrethroid insecticides on sodium channel gating in internally perfused, cultured mouse neuroblastoma cells were studied using the suction pipette, voltage clamp technique. Pyrethroids increased the amplitude of the sodium current, sometimes by more than 200%. Activation of the sodium current occurred at more hyperpolarized potentials than under control conditions. The declining phase of the sodium current during depolarization was markedly slowed down and after repolarization of the membrane a large, slowly decaying sodium tail current developed. Pyrethroids did not affect the sodium current reversal potential, steady-state sodium inactivation or recovery from sodium channel inactivation. The amplitude of the pyrethroid-induced slow tail current was always proportional to the sodium current at the end of the preceding depolarizing pulse. The rate of decay of the slow tail current strongly depended on pyrethroid structure and increased in the order deitamethrin, cyphenothrin, fenfluthrin and phenothrin. The rate of decay further depended on membrane potential and temperature. Below -85 mV the instantaneous current-voltage relationship of the slow tail current showed a negative slope conductance. The tail current decayed more slowly at low temperatures. Arrhenius plots indicated that the relaxation of open sodium channels to a closed state in,,olved a higher energy barrier for pyrethroid-affected than for normal channels. The energy barrier was higher after deitamethrin than after the non-cyano pyrethroid fenfluthrin. It is concluded that in mammalian neuronal membrane pyrethroids selectively reduce the rate of closing of sodium channels both during depolarizatio~ and after repelarization of the nerve membrane.

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