TRANSDUCER PROPERTIES OF THE RAPIDLY ADAPTING STRETCH-RECEPTOR NEURON IN THE CRAYFISH (PACIFASTACUS-LENIUSCULUS)

1. The transducer properties of the rapidly adapting stretch receptor neurone of the crayfish (Pacifastacus leniusculus) were studied using a two-microelectrode voltage clamp technique. 2. The impulse response to ramp-and-hold extensions of the receptor muscle typically consisted of a high frequency burst followed by cessation of impulses within a relatively short time depending on the amplitude of extension. The type of adaptation was consistent with earlier studies. The stimulus-response relationship for the impulse frequency was non-linear and had a slope in a log-log plot of 2.9. 3. When impulse generation was blocked by tetrodotoxin (TTX), (block of Na+ channels) the receptor potential was extension dependent and similar to that found in the slowly adapting receptor. For small extensions there was an initial peak followed by a fall to a steady potential level. For large extensions the potential response during the ramp phase consisted of a peak followed by a constant potential level lasting to the end of the ramp. When the extension changed to the hold phase the potential fell towards a steady state. The relation between extension and amplitude of receptor potential was non-linear and saturated at -40 to -30 mV (extensions > 15% of zero length, l(o)). 4. When potassium channels were blocked by TEA (50 mM) and 4-aminopyridine (4-AP, 5 mM) (and Na+ channels blocked by TTX) the shape of the generator potential become less complex with an increased amplitude for large extensions. 5. When the receptor neurone was voltage clamped at the resting potential, extension of the receptor muscle produced an inwardly directed receptor current, the stretch-induced current (SIC). The response consisted of a fast transient phase which decayed towards a steady state. The SIC peak amplitude was dependent on extension in a sigmoidal fashion and saturated at 190 nA (extensions > 25% of l(o)). The slope of the steepest part of the stimulus-response relation (between 10 and 20% extension) was 4.7+/-0.25 (mean+/-S.E.M.) in a log-log plot. 6. The peak amplitude of the SIC increased with increasing extension speed (ramp steepness), the relation between the slope of the ramp and current amplitude being a first order (hyperbolic) function. The amplitude of the receptor current was voltage dependent and had a reversal potential of +16.2+/-1.8 mV (mean+/-S.E.M., 32 cells) From the reversal potential the permeability ratio, P(Na)/P(K), of the transducer permeability system was calculated to be 1.5. The I-V curve of SIC was non-linear. 7. When the external Ca2+ concentration was lowered the receptor current amplitude increased. This led to displacement of the stimulus-response relation towards smaller extensions. No change in reversal potential was observed. 8. The receptor current amplitude was reduced when the normal bathing solution was replaced by solutions containing mainly Ca2+ or Mg2+ ions. Small changes in the reversal potential for the receptor current were seen. Calculations from the current amplitudes at normal and high divalent cation solutions resulted in the following ratios: P(Ca)/P(Na) = 0.44+/-0.06 and P(Mg)/P(Na) = 0.60+/-0.05 (mean+/-S.E.M., 4 cells). 9. It is concluded that the transducer properties of the rapidly adapting are similar to the slowly adapting stretch receptor neurone. The permeability ratios of the transducer membrane for different ions, P(Na):P(K):P(Ca):P(Mg), was 1:0.68:0.44:0.60. The time course of adaptation of the SIC was found to be faster and the stimulus-response relation for the SIC was steeper compared with the slowly adapting stretch receptor neurone, which could in part explain the more rapid adaptation.