1 Rabbit isolated irides were loaded with [3H]-noradrenaline and superfused with Tyrode solution. The inhibition by the muscarinic agonists (±)-methacholine and pilocarpine of the [3H]-noradrenaline overflow into the superfusate evoked by field stimulation (pulses of 1 ms duration, 75 mA) was measured as an index of activation of presynaptic muscarinic receptors. 2 The fractional rate of release per pulse during the first stimulation period (SI) was low with 360 pulses at 3 Hz, intermediate with 360 pulses at 10Hz and high with 1200 pulses at 10Hz. Upon repetitive stimulation (7 periods at 20 min intervals), the fractional rates of release per pulse during S7 no longer differed, suggesting a 'long-term' regulation of [3H]-noradrenaline release depending on the stimulation conditions. 5 Depending on the concentration, pilocarpine reduced the [3H]-noradrenaline overflow evoked by 360 pulses at 3Hz up to 63%. However, at 10Hz stimulation frequency the compound was inactive as an agonist but competitively antagonized the presynaptic inhibition induced by methacholine. The KB under the latter condition (0.95 Mm) was very close to the EC50 value determined at 3 Hz (0.85 pM). 6 The results demonstrate a muscarinic inhibition of noradrenaline release from the rabbit isolated iris. The activation by pilocarpine of the presynaptic receptors provides an alternative explanation for the miosis induced in the rabbit in vivo, which might be the result of a decreased sympathetic tone in the iris dilator muscle.
We investigated the contribution of rods and cones to the human pattern electroretinogram to onset and offset checkerboards of different spatial frequency and wavelength in a 39 degrees x 39 degrees field. Under strictly scotopic conditions, there was a negative potential at onset and a positive potential at offset, whereas under photopic conditions, there was a positive potential at onset and a negative/positive potential at offset. Thus, the waveform to pattern onset (offset) was that of the luminance electroretinogram to decreasing (increasing) luminances. For pattern onset, the sensitivity difference 486-601 nm under scotopic and photopic conditions closely followed the luminosity function of rods and cones. The amplitude of the scotopic onset response increased with check size up to 3 degrees 30' and that of the photopic onset response, up to 30'. With larger checks, the scotopic and photopic onset response markedly decreased. This indicates antagonistic center-surround organization of the receptive fields under both scotopic and photopic conditions. By contrast, the offset response monotonically increased with check size under scotopic and photopic conditions, which suggests a luminance component in the pattern electroretinogram. Consequently, the pattern electroretinogram to reversing checkerboards has to be regarded as a mixture of both pattern- (contrast) and luminance-specific components.
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