The modified cilium (dendrite) of epithelial mechanoreceptors of insects contains microtubules in different arrangements: (1) microtubules distributed over the entire receptor and not fixed in a special configuration, therefore called free microtubules, (2) densely packed, interconnected microtubules called the tubular body, and (3) 9 doublet microtubules. These groups of microtubules have been discussed in relation to mechanotransduction. In a preceding paper the free microtubules were proved to be not involved in mechanotransduction. In this paper the hypothesis is examined that the tubular body may be essential to mechanotransduction. For this purpose the effect of the microtubule-disassembling drug vinblastine on both the tubular body and the sensitivity is examined in a femoral mechanoreceptor of the cricket Acheta domesticus. After 6- to 26-h exposure to vinblastine the tubular body is partially or totally destroyed. Simultaneously, mechanical sensitivity decays to zero. In contrast, the pacemaker property for nerve impulses of the apical dendritic segment is only slightly altered. We conclude from these results that the tubular body is essential to mechanotransduction. Three experiments in which a (small) response persisted, despite a totally destroyed tubular body, suggest that receptor potentials can in principle be evoked without an intact tubular body. In addition to the irreversible reduction of receptor sensitivity, vinblastine causes a reversible reduction during repetitive stimulation. This adaptation is supposed to be the consequence of altered properties of the tubular body.
Microtubules (Mt) are present in the modified cilium of epithelial mechanoreceptors of insects in three different arrangements: (1) 9 doublet Mt in the proximal region of the outer segment, (2) densely packed, interconnected Mt of the tubular body in the dendritic tip receiving the adequate stimuli, and (3) Mt between ciliary neck and tubular body, which are not fixed in a special configuration and therefore called free Mt. The free Mt are considered by some authors to be elements of intracellular signal transmission. This hypothesis was examined by electrophysiological and morphological studies on a tibial hair-mechanoreceptor of a cricket (Acheta domesticus). Exposure of the receptor from the apical side to vinblastine disassembled the free Mt within 2 to 4 h, while Mt of the tubular body were only little affected during this time interval. In this state of Mt disassembly (up to 7 h of application) mechanosensitivity of the receptor is only slightly reduced or not at all. The pacemaker property of the dendrite for nerve impulse is also preserved. It is concluded that the free Mt are not elements of intracellular signal transmission and are not directly involved in mechanotransduction.
A simplified method is described for recording electrical potentials without injury from epidermal insect mechanoreceptors. For this purpose a stationary capillary-electrode is used with an anterior part made of polyethylene and filled with a hygroscopic electrolyte; its wide orifice allows the hair-shaft to keep contact when it is bent under the stimulus.
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