Sensitivity of an insect mechanoreceptor after destruction of dendritic microtubules by means of vinblastine

Cell and Tissue Research - 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...     

Secretory activities of plasmatocytes and oenocytoids during the moulting cycle in an insect (Rhodnius).

Plasmatocytes in Rhodnius appear to be the chief source of the basement membrane (basal lamina) of the abdominal epidermis. The membrane increases three-fold in thickness while the cells are applied to its surface, from 4 to 9 days after feeding. At this time irregular deposits of membrane substance appear, applied to the membrane in the vicinity of plasmatocytes. Many small vesicles perhaps undergoing exocytosis are seen at the surface of the plasmatocytes in contact with the basement membrane (basal limina). The large granular inclusions of the plasmatocytes are dispersed and their contents appear to provide the substance of the basement membrane, which has the same staining properties as these inclusions.     

After-hyperpolarization and receptor potential attenuation following bursts of action potentials in an insect mechanoreceptor

The receptor potential in the sensory neuron of the cockroach femoral tactile spine was recently observed by raising the axon into an oil bath and measuring the decrementally conducted receptor current. Although action potential discharge in this receptor adapts rapidly, there was no evidence of adaptation in the receptor potential. In the present work we report that bursts of action potentials in the neuron produce a prolonged after-hyperpolarization and attenuate the receptor potential. Both of these effects could be important in receptor adaptation and we sought to identify their origin. It was impossible to control ionic concentrations in the fluid surrounding the sensory neuron because of an effective glial barrier, but it was possible to infuse the tissues with chemical agents which are known to block ionic membrane processes. Cobalt and cadmium, which inhibit calcium influx, eliminated the effects of action potentials, and ouabain had similar effects. These results suggest that both a calcium-activated potassium conductance and an electrogenic sodium pump are involved in these phenomena. However, it is argued that the former is probably more important.     

Phentolamine selectively affects the fast sodium component of sensory adaptation in an insect mechanoreceptor

Phentolamine and related compounds have several different actions on nervous tissues in vertebrates and invertebrates, including a local anesthetic effect. However, recent work suggests that phentolamine can interfere with sensory transduction in insect mechanoreceptors at significantly lower concentrations than are required for conduction block. We tested the actions of phentolamine on sensory transduction and encoding in an insect mechanoreceptor, the cockroach tactile spine neuron and found that 500 microM phentolamine increased the action potential threshold by 50%. The passive membrane properties of the neuron were not affected, but one component of dynamic threshold change was strongly and selectively reduced. This component has previously been attributed to slowly inactivating sodium channels in the action potential initiating region, suggesting that these channels are the most phentolamine-sensitive sites.     

Neutral metal chelator-sensitive protease in insect moulting fluid

Proteolytic activity in moulting fluid from the sphingid Manduca sexta has at least two pH optima; these occur at pH 7 and at pH 7·7. The latter activity is shown to be trypsin-like in that it is susceptible to inhibition by diisopropylfluorophosphate. By contrast, the peak at neutral pH consists of proteolytic activity not hitherto described in invertebrates. This activity shows little or no inhibition with diisopropylfluorophosphate or p-hydroxymercuribenzoate but is strongly inhibited by chelators such as 1,10-phenanthroline, 8-hydroxyquinoline, and EDTA. The neutral metal chelator-sensitive activity requires calcium but the inhibitor data permit the conclusion that the metal ion inhibited by the chelators belongs to the first transition series and thus cannot be calcium. The neutral protease appears to be similar to proteases previously characterized from bacteria and snake venom. In moulting fluid from Manduca, proteolytic activity in vitro is very low in the presence of 1,10-phenanthroline at every pH studied except pH 7·7; in vivo, ecdysis is inhibited in Manduca larvae fed on diet containing a sufficient level (0·02 per cent or higher) of 1,10-phenanthroline. The metal chelator-sensitive proteolytic activity appears to be an essential moulting protease in Manduca.     

Sensory transduction in an insect mechanoreceptor: extended bandwidth measurements and sensitivity to simulus strenght

The dynamic properties of sensory transduction in an insect mechanoreceptor, the femoral tactile spine of the cockroach, Periplaneta americana, have been studied by measurement of the frequency response function between randomly varying movement of the tactile spine and afferent action potentials from the sensory neuron which innervates it. The frequency response function of the mechanoreceptor has been characterized over a frequency range which is more than ten times larger than has previously been used for this preparation. Also the effects of varying the amplitude of the stimulating signal have been studied by the use of a range of input signal strengths from about 0.5 to 10 m R.M.S. displacement. The measured frequency response functions can all be well fitted by a theoretical relationship which is a fractional exponent of complex frequency, provided that the time delay caused by conduction of the action potentials from the sensory dendrite to the recording electrodes is taken into account. Under small signal conditions the exponent of complex frequency is close to 0.5 but with larger displacements its value decreases to about half this value. The overall sensitivity of the receptor, as measured by the gain of the frequency response function at a natural frequency of 1 radian/s, is not significantly altered by changes in the input movement amplitude, so that the receptor behaves linearly in this respect. However, the mean rate of action potential occurrence is not linearly related to input movement amplitude. These results are discussed in terms of current theories of sensory transduction and the possible role of tubular bodies in the dynamic behaviour of insect cuticular mechanoreceptors.     

Electrical properties of the dendrite in an insect mechanoreceptor: Effects of antidromic or direct electrical stimulation

A study of the negative phase of the spikes recorded extra cellularly from insect mechanoreceptor has been performed in order to characterize some electrical properties of the dendrite which contains the transducing part of the sensory neuron. These properties have been investigated in mechanoreceptors of the metathoracic leg of the locust Schistocerca gregaria by firing antidromic action potentials both at rest and during mechanical or electrical stimulation. The amplitude of the negative phase of the spike appears to be correlated with the polarization of the dendritic membrane, although when bursts of action potentials are applied, the relation is more complex, including a depressive influence of a given spike on the following spike. The receptor potential and the antidromic dendritic spikes both originate in the same region of the dendrite but they involve different ionic processes. Our results indicate that the dendrite is electrically excitable. The spike which originates in the dendrite has an initial negative phase with a small superimposed positive component. A spike of this shape is never observed under natural stimulation. It is proposed that the negative phase of the antidromic impulse provides a suitable means for studying the variations in electrical polarization of the dendrite which cannot be recorded directly.     

Removal of rapid sensory adaptation from an insect mechanoreceptor neuron by oxidizing agents which affect sodium channel inactivation

1. The femoral tactile spine of the cockroach is a mechanoreceptor with a single sensory neuron. The response to a step movement is a burst of action potentials which decays to zero in about 1 s. This rapid adaptation is a property of the action potential initiating region of the neuron. 2. The oxidizing agents chloramine-T and N-chlorosuccinimide selectively and irreversibly remove sodium channel inactivation from neurons in several preparations and are believed to act by oxidation of methionine or cysteine residues in the proteins of the sodium channel. 3. Chloramine-T and N-chlorosuccinimide, applied for a controlled time period, eliminated the rapid adaptation of the tactile spine neuron to an electrical depolarization. After treatment it fired tonically in response to a steady current stimulus. Longer applications of the agents eventually raised the threshold for action potential initiation. 4. Threshold behavior in the tactile spine neuron was characterized by measuring strength-duration relationships for stimulation with extracellular current pulses at the action potential initiating region. The two oxidizing agents caused a voltage-dependent modification of the dynamic threshold properties which led to the change from rapidly adapting to tonic behavior. 5. Two stronger oxidizing agents, N-bromoacetamide and N-bromosuccinimide, raised the threshold of the neuron without removing rapid adaptation. These two agents act similarly to chloramine-T and N-chlorosuccinimide on sodium inactivation in other neurons but are believed to oxidize the tryptophan, tyrosine and histidine residues of proteins in addition to cysteine and methionine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cuticle degrading proteases from insect moulting fluid and culture filtrates of entomopathogenic fungi

Insects degrade their own cuticle during moulting, a process which is catalysed by a complex mixture of enzymes. Entomopathogenic fungi infect the insect host by penetration of the cuticle, utilizing enzymatic and/or physical mechanisms. Protein is a major component of insect cuticle and a major recyclable resource for the insect and, therefore, represents a significant barrier to the invading fungus. To this end, both insects and entomopathogenic fungi produce a variety of cuticle degrading proteases. The aim of this paper is to review these proteases and to highlight their similarities, with particular reference to the tobacco hornworm, Manduca sexta, and the entomopathogenic fungus, Metarhizium anisopliae     

Structure of the toad epidermis during the moulting cycle

Summary Skin samples were taken from 17 toads (Bufo bufo) in various phases of the moulting cycle. The phase was determined by recording a number of moulting intervals prior to sacrifice and by study of the macroscopic appearance of the skin and the moulting behaviour at sacrifice. Eight of the toads were in the moult when the samples were taken.Characteristic morphological changes were found to be restricted to a period immediately prior to and after shedding of the slough. Chemical changes of the membranes of stratum corneum (or material adhered to them) were observed already prior to adoption of the moulting posture. The separation from the underlying epidermis prior to shedding was accompanied by a swelling of the stratum corneum cells. After final detachment of the slough the replacement layer was differentiated into a new stratum corneum within 24 hours. Significant changes in the morphology of the flask cells were not observed.The findings are discussed with emphasis on the processes of separation and differentiation of the stratum corneum. Based on the morphology of the epidermis the following terminology is proposed for the phases of the moulting cycle: Intermoult phase, preparation phase, early shedding phase, late shedding phase, and differentiation phase.The authors wish to thank professor C.B. Jørgensen, Dr. D. P. Knight and Dr. E.H. Larsen for valuable discussions. The technical assistance of Miss Susanne Binzer and Mrs. Grete Budtz is gratefully acknowledged. G.B. kept records of the moults. Without her patient and careful observations during several weeks it had not been possible to obtain the present material.     

Branchial mechanoreceptor activity during spontaneous ventilation in channel catfish

Extracellular afferent neural activity was recorded in vivo from cranial nerve IX (glossopharyngeal) from mechanoreceptors in the first gill arch of anesthetized, spontaneously breathing channel catfish (Ictalurus punctatus). Single unit and paucifiber recordings show that both phasic and tonic receptors were active during normal ventilation. Phasic receptors were characterized as having a burst of activity during some phase of the ventilatory cycle. Most of these occurred during peak adduction or peak abduction. Phasic receptors were not active during spontaneous apnic periods. Tonic receptors were always active, even during apneas, firing frequency was modulated by breathing movements with peak activity occurring during adduction. Flow-sensitive mechanoreceptors were identified in anesthetized, paralyzed catfish. These receptors decreased activity when the ventilatory water flow was stopped. Hypercapnia (5% CO(2) in air) stimulated ventilatory rate and amplitude but had no effect on mechanoreceptor activity. The discharge characteristics of branchial mechanoreceptors indicate that they could be involved in the timing and coordination of ventilatory movements and maintenance of the 'gill curtain' to minimize ventilatory dead space. Unlike ventilatory mechanoreceptors in the air breathing organs of gar and lungs of lungfish and tetrapods, branchial mechanoreceptors were insensitive to hypercapnia.