Direct chemically mediated synaptic transmission from mechanosensory afferents contributes to habituation of crayfish lateral giant escape reaction

The neural mechanism of habituation of the crayfish lateral giant-mediated escape reaction was analyzed electrophysiologically and pharmacologically. Upon repeated stimulation of tailfan afferents (at 0.2–1 Hz) lateral giant showed rapid habituation and failed to spike. Upon low-intensity sensory stimulation, the lateral giant responded with two subthreshold excitatory post-synaptic potentials, the and components. A third component, the ' component, was discriminated at the boundary of excitatory post-synaptic potentials between the late and early components with stimulation just subthreshold or suprathreshold to evoke lateral giant spikes. This ' component increased in amplitude with hyperpolarizing current injected into the lateral giants, although the amplitude of both the and components remained constant. Furthermore, bath application of the nicotinic antagonist, d-tubocurarine caused a rapid reduction in the amplitude of the ' component while the amplitude of the component was reduced gradually and that of the component remained unchanged. Single-hair stimulation indicated that some sensory afferents made direct connections with the lateral giants mediated by chemical synapses and form the potential of ' component of the lateral giants. Since lateral giant inactivation was associated with a reduction of excitatory post-synaptic potential amplitude of the ' component, connection from these afferents could contribute, at least in part, to lateral giant habituation.Abbreviations EPSP excitatory post-synaptic potential - LG lateral giant     

Habituation of mechanoreceptive interneurons in the crayfish.

The effect of repetition of sensory stimuli was studied on the responses of mechanoreceptive interneurons in the optic tract of the crayfish (Procambarus bouvieri (Ortmann)). The number of spikes recorded from a given unit gradually decreased during a train of stimuli. The decrease showed a negative exponential time course, with a curvature dependent upon the frequency of stimulation, the intensity of stimuli, and the hour of the day. Habituation is selective for the intensity and rate of stimulation, and the particular spot of receptive field stimulated. Locomotor excitation results in a dimminution of the rate of decay. The effect of a single train of stimuli when leading to pronounced habituation may persist for periods longer than 24 hr.     

Lateral giant fibre activation of exopodite motor neurones in the crayfish tailfan

The uropods of decapod crustaceans play a major role in the production of thrust during escape swimming. Here we analyse the output connections of a pair of giant interneurones, that mediate and co-ordinate swimming tail flips, on motor neurones that control the exopodite muscles of the uropods. The lateral giants make short latency output connections with phasic uropod motor neurones, including the productor, the lateral abductor and adductor exopodite motor neurones that we have identified both physiologically and anatomically. On the other hand, tonic motor neurones, including the ventral abductor and reductor exopodite motor neurones, receive no input from the lateral giants. We show that there is no simple reciprocal activation of the phasic opener (lateral abductor) and closer (adductor) motor neurones of the exopodite, but instead both phasic motor neurones are activated in parallel with the productor motor neurone during a tail flip. Our results show that the neuronal pathways activating the tonic and phasic motor neurones of the exopodite are apparently independent, with phasic motor neurones being activated during escape movements and tonic motor neurones being activated during slow postural movements.     

Habituation of mechanoreceptive interneurons in the crayfish

The effect of repetition of sensory stimuli was studied on the responses of mechanoreceptive interneurons in the optic tract of the crayfish (Procambarus bouvieri (Ortmann)). The number of spikes recorded from a given unit gradually decreased during a train of stimuli. The decrease showed a negative exponential time course, with a curvature dependent upon the frequency of stimulation, the intensity of stimuli, and the hour of the day. Habituation is selective for the intensity and rate of stimulation, and the particular spot of receptive field stimulated. Locomotor excitation results in a dimminution of the rate of decay. The effect of a single train of stimuli when leading to pronounced habituation may persist for periods longer than 24 hr.     

Monosynaptic excitation of lateral giant fibres by proprioceptive afferents in the crayfish

Giant interneurones mediate a characteristic `tail flip' escape response of the crayfish, Procambarus clarkii, which move it rapidly away from the source of stimulation. We have analysed the synaptic connections of proprioceptive sensory neurones with one type of giant interneurone, the lateral giant. Spikes in sensory neurones innervating an exopodite-endopodite chordotonal organ in the tailfan, which monitors the position and movements of the exopodite, are followed at a short and constant latency by excitatory postsynaptic potentials in a lateral giant interneurone (LG) recorded in the terminal abdominal ganglion. These potentials are unaffected by manipulation of the membrane potential of LG, by bath application of saline with a low calcium concentration, or by one containing the nicotinic antagonist, curare. The potentials evoked in LG by chordotonal organ stimulation are thus thought to be monosynaptic and electrically mediated. This is the first demonstration that LG receives input from sensory receptors other than exteroceptors in the terminal abdominal ganglion. Accepted: 7 April 1997     

Extrinsic modulation of crayfish escape behaviour.

Extrinsic systems were shown to control the excitability of the neurones which mediate tail-flip escape in the crayfish. Restraint suppresses the escape mediated by giant fibres and some, but not all, categories of non-giant mediated escape; autotomy of claws increases the excitability of non-giant mediated escape without affecting the lateral giant reflex. The effects of restraint on the lateral giant reflex result from inhibition rather than reduced facilitation. The inhibition descends from thoracic and higher levels, and the lateral giant escape command neurone appears to be its primary target. Inhibition may serve to shift the control of escape behaviour from short latency 'reflex' systems to more flexible 'voluntary' ones which can produce responses at times most opportune for successful escape.     

Long-lasting potentiation of excitatory synaptic signaling to the crayfish lateral giant neuron

The neural circuit that underlies the lateral giant fiber (LG)-mediated reflex escape in crayfish has provided findings relating synaptic change to nonassociative learning such as sensitization and habituation. The LGs receive sensory inputs from the primary sensory afferents and a group of mechanosensory interneurons (MSIs). An increase of excitability by suprathreshold repetitive excitation of this circuit, which is similar to Hebbian long-term potentiation (LTP), has been reported [Miller et al. (1987) J Neurosci 7:1081]. This potentiation was previously thought to result from the enhancement of transmission at cholinergic synapses between primary afferents and MSIs but not the electrical synapses onto LG. In this study, we found that potentiation of synaptic signaling at the electrical synapse onto LG can also be induced when the synapse was activated with subthreshold repetitive pulses or with a few strong suprathreshold shocks. LG LTP was induced in the preparation which had received pulses at limited frequency range. Although whether this LTP is involved in the learning process of escape behavior in crayfish is not clear, the intensity and amount of sensory stimulation used here mimicked those that could easily be produced by a predator trying to catch a crayfish and could be of adaptive significance in life.     

Electron tomographic analysis of gap junctions in lateral giant fibers of crayfish

Innexin-gap junctions in crayfish lateral giant fibers (LGFs) have an important role in escape behavior as a key component of rapid signal transduction. Knowledge of the structure and function of characteristic vesicles on the both sides of the gap junction, however, is limited. We used electron tomography to analyze the three-dimensional structure of crayfish gap junctions and gap junctional vesicles (GJVs). Tomographic analyses showed that some vesicles were anchored to innexons and almost all vesicles were connected by thin filaments. High densities inside the GJVs and projecting densities on the GJV membranes were observed in fixed and stained samples. Because the densities inside synaptic vesicles were dependent on the fixative conditions, different fixative conditions were used to elucidate the molecules included in the GJVs. The projecting densities on the GJVs were studied by immunoelectron microscopy with anti-vesicular monoamine transporter (anti-VMAT) and anti-vesicular nucleotide transporter (anti-VNUT) antibodies. Some of the projecting densities were labeled by anti-VNUT, but not anti-VMAT. Three-dimensional analyses of GJVs and excitatory chemical synaptic vesicles (CSVs) revealed clear differences in their sizes and central densities. Furthermore, the imaging data obtained under different fixative conditions and the immunolabeling results, in which GJVs were positively labeled for anti-VNUT but excitatory CSVs were not, support our model that GJVs contain nucleotides and excitatory CSVs do not. We propose a model in which characteristic GJVs containing nucleotides play an important role in the signal processing in gap junctions of crayfish LGFs.     

Modulation of the crayfish escape reflex--physiology and neuroethology

We review here factors that control the excitability of thegiant neuron-mediated tail-flip escape behavior in crayfish,focusing especially on recent findings concerning serotonergicmodulation. Serotonin can either facilitate or inhibit escapedepending on concentration and pattern of application. Low concentrationsfacilitate while high ones inhibit; however, if high concentrationsarise gradually they facilitate instead of inhibiting. The effectsof serotonin can also be altered by social experience, withapplication regimens that cause facilitation in social isolatescoming to produce inhibition after an extended period of livingas a subordinate. Attempts to understand both the possible physiologicalbasis of some of these complexities and their possible functionare discussed. Neuroethological investigations indicate thatgiant neuron-mediated escape is inhibited during the initialfights that establish social relationships and is facilitatedin their immediate aftermath. Once the relationship of a pairis well-established, the presence of the dominant tends to suppressgiant neuron-mediated escape (but not tail-flip escape mediatedby non-giant circuitry) in the subordinate, but the presenceof the subordinate has relatively little effect on the dominant.These patterns of modulation can be seen as consistent withthe known variations in serotonin's effect as a function ofconcentration and social experience and may provide a biologicalreason for these variations.     

Thoracic output of crayfish giant fibres

Summary The thoracic homologue of the abdominal segmental giant neurone of crayfish Pacifastacus leniusculus is identified and described. It has a small cell body located in the anterior ventro-lateral quadrant of the ganglion and a large neuropil arborization, with dendrites aligned along the tracts of the giant fibres. The SG axon exits the ganglion within the major root which innervates the leg, usually in the anterior region of this root. Within 1–2 mm of the ganglion the axon terminates in a mass of fine branches, apparently randomly located within the base of the root.The SG receives suprathreshold input from the ipsilateral MG and LG fibres through rectifying electrical synapses. It makes output to FF motor neurones, also through electrical synapses. The SG also makes output to at least one corollary discharge interneurone. The SG receives depolarizing inhibitory synaptic potentials which can prevent its activation by the GFs. Some but not all of these synaptic potentials are common to similar potentials occurring in a large leg promotor motor neurone.Abbreviations AC anterior connective - GF giant fibre - IPSP inhibitory post-synaptic potential - LG lateral giant fibre - MG medial giant fibre - MoG motor giant neurone - PC posterior connective - PMM promotor motor neurone - r1 first root - r3 third root - rAD anterior distal root - rPD posterior distal root - rPM promotor muscle root - SG segmental giant neurone