Presynaptic inhibition and antidromic discharges in crayfish primary afferents

Mutual Suppression of Proximal and Distal Axonal Spike Initiation Determines the Output Patterns of a Motor Neuron

Axonal spike initiation at sites far from somatodendritic integration occurs in a range of systems, but its contribution to neuronal output activity is not well understood. We studied the interactions of distal and proximal spike initiation in an unmyelinated motor axon of the stomatogastric nervous system in the lobster, Homarus americanus. The peripheral axons of the pyloric dilator (PD) neurons generate tonic spiking in response to dopamine application. Centrally generated bursting activity and peripheral spike initiation had mutually suppressive effects. The two PD neurons and the electrically coupled oscillatory anterior burster (AB) neuron form the pacemaker ensemble of the pyloric central pattern generator, and antidromic invasion of central compartments by peripherally generated spikes caused spikelets in AB. Antidromic spikes suppressed burst generation in an activity-dependent manner: slower rhythms were diminished or completely disrupted, while fast rhythmic activity remained robust. Suppression of bursting was based on interference with the underlying slow wave oscillations in AB and PD, rather than a direct effect on spike initiation. A simplified multi-compartment circuit model of the pacemaker ensemble replicated this behavior. Antidromic activity disrupted slow wave oscillations by resetting the inward and outward current trajectories in each spike interval. Centrally generated bursting activity in turn suppressed peripheral spike initiation in an activity-dependent manner. Fast bursting eliminated peripheral spike initiation, while slower bursting allowed peripheral spike initiation to continue during the intervals between bursts. The suppression of peripheral spike initiation was associated with a small after-hyperpolarization in the sub-millivolt range. A realistic model of the PD axon replicated this behavior and showed that a sub-millivolt cumulative after-hyperpolarization across bursts was sufficient to eliminate peripheral spike initiation. This effect was based on the dynamic interaction between slow activity-dependent hyperpolarization caused by the Na⁺/K⁺-pump and inward rectification through the hyperpolarization-activated inward current, I _h . These results demonstrate that interactions between different spike initiation sites based on spike propagation can shift the relative contributions of different types of activity in an activity-dependent manner. Therefore, distal axonal spike initiation can play an important role in shaping neural output, conditional on the relative level of centrally generated activity.

State-Dependent Modification of Sensory Sensitivity via Modulation of Backpropagating Action Potentials

Neuromodulators play a critical role in sensorimotor processing via various actions, including pre- and postsynaptic signal modulation and direct modulation of signal encoding in peripheral dendrites. Here, we present a new mechanism that allows state-dependent modulation of signal encoding in sensory dendrites by neuromodulatory projection neurons. We studied the impact of antidromic action potentials (APs) on stimulus encoding using the anterior gastric receptor (AGR) neuron in the heavily modulated crustacean stomatogastric ganglion (STG). We found that ectopic AP initiation in AGR's axon trunk is under direct neuromodulatory control by the inferior ventricular (IV) neurons, a pair of descending projection neurons. IV neuron activation elicited a long-lasting decrease in AGR ectopic activity. This modulation was specific to the site of AP initiation and could be mimicked by focal application of the IV neuron co-transmitter histamine. IV neuron actions were diminished after blocking H2 receptors in AGR's axon trunk, suggesting a direct axonal modulation. This local modulation did not affect the propagation dynamics of en passant APs. However, decreases in ectopic AP frequency prolonged sensory bursts elicited distantly near AGR's dendrites. This frequency-dependent effect was mediated via the reduction of antidromic APs, and the diminishment of backpropagation into the sensory dendrites. Computational models suggest that invading antidromic APs interact with local ionic conductances, the rate constants of which determine the sign and strength of the frequency-dependent change in sensory sensitivity. Antidromic APs therefore provide descending projection neurons with a means to influence sensory encoding without affecting AP propagation or stimulus transduction.

Comparative biology of pain: What invertebrates can tell us about how nociception works

The inability to adequately treat chronic pain is a worldwide health care crisis. Pain has both an emotional and a sensory component, and this latter component, nociception, refers specifically to the detection of damaging or potentially damaging stimuli. Nociception represents a critical interaction between an animal and its environment and exhibits considerable evolutionary conservation across species. Using comparative approaches to understand the basic biology of nociception could promote the development of novel therapeutic strategies to treat pain, and studies of nociception in invertebrates can provide especially useful insights toward this goal. Both vertebrates and invertebrates exhibit segregated sensory pathways for nociceptive and nonnociceptive information, injury-induced sensitization to nociceptive and nonnociceptive stimuli, and even similar antinociceptive modulatory processes. In a number of invertebrate species, the central nervous system is understood in considerable detail, and it is often possible to record from and/or manipulate single identifiable neurons through either molecular genetic or physiological approaches. Invertebrates also provide an opportunity to study nociception in an ethologically relevant context that can provide novel insights into the nature of how injury-inducing stimuli produce persistent changes in behavior. Despite these advantages, invertebrates have been underutilized in nociception research. In this review, findings from invertebrate nociception studies are summarized, and proposals for how research using invertebrates can address questions about the fundamental mechanisms of nociception are presented.

Combined Changes in Chloride Regulation and Neuronal Excitability Enable Primary Afferent Depolarization to Elicit Spiking without Compromising its Inhibitory Effects

The central terminals of primary afferent fibers experience depolarization upon activation of GABAA receptors (GABAAR) because their intracellular chloride concentration is maintained above electrochemical equilibrium. Primary afferent depolarization (PAD) normally mediates inhibition via sodium channel inactivation and shunting but can evoke spikes under certain conditions. Antidromic (centrifugal) conduction of these spikes may contribute to neurogenic inflammation while orthodromic (centripetal) conduction could contribute to pain in the case of nociceptive fibers. PAD-induced spiking is assumed to override presynaptic inhibition. Using computer simulations and dynamic clamp experiments, we sought to identify which biophysical changes are required to enable PAD-induced spiking and whether those changes necessarily compromise PAD-mediated inhibition. According to computational modeling, a depolarizing shift in GABA reversal potential (EGABA) and increased intrinsic excitability (manifest as altered spike initiation properties) were necessary for PAD-induced spiking, whereas increased GABAAR conductance density (ḡGABA) had mixed effects. We tested our predictions experimentally by using dynamic clamp to insert virtual GABAAR conductances with different EGABA and kinetics into acutely dissociated dorsal root ganglion (DRG) neuron somata. Comparable experiments in central axon terminals are prohibitively difficult but the biophysical requirements for PAD-induced spiking are arguably similar in soma and axon. Neurons from naïve (i.e. uninjured) rats were compared before and after pharmacological manipulation of intrinsic excitability, and against neurons from nerve-injured rats. Experimental data confirmed that, in most neurons, both predicted changes were necessary to yield PAD-induced spiking. Importantly, such changes did not prevent PAD from inhibiting other spiking or from blocking spike propagation. In fact, since the high value of ḡGABA required for PAD-induced spiking still mediates strong inhibition, we conclude that PAD-induced spiking does not represent failure of presynaptic inhibition. Instead, diminished PAD caused by reduction of ḡGABA poses a greater risk to presynaptic inhibition and the sensory processing that relies upon it.

The effect of sensory feedback on crayfish posture and locomotion: II. Neuromechanical simulation of closing the loop

Neuromechanical simulation was used to determine whether proposed thoracic circuit mechanisms for the control of leg elevation and depression in crayfish could account for the responses of an experimental hybrid neuromechanical preparation when the proprioceptive feedback loop was open and closed. The hybrid neuromechanical preparation consisted of a computational model of the fifth crayfish leg driven in real time by the experimentally recorded activity of the levator and depressor (Lev/Dep) nerves of an in vitro preparation of the crayfish thoracic nerve cord. Up and down movements of the model leg evoked by motor nerve activity released and stretched the model coxobasal chordotonal organ (CBCO); variations in the CBCO length were used to drive identical variations in the length of the live CBCO in the in vitro preparation. CBCO afferent responses provided proprioceptive feedback to affect the thoracic motor output. Experiments performed with this hybrid neuromechanical preparation were simulated with a neuromechanical model in which a computational circuit model represented the relevant thoracic circuitry. Model simulations were able to reproduce the hybrid neuromechanical experimental results to show that proposed circuit mechanisms with sensory feedback could account for resistance reflexes displayed in the quiescent state and for reflex reversal and spontaneous Lev/Dep bursting seen in the active state.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Collateral actions of premotor interneurons on ventral spinocerebellar tract neurons in the cat

Strong evidence that premotor interneurons provide ventral spinocerebellar tract (VSCT) neurons with feedback information on their actions on motoneurons was previously found for Ia inhibitory interneurons and Renshaw cells, while indications for similar actions of other premotor interneurons were weaker and indirect. Therefore the aim of the present study was to reexamine this possibility with respect to interneurons relaying actions of group Ib afferents from tendon organs and group II afferents from muscle spindles. In all, 133 VSCT neurons in the L3-L5 segments (including 41 spinal border neurons) were recorded from intracellularly in deeply anesthetized cats to verify that stimuli applied in motor nuclei evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) attributable to stimulation of axon collaterals of premotor interneurons. IPSPs were found in over two thirds of the investigated neurons. When intraspinal stimuli were preceded by stimuli applied to a muscle nerve at critical intervals, IPSPs evoked from motor nuclei were considerably reduced, indicating a collision of nerve volleys in axons of interneurons activated by group I and group II afferents. In individual VSCT neurons monosynaptic IPSPs were evoked from both biceps-semitendinosus and gastrocnemius-soleus motor nuclei, in parallel with disynaptic IPSPs from group Ib and group II as well as group Ia afferents. These observations indicate that individual VSCT neurons may monitor the degree of inhibition of both flexor and extensor motoneurons by premotor interneurons in inhibitory pathways from group Ib and group II afferents to motoneurons. They may thus be providing the cerebellum with feedback information on actions of these premotor interneurons on motoneurons.

Regulation of motor patterns by the central spike-initiation zone of a sensory neuron

Sensory feedback from muscles and peripheral sensors acts to initiate, tune or reshape motor activity according to the state of the body. Yet, sensory neurons often show low levels of activity even in the absence of sensory input. Here we examine the functional role of spontaneous low-frequency activity of such a sensory neuron. The anterior gastric receptor (AGR) is a muscle-tendon organ in the crab stomatogastric nervous system whose phasic activity shapes the well-characterized gastric mill (chewing) and pyloric (filtering) motor rhythms. Phasic activity is driven by a spike-initiation zone near the innervated muscle. We demonstrate that AGR possesses a second spike-initiation zone, which is located spatially distant from the innervated muscle in a central section of the axon. This initiation zone generates tonic activity and is responsible for the spontaneous activity of AGR in vivo, but does not code sensory information. Rather, it is sensitive to the neuromodulator octopamine. A computational model indicates that the activity at this initiation zone is not caused by excitatory input from another neuron, but generated intrinsically. This tonic activity is functionally relevant, because it modifies the activity state of the gastric mill motor circuit and changes the pyloric rhythm. The sensory function of AGR is not impaired as phasic activity suppresses spiking at the central initiation zone. Our results thus demonstrate that sensory neurons are not mere reporters of sensory signals. Neuromodulators can elicit non-sensory coding activity in these neurons that shapes the state of the motor system.

Retrograde signaling from the brain to the retina modulates the termination of the light response in Drosophila

A critical factor in visual function is the speed with which photoreceptors (PRs) return to the resting state when light intensity dims. Several elements subserve this process, many of which promote the termination of the phototransduction cascade. Although the known elements are intrinsic to PRs, we have found that prompt restoration to the resting state of the Drosophila electroretinogram can require effective communication between the retina and the underlying brain. The requirement is seen more dramatically with long than with short light pulses, distinguishing the phenomenon from gross disruption of the termination machinery. The speed of recovery is affected by mutations (in the Hdc and ort genes) that prevent PRs from transmitting visual information to the brain. It is also affected by manipulation (using either drugs like neostigmine or genetic tools to inactivate neurotransmitter release) of cholinergic signals that arise in the brain. Intracellular recordings support the hypothesis that PRs are the target of this communication. We infer that signaling from the retina to the optic lobe prompts a feedback signal to retinal PRs. Although the mechanism of this retrograde signaling remains to be discerned, the phenomenon establishes a previously unappreciated mode of control of the temporal responsiveness of a primary sensory neuron.

In Vivo Analysis of Proprioceptive Coding and Its Antidromic Modulation in the Freely Behaving Crayfish

Although sensory nerves in vitro are known to convey both orthodromic (sensory) and antidromic (putatively modulating) action potentials, in most cases very little is known about their bidirectional characteristics in intact animals. Here, we have investigated both the sensory coding properties and antidromic discharges that occur during real walking in the freely behaving crayfish. The activity of the sensory nerve innervating the proprioceptor CBCO, a chordotonal organ that monitors both angular movement and position of the coxo-basipodite (CB) joint, which is implicated in vertical leg movements, was recorded chronically along with the electromyographic activity of the muscles that control CB joint movements. Two wire electrodes placed on the sensory nerve were used to discriminate orthodromic from antidromic action potentials and thus allowed for analysis of both sensory coding and antidromic discharges. A distinction is proposed between 3 main classes of sensory neuron, according to their firing in relation to levator muscle activity during free walking. In parallel, we describe 2 types of antidromic activity: one produced exclusively during motor activity and a second produced both during and in the absence of motor activity. A negative correlation was found between the activity of sensory neurons in each of the 3 classes and identified antidromic discharges during walking. Finally, a state-dependent plasticity of CBCO nerve activity has been found by which the distribution of sensory orthodromic and antidromic activity changes with the physiological state of the biomechanical apparatus.

Effects of antidromic discharges in crayfish primary afferents

Contrary to orthodromic spikes that are generated in sensory organs and conveyed to CNS, antidromic spikes are generated in the axon terminals of the sensory neurons within the CNS and are conveyed to the peripheral sensory organ. Antidromic discharges are observed in primary afferent neurons of both vertebrates and invertebrates and seem to be related to the rhythmic activity of central neural networks. In this study, we analyzed the effect of antidromic discharges on the sensory activity of a leg proprioceptor in in vitro preparations of the crayfish CNS. Intracellular microelectrodes were used both to record the orthodromic spikes and to elicit antidromic spikes by injecting squares pulses of depolarizing current at various frequencies. Experiments were performed on the three types of identified sensory afferents (tonic, phasotonic, and phasic). The main results showed a reduction of the firing frequency of the orthodromic activity in 82% of the tested afferents. In tonic afferents, during their occurrences and according to their frequency, antidromic spikes or bursts reduced or suppressed the orthodromic activity. Following their terminations, they also induced a silent period and a gradual recovery of the orthodromic activity, both of which increased as the duration and the frequency of the antidromic bursts increased. In phasotonic and phasic afferents, antidromic bursts reduced or suppressed the phasic responses as their frequency and durations increased. In phasotonic afferents, if elicited prior to the movements, long-duration bursts with increasing frequency reduced more rapidly the tonic background activity than the phasic one whereas short-duration bursts at high frequency produced strong decreases of both. The effect of antidromic bursts accumulated when they are repetitively elicited. Antidromic bursts induced a much larger decrease of the sensory activity than adaptation alone. The occurrences of antidromic spikes or bursts may have a functional role in modulating the incoming sensory messages during locomotion. The mechanisms by which antidromic spikes modulate the firing sensitivity of the primary afferents may well lie in modifications of the properties of either mecanotransduction and/or spike initiation.

Efferent controls in crustacean mechanoreceptors

Since the 1960s it has been known that central neural networks can elaborate motor patterns in the absence of any sensory feedback. However, sensory and neuromodulatory inputs allow the animal to adapt the motor command to the actual mechanical configuration or changing needs. Many studies in invertebrates, particularly in crustacea, have described several mechanisms of sensory-motor integration and have shown that part of this integration was supported by the efferent control of the mechanosensory neurons themselves. In this article, we review the findings that support such an efferent control of mechanosensory neurons in crustacea. Various types of crustacean proprioceptors feeding information about joint movements and strains to central neural networks are considered, together with evidence of efferent controls exerted on their sensory neurons. These efferent controls comprise (1) the neurohormonal modulation of the coding properties of sensory neurons by bioamines and peptides; (2) the presynaptic inhibition of sensory neurons by GABA, glutamate and histamine; and (3) the long-term potentiation of sensory-motor synapses by glutamate. Several of these mechanisms can coexist on the same sensory neuron, and the functional significance of such multiple modulations is discussed.

Functional multimodality of axonal tree in invertebrate neurons

This review, based on invertebrate neuron examples, aims at highlighting the functional consequences of axonal tree organization. The axonal organization of invertebrate neurons is very complex both morphologically and physiologically. The first part shows how the transfer of information along sensory axons is modified by presynaptic inhibition mechanisms. In primary afferents, presynaptic inhibition is involved in: 1) increasing the dynamic range of the sensory response; 2) processing the sensory information such as increasing spatial and/or temporal selectivity; 3) discriminating environmental information from sensory activities generated by the animal's own movement; and 4) modulating the gain of negative feedback (resistance reflex) during active rhythmic movements such as locomotion. In a second part, the whole organization of other types of neurons is considered, and evidence is given that a neuron may not work as a unit, but rather as a mosaic of disconnected 'integrate-and-fire' units. Examples of invertebrate neurons are presented in which several spike initiating zones exist, such as in some stomatogastric neurons. The separation of a neuron into two functionally distinct entities may be almost total with distinct arborizations existing in different ganglia. However, this functional separation is not definitive and depends on the state of the neuron. In conclusion, the classical integrate-and-fire representation of the neuron, with its dendritic arborization, its spike initiating zone, its axon and axonal tree seems to be no more applicable to invertebrate neurons. A better knowledge of the function of vertebrate neurons would probably demonstrate that it is the case for a large number of them, as suggested by the complex architecture of some reticular interneurons in vertebrates.

Flexible processing of sensory information induced by axo-axonic synapses on afferent fibers

Recent experiments indicate that afferent information is processed in the intraspinal arborisation of mammalian group I fibres. During muscle contraction, Ib inputs arising from tendon organs are filtered out by presynaptic inhibition after their entry in the spinal cord. This paper reviews the mechanisms by which GABAergic axo-axonic synapses, i.e., the morphological substrate of presynaptic inhibition, exert this filtering effect. Using confocal microscopy, axo-axonic synapses were demonstrated on segmental Ib collaterals. Most synapses were located on short preterminal and terminal branches. Using a simple compartmental model of myelinated axon, the primary afferent depolarisation (PAD), generated by such synapses, was predicted to reduce the amplitude of incoming action potentials by inactivating the sodium current, and this prediction was experimentally verified. A further theoretical work, relying on cable theory, suggests that the electrotonic structure of collaterals and the distribution of axo-axonic synapses allow large PADs (about 10 mV) to develop on some distal branches, which is likely to result in a substantial presynaptic inhibition. In addition, the electrotonic structure of group I collaterals is likely to prevent PAD from spreading to the whole arborisation. Such a non-uniform diffusion of the PAD accounts for differential presynaptic inhibition in intraspinal branches of the same fibre. Altogether, our experimental and theoretical works suggest that axo-axonic synapses can control the selective funnelling of sensory information toward relevant targets specified according to the motor task.