Shunting versus inactivation: analysis of presynaptic inhibitory mechanisms in primary afferents of the crayfish

Feedback inhibition by a descending GABAergic neuron regulates timing of escape behavior in Drosophila larvae

Escape behaviors help animals avoid harm from predators and other threats in the environment. Successful escape relies on integrating information from multiple stimulus modalities (of external or internal origin) to compute trajectories toward safe locations, choose between actions that satisfy competing motivations, and execute other strategies that ensure survival. To this end, escape behaviors must be adaptive. When a Drosophila melanogaster larva encounters a noxious stimulus, such as the focal pressure a parasitic wasp applies to the larval cuticle via its ovipositor, it initiates a characteristic escape response. The escape sequence consists of an initial abrupt bending, lateral rolling, and finally rapid crawling. Previous work has shown that the detection of noxious stimuli primarily relies on class IV multi-dendritic arborization neurons (Class IV neurons) located beneath the body wall, and more recent studies have identified several important components in the nociceptive neural circuitry involved in rolling. However, the neural mechanisms that underlie the rolling-escape sequence remain unclear. Here, we present both functional and anatomical evidence suggesting that bilateral descending neurons within the subesophageal zone of D. melanogaster larva play a crucial role in regulating the termination of rolling and subsequent transition to escape crawling. We demonstrate that these descending neurons (designated SeIN128) are inhibitory and receive inputs from a second-order interneuron upstream (Basin-2) and an ascending neuron downstream of Basin-2 (A00c). Together with optogenetic experiments showing that co-activation of SeIN128 neurons and Basin-2 influence the temporal dynamics of rolling, our findings collectively suggest that the ensemble of SeIN128, Basin-2, and A00c neurons forms a GABAergic feedback loop onto Basin-2, which inhibits rolling and thereby facilitates the shift to escape crawling.

Facilitation of sensory transmission to motoneurons during cortical or sensory-evoked primary afferent depolarization (PAD) in humans

Sensory and corticospinal tract (CST) pathways activate spinal GABAergic interneurons that have axoaxonic connections onto proprioceptive (Ia) afferents that cause long-lasting depolarizations (termed primary afferent depolarization, PAD). In rodents, sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia afferents, rather than at presynaptic terminals, and facilitates spike propagation to motoneurons by preventing branch-point failures, rather than causing presynaptic inhibition. We examined in 40 human participants whether putative activation of Ia-PAD by sensory or CST pathways can also facilitate Ia afferent activation of motoneurons via the H-reflex. H-reflexes in several leg muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar long-lasting time course (∼200 ms) to phasic PAD measured in rodent Ia afferents. Long trains of cutaneous or proprioceptive afferent conditioning produced longer-lasting facilitation of the H-reflex for up to 2 min, consistent with tonic PAD in rodent Ia afferents mediated by nodal α5-GABAA receptors for similar stimulation trains. Facilitation of H-reflexes by this conditioning was likely not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not alone facilitate the tonic firing rate of motor units. Furthermore, cutaneous conditioning increased the firing probability of single motor units (motoneurons) during the H-reflex without increasing their firing rate at this time, indicating that the underlying excitatory postsynaptic potential was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia afferent branches. KEY POINTS: Controlled execution of posture and movement requires continually adjusted feedback from peripheral sensory pathways, especially those that carry proprioceptive information about body position, movement and effort. It was previously thought that the flow of proprioceptive feedback from Ia afferents was only reduced by GABAergic neurons in the spinal cord that sent axoaxonic projections to the terminal endings of sensory axons (termed GABAaxo neurons). Based on new findings in rodents, we provide complementary evidence in humans to suggest that sensory and corticospinal pathways known to activate GABAaxo neurons that project to dorsal parts of the Ia afferent also increase the flow of proprioceptive feedback to motoneurons in the spinal cord. These findings support a new role for spinal GABAaxo neurons in facilitating afferent feedback to the spinal cord during voluntary or reflexive movements.

Bumetanide increases postsynaptic inhibition after chronic SCI and decreases presynaptic inhibition with step-training

Current anti-spastic medication significantly compromises motor recovery after spinal cord injury (SCI), indicating a critical need for alternative interventions. Because a shift in chloride homeostasis decreases spinal inhibition and contributes to hyperreflexia after SCI, we investigated the effect of bumetanide, an FDA-approved sodium-potassium-chloride intruder (NKCC1) antagonist, on presynaptic and postsynaptic inhibition. We compared its effect with step-training as it is known to improve spinal inhibition by restoring chloride homeostasis. In SCI rats, a prolonged bumetanide treatment increased postynaptic inhibition but not presynaptic inhibition of the plantar H-reflex evoked by posterior biceps and semitendinosus (PBSt) group I afferents. By using in vivo intracellular recordings of motoneurons, we further show that a prolonged bumetanide increased postsynaptic inhibition by hyperpolarizing the reversal potential for inhibitory postsynaptic potentials (IPSPs) after SCI. However, in step-trained SCI rats an acute delivery of bumetanide decreased presynaptic inhibition of the H-reflex, but not postsynaptic inhibition. These results suggest that bumetanide might be a viable option to improve postsynaptic inhibition after SCI, but it also decreases the recovery of presynaptic inhibition with step-training. We discuss whether the effects of bumetanide are mediated by NKCC1 or by off-target effects. KEY POINTS: After spinal cord injury (SCI), chloride homeostasis is dysregulated over time in parallel with the decrease in presynaptic inhibition of Ia afferents and postsynaptic inhibition of motoneurons, and the development of spasticity. While step-training counteracts these effects, it cannot always be implemented in the clinic because of comorbidities. An alternative intervention is to use pharmacological strategies to decrease spasticity without hindering the recovery of motor function with step-training. Here we found that, after SCI, a prolonged bumetanide (an FDA-approved antagonist of the sodium-potassium-chloride intruder, NKCC1) treatment increases postsynaptic inhibition of the H-reflex, and it hyperpolarizes the reversal potential for inhibitory postsynaptic potentials in motoneurons. However, in step-trained SCI, an acute delivery of bumetanide decreases presynaptic inhibition of the H-reflex, but not postsynaptic inhibition. Our results suggest that bumetanide has the potential to decrease spastic symptoms related to a decrease in postsynaptic but not presynaptic inhibition after SCI.

Activity-dependent decline and recovery of synaptic transmission in central parts of surviving primary afferents after their peripheral cut in crayfish

Axons deprived of their nucleus degenerate within a few days in mammals but survive for several months in crustaceans. However, it is not known whether central synapses from sensory axons may preserve their molecular machinery in the absence of spiking activity. To assess this, we used peripheral axotomy, which removes their nuclei combined with electrophysiology techniques and electron microscopy imaging. We report the following. (1) Electron microscopy analysis confirms previous observations that glial cell nuclei present in the sensory nerve proliferate and migrate to axon tubes, where they form close contacts with surviving axons. (2) After peripheral axotomy performed in vivo on the coxo-basipodite chordotonal organ (CBCO), the sensory nerve does not convey any sensory message, but antidromic volleys are observed. (3) Central synaptic transmission from the CBCO to motoneurons (MNs) progressively declines over 200 days (90% of monosynaptic excitatory transmission is lost after 3 weeks, whereas 60% of disynaptic inhibitory transmission persists up to 6 months). After 200 days, no transmission is observed. (4) However, this total loss is apparent only because repetitive electrical stimulation of the sensory nerve in vitro progressively restores first inhibitory post-synaptic potentials and then excitatory post-synaptic potentials. (5) The loss of synaptic transmission can be prevented by in vivo chronic sensory nerve stimulation. (6) Using simulations based on the geometric arrangements of synapses of the monosynaptic excitatory transmission and disynaptic inhibitory pathways, we show that antidromic activity in the CBCO nerve could play a role in the maintenance of synaptic function of inhibitory pathways to MNs, but not monosynaptic excitatory transmission to MNs. Our study confirms the deep changes in glial nuclei observed in axons deprived of their nucleus. We further show that the machinery for spike conduction and synaptic release persists for several months, even if there is no longer any activity. Indeed, we were able to restore, with electrical activity, spike conduction and synaptic function after long silent periods (>6 months).

GABA facilitates spike propagation through branch points of sensory axons in the spinal cord

Movement and posture depend on sensory feedback that is regulated by specialized GABAergic neurons (GAD2+) that form axo-axonic contacts onto myelinated proprioceptive sensory axons and are thought to be inhibitory. However, we report here that activating GAD2+ neurons directly with optogenetics or indirectly by cutaneous stimulation actually facilitates sensory feedback to motor neurons in rodents and humans. GABAA receptors located at or near nodes of Ranvier of sensory axons cause this facilitation by preventing spike propagation failure at the many axon branch points, which is otherwise common without GABA. In contrast, GABAA receptors are generally lacking from axon terminals and so cannot inhibit transmitter release onto motor neurons, unlike GABAB receptors that cause presynaptic inhibition. GABAergic innervation near nodes and branch points allows individual branches to function autonomously, with GAD2+ neurons regulating which branches conduct, adding a computational layer to the neuronal networks generating movement and likely generalizing to other central nervous system axons.

Modeling analysis of subthreshold voltage signaling along hippocampal mossy fiber axons

Axons are classically thought of as electrically well isolated from other parts of the neurons due to the shape of a long cable-like structure. In contrast to this classical view on axonal compartmentalization, recent studies revealed that subthreshold depolarization of soma and dendrite passively propagates to the axons for a substantial distance, as demonstrated in some experimentally accessible axons including hippocampal mossy fibers and cortical pyramidal cell axons. Passive propagation of subthreshold dendritic EPSPs to the axons, defined as EPreSPs (excitatory presynaptic potentials), has been demonstrated to affect transmitter release from the axon terminals. To further characterize and explore the functional significance of passive subthreshold voltage signaling along the axons, the model of EPreSPs along hippocampal mossy fibers, proposed by Alle and Geiger, was reconstructed on the NEURON simulator. To test the effect of EPreSPs on action potentials and transmitter release from the axon terminals, additional conductances were incorporated into the previous passive propagation model. These include the axonal sodium, potassium, and leak channels as well as presynaptic calcium channels composed of P/Q-, N-, and R-types, which are reconstructed from the properties of those recorded from mossy fiber boutons experimentally. In this revised model, the preceding subthreshold EPreSPs slightly reduced the action potential-evoked presynaptic calcium currents by a decrease in the amplitude of action potentials due to the slow depolarization. It should be mentioned that EPreSPs by themselves elicited small calcium currents during subthreshold depolarization through these high-voltage activated calcium channels. Since the previous experimental study by simultaneous pre and postsynaptic recordings demonstrated that EPreSPs enhanced action potential-evoked transmitter release from the mossy fiber terminals, it has been suggested that different mechanisms from the enhancement of action potential-evoked presynaptic calcium entry may involve enhanced transmitter release by EPreSP. Small calcium entry by subthreshold EPreSPs may enhance transmitter release from the mossy fiber terminals by acting as high-affinity calcium sensors for enhancing transmitter release. Another form of axonal subthreshold voltage signaling, GABA-EPreSPs elicited by a spillover of GABA from surrounding interneurons, was also explored. Functional consequences of the two modes of axonal subthreshold voltage signaling were discussed with the simulation results.

Soleus responses to Achilles tendon stimuli are suppressed by heel and enhanced by metatarsal cutaneous stimuli during standing

KEY POINTS

We examined the influence of cutaneous feedback from the heel and metatarsal regions of the foot sole on the soleus stretch reflex pathway during standing. We found that heel electrical stimuli suppressed and metatarsal stimuli enhanced the soleus vibration response. Follow-up experiments indicated that the interaction between foot sole cutaneous feedback and the soleus vibration response was likely not mediated by presynaptic inhibition and was contingent upon a modulation at the ⍺-motoneuron pool level. The spatially organized interaction between cutaneous feedback from the foot sole and the soleus vibration response provides information about how somatosensory information is combined to appropriately respond to perturbations during standing.

ABSTRACT

Cutaneous feedback from the foot sole provides balance-relevant information and has the potential to interact with spinal reflex pathways. In this study, we examined how cutaneous feedback from the foot sole (heel and metatarsals) influenced the soleus response to proprioceptive stimuli during standing. We delivered noisy vibration (10-115 Hz) to the right Achilles tendon while we intermittently applied electrical pulse trains (five 1-ms pulses at 200 Hz, every 0.8-1.0 s) to the skin under either the heel or the metatarsals of the ipsilateral foot sole. We analysed time-dependent (referenced to cutaneous stimuli) coherence and cross-correlations between the vibration acceleration and rectified soleus EMG. Vibration-EMG coherence was observed across a bandwidth of ∼10-80 Hz, and coherence was suppressed by heel but enhanced by metatarsal cutaneous stimuli. Cross-correlations showed soleus EMG was correlated with the vibration (∼40 ms lag) and cross-correlations were also suppressed by heel (from 104-155 ms) but enhanced by metatarsal (from 76-128 ms) stimuli. To examine the neural mechanisms mediating this reflex interaction, we conducted two further experiments to probe potential contributions from (1) presynaptic inhibition, and (2) modulations at the ⍺- and γ-motoneuron pools. Results suggest the cutaneous interactions with the stretch reflex pathway required a modulation at the ⍺-motoneuron pool and were likely not mediated by presynaptic inhibition. These findings demonstrate that foot sole cutaneous information functionally tunes the stretch reflex pathway during the control of upright posture and balance.

A sex-specific switch between visual and olfactory inputs underlies adaptive sex differences in behavior

Although males and females largely share the same genome and nervous system, they differ profoundly in reproductive investments and require distinct behavioral, morphological, and physiological adaptations. How can the nervous system, while bound by both developmental and biophysical constraints, produce these sex differences in behavior? Here, we uncover a novel dimorphism in Drosophila melanogaster that allows deployment of completely different behavioral repertoires in males and females with minimum changes to circuit architecture. Sexual differentiation of only a small number of higher order neurons in the brain leads to a change in connectivity related to the primary reproductive needs of both sexes-courtship pursuit in males and communal oviposition in females. This study explains how an apparently similar brain generates distinct behavioral repertoires in the two sexes and presents a fundamental principle of neural circuit organization that may be extended to other species.

Presynaptic Inhibition of Pain and Touch in the Spinal Cord: From Receptors to Circuits

Sensory primary afferent fibers, conveying touch, pain, itch, and proprioception, synapse onto spinal cord dorsal horn neurons. Primary afferent central terminals express a wide variety of receptors that modulate glutamate and peptide release. Regulation of the amount and timing of neurotransmitter release critically affects the integration of postsynaptic responses and the coding of sensory information. The role of GABA (γ-aminobutyric acid) receptors expressed on afferent central terminals is particularly important in sensory processing, both in physiological conditions and in sensitized states induced by chronic pain. During the last decade, techniques of opto- and chemogenetic stimulation and neuronal selective labeling have provided interesting insights on this topic. This review focused on the recent advances about the modulatory effects of presynaptic GABAergic receptors in spinal cord dorsal horn and the neural circuits involved in these mechanisms.

Nicotinic receptor modulation of primary afferent excitability with selective regulation of Aδ-mediated spinal actions

Somatosensory input strength can be modulated by primary afferent depolarization (PAD) generated predominantly via presynaptic GABA_A receptors on afferent terminals. We investigated whether ionotropic nicotinic acetylcholine receptors (nAChRs) also provide modulatory actions, focusing on myelinated afferent excitability in in vitro murine spinal cord nerve-attached models. Primary afferent stimulation-evoked synaptic transmission was recorded in the deep dorsal horn as extracellular field potentials (EFPs), whereas concurrently recorded dorsal root potentials (DRPs) were used as an indirect measure of PAD. Changes in afferent membrane excitability were simultaneously measured as direct current (DC)-shifts in membrane polarization recorded in dorsal roots or peripheral nerves. The broad nAChR antagonist d-tubocurarine (d-TC) selectively and strongly depressed Aδ-evoked synaptic EFPs (36% of control) coincident with similarly depressed A-fiber DRP (43% of control), whereas afferent electrical excitability remained unchanged. In comparison, acetylcholine (ACh) and the nAChR agonists, epibatidine and nicotine, reduced afferent excitability by generating coincident depolarizing DC-shifts in peripheral axons and intraspinally. Progressive depolarization corresponded temporally with the emergence of spontaneous axonal spiking and reductions in the DRP and all afferent-evoked synaptic actions (31%-37% of control). Loss of evoked response was long-lasting, independent of DC repolarization, and likely due to mechanisms initiated by spontaneous C-fiber activity. DC-shifts were blocked with d-TC but not GABA_A receptor blockers and retained after tetrodotoxin block of voltage-gated Na⁺ channels. Notably, actions tested were comparable between three mouse strains, in rat, and when performed in different labs. Thus, nAChRs can regulate afferent excitability via two distinct mechanisms: by central Aδ-afferent actions, and by transient extrasynaptic axonal activation of high-threshold primary afferents.NEW & NOTEWORTHY Primary afferents express many nicotinic ACh receptor (nAChR) subtypes but whether activation is linked to presynaptic inhibition, facilitation, or more complex and selective activity modulation is unknown. Recordings of afferent-evoked responses in the lumbar spinal cord identified two nAChR-mediated modulatory actions: 1) selective control of Aδ afferent transmission and 2) robust changes in axonal excitability initiated via extrasynaptic shifts in DC polarization. This work broadens the diversity of presynaptic modulation of primary afferents by nAChRs.

Axonal mechanisms mediating γ-aminobutyric acid receptor type A (GABA-A) inhibition of striatal dopamine release

Axons of dopaminergic neurons innervate the striatum where they contribute to movement and reinforcement learning. Past work has shown that striatal GABA tonically inhibits dopamine release, but whether GABA-A receptors directly modulate transmission or act indirectly through circuit elements is unresolved. Here, we use whole-cell and perforated-patch recordings to test for GABA-A receptors on the main dopaminergic neuron axons and branching processes within the striatum of adult mice. Application of GABA depolarized axons, but also decreased the amplitude of axonal spikes, limited propagation and reduced striatal dopamine release. The mechanism of inhibition involved sodium channel inactivation and shunting. Lastly, we show the positive allosteric modulator diazepam enhanced GABA-A currents on dopaminergic axons and directly inhibited release, but also likely acts by reducing excitation from cholinergic interneurons. Thus, we reveal the mechanisms of GABA-A receptor modulation of dopamine release and provide new insights into the actions of benzodiazepines within the striatum.

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in rats

KEY POINTS

Presynaptic inhibition is modulated by supraspinal centres and primary afferents in order to filter sensory information, adjust spinal reflex excitability, and ensure smooth movement. After spinal cord injury (SCI), the supraspinal control of primary afferent depolarization (PAD) interneurons is disengaged, suggesting an increased role for sensory afferents. While increased H-reflex excitability in spastic individuals indicates a possible decrease in presynaptic inhibition, it remains unclear whether a decrease in sensory-evoked PAD contributes to this effect. We investigated whether the PAD evoked by hindlimb afferents contributes to the change in presynaptic inhibition of the H-reflex in a decerebrated rat preparation. We found that chronic SCI decreases presynaptic inhibition of the plantar H-reflex through a reduction in PAD evoked by posterior biceps-semitendinosus (PBSt) muscle group I afferents. We further found that step-training restored presynaptic inhibition of the plantar H-reflex evoked by PBSt, suggesting the presence of activity-dependent plasticity of PAD pathways activated by flexor muscle group I afferents.

ABSTRACT

Spinal cord injury (SCI) results in the disruption of supraspinal control of spinal networks and an increase in the relative influence of afferent feedback to sublesional neural networks, both of which contribute to enhancing spinal reflex excitability. Hyperreflexia occurs in ∼75% of individuals with a chronic SCI and critically hinders functional recovery and quality of life. It is suggested that it results from an increase in motoneuronal excitability and a decrease in presynaptic and postsynaptic inhibitory mechanisms. In contrast, locomotor training decreases hyperreflexia by restoring presynaptic inhibition. Primary afferent depolarization (PAD) is a powerful presynaptic inhibitory mechanism that selectively gates primary afferent transmission to spinal neurons to adjust reflex excitability and ensure smooth movement. However, the effect of chronic SCI and step-training on the reorganization of presynaptic inhibition evoked by hindlimb afferents, and the contribution of PAD has never been demonstrated. The objective of this study is to directly measure changes in presynaptic inhibition through dorsal root potentials (DRPs) and its association with plantar H-reflex inhibition. We provide direct evidence that H-reflex hyperexcitability is associated with a decrease in transmission of PAD pathways activated by posterior biceps-semitendinosus (PBSt) afferents after chronic SCI. More precisely, we illustrate that the pattern of inhibition evoked by PBSt group I muscle afferents onto both L4-DRPs and plantar H-reflexes evoked by the distal tibial nerve is impaired after chronic SCI. These changes are not observed in step-trained animals, suggesting a role for activity-dependent plasticity to regulate PAD pathways activated by flexor muscle group I afferents.

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.

Extrasynaptic α5GABAA receptors on proprioceptive afferents produce a tonic depolarization that modulates sodium channel function in the rat spinal cord

Activation of GABA_A receptors on sensory axons produces a primary afferent depolarization (PAD) that modulates sensory transmission in the spinal cord. While axoaxonic synaptic contacts of GABAergic interneurons onto afferent terminals have been extensively studied, less is known about the function of extrasynaptic GABA receptors on afferents. Thus, we examined extrasynaptic α₅GABA_A receptors on low-threshold proprioceptive (group Ia) and cutaneous afferents. Afferents were impaled with intracellular electrodes and filled with neurobiotin in the sacrocaudal spinal cord of rats. Confocal microscopy was used to reconstruct the afferents and locate immunolabelled α₅GABA_A receptors. In all afferents α₅GABA_A receptors were found throughout the extensive central axon arbors. They were most densely located at branch points near sodium channel nodes, including in the dorsal horn. Unexpectedly, proprioceptive afferent terminals on motoneurons had a relative lack of α₅GABA_A receptors. When recording intracellularly from these afferents, blocking α₅GABA_A receptors (with L655708, gabazine, or bicuculline) hyperpolarized the afferents, as did blocking neuronal activity with tetrodotoxin, indicating a tonic GABA tone and tonic PAD. This tonic PAD was increased by repeatedly stimulating the dorsal root at low rates and remained elevated for many seconds after the stimulation. It is puzzling that tonic PAD arises from α₅GABA_A receptors located far from the afferent terminal where they can have relatively little effect on terminal presynaptic inhibition. However, consistent with the nodal location of α₅GABA_A receptors, we find tonic PAD helps produce sodium spikes that propagate antidromically out the dorsal roots, and we suggest that it may well be involved in assisting spike transmission in general. NEW & NOTEWORTHY GABAergic neurons are well known to form synaptic contacts on proprioceptive afferent terminals innervating motoneurons and to cause presynaptic inhibition. However, the particular GABA receptors involved are unknown. Here, we examined the distribution of extrasynaptic α₅GABA_A receptors on proprioceptive Ia afferents. Unexpectedly, these receptors were found preferentially near nodal sodium channels throughout the afferent and were largely absent from afferent terminals. These receptors produced a tonic afferent depolarization that modulated sodium spikes, consistent with their location.

Spiking Neural Networks Based on OxRAM Synapses for Real-Time Unsupervised Spike Sorting

In this paper, we present an alternative approach to perform spike sorting of complex brain signals based on spiking neural networks (SNN). The proposed architecture is suitable for hardware implementation by using resistive random access memory (RRAM) technology for the implementation of synapses whose low latency (<1μs) enables real-time spike sorting. This offers promising advantages to conventional spike sorting techniques for brain-computer interfaces (BCI) and neural prosthesis applications. Moreover, the ultra-low power consumption of the RRAM synapses of the spiking neural network (nW range) may enable the design of autonomous implantable devices for rehabilitation purposes. We demonstrate an original methodology to use Oxide based RRAM (OxRAM) as easy to program and low energy (<75 pJ) synapses. Synaptic weights are modulated through the application of an online learning strategy inspired by biological Spike Timing Dependent Plasticity. Real spiking data have been recorded both intra- and extracellularly from an in-vitro preparation of the Crayfish sensory-motor system and used for validation of the proposed OxRAM based SNN. This artificial SNN is able to identify, learn, recognize and distinguish between different spike shapes in the input signal with a recognition rate about 90% without any supervision.

A novel GABA-mediated corticotropin-releasing hormone secretory mechanism in the median eminence

Corticotropin-releasing hormone (CRH), which is synthesized in the paraventricular nucleus (PVN) of the hypothalamus, plays an important role in the endocrine stress response. The excitability of CRH neurons is regulated by γ-aminobutyric acid (GABA)-containing neurons projecting to the PVN. We investigated the role of GABA in the regulation of CRH release. The release of CRH was impaired, accumulating in the cell bodies of CRH neurons in heterozygous GAD67-GFP (green fluorescent protein) knock-in mice (GAD67(+/GFP)), which exhibited decreased GABA content. The GABAA receptor (GABAAR) and the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1), but not the K(+)-Cl(-) cotransporter (KCC2), were expressed in the terminals of the CRH neurons at the median eminence (ME). In contrast, CRH neuronal somata were enriched with KCC2 but not with NKCC1. Thus, intracellular Cl(-) concentrations ([Cl(-)]i) may be increased at the terminals of CRH neurons compared with concentrations in the cell body. Moreover, GABAergic terminals projecting from the arcuate nucleus were present in close proximity to CRH-positive nerve terminals. Furthermore, a GABAAR agonist increased the intracellular calcium (Ca(2+)) levels in the CRH neuron terminals but decreased the Ca(2+) levels in their somata. In addition, the increases in Ca(2+) concentrations were prevented by an NKCC1 inhibitor. We propose a novel mechanism by which the excitatory action of GABA maintains a steady-state CRH release from axon terminals in the ME.

Spinal presynaptic inhibition in pain control

The gate control theory proposed that the nociceptive sensory information transmitted to the brain relies on an interplay between the inputs from nociceptive and non-nociceptive primary afferent fibers. Both inputs are normally under strong inhibitory control in the spinal cord. Under healthy conditions, presynaptic inhibition activated by non-nociceptive fibers modulates the afferent input from nociceptive fibers onto spinal cord neurons, while postsynaptic inhibition controls the excitability of dorsal horn neurons, and silences the non-nociceptive information flow to nociceptive-specific (NS) projection neurons. However, under pathological conditions, this spinal inhibition may be altered and lead to chronic pain. This review summarizes our knowledge of presynaptic inhibition in pain control, with particular focus on how its alteration after nerve or tissue injury contributes to neuropathic or inflammatory pain syndromes, respectively.

Regulation of action potential waveforms by axonal GABAA receptors in cortical pyramidal neurons

GABAA receptors distributed in somatodendritic compartments play critical roles in regulating neuronal activities, including spike timing and firing pattern; however, the properties and functions of GABAA receptors at the axon are still poorly understood. By recording from the cut end (bleb) of the main axon trunk of layer -5 pyramidal neurons in prefrontal cortical slices, we found that currents evoked by GABA iontophoresis could be blocked by picrotoxin, indicating the expression of GABAA receptors in axons. Stationary noise analysis revealed that single-channel properties of axonal GABAA receptors were similar to those of somatic receptors. Perforated patch recording with gramicidin revealed that the reversal potential of the GABA response was more negative than the resting membrane potential at the axon trunk, suggesting that GABA may hyperpolarize the axonal membrane potential. Further experiments demonstrated that the activation of axonal GABAA receptors regulated the amplitude and duration of action potentials (APs) and decreased the AP-induced Ca2+ transients at the axon. Together, our results indicate that the waveform of axonal APs and the downstream Ca2+ signals are modulated by axonal GABAA receptors.

Inhibition downunder: an update from the spinal cord

Inhibitory neurons in the spinal cord perform dedicated roles in processing somatosensory information and shaping motor behaviors that range from simple protective reflexes to more complex motor tasks such as locomotion, reaching and grasping. Recent efforts examining inhibition in the spinal cord have been directed toward determining how inhibitory cell types are specified and incorporated into the sensorimotor circuitry, identifying and characterizing molecularly defined cohorts of inhibitory neurons and interrogating the functional contribution these cells make to sensory processing and motor behaviors. Rapid progress is being made on all these fronts, driven in large part by molecular genetic and optogenetic approaches that are being creatively combined with neuroanatomical, electrophysiological and behavioral techniques.

Response differences of intersegmental auditory neurons recorded close to or far away from the presumed spike-generating zone

Intracellular recordings may give valuable information about processing of a neuron and possibly its input from the network. Impalement with an electrode causes injury to the cell and depolarization from intrusion of extracellular fluid. Thus, penetration artefacts may contaminate recordings and conceal or even alter relevant information. These penetration artefacts may have the strongest impact close to the spike-generating zone near the dendrites. Recordings in axonal portions might therefore be less vulnerable while providing insufficient information about the synaptic input. In this study, we present data of five previously identified intersegmental auditory neurons of a bushcricket independently recorded in their dendrites (prothorax) and axon (brain). Generally, responses to acoustic pulses of the same parameter combination were similar within a neuronal class at the two recording sites. However, all neuronal classes showed significantly higher response variability and a tendency for higher spike activity when recorded in the dendrites. Unexpectedly, the combined activity of two neurons (Ascending Neurons 1 and 2) recorded in the brain provides a better fit to song recognition than when recorded in the thorax. Axonal recordings of T-shaped Neuron 1 revealed graded potentials originating in the brain and modulating its output in a potentially behaviourally relevant manner.

Weaker control of the electrical properties of cerebellar granule cells by tonically active GABAA receptors in the Ts65Dn mouse model of Down's syndrome

BACKGROUND

Down's syndrome (DS) is caused by triplication of all or part of human chromosome 21 and is characterized by a decrease in the overall size of the brain. One of the brain regions most affected is the cerebellum, in which the number of granule cells (GCs) is markedly decreased. GCs process sensory information entering the cerebellum via mossy fibres and pass it on to Purkinje cells and inhibitory interneurons. How GCs transform incoming signals depends on their input-output relationship, which is adjusted by tonically active GABA(A) receptor channels.

RESULTS

We report that in the Ts65Dn mouse model of DS, in which cerebellar volume and GC number are decreased as in DS, the tonic GABA(A) receptor current in GCs is smaller than in wild-type mice and is less effective in moderating input resistance and raising the minimum current required for action potential firing. We also find that tonically active GABA(A) receptors curb the height and broaden the width of action potentials in wild-type GCs but not in Ts65Dn GCs. Single-cell real-time quantitative PCR reveals that these electrical differences are accompanied by decreased expression of the gene encoding the GABA(A) receptor β3 subunit but not genes coding for some of the other GABA(A) receptor subunits expressed in GCs (α1, α6, β2 and δ).

CONCLUSIONS

Weaker moderation of excitability and action potential waveform in GCs of the Ts65Dn mouse by tonically active GABA(A) receptors is likely to contribute to atypical transfer of information through the cerebellum. Similar changes may occur in DS.

Trigeminal ganglion neurons of mice show intracellular chloride accumulation and chloride-dependent amplification of capsaicin-induced responses

Intracellular Cl⁻ concentrations ([Cl⁻]_i) of sensory neurons regulate signal transmission and signal amplification. In dorsal root ganglion (DRG) and olfactory sensory neurons (OSNs), Cl⁻ is accumulated by the Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1), resulting in a [Cl⁻]_i above electrochemical equilibrium and a depolarizing Cl⁻ efflux upon Cl⁻ channel opening. Here, we investigate the [Cl⁻]_i and function of Cl⁻ in primary sensory neurons of trigeminal ganglia (TG) of wild type (WT) and NKCC1^−/− mice using pharmacological and imaging approaches, patch-clamping, as well as behavioral testing. The [Cl⁻]_i of WT TG neurons indicated active NKCC1-dependent Cl⁻ accumulation. Gamma-aminobutyric acid (GABA)_A receptor activation induced a reduction of [Cl⁻]_i as well as Ca²⁺ transients in a corresponding fraction of TG neurons. Ca²⁺ transients were sensitive to inhibition of NKCC1 and voltage-gated Ca²⁺ channels (VGCCs). Ca²⁺ responses induced by capsaicin, a prototypical stimulus of transient receptor potential vanilloid subfamily member-1 (TRPV1) were diminished in NKCC1^−/− TG neurons, but elevated under conditions of a lowered [Cl⁻]_o suggesting a Cl⁻-dependent amplification of capsaicin-induced responses. Using next generation sequencing (NGS), we found expression of different Ca²⁺-activated Cl⁻ channels (CaCCs) in TGs of mice. Pharmacological inhibition of CaCCs reduced the amplitude of capsaicin-induced responses of TG neurons in Ca²⁺ imaging and electrophysiological recordings. In a behavioral paradigm, NKCC1^−/− mice showed less avoidance of the aversive stimulus capsaicin. In summary, our results strongly argue for a Ca²⁺-activated Cl⁻-dependent signal amplification mechanism in TG neurons that requires intracellular Cl⁻ accumulation by NKCC1 and the activation of CaCCs.

Spatial gradient in TTX sensitivity of axons at the crayfish opener neuromuscular junction

At the crayfish opener neuromuscular junction, axons branch repeatedly before synapsing onto muscle fibers as varicosities. Excitability of these axons was examined with two-electrode current clamp before and after partial block of Na(+) channels with 1 nM tetrodotoxin. 4-Aminopyridine (200 μM) was added to homogenize nonuniformity in K(+) channel density. The impact of tetrodotoxin was evaluated in terms of action potential (AP) amplitude, rate of rise, and threshold. All three parameters were more severely affected at the secondary than the primary branching point (BP). Both BPs fired continuously during 1-s current steps before tetrodotoxin. After tetrodotoxin, the secondary BP fired only in brief bursts, whereas the primary BP still fired continuously. Despite this diminished excitability at the secondary BP, no failure in orthodromic AP conduction was observed. AP waveform at terminals (AP(f)) was examined with voltage indicators. For orthodromic APs, reduction in AP amplitude and prolongation of AP rise time in tetrodotoxin were more pronounced in terminals than at the secondary BP. For APs initiated at the secondary BP, AP(f) sometimes failed to show a spikelike waveform in tetrodotoxin. This degraded AP(f) was not due to averaging variable AP invasion into terminals, because the variance of AP(f) traces did not increase in tetrodotoxin. Tetrodotoxin applied in the absence of 4-aminopyridine showed an impact on the distal axon similar but less distinct than that recorded with 4-aminopyridine. In conclusion, the distal axon is more sensitive to tetrodotoxin than the proximal axon, such that AP waveform degrades as it propagates toward terminals in tetrodotoxin.

Persistent inflammation increases GABA-induced depolarization of rat cutaneous dorsal root ganglion neurons in vitro

Persistent inflammation is associated with a shift in spinal GABA(A) signaling from inhibition to excitation such that GABA(A)-receptor activation contributes to inflammatory hyperalgesia. We tested the hypothesis that the primary afferent is the site of the persistent inflammation-induced shift in GABA(A) signaling which is due to a Na(+)-K(+)-Cl(-)-co-transporter (NKCC1)-dependent depolarization of the GABA(A) current equilibrium potential (E(GABA)). Acutely dissociated retrogradely labeled cutaneous dorsal root ganglion (DRG) neurons from naïve and inflamed (3 days after a subcutaneous injection of complete Freund's adjuvant) adult male rats were studied with Ca(2+) imaging, western blot and gramicidin-perforated patch recording. GABA evoked a Ca(2+) transient in a subpopulation of small- to medium-diameter capsaicin-sensitive cutaneous neurons. Inflammation was associated with a significant increase in the magnitude of GABA-induced depolarization as well as the percentage of neurons in which GABA evoked a Ca(2+) transient. There was no detectable change in NKCC1 protein or phosphoprotein at the whole ganglia level. Furthermore, the increase in excitatory response was comparable in both HEPES- and HCO(3)(-)-buffered solutions, but was only associated with a depolarization of E(GABA) in HCO(3)(-)-based solution. In contrast, under both recording conditions, the excitatory response was associated with an increase in GABA(A) current density, a decrease in low threshold K(+) current density, and resting membrane potential depolarization. Our results suggest that increasing K(+) conductance in afferents innervating a site of persistent inflammation may have greater efficacy in the inhibition of inflammatory hyperalgesia than attempting to drive a hyperpolarizing shift in E(GABA).

Modulation of presynaptic GABA(A) receptors by endogenous neurosteroids

BACKGROUND AND PURPOSE

Although 3α-hydroxy, 5α-reduced pregnane steroids, such as allopregnanolone (AlloP) and tetrahydrodeoxycorticosterone, are endogenous positive modulators of postsynaptic GABA(A) receptors, the functional roles of endogenous neurosteroids in synaptic transmission are still largely unknown.

EXPERIMENTAL APPROACH

In this study, the effect of AlloP on spontaneous glutamate release was examined in mechanically isolated dentate gyrus hilar neurons by use of the conventional whole-cell patch-clamp technique.

KEY RESULTS

AlloP increased the frequency of glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in a dose-dependent manner. The AlloP-induced increase in sEPSC frequency was completely blocked by a non-competitive GABA(A) receptor blocker, tetrodotoxin or Cd(2+) , suggesting that AlloP acts on presynaptic GABA(A) receptors to depolarize presynaptic nerve terminals to increase the probability of spontaneous glutamate release. On the other hand, γ-cyclodextrin (γ-CD) significantly decreased the basal frequency of sEPSCs. However, γ-CD failed to decrease the basal frequency of sEPSCs in the presence of a non-competitive GABA(A) receptor antagonist or tetrodotoxin. In addition, γ-CD failed to decrease the basal frequency of sEPSCs after blocking the synthesis of endogenous 5α-reduced pregnane steroids. Furthermore, γ-CD decreased the extent of muscimol-induced increase in sEPSC frequency, suggesting that endogenous neurosteroids can directly activate and/or potentiate presynaptic GABA(A) receptors to affect spontaneous glutamate release onto hilar neurons.

CONCLUSIONS AND IMPLICATIONS

The modulation of presynaptic GABA(A) receptors by endogenous neurosteroids might affect the excitability of the dentate gyrus-hilus-CA3 network, and thus contribute, at least in part, to some pathological conditions, such as catamenial epilepsy and premenstrual dysphoric disorder.

Activation of GABAA receptors modulates all stages of mechanoreception in spider mechanosensory neurons

GABA(A) receptors mediate mainly inhibitory effects, but there are also many examples of excitatory effects in both mammalian and invertebrate preparations. Here, we aimed to create a complete, quantitative picture of GABA(A)-mediated excitation in a mechanosensory neuron where this phenomenon has been well established. We used muscimol to activate GABA(A) receptors in spider VS-3 neurons and measured the dynamic behavior independently and separately at each of three stages of mechanoreception (receptor current, receptor potential, and action potentials) before and during modulation. We calculated frequency response functions between each stage, estimated information as signal entropy, and estimated information capacity from coherence. Since coherence is sensitive to both noise and nonlinearity, we measured signal-to-noise separately at each stage by averaging responses to repeated mechanical inputs. Muscimol depolarized VS-3 neurons and, after brief inhibition, increased their firing rates. During this excitation, we found significant changes at each stage. Receptor current was attenuated but became more selective to high frequencies. Membrane impedance and time constant fell, favoring higher frequency transmission from receptor current to receptor potential. Action potential firing increased and had higher total entropy. Information capacity from signal-to-noise was always much higher than from coherence, confirming that intracellular noise does not limit signal transmission in these neurons. We conclude that GABA(A) receptor activation shifts each stage of mechanotransduction to higher frequency sensitivity, while the elevated firing rate increases the amount of information that can be encoded. These results show that a single neurotransmitter can finely modulate a sensory neuron's sensitivity and ability to transmit information.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Random Stimulation of Spider Mechanosensory Neurons Reveals Long-Lasting Excitation by GABA and Muscimol

γ-Aminobutyric acid type A (GABAA) receptor activation inhibits many primary afferent neurons by depolarization and increased membrane conductance. Deterministic (step and sinusoidal) functions are commonly used as stimuli to test such inhibition. We found that when the VS-3 mechanosensory neurons innervating the spider lyriform slit-sense organ were stimulated by randomly varying white-noise mechanical or electrical signals, their responses to GABAA receptor agonists were more complex than the inhibition observed during deterministic stimulation. Instead, there was rapid excitation, then brief inhibition, followed by long-lasting excitation. During the final excitatory phase, VS-3 neuron sensitivity to high-frequency signals increased selectively and their linear information capacity also increased. Using experimental and simulation approaches we found that the excitatory effect could also be achieved by depolarizing the neurons without GABA application and that excitation could override the inhibitory effect produced by increased membrane conductance (shunting). When the VS-3 neurons were exposed to bumetanide, an antagonist of the Cl− transporter NKCC1, the GABA-induced depolarization decreased without any change in firing rate, suggesting that the effects of GABA can be maintained for a long time without additional Cl− influx. Our results show that the VS-3 neuron's response to GABA depends profoundly on the type of signals the neuron is conveying while the transmitter binds to its receptors.

Postnatal changes in somatic gamma-aminobutyric acid signalling in the rat hippocampus

During postnatal development of the rat hippocampus, gamma-aminobutyric acid (GABA) switches its action on CA3 pyramidal cells from excitatory to inhibitory. To characterize the underlying changes in the GABA reversal potential, we used somatic cell-attached recordings of GABA(A) and N-methyl-D-aspartate channels to monitor the GABA driving force and resting membrane potential, respectively. We found that the GABA driving force is strongly depolarizing during the first postnatal week. The strength of this depolarization rapidly declines with age, although GABA remains slightly depolarizing, by a few millivolts, even in adult neurons. Reduction in the depolarizing GABA driving force was due to a progressive negative shift of the reversal potential of GABA currents. Similar postnatal changes in GABA signalling were also observed using the superfused hippocampus preparation in vivo, and in the hippocampal interneurons in vitro. We also found that in adult pyramidal cells, somatic GABA reversal potential is maintained at a slightly depolarizing level by bicarbonate conductance, chloride-extrusion and chloride-loading systems. Thus, the postnatal excitatory-to-inhibitory switch in somatic GABA signalling is associated with a negative shift of the GABA reversal potential but without a hyperpolarizing switch in the polarity of GABA responses. These results also suggest that in adult CA3 pyramidal cells, somatic GABAergic inhibition takes place essentially through shunting rather than hyperpolarization. Apparent hyperpolarizing GABA responses previously reported in the soma of CA3 pyramidal cells are probably due to cell depolarization during intracellular or whole-cell recordings.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

Activation of ionotropic γ-aminobutyric acid type A (GABAA) receptors depolarizes neurons that have high intracellular [Cl−], causing inhibition or excitation in different cell types. The depolarization often leads to inactivation of voltage-gated Na channels, but additional ionic mechanisms may also be affected. Previously, a simulated model of spider VS-3 mechanosensory neurons suggested that although voltage-activated Na+current is partially inactivated during GABA-induced depolarization, a slowly activating and inactivating component remains and may contribute to the depolarization. Here, we confirmed experimentally, by blocking Na channels prior to GABA application, that Na+current contributes to GABA-induced depolarization in VS-3 neurons. Ratiometric Ca2+imaging experiments combined with intracellular recordings revealed a significant increase in intracellular [Ca2+] when GABAAreceptors were activated, synchronous with the depolarization and probably due to Ca2+influx via low-voltage–activated (LVA) Ca channels. In contrast, GABAB-receptor activation in these neurons was previously shown to inhibit LVA current. Blockade of voltage-gated K channels delayed membrane repolarization, extending GABA-induced depolarization. However, inhibition of Ca channels significantly increased the amplitude of GABA-induced depolarization, indicating that Ca2+-activated K+current has an even stronger repolarizing effect. Regulation of intracellular [Ca2+] is important for many cellular processes and Ca2+control of K+currents may be particularly important for some functions of mechanosensory neurons, such as frequency tuning. These data show that GABAA-receptor activation participates in this regulation.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

GABA: a pioneer transmitter that excites immature neurons and generates primitive oscillations

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

Chloride homeostasis differentially affects GABA(A) receptor- and glycine receptor-mediated effects on spontaneous circuit activity in hippocampal cell culture

The potassium-chloride cotransporter 2 (KCC2)-dependent intracellular chloride level determines whether neurons respond to GABA and/or glycine by depolarization or hyperpolarization. However, still unknown is the role of KCC2-dependent chloride homeostasis in regulating the spontaneous activity of neuronal circuits via GABA(A) receptor (GABA(A)R) and the glycine receptor (GlyR). In this study, patch-clamp recordings were performed to measure the change of spontaneous neuronal activity in cultured hippocampal neurons. Our results showed that inhibition of KCC2 with furosemide, as well as blockade of GABA(A)R with bicuculline, significantly enhanced circuit activity. Perfusion with bicuculline further enhanced the effects of furosemide on spontaneous circuit activity, while furosemide did not alter the effects of bicuculline. Surprisingly, blockade of GlyR not only induced obvious tonic currents, but also significantly decreased spontaneous synaptic activity. Moreover, inhibition of KCC2 did not change the depressive effect of strychnine on neuronal circuits. Our findings suggest that KCC2-dependent chloride homeostasis is mainly involved in GABA(A)R-mediated synaptic inhibition whereas GlyR-mediated tonic action plays a totally different role in regulating hippocampal circuit activity.

Kv7/KCNQ/M-channels in rat glutamatergic hippocampal axons and their role in regulation of excitability and transmitter release

M-current (I(M)) plays a key role in regulating neuronal excitability. Mutations in Kv7/KCNQ subunits, the molecular correlates of I(M), are associated with a familial human epilepsy syndrome. Kv7/KCNQ subunits are widely expressed, and I(M) has been recorded in somata of several types of neurons, but the subcellular distribution of M-channels remains elusive. By combining field-potential, whole-cell and intracellular recordings from area CA1 in rat hippocampal slices, and computational modelling, we provide evidence for functional M-channels in unmyelinated axons in the brain. Our data indicate that presynaptic M-channels can regulate axonal excitability and synaptic transmission, provided the axons are depolarized into the I(M) activation range (beyond approximately -65 mV). Here, such depolarization was achieved by increasing the extracellular K(+) concentration ([K(+)](o)). Extracellular recordings in the presence of moderately elevated [K(+)](o) (7-11 mm), showed that the specific M-channel blocker XE991 reduced the amplitude of the presynaptic fibre volley and the field EPSP in a [K(+)](o)-dependent manner, both in stratum radiatum and in stratum lacknosum moleculare. The M-channel opener, retigabine, had opposite effects. The higher the [K(+)](o), the greater the effects of XE991 and retigabine. Similar pharmacological modulation of EPSPs recorded intracellularly from CA1 pyramidal neurons, while blocking postsynaptic K(+) channels with intracellular Cs(+), confirmed that active M-channels are located presynaptically. Computational analysis with an axon model showed that presynaptic I(M) can control Na(+) channel inactivation and thereby affect the presynaptic action potential amplitude and Ca(2+) influx, provided the axonal membrane potential is sufficiently depolarized. Finally, we compared the effects of blocking I(M) on the spike after-depolarization and bursting in CA3 pyramidal neuron somata versus their axons. In standard [K(+)](o) (2.5 mm), XE991 increased the ADP and promoted burst firing at the soma, but not in the axons. However, I(M) contributed to the refractory period in the axons when spikes were broadened by a low dose 4-aminopyridine (200 microm). Our results indicate that functional Kv7/KCNQ/M-channels are present in unmyelinated axons in the brain, and that these channels may have contrasting effects on excitability depending on their subcellular localization.

Tests for presynaptic modulation of corticospinal terminals from peripheral afferents and pyramidal tract in the macaque

The efficacy of sensory input to the spinal cord can be modulated presynaptically during voluntary movement by mechanisms that depolarize afferent terminals and reduce transmitter release. It remains unclear whether similar influences are exerted on the terminals of descending fibres in the corticospinal pathway of Old World primates and man. We investigated two signatures of presynaptic inhibition of the macaque corticospinal pathway following stimulation of the peripheral nerves of the arm (median, radial and ulnar) and the pyramidal tract: (1) increased excitability of corticospinal axon terminals as revealed by changes in antidromically evoked cortical potentials, and (2) changes in the size of the corticospinal monosynaptic field potential in the spinal cord. Conditioning stimulation of the pyramidal tract increased both the terminal excitability and monosynaptic fields with similar time courses. Excitability was maximal between 7.5 and 10 ms following stimulation and returned to baseline within 40 ms. Conditioning stimulation of peripheral nerves produced no statistically significant effect in either measure. We conclude that peripheral afferents do not exert a presynaptic influence on the corticospinal pathway, and that descending volleys may produce autogenic terminal depolarization that is correlated with enhanced transmitter release. Presynaptic inhibition of afferent terminals by descending pathways and the absence of a reciprocal influence of peripheral input on corticospinal efficacy would help to preserve the fidelity of motor commands during centrally initiated movement.

Shunting versus inactivation: simulation of GABAergic inhibition in spider mechanoreceptors suggests that either is sufficient

Afferent neurons entering the central nervous systems of vertebrates and invertebrates receive presynaptic inhibition on their axon terminals. This usually involves an increase in membrane conductance (shunting) and depolarization (primary afferent depolarization, PAD). In arachnids and crustaceans the peripherally located parts of afferent neurons also receive efferent synapses. GABA (gamma-aminobutyric acid) plays a major role in both central and peripheral inhibition, activating chloride channels that depolarize the membrane and increase its conductance. Although both central and peripheral inhibition have been widely investigated, debate continues about the mechanisms involved, especially concerning the relative contributions of shunting versus inactivation of sodium channels by depolarization. Sensory neurons innervating spider VS-3 slit sensilla are accessible to intracellular recordings during mechanical or electrical stimulation. These neurons are inhibited by GABA, and both the electrophysiology and pharmacology of this inhibition have been studied previously. Here, we developed a Hodgkin-Huxley style model to simulate VS-3 neuron activity before and after GABA treatment. The model indicates that GABA-activated chloride current can entirely account for action potential suppression, and that either shunting or inactivation are sufficient to produce inhibition. This model also demonstrates that slowing of sodium current contributes to inhibition.

Depression of presynaptic excitation by the activation of vanilloid receptor 1 in the rat spinal dorsal horn revealed by optical imaging

In this study, we show that capsaicin (CAP) depresses primary afferent fiber terminal excitability by acting on vanilloid receptor 1 (TRPV1 channels) of primary afferent fibers in adenosine 5'-triphosphate (ATP)- and temperature-dependent manner using two optical imaging methods. First, transverse slices of spinal cord were stained with a voltage-sensitive dye and the net excitation in the spinal dorsal horn was recorded. Prolonged treatment (>20 min) with the TRPV1 channel agonist, CAP, resulted in a long-lasting inhibition of the net excitation evoked by single-pulse stimulation of C fiber-activating strength. A shorter application of CAP inhibited the excitation in a concentration-dependent manner and the inhibition was reversed within several minutes. This inhibition was Ca⁺⁺-dependent, was antagonized by the TRPV1 channel antagonist, capsazepine (CPZ), and the P₂X and P₂Y antagonist, suramin, and was facilitated by the P₂Y agonist, uridine 5'-triphosphate (UTP). The inhibition of excitation was unaffected by bicuculline and strychnine, antagonists of GABA_A and glycine receptors, respectively. Raising the perfusate temperature to 39°C from 27°C inhibited the excitation (-3%/°C). This depressant effect was antagonized by CPZ and suramin, but not by the P₂X antagonist, 2', 3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP). Second, in order to record the presynaptic excitation exclusively, we stained the primary afferent fibers anterogradely from the dorsal root. CAP application and a temperature increase from 27°C to 33°C depressed the presynaptic excitation, and CPZ antagonized these effects. Thus, this study showed that presynaptic excitability is modulated by CAP, temperature, and ATP under physiological conditions, and explains the reported central actions of CAP. These results may have clinical importance, especially for the control of pain.

Presynaptic GABAA receptors facilitate spontaneous glutamate release from presynaptic terminals on mechanically dissociated rat CA3 pyramidal neurons

Mossy fiber-derived giant spontaneous miniature excitatory postsynaptic currents have been suggested to be large enough to generate action potentials in postsynaptic CA3 pyramidal neurons. Here we report on the functional roles of presynaptic GABA(A) receptors on excitatory terminals in contributing to spontaneous glutamatergic transmission to CA3 neurons. In mechanically dissociated rat hippocampal CA3 neurons with adherent presynaptic nerve terminals, spontaneous excitatory postsynaptic currents were recorded using conventional whole-cell patch clamp recordings. In most recordings, unusually large spontaneous excitatory postsynaptic currents up to 500 pA were observed. These large spontaneous excitatory postsynaptic currents were highly sensitive to group II metabotropic glutamate receptor activation, and were still observed even after the blockade of voltage-dependent Na(+) or Ca(2+) channels. Exogenously applied muscimol (0.1-3 microM) significantly increased the frequency of spontaneous excitatory postsynaptic currents including the large ones. This facilitatory effect of muscimol was completely inhibited in the presence of 10 microM 6-imino-3-(4-methoxyphenyl)-1(6H)-pyridazinebutanoic acid HBr, a specific GABA(A) receptor antagonist. Pharmacological data suggest that activation of presynaptic GABA(A) receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release. In the current-clamp condition, a subset of large spontaneous excitatory postsynaptic potentials triggered action potentials, and muscimol greatly increased the frequency of spontaneous excitatory postsynaptic potential-triggered action potentials in postsynaptic CA3 pyramidal neurons. The results suggest that presynaptic GABA(A) receptors on glutamatergic terminals play an important role in the excitability of CA3 neurons as well as in the presynaptic modulation of glutamatergic transmission onto hippocampal CA3 neurons.

Inhibitory glutamate receptors in spider peripheral mechanosensory neurons

Most mechanosensory neurons are inhibited by GABAergic efferent neurons. This inhibition is often presynaptic and mediated by ionotropic GABA receptors at the axon terminals. GABA receptor activation opens Cl- channels, leading to membrane depolarization and an increase in membrane conductance. In many invertebrate preparations, efferent neurons that innervate mechanosensory afferents contain glutamate in addition to GABA, suggesting that the sensory neurons are also modulated by glutamate. However, the effects of glutamate on these neurons are not well understood. Peripheral parts of the spider (Cupiennius salei) mechanosensory neurons are surrounded by efferent fibers immunoreactive to antibodies against GABA and glutamate. GABA and its analogue muscimol were shown to effectively inhibit spider mechanosensory neurons innervating lyriform slit sensilla VS-3 that detects cuticular strains in the leg. Here, we show that glutamate also inhibits the VS-3 neurons, but its effects are different from those of GABA or muscimol, suggesting that it acts on a different group of receptors. GABA and muscimol always depolarized these neurons and the inhibitory effect was strongly correlated with the amount of depolarization. In contrast, glutamate inhibited the VS-3 neurons even when it did not depolarize them. In addition, while glutamate inhibited both the axonal action potentials elicited with electrical stimulation and dendritic action potentials produced by mechanical stimulation, muscimol only inhibited the axonal action potentials. Therefore, the inhibitory glutamate receptors in the VS-3 neurons are distinct from and differently distributed than the GABA receptors, providing a subtle control of the neurons' sensitivity in varying behavioural situations.

Modulation of Transmitter Release by Presynaptic Resting Potential and Background Calcium Levels

Activation of presynaptic ion channels alters the membrane potential of nerve terminals, leading to changes in transmitter release. To study the relationship between resting potential and exocytosis, we combined pre- and postsynaptic electrophysiological recordings with presynaptic Ca(2+) measurements at the calyx of Held. Depolarization of the membrane potential to between -60 mV and -65 mV elicited P/Q-type Ca(2+) currents of < 1 pA and increased intraterminal Ca(2+) by < 100 nM. These small Ca(2+) elevations were sufficient to enhance the probability of transmitter release up to 2-fold, with no effect on the readily releasable pool of vesicles. Moreover, the effects of mild depolarization on release had slow kinetics and were abolished by 1 mM intraterminal EGTA, suggesting that Ca(2+) acted through a high-affinity binding site. Together, these studies suggest that control of resting potential is a powerful means for regulating synaptic function at mammalian synapses.

Neural mechanisms controlling jaw-jerk reflex in the cat

Signal substances of axon terminals presynaptic to jaw spindle Ia afferents and their ultrastructural features were examined using a combination of intra-axonal horseradish peroxidase injection and postembedding immunogold-labeling techniques in cats. A total of 35 axon terminals presynaptic to 22 horseradish peroxidase-labeled Ia boutons were examined. Of the 35 presynaptic axon terminals, 14 (40%) were immunoreactive for both gamma-aminobutyric acid and glycine, 9 (26%) for gamma-aminobutyric acid alone and 9 (26%) for glycine alone. The bouton volume, mitochondrial volume, active zone area, and apposed surface area were larger for Ia boutons than for presynaptic axon terminals, while each of the values is similar among the three types of presynaptic axon terminals. These results suggest that gamma-aminobutyric acid and glycine play an important role for modulating the jaw-jerk reflex presynaptically and that the smaller size of presynaptic axon terminals is important to prevent action potential generation from Ia afferents.

Feed-forward facilitation of glutamate release by presynaptic GABA(A) receptors

Disynaptic GABAergic inputs from Schaffer collateral (SC) afferents on to the soma of glutamatergic CA1 pyramidal neurons are involved in feed-forward inhibition in the hippocampal neural circuits. Here we report the functional roles of presynaptic GABA(A) receptors on SC afferents projecting to CA1 pyramidal neurons. Muscimol (0.5 microM), a selective GABA(A) receptor agonist, increased SC-evoked EPSC amplitude and decreased paired-pulse ratio in the slice preparation, in addition, it facilitated spontaneous glutamate release on to mechanically dissociated CA1 pyramidal neurons in an external Ca2+-dependent manner. In field recordings, muscimol at low concentrations (< or = 0.5 microM) increased not only the excitability of SC afferents but glutamate release, however, it at high concentrations (> or = 1 microM) changed bidirectionally. These results suggest that the moderate activation of presynaptic GABA(A) receptors depolarizes SC afferents and enhances SC-mediated glutamatergic transmission. When endogenous GABA was disynaptically released by brief trains of stimulation of SC afferents, the axonal excitability in addition to glutamate release was increased. The effects of endogenous GABA on the excitability of SC afferents were blocked by either SR95531 or AMPA receptor blockers, which would be expected to block disynaptic feed-forward neural circuits. The present results provide a novel form of presynaptic modulation (feed-forward facilitation) of glutamatergic transmission by presynaptic GABA(A) receptors within the intrinsic hippocampal neural circuits.

Central inhibitory microcircuits controlling spike propagation into sensory terminals

The phenomenon of afferent presynaptic inhibition has been intensively studied in the sensory neurons of the chordotonal organ from the coxobasal joint (CBCO) of the crayfish leg. This has revealed that it has a number of discrete roles in these afferents, mediated by distinct populations of interneurons. Here we examine further the effect of presynaptic inhibition on action potentials in the CBCO afferents and investigate the nature of the synapses that mediate it. In the presence of picrotoxin, the action potential amplitude is increased and its half-width decreased, and a late depolarizing potential following the spike is increased in amplitude. Ultrastructural examination of the afferent terminals reveals that synaptic contacts on terminal branches are particularly abundant in the neuropil close to the main axon. Many of the presynaptic terminals contain small agranular vesicles, are of large diameter, and are immunoreactive for gamma-aminobutyric acid (GABA). These terminals are sometimes seen to make reciprocal connections with the afferents. Synaptic contacts from processes immunoreactive for glutamate are found on small-diameter afferent terminals. A few of the presynaptic processes contain numerous large granular vesicles and are immunoreactive for neither GABA nor glutamate. The effect that the observed reciprocal synapses might have was investigated by using a multicompartmental model of the afferent terminal.

Presynaptic, extrasynaptic and axonal GABAA receptors in the CNS: where and why?

Although GABA(A) receptors are widely distributed at inhibitory synapses on dendrites and cell bodies of neurons, they also occur in other places, in particular at synapses made on axons and in extrasynaptic membranes. This review summarises some of the evidence that presynaptic receptors modulate transmission not only at primary afferents in the spinal cord, but also at a variety of sites in the brain, including hippocampal mossy fibres. These receptors modulate transmitter release via several different mechanisms. Another form of unconventional GABA(A) receptor-mediated signalling is the mediation of a tonic conductance, seen in granule cells of the cerebellum and dentate gyrus and also in hippocampal interneurons. Tonic signalling appears to be mediated by extrasynaptic receptors. The adaptive significance of this form of signalling remains poorly understood.

Dendritic excitability and localization of GABA-mediated inhibition in spider mechanoreceptor neurons

GABAergic inhibition of mechanosensory afferent axon terminals is a widespread phenomenon in vertebrates and invertebrates. Spider mechanoreceptor neurons receive efferent innervation on their peripherally located axons, somata and sensory dendrites, and the dendrites have recently been shown to be excitable. Excitability of the spider sensory neurons is inhibited by muscimol and GABA, agonists of ionotropic GABA receptors. Here we asked where in the neurons this inhibition occurs. We found no evidence for inhibition of action potentials in the sensory dendrites, but axonal action potentials were rapidly suppressed by both agonists. Earlier work showed that metabotropic GABA(B) receptors are located on the dendrites and distal somata of the spider sensory neurons, where they modulate voltage-activated conductances and may provide slower, prolonged inhibition. Therefore, GABA released from single peripheral efferents may activate both ionotropic and metabotropic receptor types, providing rapid suppression of axonal activity followed by slower inhibition that eventually prevents action potential initiation in the distal dendrites.