Superficial dorsal horn neurons with double spike activity in the rat

Noise or signal? Spontaneous activity of dorsal horn neurons: patterns and function in health and disease

Spontaneous activity refers to the firing of action potentials by neurons in the absence of external stimulation. Initially considered an artifact or "noise" in the nervous system, it is now recognized as a potential feature of neural function. Spontaneous activity has been observed in various brain areas, in experimental preparations from different animal species, and in live animals and humans using non-invasive imaging techniques. In this review, we specifically focus on the spontaneous activity of dorsal horn neurons of the spinal cord. We use a historical perspective to set the basis for a novel classification of the different patterns of spontaneous activity exhibited by dorsal horn neurons. Then we examine the origins of this activity and propose a model circuit to explain how the activity is generated and transmitted to the dorsal horn. Finally, we discuss possible roles of this activity during development and during signal processing under physiological conditions and pain states. By analyzing recent studies on the spontaneous activity of dorsal horn neurons, we aim to shed light on its significance in sensory processing. Understanding the different patterns of activity, the origins of this activity, and the potential roles it may play, will contribute to our knowledge of sensory mechanisms, including pain, to facilitate the modeling of spinal circuits and hopefully to explore novel strategies for pain treatment.

Synchronous firing of dorsal horn neurons at the origin of dorsal root reflexes in naïve and paw-inflamed mice

Spinal interneurons located in the dorsal horn induce primary afferent depolarization (PAD) controlling the excitability of the afferent’s terminals. Following inflammation, PAD may reach firing threshold contributing to maintain inflammation and pain. Our aim was to study the collective behavior of dorsal horn neurons, its relation to backfiring of primary afferents and the effects of a peripheral inflammation in this system. Experiments were performed on slices of spinal cord obtained from naïve adult mice or mice that had suffered an inflammatory pretreatment. Simultaneous recordings from groups of dorsal horn neurons and primary afferents were obtained and machine-learning methodology was used to analyze effective connectivity between them. Dorsal horn recordings showed grouping of spontaneous action potentials from different neurons in “population bursts.” These occurred at irregular intervals and were formed by action potentials from all classes of neurons recorded. Compared to naïve, population bursts from treated animals concentrated more action potentials, had a faster onset and a slower decay. Population bursts were disrupted by perfusion of picrotoxin and held a strong temporal correlation with backfiring of afferents. Effective connectivity analysis allowed pinpointing specific neurons holding pre- or post-synaptic relation to the afferents. Many of these neurons had an irregular fast bursting pattern of spontaneous firing. We conclude that population bursts contain action potentials from neurons presynaptic to the afferents which are likely to control their excitability. Peripheral inflammation may enhance synchrony in these neurons, increasing the chance of triggering action potentials in primary afferents and contributing toward central sensitization.

Origin and classification of spontaneous discharges in mouse superficial dorsal horn neurons

Superficial laminae of the spinal cord possess a considerable number of neurons with spontaneous activity as reported in vivo and in vitro preparations of several species. Such neurons may play a role in the development of the nociceptive system and/or in the spinal coding of somatosensory signals. We have used electrophysiological techniques in a horizontal spinal cord slice preparation from adult mice to investigate how this activity is generated and what are the main patterns of activity that can be found. The results show the existence of neurons that fire regularly and irregularly. Within each of these main types, it was possible to distinguish patterns of spontaneous activity formed by single action potentials and different types of bursts according to intra-burst firing frequency. Activity in neurons with irregular patterns was blocked by a mixture of antagonists of the main neurotransmitter receptors present in the cord. Approximately 82% of neurons with a regular firing pattern were insensitive to synaptic antagonists but their activity was inhibited by specific ion channel blockers. It is suggested that these neurons generate endogenous activity due to the functional expression of hyperpolarisation-activated and persistent sodium currents driving the activity of irregular neurons.

Analysis of spontaneous activity of superficial dorsal horn neurons in vitro: neuropathy-induced changes

The superficial dorsal horn contains large numbers of interneurons which process afferent and descending information to generate the spinal nociceptive message. Here, we set out to evaluate whether adjustments in patterns and/or temporal correlation of spontaneous discharges of these neurons are involved in the generation of central sensitization caused by peripheral nerve damage. Multielectrode arrays were used to record from discrete groups of such neurons in slices from control or nerve damaged mice. Whole-cell recordings of individual neurons were also obtained. A large proportion of neurons recorded extracellularly showed well-defined patterns of spontaneous firing. Clock-like neurons (CL) showed regular discharges at ∼6 Hz and represented 9 % of the sample in control animals. They showed a tonic-firing pattern to direct current injection and depolarized membrane potentials. Irregular fast-burst neurons (IFB) produced short-lasting high-frequency bursts (2-5 spikes at ∼100 Hz) at irregular intervals and represented 25 % of the sample. They showed bursting behavior upon direct current injection. Of the pairs of neurons recorded, 10 % showed correlated firing. Correlated pairs always included an IFB neuron. After nerve damage, the mean spontaneous firing frequency was unchanged, but the proportion of CL increased significantly (18 %) and many of these neurons appeared to acquire a novel low-threshold A-fiber input. Similarly, the percentage of IFB neurons was unaltered, but synchronous firing was increased to 22 % of the pairs studied. These changes may contribute to transform spinal processing of nociceptive inputs following peripheral nerve damage. The specific roles that these neurons may play are discussed.

Intrinsic neuronal excitability: implications for health and disease

The output of a single neuron depends on both synaptic connectivity and intrinsic membrane properties. Changes in both synaptic and intrinsic membrane properties have been observed during homeostatic processes (e.g., vestibular compensation) as well as in several central nervous system (CNS) disorders. Although changes in synaptic properties have been extensively studied, particularly with regard to learning and memory, the contribution of intrinsic membrane properties to either physiological or pathological processes is much less clear. Recent research, however, has shown that alterations in the number, location or properties of voltage- and ligand-gated ion channels can underlie both normal and abnormal physiology, and that these changes arise via a diverse suite of molecular substrates. The literature reviewed here shows that changes in intrinsic neuronal excitability (presumably in concert with synaptic plasticity) can fundamentally modify the output of neurons, and that these modifications can subserve both homeostatic mechanisms and the pathogenesis of CNS disorders including epilepsy, migraine, and chronic pain.

Ion channel density regulates switches between regular and fast spiking in soma but not in axons

The threshold firing frequency of a neuron is a characterizing feature of its dynamical behaviour, in turn determining its role in the oscillatory activity of the brain. Two main types of dynamics have been identified in brain neurons. Type 1 dynamics (regular spiking) shows a continuous relationship between frequency and stimulation current (f-I_stim) and, thus, an arbitrarily low frequency at threshold current; Type 2 (fast spiking) shows a discontinuous f-I_stim relationship and a minimum threshold frequency. In a previous study of a hippocampal neuron model, we demonstrated that its dynamics could be of both Type 1 and Type 2, depending on ion channel density. In the present study we analyse the effect of varying channel density on threshold firing frequency on two well-studied axon membranes, namely the frog myelinated axon and the squid giant axon. Moreover, we analyse the hippocampal neuron model in more detail. The models are all based on voltage-clamp studies, thus comprising experimentally measurable parameters. The choice of analysing effects of channel density modifications is due to their physiological and pharmacological relevance. We show, using bifurcation analysis, that both axon models display exclusively Type 2 dynamics, independently of ion channel density. Nevertheless, both models have a region in the channel-density plane characterized by an N-shaped steady-state current-voltage relationship (a prerequisite for Type 1 dynamics and associated with this type of dynamics in the hippocampal model). In summary, our results suggest that the hippocampal soma and the two axon membranes represent two distinct kinds of membranes; membranes with a channel-density dependent switching between Type 1 and 2 dynamics, and membranes with a channel-density independent dynamics. The difference between the two membrane types suggests functional differences, compatible with a more flexible role of the soma membrane than that of the axon membrane.

All activity of the brain is manifested in electrical oscillatory patterns, shaped by the firing dynamics of the many neurons forming the brain networks. The underlying mechanisms of the firing pattern in the single neurons are still not fully understood. The distribution and identity of different channel types have been suggested as critical factors. We have suggested that the density of channels in the membrane is a fundamental complementary mechanism. In a hippocampal soma membrane model study we have shown that altering the ion channel densities can cause the membrane to switch between two qualitatively different firing patterns. Here we extend the analysis to two axon membranes. Unexpectedly, both show that channel density alterations do not cause switches between different firing behaviours. We believe that this is an important property of axon membranes, explaining their limited flexibility.