16 p., 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., Ministerio de Ciencia e Innovación, Unión Europea, FEDER

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., Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación, Universidad de Alcalá

IBRO 2023. 11th Word Congress of Neuroscience, 9-13 september 2023, Granada, Spain, Esta publicación es parte del proyecto de I+D+i PID2021-126330OB-I00, financiado por MICIU/AEI/10.13039/501100011033/ y por “FEDER Una manera de hacer Europa”, The spinal dorsal horn receives and integrates a wide variety of somatosensory inputs and is particularly relevant for nociceptive information. This area receives signals directly from the periphery via primary afferents, but also descending inputs from higher centres that modulate the incoming information and the processing by intraspinal circuits (1). Serotoninergic and adrenergic descending systems are two modulatory systems that can have both facilitatory and inhibitory effects on dorsal horn neurons (DHNs) and nociceptive processing (2). The presence of spontaneous activity in spinal circuits has been reported under in vivo and in vitro conditions and may play a fundamental role in the processing of nociceptive information by modulating the excitability of the spinal circuits. Our aim was to investigate the modulatory role of the adrenergic and serotonergic descending systems on the spontaneous activity of DHNs, the occurrence of coordinated activity in spinal circuits and the generation of dorsal root potentials in primary afferents., Agencia Estatal de Investigación

Central terminals of primary afferents and dorsal horn neurons usually exhibit spontaneous activity, the two phenomena being interrelated. Spontaneous activity may constitute a system for adjusting the level of excitation of spinal circuits and the processing of somatosensory information. Superficial dorsal horn neurons fire action potentials in a coordinated form, giving rise to population events. These population events are altered by peripheral inflammation, suggesting their implication in central sensitisation. In this work, we aimed to define the role of primary afferents in the occurrence of this coordinated activity. Channelrhodopsin-2, archaerhodopsin-3 or the hM4Di-DREADD receptor were expressed in primary afferents by Cre-recombination under control of the advillin promoter. Dorsal roots and superficial dorsal horn neurons were simultaneously recorded using in vitro spinal cord slices from neonatal mice. Depolarisation of primary afferents by activation of channelrhodopsin-2 inhibited dorsal root activity and the coordinated firing of dorsal horn neurons. DREADD activation reduced the activity in the afferents and depressed coordinated activity in dorsal horn neurons. In contrast, hyperpolarisation of afferents by archaerhodopsin-3 augmented dorsal root responses and increased the coordinated activity of spinal neurons. The present results demonstrate a direct implication of primary afferents in the generation of coordinated spontaneous firing in superficial dorsal horn neurons., Agencia Estatal de Investigación

Neuroscience 2024, 05/10/2024-09/10/2024, Chicago, Estados Unidos., Primary afferents are responsible for transmitting the sensory information generated at peripheral receptors to the spinal cord. However, before this information reaches second-order neurons, it can be modulated by presynaptic contacts from dorsal horn neurons. These presynaptic contacts can regulate neurotransmitter release by modifying the excitability of synaptic terminals and thus the efficiency of information transmission. Using opto- and chemogenetic techniques, we wanted to investigate how changes in the excitability of primary afferent terminals can modify spontaneous activity in spinal cord circuits. These studies were carried out using in vitro spinal cord preparations from neonatal mice expressing channelrhodopsin-2 (ChR2), Archaerhodopsin-3 (ArchT) and hM4Di-DREADD in primary afferent neurons under the control of the advillin promoter. Glass suction electrodes were used to record dorsal root activity, while multi-electrode arrays were used to record the activity of neurons in the dorsal horn. An adjacent dorsal root was electrically stimulated to activate primary afferents and generate synaptic responses in both the recording root and dorsal horn neurons. ChR2 activation by light at 455nm caused a depolarization in the afferents contained within the root, together with abolition of spontaneous dorsal root potentials (sDRPs). There was also an increase in the spontaneous firing of most dorsal horn neurons, but this enhancement was accompanied by a loss of coordination between neurons. Activation of hM4Di-DREADD with clozapine N-oxide led to a reduction in sDRP and in the firing frequency of dorsal horn neurons, with a concomitant loss of coordinated activity. On the other hand, ArchT stimulation with light at 567nm hyperpolarized the afferents in the dorsal root and increased sDRP amplitude. The activity of dorsal horn neurons showed a moderate increase during afferent hyperpolarization with an enhancement of coordinated activity. The responses produced by the electrical stimulation of an adjacent dorsal root were inhibited by both depolarization and hM4Di-DREADD activation, but were unchanged during the hyperpolarization induced by ArchT. These results show that the ability of primary afferents to transmit information can be strongly regulated by acting at their central terminals inside the cord. Primary afferents terminals work as an essential component in maintaining coordination in the spontaneous activity of spinal circuits, contributing to adjust their excitability., Agencia Estatal de Investigación

IBRO 2023. 11th Word Congress of Neuroscience, 9-13 september 2023, Granada, Spain, Esta publicación es parte del proyecto de I+D+i PID2021-126330OB-I00, financiado por MICIU/AEI/10.13039/501100011033/ y por “FEDER Una manera de hacer Europa”., The excitability of the central terminals of primary afferents (PA) is critical in regulating neurotransmitter release to second-order neurons
located in the spinal cord. Primary afferent depolarisation (PAD) is an important mechanism of presynaptic inhibition that regulates
somatosensory inputs to the spinal cord, and primary afferent
hyperpolarisation (PAH) can produce opposite effects (1,2). In
addition, activity in primary afferents may serve as a mechanism for
spreading excitation to spinal cord neurons, thereby maintaining the
excitability of spinal cord circuits (3,4).
Our aim was to investigate how changes in the excitability at PA
terminals modify activity in spinal circuits. By expressing opto- and
chemogenetic tools under the control of the advillin promoter, the
excitability of central terminals of PA was manipulated to test the
influence of these manoeuvres on the rhythmic spontaneous and
induced activity recorded from dorsal roots and dorsal horn neurons., Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación

The dorsal horn of the spinal cord represents the first site in the central nervous system (CNS) where nociceptive signals are integrated. As a result, there has beenarapidgrowthinthenumberofstudiesinvestigating the ionic mechanisms regulating the excitability of dorsal horn neurons under normal and pathological conditions. We believe that it is time to look back and to critically examine what picture emerges from this wealth of studies. What are the actual types of neurons described in the literature based on electrophysiological criteria? Are these electrophysiologically-defined subpopulations strongly linked to specific morphological, functional, or molecular traits? Are these electrophysiological properties stable, or can they change during development or in response to peripheral injury? Here we provide an in-depth overview of both early and recent publications that explore the factors influencing dorsal horn neuronal excitability (including intrinsic membrane properties and synaptic transmission), how these factors vary across distinct subtypes of dorsal horn neurons, and how such factors are altered by peripheral nerve or tissue damage. The metaresearch presented below leads to the conclusion that the dorsal horn is comprised of highly heterogeneous subpopulations in which the observed electrophysiological properties of a given neuron often fail to easily predict other properties such as biochemical phenotype or morphology. This highlights the need for future studies which can more fully interrogate the properties of dorsal horn neurons in a multi-modal manner., Agencia Estatal de Investigación

Datos analizados de registros electrofisiológicos crudos que estudian actividad espontánea de neuronas espinales in vitro. Datos de test de comportamiento realizados en los animales estudiados.

El proyecto estudia las características funcionales de neuronas de la médula espinal de animales de experimentación. Estos datos servirán para conocer como funcionan los circuitos medulares en los que participan estas neuronas e identificar diferencias en su funcionamiento individual o conjunto en respuesta a procesos patológicos producidos por inflamación. Como complemento a esta información también se podrán recopilar datos histológicos y sobre comportamiento de los animales.

<p>El proyecto estudia las características funcionales de neuronas de la médula espinal de animales de experimentación. Estos datos servirán para conocer como funcionan los circuitos medulares en los que participan estas neuronas e identificar las características de su funcionamiento individual o conjunto. Como complemento a esta información también se han recopilado datos histológicos.</p>
<p>Datos analizados de registros electrofisiológicos crudos que estudian actividad espontánea e inducida de neuronas espinales y aferentes primarias in vitro, en respuesta a la modulación optogenética de la actividad de las aferentes.<p>
<p>Metolodogía (empleada para la recogida o generación de los datos)</p>
<p>La actividad eléctrica de neuronas del asta dorsal y de aferentes primarias fueron obtenidas de preparaciones in vitro de médula espinal de ratón.</p>
<p>Las señales se obtuvieron con matrices de multielectrodos y electrodos de succión conectados a amplificadores, y se almacenaron utilizando el software Spike2 (Cambridge Electronic Design, UK).</p>
<p>Para el aislamiento de potenciales de acción se utilizó el programa kilosort2 (https://github.com/MouseLand/Kilosort)</p>