Anders Victor Petersen

Modulation of the intrinsic properties of motoneurons by serotonin.

Serotonin (5-HT) is one of the main transmitters in the nervous system. Serotonergic neurons in the raphe nuclei in the brainstem innervate most parts of the central nervous system including motoneurons in the spinal cord and brainstem. This review will focus on the modulatory role that 5-HT exerts on motoneurons and its physiological consequences. The somato-dendritic compartments of motoneurons are densely innervated by serotonergic synaptic boutons and several receptors are expressed in the membrane of motoneurons including 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2A, 5-HT2B, 5-HT2C and 5-HT5A. The activation of serotonergic receptors induces a general increase of the excitability of motoneurons through the modulation of several classes of ion channels. 5-HT depolarizes motoneurons towards the threshold for action potentials by inhibiting leak conductances and promoting a hyperpolarization activated cationic current. At the same time, 5-HT increases the firing frequency by inhibiting the small Ca2+ activated K+ conductance (SK) responsible for the medium afterhyperpolarization (AHP) following action potentials. 5-HT also promotes persistent inward currents mediated by voltage sensitive Ca2+ and Na+ conductances, producing a sustained depolarization and an amplification of synaptic inputs. Under pathological conditions, such as after a spinal cord injury, the promotion of persistent inward currents by serotonin and/or the overexpression of autoactive serotonergic receptors may contribute to motoneuronal excitability, muscle spasms and spasticity and hence, impairment of stereotyped motor behaviors such as locomotion, ejaculation and micturition.

How do Glial Cells Contribute to Motor Control

For many years, glial cells from the central nervous system have been considered as support cells involved in the homeostasis of the brain. However, a series of key-findings obtained during the past two decades has put light on unexpected roles for glia and it is getting more and more admitted that glia play an active role in several physiological functions. The discovery that a bidirectional communication takes place between astrocytes (the star shaped glial cell of the brain) and neurons, was a major breakthrough in the field of synaptic physiology. Astrocytes express receptors that get activated by neurotransmitters during synaptic transmission. In turn they release other transmitters - called gliotransmitters - that bind to neuronal receptors and modulate synaptic transmission. This feedback, which led to the concept of the tripartite synapse, has been reported with various transmitters including glutamate, ATP, GABA or serine. In the present review we will focus on astrocytes and review the evidence suggesting and demonstrating their role in motor control. Rhythmic motor behaviors such as locomotion, swimming or chewing are generated by networks of neurons termed central pattern generators (CPG). These networks are highly flexible and adjust the frequency of their output to the external environment. In the case of respiration, the CPG reacts when changes in the pH of the blood occur. The chemosensory control of breathing is ensured by astrocytes, which react to variation of the blood pH by releasing ATP on neurons that in turn adapt the frequency of respiration. In the spinal cord, diverse transmitters such as ATP, adenosine or endocannabinoids modulate the CPG responsible for locomotion. A growing body of evidence suggests that glial cells release some of these molecules. These data suggest that astrocytes play an essential role in motor control and we believe that a range of studies will confirm this view in the near future.

Plasticity of the Axon Initial Segment Fast and Slow Processes with Multiple Functional Roles

The axon initial segment (AIS) is a key neuronal compartment because it is responsible for action potential initiation. The local density of Na+ channels, the biophysical properties of K+ channels, as well as the length and diameter of the AIS determine the spiking of neurons. These parameters undergo important modifications during development. The development of the AIS is governed by intrinsic mechanisms. In addition, surrounding neuronal networks modify its maturation. As a result, neurons get tuned to particular physiological functions. Neuronal activity also influences the morphology of the mature AIS. When excitatory neurons are hyperactive, their AIS undergo structural changes that decrease their excitability and thereby maintain the activity within a given range. These slow homeostatic regulatory mechanisms occur on a time scale of hours or days. In contrast, the activation of metabotropic receptors modulates the properties of ion channels expressed at the AIS within seconds and consequently produces fast adjustments of neuronal excitability. Recent results suggest that this plasticity plays important roles in physiological functions as diverse as memory formation, hearing, and motor control.

Serotonin Regulates the Firing of Principal Cells of the Subiculum by Inhibiting a T-type Ca2+ Current

The subiculum is the main output of the hippocampal formation. A high proportion of its principal neurons fire action potentials in bursts triggered by the activation of low threshold calcium currents. This firing pattern promotes synaptic release and regulates spike-timing-dependent plasticity. The subiculum receives a high density of fibers originating from the raphe nuclei, suggesting that serotonin (5-HT) modulates subicular neurons. Here we investigated if and how 5-HT modulates the firing pattern of bursting neurons. By combining electrophysiological analysis with pharmacology, optogenetics and calcium imaging, we demonstrate that 5-HT2C receptors reduce bursting activity by inhibiting a low-threshold calcium current mediated by T-type Ca2+ channels in principal cells of the subiculum. In addition, we show that the activation of this novel pathway decreases bursting activity and the occurrence of epileptiform discharges induced in in vitro models for temporal lobe epilepsy (TLE).

Fast detection of extrasynaptic GABA with a whole-cell sniffer.

Gamma-amino-butyric acid (GABA) is the main inhibitory transmitter of the brain. It operates by binding to specific receptors located both inside and outside synapses. The extrasynaptic receptors are activated by spillover from GABAergic synapses and by ambient GABA in the extracellular space. Ambient GABA is essential for adjusting the excitability of neurons. However, due to the lack of suitable methods, little is known about its dynamics. Here we describe a new technique that allows detection of GABA transients and measurement of the steady state GABA concentration with high spatial and temporal resolution. We used a human embryonic kidney (HEK) cell line that stably expresses GABAA receptors composed of α1, β2, and γ2 subunits. We recorded from such a HEK cell with the whole-cell patch-clamp technique. The presence of GABA near the HEK cell generated a measurable electric current whose magnitude increased with concentration. A fraction of the current did not inactivate during prolonged exposition to GABA. This technique, which we refer to as a “sniffer” allows the measurement of ambient GABA concentration inside nervous tissue with a resolution of few tens of nanomolars. In addition, the sniffer detects variations in the extrasynaptic GABA concentration with millisecond time resolution. Pilot experiments demonstrate that the sniffer is able to report spillover of GABA induced by synaptic activation in real time. This is the first report on a GABA sensor that combines the ability to detect fast transients and to measure steady concentrations.

A neuroligin-1-derived peptide stimulates phosphorylation of the NMDA receptor NR1 subunit and rescues MK-801-induced decrease in long-term potentiation and memory impairment

Neuroligins (NLs) are postsynaptic adhesion molecules, interacting with presynaptic neurexins (NXs), which determine the differential formation of excitatory (glutamatergic, NL1) and inhibitory (GABAergic, NL2) synapses. We have previously demonstrated that treatment with a NL2‐derived peptide, neurolide‐2, reduces sociability and increase animal aggression. We hypothesized that interfering with NL1 function at the excitatory synapses might regulate synaptic plasticity and learning, and counteract memory deficits induced by N‐methyl‐d‐aspartate (NMDA) receptor inhibition. First, neuronal NMDA receptor phosphorylation after treatment with NL1 or a mimetic peptide, neurolide‐1, was quantified by immunoblotting. Subsequently, we investigated effects of neurolide‐1 on long‐term potentiation (LTP) induction in hippocampal slices compromised by NMDA receptor inhibitor MK‐801. Finally, we investigated neurolide‐1 effects on short‐ and long‐term social and spatial memory in social recognition, Morris water‐maze, and Y‐maze tests. We found that subcutaneous neurolide‐1 administration, restored hippocampal LTP compromised by NMDA receptor inhibitor MK‐801. It counteracted MK‐801‐induced memory deficit in the water‐maze and Y‐maze tests after long‐term treatment (24 h and 1–2 h before the test), but not after short‐term exposure (1–2 h). Long‐term exposure to neurolide‐1 also facilitated social recognition memory. In addition, neurolide‐1‐induced phosphorylation of the NMDA receptor NR1 subunit on a site important for synaptic trafficking, potentially favoring synaptic receptor retention. Our findings emphasize the role of NL1–NMDA receptor interaction in cognition, and identify neurolide‐1, as a valuable pharmacological tool to examine the in vivo role of postsynaptic NL1 in cognitive behavior in physiological and pathological conditions.

Fast and reliable identification of axons, axon initial segments and dendrites with local field potential recording.

The axon initial segment (AIS) is an essential neuronal compartment. It is usually where action potentials are initiated. Recent studies demonstrated that the AIS is a plastic structure that can be regulated by neuronal activity and by the activation of metabotropic receptors. Studying the AIS in live tissue can be difficult because its identification is not always reliable. Here we provide a new technique allowing a fast and reliable identification of the AIS in live brain slice preparations. By simultaneous recording of extracellular local field potentials and whole-cell patch-clamp recording of neurons, we can detect sinks caused by inward currents flowing across the membrane. We determine the location of the AIS by comparing the timing of these events with the action potential. We demonstrate that this method allows the unequivocal identification of the AIS of different types of neurons from the brain.

Soluble Ectodomain of Neuroligin 1 Decreases Synaptic Activity by Activating Metabotropic Glutamate Receptor 2

Synaptic cell adhesion molecules represent important targets for neuronal activity-dependent proteolysis. Postsynaptic neuroligins (NLs) form trans-synaptic complexes with presynaptic neurexins (NXs). Both NXs and NLs are cleaved from the cell surface by metalloproteases in an activity-dependent manner, releasing a soluble extracellular fragment and membrane-tethered C-terminal fragment. The cleavage of NL1 depresses synaptic transmission, but the mechanism by which this occurs is unknown. Metabotropic glutamate receptor 2 (mGluR2) are located primarily at the periphery of presynaptic terminals, where they inhibit the formation of cyclic adenosine monophosphate (cAMP) and consequently suppress the release of glutamate and decrease synaptic transmission. In the present study, we found that the soluble ectodomain of NL1 binds to and activates mGluR2 in both neurons and heterologous cells, resulting in a decrease in cAMP formation. In a slice preparation from the hippocampus of mice, NL1 inhibited the release of glutamate from mossy fibers that project to CA3 pyramidal neurons. The presynaptic effect of NL1 was abolished in the presence of a selective antagonist for mGluR2. Thus, our data suggest that the soluble extracellular domain of NL1 functionally interacts with mGluR2 and thereby decreases synaptic strength.

Locomotor- and Reward-Enhancing Effects of Cocaine Are Differentially Regulated by Chemogenetic Stimulation of Gi-Signaling in Dopaminergic Neurons

Abstract Dopamine plays a key role in the cellular and behavioral responses to drugs of abuse, but the implication of metabotropic regulatory input to dopaminergic neurons on acute drug effects and subsequent drug-related behavior remains unclear. Here, we used chemogenetics [Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)] to modulate dopamine signaling and activity before cocaine administration in mice. We show that chemogenetic inhibition of dopaminergic ventral tegmental area (VTA) neurons differentially affects locomotor and reward-related behavioral responses to cocaine. Stimulation of Gi-coupled DREADD (hM4Di) expressed in dopaminergic VTA neurons persistently reduced the locomotor response to repeated cocaine injections. An attenuated locomotor response was seen even when a dual-viral vector approach was used to restrict hM4Di expression to dopaminergic VTA neurons projecting to the nucleus accumbens. Surprisingly, despite the attenuated locomotor response, hM4Di-mediated inhibition of dopaminergic VTA neurons did not prevent cocaine sensitization, and the inhibitory effect of hM4Di-mediated inhibition was eliminated after withdrawal. In the conditioned place-preference paradigm, hM4Di-mediated inhibition did not affect cocaine-induced place preference; however, the extinction period was extended. Also, hM4Di-mediated inhibition had no effect on preference for a sugar-based reward over water but impaired motivation to work for the same reward in a touchscreen-based motivational assay. In addition, to support that VTA dopaminergic neurons operate as regulators of reward motivation toward both sugar and cocaine, our data suggest that repeated cocaine exposure leads to adaptations in the VTA that surmount the ability of Gi-signaling to suppress and regulate VTA dopaminergic neuronal activity.

Spinal dorsal horn astrocytes release GABA in response to synaptic activation

GABA is an essential molecule for sensory information processing. It is usually assumed to be released by neurons. Here we show that in the dorsal horn of the spinal cord, astrocytes respond to glutamate by releasing GABA. Our findings suggest a novel role for astrocytes in somatosensory information processing.