Spinal dorsal horn astrocytes release GABA in response to synaptic activation.

KEY POINTS: 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. ABSTRACT: Astrocytes participate in neuronal signalling by releasing gliotransmitters in response to neurotransmitters. We investigated if astrocytes from the dorsal horn of the spinal cord of adult red-eared turtles (Trachemys scripta elegans) release GABA in response to glutamatergic receptor activation. For this, we developed a GABA sensor consisting of HEK cells expressing GABAA receptors. By positioning the sensor recorded in the whole-cell patch-clamp configuration within the dorsal horn of a spinal cord slice, we could detect GABA in the extracellular space. Puff application of glutamate induced GABA release events with time courses that exceeded the duration of inhibitory postsynaptic currents by one order of magnitude. Because the events were neither affected by extracellular addition of nickel, cadmium and tetrodotoxin nor by removal of Ca2+ , we concluded that they originated from non-neuronal cells. Immunohistochemical staining allowed the detection of GABA in a fraction of dorsal horn astrocytes. The selective stimulation of A∂ and C fibres in a dorsal root filament induced a Ca2+ increase in astrocytes loaded with Oregon Green BAPTA. Finally, chelating Ca2+ in a single astrocyte was sufficient to prevent the GABA release evoked by glutamate. Our results indicate that glutamate triggers the release of GABA from dorsal horn astrocytes with a time course compatible with the integration of sensory inputs.

Live Imaging of Kv7.2/7.3 Cell Surface Dynamics at the Axon Initial Segment: High Steady-State Stability and Calpain-Dependent Excitotoxic Downregulation Revealed.

The voltage-gated K(+) channels Kv7.2 and Kv7.3 are located at the axon initial segment (AIS) and exert strong control over action potential generation. Therefore, changes in their localization or cell surface numbers are likely to influence neuronal signaling. However, nothing is known about the cell surface dynamics of Kv7.2/7.3 at steady state or during short-term neuronal stimulation. This is primarily attributable to their membrane topology, which hampers extracellular epitope tagging. Here we circumvent this limitation by fusing an extra phluorin-tagged helix to the N terminus of human Kv7.3. This seven transmembrane chimera, named super ecliptic phluorin (SEP)-TAC-7.3, functions and traffics as a wild-type (WT) channel. We expressed SEP-TAC-7.3 in dissociated rat hippocampal neurons to examine the lateral mobility, surface numbers, and localization of AIS Kv7.2/7.3 heteromers using live imaging. We discovered that they are extraordinarily stable and exhibit a very low surface mobility both during steady state and neuronal stimulation. In the latter case, we also found that neither localization nor cell surface numbers were changed. However, at high glutamate loads, we observed a rapid irreversible endocytosis of Kv7.2/7.3, which required the activation of NR2B-containing NMDA receptors, Ca(2+) influx, and calpain activation. This excitotoxic mechanism may be specific to ankyrin G-bound AIS proteins because Nav1.2 channels, but not AIS GABAA receptors, were also endocytosed. In conclusion, we have, for the first time, characterized the cell surface dynamics of a full-length Kv7 channel using a novel chimeric strategy. This approach is likely also applicable to other Kv channels and thus of value for the additional characterization of this ion channel subfamily. SIGNIFICANCE STATEMENT: The voltage-gated K(+) channels Kv7.2 and Kv7.3 exert strong control over action potential generation, but little is known about their cell surface dynamics. Using a novel phluorin-based approach, we here show that these channels are highly stable at steady state and different types of neuronal stimulation. However, at high glutamate loads, they undergo a rapid calpain-dependent endocytosis that likely represents an early response during excitotoxic states.

White Noise Background Improves Tone Discrimination by Suppressing Cortical Tuning Curves.

The brain faces the difficult task of maintaining a stable representation of key features of the outside world in noisy sensory surroundings. How does the sensory representation change with noise, and how does the brain make sense of it? We investigated the effect of background white noise (WN) on tuning properties of neurons in mouse A1 and its impact on discrimination performance in a go/no-go task. We find that WN suppresses the activity of A1 neurons, which surprisingly increases the discriminability of tones spectrally close to each other. To confirm the involvement of A1, we optogenetically excited parvalbumin-positive (PV+) neurons in A1, which have similar effects as WN on both tuning properties and frequency discrimination. A population model suggests that the suppression of A1 tuning curves increases frequency selectivity and thereby improves discrimination. Our findings demonstrate that the cortical representation of pure tones adapts during noise to improve sensory acuity.

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.