Inhibitory Brainstem Research Articles

Neurexins control the strength and precise timing of glycinergic inhibition in the auditory brainstem.

Neurexins play diverse functions as presynaptic organizers in various glutamatergic and GABAergic synapses. However, it remains unknown whether and how neurexins are involved in shaping functional properties of the glycinergic synapses, which mediate prominent inhibition in the brainstem and spinal cord. To address these issues, we examined the role of neurexins in a model glycinergic synapse between the principal neuron in the medial nucleus of the trapezoid body (MNTB) and the principal neuron in the lateral superior olive (LSO) in the auditory brainstem. Combining RNAscope with stereotactic injection of AAV-Cre in the MNTB of neurexin1/2/3 conditional triple knockout mice, we showed that MNTB neurons highly express all isoforms of neurexins although their expression levels vary remarkably. Selective ablation of all neurexins in MNTB neurons not only reduced the amplitude but also altered the kinetics of the glycinergic synaptic transmission at LSO neurons. The synaptic dysfunctions primarily resulted from an impaired Ca2+ sensitivity of release and a loosened coupling between voltage-gated Ca2+ channels and synaptic vesicles. Together, our current findings demonstrate that neurexins are essential in controlling the strength and temporal precision of the glycinergic synapse, which therefore corroborates the role of neurexins as key presynaptic organizers in all major types of fast chemical synapses.

Systemic Glucose Regulation by a Hindbrain Inhibitory Circuit in a Mouse Model of Type 1 Diabetes

Introduction: Previous work showed that increasing the electrical activity of inhibitory neurons in the dorsal vagal complex (DVC) is sufficient to increase whole-body glucose concentration in normoglycemic mice. Here we tested the hypothesis that deactivating GABAergic neurons in the dorsal hindbrain of hyperglycemic mice decreases synaptic inhibition of parasympathetic motor neurons in the dorsal motor nucleus of the vagus (DMV) and reduces systemic glucose levels. Methods: Chemogenetic activation or inactivation of GABAergic neurons in the nucleus tractus solitarius (NTS) was used to assess effects of modulating parasympathetic output on blood glucose concentration in normoglycemic and hyperglycemic mice. Patch-clamp electrophysiology in vitro was used to assess cellular effects of chemogenetic manipulation of NTS GABA neurons. Results: Chemogenetic activation of GABAergic NTS neurons in normoglycemic mice increased their action potential firing, resulting in increased inhibitory synaptic input to DMV motor neurons and elevated blood glucose concentration. Deactivation of GABAergic DVC neurons in normoglycemic mice altered their electrical activity but did not alter systemic glucose levels. Conversely, stimulation of GABAergic DVC neurons in mice that were hyperglycemic subsequent to treatment with streptozotocin changed their electrical activity but did not alter whole-body glucose concentration, while deactivation of this inhibitory circuit significantly decreased circulating glucose concentration. Peripheral administration of a brain impermeant muscarinic acetylcholine receptor antagonist abolished these effects. Conclusion: Disinhibiting vagal motor neurons decreases hyperglycemia in a mouse model of type 1 diabetes. This inhibitory brainstem circuit emerges as a key parasympathetic regulator of whole-body glucose homeostasis that undergoes functional plasticity in hyperglycemic conditions.

Temporal hyper-precision of brainstem neurons alters spatial sensitivity of binaural auditory processing with cochlear implants.

The ability to localize a sound source in complex environments is essential for communication and navigation. Spatial hearing relies predominantly on the comparison of differences in the arrival time of sound between the two ears, the interaural time differences (ITDs). Hearing impairments are highly detrimental to sound localization. While cochlear implants (CIs) have been successful in restoring many crucial hearing capabilities, sound localization via ITD detection with bilateral CIs remains poor. The underlying reasons are not well understood. Neuronally, ITD sensitivity is generated by coincidence detection between excitatory and inhibitory inputs from the two ears performed by specialized brainstem neurons. Due to the lack of electrophysiological brainstem recordings during CI stimulation, it is unclear to what extent the apparent deficits are caused by the binaural comparator neurons or arise already on the input level. Here, we use a bottom-up approach to compare response features between electric and acoustic stimulation in an animal model of CI hearing. Conducting extracellular single neuron recordings in gerbils, we find severe hyper-precision and moderate hyper-entrainment of both the excitatory and inhibitory brainstem inputs to the binaural comparator neurons during electrical pulse-train stimulation. This finding establishes conclusively that the binaural processing stage must cope with highly altered input statistics during CI stimulation. To estimate the consequences of these effects on ITD sensitivity, we used a computational model of the auditory brainstem. After tuning the model parameters to match its response properties to our physiological data during either stimulation type, the model predicted that ITD sensitivity to electrical pulses is maintained even for the hyper-precise inputs. However, the model exhibits severely altered spatial sensitivity during electrical stimulation compared to acoustic: while resolution of ITDs near midline was increased, more lateralized adjacent source locations became inseparable. These results directly resemble recent findings in rodent and human CI listeners. Notably, decreasing the phase-locking precision of inputs during electrical stimulation recovered a wider range of separable ITDs. Together, our findings suggest that a central problem underlying the diminished ITD sensitivity in CI users might be the temporal hyper-precision of inputs to the binaural comparator stage induced by electrical stimulation.

A brainstem map for visceral sensations

The nervous system uses various coding strategies to process sensory inputs. For example, the olfactory system uses large receptor repertoires and is wired to recognize diverse odours, whereas the visual system provides high acuity of object position, form and movement1–5. Compared to external sensory systems, principles that underlie sensory processing by the interoceptive nervous system remain poorly defined. Here we developed a two-photon calcium imaging preparation to understand internal organ representations in the nucleus of the solitary tract (NTS), a sensory gateway in the brainstem that receives vagal and other inputs from the body. Focusing on gut and upper airway stimuli, we observed that individual NTS neurons are tuned to detect signals from particular organs and are topographically organized on the basis of body position. Moreover, some mechanosensory and chemosensory inputs from the same organ converge centrally. Sensory inputs engage specific NTS domains with defined locations, each containing heterogeneous cell types. Spatial representations of different organs are further sharpened in the NTS beyond what is achieved by vagal axon sorting alone, as blockade of brainstem inhibition broadens neural tuning and disorganizes visceral representations. These findings reveal basic organizational features used by the brain to process interoceptive inputs.

Modulation of recombinant human alpha 1 glycine receptor by flavonoids and gingerols.

The inhibitory glycine receptor (GlyR) is a principal mediator of fast synaptic inhibition in mammalian spinal cord, brainstem, and higher brain centres. Flavonoids are secondary plant metabolites that exhibit many beneficial physiological effects, including modulatory action on neuronal receptors. Using whole-cell current recordings from recombinant human α1 GlyRs, expressed in HEK293cells, we compared the flavonols kaempferol and quercetin, the flavanone naringenin, the flavones apigenin and nobiletin, the isoflavone genistein, and two gingerols, 6-gingerol and 8-gingerol for their modulation of receptor currents. All compounds were inhibitors of the GlyR with IC50 values ranging between 9.3±2.6µM (kaempferol) and 46.7±6.5µM (genistein), following a mixed mode of inhibition. Co-application of two inhibitors revealed distinct binding sites for flavonoids and gingerols. Pore-lining mutants T258A and T258S were strongly inhibited by quercetin and naringenin, but not by 6-gingerol, confirming the existence of distinct binding sites for flavonoids and gingerols. Apigenin, kaempferol, nobiletin, naringenin and 6-gingerol showed biphasic action, potentiating glycine-induced currents at low concentration of both, modulator and glycine, and inhibiting at higher concentrations. Identification of distinct modulatory sites for flavonoids and related compounds may present pharmacological target sites and aid the discovery of novel glycinergic drugs.

When a Gain Becomes a Loss: Gain-of-Function Glycine Receptors and Hyperekplexia

Glycine receptors (GlyRs) mediate rapid synaptic inhibition in mammalian spinal cord and brainstem. GlyRs mutations often produce abnormalities that result in hyperekplexia. Most mutations cause a loss of GlyR function such as decreased expression, conductance or ligand sensitivity. It is easily seen how a reduction in inhibition could result in the increased excitatory symptoms of hyperekplexia. Paradoxically, the α1V280M mutation triggers hyperekplexia but exhibits gain of GlyR function. The α1V280M mutation increases GlyR sensitivity to glycine and produces spontaneous channel activity but does not alter GlyR expression. Because CSF contains resting levels of 10µM glycine and because GlyRs undergo desensitization, we sought to determine the degree of desensitization induced by resting glycine levels for wild-type (α1WT) and α1V280M GlyRs. HEK-293 cells transfected with cDNA for either human α1WT or α1V280M GlyR were prepared for outside-out patch clamp recording. Currents were activated by five, 300ms applications of 3mM glycine every 5s (–50mV). Between applications (background), patches were perfused with glycine-free solution. Subsequently, the background was switched to a solution containing between 3nM and 60µM glycine; and currents recorded. Finally, recovery currents were obtained. Desensitization was calculated as the peak current in the presence of glycine background relative to that in glycine-free background. Background glycine decreased peak currents through both α1WT and α1V280M GlyR with a sigmoidal concentration dependence. IC50s for current inhibition were 14µM for α1WT GlyR and 0.3µM for α1V280M GlyR. The 10µM glycine in CSF would desensitize about half of α1WT GlyRs. In contrast, 98% of α1V280M GlyR would be desensitized. Thus, even though the V280M mutation produces a gain-of-function at the receptor level, it results in a loss-of-function at the synaptic level. This may explain why patients with this mutation exhibit the symptoms of hyperekplexia.

Novel inhibitory brainstem neurons with selective projections to spinal lamina I reduce both pain and itch.

Sensory information is transmitted from peripheral nerves, through the spinal cord, and up to the brain (“bottom up” pathway). Some of this information may be modulated by “top‐down” projections from the brain to the spinal cord. Discovering endogenous mechanisms for reducing pain and itch holds enormous potential for developing new treatments. However, neurons mediating the top‐down inhibition of pain are not well understood, nor has any such pathway been identified for itch sensation. Here, we identify a novel population of GABAergic neurons in the ventral brainstem, distinguished by prodynorphin expression, which we named LJA5. LJA5 neurons provide the only known inhibitory projection specifically to lamina I of the spinal cord, which contains sensory neurons that transmit pain and itch information up to the brain. Chemogenetically activating LJA5 neurons in male mice reduces capsaicin‐induced pain and histamine‐induced itch. Identifying this new pathway opens new treatment opportunities for chronic, refractory pain, and pruritis.

Novel Inhibitory Brainstem Neurons with Selective Projections to Spinal Lamina I Reduce Both Pain and Itch

Chronic pain and itch afflict millions of patients, and current treatments are largely ineffective. We do not fully understand the neural control of pain and itch, and are thus limited in developing therapies targeting the underlying circuitry in the brain or spinal cord. Current models show that lamina I of the spinal cord contains the neurons that transmit pain and itch information from the skin up to the brain. However, it is not well understood how the brain modulates pain and itch sensations in a top-down fashion. The simplest and most direct way for the brain to suppress pain and itch would be a direct, inhibitory projection to lamina I sensory neurons, but prior investigations have focused on brainstem regions that project axons nonspecifically across many laminae of the spinal cord. In addition to top-down inhibition of pain, pharmacological studies suggest the possibility of top-down itch control, but this has no known neural substrate. Therefore, the specific neural pathway the brain uses to modulate pain remains incompletely understood, and whether the brain can inhibit itch in a top-down fashion is unknown. Here we identify a novel population of GABAergic neurons in the ventral brainstem, distinguished by prodynorphin expression, which we named LJA5. LJA5 neurons provide the only known inhibitory projection specifically to lamina I of the spinal cord, which contains sensory neurons that transmit pain and itch information up to the brain. Chemogenetically activating LJA5 neurons reduces pain and itch. Identifying this new top-down pathway opens new treatment opportunities for chronic, refractory pain and pruritis.

Abnormal Blink Reflex and Intermuscular Coherence in Writer's Cramp.

Background: Writer's cramp (WC) is a task-specific focal hand dystonia presenting with pain, stiffness and/or tremor while writing. We explored the involvement of cortical and brainstem circuits by measuring intermuscular coherence (IMC) and pre-pulse inhibition (PPI) of the blink reflex.Methods: IMC was measured in 10 healthy controls and 20 WC patients (10 with associated tremor) while they performed a precision grip task at different force levels. Blink responses were evaluated in 9 healthy controls and 10 WC patients by stimulating the right supraorbital nerve and recording surface EMG from the orbicularis oculi muscles bilaterally. PPI involved conditioning this stimulation with a prior shock to the right median nerve (100 ms interval), and measuring the reduction in the R2 component of the blink reflex.Results: Significant IMC at 3–7 Hz was present in WC patients, but not in healthy controls. Compared to healthy controls, in WC patients the R2 component of the blink reflex showed significantly less PPI. IMC at 3–7 Hz could reliably discriminate WC patients from healthy controls.Conclusion: Cortical or sub-cortical circuits generating theta (3–7 Hz) oscillations might play an important role in the pathogenesis of WC. Moreover, the lack of PPI implicates abnormalities in brainstem inhibition in the emergence of WC. IMC may merit further development as an electrodiagnostic test for focal dystonia.

Intrinsic physiology of inhibitory neurons changes over auditory development.

During auditory development, changes in membrane properties promote the ability of excitatory neurons in the brain stem to code aspects of sound, including the level and timing of a stimulus. Some of these changes coincide with hearing onset, suggesting that sound-driven neural activity produces developmental plasticity of ion channel expression. While it is known that the coding properties of excitatory neurons are modulated by inhibition in the mature system, it is unknown whether there are also developmental changes in the membrane properties of brain stem inhibitory neurons. We investigated the primary source of inhibition in the avian auditory brain stem, the superior olivary nucleus (SON). The present studies test the hypothesis that, as in excitatory neurons, the membrane properties of these inhibitory neurons change after hearing onset. We examined SON neurons at different stages of auditory development: embryonic days 14-16 (E14-E16), a time at which cochlear ganglion neurons are just beginning to respond to sound; later embryonic stages (E18-E19); and after hatching (P0-P2). We used in vitro whole cell patch electrophysiology to explore physiological changes in SON. Age-related changes were observed at the level of a single spike and in multispiking behavior. In particular, tonic behavior, measured as a neuron's ability to sustain tonic firing over a range of current steps, became more common later in development. Voltage-clamp recordings and biophysical models were employed to examine how age-related increases in ion currents enhance excitability in SON. Our findings suggest that concurrent increases in sodium and potassium currents underlie the emergence of tonic behavior. NEW & NOTEWORTHY This article is the first to examine heterogeneity of neuronal physiology in the inhibitory nucleus of the avian auditory system and demonstrate that tonic firing here emerges over development. By pairing computer simulations with physiological data, we show that increases in both sodium and potassium channels over development are necessary for the emergence of tonic firing.

Glycinergic Input to the Mouse Basal Forebrain Cholinergic Neurons.

The basal forebrain (BF) receives afferents from brainstem ascending pathways, which has been implicated first by Moruzzi and Magoun (1949) to induce forebrain activation and cortical arousal/waking behavior; however, it is very little known about how brainstem inhibitory inputs affect cholinergic functions. In the current study, glycine, a major inhibitory neurotransmitter of brainstem neurons, and gliotransmitter of local glial cells, was tested for potential interaction with BF cholinergic (BFC) neurons in male mice. In the BF, glycine receptor α subunit-immunoreactive (IR) sites were localized in choline acetyltransferase (ChAT)-IR neurons. The effect of glycine on BFC neurons was demonstrated by bicuculline-resistant, strychnine-sensitive spontaneous IPSCs (sIPSCs; 0.81 ± 0.25 × 10-1 Hz) recorded in whole-cell conditions. Potential neuronal as well as glial sources of glycine were indicated in the extracellular space of cholinergic neurons by glycine transporter type 1 (GLYT1)- and GLYT2-IR processes found in apposition to ChAT-IR cells. Ultrastructural analyses identified synapses of GLYT2-positive axon terminals on ChAT-IR neurons, as well as GLYT1-positive astroglial processes, which were localized in the vicinity of synapses of ChAT-IR neurons. The brainstem raphe magnus was determined to be a major source of glycinergic axons traced retrogradely from the BF. Our results indicate a direct effect of glycine on BFC neurons. Furthermore, the presence of high levels of plasma membrane glycine transporters in the vicinity of cholinergic neurons suggests a tight control of extracellular glycine in the BF.SIGNIFICANCE STATEMENT Basal forebrain cholinergic (BFC) neurons receive various activating inputs from specific brainstem areas and channel this information to the cortex via multiple projections. So far, very little is known about inhibitory brainstem afferents to the BF. The current study established glycine as a major regulator of BFC neurons by (1) identifying glycinergic neurons in the brainstem projecting to the BF, (2) showing glycine receptor α subunit-immunoreactive (IR) sites in choline acetyltransferase (ChAT)-IR neurons, (3) demonstrating glycine transporter type 2 (GLYT2)-positive axon terminals synapsing on ChAT-IR neurons, and (4) localizing GLYT1-positive astroglial processes in the vicinity of synapses of ChAT-IR neurons. The effect of glycine on BFC neurons was demonstrated by bicuculline-resistant, strychnine-sensitive spontaneous IPSCs recorded in whole-cell conditions.

Role of the locus coeruleus in the emergence of power law wake bouts in a model of the brainstem sleep-wake system through early infancy

Infant rats randomly cycle between the sleeping and waking states, which are tightly correlated with the activity of mutually inhibitory brainstem sleep and wake populations. Bouts of sleep and wakefulness are random; from P2–P10, sleep and wake bout lengths are exponentially distributed with increasing means, while during P10–P21, the sleep bout distribution remains exponential while the distribution of wake bouts gradually transforms to power law. The locus coeruleus (LC), via an undeciphered interaction with sleep and wake populations, has been shown experimentally to be responsible for the exponential to power law transition. Concurrently during P10–P21, the LC undergoes striking physiological changes – the LC exhibits strong global 0.3 Hz oscillations up to P10, but the oscillation frequency gradually rises and synchrony diminishes from P10–P21, with oscillations and synchrony vanishing at P21 and beyond. In this work, we construct a biologically plausible Wilson Cowan-style model consisting of the LC along with sleep and wake populations. We show that external noise and strong reciprocal inhibition can lead to switching between sleep and wake populations and exponentially distributed sleep and wake bout durations as during P2–P10, with the parameters of inhibition between the sleep and wake populations controlling mean bout lengths. Furthermore, we show that the changing physiology of the LC from P10–P21, coupled with reciprocal excitation between the LC and wake population, can explain the shift from exponential to power law of the wake bout distribution. To our knowledge, this is the first study that proposes a plausible biological mechanism, which incorporates the known changing physiology of the LC, for tying the developing sleep-wake circuit and its interaction with the LC to the transformation of sleep and wake bout dynamics from P2–P21.

Modulation of Recombinant Human α1 Glycine Receptors by Mono- and Disaccharides: A Kinetic Study.

Glycine receptors (GlyRs) mediate fast synaptic inhibition in spinal cord, brainstem, and higher brain centers. Recently, glucose was identified as a positive modulator of GlyR-mediated currents. Here, we investigated extent and kinetics of the positive modulation of recombinant human α1 glycine receptors by different mono- and disaccharides and sorbitol using patch-clamp recording techniques. Glucose and fructose augmented glycine-mediated whole-cell currents with an EC50 of 6-7 mM. At concentrations > 10 mM, the maximum of current enhancement was reached within ∼30 min. Kinetics of GlyR modulation resemble those of protein glycation. On-rates were <0.5 h for saturating concentrations of monosaccharides and ∼1.5 h for disaccharides. Off-rates were considerably slower (>24 h). Galactose, the C4-epimer of glucose, and the sugar alcohol sorbitol had no effect on GlyR currents. Recent cryoelectron microscopy structures were used to identify a potential binding site for saccharides near the ivermectin binding pocket with lysine 143 as possible attachment site. The GlyR mutant α1(K143A) was not potentiated by glucose, suggesting an involvement of this residue in glycine receptor modulation by saccharides.

Effect of high frequency repetitive transcranial magnetic stimulation on brainstem excitability in SCI

Background We studied the effect of one session of high-frequency repetitive transcranial magnetic stimulation (rTMS) on brainstem stem excitability in patients with incomplete SCI. Methods The study was a randomized, double-blind, sham-controlled trial. We recruited 27 patients with SCI to be randomly distributed in two study groups: active or sham rTMS. We recorded the blink reflex (BR) induced with electrical stimulation at 15 times sensory threshold (ST) to the supraorbital nerve (SON). A prepulse electrical stimulus on BR (PP-BR), was delivered with ring electrodes to the right index finger at 2 times ST and applied 100 ms before the SON stimulus. We also examined the excitability recovery curve of the blink reflex (RC-BR), with pairs of stimuli at interstimuli intervals (ISI) of 160, 300, 500, and 1000 ms. Patients were examined at baseline, and after high-frequency vertex rTMS (20 Hz; 40 pulses/burst, 1800 pulses total over 20 min). Results Fifteen patients received active rTMS and 12 sham rTMS. The age between groups was not significant. At baseline, R2 area of BR and percentage changes (%) in RC-BR at all intervals were similar in both groups. Active but not sham rTMS significantly reduced the area of R2 in BR and decreased the % inhibition with respect to baseline (p ⩽ 0.05), RC-BR did not change with any rTMS stimulation at any time. Conclusions High frequency-rTMS over motor cortex induces a decrease in brainstem reflex excitability and reduces inhibitory brainstem activity in patients with SCI.

Glucose is a positive modulator for the activation of human recombinant glycine receptors.

The inhibitory glycine receptor (GlyR), a cys-loop ion channel receptor, mediates rapid synaptic inhibition in spinal cord, brainstem and higher centres of the mammalian central nervous system. Here, modulation of GlyR function by glucose and fructose was examined in recombinant alpha1 and alpha1/beta GlyRs using patch-clamp methods. Glucose was a positive modulator of the receptor, reducing the average EC50 for glycine up to 4.5-fold. Glucose reduced cell-to-cell variability of glycine-mediated currents by stabilizing receptors with low EC50. Pre-incubation with sugars for several hours also produced augmentation of current responses that persisted after sugar removal. Potentiation by sugars was most significant in the range between 5 and 20 mM, with EC50 values ~ 10 mM, i.e. at physiological levels. Addition of glucose had no significant influence on responses mediated by the other GlyR agonists like taurine, β-alanine or ivermectin, indicating that glucose specifically augmented glycine receptor-mediated responses, and did not act through indirect metabolic effects. Receptor modulation by glucose may account for differences in constants reported in the literature and may be clinically relevant for disorders with elevated blood glucose levels. Glucose and related sugars are essential metabolites. We identified glucose and fructose as positive modulators of the human inhibitory glycine receptor, a neuronal ligand-gated ion channel. Receptor-mediated currents were enhanced at physiological concentrations (~ 10 mM of sugar). Direct modulation of a synaptic receptor by glucose is relevant in clinical cases of elevated blood glucose, and may be considered in experimental protocols.

In vivo protection against strychnine toxicity in mice by the glycine receptor agonist ivermectin.

The inhibitory glycine receptor, a ligand-gated ion channel that mediates fast synaptic inhibition in mammalian spinal cord and brainstem, is potently and selectively inhibited by the alkaloid strychnine. The anthelminthic and anticonvulsant ivermectin is a strychnine-independent agonist of spinal glycine receptors. Here we show that ivermectin is an effective antidote of strychnine toxicity in vivo and determine time course and extent of ivermectin protection. Mice received doses of 1 mg/kg and 5 mg/kg ivermectin orally or intraperitoneally, followed by an intraperitoneal strychnine challenge (2 mg/kg). Ivermectin, through both routes of application, protected mice against strychnine toxicity. Maximum protection was observed 14 hours after ivermectin administration. Combining intraperitoneal and oral dosage of ivermectin further improved protection, resulting in survival rates of up to 80% of animals and a significant delay of strychnine effects in up to 100% of tested animals. Strychnine action developed within minutes, much faster than ivermectin, which acted on a time scale of hours. The data agree with a two-compartment distribution of ivermectin, with fat deposits acting as storage compartment. The data demonstrate that toxic effects of strychnine in mice can be prevented if a basal level of glycinergic signalling is maintained through receptor activation by ivermectin.

Alterations in excitatory and inhibitory brainstem interneuronal circuits in fibromyalgia: Evidence of brainstem dysfunction

ObjectivePatients with fibromyalgia syndrome (FMS) perceive stimuli differently and show altered cortical sensory representation maps following peripheral stimulation. Altered sensory gating may play a causal role. MethodsBlink reflex, blink reflex excitability recovery, and prepulse inhibition of the blink reflex – representing brainstem excitability – were assessed in 10 female patients with FMS and 26 female healthy controls. ResultsUnconditioned blink reflex characteristics (R1 latency and amplitude, R2 and R2c latency and area-under-the-curve) did not differ significantly between patients and controls. Blink reflex excitability recovery was enhanced in patients versus controls at all intervals tested. Prepulses significantly suppressed R2 area and increased R2 latency in patients and controls. However, R2 area suppression was significantly less in patients than in controls (patients: to 80.0±28.9%, controls: to 47.8±21.7%). The general pattern of corresponding changes in R2c was similar. ConclusionsBlink reflex is normal, whereas blink reflex excitability recovery is enhanced and blink reflex prepulse inhibition is reduced in patients with FMS, suggesting functional changes at the brainstem level in FMS. SignificanceReduced blink reflex prepulse inhibition concurs with altered sensory gating in patients with FMS.

The kinetic properties of the α3 rat glycine receptor make it suitable for mediating fast synaptic inhibition

Glycine receptors mediate fast synaptic inhibition in spinal cord and brainstem. Two α subunits are present in adult neurones, α1, which forms most of the synaptic glycine receptors, and α3. The physiological role of α3 is not known, despite the fact that α3 expression is concentrated in areas involved in nociceptive processing, such as the superficial dorsal horn. In the present study, we characterized the kinetic properties of rat homomeric α3 glycine receptors heterologously expressed in HEK293 cells. We analysed steady state single channel activity at a range of different glycine concentrations by fitting kinetic schemes and found that α3 channels resemble α1 receptors in their high maximum open probability (99.1% cf. 98% for α1), but differ in that maximum open probability is reached when all five binding sites are occupied by glycine (cf. three out of five sites for α1). α3 activation was best described by kinetic schemes that allow the channel to open also when partially liganded and that contain more than the minimum number of shut states, either as desensitized distal states (Jones and Westbrook scheme) or as pre-open gating intermediates (flip scheme). We recorded also synaptic-like α3 currents elicited by the rapid application of 1 ms pulses of high concentration glycine to outside-out patches. These currents had fast deactivation, with a time constant of decay of 9 ms. Thus, if native synaptic currents can be mediated by α3 glycine receptors, they are likely to be very close in their kinetics to α1-mediated synaptic events.

Impairment of Habituation of the Auditory Brain Stem Response in Migrainous Vertigo

We evaluated the auditory brain stem response (ABR) in migrainous vertigo (MV). Four subjects who met clinical criteria for definite MV and 4 subjects with non-vertiginous migraine (NVM) underwent ABR testing while asymptomatic and within 16 hours of a symptomatic episode. Four control subjects were also tested. A set of 4 consecutive 750-click series was administered at 50-, 60-, and 70-dB intensities. We compared the groups in terms of habituation of the amplitude of wave IV-V (habituation of IV-V) from the first through fourth series for each set. The habituation of IV-V amplitude to 50-dB stimuli was significantly less (p = 0.047) in the symptomatic MV group (5.08% +/- 22.32%) than in the symptomatic NVM group (-21.44% +/- 13.50%) or the control group (-26.06% +/- 9.76%). The habituation of IV-V amplitude to 70-dB stimuli in the MV group was significantly less (p = 0.031) during symptomatic testing (-3.43% +/- 8.89%) than during asymptomatic testing (-21.23% +/- 6.41%). The habituation of IV-V amplitude is reduced during MV attacks. This finding suggests impaired brain stem inhibition at the level of the inferior colliculus, which shares serotonergic connections with the dorsal raphe nucleus, an area hyperactive in migraine.

Rivastigmine Alleviates Experimentally Induced Colitis in Mice and Rats by Acting at Central and Peripheral Sites to Modulate Immune Responses

The cholinergic anti-inflammatory system and α7 nicotinic receptors in macrophages have been proposed to play a role in neuroimmunomodulation and in the etiology of ulcerative colitis. We investigated the ability of a cholinesterase (ChE) inhibitor rivastigmine, to improve the pathology of ulcerative colitis by increasing the concentration of extracellular acetylcholine in the brain and periphery. In combination with carbachol (10 µM), rivastigmine (1 µM) significantly decreased the release of nitric oxide, TNF-α, IL-1β and IL-6 from lipopolysaccharide-activated RAW 264.7 macrophages and this effect was abolished by α7 nicotinic receptor blockade by bungarotoxin. Rivastigmine (1 mg/kg) but not (0.5 mg/kg), injected subcutaneously once daily in BALB/c mice with colitis induced by 4% dextran sodium sulphate (DSS), reduced the disease activity index (DAI) by 60% and damage to colon structure. Rivastigmine (1 mg/kg) also reduced myeloperoxidase activity and IL-6 by >60%, and the infiltration of CD11b expressing cells by 80%. These effects were accompanied by significantly greater ChE inhibition in cortex, brain stem, plasma and colon than that after 0.5 mg/kg. Co-administration of rivastigmine (1 mg/kg) with the muscarinic antagonist scopolamine significantly increased the number of CD11b expressing cells in the colon but did not change DAI compared to those treated with rivastigmine alone. Rivastigmine 1 and 2 mg given rectally to rats with colitis induced by rectal administration of 30 mg dintrobezene sulfonic acid (DNBS) also caused a dose related reduction in ChE activity in blood and colon, the number of ulcers and area of ulceration, levels of TNF-α and in MPO activity. The study revealed that the ChE inhibitor rivastigmine is able to reduce gastro-intestinal inflammation by actions at various sites at which it preserves ACh. These include ACh released from vagal nerve endings that activates alpha7 nicotinic receptors on circulating macrophages and in brainstem neurons.