Journal of Physiology

1-Subunits enhance the gating properties of large-conductance Ca2+-activated K+ channels (BKCa) formed by -subunits. In arterial vascular smooth muscle cells (VSMCs), 1-subunits are vital in coupling SR-generated Ca2+ sparks to BKCa activation, affecting contractility and blood pressure. Studies in cremaster and cerebral VSMCs show heterogeneity of BKCa activity due to apparent differences in the functional 1-subunit:-subunit ratio. To define these differences, studies were conducted at the single-channel level while siRNA was used to manipulate specific subunit expression. 1 modulation of the -subunit Ca2+ sensitivity was studied using patch-clamp techniques. BKCa channel normalized open probability (NPo) versus membrane potential (Vm) curves were more left-shifted in cerebral versus cremaster VSMCs as cytoplasmic Ca2+ was raised from 0.5 to 100 m. Calculated V1/2 values of channel activation decreased from 72.0 ± 6.1 at 0.5 m Ca2+i to –89 ± 9 mV at 100 m Ca2+i in cerebral compared with 101 ± 10 to –63 ± 7 mV in cremaster VSMCs. Cremaster BKCa channels thus demonstrated an ~2.5-fold weaker apparent Ca2+ sensitivity such that at a value of Vm of –30 mV, a mean value of [Ca2+]i of 39 m was required to open half of the channels in cremaster versus 16 m [Ca2+]i in cerebral VSMCs. Further, shortened mean open and longer mean closed times were evident in BKCa channel events from cremaster VSMCs at either –30 or 30 mV at any given [Ca2+]. 1-Subunit-directed siRNA decreased both the apparent Ca2+ sensitivity of BKCa in cerebral VSMCs and the appearance of spontaneous transient outward currents. The data are consistent with a higher ratio of 1-subunit:-subunit of BKCa channels in cerebral compared with cremaster VSMCs. Functionally, this leads both to higher Ca2+ sensitivity and NPo for BKCa channels in the cerebral vasculature relative to that of skeletal muscle.

Cholecystokinin (CCK) is a hypothetical controller for suckling and infancy body weight, although the underlying mechanisms remain unclear. Therefore, the present study analysed the mechanisms using mice lacking the CCK-1 receptor (CCK1R–/–). Although CCK1R–/– mice displayed normal weights at birth and adulthood, CCK1R–/– pups had enlarged adipocytes and were overweight from the first to second week after birth, regardless of maternal genotype. The lacZ reporter gene assay and/or calcium imaging analysis demonstrated that CCK-1 receptors were abundant in satiety-controlling regions such as the hypothalamus, brainstem, nodose ganglion and pylorus in adults, whereas these signals were few to lacking at pre-weanling stages. At postnatal day (PD) 6, the increase in cFos expression in the medullary nucleus tractus solitarius was similarly triggered by gastrointestinal milk- or saline filling in both genotypes, further indicating immature CCK-1 receptor function in an ascending satiety-controlling system during infancy. Conversely, third ventricle ependymal tanycyte-like cells expressed CCK-1 receptors with expression peaking at PD6. At PD6, wild-type but not CCK1R–/– mice had increased cFos immunoreactivity in ependymal cells following gastrointestinal milk filling whereas the response became negligible at PD12. In addition, ependymal cFos was not increased by saline filling, indicating that these responses are dependent on CCK-1 receptors, developmental stage and nutrients. Furthermore, body weights of wild-type pups were transiently increased by blocking ependymal CCK receptor function with microinjection of a CCK-1 antagonist, but not a CCK-2 antagonist. Hence, we demonstrate de novo functions of ependymal CCK-1 receptors and reveal a new aspect of infant satiety-controlling mechanisms.

Exercise-induced vascular endothelial adaptations in the kidney are not well understood. Therefore, we investigated the impact of voluntary wheel running (VWR) on the abundance of endothelial nitric oxide synthase (eNOS) and extracellular superoxide dismutase (EC SOD), in kidney and lung, and other SOD isoforms and total antioxidant capacity (TAC), in kidney. We also determined whether VWR influences susceptibility to acute kidney injury (AKI). Male Sprague–Dawley and Fisher 344 rats, VWR or sedentary for 12 weeks, were subjected to AKI (uninephrectomy (UNX) and 35 min of left kidney ischaemia–24 h reperfusion, IR). We measured glomerular filtration rate (GFR) and renal plasma flow (RPF), and analysed renal structural injury. Running was comparable between strains and VWR reduced body weight. In Sprague–Dawley rats, VWR reduced eNOS and EC SOD, but increased Mn SOD in kidney. Similar changes were seen after 6 weeks of VWR in Sprague–Dawley rats. In Fisher 344 rats, VWR increased eNOS, all SOD isoforms and TAC in kidney. Both strains increased eNOS and EC SOD in lung with VWR. Compared to UNX alone, UNX-IR injury markedly reduced renal function for both strains; however, in the Sprague–Dawley rats, VWR exacerbated falls in GFR and RPF due to UNX-IR, whereas in the Fisher 344 rats, GFR was unaffected by VWR. Some indices of renal structural injury due to UNX-IR tended to be worse in SD vs. F344. Our study demonstrates that genetic background influences the effect of exercise on kidney eNOS and EC SOD, which in turn influence the susceptibility to AKI.

-Cell mass increases during pregnancy in adaptation to the insulin resistance of pregnancy. This increase is accompanied by an increase in -cell proliferation, a process that requires intact prolactin receptor (Prlr) signalling. Previously, it was found that during pregnancy, heterozygous prolactin receptor-null (Prlr+/–) mice had lower number of -cells, lower serum insulin and higher blood glucose levels than wild-type (Prlr+/+) mice. An unexpected observation was that the glucose homeostasis of the experimental mouse depends on the genotype of her mother, such that within the Prlr+/+ group, the Prlr+/+ offspring derived from Prlr+/+ mothers (Prlr+/+(+/+)) had higher -cell mass and lower blood glucose than those derived from Prlr+/– mothers (Prlr+/+(+/–)). Pathways that are known to regulate -cell proliferation during pregnancy include insulin receptor substrate-2, Akt, menin, the serotonin synthetic enzyme tryptophan hydroxylase-1, Forkhead box M1 and Forkhead box D3. The aim of the present study was to determine whether dysregulation in these signalling molecules in the islets could explain the maternal effect on the phenotype of the offspring. It was found that the pregnancy-induced increases in insulin receptor substrate-2 and Akt expression in the islets were attenuated in the Prlr+/+(+/–) mice in comparison to the Prlr+/+(+/+) mice. The expression of Forkhead box D3, which plays a permissive role for -cell proliferation during pregnancy, was also lower in the Prlr+/+(+/–) mice. In contrast, the pregnancy-induced increases in phospho-Jak2, tryptophan hydroxylase-1 and FoxM1, as well as the pregnancy-associated reduction in menin expression, were comparable between the two groups. There was also no difference in expression levels of genes that regulate insulin synthesis and secretion (i.e. glucose transporter 2, glucokinase and pancreatic and duodenal homeobox-1) between these two groups. Taken together, these results suggest that the in utero environment of the Prlr+/– mother confers long-term changes in the pancreatic islets of her offspring such that when the offspring themselves became pregnant, they cannot adapt to the increased insulin demands of their own pregnancy.

The time course of muscular fatigue that develops during and after an intense bout of self-paced dynamic exercise was characterized by using different forms of electrical stimulation (ES) of the exercising muscles. Ten active subjects performed a time trial (TT) involving repetitive concentric extension/flexion of the right knee using a Biodex dynamometer. Neuromuscular function (NMF), including ES and a 5 s maximal isometric voluntary contraction (MVC), was assessed before the start of the TT and immediately (<5 s) after each 20% of the TT had been completed, as well as 1, 2, 4 and 8 min after TT termination. The TT time was 347 ± 98 s. MVCs were 52% of baseline values at TT termination. Torque responses from ES were reduced to 33–68% of baseline using different methods of stimulation, suggesting that the extent to which peripheral fatigue is documented during exercise depends upon NMF assessment methodology. The major changes in muscle function occurred within the first 40% of exercise. Significant recovery in skeletal muscle function occurs within the first 1–2 min after exercise, showing that previous studies may have underestimated the extent to which peripheral fatigue develops during exercise.

We combine electrophysiological and optical techniques to investigate the role that the expression of chloride channels (ClC-1) plays on the age-dependent electrical properties of mammalian muscle fibres. To this end, we comparatively evaluate the magnitude and voltage dependence of chloride currents (ICl), as well as the resting resistance, in fibres isolated from control and human skeletal actin (HSA)LR mice (a model of myotonic dystrophy) of various ages. In control mice, the maximal peak chloride current ([peak-ICl]max) increases from –583 ± 126 to –956 ± 260 A cm–2 (mean ± SD) between 3 and 6 weeks old. Instead, in 3-week-old HSALR mice, ICl are significantly smaller (–153 ± 33 A cm–2) than in control mice, but after a long period of ~14 weeks they reach statistically comparable values. Thus, the severe ClC-1 channelopathy in young HSALR animals is slowly reversed with aging. Frequency histograms of the maximal chloride conductance (gCl,max) in fibres of young HSALR animals are narrow and centred in low values; alternatively, those from older animals show broad distributions, centred at larger gCl,max values, compatible with mosaic expressions of ClC-1 channels. In fibres of both animal strains, optical data confirm the age-dependent increase in gCl, and additionally suggest that ClC-1 channels are evenly distributed between the sarcolemma and transverse tubular system membranes. Although gCl is significantly depressed in fibres of young HSALR mice, the resting membrane resistance (Rm) at –90 mV is only slightly larger than in control mice due to upregulation of a Rb-sensitive resting conductance (gK,IR). In adult animals, differences in Rm are negligible between fibres of both strains, and the contributions of gCl and gK,IR are less altered in HSALR animals. We surmise that while hyperexcitability in young HSALR mice can be readily explained on the basis of reduced gCl, myotonia in adult HSALR animals may be explained on the basis of a mosaic expression of ClC-1 channels in different fibres and/or on alterations of other conductances.

The assessment of voluntary activation of human muscles usually depends on measurement of the size of the twitch produced by an interpolated nerve or cortical stimulus. In many forms of fatiguing exercise the superimposed twitch increases and thus voluntary activation appears to decline. This is termed ‘central' fatigue. Recent studies on isolated mouse muscle suggest that a peripheral mechanism related to intracellular calcium sensitivity increases interpolated twitches. To test whether this problem developed with human voluntary contractions we delivered maximal tetanic stimulation to the ulnar nerve (60 s at physiological motoneuronal frequencies, 30 and 15 Hz). During the tetani (at 30 Hz) in which the force declined by 42%, the absolute size of the twitches evoked by interpolated stimuli (delivered regularly or only in the last second of the tetanus) diminished progressively to less than 1%. With stimulation at 30 Hz, there was also a marked reduction in size and area of the interpolated compound muscle action potential (M wave). With a 15 Hz tetanus, a progressive decline in the interpolated twitch force also occurred (to ~10%) but did so before the area of the interpolated M wave diminished. These results indicate that the increase in interpolated twitch size predicted from the mouse studies does not occur. Diminution in superimposed twitches occurred whether or not the M wave indicated marked impairment at sarcolemmal/t-tubular levels. Consequently, the increase in superimposed twitch, which is used to denote central fatigue in human fatiguing exercise, is likely to reflect low volitional drive to high-threshold motor units, which stop firing or are discharging at low frequencies.

The response to mechanical stimuli, i.e. tensegrity, plays an important role in regulating cell physiological and pathophysiological function, and the mechanical silencing observed in intensive care unit (ICU) patients leads to a severe and specific muscle wasting condition. This study aims to unravel the underlying mechanisms and the effects of passive mechanical loading on skeletal muscle mass and function at the gene, protein and cellular levels. A unique experimental rat ICU model has been used allowing long-term (weeks) time-resolved analyses of the effects of standardized unilateral passive mechanical loading on skeletal muscle size and function and underlying mechanisms. Results show that passive mechanical loading alleviated the muscle wasting and the loss of force-generation associated with the ICU intervention, resulting in a doubling of the functional capacity of the loaded versus the unloaded muscles after a 2-week ICU intervention. We demonstrate that the improved maintenance of muscle mass and function is probably a consequence of a reduced oxidative stress revealed by lower levels of carbonylated proteins, and a reduced loss of the molecular motor protein myosin. A complex temporal gene expression pattern, delineated by microarray analysis, was observed with loading-induced changes in transcript levels of sarcomeric proteins, muscle developmental processes, stress response, extracellular matrix/cell adhesion proteins and metabolism. Thus, the results from this study show that passive mechanical loading alleviates the severe negative consequences on muscle size and function associated with the mechanical silencing in ICU patients, strongly supporting early and intense physical therapy in immobilized ICU patients.

The chloride channel CLC-3 is expressed in the brain on synaptic vesicles and postsynaptic membranes. Although CLC-3 is broadly expressed throughout the brain, the CLC-3 knockout mouse shows complete, selective postnatal neurodegeneration of the hippocampus, suggesting a crucial role for the channel in maintaining normal brain function. CLC-3 channels are functionally linked to NMDA receptors in the hippocampus; NMDA receptor-dependent Ca2+ entry, activation of Ca2+/calmodulin kinase II and subsequent gating of CLC-3 link the channels via a Ca2+-mediated feedback loop. We demonstrate that loss of CLC-3 at mature synapses increases long-term potentiation from 135 ± 4% in the wild-type slice preparation to 154 ± 7% above baseline (P < 0.001) in the knockout; therefore, the contribution of CLC-3 is to reduce synaptic potentiation by ~40%. Using a decoy peptide representing the Ca2+/calmodulin kinase II phosphorylation site on CLC-3, we show that phosphorylation of CLC-3 is required for its regulatory function in long-term potentiation. CLC-3 is also expressed on synaptic vesicles; however, our data suggest functionally separable pre- and postsynaptic roles. Thus, CLC-3 confers Cl– sensitivity to excitatory synapses, controls the magnitude of long-term potentiation and may provide a protective limit on Ca2+ influx.

The natural target of the botulinum neurototoxin type A (BoNT-A) is the neuromuscular junction. When injected into a muscle, BoNT-A is internalized by motoneurone terminals where it functions as an endopeptidase, cleaving protein components of the synaptic machinery responsible for vesicle docking and exocytosis. As a result, BoNT-A induces a characteristic flaccid paralysis of the affected muscle. In animal models, BoNT-A applied in the periphery can also influence central activity via retrograde transport and transcytosis. An analogous direct central effect in humans is still debated. The present study was designed to address whether BoNT-A modifies the activity of the spinal recurrent inhibitory pathways, when injected at muscular level, in humans. To avoid methodological bias, the recurrent inhibition from an injected muscle (soleus) was investigated on an untreated muscle (quadriceps), and stimulation parameters (producing recurrent inhibition) were monitored on a third non-injected muscle but innervated by the same nerve as the soleus (flexor digitorum brevis). The experiments were performed on 14 post-stroke patients exhibiting spasticity in ankle plantarflexors, candidates for BoNT-A. One month after BoNT-A, the level of recurrent inhibition was depressed. It is suggested that the depression of recurrent inhibition was induced by BoNT-A, injected peripherally, through axonal transport and blockade of the cholinergic synapse between motoneurone recurrent collaterals and Renshaw cells.

Type A botulinum toxin blocks not only ACh release from motor nerve terminals but also central synaptic transmission, including glutamate, noradrenaline, dopamine, ATP, GABA and glycine. Neurotoxins (NTXs) are transported by both antero- and retrogradely along either motor or sensory axons for bidirectional delivery between peripheral tissues or the CNS. A newly developed type A2 NTX (A2NTX) injected into one rat foreleg muscle was transported to the contralateral muscle. This finding was consistent with the NTX traveling retrogradely via spinal neurons and then transsynaptically through motor neurons to the contralateral motor neurons within the spinal cord and on to the soleus muscle. In the present study we found that toxin injection into the rat left soleus muscle clearly induced bilateral muscle relaxation in a dose-dependent fashion, although the contralateral muscle relaxation followed the complete inhibition of toxin-injected ipsilateral muscles. The toxin-injected ipsilateral muscle relaxation was faster and stronger in A2NTX-treated rats than A1LL (BOTOX). A1LL was transported almost equally to the contralateral muscle via neural pathways and the bloodstream. In contrast, A2NTX was mainly transported to contralateral muscles via the blood. A1LL was more successfully transported to contralateral spinal neurons than A2NTX. We also demonstrated that A1LL and A2NTX were carried from peripheral to CNS and vice versa by dual antero- and retrograde axonal transport through either motor or sensory neurons.

Time representation is an important element of cerebellar neural processing, but the mechanisms involved are poorly understood. We demonstrate that the major mossy fibre input system originating from the lateral reticular nucleus (LRN) can represent sensory event timing over hundreds of milliseconds. In vivo, cerebellar-projecting LRN neurons discharge extremely regularly with a clock-like rhythm. In response to stimulation of a wide peripheral receptive field, firing briefly pauses then resumes with precise timing. The precision of post-stimulus spikes and the regularity of firing mean that the stimulus timing is represented by LRN spike timing over hundreds of milliseconds. In an arithmetic progression model of LRN neuron firing, highly predictable post-stimulus spike timing is modulated by changing the variability of the first post-inhibitory spike and of the subsequent interspike intervals. From in vitro analysis we show that the Ca2+-activated small-conductance K+ current (SK) contributes to interspike interval regularity and that the hyperpolarization-activated cation current (Ih) contributes to short-latency, high-precision post-hyperpolarisation spike timing. Consistent with this, we demonstrate in vivo that resumption of firing becomes more sharply timed after longer stimulus-evoked pauses. Thus, Ih is a potential conductance that could mediate the precisely timed resumption of firing after the pause. Through the widespread projections of LRN neurons, these properties may enable the LRN to provide precisely timed signals to the cerebellum over a prolonged period following a stimulus, which may also both activate and sustain oscillatory processes in the cerebellar cortex.

Within the core molecular clock, protein phosphorylation and degradation play a vital role in determining circadian period. The ‘after-hours' (Afh) mutation in mouse slows the degradation of the core clock protein Cryptochrome, lengthening the period of the molecular clock in the suprachiasmatic nuclei (SCN) and behavioural wheel-running rhythms. However, we do not yet know how the Afh mutation affects other aspects of physiology or the activity of circadian oscillators in other brain regions. Here we report that daily rhythms of metabolism and ingestive behaviours are altered in these animals, as are PERIOD2::LUCIFERASE (PER2::LUC) rhythms in mediobasal hypothalamic nuclei, which influence these behaviours. Overall there is a trend towards period lengthening and a decrease in amplitude of PER2::LUC rhythms throughout the brain. Imaging of single cells from the arcuate and dorsomedial hypothalamic nuclei revealed this reduction in tissue oscillator amplitude to be due to a decrease in the amplitude, rather than a desynchrony, of single cells. Consistent with existing models of oscillator function, this cellular phenotype was associated with a greater susceptibility to phase-shifting stimuli in vivo and in vitro, with light evoking high-amplitude Type 0 resetting in Afh mutant mice. Together, these findings reveal unexpected consequences of the Afh mutation on the amplitude and synchrony of individual cellular oscillators in the SCN.

Many theories of motor control suggest that we select our movements to reduce energy use. However, it is unclear whether this process underlies short-term motor adaptation to novel environments. Here we asked whether adaptation to walking on a split-belt treadmill leads to a more economical walking pattern. We hypothesized that adaptation would be accompanied by a reduction in metabolic power and muscle activity and that these reductions would be temporally correlated. Eleven individuals performed a split-belt adaptation task where the belt speeds were set at 0.5 and 1.5 m s-1. Adaptation was characterized by step length symmetry, which is the normalized difference in step length between the legs. Metabolic power was calculated based on expired gas analysis, and surface EMG was used to record the activity of four bilateral leg muscles (tibialis anterior, lateral gastrocnemius, vastus lateralis and biceps femoris). All participants initially walked with unequal step lengths when the belts moved at different speeds, but gradually adapted to take steps of equal length. Additionally, net metabolic power was reduced from early adaptation to late adaptation (early, 3.78 ± 1.05 W kg–1; and late, 3.05 ± 0.79 W kg–1; P < 0.001). This reduction in power was also accompanied by a bilateral reduction in EMG throughout the gait cycle. Furthermore, the reductions in metabolic power occurred over the same time scale as the improvements in step length symmetry, and the magnitude of these improvements predicted the size of the reduction in metabolic power. Our results suggest that increasing economy may be a key criterion driving locomotor adaptation.

We previously demonstrated that genetic and/or pharmacological ablation of the TRPV1+/peptidergic or the MrgprD+/non-peptidergic subset of nociceptors produced selective, modality-specific deficits in the behavioural responses to heat and mechanical stimuli, respectively. To assess whether this modality-specific contribution is also manifest at the level of spinal cord neuron responsiveness, here we made extracellular recordings from lumbar dorsal horn neurons of the mouse in response to graded thermal and mechanical stimulation. We found that, following intrathecal injection of capsaicin to eliminate the central terminals of TRPV1+ nociceptors, neurons in the region of laminae I and V of the spinal cord lost responsiveness to noxious heat (whether generated by a contact heat probe or diode laser), with no change in their response to noxious mechanical stimulation. In contrast, ablation of MrgprD+ afferents did not alter the response to noxious heat, but reduced the firing of superficial dorsal horn nociceptive-specific neurons in response to graded mechanical stimulation and decreased the relative number of wide dynamic range neurons that were exclusively mechanosensitive. Neither ablation procedure reduced the number of dorsal horn neurons that responded to noxious cold. These findings support the conclusion that TRPV1+ nociceptors are necessary and probably sufficient for noxious heat activation of dorsal horn neurons and that, despite their polymodal properties, TRPV1+ and MrgprD+ nociceptors provide modality-specific contributions to the response properties of spinal cord neurons.

The T-junction of sensory neurons in the dorsal root ganglion (DRG) is a potential impediment to action potential (AP) propagation towards the CNS. Using intracellular recordings from rat DRG neuronal somata during stimulation of the dorsal root, we determined that the maximal rate at which all of 20 APs in a train could successfully transit the T-junction (following frequency) was lowest in C-type units, followed by A-type units with inflected descending limbs of the AP, and highest in A-type units without inflections. In C-type units, following frequency was slower than the rate at which AP trains could be produced in either dorsal root axonal segments or in the soma alone, indicating that the T-junction is a site that acts as a low-pass filter for AP propagation. Following frequency was slower for a train of 20 APs than for two, indicating that a cumulative process leads to propagation failure. Propagation failure was accompanied by diminished somatic membrane input resistance, and was enhanced when Ca2+-sensitive K+ currents were augmented or when Ca2+-sensitive Cl– currents were blocked. After peripheral nerve injury, following frequencies were increased in axotomized C-type neurons and decreased in axotomized non-inflected A-type neurons. These findings reveal that the T-junction in sensory neurons is a regulator of afferent impulse traffic. Diminished filtering of AP trains at the T-junction of C-type neurons with axotomized peripheral processes could enhance the transmission of activity that is ectopically triggered in a neuroma or the neuronal soma, possibly contributing to pain generation.

Fragile X syndrome (FXS) is the most common form of inheritable mental retardation caused by transcriptional silencing of the Fmr1 gene resulting in the absence of fragile X mental retardation protein (FMRP). The role of this protein in neurons is complex and its absence gives rise to diverse alterations in neuronal function leading to neurological disorders including mental retardation, hyperactivity, cognitive impairment, obsessive-compulsive behaviour, seizure activity and autism. FMRP regulates mRNA translation at dendritic spines where synapses are formed, and thus the lack of FMRP can lead to disruptions in synaptic transmission and plasticity. Many of these neurological deficits in FXS probably involve the prefrontal cortex, and in this study, we have focused on modulatory actions of dopamine in the medial prefrontal cortex. Our data indicate that dopamine produces a long-lasting enhancement of evoked inhibitory postsynaptic currents (IPSCs) mediated by D1-type receptors seen in wild-type mice; however, such enhancement is absent in the Fmr1 knock-out (Fmr1 KO) mice. The facilitation of IPSCs produced by direct cAMP stimulation was unaffected in Fmr1 KO, but D1 receptor levels were reduced in these animals. Our results show significant disruption of dopaminergic modulation of synaptic transmission in the Fmr1 KO mice and this alteration in inhibitory activity may provide insight into potential targets for the rescue of deficits associated with FXS.

Changes in the activity of striatal output neurons (SONs) have been implicated in the pathogenesis of Huntington's disease (HD). In this inherited polyglutamine disorder, accumulation of intracellular toxins causes a variety of deficits, including synaptic dysfunction, but it is still unclear to what extent striatal GABA release is afflicted as well. Two murine HD models were used, a recently created knock-in mouse (Z_Q175_KI) and an established model of HD (R6/2). In sagittal slices with relatively well-preserved glutamatergic connections throughout the basal ganglia, we have characterized the following: (i) the excitability of SONs; (ii) their spontaneous action potential-dependent GABAergic synaptic activity; (iii) the capacity of exogenous GABA to inhibit spontaneous action potential generation; and (iv) the properties of GABAergic unitary evoked responses (eIPSCs) in response to intrastriatal minimal stimulation at low and high frequency. The HD SONs exhibited enhanced intrisic excitability and higher levels of GABAergic spontaneous activity without presenting evidence for homeostatic upregulation of endogenous or exogenous GABA actions. Unitary eIPSC amplitudes were reduced, with a clear deficit in the probability of release, as indicated by a higher paired-pulse ratio, failure rate and coefficient of variation. In conditions of high-frequency activation, GABAergic connections of HD SONs were prone to asynchronous release and delayed IPSC generation at the expense of synchronized release. Both in wild-type and in HD SONs, GABA was inhibitory. Our results support the conclusion that the enhanced spontaneous synaptic activity in the HD striatum reflects disinhibition. Pharmacological tests identified the HD-related tonic suppression of synaptic inhibition as a glutamate- and endocannabinoid-dependent process.