Optimizing GABAA receptor antagonism for the induction of long-term potentiation in the mouse and rat dentate gyrus in vitro.

The reliable induction of long-term potentiation (LTP) in the dentate gyrus (DG) in vitro requires the blockade of the γ-aminobutyric acid A (GABAA) receptor. In these studies we examined the effectiveness of the specific GABAA receptor antagonist bicuculline methiodide (BMI) in facilitating LTP in the DG from hippocampal slices obtained from either C57Bl/6 mice or Sprague-Dawley rats, two species commonly used for electrophysiology. In the C57Bl/6 mice, maximal short-term potentiation and LTP in the DG were produced with a concentration of 5 µM BMI. In contrast, a concentration of 10 µM BMI was required to produce maximal short-term potentiation and LTP in the DG of Sprague-Dawley rats. These results reveal that there are species differences in the optimal amount of BMI required to produce robust and reliable LTP in the rodent DG in vitro and highlight the need to take consideration of the species being used when choosing concentrations of pharmacological agents to employ for electrophysiological use.NEW & NOTEWORTHY In this report we provide specific neurophysiological evidence for concentrations of GABAA antagonist required to study long-term potentiation in the medial perforant pathway of the dentate gyrus. Two commonly used species, Sprague-Dawley rats and C57Bl/6 mice, require different concentrations of bicuculline methiodide to induce optimal short-term and long-term potentiation.

Neuroinflammation alters GABAergic neurotransmission in hyperammonemia and hepatic encephalopathy, leading to motor incoordination. Mechanisms and therapeutic implications.

Enhanced GABAergic neurotransmission contributes to impairment of motor coordination and gait and of cognitive function in different pathologies, including hyperammonemia and hepatic encephalopathy. Neuroinflammation is a main contributor to enhancement of GABAergic neurotransmission through increased activation of different pathways. For example, enhanced activation of the TNFα-TNFR1-NF-κB-glutaminase-GAT3 pathway and the TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway in cerebellum of hyperammonemic rats enhances GABAergic neurotransmission. This is mediated by mechanisms affecting GABA synthesizing enzymes GAD67 and GAD65, total and extracellular GABA levels, membrane expression of GABAA receptor subunits, of GABA transporters GAT1 and GAT three and of chloride co-transporters. Reducing neuroinflammation reverses these changes, normalizes GABAergic neurotransmission and restores motor coordination. There is an interplay between GABAergic neurotransmission and neuroinflammation, which modulate each other and altogether modulate motor coordination and cognitive function. In this way, neuroinflammation may be also reduced by reducing GABAergic neurotransmission, which may also improve cognitive and motor function in pathologies associated to neuroinflammation and enhanced GABAergic neurotransmission such as hyperammonemia, hepatic encephalopathy or Parkinson's disease. This provides therapeutic targets that may be modulated to improve cognitive and motor function and other alterations such as fatigue in a wide range of pathologies. As a proof of concept it has been shown that antagonists of GABAA receptors such as bicuculline reduces neuroinflammation and improves cognitive and motor function impairment in rat models of hyperammonemia and hepatic encephalopathy. Antagonists of GABAA receptors are not ideal therapeutic tools because they can induce secondary effects. As a more effective treatment to reduce GABAergic neurotransmission new compounds modulating it by other mechanisms are being developed. Golexanolone reduces GABAergic neurotransmission by reducing the potentiation of GABAA receptor activation by neurosteroids such as allopregnanolone. Golexanolone reduces neuroinflammation and GABAergic neurotransmission in animal models of hyperammonemia, hepatic encephalopathy and cholestasis and this is associated with improvement of fatigue, cognitive impairment and motor incoordination. This type of compounds may be useful therapeutic tools to improve cognitive and motor function in different pathologies associated with neuroinflammation and increased GABAergic neurotransmission.

Neuroprotective effects of platinum nanoparticle-based microreactors in bicuculline-induced seizures.

Epilepsy designates a group of chronic brain disorders, characterized by the recurrence of hypersynchronous, repetitive activity, of neuronal clusters. Epileptic seizures are the hallmark of epilepsy. The primary goal of epilepsy treatment is to eliminate seizures with minimal side effects. Nevertheless, approximately 30% of patients do not respond to the available drugs. An imbalance between excitatory/inhibitory neurotransmission, that leads to excitotoxicity, seizures, and cell death, has been proposed as an important mechanism regarding epileptogenesis. Recently, it has been shown that microreactors composed of platinum nanoparticles (Pt-NP) and glutamate dehydrogenase possess in vitro and in vivo activity against excitotoxicity. This study investigates the in vivo effects of these microreactors in an animal model of epilepsy induced by the administration of the GABAergic antagonist bicuculline. Male Wistar rats were administered intracerebroventricularly (i.c.v.) with the microreactors or saline and, five days later, injected with bicuculline or saline. Seizure severity was evaluated in an open field. Thirty min after behavioral measurements, animals were euthanized, and their brains processed for neurodegeneration evaluation and for neurogenesis. Treatment with the microreactors significantly increased the time taken for the onset of seizures and for the first tonic-clonic seizure, when compared to the bicuculline group that did not receive the microreactor. The administration of the microreactors also increased the time spent in total exploration and grooming. Treatment with the microreactors decreased bicuculline-induced neurodegeneration and increased neurogenesis in the dorsal and ventral hippocampus. These observations suggest that treatment with Pt-NP-based microreactors attenuates the behavioral and neurobiological consequences of epileptiform seizure activity.

Synergistic effect between citalopram and muscimol upon induction of anxiolytic- and antidepressant-like effects in male mice: An isobologram analysis.

Background: There is evidence that both the GABAergic system and serotonin reuptake inhibitor (SSRI) such as citalopram are involved in the modulation of anxiety and depression processes. In this research, we examined the effects of GABAA receptor agents and citalopram on anxiety- and depression-related behaviors and their interaction in male mice. Methods: For intracerebroventricular (i.c.v.) infusion, a guide cannula was implanted in the left lateral ventricle. Anxiety and depression behaviors were evaluated using the elevated plus-maze (EPM) and forced swimming test (FST). Results: The results revealed that i.c.v. microinjection of muscimol (1 µg/mouse) enhanced % OAT (open arm time) and % OAE (open arm entries) in the EPM test and decreased immobility time in the FST without affecting locomotor activity, presenting anxiolytic- and antidepressant-like behaviors in the EPM and FST, respectively. On the other hand, i.c.v. microinjection of bicuculline (1 µg/mouse) reduced % OAT and % OAE without affecting locomotor activity and immobility time, presenting an anxiogenic-like effect. Moreover, i.p. administration of citalopram (8 mg/kg) increased %OAT and %OAE and reduced immobility time with no effect on locomotor activity, showing anxiolytic- and antidepressant-like responses in male mice. Furthermore, i.c.v. infusion of an ineffective dosage of muscimol potentiated the anxiolytic- and antidepressant-like responses induced by i.p. injection of citalopram in male mice. When citalopram and bicuculline were co-injected, a non-significant dose of bicuculline reversed the anxiolytic-like effect of citalopram in male mice. Also, the data revealed synergistic anxiolytic- and antidepressant-like behaviors between citalopram and muscimol in male mice. Conclusions: The results suggested an interaction between citalopram and GABAergic agents on the modulation of anxiety and depression behaviors in male mice.

Cross state-dependent memory retrieval between tramadol and ethanol: involvement of dorsal hippocampal GABAA receptors.

RATIONALE: Tramadol and ethanol, as psychoactive agents, are often abused. Discovering the molecular pathways of drug-induced memory creation may contribute to preventing drug addiction and relapse. OBJECTIVE: The tramadol- and ethanol-induced state-dependent memory (SDM) and cross-SDM retrieval between tramadol and ethanol were examined in this study. Moreover, because of the confirmed involvement of GABAA receptors and GABAergic neurotransmission in memory retrieval impairment, we assessed cross-SDM retrieval between tramadol and ethanol with a specific emphasis on the role of the GABAA receptors. The first hypothesis of this study was the presence of cross-SDM between tramadol and ethanol, and the second hypothesis was related to possible role of GABAA receptors in memory retrieval impairment within the dorsal hippocampus. The cannulae were inserted into the hippocampal CA1 area of NMRI mice, and a step-down inhibitory avoidance test was used to evaluate state dependence and memory recovery. RESULTS: The post-training and/or pre-test administration of tramadol (2.5 and 5 mg/kg, i.p.) and/or ethanol (0.5 and 1 g/kg, i.p.) induced amnesia, which was restored after the administration of the drugs 24 h later during the pre-test period, proposing ethanol and tramadol SDM. The pre-test injection of ethanol (0.25 and 0.5 g/kg, i.p.) with tramadol at an ineffective dose (1.25 mg/kg) enhanced tramadol SDM. Moreover, tramadol injection (1.25 and 2.5 mg/kg) with ethanol at the ineffective dose (0.25 g/kg) promoted ethanol SDM. Furthermore, the pre-test intra-CA1 injection of bicuculline (0.0625, 0.125, and 0.25 µg/mouse), a GABAA receptor antagonist, 5 min before the injection of tramadol (5 mg/kg) or ethanol (1 g/kg) inhibited tramadol- and ethanol-induced SDM dose-dependently. CONCLUSION: The findings strongly confirmed cross-SDM between tramadol and ethanol and the critical role of dorsal hippocampal GABAA receptors in the cross-SDM between tramadol and ethanol.

Pharmacokinetics, tissue distribution, and plasma protein binding ratio of bicuculline following intragastric and intravenous administration in rats using ultra-high-performance liquid chromatography-tandem mass spectrometry.

Bicuculline is a natural isoquinoline alkaloid that works as a gamma-aminobutyric acid receptor antagonist. It is widely found in Papaveraceae plants used in traditional Chinese medicines. Bicuculline not only has been shown to have favorable analgesic, memory-improving, and anxiolytic effects but may also cause adverse effects such as convulsions and epilepsy. A simple, rapid, and sensitive method was developed and validated for the determination of bicuculline in the plasma and tissue samples in rats by ultra-high-performance liquid chromatography-tandem mass spectrometry (MS/MS). The chromatographic separation was performed on a Thermo Scientific C18 column. The MS/MS system was operated in the positive multiple reaction monitoring mode, and the precursor-product ion transitions were optimized as m/z 368.0 → 307.1 for bicuculline and as 354.1 → 188.1 for protopine (internal standard). The linearity, accuracy, precision, recovery, and matrix effect were within acceptable limits. The experimental data showed that bicuculline was rapidly absorbed and eliminated in rats, with a moderate plasma protein binding ratio and low bioavailability. The main tissues of distribution were the kidney, liver, and brain; bicuculline could exert its pharmacological effects across the blood-brain barrier. This study has positive implications for the clinical use of herbal medicines containing bicuculline and for further development.

Frequency-coded patterns of sympathetic vasomotor activity are differentially evoked by the paraventricular nucleus of the hypothalamus in the Goldblatt hypertension model.

Introduction: The paraventricular nucleus of the hypothalamus (PVN) contains premotor neurons involved in the control of sympathetic vasomotor activity. It is known that the stimulation of specific areas of the PVN can lead to distinct response patterns at different target territories. The underlying mechanisms, however, are still unclear. Recent evidence from sympathetic nerve recording suggests that relevant information is coded in the power distribution of the signal along the frequency range. In the present study, we addressed the hypothesis that the PVN is capable of organizing specific spectral patterns of sympathetic vasomotor activation to distinct territories in both normal and hypertensive animals. Methods: To test it, we investigated the territorially differential changes in the frequency parameters of the renal and splanchnic sympathetic nerve activity (rSNA and sSNA, respectively), before and after disinhibition of the PVN by bicuculline microinjection. Subjects were control and Goldblatt rats, a sympathetic overactivity-characterized model of neurogenic hypertension (2K1C). Additionally, considering the importance of angiotensin II type 1 receptors (AT1) in the sympathetic responses triggered by bicuculline in the PVN, we also investigated the impact of angiotensin AT1 receptors blockade in the spectral features of the rSNA and sSNA activity. Results: The results revealed that each nerve activity (renal and splanchnic) presents its own electrophysiological pattern of frequency-coded rhythm in each group (control, 2K1C, and 2K1C treated with AT1 antagonist losartan) in basal condition and after bicuculline microinjection, but with no significant differences regarding total power comparison among groups. Additionally, the losartan 2K1C treated group showed no decrease in the hypertensive response triggered by bicuculline when compared to the non-treated 2K1C group. However, their spectral patterns of sympathetic nerve activity were different from the other two groups (control and 2K1C), suggesting that the blockade of AT1 receptors does not totally recover the basal levels of neither the autonomic responses nor the electrophysiological patterns in Goldblatt rats, but act on their spectral frequency distribution. Discussion: The results suggest that the differential responses evoked by the PVN were preferentially coded in frequency, but not in the global power of the vasomotor sympathetic responses, indicating that the PVN is able to independently control the frequency and the power of sympathetic discharges to different territories.

Involvement of the spinal γ-aminobutyric acid receptor in the analgesic effects of intrathecally injected hypertonic saline in spinal nerve-ligated rats.

Background: : Hypertonic saline is used for treating chronic pain; however, clinical studies that aid in optimizing therapeutic protocols are lacking. We aimed to determine the concentration of intrathecally injected hypertonic saline at which the effect reaches its peak as well as the underlying γ-aminobutyric acid (GABA) receptor-related antinociceptive mechanism. Methods: : Spinal nerve ligation (SNL; left L5 and L6) was performed to induce neuropathic pain in rats weighing 250-300 g. Experiment 1: one week after implanting the intrathecal catheter, 60 rats were assigned randomly to intrathecal injection with 0.45%, 0.9%, 2.5%, 5%, 10%, and 20% NaCl, followed by behavioral testing at baseline and after 30 minutes, 2 hours, 1 day, and 1 week to determine the minimal concentration which produced maximal analgesia. Experiment 2: after determining the optimal intrathecal hypertonic saline concentration, 60 rats were randomly divided into four groups: Sham, hypertonic saline without pretreatment, and hypertonic saline after pretreatment with one of two GABA receptor antagonists (GABAA [bicuculline], or GABAB [phaclofen]). Behavioral tests were performed at weeks 1 and 3 following each treatment. Results: : Hypertonic saline at concentrations greater than 5% alleviated SNL-induced mechanical allodynia and had a significant therapeutic effect, while showing a partial time- and dose-dependent antinociceptive effect on thermal and cold hyperalgesia. However, pretreatment with GABA receptor antagonists inhibited the antinociceptive effect of 5% NaCl. Conclusions: : This study indicates that the optimal concentration of hypertonic saline for controlling mechanical allodynia in neuropathic pain is 5%, and that its analgesic effect is related to GABAA and GABAB receptors.

Adenosine and γ-aminobutyric acid partially regulate metabolic and ventilatory responses of Damaraland mole-rats to acute hypoxia.

Fossorial Damaraland mole-rats (Fukomys damarensis) mount a robust hypoxic metabolic response (HMR) but a blunted hypoxic ventilatory response (HVR) to acute hypoxia. Although these reflex physiological responses have been described previously, the underlying signalling pathways are entirely unknown. Of particular interest are contributions from γ-aminobutyric acid (GABA), which is the primary inhibitory neurotransmitter in the nervous system of most adult mammals, and adenosine, the accumulation of which increases during hypoxia as a breakdown product of ATP. Therefore, we hypothesized that GABAergic and/or adenosinergic signalling contributes to the blunted HVR and robust HMR in Damaraland mole-rats. To test this hypothesis, we injected adult animals with saline alone (controls), or 100 mg kg-1 aminophylline or 1 mg kg-1 bicuculline, to block adenosine or GABAA receptors, respectively. We then used respirometry, plethysmography and thermal RFID probes to non-invasively measure metabolic, ventilator and thermoregulatory responses, respectively, to acute hypoxia (1 h in 5 or 7% O2) in awake and freely behaving animals. We found that bicuculline had relatively minor effects on metabolism and thermoregulation but sensitized ventilation such that the HVR became manifest at 7% instead of 5% O2 and was greater in magnitude. Aminophylline increased metabolic rate, ventilation and body temperature in normoxia, and augmented the HMR and HVR. Taken together, these findings indicate that adenosinergic and GABAergic signalling play important roles in mediating the robust HMR and blunted HVR in Damaraland mole-rats.

Differential inhibition of cough by GABAA and GABAB receptor antagonists in the nucleus of the solitary tract in cats.

Bicuculline and saclofen were microinjected into the rostral (rNTS) and caudal nucleus of the solitary tract (cNTS) in 17 anesthetized cats. Electromyograms (EMGs) of the diaphragm (DIA) and abdominal muscles (ABD), esophageal pressures (EP), and blood pressure were recorded and analyzed. Bilateral microinjections of 1 mM bicuculline in the rNTS significantly reduced the number of coughs (CN), amplitudes of DIA and ABD EMG, inspiratory and expiratory EP, and prolonged the duration of the cough expiratory phase (CTE) as well as the total cough cycle duration (CTtot). Bilateral microinjections of 2 mM saclofen reduced only cough expiratory efforts. Bilateral microinjection of bicuculline in the cNTS significantly reduced CN and amplitudes of ABD EMG and elongated CTE and CTtot. Bilateral microinjections of saclofen in cNTS had no significant effect on analyzed cough parameters. Our results confirm a different GABAergic inhibitory system in the rNTS and cNTS acting on mechanically induced cough in cats.

Epileptiform activity influences theta-burst induced LTP in the adult hippocampus: a role for synaptic lipid raft disruption in early metaplasticity?

Non-epileptic seizures are identified as a common epileptogenic trigger. Early metaplasticity following seizures may contribute to epileptogenesis by abnormally altering synaptic strength and homeostatic plasticity. We now studied how in vitro epileptiform activity (EA) triggers early changes in CA1 long-term potentiation (LTP) induced by theta-burst stimulation (TBS) in rat hippocampal slices and the involvement of lipid rafts in these early metaplasticity events. Two forms of EA were induced: (1) interictal-like EA evoked by Mg2+ withdrawal and K+ elevation to 6 mM in the superfusion medium or (2) ictal-like EA induced by bicuculline (10 µM). Both EA patterns induced and LTP-like effect on CA1 synaptic transmission prior to LTP induction. LTP induced 30 min post EA was impaired, an effect more pronounced after ictal-like EA. LTP recovered to control levels 60 min post interictal-like EA but was still impaired 60 min after ictal-like EA. The synaptic molecular events underlying this altered LTP were investigated 30 min post EA in synaptosomes isolated from these slices. EA enhanced AMPA GluA1 Ser831 phosphorylation but decreased Ser845 phosphorylation and the GluA1/GluA2 ratio. Flotillin-1 and caveolin-1 were markedly decreased concomitantly with a marked increase in gephyrin levels and a less prominent increase in PSD-95. Altogether, EA differentially influences hippocampal CA1 LTP thorough regulation of GluA1/GluA2 levels and AMPA GluA1 phosphorylation suggesting that altered LTP post-seizures is a relevant target for antiepileptogenic therapies. In addition, this metaplasticity is also associated with marked alterations in classic and synaptic lipid raft markers, suggesting these may also constitute promising targets in epileptogenesis prevention.

Modularity and neuronal heterogeneity: Two properties that influence in vitro neuropharmacological experiments.

Introduction: The goal of this work is to prove the relevance of the experimental model (in vitro neuronal networks in this study) when drug-delivery testing is performed. Methods: We used dissociated cortical and hippocampal neurons coupled to Micro-Electrode Arrays (MEAs) arranged in different configurations characterized by modularity (i.e., the presence of interconnected sub-networks) and heterogeneity (i.e., the co-existence of neurons coming from brain districts). We delivered increasing concentrations of bicuculline (BIC), a neuromodulator acting on the GABAergic system, and we extracted the IC50 values (i.e., the effective concentration yielding a reduction in the response by 50%) of the mean firing rate for each configuration. Results: We found significant lower values of the IC50 computed for modular cortical-hippocampal ensembles than isolated cortical or hippocampal ones. Discussion: Although tested with a specific neuromodulator, this work aims at proving the relevance of ad hoc experimental models to perform neuropharmacological experiments to avoid errors of overestimation/underestimation leading to biased information in the characterization of the effects of a drug on neuronal networks.

Germinated brown rice protects against glutamate toxicity in HT22 hippocampal neurons through the jnk-mediated apoptotic pathway via the GABAA receptor.

The anti-apoptosis effect of germinated brown rice (GBR) focusing on differentiated HT22 cells results in improved nutritional values after the germination process of GBR which contains total phenolic compounds and γ-aminobutyric acid (GABA). Cell death induced by 5 mM glutamate was investigated for 24 h to determine whether GBR mediates cell death through GABA receptors by using antagonists. The results showed that GBR (100 µg/ml) suppressed glutamate-induced cytotoxicity and caused arrest at the G1/S phase of the cell cycle in differentiated HT22 cells. Furthermore, GBR significantly decreased the expression level of c-Jun, while its active form, p-c-Jun, is the downstream product of the JNK-mediated apoptotic pathway and causes subsequent cell death. In addition, bicuculline (12.5 nM), a GABAA antagonist, could eliminate GBR effects, but phaclofen (1 mM), a GABAB antagonist, could not. Surprisingly, GBR exhibited a better neuroprotective effect than a pure commercial GABA compound (0.115 µM). These results indicated that GBR possessed high anti-apoptotic activity and inhibited cell death in differentiated HT22 cells by perturbing re-entry of the cell cycle and apoptosis via the GABAA receptor. Hence, GBR could be further used as a valuable nutritional compound to prevent apoptosis-induced neurodegenerative diseases.

Impulsive and compulsive behaviors can be induced by opposite GABAergic dysfunctions inside the primate ventral pallidum.

Introduction: The ventral pallidum (VP) is central in the limbic Basal Ganglia circuit, controlling both appetitive (approach) and aversive (avoidance) motivated behaviors. Nevertheless, VP involvement in pathological aspects remains unclear, especially in the behavioral expression of different motivational dysfunctions. This study aimed to investigate how the VP contributes to the expression of abnormal behaviors via opposite GABAergic dysfunctions. Methods: Opposite GABAergic dysfunctions were induced by injecting muscimol (a GABAA agonist) and bicuculline (a GABAA antagonist) into monkeys. We determined the effects of both substances on self-initiated behaviors in lab-chair and in free-moving home-cage contexts in six monkeys, and in two animals performing an approach-avoidance task in appetitive and aversive contexts. Results: While the self-initiated behaviors induced by bicuculline injections in VP were characterized by compulsive behaviors such as repetitive grooming and self-biting, muscimol injections induced impulsive behaviors including limb movements in a lab-chair context and exploration behaviors in a free-moving context. More specific behavioral effects were observed in the approach-avoidance task. The muscimol injections induced premature responses and erroneous screen touches, which characterize impulsive and attention disorders, while the bicuculline injections into the VP increased passive avoidance (non-initiated action) and task-escape in an aversive context, suggesting an anxiety disorder. Conclusions: These results show that activating or blocking GABAergic transmission in the VP impairs motivated behaviors. Furthermore, the behavioral expressions produced by these opposite disturbances show that the VP could be involved in anxiety-driven compulsive disorders, such as OCD, as well as in impulsive disorders motivated by attention deficits or reward-seeking, as seen in ADHD or impulse control disorders.

Reduced mechanical hypersensitivity by inhibition of the amygdala in experimental neuropathy: Sexually dimorphic contribution of spinal neurotransmitter receptors.

Here we studied spinal neurotransmitter mechanisms involved in the reduction of mechanical hypersensitivity by inhibition of the amygdaloid central nucleus (CeA) in male and female rats with spared nerve injury (SNI) model of neuropathy. SNI induced mechanical hypersensitivity that was stronger in females. Reversible blocking of the CeA with muscimol (GABAA receptor agonist) induced a reduction of mechanical hypersensitivity that did not differ between males and females. Following spinal co-administration of atipamezole (α2-adrenoceptor antagonist), the reduction of mechanical hypersensitivity by CeA muscimol was attenuated more in males than females. In contrast, following spinal co-administration of raclopride (dopamine D2 receptor antagonist) the reduction of hypersensitivity by CeA muscimol was attenuated more in females than males. The reduction of mechanical hypersensitivity by CeA muscimol was equally attenuated in males and females by spinal co-administration of WAY-100635 (5-HT1A receptor antagonist) or bicuculline (GABAA receptor antagonist). The CeA muscimol induced attenuation of ongoing pain-like behavior (conditioned place preference test) that was reversed by spinal co-administration of atipamezole in both sexes. The results support the hypothesis that CeA contributes to mechanical hypersensitivity and ongoing pain-like behavior in SNI males and females. Disinhibition of descending controls acting on spinal α2-adrenoceptors, 5-HT1A, dopamine D2 and GABAA receptors provides a plausible explanation for the reduction of mechanical hypersensitivity by CeA block in SNI. The involvement of spinal dopamine D2 receptors and α2-adrenoceptors in the CeA muscimol-induced reduction of mechanical hypersensitivity is sexually dimorphic, unlike that of spinal α2-adrenoceptors in the reduction of ongoing neuropathic pain.

Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories.

The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (EMG) responses and resulting limb movement were investigated. In ketamine-anesthetized cats, paw movement kinematics in 3D and EMG activity from 8 to 12 forelimb muscles evoked by ICMS applied to the forelimb area of the cat motor cortex (MCx) were recorded. The EMG responses evoked by ICMS were also compared to those evoked by focal ictal bursts induced by the iontophoretic ejection of the GABAA receptor antagonist bicuculline methochloride (BIC) at the same cortical point. The effects of different initial limb starting positions on movement trajectories resulting from long-duration ICMS were also studied. The ICMS duration did not affect the evoked muscle activation pattern (MAP). Short (50 ms) and long (500 ms) stimulus trains activated the same muscles in the same proportions. MAPs could, however, be modified by gradually increasing the stimulus intensity. MAPs evoked by focal ictal bursts were also highly correlated with those obtained by ICMS at the same cortical point. Varying the initial position of the forelimb did not change the MAPs evoked from a cortical point. Consequently, the evoked movements reached nearly the same final end point and posture, with variability. However, the movement trajectories were quite different depending on the initial limb configuration and starting position of the paw. The evoked movement trajectory was most natural when the forelimb lay pendant ~ perpendicular to the ground (i.e., in equilibrium with the gravitational force). From other starting positions, the movements did not appear natural. These observations demonstrate that while the output of the cortical point evokes a seemingly coordinated limb movement from a rest position, it does not specify a particular movement direction or a controlled trajectory from other initial positions.

α-Pinene Influence on Pulpal Pain-Induced Learning and Memory Impairment in Rats Via Modulation of the GABAA Receptor.

Background: This study investigated the effect of central administration of α-pinene and the interaction of α-pinene with GABAA receptor on pulpal nociception-induced changes in learning and memory performances in rats. Materials and Methods: Sixty-six adult male Wistar rats were used. Pulpal nociception was induced by intradental application of capsaicin (100 µg/rat). α-pinene (0.1, 0.2, and 0.4 µg/rat) was injected centrally 10 min before the administration of capsaicin. In addition, α-pinene (0.4 µg/rat) was co-injected with bicuculline (0.5 µg/rat). Spatial and passive avoidance learning and memory were assessed using Morris water maze (MWM) and shuttle box tasks, respectively. Results: Experimental results of the MWM test showed that capsaicin increases escape latency and distance traveled to the hidden platform (P < 0.01). The effect was prohibited by α-pinene at the dose of 0.4 µg/rat. Moreover, capsaicin-treated animals spent less time in the target zone than capsaicin + α-pinene (0.4 µg/rat)-treated rats (P < 0.05). In the shuttle box test, α-pinene (0.2 µg and 0.4 µg) prevented an increased number of acquisition trials and time spent in the dark chamber induced by capsaicin, whereas it increased step-through latency (P < 0.01). However, the effects of α-pinene (0.4 µg/rat) in both tests were prohibited by bicuculline (0.5 µg/rat). Conclusion: The data showed that central administration of α-pinene might reduce pulpalgia-induced learning and memory impairment, at least partially, via modulation of GABAA receptors.

Positive allosteric modulators of GABAA receptor restore chloride current from blockade by competitive antagonists in a ligand-dependent manner.

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter. GABA receptor type A (GABAAR) possesses binding sites for a large group of pharmacological agents which are supposed to interact allosterically with each other. The aim of this work was to study the interaction between the positive allosteric modulators (PAMs) and the competitive antagonists of GABAARs. The GABA-induced chloride current (IGABA) was measured in isolated Purkinje cells of rat cerebellum using the patch-clamp technique. PAMs, neurosteroid allopregnanolone (Allo) and zolpidem (Zolp), a drug that positively modulates the GABAAR through interaction with the benzodiazepine (BDZ) site, doubled the IGABA amplitude in the control solution. Competitive antagonist of GABAARs, bicuculline (Bic, 5 µM) blocked the IGABA by 90%. The addition of 1 µM Allo or 0.5 µM Zolp to the Bic solution caused an unblocking effect, so that the IGABA amplitude increased 10 and 4 times from control value, correspondingly. This unblocking effect developed slowly, as evidenced by a threefold increase in the current rise time. Competitive antagonist of GABAARs, gabazine (GBZ, 0.5 µM) blocked the IGABA by 87%. The addition of 1 µM Allo to the GBZ solution caused an unblocking effect, so that the IGABA amplitude increased 7-fold. However, the addition of 0.5 µM Zolp to the GBZ solution did not cause an unblocking effect. So, Allo appeared to have a stronger unblocking potential than Zolp, and Bic binding site showed a higher sensitivity to the action of unblocking PAMs than GBZ binding site. The results indicate for the first time the existence of an allosteric relationship between the sites binding PAMs and the competitive antagonists of GABAAR.

Bicuculline restores frequency-dependent hippocampal I/E ratio and circuit function in PGC-1ɑ null mice.

Altered inhibition/excitation (I/E) balance contributes to various brain disorders. Dysfunctional GABAergic interneurons enhance or reduce inhibition, resulting in I/E imbalances. Differences in short-term plasticity between excitation and inhibition cause frequency-dependence of the I/E ratio, which can be altered by GABAergic dysfunction. However, it is unknown whether I/E imbalances can be rescued pharmacologically using a single dose when the imbalance magnitude is frequency-dependent. Loss of PGC-1α (peroxisome proliferator activated receptor γ coactivator 1α) causes transcriptional dysregulation in hippocampal GABAergic interneurons. PGC-1α-/- slices have enhanced baseline inhibition onto CA1 pyramidal cells, causing increased I/E ratio and impaired circuit function. High frequency stimulation reduces the I/E ratio and recovers circuit function in PGC-1α-/- slices. Here we tested if using a low dose of bicuculline that can restore baseline I/E ratio can also rescue the frequency-dependent I/E imbalances in these mice. Remarkably, bicuculline did not reduce the I/E ratio below that of wild type during high frequency stimulation. Interestingly, bicuculline enhanced the paired-pulse ratio (PPR) of disynaptic inhibition without changing the monosynaptic inhibition PPR, suggesting that bicuculline modifies interneuron recruitment and not GABA release. Bicuculline improved CA1 output in PGC-1α-/- slices, enhancing EPSP-spike coupling to wild type levels at high and low frequencies. Our results show that it is possible to rescue frequency-dependent I/E imbalances in an animal model of transcriptional dysregulation with a single treatment.

Dihydromyricetin Protects Against Ethanol-Induced Toxicity in SH-SY5Y Cell Line: Role of GABAA Receptor.

Toxicity induced by binge alcohol drinking, particularly in adolescent and young adults, is of major medical and social consequence. Recently, we reported that butyrate, a short chain fatty acid, can protect against ethanol (ETOH)-induced toxicity in an in vitro model. In this study, we sought to evaluate the potential effectiveness of dihydromyricetin (DHM), a natural bioactive flavonoid, alone or in combination with butyrate in the same model. Exposure of SH-SY5Y cells for 24 h to 500 mM ETOH resulted in approximately 40% reduction in cell viability, which was completely prevented by 0.1 µM DHM. Combinations of DHM and butyrate provided synergistic protection against alcohol toxicity. Whereas butyrate effect was shown to be mediated primarily through fatty acid receptor 3 activation, DHM protection appears to be mediated primarily via benzodiazepine receptor site of GABAA receptor. This is based on the finding that DHM's effect could be completely prevented by pretreatment with flumazenil, a selective antagonist at this site, but not by bicuculline, a selective antagonist at the actual GABAA receptor binding site. These findings suggest potential utility of DHM alone or in combination with butyrate against ETOH-induced toxicity.