Microinjection Of Bicuculline Research Articles

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus.

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.

Opioidergic, GABAergic and serotonergic neurotransmission in the dorsal raphe nucleus modulates tonic immobility in guinea pigs

Tonic immobility (TI) is an innate defensive behavior that can be elicited by physical restriction and postural inversion and is characterized by a profound and temporary state of akinesis. Our previous studies demonstrated that the stimulation of serotonin receptors in the dorsal raphe nucleus (DRN) appears to be biphasic during TI responses in guinea pigs (Cavia porcellus). Serotonin released by the DRN modulates behavioral responses and its release can occur through the action of different neurotransmitter systems, including the opioidergic and GABAergic systems. This study examines the role of opioidergic, GABAergic and serotonergic signaling in the DRN in TI defensive behavioral responses in guinea pigs. Microinjection of morphine (1.1nmol) or bicuculline (0.5nmol) into the DRN increased the duration of TI. The effect of morphine (1.1nmol) was antagonized by pretreatment with naloxone (0.7nmol), suggesting that the activation of μ opioid receptors in the DRN facilitates the TI response. By contrast, microinjection of muscimol (0.5nmol) into the DRN decreased the duration of TI. However, a dose of muscimol (0.26nmol) that alone did not affect TI, was sufficient to inhibit the effect of morphine (1.1nmol) on TI, indicating that GABAergic and enkephalinergic neurons interact in the DRN. Microinjection of alpha-methyl-5-HT (1.6nmol), a 5-HT2 agonist, into the DRN also increased TI. This effect was inhibited by the prior administration of naloxone (0.7nmol). Microinjection of 8-OH-DPAT (1.3nmol) also blocked the increase of TI promoted by morphine (1.1nmol). Our results indicate that the opioidergic, GABAergic and serotonergic systems in the DRN are important for modulation of defensive behavioral responses of TI. Therefore, we suggest that opioid inhibition of GABAergic neurons results in disinhibition of serotonergic neurons and this is the mechanism by which opioids could enhance TI. Conversely, a decrease in TI could occur through the activation of GABAergic interneurons.

Interaction between Angiotensinergic System and GABAergic System on Thirst in Adult Male Rats

Thirst is a subjective perception that provides the urge for human and animals to drink fluids and it is important for maintaining body fluid homeostasis and may arise from deficits in either intracellular or extracellular fluid volume. Gamma-aminobutyric acid (GABA) and Angiotensin (Ang) receptors in the brain are involved with thirst, water intake and balance of body liquid. The present study investigated the interaction between Angiotensinergic and GABAergic systems on water intake in adult male rats. Intracerebroventricular (i.c.v.) injections were carried out in all experiments after 24 h deprivation of water intake. After deprivation the volume of consumed water was measured for 1 h. Administration of Losartan (45 μg/rat), Muscimol (0.1 μg/rat) significantly decreased water intake while, i.c.v. microinjection of Bicuculline (1 μg/rat) significantly increased it as compared to Saline-treated controls. I.C.V. microinjection of Muscimol 15 min after Losartan administration decreased water intake significantly, while, i.c.v. microinjection of Bicuculline 15min after Losartan administration could attenuate increasing effect of Bicuculline on water intake. It is concluded that Angiotensinergic system have interaction with GABAergic system on water intake.

Reduction of Nitric Oxide-Mediated &amp;gamma;-Amino Butyric Acid Release in Rostral Ventrolateral Medulla Is Involved in Superoxide-Induced Sympathoexcitation of Hypertensive Rats

The rostral ventrolateral medulla (RVLM) in the brainstem is responsible for regulation of the sympathetic nervous system. In the RVLM, nitric oxide (NO)-mediated γ-amino butyric acid (GABA) is a major sympathoinhibitory amino acid neurotransmitter and superoxide is a major sympathoexcitatory factor. In this study, we investigated whether or not NO-mediated GABA release is involved in superoxide-induced sympathoexcitation in the RVLM of hypertensive rats. For our model hypertensive rats with sympathoexcitation, we used stroke-prone spontaneously hypertensive rats (SHRSP). GABA levels in the RVLM were measured by in vivo microdialysis. Microinjection of tempol, a superoxide scavenger, into the RVLM decreased arterial pressure (AP), heart rate (HR), and renal sympathetic nerve activity (RSNA) with an increase in GABA release in the RVLM. Microinjection of N(G)-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor, into the RVLM increased AP, HR, and RSNA with a decrease in GABA release in the RVLM. Prior microinjection of L-NMMA into the RVLM attenuated the tempol-induced changes in AP, HR, RSNA, and GABA release in the RVLM. Microinjection of bicuculline, a GABA receptor blocker, into the RVLM attenuated the tempol- and L-NMMA-induced changes in AP, HR, and RSNA. The findings suggest that reduction of NO-mediated GABA release in the RVLM is partly involved in superoxide-induced sympathoexcitation of SHRSP.

Modulation of the cough reflex by GABAA receptors in the caudal ventral respiratory group of the rabbit

We have previously shown that the caudal ventral respiratory group (cVRG) is a possible site of action of some antitussive drugs and plays a crucial role in determining both the expiratory and inspiratory components of the cough motor pattern. In addition, it has been reported that medullary expiratory neurons of the cVRG are subject to potent GABAergic gain modulation. This study was devoted to investigate the role of cVRG GABAA receptors in the control of baseline respiratory activity and cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. To this purpose, bilateral microinjections (30–50 nl) of bicuculline or muscimol were performed into the cVRG of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bicuculline (1 mM) increased peak abdominal activity and respiratory frequency due to decreases in TE. Cough responses were potentiated mainly owing to increases in the cough number. The recovery was observed within ~2 h. On the contrary, muscimol (0.3 mM) abolished abdominal activity and decreased respiratory frequency due to increases in TE. In addition, cough responses were progressively reduced and completely suppressed within ~20 min. Partial recovery of cough responses was achieved after ~3 h or within ~5 min following bicuculline microinjections at the same locations. The sneeze reflex induced by mechanical stimulation of the nasal mucosa persisted following bicuculline and muscimol microinjections. However, the number and intensity of expiratory thrusts were enhanced by bicuculline and suppressed by muscimol. The results provide evidence that a potent GABAA-mediated inhibitory modulation is exerted at the level of the cVRG not only on respiratory activity, but also on cough and sneeze reflex responses.

Involvement of basolateral amygdala GABAA receptors in the effect of dexamethasone on memory in rats

In this study we investigated whether GABA(A) receptors of the basolateral amygdala (BLA) interact with the effect of dexamethasone on the retrieval stage of memory. Adult male Wistar rats were bilaterally cannulated in the BLA by stereotaxic surgery. The animals were trained in step-through apparatus by induction of electric shock (1.5 mA, 3 s) and were tested for memory retrieval after 1 d. The time of latency for entering the dark compartment of the instrument and the time spent by rats in this chamber were recorded for evaluation of the animals' retrieval in passive avoidance memory. Administration of dexamethasone (0.3 and 0.9 mg/kg, subcutaneously (s.c.)), immediately after training, enhanced memory retrieval. This effect was reduced by intra-BLA microinjection of muscimol (0.125, 0.250 and 0.500 µg/rat), when administered before 0.9 mg/kg of dexamethasone. Microinjection of bicuculline (0.75 µg/rat, intra-BLA) with an ineffective dose of dexamethasone (0.1 mg/kg, s.c.) increased memory retrieval. However, the same doses of muscimol and bicuculline without dexamethasone did not affect memory processes. Our data support reports that dexamethasone enhances memory retrieval. It seems that GABA(A) receptors of the BLA mediate the effect of dexamethasone on memory retrieval in rats.

The effects of hippocampal opioidergic and septal GABAergic system interactions on anxiety-like behavior in rats

AimsPrevious studies have shown that the septum and hippocampus are connected and play a key role in anxiety-related behavior. In the present study, the effects of the interactions between the opioidergic system in the dorsal hippocampus (CA1) and the GABAergic system in the medial septum on anxiety in male Wistar rats were investigated. Main methodsAn elevated plus maze was used to study the effects of anxiogenic and anxiolytic drugs in rodents. Male Wistar rats were used for this test. We injected morphine (2.5, 5.0, and 7.5μg/rat) into the CA1 and GABAA agonist muscimol (2.5, 5.0, and 10.0ng/rat) into the intra-medial septum (MS). Conversely, we detected the microinjection of bicuculline, a GABAA antagonist (10, 20, and 30ng/rat), into the MS. OAT%, OAE% and locomotor activity were determined by this behavioral test. Key findingsWe found that the simultaneous administration of intra-CA1 morphine (2.5μg/rat) and intra-MS muscimol (2.5ng/rat) increased the magnitude of anxiolysis. On the other hand, simultaneous administration of intra-CA1 morphine (7.5μg/rat) and intra-MS bicuculline decreased the anxiolytic effect. SignificanceIn conclusion, our data suggest that the opioidergic system in the CA1 is involved in anxiety-related behavior and influences the GABAergic system within the MS. In addition, we observed that the interactions between the opioidergic and GABAergic systems within the septohippocampus increased anxiety-related behavior compared to stimulation of these receptor systems independently.

P.1.d.019 Selective ablation of mGluR5 receptors on dopamine D1-expressing neurons abolishes novelty-seeking behaviours

Considering the influence of the substantia nigra on mesencephalic neurons involved with fear-induced reactions organized in rostral aspects of the dorsal midbrain, the present work investigated the topographical and functional neuroanatomy of similar influence on caudal division of the corpora quadrigemina, addressing: (a) the neural hodology connecting the neostriatum, the substantia nigra, periaqueductal gray matter and inferior colliculus (IC) neural networks; (b) the influence of the inhibitory neostriatonigral–nigrocollicular GABAergic links on the control of the defensive behavior organized in the IC. The effects of the increase or decrease of activity of nigrocollicular inputs on defensive responses elicited by either electrical or chemical stimulation of the IC were also determined. Electrolytic or chemical lesions of the substantia nigra, pars reticulata (SNpr), decreased the freezing and escape behaviors thresholds elicited by electrical stimulation of the IC, and increased the behavioral responses evoked by the GABAA blockade in the same sites of the mesencephalic tectum (MT) electrically stimulated. These findings were corroborated by similar effects caused by microinjections of the GABAA-receptor agonist muscimol in the SNpr, followed by electrical and chemical stimulations of the IC. The GABAA blockade in the SNpr caused a significant increase in the defensive behavior thresholds elicited by electrical stimulation of the IC and a decrease in the mean incidence of panic-like responses induced by microinjections of bicuculline in the mesencephalic tectum (inferior colliculus). These findings suggest that the substantia nigra receives GABAergic inputs that modulate local and also inhibitory GABAergic outputs toward the IC. In fact, neurotracing experiments with fast blue and iontophoretic microinjections of biotinylated dextran amine either into the inferior colliculus or in the reticular division of the substantia nigra demonstrated a neural link between these structures, as well as between the neostriatum and SNpr.

Spatial and temporal properties of tic-related neuronal activity in the cortico-basal ganglia loop.

Motor tics are involuntary brief muscle contractions that interfere with ongoing behavior and appear as a symptom in several human disorders. While the pathophysiology of tics is still largely unknown, multiple lines of evidence suggest the involvement of the corticobasal ganglia loop in tic disorders. We administered local microinjections of bicuculline into the putamen of Macaca fascicularis monkeys to induce motor tics, while simultaneously recording neuronal activity from the primary motor cortex, putamen, and globus pallidus. These data were used to explore the spatial and temporal properties of tic-related neuronal activity within the cortico-basal ganglia system. In the putamen, tics were associated with brief bursts of activity of phasically active neurons (presumably the projection neurons) and complex excitation-inhibition patterns of tonically active neurons. Tic-related activity within the putamen was spatially focused and somatotopically organized. In the globus pallidus, tic-related activity was diffusely distributed throughout the motor territory. Tic-related activity in the putamen usually preceded the tic-related activations in the cortex, but in the globus pallidus, tic-related activity was mostly later than the cortex. These findings shed new light on the role of the different basal ganglia nuclei in the generation of motor tics. Despite the early and somatotopically focused nature of tic-related activity in the input stage of the basal ganglia, tic-related activity in the output nucleus is temporally late and diffusely distributed, making it incompatible with a role in tic initiation. Instead, abnormal basal ganglia activity may serve to modulate motor patterns or activate learning mechanisms, thus augmenting further tic expression.

Serotonin-2A receptor regulation of panic-like behavior in the rat dorsal periaqueductal gray matter: the role of GABA

Electrical stimulation of the dorsal periaqueductal gray (dPAG) evokes escape, a defensive response associated with panic attacks. Stimulation of 5-HT1A or 5-HT2A receptors in this midbrain area equally inhibits escape performance, even though at the molecular level these receptors cause opposite effects, i.e., activation of the former hyperpolarizes the cell membrane, while the latter excites it. A proposal has been made that 5-HT2A receptor agonists exert their inhibitory effect on escape by activating GABAergic interneurons located in the dPAG. In the present study, we evaluated this hypothesis by investigating whether previous intra-dPAG administration of the GABAA receptor antagonist bicuculline blocks the anti-escape effect caused by the local injection of different 5-HT2A/2C receptor agonists. Intra-dPAG administration of 5-HT, the preferential 5-HT2A receptor agonist DOI, the nonselective 5-HT2C receptor agonist mCPP or the 5-HT2C receptor agonist RO 60-0175 significantly inhibited the escape reaction induced by electrical stimulation of the same brain area. In all cases, this panicolytic-like effect was blocked by previous microinjection of bicuculline. This GABAA antagonist, however, failed to antagonize the anti-escape effect caused by the 5-HT1A receptor agonist 8-OH-DPAT. The inhibitory effect caused by DOI, RO 60-0175, and mCPP was also blocked by previous intra-dPAG injection of the preferential 5-HT2A receptor antagonist ketanserin. Pre-administration of the 5-HT2C receptor antagonist SB-242084 in the dPAG did not block the anti-escape effect of RO 60-0175. Stimulation of 5-HT2A but not 5-HT2C receptors in the dPAG causes a panicolytic-like effect that is mediated by facilitation of GABAergic neurotransmission.

SYMPATHETIC AND RESPIRATORY CONTROL BY HYPOTHALAMIC PERIFORNICAL AREA: ROLE OF HYPOCRETIN NEURONS

Hypocretin (orexin) neurons within the hypothalamic perifornical area (PeF) are believed to contribute to the cardiorespiratory responses associated with stress or arousal. In this study we first showed that microinjections of bicuculline into the PeF of urethane-anesthetised rats evoked increases in renal sympathetic nerve activity and phrenic nerve activity burst rate that were significantly greater than the increases in these variables evoked from regions dorsal, ventral and lateral to the PeF. Systemic administration of almorexant (a hypocretin receptor antagonist) significantly reduced the increases in these variables evoked by bicuculline microinjection into the PeF, but had little effect on the sympathetic and respiratory response to stimulation of peripheral chemoreceptors. The results indicate that blockade of central hypocretin receptors powerfully inhibit the respiratory and sympathetic response evoked by activation of PeF neurons.

Effects of nitric oxide synthase inhibition on dorsal vagal stimulation‐induced pancreatic insulin secretion

Pancreatic islets receive parasympathetic input from the dorsal motor nucleus of the vagus (DMV). This study examined whether chemical activation of the DMV modulates pancreatic endocrine secretion and whether this process involves nitric oxide (NO). All experiments were conducted in isoflurane/urethane-anaesthetized Sprague Dawley rats after over-night fast. Stimulation of the DMV using bicuculline (Bic; GABAA antagonist, 100 pmol/25 nl) microinjections resulted in significant and rapid increases in glucose-mediated insulin secretion (9.2 ± 0.1 ng/ml peak) compared to vehicle (5.5 ± 2.0 ng/ml; P<0.05). A repeat microinjection of BIM made 40 min later did not increase insulin secretion significantly. Microinjection of Bic outside the DMV or microinjection of vehicle into the DMV did not affect insulin secretion. Blockade of muscarinic receptors (atropine methonitrate; 100μg/kg min, i.v. for 20 minutes) reduced the activation of glucose-mediated insulin secretion produced by stimulation of the DMV from 9.2 ± 0.9 ng/ml to 1.6 ± 0.1 ng/ml (P<0.05). On the other hand, L-nitroarginine (NO synthase inhibitor, 30 mg/kg, i.v.) increased the excitatory effect of DMV stimulation on glucose-mediated insulin secretion (from 8.9 ± 1.2 ng/ml to 15.3 ± 3.0 ng/ml; P<0.05). This suggests that NO may play an inhibitory role in the vagal regulation of pancreatic endocrine secretion. Supported by AHMRF.

Differential organization of excitatory and inhibitory synapses within the rat dorsal vagal complex

The dorsal motor nucleus of the vagus (DMV) is pivotal in the regulation of upper gastrointestinal functions, including motility and both gastric and pancreatic secretion. DMV neurons receive robust GABA- and glutamatergic inputs. Microinjection of the GABA(A) antagonist bicuculline (BIC) into the DMV increases pancreatic secretion and gastric motility, whereas the glutamatergic antagonist kynurenic acid (KYN) is ineffective unless preceded by microinjection of BIC. We used whole cell patch-clamp recordings with the aim of unveiling the brain stem neurocircuitry that uses tonic GABA- and glutamatergic synapses to control the activity of DMV neurons in a brain stem slice preparation. Perfusion with BIC altered the firing frequency of 71% of DMV neurons, increasing firing frequency in 80% of the responsive neurons and decreasing firing frequency in 20%. Addition of KYN to the perfusate either decreased (52%) or increased (25%) the firing frequency of BIC-sensitive neurons. When KYN was applied first, the firing rate was decreased in 43% and increased in 21% of the neurons; further perfusion with BIC had no additional effect in the majority of neurons. Our results indicate that there are several permutations in the arrangements of GABA- and glutamatergic inputs controlling the activity of DMV neurons. Our data support the concept of brain stem neuronal circuitry that may be wired in a finely tuned organ- or function-specific manner that permits precise and discrete modulation of the vagal motor output to the gastrointestinal tract.

Neural mechanisms mediating circadian phase resetting by activation of 5-HT 7 receptors in the dorsal raphe: Roles of GABAergic and glutamatergic neurotransmission

5-HT 7 receptors in the dorsal raphe nucleus (DRN) influence circadian rhythms, sleep, and serotonin release. Because interactions between 5-HT 7 receptors and glutamatergic and GABAergic neurons have been demonstrated previously, the current studies tested the hypothesis that GABAergic and/or glutamatergic neurons mediate phase shifts induced by activation of DRN 5-HT 7 receptors. Hamsters were fitted with guide cannulae aimed at the DRN, housed in cages with running wheels, and exposed to 14 h light (L):10 h dark (D). In Experiment 1, hamsters received DRN pretreatment with muscimol (87.6 pmol) or vehicle before DRN 8-OH-DPAT (6 pmol) microinjections at ZT6. After exposure to constant darkness (10 days), phase shifts were calculated and animals were re-exposed to 14 L:10D. The procedure was repeated to give each animal the alternate pretreatment. In Experiment 2, hamsters received DRN pretreatment with NMDA (20 pmol) or vehicle before 8-OH-DPAT at ZT 6. Other experiments tested the effects of single DRN microinjections of muscimol, bicuculline (136 pmol), NMDA, MK-801 (10 pmol) or vehicle. Phase shifts (mean ± S.E.M., h) in muscimol/8-OH-DPAT-microinjected hamsters (1.02 ± 0.30) were not different ( P = 0.11) from those in vehicle/8-OH-DPAT-microinjected hamsters (1.34 ± 0.30), while those in NMDA/8-OH-DPAT-microinjected hamsters (0.67 ± 0.17) were smaller ( P < 0.05) than those in vehicle/8-OH-DPAT-microinjected hamsters (0.97 ± 0.10). DRN single microinjections of bicuculline, but not muscimol, NMDA, or MK-801 induced phase advances. Bicuculline also potentiated 8-OH-DPAT-induced phase advances ( P < 0.05). These finding suggest that the mechanism mediating DRN 5-HT 7 receptor induction of phase advances involves decreased glutamatergic neurotransmission, and furthermore, that inhibition of DRN GABAergic neurotransmission causes a phase advance.

Involvement of GABAergic transmission in the midbrain ventral tegmental area in the regulation of hippocampal theta rhythm

Previously we indicated that the ventral tegmental area (VTA) may belong to the system regulating hippocampal theta rhythm. In the present study, we aimed at assessing the role of the GABAergic system of the VTA in regulation of hippocampal electric activity. Male Wistar rats received unilateral intra-VTA microinjection of either bicuculline (50 ng/0.5 μl, n = 9), muscimol (100 ng/0.5 μl, n = 10) or phaclofen (500 ng/0.5 μl, n = 9). 1-min tail pinch stimulations were applied at 10-min intervals to evoke theta rhythm episodes in hippocampus. We analysed peak power ( P max) and corresponding frequency ( F max) of EEG signal at delta and theta bands. Bicuculline induced theta rhythm in both hippocampi with 0 latency, continuous for ca. 33 min. Phaclofen also induced theta but in this group it appeared with latency (17.45 ± 3.16 min on average), lasted for ca. 33.6 min and during this time was interrupted by periods of irregular activity of variable length. Tail pinch was not applied in these groups. Muscimol induced an opposite effect: depression of theta P max with simultaneous increase in delta P max and a decrease in F max delta during episodes of tail pinch-evoked theta. This effect had variable latency and no return to the control EEG could be observed. We propose that GABA activity in the VTA is of tonic character, so that abolition of this mechanism produces immediate effect, i.e. theta induction (strong by GABA A and weak by GABA B receptors blockade), whereas enhancing the already present GABAergic inhibition causes delayed, prolonged changes expressed as gradual loss of theta synchronisation.

The viscerosympathetic response in rabbits is mediated by GABAergic and glutamatergic inputs into the sympathetic premotor neurons of the rostral ventrolateral medulla

Neurons in the rostral ventrolateral medulla (RVLM) receive inputs from various sources, including baroreceptors, and then regulate the activity of sympathetic preganglionic neurons in the spinal cord. Whether RVLM neurons mediate the viscerosympathetic reflex has yet to be clarified. In the present study, we investigated the role of RVLM neurons in the viscerosympathetic reflex in anaesthetized and vagotomized rabbits. Electrical stimulation of the greater splanchnic nerve (SplN) evoked reflex responses in renal sympathetic activity that were composed of inhibitory and/or excitatory components. Bilateral microinjection of muscimol, a GABA(A) receptor agonist, into the RVLM blocked the reflex responses. Bilateral microinjection of bicuculline, a GABA(A) receptor antagonist, largely attenuated the inhibitory component, whereas kynurenic acid, a glutamate receptor antagonist, eliminated the excitatory component. The activities of 21 RVLM barosensitive bulbospinal neurons were recorded. Twenty of the neurons responded to the SplN stimulation. The responses also consisted of inhibitory and/or excitatory components. The excitatory component of these neurons preceded that of the renal sympathetic nerve activity by about 100 ms. This latency difference was almost the same as that of the inhibitory responses evoked by aortic nerve stimulation. Therefore, the renal sympathetic reflex responses evoked by SplN stimulation are mediated by RVLM neurons, and GABAergic and glutamatergic transmission in the RVLM are related to this reflex.

Bicuculline-Induced Chorea Manifests in Focal Rather Than Globalized Abnormalities in the Activation of the External and Internal Globus Pallidus

Chorea is a basal-ganglia (BG) related hyperkinetic movement disorder characterized by irregular continuous involuntary movements. Chorea and related hyperbehavioral disorders may be induced in behaving primates by local microinjections of the GABA(A) antagonist bicuculline to the globus pallidus externus (GPe). We performed multielectrode extracellular recordings in the GPe and in the globus pallidus internus (GPi) before, during, and after bicuculline microinjections. Bicuculline led to an increase in the firing rate and a change in the firing pattern of GPe neurons. Two types of abnormal neuronal firing patterns were detected in GPe neurons close to the bicuculline microinjection site: continuous high-frequency activity and bistable activity, in which neurons transitioned between high-frequency and complete cessation of firing. Neuronal activity remained uncorrelated within and between the GPe and the GPi, with no evidence for propagation of the focal GPe abnormal activity downstream to the GPi. Despite reduction in the information capacity of bicuculline-affected GPe neurons, the ability to encode behavioral events was maintained. We found similar responses of GPe neurons to bicuculline in vitro in the rat, suggesting a basic cellular mechanism underlying these abnormal firing patterns. These results demonstrate that chorea is associated with focal neuronal changes that are not complemented by global changes in the BG nuclei. This suggests a mechanism of stochastic phasic alteration of BG control leading to the chaotic nature of chorea. Thus rather than imposing a globalized state of cortical excitability, chorea might be associated with changes in internal information processing within the BG.

Respiratory responses induced by blockades of GABA and glycine receptors within the Bötzinger complex and the pre-Bötzinger complex of the rabbit

The respiratory role of GABA A, GABA B and glycine receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (preBötC) was investigated in α-chloralose-urethane anesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30–50 nl) of GABA and glycine receptor agonists and antagonists. GABA A receptor blockade by bicuculline (5 mM) or gabazine (2 mM) within the BötC induced strong depression of respiratory activity up to apnea. The latter was reversed by hypercapnia. Glycine receptor blockade by strychnine (5 mM) within the BötC decreased the frequency and amplitude of phrenic bursts. Bicuculline microinjections into the preBötC caused decreases in respiratory frequency and the appearance of two alternating different levels of peak phrenic activity. Strychnine microinjections into the preBötC increased respiratory frequency and decreased peak phrenic amplitude. GABA A, but not glycine receptor antagonism within the preBötC restored respiratory rhythmicity during apnea due to bicuculline or gabazine applied to the BötC. GABA B receptor blockade by CGP-35348 (50 mM) within the BötC and the preBötC did not affect baseline respiratory activity, though microinjections of the GABA B receptor agonist baclofen (1 mM) into the same regions altered respiratory activity. The results show that only GABA A and glycine receptors within the BötC and the preBötC mediate a potent control on both the intensity and frequency of inspiratory activity during eupneic breathing. This study is the first to provide evidence that these inhibitory receptors have a respiratory function within the BötC.

Role of the central amygdala GABA-A receptors in morphine state-dependent memory

Aims In the present experiments, the effects of bilateral microinjections of the GABA-A receptor agonist and/or antagonist into the central amygdala (CeA) on morphine state-dependent memory were examined. Main methods In order to assess memory retrieval, a step-through passive avoidance task was used in adult male Wistar rats. Key findings Subcutaneous (s.c.) administration of morphine (5 and 7.5 mg/kg) immediately after training (post-training) decreased the memory retrieval. Pre-test administration of the opioid (7.5 mg/kg) also induced amnesia. The response induced by post-training morphine (7.5 mg/kg) was significantly reversed by pre-test administration of the drug (5 and 7.5 mg/kg), indicating morphine state-dependent memory. Pre-test intra-CeA microinjection of muscimol, a GABA-A receptor agonist (0.01, 0.02 and 0.03 µg/rat) reduced morphine state-dependent memory. However, the same doses of muscimol by itself had no effect on memory retrieval. Furthermore, pre-test intra-CeA microinjection of bicuculline, a GABA-A receptor antagonist by itself did not alter memory retrieval. The antagonist also did not change post-training morphine (7.5 mg/kg)-induced amnesia, but in combination with a lower dose of morphine (0.5 mg/kg), improved memory performance. Moreover, muscimol's ability to interfere with morphine state-dependent memory was reversed by co-injection of bicuculline. Significance The results suggest that GABA-A receptor mechanism of the CeA may influence morphine state-dependent memory.

Opioidergic and GABAergic mechanisms in the rostral ventromedial medulla modulate the nociceptive response of vocalization in guinea pigs

Vocalization generated by the application of a noxious stimulus is an integrative response related to the affective-motivational component of pain. The rostral ventromedial medulla (RVM) plays an important role in descending pain modulation, and opiates play a major role in modulation of the antinociception mediated by the RVM. Further, it has been suggested that morphine mediates antinociception indirectly, by inhibition of tonically active GABAergic neurons. The current study evaluated the effects of the opioids and GABA agonists and antagonists in the RVM on an affective-motivational pain model. Additionally, we investigated the opioidergic–GABAergic interaction in the RVM in the vocalization response to noxious stimulation. Microinjection of either morphine (4.4 nmol/0.2 μl) or bicuculline (0.4 nmol/0.2 μl) into the RVM decreased the vocalization index, whereas application of the GABA A receptor agonist, muscimol (0.5 nmol/0.2 μl) increased the vocalization index during noxious stimulation. Furthermore, prior microinjection of either the opioid antagonist naloxone (2.7 nmol/0.2 μl) or muscimol (0.25 nmol/0.2 μl) into the RVM blocked the reduction in vocalization index induced by morphine. These observations suggest an antinociceptive and pro-nociceptive role of the opioidergic and GABAergic neurotransmitters in the RVM, respectively. Our data show that opioids have an antinociceptive effect in the RVM, while GABAergic neurotransmission is related to the facilitation of nociceptive responses. Additionally, our results indicate that the antinociceptive effect of the opioids in the RVM could be mediated by a disinhibition of tonically active GABAergic interneurons in the downstream projection neurons of the descending pain control system; indicating an interaction between the opioidergic and GABAergic pathways of pain modulation.