strychnine and bicuculline-methiodide

Minocycline is a widely used glial activation inhibitor that could suppress pain-related behaviors in a number of different pain animal models, yet, its analgesic mechanisms are not fully understood. Hyperpolarization-activated cation channel-induced Ih current plays an important role in neuronal excitability and pathological pain. In this study, we investigated the possible effect of minocycline on Ih of substantia gelatinosa neuron in superficial spinal dorsal horn by using whole-cell patch-clamp recording. We found that extracellular minocycline rapidly decreases Ih amplitude in a reversible and concentration-dependent manner (IC50 = 41 μM). By contrast, intracellular minocycline had no effect. Minocycline-induced inhibition of Ih was not affected by Na(+) channel blocker tetrodotoxin, glutamate-receptor antagonists (CNQX and D-APV), GABAA receptor antagonist (bicuculine methiodide), or glycine receptor antagonist (strychnine). Minocycline also caused a negative shift in the activation curve of Ih, but did not alter the reversal potential. Moreover, minocycline slowed down the inter-spike depolarizing slope and produced a robust decrease in the rate of action potential firing. Together, these results illustrate a novel cellular mechanism underlying minocycline's analgesic effect by inhibiting Ih currents of spinal dorsal horn neurons.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Bicuculline; Dose-Response Relationship, Drug; Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; Lumbosacral Region; Male; Membrane Potentials; Membrane Transport Modulators; Minocycline; Patch-Clamp Techniques; Posterior Horn Cells; Rats, Sprague-Dawley; Strychnine; Substantia Gelatinosa; Tetrodotoxin; Tissue Culture Techniques

Partial agonists at the NMDA receptor co-agonist binding site may have potential therapeutic efficacy in a number of cognitive and neurological conditions. The entorhinal cortex is a key brain area in spatial memory and cognitive processing. At synapses in the entorhinal cortex, NMDA receptors not only mediate postsynaptic excitation but are expressed in presynaptic terminals where they tonically facilitate glutamate release. In a previous study we showed that the co-agonist binding site of the presynaptic NMDA receptor is endogenously and tonically activated by D-serine released from astrocytes. In this study we determined the effects of two co-agonist site partial agonists on both presynaptic and postsynaptic NMDA receptors in layer II of the entorhinal cortex. The high efficacy partial agonist, D-cycloserine, decreased the decay time of postsynaptic NMDA receptor mediated currents evoked by electrical stimulation, but had no effect on amplitude or other kinetic parameters. In contrast, a lower efficacy partial agonist, 1-aminocyclobutane-1-carboxylic acid, decreased decay time to a greater extent than D-cycloserine, and also reduced the peak amplitude of the evoked NMDA receptor mediated postsynaptic responses. Presynaptic NMDA receptors, (monitored indirectly by effects on the frequency of AMPA receptor mediated spontaneous excitatory currents) were unaffected by D-cycloserine, but were reduced in effectiveness by 1-aminocyclobutane-1-carboxylic acid. We discuss these results in the context of the effect of endogenous regulation of the NMDA receptor co-agonist site on receptor gating and the potential therapeutic implications for cognitive disorders.

Topics: Algorithms; Amino Acids, Cyclic; Animals; Astrocytes; Bicuculline; Binding Sites; Cycloserine; Entorhinal Cortex; Excitatory Postsynaptic Potentials; Female; Glutamic Acid; Hippocampus; Male; Neurons; Patch-Clamp Techniques; Picrotoxin; Presynaptic Terminals; Rats; Rats, Wistar; Receptors, GABA-A; Receptors, N-Methyl-D-Aspartate; Strychnine

β-Amyloid peptide (Aβ) plays a central role in the pathogenesis of Alzheimer׳s disease, but in lower amounts it is found in normal brains where it participates in physiological processes and probably regulates synaptic plasticity. This study investigated the effects of physiologically relevant concentrations of Aβ (1 pM-100 nM), fragment 25-35, on glycine-mediated membrane current in acutely isolated rat hippocampal pyramidal neurons using whole-cell patch-clamp technique. We have found that short (600 ms) co-application of glycine with Aβ caused reversible dose-dependent and voltage-independent acceleration of desensitization of glycine current. The peak amplitude of the current remained unchanged. The effect of picomolar Aβ concentrations persisted in the presence of 1 µM Aβ in the pipette solution, implying that Aβ bounds to extracellular site(s). Concentration-dependence curve was N-shaped with maximums at 100 pM and 100 nM, suggesting the existence of two binding sites, which may interact with each other. Glycine current resistant to 100 µM picrotoxin, was insensitive to Aβ, which suggests that Aβ affected mainly homomeric glycine receptors. When Aβ was added to bath solution, besides acceleration of desensitization, it caused reversible dose-dependent reduction of glycine current peak amplitude. These results demonstrate that physiological (picomolar) concentrations of Aβ reversibly augment the desensitization of glycine current, probably by binding to external sites on homomeric glycine receptors. Furthermore, Aβ can suppress the peak amplitude of glycine current, but this effect develops slowly and may be mediated through some intracellular machinery.

Topics: Action Potentials; Amyloid beta-Peptides; Animals; Bicuculline; Cells, Cultured; Dose-Response Relationship, Drug; GABA Antagonists; Glycine Agents; Hippocampus; Patch-Clamp Techniques; Peptide Fragments; Picrotoxin; Pyramidal Cells; Rats; Rats, Wistar; Receptors, Glycine; Strychnine

Unitary excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials (PSPs) were evoked between neurons in Rexed's laminae (L)II-V of spinal slices from young hamsters (7-24 days old) at 27°C using paired whole cell recordings. Laminar differences in synaptic efficacy were observed: excitatory connections were more secure than inhibitory connections in LII and inhibitory linkages in LII were less reliable than those in LIII-V. A majority of connections displayed paired-pulse facilitation or depression. Depression was observed for both EPSPs and IPSPs, but facilitation was seen almost exclusively for IPSPs. There were no frequency-dependent shifts between facilitation and depression. Synaptic depression was associated with an increased failure rate and decreased PSP half-width for a majority of connections. However, there were no consistent changes in failure rate or PSP time course at facilitating connections. IPSPs evoked at high-failure synapses had consistently smaller amplitude and showed greater facilitation than low-failure connections. Facilitation at inhibitory connections was positively correlated with synaptic jitter and associated with a decrease in latency. At many connections, the paired-pulse ratio varied from trial to trial and depended on the amplitude of the first PSP; dependence was greater for inhibitory synapses than excitatory synapses. Paired-pulse ratios for connections onto neurons with rapidly adapting, "phasic" discharge to depolarizing current injection were significantly greater than for connections onto neurons with tonic discharge properties. These results are evidence of diversity in synaptic transmission between dorsal horn neurons, the nature of which may depend on the types of linkage, laminar location, and intrinsic firing properties of postsynaptic cells.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Animals, Newborn; Bicuculline; Cricetinae; Electric Stimulation; Excitatory Amino Acid Antagonists; Female; GABA-A Receptor Antagonists; Glycine Agents; In Vitro Techniques; Lysine; Male; Mesocricetus; Neural Pathways; Patch-Clamp Techniques; Posterior Horn Cells; Reaction Time; Spinal Cord; Strychnine; Synapses; Synaptic Transmission; Time Factors

Chemical transmission at inhibitory synapses in thalamus may involve receptor activation by beta-amino acids and glycine, as well as GABA. Given their hypothesized roles, we investigated effects of the putative beta-amino acid antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide (TAG) on synaptic inhibition in dorsal thalamus. We performed whole-cell recordings in 200-250 microm sections and immunocytochemical (ICC) studies in ventrobasal thalamus of rat brain (P12-P14). Stimulation of medial lemniscus evoked inhibitory postsynaptic currents (IPSCs) which were purely glycinergic or GABA(A)ergic, or most commonly mixed glycinergic and GABA(A)ergic responses, based on abolition by strychnine, bicuculline, or combined antagonism. TAG antagonized mixed IPSCs (IC(50) approximately 70 microM) in a manner distinguishable from classical glycine and GABA(A) receptor antagonists. TAG (250 microM) reduced the amplitude of glycinergic components which had a decay time constant of approximately 9 ms or approximately 230 ms by 45-50%, and a GABA(A)ergic component which had a decay time constant of approximately 40 ms by approximately 60%. As in the glycinergic component, TAG reduced the amplitude of infrequently occurring, pure glycinergic IPSCs. Surprisingly, TAG had no effect on pure GABA(A)ergic IPSCs, with a decay time constant of approximately 20 ms that correlated to kinetics of GABA-activated channels. ICC studies showed co-localization of alpha(1/2) glycine and alpha(4) GABA(A) receptors at inhibitory synapses. Activation of alpha(4) receptors by beta-amino acids may contribute to the GABA(A)ergic component of mixed IPSCs. The short and long-duration glycinergic IPSCs had decay time constants that correlated to the burst durations of single channels opened by beta-amino acids and glycine. Overall, the effects of TAG implicate beta-amino acid involvement in GABA(A)ergic and glycinergic transmission.

Topics: Animals; Benzothiadiazines; beta-Alanine; Bicuculline; Dose-Response Relationship, Drug; Glycine; Inhibitory Postsynaptic Potentials; Kynurenic Acid; Membrane Potentials; Patch-Clamp Techniques; Protein Subunits; Rats; Rats, Sprague-Dawley; Receptors, GABA-A; Receptors, Glycine; Strychnine; Taurine; Thalamic Nuclei

Despite evidence of local glycinergic circuits in the mature cerebellar nuclei the result of their activation remains unknown. Here, using whole cell recordings in rat cerebellar slices we demonstrated that after postnatal day 17 (>P17) glycinergic IPSCs can be readily evoked in large deep cerebellar nuclear neurons (DCNs), in the same way as in neonatal DCNs (P7-P10). Spontaneous glycinergic IPSCs were very rare but direct presynaptic depolarization by superfusion with elevated potassium concentration or application of 4-aminopyridine consistently evoked strychnine sensitive IPSCs. Glycinergic IPSCs showed fast kinetics in >P17 DCNs while were significantly slower in neonatal DCNs. Immuno-histochemical investigations using a specific marker for glycinergic fibers and terminals showed low density of immuno-fluorescent puncta, putative glycinergic boutons surrounding P18-P23 DCNs, in agreement with the rare spontaneous synaptic activity. But putative glycinergic boutons were present in critical areas for the control of spike generation. In contrast to adult and neonatal DCNs, glycinergic IPSCs could not be induced in juvenile DCNs (P13-P17) despite similar perisomatic immuno-staining pattern and expression of glycinergic receptors to >P17 DCNs. The latter results demonstrate substantial postnatal development of glycinergic cerebellar nuclei circuits. The cerebellum is involved in rapidly controlling ongoing movements. For that function, it is thought important the temporal and spatial precision of its output, which is carried to target structures by DCNs. The present study, by demonstrating fast glycinergic IPSCs in mature DCNs, points to the activation of glycinergic microcircuits as one of the possible mechanism involved in the spatio-temporal control of cerebellar output.

Topics: Age Factors; Animals; Animals, Newborn; Bicuculline; Cerebellar Nuclei; Drug Interactions; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA Antagonists; Glycine; Glycine Agents; Glycine Plasma Membrane Transport Proteins; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Kynurenic Acid; Neurons; Patch-Clamp Techniques; Pyridazines; Rats; Rats, Sprague-Dawley; Strychnine; Synapses

Previous work has shown that latent respiratory motor pathways known as crossed phrenic pathways are inhibited via a spinal inhibitory process; however, the underlying mechanisms remain unknown. The present study investigated whether spinal GABA-A and/or glycine receptors are involved in the inhibition of the crossed phrenic pathways after a C2 spinal cord hemisection injury. Under ketamine/xylazine anesthesia, adult, female, Sprague-Dawley rats were hemisected at the C2 spinal cord level. Following 1 week post injury, rats were anesthetized with urethane, vagotomized, paralyzed and ventilated. GABA-A receptor (bicuculline and Gabazine) and glycine receptor (strychnine) antagonists were applied directly to the cervical spinal cord (C3-C7), while bilateral phrenic nerve motor output was recorded. GABA-A receptor antagonists significantly increased peak phrenic amplitude bilaterally and induced crossed phrenic activity in spinal-injured rats. Muscimol, a specific GABA-A receptor agonist, blocked these effects. Glycine receptor antagonists applied directly to the spinal cord had no significant effect on phrenic motor output. These results indicate that phrenic motor neurons are inhibited via a GABA-A mediated receptor mechanism located within the spinal cord to inhibit the expression of crossed phrenic pathways.

Topics: Animals; Bicuculline; Body Weight; Efferent Pathways; Female; GABA Antagonists; gamma-Aminobutyric Acid; Glycine; Glycine Agents; Phrenic Nerve; Pyridazines; Rats; Rats, Sprague-Dawley; Receptors, Glycine; Respiratory Mechanics; Respiratory Muscles; Spinal Cord Injuries; Stereotaxic Techniques; Strychnine; Vagotomy

Taurine (2-aminoethanesulfonic acid) is the most abundant free amino acid in the developing mammalian cerebral cortex, however, few studies have reported its neurobiological functions during development. In this study, by means of whole-cell patch-clamp recordings, we examined the effects of taurine on chloride channel receptors in neocortical neurons from early to late postnatal stages, which cover a critical period in cortical circuit formation. We show here that taurine activates chloride channels in cortical neurons throughout the postnatal stages examined (from postnatal day 2 to day 36). The physiological effects of taurine changed from excitatory to inhibitory due to variations in the intracellular Cl- concentration during development. An antagonist blocking analysis also demonstrated a developmental shift in the receptor target of taurine, from glycine receptors to GABAA receptors. Taken together, these results may reflect genetically programmed, bidirectional functions of taurine. At the early developmental stage, taurine acting on glycine receptors would serve to promote cortical circuit formation. As cortical circuit has to be regulated in the later stages, taurine would serve as a safeguard against hyperexcitable circuit.

Topics: Aging; Animals; Animals, Newborn; Bicuculline; Cerebral Cortex; Chloride Channels; Chlorides; Critical Period, Psychological; Dose-Response Relationship, Drug; Female; GABA Antagonists; Glycine Agents; Immunohistochemistry; In Vitro Techniques; Male; Membrane Potentials; Mice; Mice, Inbred ICR; Neurons; Patch-Clamp Techniques; Pregnancy; Receptors, GABA-A; Receptors, Glycine; Sodium-Potassium-Chloride Symporters; Solute Carrier Family 12, Member 2; Strychnine; Taurine

Neurons in the inferior colliculus (IC) that are excited by one ear and inhibited by the other [excitatory-inhibitory (EI) neurons] can code interaural intensity disparities (IIDs), the cues animals use to localize high frequencies. Although EI properties are first formed in a lower nucleus and imposed on some IC cells via an excitatory projection, many other EI neurons are formed de novo in the IC. By reversibly inactivating the dorsal nucleus of the lateral lemniscus (DNLL) in Mexican free-tailed bats with kynurenic acid, we show that the EI properties of many IC cells are formed de novo via an inhibitory projection from the DNLL on the opposite side. We also show that signals excitatory to the IC evoke an inhibition in the opposite DNLL that persists for tens of milliseconds after the signal has ended. During that period, strongly suppressed EI cells in the IC are deprived of inhibition from the DNLL and respond to binaural signals as weakly inhibited or monaural cells. By relieving inhibition at the IC, we show that an initial binaural signal essentially reconfigures the circuit and thereby allows IC cells to respond to trailing binaural signals that were inhibitory when presented alone. Thus, DNLL innervation creates a property in the IC that is not possessed by lower neurons or by collicular EI neurons that are not innervated by the DNLL. That property is a change in responsiveness to binaural signals, a change dependent on the reception of an earlier sound. These features suggest that the circuitry linking the DNLL with the opposite central nucleus of the IC is important for the processing of IIDs that change over time, such as the IIDs generated by moving stimuli or by multiple sound sources that emanate from different regions of space.

Topics: Acoustic Stimulation; Action Potentials; Animals; Aspartic Acid; Auditory Pathways; Auditory Threshold; Bicuculline; Chiroptera; Echolocation; GABA Antagonists; Glutamic Acid; Inferior Colliculi; Iontophoresis; Kynurenic Acid; Neural Inhibition; Neurons; Pons; Reaction Time; Receptors, Glycine; Sound Localization; Strychnine

This study aimed to show the presence of deglutitive and distal inhibition in the rat esophagus and to differentiate the underlying neural mechanisms.. Under urethane anesthesia, the pharyngoesophageal tract was fitted with water-filled balloons for luminal distention and pressure recording. Neural activity was recorded in the nucleus tractus solitarii subnucleus centralis and rostral nucleus ambiguous.. Distal esophageal distention evoked both rhythmic local contractions and burst discharges of ambiguous neurons that were simultaneously inhibited by a swallow or proximal esophageal distention. In subnucleus centralis interneurons, type I rhythmic burst discharges correlated with distal esophageal pressure waves and were suppressed by midthoracic esophageal distention; type II non-rhythmic excitatory responses, like type III inhibitory responses, were evoked by distention of either the thoracic or distal esophagus. When applied to the surface of the solitarius complex, bicuculline and, less effectively, strychnine suppressed distal inhibition, and 2-(OH)-saclofen and 3-aminopropylphosphonic acid were ineffective. None of the drugs tested, including systemic picrotoxin, affected deglutitive inhibition.. Distal and deglutitive inhibition are present in the rat esophagus. The former, unlike the latter, depends on activation of ligand-gated chloride channels associated with subnucleus centralis premotor neurons. Inhibitory aminoacidergic local interneurons are a probable source of type II responses.

Topics: Animals; Baclofen; Bicuculline; Deglutition; Esophagus; GABA-A Receptor Antagonists; Male; Models, Biological; Muscle, Smooth; Propylamines; Rats; Rats, Sprague-Dawley; Solitary Nucleus; Strychnine; Synaptic Transmission

Activation of baroreceptors causes efferent sympathetic nerve activity (SNA) to fall. Two mechanisms could account for this sympathoinhibition: disfacilitation of sympathetic preganglionic neurons (SPN) and/or direct inhibition of SPN. The roles that spinal GABA and glycine receptors play in the baroreceptor reflex were examined in anesthetized, paralyzed, and artificially ventilated rats. Spinal GABA(A) receptors were blocked by an intrathecal injection of bicuculline methiodide, whereas glycine receptors were blocked with strychnine. Baroreceptors were activated by stimulation of the aortic depressor nerve (ADN), and a somatosympathetic reflex was used as control. After an intrathecal injection of vehicle, there was no effect on any measured variable or evoked reflex. In contrast, bicuculline caused a dose-dependent increase in arterial pressure, SNA, phrenic nerve discharge, and it significantly facilitated the somatosympathetic reflex. However, bicuculline did not attenuate either the depressor response or sympathoinhibition evoked after ADN stimulation. Similarly, strychnine did not affect the baroreceptor-induced depressor response. Thus GABA(A) and glycine receptors in the spinal cord have no significant role in baroreceptor-mediated sympathoinhibition.

Topics: Adrenergic Fibers; Animals; Aorta; Baroreflex; Bicuculline; Blood Pressure; Carbon Dioxide; Dose-Response Relationship, Drug; Electric Stimulation; GABA-A Receptor Antagonists; Injections, Intravenous; Injections, Spinal; Phrenic Nerve; Physical Stimulation; Pulmonary Gas Exchange; Rats; Rats, Sprague-Dawley; Rats, Wistar; Reaction Time; Receptors, GABA-A; Spinal Cord; Splanchnic Nerves; Strychnine

The neuronal plasticity in the spinal dorsal horn induced after conditioning low-frequency stimulation of afferent A fibers, and its relationship with spinal inhibitory networks, was investigated with an optical-imaging method that detects neuronal excitation. High-intensity single-pulse stimulation of the dorsal root activating both A and C fibers evoked an optical response in the dorsal horn in transverse slices of 12- to 25-day-old rat spinal cords stained with a voltage-sensitive dye, RH-482. The optical response, reflecting the net excitation of neuronal elements along the thickness of each slice, was suppressed after a conditioning low-frequency stimulation (0.2-1 Hz for 10 min) to A fibers in the dorsal root. The degree of suppression was largest in the lamina II of the dorsal horn (48% reduction), where the majority of C fibers terminate, and much less in the deeper dorsal horn (5% reduction in laminae III-IV). The onset of suppression was somewhat slow; after the low-frequency stimulation, the magnitude of excitation gradually decreased, reached the maximum effect 30 min after the conditioning, and remained at the suppressed level for >1 h. Suppression was not observed when the low-frequency stimulation was given during a 20-min perfusion with a solution containing an NMDA-receptor antagonist, DL-2-amino-5-phosphonovaleric acid (30 microM). A brief application of an opioid-receptor antagonist, naloxone (0.5 microM), inhibited the induction, but not the maintenance, of low-frequency stimulus-induced suppression. However, treatments with the GABA(A) receptor antagonist bicuculline (1 microM) and the glycine receptor antagonist strychnine (0.3 microM) did not affect suppression induction and maintenance. In conclusion, conditioning low-frequency stimulation to A fibers interferes with the afferent-induced excitation in the dorsal horn. The low-frequency stimulation-induced suppression is maintained by a reduction of glutamatergic excitatory transmissions in the dorsal horn, not by an enhanced inhibition. Activation of the spinal opioid-mediated system by low-frequency stimulation, but not the inhibitory amino acid-mediated system, is necessary to initiate robust suppression. The long-term depression of afferent synaptic efficacy onto excitatory interneurons likely takes the primary role in the robust suppression of neuronal excitation in the dorsal horn.

Topics: 2-Amino-5-phosphonovalerate; Afferent Pathways; Animals; Bicuculline; Electric Stimulation; Evoked Potentials; Excitatory Amino Acid Antagonists; In Vitro Techniques; Naloxone; Nerve Fibers; Neuronal Plasticity; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Reaction Time; Strychnine

We used whole-cell patch clamp recordings in a rat brainstem slice preparation to characterize the properties of miniature excitatory postsynaptic currents (mEPSCs) in hypoglossal motoneurons. The distinct kinetic characteristics of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated synaptic responses allowed us to study dual component mEPSCs mediated by the two receptor types. Using this approach, NMDA and non-NMDA receptors were found to be co-localized at the same synaptic locations. In addition, some sites contain only NMDA receptors since a large proportion of mEPSCs were apparently mediated by NMDA receptors only. Furthermore, the amplitudes of pharmacologically isolated NMDA receptor-mediated mEPSCs were highly variable in individual cells and their decay kinetics were modulated by membrane potential.

Topics: Animals; Animals, Newborn; Bicuculline; Evoked Potentials; Excitatory Amino Acid Antagonists; Hypoglossal Nerve; In Vitro Techniques; Motor Neurons; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, N-Methyl-D-Aspartate; Strychnine; Synapses; Tetrodotoxin

The depression of upper airway motor activity that develops during the rapid eye movement (REM) stage of sleep is a major factor allowing upper airway obstructions to occur in patients with sleep apnea syndrome. Microinjections of carbachol, a cholinergic agonist, into the dorsal pontine tegmentum of chronically instrumented cats produce REM sleep. In acutely decerebrate cats, carbachol induces postural atonia, eye movements and a depression of the motor output to respiratory pump and upper airway muscles. In lumbar motoneurons, the depression of activity is due to a glycinergic inhibition that has the same characteristics during natural REM sleep in chronic cats and carbachol-induced atonia in decerebrate cats (Neurophysiology, 57 (1987) 1118-1129). The mechanisms that lead to the suppression of upper airway motoneuronal activity during REM sleep are unknown. In this study, we assessed whether the depression of hypoglossal (XII) nerve activity induced by pontine carbachol injections is caused by inhibitory amino acids acting within the XII nucleus. In decerebrate, paralyzed and artificially ventilated cats, we recorded the activities of both XII nerves (genioglossal branches), one phrenic and a cervical motor branch (to monitor postural activity). Postural atonia and respiratory depression were induced by pontine carbachol injections. The inhibitory amino acid receptor antagonists, strychnine (glycine receptors) or bicuculline (GABAA receptors), were injected (100-250 nl; 1.0-2.5 mM) into one XII nucleus (the other served as control) in an attempt to reduce or abolish the depression subsequently induced by pontine carbachol. Prior to the carbachol injections, both antagonists caused similar elevations of XII nerve activity on the treated side (30-40%). However, following carbachol, the XII nerve activity on the treated side was depressed to about 25% of the (pre-antagonist and pre-carbachol) control level, whereas the depression on the untreated side was slightly greater, to 10-15% of the control. Additional injections of antagonists during the carbachol-induced depression produced no further increase in nerve activity. This minor effect of the antagonists on the carbachol-induced depression of XII nerve activity was in contrast to the marked disinhibitory effects that both antagonists had on the XII nerve response to electrical stimulation of the lingual nerve. The latter was used as a control for the ability of strychnine and bicuculline to exert disin

Topics: Animals; Bicuculline; Carbachol; Cats; Decerebrate State; Female; Functional Laterality; Hypoglossal Nerve; Male; Motor Neurons; Muscimol; Muscle Tonus; Reference Values; Sleep, REM; Strychnine; Synapses; Time Factors

Experiments were conducted in urethan-anesthetized rabbits to determine whether vasomotor effects elicited by activation or inhibition of the caudal ventrolateral medulla depend on gamma-aminobutyric acid (GABA)ergic, glycinergic, or alpha-adrenergic receptors in the region of the rostral ventrolateral medulla, which contains the bulbospinal sympathoexcitatory neurons. Bilateral injection of bicuculline methiodide into the rostral medulla caused a dose-related reduction in the fall in arterial pressure and in the inhibition of renal sympathetic nerve activity normally elicited by chemical stimulation of neurons in the caudal medulla using local injection of L-glutamate. When both bicuculline and muscimol were injected into the rostral medulla at the same time, resting arterial pressure was maintained at base-line levels, and the sympathoexcitatory neurons remained normally excitable by local injection of L-glutamate into the rostral medulla. In the presence of this mixed antagonist-agonist GABAergic blockade, both decreases and increases in arterial pressure elicited by excitation or inhibition of neuronal function in the caudal medulla were abolished. Similar effects were not observed after blockade of glycinergic or alpha-adrenergic receptors in the rostral ventrolateral medulla. Results suggest that the depressor neurons in the caudal ventrolateral medulla alter peripheral sympathetic vasomotor activity almost entirely by an action on GABAergic receptors in the rostral ventrolateral medulla.

Topics: Animals; Bicuculline; Blood Pressure; Functional Laterality; Heart Rate; Kidney; Male; Medulla Oblongata; Muscimol; Neurons; Phentolamine; Rabbits; Receptors, GABA-A; Strychnine; Sympathetic Nervous System

We studied the cardiovascular effects of altering GABA-ergic tone in the posterior hypothalamus in rats. Animals were equipped with chronic guide cannulas placed in the posterior hypothalamus, arterial and venous catheters, and a bipolar electrode on the splanchnic nerve. Microinjected bilaterally into the posterior hypothalamus in conscious rats, the postsynaptic gamma-aminobutyric acid (GABA) antagonists bicuculline methiodide and picrotoxin rapidly increased heart rate, blood pressure, and sympathetic nerve activity. Microinjection of the GABA agonist muscimol into this same region decreased heart rate, blood pressure, and sympathetic nerve activity in conscious rats. In contrast, muscimol infused into the posterior hypothalamus of anesthetized rats failed to alter heart rate or blood pressure. We conclude that 1) the posterior hypothalamus contains a sympathoexcitatory system that is modulated by changes in GABA-ergic tone and 2) tonic GABA-ergic inhibition is sufficient to completely suppress the activity of this hypothalamic sympathoexcitatory system in anesthetized animals but not in conscious rats.

Topics: Animals; Bicuculline; Blood Pressure; GABA Antagonists; Heart Rate; Hypothalamus; Male; Muscimol; Nitroprusside; Phenylephrine; Picrotoxin; Rats; Rats, Inbred Strains; Strychnine; Sympathetic Nervous System

Previous experiments have suggested that the red nucleus is an essential structure in the neural pathways subserving the conditioned responses (CRs) elicited in several simple associative learning paradigms. The present investigation confirms the involvement of the magnocellular red nucleus in production of the classically conditioned nictitating membrane response in the rabbit and suggests that gamma-aminobutyric acid (GABA) processes within this structure are involved in expression of the CR. Specifically, these studies demonstrate that microinfusion of a GABA antagonist (either picrotoxin or bicuculline methiodide) into the magnocellular red nucleus can selectively and reversibly reduce or abolish retention of the CR, without altering the unconditioned reflex response. Furthermore, these pharmacological manipulations that disrupt the CR are both anatomically and pharmacologically specific, and demonstrate a predictable dose-dependent function. These findings suggest that GABAergic processes within the magnocellular red nucleus are part of the critical circuitry subserving the CR.

Topics: Animals; Bicuculline; Conditioning, Classical; Electric Stimulation; Male; Nictitating Membrane; Picrotoxin; Rabbits; Red Nucleus; Retention, Psychology; Strychnine

Two pyridazinyl GABA derivatives, SR 95103 and SR 42641 have recently been described as selective GABAA receptor antagonists. We have now investigated the behavioural effects of SR 95103 and SR 42641 after intrastriatal injection in mice. When injected into the right striatum, SR 95103 (0.01-0.5 microgram), SR 42641 (0.0001-0.01 microgram) and bicuculline methiodide (0.005-0.05 microgram) induced contralateral rotations which were antagonized by intraperitoneal injection of muscimol. In contrast, the intrastriatal injection of the GABAA receptor agonist muscimol induced ipsilateral rotations. Muscimol-induced turning was antagonized by SR 95103 (10-30 mg/kg), SR 42641 (1-10 mg/kg) and (+)-bicuculline (0.125-0.5 mg/kg) injected intraperitoneally, but not by strychnine. Intrastriatal glycine also induced ipsilateral rotations which were antagonized by strychnine (0.01-0.3 mg/kg i.p.) but not by (+)-bicuculline, SR 95103 or SR 42641. These results suggest that SR 95103 and SR 42641 induce turning through a selective blockade of GABAA receptors within the striatum.

Topics: Analysis of Variance; Animals; Behavior, Animal; Bicuculline; Corpus Striatum; Dopamine; Dose-Response Relationship, Drug; Female; Glycine; Mice; Muscimol; Pyridazines; Receptors, GABA-A; Strychnine

Recent studies have shown that blockade of gamma-aminobutyric acid (GABA) in the posterior hypothalamus in anesthetized rats elicits cardio-respiratory stimulation similar to that seen in emotional defense reactions and, in conscious rats, locomotor arousal suggesting a flight response. The present study was conducted in order to test the hypothesis that the behavioral effects elicited by GABA blockade in the posterior hypothalamus were the results of disinhibiting a mechanism whose activation selectively enhances reactivity to aversive stimuli. Male rats were trained on a Sidman shock avoidance schedule (RS20:SS10) as well as a food-reinforced approach schedule (VI 1). Under anesthesia, guide cannulae were stereotaxically implanted bilaterally in the posterior hypothalamus at sites where microinjection of the GABA antagonist, bicuculline methiodide (BMI) 25 ng, increased heart rate. After recovery, rats were tested in both the avoidance and VI 1 schedules after hypothalamic microinjection of saline, BMI 25 ng, and the GABA agonist, muscimol 25 ng. Microinjection of BMI significantly increased the avoidance responses but had no effect on the approach responses. Muscimol decreased both the avoidance and approach responses. When microinjected into the lateral hypothalamic area, BMI had no effect on the response rates in either schedule while muscimol decreased the approach responding only. Therefore, GABA blockade at the discrete area of the posterior hypothalamic nucleus appears to elicit a selective enhancement of avoidance responses. These results suggest that an endogenous GABAergic system in the posterior hypothalamus may tonically inhibit a constellation of autonomic, locomotor and motivational responses that are necessary for some types of defense reaction.

Topics: Animals; Avoidance Learning; Bicuculline; Electroshock; gamma-Aminobutyric Acid; Heart Rate; Hypothalamus; Hypothalamus, Posterior; Male; Muscimol; Rats; Rats, Inbred Strains; Strychnine

A total of 45 cells with spinal projections, pulse-synchronous discharges, and extreme sensitivity to changes in arterial pressure were recorded in the retrofacial portion of nucleus paragigantocellularis lateralis (PGCL) in halothane-anesthetized rats. Every unit tested was inhibited by iontophoretic applications of gamma-aminobutyric acid (GABA), glycine, and taurine and excited by glutamate or the GABA receptor antagonist bicuculline methiodide. The inhibitory effect of GABA was antagonized by bicuculline methiodide but not by strychnine; the effect of glycine was eliminated by strychnine but unaffected by bicuculline. Iontophoretically applied bicuculline completely antagonized the time-locked reduction in unit activity resulting from increases in arterial pressure while strychnine was ineffective. 1-Glutamate, applied in a dose that produced a larger increase in the unit discharge rate than bicuculline, did not significantly alter the magnitude of the inhibition associated with increases in arterial pressure. It is concluded that GABA or a related substance mediates the inhibition of PGCL reticulospinal neurons resulting from the activation of arterial baroreceptors.

Topics: Animals; Bicuculline; Blood Pressure; Cardiovascular System; Electric Stimulation; gamma-Aminobutyric Acid; Glutamates; Glutamic Acid; Glycine; Iontophoresis; Male; Neurons; Pressoreceptors; Rats; Rats, Inbred Strains; Spinal Cord; Strychnine; Sympathetic Nervous System; Taurine