Visceral afferent stimulation-evoked changes in the release of peptides into the parabrachial nucleus in vivo

OBJECTIVE: Although there is evidence from postmortem studies suggestive of deficient inhibitory neurotransmission of gamma-aminobutyric acid (GABA) in schizophrenia, no direct in vivo evidence has been obtained to date. The authors used single photon emission computed tomography (SPECT) with iodine-123-labeled iomazenil ([123I]iomazenil), a radioligand that selectively binds with high affinity to the benzodiazepine subunit of the GABAA receptor complex in the human brain, to investigate the presence of benzodiazepine receptor abnormalities in the cerebral cortex of living subjects with schizophrenia. METHOD: Dynamic [123I]iomazenil SPECT was performed in 15 patients (14 patients with DSM-III-R schizophrenia and one with schizophreniform disorder) and 12 healthy subjects over a period of 2 hours. The time-integral method was used to generate ratios of "specific" to "nonspecific" [123I]iomazenil binding at equilibrium for several cortical regions. RESULTS: No overall between-group differences in benzodiazepine receptor binding were found, but significant correlations emerged between the severity of schizophrenic symptoms and [123I]iomazenil binding in limbic cortical regions: positive symptom scores were negatively correlated with benzodiazepine receptor binding in the left medial temporal region, and negative symptoms were inversely related to receptor binding in the medial frontal region. These correlations were not significant when a Bonferroni correction for multiple comparisons was applied. CONCLUSIONS: These preliminary results are consistent with previous research implicating limbic cortical regions in the pathophysiology of schizophrenia, suggesting that reduced inhibitory GABAergic tone in these areas may contribute to the appearance of schizophrenic symptoms.     

Unchanged [3H]MK-801 binding and increased [3H]flunitrazepam binding in turtle forebrain during anoxia

In order to determine if functional changes in N-methyl-D-aspartate receptors and GABAA receptors play a role in the remarkable anoxia tolerance of freshwater turtle brain, we used autoradiographic techniques to assay [3H]MK-801 and [3H]flunitrazepam binding in turtle forebrain after turtles had been subjected to anoxia for 2 or 6 h. The effects of glutamate, glycine, competitive N-methyl-D-aspartate antagonists, glycine antagonists, polyamines, magnesium, and zinc on [3H]MK-801 binding were the same in anoxic and control turtle forebrains. These results indicate that NMDA receptor regulation plays no role in the adaptive responses to anoxia in turtle brain. In contrast, [3H]flunitrazepam binding was significantly increased in the anoxic dorsal cortex and striatum. The most parsimonious explanation for elevated benzodiazepine receptor binding is that the rise in extracellular GABA levels known to accompany anoxia enhances benzodiazepine receptor affinity. It is possible, however, that GABAA receptor upregulation during anoxia increases the effectiveness of the inhibitory action of released GABA and contributes to the anoxia tolerance of turtles.     

Key amino acids in the gamma subunit of the gamma-aminobutyric acidA receptor that determine ligand binding and modulation at the benzodiazepine site

Pharmacological analyses of gamma-aminobutyric acidA (GABAA) receptor subtypes have suggested that both the alpha and gamma subunits, but not the beta subunit, contribute to the benzodiazepine binding site. We took advantage of the different pharmacological properties conferred by the inclusion of different gamma subunits in the receptor macromolecule to identify amino acids gamma2Phe77 and gamma2Met130 as key determinants of the benzodiazepine binding site. gamma2Phe77 was required for high affinity binding of the benzodiazepine site ligands flumazenil, CL218,872, and methyl-beta-carboline-3-carboxylate but not flunitrazepam. This amino acid was, however, required for allosteric modulation by flunitrazepam, as well as other benzodiazepine site ligands. In contrast, gamma2Met130 was required for high affinity binding of flunitrazepam, clonazepam, and triazolam but not flumazenil, CL218, 872, or methyl-beta-carboline-3-carboxylate and did not affect benzodiazepine efficacy. Introduction of the phenylalanine and methionine into the appropriate positions of gamma1 was not sufficient to confer high affinity for the benzodiazepine site ligand zolpidem. These data show that gamma2Phe77 and gamma2Met130 are necessary for high affinity binding of a number of benzodiazepine site ligands. Although most previous studies have focused on the contribution of the alpha subunit, we demonstrated a critical role for the gamma subunit at the benzodiazepine binding site, indicating that this modulatory site is located at the interface of these two subunits. Furthermore, gamma2Phe77 is homologous to alpha1Phe64, which has been previously shown to be a key determinant of the GABA binding site, suggesting a conservation of motifs between different ligand binding sites on the GABAA receptor.     

Modulation of GABAA receptor function by alcohols: effects of subunit composition and differential effects of ethanol

We sought to test the hypotheses that closely related alcohols would have effects on GABAA receptor function that were not predicted by differences in lipid solubility, and that the subunit structure of the GABAA receptor would significantly affect the actions of different alcohols. Cloned subunits of human GABAA receptors were expressed in Xenopus oocytes, and two-electrode voltage-clamp recording was used to quantify the membrane current response to GABA in the presence and absence of different alcohols. 1-Butanol and 2-butanol differentially potentiated the response to 20 microM GABA in oocytes expressing the alpha 1 beta 2 gamma 2L and alpha 2 beta 2 gamma 2L receptor isoforms. In the alpha 1 beta 2 gamma 2L receptor construct, 1-butanol was more potent than 2-butanol to potentiate GABAA receptor function, but 2-butanol had a greater efficacy. In the alpha 2 beta 2 gamma 2L receptor construct, 1-butanol and 2-butanol were equipotent, but 2-butanol again had a greater efficacy. In the alpha 2 beta 2 receptor construct, both 1-butanol and 2-butanol produced large potentiations of the current response to 3 microM GABA. The efficacy for butanol potentiation of GABA responses in the absence of a gamma 2L subunit was greater, but the potency was greatly reduced. Low concentrations (20 mM) of ethanol potentiated GABA responses in the alpha 1 beta 2 gamma 2L receptor construct. Ethanol potentiation of GABAA receptor function was completely blocked by the benzodiazepine receptor partial inverse agonist RO15-4513 at a concentration (0.5 microM) that did not alter the control GABA response. In contrast, RO15-4513 did not block potentiation of GABAA receptor activity induced by n-propanol, 1-butanol, 2-butanol, 1-heptanol, or propofol (2,6-diisopropylphenol). These results suggest that alcohols have specific interactions with GABAA receptors, and that ethanol may have unique effects not shared by other longer chain alcohols.     

Swim stress selectively alters the specific binding of a benzodiazepine antagonist in mice

The ability of flurazepam to antagonize the electrical precipitation of tonic hindlimb extension is reduced 24 h after mice are forced to swim for 10 min in cold water (6 degrees C). Presumably, this reduction in flurazepam's antiseizure efficacy reflects an environmental stress-induced modification of the GABAA receptor complex. The current study employed a variety of complementary in vitro approaches to characterize the delayed effects of cold-water swim stress on binding parameters of the GABAA receptor complex that may be associated with flurazepam's reduced antiseizure efficacy. The specific binding of [3H]flunitrazepam and the potentiation of this binding by chloride ions did not change after stress in the cerebral cortex, hippocampus, and cerebellum. Moreover, swim stress did not alter the ability of GABA to inhibit the binding of [35S]t-butylbicyclophosphorothionate (TBPS), a ligand that is a specific biochemical marker of the GABA-associated chloride ionophore, to crude membranes prepared from the cerebral cortex and cerebellum. Swim stress was associated with alterations of the specific binding of [3H]Ro 15-1788, a benzodiazepine receptor antagonist, to crude hippocampal and cerebellar membranes. The results are considered in the context of new insights derived from molecular cloning studies of the GABAA receptor complex.     

Astroglia-released factor with negative allosteric modulatory properties at the GABA A receptor

We have previously shown, using whole-cell patch-clamp techniques, that astrocytes release a negative allosteric modulator of the gamma-aminobutyric acid type A receptor (GABAA receptor) with beta-carboline-like properties, thus, likely to act at the benzodiazepine site. Here, using patch-clamp and binding techniques, we confirm that the low-molecular-weight fraction of astroglia-conditioned medium (ACM lmf) contains a factor(s) that negatively modulates GABAA-receptor function. This factor, like beta-carbolines, enhances the specific binding of [35S]t-butyl bicyclophosphorothionate (TBPS) to adult rat cortical membranes in the presence of GABA. However, it fails to interact with various ligands of the benzodiazepine (BZD) site of the GABAA receptor ([3H]flunitrazepam, [3H]Ro 15-1788 and [3H]Ro 15-4513). The question of the actual binding site of the astroglia-derived factor on the GABAA receptor, thus, remains open and can be addressed only after the purification of the active molecule(s) of ACM Imf has been completed, and a labeled form of the endogenous ligand becomes available. Taken together, however, the data suggest that type 1 astrocytes are able to modulate the effects of the main inhibitory neurotransmission in the central nervous system.     

Antisense inhibition of striatal GABAA receptor proteins decreases GABA-stimulated chloride uptake and increases cocaine sensitivity in rats

The functional status of striatal GABAA receptors appears to be inversely related to the magnitude of cocaine-induced behaviors. Exposure of striatum to antisense oligodeoxynucleotides (ASODNs) targeted to the mRNAs for the alpha 2 and the beta 3 subunits of the GABAA receptor should decrease expression of receptor proteins and therefore might be expected to increase cocaine sensitivity. ASODNs, scrambled ODNs or saline were injected into right lateral ventricle of rats and behavioral responses to cocaine were tested 18-20 h after treatment. Animals injected separately with alpha 2 or beta 3 ASODNs exhibited increased behavioral sensitivity to cocaine compared to rats injected with saline or scrambled ODNs including performing more 360 degrees turns to the left than to the right. There was significantly less GABA-stimulated Cl uptake in right striatum compared to left striatum of ASODN-treated rats with no significant difference between sides in control animals. Specific binding to benzodiazepine and convulsant sites on the GABAA receptor was not selectively altered by ASODN treatment. Combined alpha 2 beta 3 ASODN treatment did not affect either cocaine sensitivity or GABAA receptor function. There was no difference between the density of Nissl stained cells in the left and right edges of striatum in control or ASODN-treated rats indicating the absence of significant neurotoxic effects of the ASODN treatment. Injection of fluorescein-conjugated ASODNs indicated that ASODN is present in striatum at times during which behavioral and neurochemical indices of GABA receptor function are decreased. Thus, the functional status of GABAA receptors in striatum may be involved in determining cocaine sensitivity.     

Benzodiazepine receptors in the post-mortem brain of suicide victims and schizophrenic subjects

To examine the role of benzodiazepine (BZ) receptors in suicide and schizophrenia, we determined BZ receptors in post-mortem brain (Brodmann's area 10) obtained from suicide victims, schizophrenic patients, and control subjects using [3H]RO15-1788 as the radioligand. The maximum number of binding sites (Bmax) of BZ receptors in the cortex of suicide victims was significantly higher compared with controls, but this increase was mainly due to those suicide victims who died by violent means and whose Bmax was significantly higher than of those who died by non-violent means or control subjects. In schizophrenic patients, Bmax was not significantly different from that of control subjects. When the schizophrenic subjects were separated into two groups, those on neuroleptics and those off neuroleptics for at least 12 months, however, the mean Bmax of BZ receptors in the prefrontal cortex in post-mortem brain obtained from schizophrenic patients on neuroleptics was significantly lower than Bmax in drug-free schizophrenic patients or normal controls. There were no significant differences among groups in values of the apparent dissociation constant (KD) of [3H]RO15-1788 binding. These results suggest that BZ receptors are up-regulated in the cortex of suicide victims, specifically those who used violent means, and that neuroleptic treatment may result in decreased central BZ receptor binding in the cortex of schizophrenic patients. Thus, the method of suicide and previous exposure to neuroleptics should be considered in the interpretation of data on BZ receptors.     

Direct activation of GABAA receptors by loreclezole, an anticonvulsant drug with selectivity for the beta-subunit

Loreclezole, an anticonvulsant and antiepileptic compound, potentiates gamma-aminobutyric acid (GABA) type A receptor function, by interacting with a specific allosteric modulatory site on receptor beta-subunits. A similar selectivity for GABAA receptor beta-subunits is apparent for the direct activation of receptor-operated Cl- channels, by the general anesthetics propofol and pentobarbital. The ability of loreclezole to activate GABAA receptors directly has now been compared, biochemically and electrophysiologically, with that of propofol. In well-washed rat cortical membranes (devoid of endogenous GABA), loreclezole and propofol increased t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding by up to 28% (at 5 microM) and 80% (at 10 microM), respectively. Higher concentrations (50-100 microM) of both compounds inhibited [35S]TBPS binding with great efficacy, an effect mimicked by GABA. In contrast, the benzodiazepine diazepam increased [35S]TBPS binding, but failed to inhibit this parameter, even at high concentrations. At concentrations of 50-100 microM, loreclezole induced inward Cl- currents in the absence of GABA, in Xenopus oocytes expressing human recombinant GABAA receptors, comprised of alpha 1-, beta 2- and gamma 2S-subunits. At 100 microM, the current evoked by loreclezole was 26% of that induced by 5 microM GABA. The current evoked by 100 microM propofol was 98% of that induced by 5 microM GABA. Currents induced by loreclezole, like those evoked by propofol, were potentiated by diazepam in a flumazenil-sensitive manner and blocked by either bicuculline or picrotoxin. These data suggest that loreclezole shares, with propofol, an agonistic action at GABAA receptors containing the beta 2-subunit and that the different efficacies of the two compounds in this regard, may underlie the difference in their pharmacological profiles. The failure of loreclezole to activate GABAA receptors containing the beta 1-subunit may be responsible for its lack of hypnotic effect.     

Protein tyrosine kinase activity as a prognostic parameter in breast cancer, a pilot study

The interaction of omega (benzodiazepine) modulatory drugs with transiently expressed alpha 1 beta 2 gamma 2 and alpha 5 beta 2 gamma 2 forms of the rat GABAA receptor was investigated using [3H]flumazenil as a probe in in vitro radioligand binding assays. The imidazopyridines alpidem and zolpidem exhibited pronounced selectivity for the alpha 1- compared to the alpha 5-containing construct, whereas omega (benzodiazepine) site modulatory compounds from other chemical series including diazepam, tetrazepam, zopiclone, triazolam, bretazenil and midazolam behaved as relatively non-selective drugs. In the presence of 10 microM gamma-aminobutyric acid (GABA) the potencies of diazepam, flunitrazepam and midazolam to inhibit [3H]flumazenil binding to the alpha 1-construct were increased 3 to 5 fold, whereas with 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate methyl ester a 2.5-fold reduction in potency was observed. Similar modulatory effects of GABA were obtained with these drugs, using the alpha 5-construct. We suggest that these GABA shift determinations of [3H]flumazenil binding can be used as a rapid test to evaluate the intrinsic activities of omega modulatory compounds.     

Functional heterogeneity of hippocampal GABAA receptors

gamma-Aminobutyric acid type A (GABAA) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose-response curves of rapidly and slowly desensitizing cells revealed EC50 values of 8.5 and 37.3 microM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABAA receptor channels was 28-31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn2+. These data suggest the presence of physiologically and pharmacologically distinct GABAA receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.     

Neurocognitive basis of impaired empathy in schizophrenia.

Patients with schizophrenia show impaired emotional and social behavior, such as lack of theory of mind and misinterpretation of social situations. However, there is a paucity of work focusing on the empathic abilities of these patients. The present study was designed to examine the degree of impairment in cognitive and affective empathy in schizophrenia and to evaluate the contribution of executive prefrontal functions to empathy in these patients. To explore the neurocognitive processes that underlie the empathic ability in schizophrenic patients, the relationship between empathy scores and the performance on a cognitive flexibility task that assesses dorsolateral and orbitofrontal functioning (set shifting and reversal, respectively) was examined in 26 patients with schizophrenia and 31 healthy control subjects. Results indicated that patients with schizophrenia were significantly impaired in both cognitive and affective empathy compared with healthy control subjects. The degree of impaired empathy related to the severity of negative symptoms. In addition, patients showed impaired performance on measures of both shifting and reversal. However, while cognitive empathy was particularly related to measurements of orbitofrontal (rather than dorsolateral) functioning, affective empathy was related to measures of social functioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Enhancement of gamma-aminobutyric acidA receptor activity by alpha-chloralose

alpha-Chloralose is widely used as an anesthetic in the laboratory due to its minimal effects on autonomic and cardiovascular systems, yet little is known about its mechanism of action. We examined the effects of alpha-chloralose on gamma-aminobutyric acid type A (GABAA) receptor activity because recent studies have shown that several classes of general anesthetics modulate the function of this receptor. GABAA receptor activity was assayed by measuring the GABA-induced current in Xenopus oocytes expressed with human GABAA receptor alpha-1, beta-1 and gamma-2L subunits. alpha-Chloralose produced a concentration-dependent potentiation of the GABA-induced current with an EC50 value of 49 microM and a maximal effect of 239% of control. Membrane current was not affected by alpha-chloralose in the absence of GABA. alpha-Chloralose (100 microM) increased the affinity for GABA 5-fold and produced a small (17%) increase in the efficacy of GABA. Measurement of the reversal potentials for the alpha-chloralose response suggested that the effect is mediated through increased Cl- conductance. Studies of alpha-chloralose interactions with other allosteric modulators determined that alpha-chloralose binds to a site on the GABAA receptor complex distinct from the benzodiazepine, neurosteroid and barbiturate sites. Chloral hydrate, trichloroethanol and urethane also augmented GABA-induced currents. alpha-Chloralose had no effect on the hydroxytryptamine-induced currents in oocytes expressed with the 5-hydroxytryptamine3 receptor. These data extend the number of classes of anesthetics that allosterically modulate GABAA receptor activity and indicate that GABAA receptors may be a common site of action for diverse classes of general anesthetics.     

Effect of peripherally and centrally administered calcitonin on gallbladder emptying in dogs

The vast molecular heterogeneity of brain gamma-aminobutyric acid type A (GABAA) receptors forms the basis for receptor subtyping. Using autoradiographic techniques, we established the characteristics of cerebellar granule cell GABAA receptors by comparing wild-type mice with those with a targeted disruption of the alpha6 subunit gene. Cerebellar granule cells of alpha6(-/-) animals have severe deficits in high affinity [3H]muscimol and [3H]SR 95531 binding to GABA sites, in agonist-insensitive [3H]Ro 15-4513 binding to benzodiazepine sites, and in furosemide-induced increases in tert-[35S]butylbicyclophosphorothionate binding to picrotoxin-sensitive convulsant sites. These observations agree with the known specific properties of these sites on recombinant alpha6beta2/3gamma2 receptors. In the presence of GABA concentrations that fail to activate alpha1 subunit-containing receptors, methyl-6,7-dimethoxy-4-ethyl-beta-carboline (30 microM), allopregnanolone (100 nM), and Zn2+ (10 microM) are less efficacious in altering tert-[35S]butylbicyclophosphorothionate binding in the granule cell layer of the alpha6(-/-) than alpha6(+/+) animals. These data concur with the deficiency of the cerebellar alpha6 and delta subunit-containing receptors in the alpha6(-/-) animals and could also account for the decreased affinity of [3H]muscimol binding to alpha6(-/-) cerebellar membranes. Predicted additional alterations in the cerebellar receptors of the mutant mice may explain a surplus of methyl-6,7-dimethoxy-4-ethyl-beta-carboline-insensitive receptors in the alpha6(-/-) granule cell layer and an increased diazepam-sensitivity in the molecular layer. These changes may be adaptive consequences of altered GABAA receptor subunit expression patterns in response to the loss of two subunits (alpha and delta) from granule cells.     

Rat and human hippocampal alpha5 subunit-containing gamma-aminobutyric AcidA receptors have alpha5 beta3 gamma2 pharmacological characteristics

The gamma-aminobutyric acid (GABA)A receptor is a hetero-oligomer consisting of five subunits, the combination of which confers unique pharmacological properties to the receptor. To understand the physiological role of native GABAA receptors, it is critical to determine their subunit compositions. The pharmacological characteristics of human alpha5 beta3 gamma2 and alpha5beta3gamma3 GABAA receptors stably expressed in L(tk-) cells were characterized with the alpha5-selective ligand [3H]L-655,708 and compared with the pharmacological characteristics of [3H]L-655,708 binding sites from rat and human hippocampus. Saturation analyses revealed a 9-fold selective affinity of [3H]L-655,708 for alpha5 beta3 gamma2 receptors (Kd = 1.7 +/- 0.4 nM), compared with alpha5 beta3 gamma3 receptors (Kd = 15 +/- 3 nM). Rat and human hippocampal [3H]L-655,708 binding sites had affinities of 2.2 +/- 0.6 and 1.0 +/- 0.2 nM, respectively, comparable to the affinity of alpha5 beta3 gamma2 receptors. Pharmacological analysis of [3H]L-655,708 binding sites in rat and human hippocampi revealed a strong correlation with the affinities of seven benzodiazepine site ligands for alpha5 beta3 gamma2 but not alpha5 beta3 gamma3 receptors. Immunoprecipitation of [3H]L-655,708 binding sites from rat hippocampus with a gamma2-selective antibody yielded 19 +/- 4% of total benzodiazepine binding sites measured using [3H]Ro15-1788, whereas no specific binding was measured after immunoprecipitation with an anti-gamma3 antibody. Combinatorial immunoprecipitations of [3H]muscimol binding sites with anti-alpha5 and anti-gamma2 or anti-alpha5 and anti-gamma3 antibodies established the preferential expression of alpha5 gamma2 receptors, accounting for 22 +/- 2% of total rat hippocampal GABAA receptors. These observations provide pharmacological and structural evidence for the prevalence of alpha5 beta3 gamma2 GABAA receptors in rat hippocampus, despite the clustering of alpha5 and gamma3 loci on the same chromosome.     

Pharmacological modulation of the diazepam-insensitive recombinant gamma-aminobutyric acidA receptors alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2

We characterized modulation of the gamma-aminobutyric acid (GABA)-evoked responses of the diazepam-insensitive alpha 4 beta 2 gamma2 and alpha 6 beta 2 gamma 2 recombinant GABAA receptors. The partial agonist bretazenil potentiated the responses of both receptors with similar dose dependence but with a higher maximal enhancement at the alpha 4 beta 2 gamma 2 receptor. The bretazenil-induced potentiation was reduced by the benzodiazepine antagonist flumazenil. At a high concentration (10 microM), flumazenil was a weak potentiator of the GABA response. The partial agonist imidazenil was inactive. The imidazobenzodiazepine inverse agonist Ro 15-4513, which is known to bind with high affinity to the alpha 6 beta 2 gamma 2 receptor, potentiated the GABA responses of the alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor subtypes with similar dose dependence over the concentration range of 0.1-10 microM. Methyl-6, 7-dimethoxy-4-ethyl-beta-carboline, a beta-carboline inverse agonist, had a similar potentiating effect when tested at a concentration of 10 microM. The alpha 4 beta 2 gamma 2 and alpha 6 beta 2 gamma 2 receptor-mediated currents had equal sensitivities to furosemide and Zn2+ ions, both of which reduced the GABA-evoked responses. The alpha 6 beta 2 gamma 2 receptor but not the alpha 4 beta 2 gamma 2 receptor exhibited a low level of spontaneous activity in the absence of GABA; this resting current could be directly potentiated by Ro 15-4513, methyl-6,7-dimethoxy-4-ethyl-beta-carboline, bretazenil and flumazenil and was blocked by picrotoxin. Thus, although the alpha 4 beta 2 gamma 2 receptors are insensitive to benzodiazepine binding site full agonists, such as diazepam, they can be modulated by certain ligands acting as partial and inverse agonists at diazepam-sensitive receptors and thereby contribute to the respective pharmacological profiles.     

Ventral tegmental area dopamine neurons mediate the shock sensitization of acoustic startle: A potential site of action for benzodiazepine anxiolytics.

Dopamine (DA)-containing neurons in the ventral tegmental area (VTA) are thought to play an important role in fear motivation. The primary objective of the present study was to determine the connection between DA D?, gamma aminobutyric acid (GABA)A, and benzodiazepine receptors in the VTA and footshock-associated emotionality. Microinfusion of the DA D? receptor agonist quinpirole, the GABAA receptor agonist muscimol, and the benzodiazepine receptor agonist flurazepam into the VTA was observed to suppress the shock enhancement of acoustic startle amplitudes. None of the drugs depressed baseline startle responding or footshock reactivity. The results indicate the involvement of VTA DA neurons in the fear-arousing properties of footshock and implicate the VTA as a possible neural site for the anxiolytic actions of benzodiazepines. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

L-lysine is a barbiturate-like anticonvulsant and modulator of the benzodiazepine receptor

Our earlier observations showed that L-lysine enhanced the activity of diazepam against seizures induced by pentylenetetrazol (PTZ), and increased the affinity of benzodiazepine receptor binding in a manner additive to that caused by gamma-aminobutyric acid (GABA). The present paper provides additional evidence to show that L-lysine has central nervous system depressant-like characteristics. L-lysine enhanced [3H]flunitrazepam (FTZ) binding in brain membranes was dose-dependent and stimulated by chloride, bromide and iodide, but not fluoride. Enhancement of [3H]FTZ binding by L-lysine at a fixed concentration was increased by GABA but inhibited by pentobarbital between 10(-7) to 10(-3)M. While GABA enhancement of [3H]FTZ binding was inhibited by the GABA mimetics imidazole acetic acid and tetrahydroisoxazol pyridinol, the enhancement by pentobarbital and L-lysine of [3H]FTZ binding was dose-dependently increased by these two GABA mimetics. The above results suggest that L-lysine and pentobarbital acted at the same site of the GABA/benzodiazepine receptor complex which was different from the GABA binding site. The benzodiazepine receptor antagonist imidazodiazepine Ro15-1788 blocked the antiseizure activity of diazepam against PTZ. Similar to pentobarbital, the anti-PTZ effect of L-lysine was not blocked by Ro15-1788. Picrotoxinin and the GABA, receptor antagonist bicuculline partially inhibited L-lysine's enhancement of [3H]FTZ binding with the IC50s of 2 microM and 0.1 microM, respectively. The convulsant benzodiazepine Ro5-3663 dose-dependently inhibited the enhancement of [3H]FTZ binding by L-lysine. This article shows the basic amino acid L-lysine to have a central nervous system depressant characteristics with an anti-PTZ seizure activity and an enhancement of [3H]FTZ binding similar to that of barbiturates but different from GABA.