Enhanced sensitivity to beta-carboline inverse agonists in rats chronically treated with FG 7142

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Neuroplasticity in specific limbic system circuits may mediate specific kindling induced changes in animal affect-implications for understanding anxiety associated with epilepsy

In two complementary experiments, we studied the effects of low frequency stimulation (LFS) of the amygdala on behavioral effects of kindling in rats and cats. These studies tested the hypothesis that kindling induced long term potentiation (KLTP) in amygdala circuits underlies interictal behavioral change. Since LFS can depotentiate LTP, it was predicted that LFS should both depotentiate KLTP and reverse behavioral effects of kindling. In cats, the effects of LFS on KLTP of amygdala efferents was studied, and related to behavioral effects. Partial ventral hippocampal kindling in cats and right amygdala kindling in rodents lastingly increased defensive response to rats in cats, and anxiety-like behavior (ALB) in the elevated plus-maze in rats. In addition, partial kindling reduced predatory attack behavior in cats independent of its effects on defensive response. Partial kindling also induced KLTP of amygdala efferent transmission to ventromedial hypothalamus (VMH) and periaqueductal gray (PAG) in left and right hemispheres. Depotentiation of amygdala efferent KLTP by bilateral amygdala LFS selectively reduced KLTP in right amygdala efferents. At the same time, defensive behavior, but not attack behavior, was returned to levels seen prior to partial kindling. Defensiveness returned to post kindling levels between 44 and 76days after LFS. At the same time, LTP was restored in the right Amygdalo-PAG pathway only. These findings suggest that lasting change in affect produced by kindling depends on LTP of right amygdala efferent transmission to PAG, replicating studies of the effects of FG-7142 on brain and behavior in the cat. The findings suggest further that the spectrum of behavioral changes produced by partial kindling are dependent on changes in a variety of neural circuits, and that amygdala efferent transmission changes are responsible for changes in defensive behavior, but not attack behavior. Effects of LFS were not due to damage, as thresholds to evoke amygdala efferent response were unchanged. Other data suggest KLTP and depotentiation in right Amygdalo-PAG may reflect changes in glutamate receptor density/synapse number. Kindling effects on rat ALB persisted for at least 1month. Bilateral but not unilateral amygdala LFS reversed the effects of kindling on risk assessment in the plus maze for at least 3weeks. Bilateral LFS also reversed the effects of kindling on open arm exploration, but effects were shorter lived, appearing at 1day but not 3weeks after kindling and LFS. These findings are consistent with other studies which suggest that amygdala neuroplasticity in separable amygdala circuits mediates lasting changes in open arm avoidance and risk assessment. Taken together, the findings of both studies support the hypothesis that a form of LTP of specific amygdala circuits underlies lasting changes in affect produced by limbic kindling. Clinical implications of these findings are discussed.

Naloxone reverses disinhibitory/aggressive behavior in 5,7-DHT-lesioned rats; involvement of GABA(A) receptor blockade?

Effective medical treatment for impulsive aggression and several impulse control disorders is needed. Disinhibited, impulsive behavior of e.g. murderers, arsonists, suicidal patients, and patients suffering from antisocial personality or substance abuse disorders has been associated with signs of a deficiency in brain serotonin (5-HT) systems. Depletion of brain 5-HT consistently produces disinhibition and aggression also in experimental animals. The present series of experiments using a modified Vogel's conflict test indicates that the disinhibitory behavior of 5-HT-lesioned rats can be reversed by the commonly used opiate receptor antagonist naloxone at doses (0.1-5.0 mg/kg, s.c.) that do not significantly affect behavior in sham-lesioned controls. Moreover, this effect of naloxone, which resembles that previously observed after administration of negative modulators of gamma-aminobutyric acidA (GABA(A))/benzodiazepine receptor complexes, was reversed by a low inert dose (2.0 mg/kg, i.p.) of amobarbital. Furthermore, both naloxone (5.0 mg/kg, s.c.) and Ro 15-4513 (1.0 mg/kg, p.o.; a partial inverse agonist at benzodiazepine receptors) significantly decreased the number of attacks and the time spent in aggressive acts in 5,7-DHT-lesioned male residents. These results taken together with previous behavioral and neurochemical data suggest that the behavioral effects of naloxone observed here may involve an antagonistic action at brain gamma-aminobutyric acidA (GABA(A))/benzodiazepine receptor complexes. Thus, naloxone, its stable analogue naltrexone or other weak negative modulators of brain GABA(A)/benzodiazepine receptor complexes may represent a new pharmacological principle for the treatment of impulse control disorders.

Involvement of diazepam binding inhibitor and its fragment octadecaneuropeptide in social isolation stress-induced decrease in pentobarbital sleep in mice

Diazepam binding inhibitor (DBI) and its fragment, octadecaneuropeptide (ODN), are putative endogenous ligands for benzodiazepine (BZD) receptors and have been shown to act as an inverse BZD receptor agonist in the brain. A previous study suggested that the social isolation stress-induced decrease in pentobarbital sleep in mice was partly due to endogenous substances with an inverse BZD receptor agonist-like property. In this study, we examined the effects of DBI and ODN on pentobarbital sleep in group-housed and socially isolated mice to test the possible involvement of DBI and ODN in a social isolation-induced decrease in pentobarbital sleep. The socially isolated mice showed significantly shorter durations of pentobarbital (50 mg/kg, intraperitoneally, i. p.) sleep compared to the group-housed animals. When injected intracerebroventricularly (i.c.v.), DBI and ODN (3 and 10 nmol) dose-dependently shortened the pentobarbital-induced sleeping time in group-housed mice at the same dose range, but these peptides had no effect on the sleeping time in socially isolated animals. In contrast, flumazenil (16.5-33 nmol, i.c.v.), a BZD receptor antagonist, reversed the pentobarbital sleeping time in socially isolated mice to the level of group-housed animals without affecting the sleeping time in group-housed animals. The effects of DBI and ODN in group-housed mice were significantly blocked by flumazenil (33 nmol, i.c.v.). Moreover, the effect of flumazenil in socially isolated mice was significantly attenuated by DBI and ODN (10 nmol, i.c.v.). These results suggest that the changes in the activity of DBI and/or ODN are partly involved in the social isolation-induced decrease in the hypnotic action of pentobarbital in mice.

Evidence that NMDA-dependent limbic neural plasticity in the right hemisphere mediates pharmacological stressor (FG-7142)-induced lasting increases in anxiety-like behavior. Study 1--Role of NMDA receptors in efferent transmission from the cat amygdala

The anxiogenic beta-carboline, FG-7142, produces intense anxiety in humans and anxiety-like behavior in animals. FG-7142 also mimics the effects of exogenous stressors. In cats, FG-7142 lastingly changes defensive and aggressive behavior. Long-term potentiation (LTP) of neural transmission between limbic structures known to modulate feline defensive response to threat accompany behavioral changes. A series of three reports describes experiments designed to test the hypothesis that behavioral changes depend upon an N-methyl-D-aspartate (NMDA) receptor-based LTP of efferent transmission from the amygdala. This first study characterizes the dose and time effects of injection of the NMDA receptor blocker 7-amino-phosphono-heptanoic acid (AP7) on efferent transmission from the cat amygdala to the ventromedial hypothalamus (VMH). Effects of doses of 0.5-10mg/kg (i.v.) of AP7 on potentials evoked in the VMH by single pulse stimulation of the basal amygdala were examined. In order to localize the action of the drug, concurrent measurements were taken of potentials evoked in the VMH by stimulation of the efferent fibers from the amygdala to the VMH (ventral amygdalofugal pathway, VAF). There was a dose-dependent reduction in the amygdalo-VMH evoked potential. The greatest reduction occurred at 5 mg/kg. Effects peaked at 10 min, and persisted for at least 1 h after injection. In contrast, AP7 increased the VAF-VMH-evoked potential at 10 min after injection, with a maximal increase at 5mg/kg. The data suggest that NMDA receptors intrinsic to the amygdala modulate excitatory efferent transmission from amygdala to VMH in the cat. It is speculated that a glutamatergic projection to gamma-aminobutyric acid tonic inhibitory systems in the VMH accounts for the VAF-VMH results.

GABAergic deafferentation hypothesis of brain aging and Alzheimer's disease revisited

Considering the mechanisms responsible for age- and Alzheimer's disease (AD)-related neuronal degeneration, little attention was paid to the opposing relationships between the energy-rich phosphates, mainly the availability of the adenosine triphosphate (ATP), and the activity of the glutamic acid decarboxylase (GAD), the rate-limiting enzyme synthesizing the gamma-amino butyric acid (GABA). Here, it is postulated that in all neuronal phenotypes the declining ATP-mediated negative control of GABA synthesis gradually declines and results in age- and AD-related increases of GABA synthesis. The Ca2+-independent carrier-mediated GABA release interferes with Ca2+-dependent exocytotic release of all transmitter-modulators, because the interstitial (ambient) GABA acts on axonal preterminal and terminal varicosities endowed with depolarizing GABA(A)-benzodiazepine receptors; this makes GABA the "executor" of virtually all age- and AD-related neurodegenerative processes. Such a role of GABA is diametrically opposite to that in the perinatal phase, when the carrier-mediated GABA release, acting on GABA(A)/chloride ionophore receptors, positively controls chemotactic migration of neuronal precursor cells, has trophic actions and initiates synaptogenesis, thereby enabling retrograde axonal transport of target produced factors that trigger differentiation of neuronal phenotypes. However, with advancing age, and prematurely in AD, the declining mitochondrial ATP synthesis unleashes GABA synthesis, and its carrier-mediated release blocks Ca2+-dependent exocytotic release of all transmitter-modulators, leading to dystrophy of chronically depolarized axon terminals and block of retrograde transport of target-produced trophins, causing "starvation" and death of neuronal somata. The above scenario is consistent with the following observations: 1) a 10-month daily administration to aging rats of the GABA-chloride ionophore antagonist, pentylenetetrazol, or of the BDZ antagonist, flumazenil (FL), each forestalls the age-related decline in cognitive functions and losses of hippocampal neurons; 2) the brains of aging rats, relative to young animals, and the postmortem brains of AD patients, relative to age-matched controls, show up to two-fold increases in GABA synthesis; 3) the aging humans and those showing symptoms of AD, as well as the aging nonhuman primates and rodents--all show in the forebrain dystrophic axonal varicosities, losses of transmitter vesicles, and swollen mitochondria. These markers, currently regarded as the earliest signs of aging and AD, can be reproduced in vitro cell cultures by 1 microM GABA; the development of these markers can be prevented by substituting Cl- with SO4(2-); 4) the extrasynaptic GABA suppresses the membrane Na+, K+-ATPase and ion pumping, while the resulting depolarization of soma-dendrites relieves the "protective" voltage-dependent Mg2+ control of the N-methyl-D-aspartate (NMDA) channels, thereby enabling Ca2+-dependent persistent toxic actions of the excitatory amino acids (EAA); and 5) in whole-cell patch-clamp recording from neurons of aging rats, relative to young rats, the application of 3 microM GABA, causes twofold increases in the whole-cell membrane Cl- conductances and a loss of the physiologically important neuronal ability to desensitize to repeated GABA applications. These age-related alterations in neuronal membrane functions are amplified by 150% in the presence of agonists of BDZ recognition sites located on GABA receptor. The GABA deafferentation hypothesis also accounts for the age- and AD-related degeneration in the forebrain ascending cholinergic, glutamatergic, and the ascending mesencephalic monoaminergic system, despite that the latter, to foster the distribution-utilization of locally produced trophins, evolved syncytium-like connectivities among neuronal somata, axon collaterals, and dendrites, to bidirectionally transport trophins. (ABSTRACT TRUNCATED)

Transmitter systems involved in neural plasticity underlying increased anxiety and defense--implications for understanding anxiety following traumatic stress

Lasting changes in anxiety-like behavior (ALB) may be produced in several ways. These include partial limbic kindling, injection of the beta-carboline FG-7142, and brief, non-injurious, exposure of rodents to cats (predator stress). Both seizures and FG-7142 induce long-term potentiation (LTP) in efferent pathways of the amygdala known to participate in feline defensive behavior. By comparing the behavioral and physiological effects of partial kindling and injection of FG-7142, NMDA-dependent LTP in the right amygdalo-periacqueductal gray (PAG) pathway emerges as being critical to maintained increases in feline ALB. A similar dependence on NMDA-mediated processes is described for lasting increases in rodent ALB following predator stress. The lasting aftereffects of predator stress on a variety of measures parallel many of the symptoms of post-traumatic stress disorder (PTSD). Support is provided for the idea that behavioral changes following FG-7142 and predator stress may model anxiety associated with PTSD. Moreover, it is suggested that both models share mechanisms in common involving the PAG. These mechanisms likely involve initiation of LTP by NMDA receptors, and prolongation of LTP by CCKB receptors. To the extent that response to the stressors reviewed here mimics the symptoms of PTSD, the data implicate NMDA-mediated processes in the creation of what van der Kolk has called permanent emotional memories in PTSD. Their representation may be in the form of NMDA-dependent LTP of transmission within the amygdala and between the amygdala and its efferents. CCK may play a pivotal role in prolonging limbic LTP and anxiety following traumatic stress. Since block of CCKB receptors before and after the stressor prevents lasting increases in ALB, pharmacological intervention to block CCK receptors shortly after a traumatic stressor might be efficacious in mitigating the permanence of these emotional memories.

Evaluation of exposure level of N-methyl-beta-carboline-3-carboxamide (FG 7142), an anxiogenic agent in humans

beta-Carboline-3-carboxylic acid methylamide (FG 7142), an anxiogenic agent, has been detected in human urine. The urinary excretion level of FG 7142 in non-smokers was found to be 0.503 +/- 0.25 ng per day (mean +/- S.D., n=10), while that in smokers was found to be 2.418 +/- 0.384 ng per day (n=10). This suggests that humans are exposed to FG 7142 and that smokers are exposed to higher levels of FG 7142 than non-smokers. Considering the previous findings that FG 7142 is present in cigarette smoke, indoor and outdoor air and smoke condensate of tree leaves, humans may be exposed to FG 7142 via the airways and lungs, although we cannot exclude the possibility of exposure via foodstuffs. Results of animal experiments suggest that FG 7142 which enters the bloodstream is excreted into the urine via the kidneys within 24 h in an unchanged form. Therefore, monitoring the level of FG 7142 in urine may be useful for monitoring the exposure level of FG 7142.

Stimulation of cortical acetylcholine efflux by FG 7142 measured with repeated microdialysis sampling

The effects of the benzodiazepine receptor partial inverse agonist beta-carboline FG 7142 on cortical ACh efflux were determined using in vivo microdialysis in freely-moving rats. Additionally, a within-subjects, repeated-dialysis experimental design (four microdialysis sessions; removable dialysis probe) was evaluated as a method for measuring changes in basal and FG 7142-stimulated ACh efflux in the frontoparietal cortex. FG 7142 (4.0, 8.0, and 16.0 mg/kg) produced a 150-470% increase in cortical ACh efflux, with a dose-dependent effect on the duration of the increase in efflux. Basal cortical ACh efflux was lower in session 4 than in session 1. However, the ability of FG 7142 to stimulate efflux was unchanged by repeated dialysis testing. The ability of tetrodotoxin (1.0 microM) to suppress both basal and FG 7142-stimulated ACh efflux was also unaffected by repeated dialysis testing. These results demonstrate that systemically administered benzodiazepine receptor inverse agonists stimulate cortical ACh efflux, and that repeated-measures experimental designs can be valid for determining certain changes in cortical ACh efflux with in vivo microdialysis.

Functional diversity of GABAA receptor ligand-gated chloride channels in rat synaptoneurosomes

Experiments were performed to examine neurochemically the functional diversity of GABAA receptors as measured by muscimol-, 5 alpha-pregnane-3 alpha,21-diol-20-one (THDOC)-, and pentobarbital-stimulated 36Cl- uptake, and region-specific changes in muscimol-, THDOC- and THDOC-induced potentiation of muscimol-stimulated 36Cl- uptake in rats treated acutely or subacutely with a subconvulsive dose of bicuculline. The data for stimulation of 36Cl- uptake by muscimol showed a single binding site interaction in the cerebral cortex, hippocampus and cerebellum. The concentration-response curves for muscimol in the cerebral cortex and hippocampus were steep and indicated an increase of approximately 130% at the maximum concentration. In contrast, the curve for the cerebellum was shallow and exhibited a smaller maximal response (approximately 60%). Apparent affinity for muscimol also differed among these brain regions. The regional differences in 36Cl- uptake induced by THDOC and pentobarbital were not as apparent as those induced by muscimol; however, the maximal modulatory effect of pentobarbital in the hippocampus was significantly higher than that in the cerebellum. In rats treated subacutely with a subconvulsive dose of bicuculline, a significant increase in muscimol-stimulated 36Cl- uptake was observed in the cerebellum but not in the frontal cortex or hippocampus. Analysis of the concentration-response curves for muscimol-stimulated 36Cl- uptake in the cerebellum revealed that the Vmax for muscimol in the subacutely treated group was significantly higher than those for muscimol in the control and acutely treated groups without any differences in the KD value.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of flumazenil increases life span and protects rats from age-related loss of cognitive functions: a benzodiazepine/GABAergic hypothesis of brain aging

Under barrier condition and with ad lib access to food and water, 20 Fischer-344 rats were chronically treated for 10 months with the benzodiazepine (BDZ) antagonist, flumazenil (FL; 4 mg/kg/day in drinking water acidified to pH = 3.0), beginning at the age of 13 months, while the group of 20 control age-matched rats received plain acidified water. The life span of the first 8 deceased rats treated with FL was significantly longer than that of the first 8 deceased rats in the age-matched control group. In tests for spontaneous ambulation and exploratory behavior in the Holeboard apparatus, conducted during the 3rd and the 8th month of treatment, the FL group, relative to controls, had significantly higher scores for the ambulation and exploratory behavior. In tests for unrewarded spontaneous alternation in the T maze, conducted at days 7, 39, 42, and 47 through 54 after drug withdrawal, i.e., at the age of 24-25 months, the FL-exposed group, compared to age-matched controls, showed a significantly higher percent of alternating choices, a behavior that was statistically comparable to that of the "young" 6-month-old controls. In the Radial Maze tests conducted 2 months after drug withdrawal, the FL group made significantly less "working memory" errors and "reference memory" errors, relative to the age-matched 25-month-old control group, a performance that was comparable to that of the young 7-month-old control group. In conclusion, chronic FL significantly protected rats from age-related loss of cognitive functions. It is postulated that the age-related alterations in brain function may be attributable to the negative metabolic/trophic influences of the "endogenous" benzodiazepine (BDZ) ligands and/or those ingested with food. A BDZ/GABAergic hypothesis of brain aging has been formulated which assumes that age-related and abnormally strong BDZ/GABAergic influences promote neurodegeneration by suppressing trophic functions of the aminergic and peptidergic neurons through opening of chloride channels in soma membrane and axon terminals, causing excessive hyperpolarizing and depolarizing inhibition, respectively. The review of human clinical and animal data indicates that FL has nootropic actions by enhancing vigilance cognitive and habituation processes.

Kindling to the benzodiazepine receptor inverse agonist, FG 7142: evidence for involvement of NMDA, but not non-NMDA, glutamatergic receptors

Repeated administration of the beta-carboline FG 7142 to mice leads to the development of kindled convulsions. In order to investigate a role for glutamatergic mechanisms in the processes underlying FG 7142 kindling, the N-methyl-D-aspartate (NMDA) antagonist, 2-amino-7-phosphono-heptanoic acid (AP7; 25 nmol), was administered intracerebroventricularly (i.c.v.) daily before administration of FG 7142 (40 mg/kg, i.p.). Under these conditions, kindling to FG 7142 did not occur. Administration of two antagonists at non-NMDA excitatory amino acid receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and gamma-D-glutamylaminomethylsulphonic acid (gamma-D-GAMS; both 25 nmol) did not prevent the development of seizures; these doses were, however, adequate and selective in protecting against seizures induced by respectively quisqualic and kainic acids given by i.c.v. The susceptibility of mice kindled with FG 7142 to seizures induced by NMDA, or kainate or quisqualate was similar in mice which had shown 5 kindled seizures to that seen in drug-naive mice; mice which had shown 10 kindled seizures showed a decreased sensitivity to NMDA-induced convulsions (ED50 was increased from 0.24 to 0.31 nmol). No changes were seen in the convulsant thresholds of either NMDA or non-NMDA agonists. These observations suggest that although NMDA receptors appear to be involved in the processes underlying FG 7142 kindling, such kindling is not necessarily associated with an increased sensitivity of glutamate receptors, and in animals which have convulsed, a decreased sensitivity to NMDA agonists occurs.

FG7142 causes opposite changes in [3H]GABA release from nigrocollicular regions

The activity of GABAergic neurons projecting from the striatum to the substantia nigra (SN) and from the SN to the superior colliculus (SC) may be involved in regulating seizure sensitivity such that striatonigral transmission is decreased and nigrocollicular transmission is increased in proconvulsant states. To test whether these changes occur in FG7142-treated rats, GABA transmission was assessed by measuring [3H]GABA release from superfused slices of the SN and SC and measuring [35S]TBPS binding to GABAA receptors throughout the brain. Nine daily injections of FG7142 (30 mg/kg IP) greatly increased myoclonic seizures in about one half of the animals. These animals exhibited a decrease in stimulated [3H]GABA release from the SN and an increase in both basal and stimulated release from the SC. Animals that were less sensitive to FG7142 treatment also had increased collicular release but not decreased nigral release. [35S]TBPS binding was unchanged by FG7142 treatment. Thus, decreased nigral GABA release may contribute to decreased striatonigral transmission after seizure occurrence whereas increased collicular GABA release may contribute to increased nigrocollicular transmission preceding multiple-seizure occurrence.

Lasting effects of FG-7142 on anxiety, aggression and limbic physiology in the cat

The effect of the anxiogenic β-carboline, FG-7142, on behavior and limbic physiology was investigated. A single systemic injection of FG-7142 (10 mg/kg) changed behavior for at least 43 days. Two days after drug administration, defensive response to rats was increased. In addition, predatory attack was suppressed in some, but not all, animals. Cats that were more predatory (killers, n=2) before drug administration showed suppression of predatory attack after FG-7142. The attack of less predatory cats (non-killers, n=3) was unaffected by FG-7142. Suppression of attack was not correlated with changes in defense. This finding suggests FG-7142 acts on separate substrates of defense and attack suppression to change these behaviors lastingly. FG-7142 produced long-term potentiation (LTP) in two of three limbic pathways investigated. LTP was observed in the amygdalo-ventromedial hypothalamic (AM-VMH) pathway. AM-VMH LTP depended on changes within the amygdala and not in the efferent synapses or in the VMH. LTP lasted 6 days, returning to baseline by 21 days after FG-7142. The only behavioral change correlated with AM-VMH LTP was defensive response at 2 days after FG-7142. Increased defense from 6 to 43 days after the drug was not correlated with AM-VMH LTP. Therefore, AM-VMH LTP may be necessary for the initiation, but not maintenance, of increased defensive response. LTP of the population spike of the perforant path-CA3 (PP-CA3) field potential in the ventral hippocampus was also seen. LTP appeared 6 days after drug administration, after cats had been exposed to rats. Before the appearance of PP-CA3 LTP, there was a transient failure of recurrent inhibition in area CA3. It is likely that exposure to a rat during the period of failed inhibition facilitated the PP-CA3 LTP. In addition, following FG-7142, facilitation in area CA3 increased lastingly, consistent with a lasting increase in the duration of excitatory neurotransmitter release. Changes in hippocampal inhibition and facilitation correlated with changes in attack behavior, but not with changes in defensive response. Systemic flumazenil (10 mg/kg) partially reversed the lasting changes in defense and approach-attack in a drug-dependent manner. Limbic physiology was unchanged by flumazenil. Flumazenil probably did not affect AM-VMH transmission in the present study because AM-VMH LTP had returned to baseline when flumazenil was given. These findings suggest flumazenil only acts on potentiated AM-VMH transmission.

Chronic administration of flumazenil (Ro 15-1788) enhances non-appetitive exploratory behavior of rats

The effects of chronic administration of the benzodiazepine receptor antagonist, flumazenil (Ro 15-1788; 4 mg/kg/day for 14 days in drinking water) on the performance of adult rats in the 12-arm radial maze were studied. Relative to controls, the animals treated with flumazenil showed an increase (P less than 0.002) in non-appetitively motived exploratory behavior, so called because it occurred in 88% of instances in non-baited alleys, facing the well-illuminated "enriched environment" of the center of the room, as opposed to the baited alleys, facing the "dull" corner of the room. This behavior emerged between day 5 and 7 of treatment with the drug, it continued to increase over the period of treatment with drug (P less than 0.002), and reached its peak at day 3, after withdrawal of the drug (P less than 0.008; a longer duration was not investigated). The occurrence of non-appetitively motivated exploratory behavior was inversely correlated with the scores for urination/defecation (P less than 0.003) and, therefore, most likely reflected the anxiolytic action of flumazenil. During treatment with drug or vehicle, the control and the drug groups made comparable numbers of "working memory" errors (P = 0.17). However, upon withdrawal of drug and introduction of alley gates (to confine the animal for 10 sec to the center platform, after an alley was explored), the working memory errors of the rats exposed to the drug, remained unchanged (P = 0.35), relative to the preceding three trials, while the performance of the control group was disrupted, as shown by an increase in the numbers of errors (P less than 0.004). At day seven of treatment with drug, the emergence of exploratory behavior was associated with an increased density and/or affinity of benzodiazepine receptors in cortex, hippocampus and brain stem, while three days after withdrawal of drug, when the exploratory behavior reached its peak, there was a reduction in GABA-enhanced binding of [3H]flunitrazepam in the cortex.

Augmentation of GABAA receptor function by chronic exposure to GABA-neutral and GABA-negative benzodiazepine ligands in cultured cortical neurons

Chronic benzodiazepine agonist administration may lead to decreases in gamma-aminobutyric acidA (GABAA) receptor binding and function, but little information is available concerning chronic GABA-neutral or GABA-negative benzodiazepine exposure. We evaluated effects of chronic exposure to flumazenil (Ro15-1788) and FG 7142 (N-methyl-beta-carboline-3-carboxamide) on GABA-dependent chloride uptake in chick cerebral cortical neurons in primary culture. Acute flumazenil treatment (1 microM) had no effect on chloride uptake, but uptake was increased after 2 days of exposure. Similar increases were observed after 4 and 10 days. Flumazenil, 0.1 microM, had no effect after 10 days, and a 10 microM concentration had a similar effect as the 1 microM concentration. Acute FG 7142 (1 microM) decreased chloride uptake, but uptake was increased markedly after 2, 4, and 10 days of treatment. No effect was observed after treatment for 10 days with 0.1 microM, but a 10 microM concentration showed similar enhancement to the 1 microM concentration. Concurrent treatment with 0.3 microM flumazenil which did not affect chloride uptake and 1 microM FG 7142 for 10 days substantially attenuated the effects of FG 7142, suggesting that FG 7142 effects are mediated at the benzodiazepine site. Benzodiazepine receptor binding was increased in cultures treated for 10 days with 1 microM flumazenil or FG 7142, with an increase in receptor number in both cases but no change in apparent affinity. Neither flumazenil nor FG 7142 (1 microM for 10 days) altered GABA-independent chloride uptake, total cellular protein, protein synthesis or degradation, or neuronal survival. These results indicate that both chronic GABA-neutral and GABA-negative benzodiazepine exposures in cultured cortical neurons lead to increases in GABA-dependent chloride uptake and benzodiazepine binding. Effects of GABA-negative benzodiazepine exposure appear to be greater than those observed with GABA-neutral benzodiazepine exposure.

GABAergic and dopaminergic transmission in the rat cerebral cortex: effect of stress, anxiolytic and anxiogenic drugs

Benzodiazepines produce their pharmacological effects by regulating the interaction of GABA with its recognition site on the GABAA receptor complex. In fact, the anxiolytic effect of benzodiazepines may be considered the consequence of the activation of the GABAA receptors induced by these drugs. On the contrary, beta-carboline derivatives which bind with high affinity to benzodiazepine recognition sites modulate the GABAergic transmission in a manner opposite to that of benzodiazepines. Thus, these compounds reduce the function of the GABA-coupled chloride channel and produce pharmacological effects (anxiogenic, proconvulsant and convulsant) opposite to those of benzodiazepines. Taken together, these data strongly indicate that the GABAA receptor complex plays a major role in the pharmacology, neurochemistry and physiopathology of stress and anxiety. This conclusion is further supported by the finding that the function of the GABAA/benzodiazepine receptor complex may be modified by the emotional state of the animals before sacrifice. Accordingly, using an unstressed animal model, the 'handling-habituated' rats, it has been demonstrated that stress, like anxiogenic drugs, decreases the function of GABAA receptor complex, an effect mimicked by the in vivo administration of different inhibitors of GABAergic transmission and antagonized by anxiolytic benzodiazepines. Moreover, a long-lasting down regulation of GABAergic synapses can be obtained after repeated administration of anxiogenic, proconvulsant and convulsant negative modulators of GABAergic transmission. The latter finding further suggests that GABAergic synapses undergo rapid and persistent plastic changes when the GABAergic transmission is persistently inhibited. Finally, the evidence that the activity of mesocortical dopaminergic pathways is altered in opposite manner by drugs that either inhibit or enhance the GABAergic transmission indicates that GABA has a functional role in regulation of dopaminergic neurons in the rat cerebral cortex. Altogether these results suggest that cortical GABAergic and dopaminergic transmission play a major role in the pharmacology, neurochemistry and pathology of the emotional states and fear.

Drug discrimination models in anxiety and depression

Drug discrimination is a technique for investigating the stimulus properties of centrally active drugs. Although many studies have employed animals to investigate the stimulus properties of substances used clinically for the treatment of anxiety and depression, it would be a mistake to consider the internal discriminative stimuli as being related specifically to the anxiolytic or antidepressant properties of these drugs. Rather drug cues are better considered as relating to the pharmacological action of classes of compounds. Thus, benzodiazepine cues generalize to other compounds acting at benzodiazepine receptors, but not to substances (anxiolytic or otherwise) acting at 5-HT1A receptors. Similarly, antidepressants with different pharmacological properties, for example the tricyclic imipramine, or the phenylaminoketone buproprion produce distinct, unrelated discriminative stimuli. For this reason, the limits of drug discrimination techniques for investigating novel anxiolytic or antidepressant drugs should be clearly recognized. Attempts to identify an anxiogenic discriminative stimulus using pentylenetetrazole have also been misguided. In this technique it has proven difficult to separate unequivocally the pharmacological proconvulsant effects of the drug from the psychological construct anxiety. Nevertheless, drug discrimination remains a valuable technique for investigating pharmacological interactions in animals and man.

Behavioral differentiation of benzodiazepine ligands after repeated administration in baboons

Baboons received repeated daily administration of saline, 5.6 mg/kg midazolam, 5.0 mg/kg flumazenil (Ro15-1788), 3.2 mg/kg 3-carboethoxy-beta-carboline hydrochloride (beta CCE) or 10 mg/kg beta CCE for 5 days. Behavioral signs of sedation and excitation were scored for 1 h after i.m. injections. Daily administration of these benzodiazepine-receptor ligands differentiated their behavioral effects; repeated midazolam resulted in tolerance to the sedative and ataxic effects; repeated beta CCE resulted in sensitization to its convulsant properties; and repeated flumazenil or saline produced no changes in behavior. In a second study, baboons received repeated injections of midazolam (5.6, 11.2 or 20 mg/kg per day) for 6 days. All three groups became tolerant to the sedative and ataxic effects of midazolam. Acute injections of flumazenil (5.0 mg/kg) on day 5 produced a dose-dependent withdrawal syndrome. This flumazenil treatment produced a slight attenuation in the degree of tolerance to midazolam on day 6, suggesting that receptor sensitivity to the benzodiazepine agonist may have partially reset.

Blockade of FG 7142 kindling by anticonvulsants acting at sites distant from the benzodiazepine receptor

Repeated administration of the beta-carboline FG 7142 results in sensitisation to its convulsant effects (chemical kindling); acutely FG 7142 is not convulsant, but following several treatments full seizures develop. It has been suggested that the increased sensitivity results from changes in benzodiazepine (BZ)/GABA receptor function. The present experiments studied the ability of BZ receptor ligands and anticonvulsant drugs with diverse mechanisms of action to block the expression and development of kindling to once daily injection of FG 7142 (40 mg/kg, i.p.) in mice. In fully kindled mice, the BZ receptor agonists clonazepam, ZK 93,423 and CL 218,872, and the antagonists flumazenil and ZK 93,426 prevented FG 7142 convulsions, as did 2 anticonvulsants, sodium valproate, possibly acting by influencing GABAergic transmission, and ethosuximide. A further two substances, MK 801 and 2-chloradenosine which act respectively via glutamatergic and purinergic mechanisms were also effective. When administered concomitantly with repeated FG 7142, all of these substances prevented or strongly reduced the development of kindling. Phenytoin and carbamazepine were ineffective in protecting against FG 7142 convulsions in kindled mice, and in preventing the development of kindling when administered repeatedly together with FG 7142. Since MK 801 and 2-chloradenosine prevented kindling, these results suggest that an interaction of FG 7142 with BZ receptors is not sufficient to induce kindling, which may instead result from secondary changes in sites distant from BZ/GABA receptors.

Diazepam sensitizes mice to FG 7142 and reduces muscimol-stimulated 36Cl- flux

Chronic treatment with benzodiazepine receptor agonists increases sensitivity to the convulsant action of FG 7142, an inverse agonist. We investigated whether or not changes in the number and function of GABA-gated chloride channels accompanies this increased sensitivity. Diazepam, 5 mg.kg-1, was administered to mice daily for five days, and mice were then tested with a single injection of FG 7142, 40 mg.kg-1, at several intervals thereafter. At 24 hours after the last diazepam dose, 10 of 15 mice had clonic seizures following FG 7142 and four of the remaining five had myoclonic jerks. At 48 hours, only one of six mice developed a clonic seizure, and none were observed in mice tested at 96 or 144 hours. Muscimol-stimulated chloride flux was reduced in cortical synaptosomes from diazepam-treated mice at 24 hours but not at 48 or 96 hours. However, the binding of [35S]TBPS, a ligand closely associated with the chloride channel, was unchanged at 24 hours. These results suggest that a transient diminution in GABA-gated chloride channel function; unaccompanied by a reduction in channel number, may underlie the sensitization to the convulsant action of FG 7142 observed after withdrawal from chronic diazepam treatment.

Chemical kindling decreases GABA-activated chloride channels of mouse brain

The repeated administration of N-methyl-beta-carboline-3-carboxamide (FG 7142) to mice leads to 'chemical kindling', i.e. the development of seizures in response to doses which were initially insufficient to produce convulsive activity. To determine if chemical kindling produced changes in the GABAA receptor/chloride channel complex, we measured the binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) to the convulsant site of the complex by quantitative autoradiography. As a measure of chloride channel function, we studied muscimol-stimulated uptake of 36Cl- by isolated brain synaptosomes. Kindling decreased the Bmax of [35S]TBPS binding in cortex but not in cerebellum or hippocampus. Kindling did not alter binding affinities in any of these brain regions. Some mice injected with FG 7142 did not kindle despite receiving the same treatment as kindled mice. These 'injected but not kindled' mice did not display decreased receptor binding in any of these brain areas. Muscimol-stimulated 36Cl- uptake into cortical synaptosomes was also diminished by chemical kindling. These findings suggest that a decrease in functioning GABA-regulated chloride channels may be responsible for chemical kindling with FG 7142.

Bidirectional effects of beta-carbolines in reflex epilepsy

Derivatives of ethyl-beta-carboline-3-carboxylate, ZK 91296, ZK 93423 and ZK 95962 have potent anticonvulsant activity against sound-induced seizures in audiogenic DBA/2 mice and against photically-induced seizures in the baboon, Papio papio. The convulsant beta-carbolines, DMCM and beta-CCM, have proconvulsant and convulsant activity in the same animal models. DMCM and beta-CCM are similar in potency as convulsants in DBA/2 mice (ED50 value for DMCM: 1.3 mg/kg; ED50 value for beta-CCM; 0.8 mg/kg), but differ with respect to their profiles for protection by anticonvulsant drugs. The anticonvulsant potencies of diazepam and clobazam are similar against both types of beta-carboline-induced seizures, whereas quazepam protects better against beta-CCM seizures (4 fold elevation in ED50 value at 1 mg/kg quazepam IP) than against DMCM seizures (1.7 fold elevation in ED50 value), supporting a preferential action of beta-CCM on BZ1 receptors. Valproate (400 mg/kg) and gamma-vinyl-GABA (1.5 g/kg) protect better against beta-CCM seizures (9.5 and 5.9 fold elevations in ED50 values respectively) than against DMCM seizures (1.8 and 2.7 fold elevations in ED50 values respectively). The excitatory amino acid antagonist, 2-amino-7-phosphonoheptanoic acid, has significant anticonvulsant activity against DMCM seizures. The elevated regional GABA levels in brains of DBA/2 mice observed during beta-CCM seizures are eliminated by the pretreatment with Ro 15-1788, which also blocks the seizure activity.