GABAA receptor modulation of memory: the role of endogenous benzodiazepines

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.

Naturally occurring benzodiazepines and benzodiazepine-like molecules in brain

Great progress has been made in the last five years in demonstrating the presence of benzodiazepines (BZDs) in mammalian tissues, in beginning studies on the origin of these natural compounds and in elucidating their possible biological roles. Many unanswered questions remain regarding the sources and the biosynthetic pathways responsible for the presence of BZDs in brain and their different physiological and/or biochemical actions. This essay will focus on recent findings supporting that: (1) BZDs are of natural origin; (2) mammalian brain contains BZDs in concentrations ranging between 5.10(-10) to 10(-8) M; (3) BZDs and BZD-like molecules are unevenly distributed in brain; the highest concentration is found in limbic structures (4) dietary source of BZDs might be a plausible explanation for their occurrence in animal tissues, including man; (5) the formation of BZDs-like molecules in brain is a possibility, experimentally supported; (6) BZDs like molecules including diazepam and N desmethyldiazepam are elevated in hepatic encephalopathy; (7) natural BZDs in the brain are involved in the modulation of memory processes. Future studies using the full range of biochemical, physiological, behavioral and molecular biological techniques available to the neuroscientist will hopefully continue to yield new and exciting information concerning the biological roles that BZDs might play in the normal and pathological functioning of the brain.

Memory processing by the limbic system: role of specific neurotransmitter systems

Experiments using localized infusions into selected brain structures of agonists and antagonists of various synaptic receptors, given before or after behavioral training, have led to the following conclusions: (1) Memory is processed shortly after training in the amygdala, medial septum and hippocampus by glutamatergic NMDA and AMPA receptors activated in that sequence. Cholinergic muscarinic receptors are activated concurrently with the former. GABAA receptors modulated by brain benzodiazepines and by beta-noradrenergic receptors inhibit the process. (2) The sequential involvement of NMDA and AMPA receptors suggests that long-term potentiation (LTP) of the synapses activated by the learning experiences in the hippocampus and/or amygdala and medial septum is the crucial event. Expression of this LTP at the time of testing is necessary for retrieval: AMPA receptor blockade in the hippocampus and amygdala at the time of testing hinders retrieval. This suggests that the LTP underlies the memory process itself. (3) The amygdala, medial septum and hippocampus mediate different types of memory and/or different components of memories. The entorhinal cortex, through mechanisms that require intact NMDA receptors and are inhibited by GABAA receptors, intervenes in post-training memory processing 90-180 min after the other limbic regions. The entorhinal cortex integrates consecutively acquired memories; this role could be maintained by the LTP that is generated after training in the amygdala, hippocampus and medial septum. Post-training intervention of the entorhinal cortex does not occur if this region is inhibited at the time of training.

Differential usage of multiple brain-derived neurotrophic factor promoters in the rat brain following neuronal activation

The rat brain-derived neurotropic factor (BDNF) gene consists of four 5' exons linked to separate promoters and one 3' exon encoding the prepro-BDNF protein. To gain insights into the regulation of BDNF mRNA expression, probes specific for the different 5' exons were used to study the expression of BDNF mRNA in the brain. Following a systemic injection of the glutamate analog kainic acid, exon I, II, and III mRNAs increased transiently in hippocampus and cerebral cortex. A modest increase was seen for exon IV, where a new transcription initiation site was induced by this treatment. Pretreatments with the N-methyl-D-aspartate (NMDA) receptor antagonist MK801 or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 2,3-dihydroxy-6-nitrosulfanoylbenzo(f)quinoxaline revealed two region-specific patterns of glutamate receptor-mediated regulation. The first pattern found in neocortex, piriform cortex, and amygdala involves regulation of BDNF exon I, II, and III mRNAs through NMDA and AMPA/kainate receptors. The second pattern found in the hippocampus involves regulation of BDNF exon I, II, and III mRNAs by high-affinity kainate or metabotropic receptors. Treatment with the gamma-aminobutyric acid subtype A (GABAA) receptor antagonist bicuculline increased exon I and III mRNAs in the denate gyrus, and the muscarinic receptor agonist pilocarpine increased exon I mRNA mainly in the neocortex. These data show that the four BDNF promoters allow multiple points of BDNF mRNA regulation and suggest that the activation of different subtypes of glutamate receptors differentially regulates the expression of BDNF exon-specific mRNAs in the brain.

Melatonin as an anxiolytic in rats: time dependence and interaction with the central GABAergic system

Anxiolytic and pro-exploratory melatonin properties were assessed in rats using a plus-maze procedure. Both melatonin (1 mg/kg) and diazepam (0.5 mg/kg) showed a significant diurnal variation to decrease anxiety and to promote exploratory behavior. Melatonin displayed anxiolytic activity at night, with absence of effects at noon and a weak activity at the beginning of the light phase. Melatonin pro-exploratory activity was found only at night. Diazepam exerted significant anxiolysis during the night, with less activity during the day. Diazepam pro-exploratory activity was found during the night only. A dose-response study carried out by injecting 1-20 mg/kg melatonin at 12:00 or 18:00 h indicated that melatonin activity was greatest at 18:00 h. Diazepam was anxiolytic at both times, and pro-exploratory at 18:00 h only. Melatonin activity was blunted by administration of the benzodiazepine antagonist, flumazenil.

Limits of habituation and extinction: implications for relapse prevention programs in addictions

Problems in the application of exposure techniques to the management of long term dishabituation in addicts are discussed in the light of human and animal evidence. Extinction and habituation of responses to drug cues or drug aftereffects are unstable and strongly dependent on context, thus limiting the effectiveness of cue exposure treatments in the prevention of relapse. Several strategies are suggested to improve the stability of extinction and habituation in order to enduringly prevent relapse in addictions. (i) Warning patients about the episodic resurgence of unexpected urges or cravings precipitated by conditioned contexts and exposing them to such contexts. (ii) To obtain a maximum protection against relapse, extinction should 'recreate' all the original learning contexts (i.e. all possible drug cues). (iii) The behavioral chains involved in self administering drugs ought to be incorporated into cue exposure treatments (without permitting consummatory responses) in order to decrease their signal value as cues for drugs.

Cholecystokinin receptors and memory: a radial maze study

CCK receptor agonists and antagonists have repeatedly been demonstrated to improve and impair, respectively, learning and memory functions. However, all studies to date have exploited avoidance paradigms. In the present study, the effect of some CCK receptor agonists and antagonists on the ability to learn an appetitively motivated task and to influence spatial working memory was investigated. In the first experiment, drugs were given immediately after each training session in the radial maze and the animals were tested, drug-free, during a 2-week period. After the initial treatments with caerulein, an unselective CCK receptor agonist (100 ng/kg SC), the animals were slightly less successful to obtain food pellets during the sessions on the first 2 days; whereas proglumide, an unselective CCK receptor antagonist (1 mg/kg SC) was without any effect. However, on the following days, all the three groups of rats (saline, caerulein, and proglumide) performed in a similar way. In the second experiment, drugs were given before each test session to well-trained animals. Scopolamine (0.15 and 0.3 mg/kg IP), the reference amnestic drug, produced dose-dependent impairment of working memory in the radial maze test. Proglumide (1 and 10 mg/kg SC) and devazepide, (a selective CCK-A receptor antagonist; 0.01 and 1 mg/kg SC), as well as caerulein (0.01, 0.1 and 1 microgram/kg SC) and CCK-4 (a selective CCK-B receptor agonist; 25 and 50 micrograms/kg SC) had no reliable effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzodiazepine inverse agonists augment long-term potentiation in CA1 and CA3 of guinea pig hippocampal slices

The effects of benzodiazepine inverse agonists on the long-term potentiation of synaptic transmission in hippocampal slices of the guinea pig were examined using an extracellular recording technique. Benzodiazepine inverse agonists, beta-carboline-3-carboxylate (beta-CCE), 2-phenylpyrazolo [4,3-c]quinolin-3(5H)-one (CGS-8216) and 2-[5-methylthien-3-yl]-2,5-dihydro-3H-pyrazolo [4,3-c]quinolin-3-one (S-135), augmented the magnitude of long-term potentiation induced by tetanic stimulation of input fibers in both the CA1 and the CA3 regions. beta-Carboline-3-carboxylate was more effective in augmenting long-term potentiation in CA1 than in CA3. Augmentation of long-term potentiation produced by beta-CCE was antagonized by concomitant application of flumazenil, a benzodiazepine receptor antagonist. Therefore, the enhancing action of benzodiazepine inverse agonists on long-term potentiation, which is suggested to be a specific action, mediated by the GABA/benzodiazepine receptor complex, might help to explain the mechanism of the memory-enhancing effects of benzodiazepine inverse agonists, observed in some in vivo behavioral paradigms.

Chronic brief restraint decreases in vivo binding of benzodiazepine receptor ligand to mouse brain

This study examines the effects of chronic brief restraint on in vivo benzodiazepine (BZD) receptor binding in mouse brain. Three groups of mice were used. Mice in group 1 were neither restrained nor injected (ACUTE control). Mice in group 2 were restrained for 5-6 s by grabbing the back skin and holding the subject upside-down at a 45 degrees angle as if to be injected (CHRONIC SHAM control) for 7 d. Mice in group 3 (CHRONIC SALINE) received daily single intraperitoneal (ip) injections of saline (5 mL/kg) for 7 d. On d 8 BZD receptors were labeled in vivo by administration of 3 microCi [3H]flumazenil (ip). The levels of ligand bound in vivo to cerebral cortex (CX), cerebellum (CB), brain stem (BS), striatum (ST), hippocampus (HP), and hypothalamus (HY) were determined. Results indicated that the level of binding was significantly (p < 0.01) lower by 30-50% (depending on the brain region) in saline-injected or sham control groups compared to acute control animals. Furthermore, the values for sham control were similar to the saline-treated group. Our data suggest that exposure to chronic mild restraint produces a decrease in in vivo binding of [3H]flumazenil in mouse brain and supports the hypothesis that chronic mild stress produces a decrease in BZD receptor binding sites.

Effect of various training procedures on performance in an elevated plus-maze: possible relation with brain regional levels of benzodiazepine-like molecules

Rats submitted to one, two, or seven sessions of exploration to a new environment (habituation) or exposed to an inhibitory avoidance training showed different degrees of anxiety, evaluated by the elevated plus-maze test. Also, the brain regional levels of benzodiazepine (BDZ)-like molecules in rats submitted to one, two, or seven sessions of habituation were differentially decreased with respect to nontrained rats. The percentage of time spent in the open arms of the elevated plus-maze for each group correlates with the data of decrease in the BDZ-like immunoreactivity in amygdala (r = 0.77, p < 0.0005), hippocampus (r = 0.68, p < 0.0005), and septum (r = 0.57, p < 0.005). These results suggest that the limbic system responds to anxiogenic experiences by changing the BDZ-like molecule levels in relation to the degree of anxiety and/or stress that accompany these experiences.

Effects of benzodiazepine receptor inverse agonists on locomotor activity and exploration in mice

This study investigates the effects of benzodiazepine receptor inverse agonists on the locomotor and exploratory behaviour of mice when tested in a familiar environment. The weak partial inverse agonist Ro 15-3505 (0.3, 1, 3 mg/kg i.p.) significantly increased locomotion and hole-dipping in habituated mice. However, the more efficacious partial inverse agonists Ro 15-4513 (0.3, 1, 3 mg/kg i.p.) and Ro 19-4603 (0.03, 0.1, 0.3 mg/kg i.p.) had no effect on these parameters. The benzodiazepine receptor antagonist flumazenil (3, 10, 20 mg/kg i.p.) also increased locomotion and hole-dipping in habituated mice, although like Ro 15-3505, these effects were of short duration occurring largely in the first 15 min following injection. Opposite effects were obtained with the partial benzodiazepine agonist Ro 17-1812 (1, 3, 10 mg/kg i.p.) which produced a longer-lasting significant decrease in hole-dipping behaviour in habituated mice without altering locomotion. Finally, in contrast to its effects in habituated animals, Ro 15-3505 (0.3, 1, 3 mg/kg i.p.) did not modify either locomotion or exploration in mice which were tested in a novel environment, showing that the effects of the inverse agonist were state-dependent. This demonstration that, under certain conditions, the weak benzodiazepine receptor inverse agonist Ro 15-3505 and the antagonist flumazenil, produce behavioural activation is in accordance with the work of others suggesting that these classes of compound may increase arousal and may therefore be of some value in treatment of memory disorders.

Cognitive enhancing properties of beta-CCM infused into the nucleus basalis magnocellularis of the rat

Peripheral administration of various benzodiazepine derivatives or beta-carbolines (inverse agonists at benzodiazepine receptors), has been shown to affect memory. In this study, the effect of local infusion of a beta-carboline-methyl beta carboline-3-carboxylate (beta-CCM) into the nucleus basalis magnocellularis (NBM) of rats was examined in a two-trial recognition task. The results show that beta-CMM (3 micrograms/0.5 microliter) enhances recognition performance when injected both before or immediately after the acquisition trial. These effects appear to be mediated by a benzodiazepine (BZD) receptor since they were blocked by pretreatment with Ro 15-1788, a BZD receptor antagonist. This study supports the involvement of the NBM in cognitive processes, and demonstrates that these processes can be influenced by alteration of GABAergic neurotransmission.

Neurotransmitter receptors involved in post-training memory processing by the amygdala, medial septum, and hippocampus of the rat

Rats were trained and tested in habituation to a novel environment and step-down inhibitory avoidance. Immediately after training in each task the animals received intra-amygdala, intraseptal, or intrahippocampal micro-injections of agonists and antagonists of various neurotransmitter receptors. In the habitation task, intrahippocampal, but not intra-amygdala or intraseptal administration of the NMDA receptor antagonist aminophosphornopentanoic acid (AP5, 5.0 micrograms) or of the muscarinic receptor antagonist, scopolamine (2.0 micrograms) caused amnesia and the indirect antagonist of GABA-A receptors, picrotoxin (0.08 microgram), caused retrograde facilitation. Intrahippocampal administration of the respective agonists, glutamate, oxotremorine, and muscimol, had effects of their own opposite to those of the blockers, and norepinephrine (0.3 microgram) caused memory facilitation. In the avoidance task, results obtained with drug infusions given into the three structures were very similar: in all cases, AP5, scopolamine, and muscimol were amnestic, and glutamate, oxotremorine, norepinephrine, and picrotoxin caused memory facilitation. In addition, also in the three structures, picrotoxin counteracted the amnestic effect of AP5 and/or scopolamine and the beta-adrenoceptor blocker, timolol (0.3 microgram), while ineffective on its own, attenuated all the effects of picrotoxin. The results suggest that similar synaptic mechanisms in the amygdala, medial septum, and hippocampus are involved in memory consolidation: NMDA, muscarinic, and beta-noradrenergic receptors stimulate and GABA-A receptors inhibit this process, and beta-noradrenergic receptors modulate the GABAergic synapses. In the avoidance task these mechanisms operate in the three structures: in habituation only those in the hippocampus are operative. Possibly in each structure these mechanisms regulate, if not actually consolidate, a different aspect, component, or form of memory.

Late posttraining memory processing by entorhinal cortex: Involvement of NMDA and GABAergic receptors

The NMDA receptor antagonist, D-2-amino-5-phosphonopentanoic acid (AP5) (5 micrograms) and the GABAA receptor agonist, muscimol (0.03 microgram) were infused bilaterally into the entorhinal cortex of rats 0, 90, 180, or 360 min after training in habituation to a novel environment or in step-down inhibitory avoidance. Animals were tested for retention 22 h after training in each task. AP5 and muscimol were amnestic for both tasks when given 90 or 180 min after training, but had no effect when given 0 or 360 min after training. In contrast, intraamygdala injections or AP5 or muscimol were amnestic when given 0 but not 90 min after inhibitory avoidance training. The results indicate that the entorhinal cortex plays a late but important role in posttraining memory processing; this role involves glutamatergic NMDA receptors and is inhibited by GABAA receptors. The intervention of the entorhinal cortex in posttraining memory processing is subsequent, and could be secondary, to that of the amygdala and other limbic structures.

Formation of benzodiazepine-like molecules in rat brain

The possible biosynthetic origin of benzodiazepine-like molecules was investigated in mammalian tissue. Rat brain homogenates or cortical slices incubated under physiological conditions showed a 4 to 7 fold increase in the content of BZD-like compounds as compared with control non incubated or boiled tissue. The quantitative analysis was performed by a radioimmunoassay with a specific monoclonal antibody. The active fraction eluting just before diazepam exhibited a Mr lower than 1300 and inhibited the [3H]flunitrazepam binding to the central benzodiazepine receptor. No activity was measured in the absence of tissue. These data suggest that under our experimental conditions, low molecular weight substances similar to benzodiazepines are formed in rat brain.