Effects of GABAA and GABAB receptor agonists on reticular-elicited hippocampal rhythmical slow activity

Effects of a Flavonoid-Rich Fraction on the Acquisition and Extinction of Fear Memory: Pharmacological and Molecular Approaches

The effects of flavonoids have been correlated with their ability to modulate the glutamatergic, serotoninergic, and GABAergic neurotransmission; the major targets of these substances are N-methyl-D-aspartic acid receptor (NMDARs), serotonin type1A receptor (5-HT_1ARs), and the gamma-aminobutyric acid type A receptors (GABA_ARs). Several studies showed that these receptors are involved in the acquisition and extinction of fear memory. This study assessed the effects of treatment prior to conditioning with a flavonoid-rich fraction from the stem bark of Erythrina falcata (FfB) on the acquisition and extinction of the conditioned suppression following pharmacological manipulations and on gene expression in the dorsal hippocampus (DH). Adult male Wistar rats were treated before conditioned fear with FfB, vehicle, an agonist or antagonist of the 5-HT_1AR, GABA_ARs or the GluN2B-NMDAR or one of these antagonists before FfB treatment. The effects of these treatments on fear memory retrieval, extinction training and extinction retrieval were evaluated at 48, 72, and 98 h after conditioning, respectively. We found that activation of GABA_ARs and inactivation of GluN2B-NMDARs play important roles in the acquisition of lick response suppression. FfB reversed the effect of blocking GluN2B-NMDARs on the conditioned fear and induced the spontaneous recovery. Blocking the 5-HT_1AR and the GluN2B-NMDAR before FfB treatment seemed to be associated with weakening of the spontaneous recovery. Expression of analysis of DH samples via qPCR showed that FfB treatment resulted in the overexpression of Htr1a, Grin2a, Gabra5, and Erk2 after the retention test and of Htr1a and Erk2 after the extinction retention test. Moreover, blocking the 5-HT1ARs and the GluN2B-NMDARs before FfB treatment resulted in reduced Htr1a and Grin2b expression after the retention test, but played a distinct role in Grin2a and Erk2 expression, according session evaluated. We show for the first time that the serotoninergic and glutamatergic receptors are important targets for the effect of FfB on the conditioned fear and spontaneous recovery, in which the ERK signaling pathway appears to be modulated. Further, these results provide important information regarding the role of the DH in conditioned suppression. Taken together, our data suggest that FfB represents a potential therapy for preventing or treating memory impairments.

Development of a theoretically-derived human anxiety syndrome biomarker

“Anxiety disorders” are extremely common; and are a major source of health costs and lost work days. Their diagnosis is currently based on clinical symptom check lists and there are no biological markers to diagnose specific syndromal causes. This paper describes: 1) a detailed theory of the brain systems controlling anxiolytic-insensitive threat-avoidance and anxiolytic-sensitive threat-approach — where, in specific brain structures, activity generates specific normal behaviours, hyperactivity generates abnormal behaviours, and hyper-reactivity (hypersensitivity to input) generates specific clinical syndromes; 2) a rodent model of systemic anxiolytic action (rhythmical slow activity), linked to the theory, that over a period of 40 years has shown predictive validity with no false positives or false negatives — and which is likely to assay the sensitivity of endogenous systems that control anxiety; and, 3) derivation from this rodent-based theory of a specific non-invasive biomarker (goal-conflict-specific rhythmicity) for the threat-approach system in humans. This new biomarker should allow division of untreated “anxiety” patients, with superficially similar clusters of symptoms, into distinct high scoring (syndromal) and low scoring groups with different treatment-responses. This would be the first theoretically-derived biomarker for any mental disorder and should: 1) predict treatment efficacy better than current symptom-based diagnoses; 2) provide a human single dose test of novel anxiolytics; 3) provide a starting point for developing biomarkers for other “anxiety” syndromes; and so, 4) greatly improve treatment outcomes and cost-effectiveness.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

Effects of intrahippocampal injection of GABAergic drugs on memory retention of passive avoidance learning in rats

The effect of post-training intrahippocampal injection of gamma-aminobutyric acid (GABA) receptor agonists and antagonists, immediately after a training session on memory retention of passive avoidance learning in rats, was measured in the presence and absence of physostigmine. Post-training treatments were carried out in all the experiments. The different doses of the GABAA receptor agonist muscimol (2, 4 and 6 microg/rat) decreased memory retention in rats dose-dependently. The higher response was obtained with 6 microg/rat of the drug. When the GABAA receptor antagonist bicuculline (0.5, 1, 2 and 4 microg/rat) was administered, only one dose of the drug (1 microg/rat) increased memory retention; however, the antagonist reduced the effect of muscimol. The GABAB receptor agonist, baclofen (0.25, 0.5, 1 and 2 microg/rat) also reduced memory retention in the animals. Intrahippocampal injection of lower doses of the GABAB receptor antagonist CGP35348 (P-[3-aminopropyl]-p-diethoxymethyl-phosphinic acid) (2.5, 5, 10 microg/rat) did not effect memory retention, although the higher doses of the drug (25 and 50 microg/rat) decreased memory retention. The doses of antagonist (2.5, 5 and 10 microg/rat), which did not elicit any response alone, reduced the effect of baclofen. The inhibitory response of CGP35348 was also decreased by bicuculline. In another series of experiments, physostigmine improved memory retention. The GABA receptor agonists, muscimol and baclofen, as well as the GABA receptor antagonists bicuculline and CGP35348, decreased the effect of physostigmine. Atropine decreased memory retention by itself and potentiated the response of muscimol and baclofen. It is concluded that GABAA and GABAB receptor activation may be involved in the impairment of memory retention.

Bridging the gap: integrating cellular and functional magnetic resonance imaging studies of the hippocampus

The importance of the medial temporal lobe in memory has been studied extensively at the neuronal, neural ensemble, and systems level. In this report, we discuss recent systems level neuroimaging results in relation to neurophysiological studies of the hippocampus and related structures within the medial temporal lobe. By combining our knowledge across the cellular and systems levels we sought to gain theoretical insight and a better understanding of the function of the hippocampus and related medial temporal lobe structures. The integration of information from studies carried out at the cellular and neural ensemble level with studies at the systems level is difficult because of the vast differences in spatial and temporal resolution of the different research methodologies, differences in neuroanatomy across species, and differences in the types of behavioral and cognitive paradigms used in rat, nonhuman primate, and human studies. Despite these methodological and species-related differences, the neurophysiological studies offer insight into many of the questions raised by recent neuroimaging studies. For instance, there is physiological evidence that suggests that the hippocampal memory system is functionally heterogeneous, which may explain some of the discrepancies in the location and extent of activation reported by different imaging studies of the medial temporal lobe. In addition, we describe recent computational models of the hippocampus which may be useful for bridging the gap between neurophysiological and neuroimaging data.

Baclofen, a selective GABAB receptor agonist, dose-dependently impairs spatial learning in rats

The present investigation assessed the effects of the selective GABAB receptor agonist baclofen (1, 3, and 6 mg/kg) on spatial learning in the Morris water maze, an aversively motivated spatial learning task. Potential anxiolytic and sedative effects of baclofen were also assessed in an open field. Baclofen dose-dependently reduced locomotion in the open field but had little effect on thigmotaxia (anxiety). In the water maze, baclofen dose-dependently impaired spatial learning and reduced swim speed. During the probe trial given after training, only rats treated with the highest dose of baclofen (6 mg/kg) failed to show a bias for the correct quadrant. Following four additional retraining trials, a second drug-reversal probe trial was given and it was found that rats switched from saline to the highest dose of baclofen (6 mg/kg) showed a bias for the correct quadrant, as did rats switched from the two lowest doses of baclofen (1 and 3 mg/kg) to saline. Rats switched from the highest dose of baclofen (6 mg/kg) to saline failed to show a quadrant bias. Performance on a visible platform task was not impaired by baclofen at any dose. Together these results suggest that baclofen resembles GABAA agonists/positive modulators in that it impairs spatial learning, but not performance of a previously acquired escape response; but differs in that it does not reduce thigmotaxia (anxiety). Potential mechanisms by which baclofen impairs mnemonic processes are discussed.

A comparison of the acute effects of a tricyclic and a MAOI antidepressant on septal driving of hippocampal rhythmical slow activity

In free-moving male rats, the function relating frequency to the threshold current required to drive hippocampal rhythmical slow activity (RSA) with septal stimulation has a minimum at 130 ms. Both classical anxiolytics (e.g. benzodiazepines) and the novel anxiolytic buspirone show similar effects on septal driving of RSA. The tricyclic antidepressant imipramine may be as effective as anxiolytic drugs in treatment of generalized anxiety disorder. The antidepressant monoamine oxidase inhibitor phenelzine has also been reported to be effective in treating anxiety, but this may reflect an action on "atypical depression" rather than "anxiety". The present study therefore compared the effects of acute administration of imipramine and phenelzine on septal driving of RSA to determine whether either would mimic anxiolytics in this test. Rats were chronically implanted with septal stimulating electrodes and subicular recording electrodes. Three groups of rats received IP injection of either imipramine (5.9-13.3 mg/kg or 13.3-30 mg/kg) or phenelzine (0.2-5.4 mg/kg). The effects produced by imipramine were very similar to the effects produced by anxiolytic drugs. In contrast, the effects produced by phenelzine were essentially opposite to those of both anxiolytic drugs and imipramine. The present experiment suggests that imipramine may act as a true anxiolytic, in addition to its conventional antidepressant properties. In contrast, phenelzine may be effective in cases where the etiology is essentially that of depression even when the symptomatology appears to be that of anxiety.

Naloxone-induced hypoalgesia: lack of involvement of the GABA-benzodiazepine receptor complex

Previous evidence has demonstrated that repeated daily administration of the opiate receptor antagonist naloxone prior to assessment of pain sensitivity provokes the development of a nonopioid form of hypoalgesia. The present experiments assessed whether the GABA-benzodiazepine receptor complex may be involved in the mediation of this effect. Male Wistar rats were administered 10 mg/kg naloxone prior to hot-plate tests (48.5 degrees C) for pain sensitivity for 8 consecutive days. Control animals were administered saline prior to, and naloxone 2-4 h after, assessment of pain reactivity. Beginning on the fourth or fifth day of this regimen, animals tested under the influence of naloxone displayed longer paw-lick latencies than controls. Preadministration of the GABAA agonist muscimol (1.0-5.0 mg/kg) and GABAA antagonist bicuculline (0.25-1.0 mg/kg) failed to affect paw-lick latencies in naloxone-tested and control rats. The GABAB receptor agonist baclofen (1.0-5.0 mg/kg) and the benzodiazepine receptor agonist diazepam (1.0-5.0 mg/kg) both elevated paw-lick latencies to the same degree in both groups of animals. These results suggest that the GABA-benzodiazepine receptor complex is not involved in the mediation of naloxone-induced hypoalgesia.

Pindolol antagonizes the effects on hippocampal rhythmical slow activity of clonidine, baclofen and 8-OH-DPAT, but not chlordiazepoxide and sodium amylobarbitone

Buspirone, benzodiazepines, barbiturates and ethanol all reliably reduce the frequency of reticular-elicited hippocampal rhythmical slow activity. In the present experiments we tested a number of drugs which are not usually used for treating generalized anxiety disorders but which have been reported to have some anxiolytic properties. Clonidine (0.3 mg/kg, i.p.), baclofen (6 mg/kg, i.p.) and 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) (2.5 mg/kg, i.p.) all reduced the frequency of rhythmical slow activity. The effect of all three drugs was reduced by the 5-hydroxytryptamine 1a antagonist pindolol (2 mg/kg, i.p.). Pindolol had no effect on the reduction in rhythmical slow activity produced by sodium amylobarbitone, as has been previously reported for the benzodiazepine chlordiazepoxide. Flumazenil (10 mg/kg, i.p.), a benzodiazepine receptor antagonist, reduced the effects of chlordiazepoxide (5 mg/kg, i.p.), but not buspirone (10 mg/kg, i.p.). A combination of the selective beta 1 adrenergic receptor antagonist metoprolol (20 mg/kg, i.p.) and the beta 2 adrenergic receptor antagonist ICI 118,551 (4 mg/kg, i.p.) did not reduce the effects of either buspirone (10 mg/kg, i.p.) or diazepam (1 mg/kg, i.p.). These data show that there are at least two separate routes through which anxiolytic agents reduce the frequency of hippocampal rhythmical slow activity. Buspirone, clonidine, baclofen and 8-OH-DPAT act via a system dependent on 5-hydroxytryptamine 1a receptor activation. Benzodiazepines act via activation of the benzodiazepine receptor and probably share with barbiturates action at the GABA-benzodiazepine-chloride ionophore complex but do not produce their effects, directly or indirectly, by 5-hydroxytryptamine 1a receptor activation.