Septal driving of hippocampal theta rhythm: role of gamma-aminobutyrate-benzodiazepine receptor complex in mediating effects of anxiolytics

The imidazodiazepine, KRM-II-81: An example of a newly emerging generation of GABAkines for neurological and psychiatric disorders

GABAkines, or positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors, are used for the treatment of anxiety, epilepsy, sleep, and other disorders. The search for improved GABAkines, with reduced safety liabilities (e.g., dependence) or side-effect profiles (e.g., sedation) constituted multiple discovery and development campaigns that involved a multitude of strategies over the past century. Due to the general lack of success in the development of new GABAkines, there had been a decades-long draught in bringing new GABAkines to market. Recently, however, there has been a resurgence of efforts to bring GABAkines to patients, the FDA approval of the neuroactive steroid brexanolone for post-partum depression in 2019 being the first. Other neuroactive steroids are in various stages of clinical development (ganaxolone, zuranolone, LYT-300, Sage-324, PRAX 114, and ETX-155). These GABAkines and non-steroid compounds (GRX-917, a TSPO binding site ligand), darigabat (CVL-865), an α2/3/5-preferring GABAkine, SAN711, an α3-preferring GABAkine, and the α2/3-preferring GABAkine, KRM-II-81, bring new therapeutic promise to this highly utilized medicinal target in neurology and psychiatry. Herein, we also discuss possible conditions that have enabled the transition to a new age of GABAkines. We highlight the pharmacology of KRM-II-81 that has the most preclinical data reported. KRM-II-81 is the lead compound in a new series of orally bioavailable imidazodiazepines entering IND-enabling safety studies. KRM-II-81 has a preclinical profile predicting efficacy against pharmacoresistant epilepsies, traumatic brain injury, and neuropathic pain. KRM-II-81 also produces anxiolytic- and antidepressant-like effects in rodent models. Other key features of the pharmacology of this compound are its low sedation rate, lack of tolerance development, and the ability to prevent the development of seizure sensitization.

Minimal changes with long-term administration of anxiolytics 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; "theta") with septal stimulation has a minimum at 7.7 Hz. Classical anxiolytics all increase thresholds in the region of 7.7 Hz, and so does the novel anxiolytic buspirone. However, unlike classical anxiolytics, 2 or 3 weeks are normally required for the onset of the clinical effects of buspirone. This study tested the effects of long-term administration of chlordiazepoxide and buspirone on septal driving of RSA. Separate groups of naive rats received three IP injections per day of chlordiazepoxide (0.4 mg/kg), buspirone (0.1 mg/kg) or saline for 50 days. Both chlordiazepoxide and buspirone increased thresholds at 7.7 Hz, as expected. These acute effects were not significantly changed with chronic administration. Buspirone and chlordiazepoxide produced similar, statistically significant, but small cumulative reductions in thresholds at 6.9 Hz. The present experiments suggest that if the effects of anxiolytic drugs on septally driven RSA provide any basis for their clinical action, then classical anxiolytics may have two actions: an immediate effect on euphoria and tension and a delayed effect on anxiety proper--with buspirone sharing only the latter effect.

Effects of long-term administration of phenelzine on reticular-elicited hippocampal rhythmical slow activity

All anxiolytics so far tested show a common reduction in the frequency of reticular-elicited hippocampal rhythmical slow activity (RSA). Acute administration of the tricyclic antidepressant imipramine shares this effect with anxiolytics. The present experiment tested whether the MAO inhibitor antidepressant phenelzine shares this common effect of anxiolytics and imipramine on hippocampal RSA. Rats, implanted with reticular stimulating electrodes and subicular recording electrodes, received four acute doses (0.2, 2.0, 6.0 and 18 mg/kg) or one chronic dose (2 mg/kg/day for 35 days) of phenelzine. Acute administration of phenelzine failed to systematically affect RSA frequency. Chronic injections of phenelzine eventually produced a reduction in RSA frequency combined with a gradual increase in baseline RSA frequency. The absence of immediate action and the production of a chronic reduction in RSA frequency are distinct from the documented effects of anxiolytics and imipramine, whereas the increase in baseline RSA frequency is similar to imipramine. The different influence of phenelzine on RSA frequency compared with anxiolytics (including imipramine) is consistent with the different clinical profiles of these drugs.

Effects of long-term administration of antidepressants on septal driving of hippocampal RSA

All classical anxiolytics raise the thresholds of septal-elicited hippocampal RSA overall, but do so mostly at 7.7 Hz (130 ms). The novel anxiolytic/antidepressant buspirone shows partial similarity with classical anxiolytics on septal driving thresholds. These effects of both the classical and novel anxiolytics are unchanged by long-term administration. The present experiment tested whether the tricyclic antidepressant imipramine and the monoamine oxidase inhibitor antidepressant phenelzine share these common effects of classical and novel anxiolytics with long-term administration. Rats, implanted with septal stimulating electrodes and subicular recording electrodes, received 15 mg/kg imipramine (twice per day) and 2 mg/kg phenelzine (once per day) for 28 days. Chronic administration of imipramine mimicked the documented effects of anxiolytics while chronic administration of phenelzine produced essentially opposite effects to the effects of anxiolytics on septal driving of RSA. Since both acute and chronic administration of imipramine but not phenelzine also produce similar effects to anxiolytics on the frequency of reticular-elicited hippocampal RSA, we suggest that (1) imipramine has a separate anxiolytic action, in addition to its antidepressant action; and (2) phenelzine may have no central anxiolytic action despite its capacity to relieve somatic symptoms in atypical depression.

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.

The effect of Ro 15-4513, an inverse agonist at the benzodiazepine receptor, on the exploratory response to novelty in the playground maze

The effects of chlordiazepoxide and the inverse agonist, Ro 15-4513, were compared on the exploratory response of rats to a novel object introduced into a familiar environment containing seven familiar objects. While chlordiazepoxide (5 mg/kg) increased the novelty response, Ro 15-4513 reduced the response in a dose-dependent manner (0.5-5.0 mg/kg). This action was specific to novelty since the response to the familiar objects was unaffected. Both drugs produced some reduction in ambulation. The effects of both drugs were blocked by flumazenil (10 mg/kg), which at this dose did not itself have any intrinsic effect on the response. Muscimol (0.001 mg/kg) had a weak chlordiazepoxide-like effect and baclofen (3 mg/kg) had a weak effect in the opposite direction.

Immunolocalization of meprobamate-like molecules in rat cerebellar cortex

The localization of meprobamate-like (MPB-like) molecules in the cerebellar cortex of the rat was investigated with the peroxidase-antiperoxidase (PAP) immunocytochemical method using an antiserum raised in rabbits. The positive immunoreaction for MPB in several nervous structures and in the wall of blood capillaries suggest the presence of endogenous MPB-like molecules.

The action of agonists and an antagonist of GABA on the frequency composition of the electrical activity of various brain structures of rats

The frequency composition of the electrograms (EG) of the visual area of the cortex and deep brain structures (putamen, dorsal hippocampus, medial portion of the mid-hypothalamus) were investigated in chronic experiments in 36 awake rats under the conditions of the separate and combined introduction of GABA, its agonists (muscimol, baclofen), and an antagonist (bicuculline) into the lateral ventricle of the brain. The principal effects were manifested primarily in the form of a decrease in the power of the oscillations in the 7-16 Hz region of the range (1-25 Hz) of EG frequencies analyzed. Even though these changes were unidirectional following the administration of the agonists, they nevertheless differed with respect to the degree of expressivity in the different structures. Bicuculline, injected against the background of the action of muscimol and baclofen, attenuated the effects induced by them. The possible mechanisms of the phenomena discovered are discussed.

Effects of ethanol and Ro 15-4513 in an electrophysiological model of anxiolytic action

Previous research has implicated hippocampal rhythmical slow activity in the mechanisms of action of the anxiolytic drugs. In this study ethanol and a putative ethanol antagonist, Ro 15-4513, were investigated with reticular elicitation of rhythmical slow activity. Doses of ethanol between 0.6 and 3.1 g/kg were used. Ethanol reduced the frequency of reticular-elicited rhythmical slow activity in the same way as has been reported for anxiolytic barbiturates and benzodiazepines. This effect was linearly related to log dose of ethanol in the range of 1.7-3.1 g/kg. Ro 15-4513 at a dose of 2 mg/kg reduced the effect of ethanol (2.0 g/kg) but had no action itself. Ethanol also decreased the slope of the stimulation voltage-rhythmical slow activity frequency function but this effect was not reduced by Ro 15-4513. These results show that ethanol acts in a similar manner to conventional anxiolytic drugs but that only one component of this action can be reduced by Ro 15-4513.

Hippocampal rhythmic slow activity in rat lines selected for differences in ethanol-induced motor impairment

Hippocampal rhythmic slow activity (RSA) was recorded during rotation and vibration stimulation after saline and ethanol (2 g/kg) administration in restrained alcohol-sensitive (ANT) and alcohol-insensitive (AT) rats implanted with chronic bipolar electrodes in the dorsal hippocampus. The saline-treated ANT rats had more high-frequency RSA than the AT rats, especially during the rotational stimulation of the optovestibular mechanisms. The difference was not found during ethanol sessions. Plasma corticosterone levels were significantly higher in the AT than the ANT rats after the recording sessions. This first electrophysiological demonstration of an alcohol-sensitivity difference in the brain between these rat lines is discussed in relation to behavioral tilting plane test used in the development of the lines, to the different innate responses of the lines to acute stress, and to the plausible line differences in brain GABAergic and serotonergic mechanisms that are known to modulate hippocampal EEG in rodents.

GABA-B receptor activation and conflict behaviour

Baclofen and oxazepam enhance extinction of conflict behaviour in the Geller-Seifter test while baclofen and diazepam release punished behaviour in Vogel's conflict test. In order to investigate the possibility that the effect of the selective GABA-B receptor agonist baclofen is mediated indirectly via the GABA-A/benzodiazepine receptor complex, the effect of pretreatment of rats with baclofen on [3H]-diazepam binding to washed and unwashed cortical and cerebellar membranes of rats has been studied. Baclofen pretreatment increased Bmax in washed cerebellar membranes when bicuculline was present in the incubation mixture. No effect was seen in cortical membranes. The present results render it unlikely that the effect of baclofen on extinction of conflict behaviour and punished drinking is mediated via the GABA-A/benzodiazepine receptor complex.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)