Behavioural evidence for GABAergic activity of the benzodiazepine flurazepam

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

Postural and anticonvulsant effects of inhibition of the rat subthalamic nucleus

The subthalamic nucleus (STN) plays a crucial role as a regulator of basal ganglia outflow by providing excitatory glutamatergic input into the two output nuclei of the basal ganglia, substantia nigra pars reticulata (SNpr), and entopeduncular nucleus. This study examined the effects of suppressing activity in the STN of the awake, behaving rat. Specifically, we evaluated the effects of unilateral and bilateral focal inhibition of STN on posture, locomotion, and susceptibility to limbic motor seizures. Unilateral microinjection of a GABA(A) receptor agonist (muscimol, 200 pmol) into STN produced a site-dependent contralaterally directed postural asymmetry without locomotor activation. This effect differed from responses produced by the same dose of muscimol placed into SNpr, which included locomotor activation in addition to contralaterally directed postural asymmetry. Locomotor activation and postural asymmetry were obtained also after blockade of glutamate transmission in SNpr by the unilateral application of kynurenate (100 nmol). Our observation that STN inhibition did not induce the locomotor activation characteristic of SNpr inhibition suggests that there are glutamatergic inputs to SNpr, other than those from STN, that are responsible for controlling locomotion. Bilateral, but not unilateral, injection of muscimol (200 pmol) into STN protected against limbic motor seizures evoked either by intravenous bicuculline or by focal application of bicuculline into anterior piriform cortex (area tempestas). These results demonstrate that focal inhibition of STN reproduces the postural asymmetry and anticonvulsant actions that are obtained with the inhibition of SNpr. This provides behavioral support for the concept that STN contributes a crucial tonic excitatory (glutamatergic) drive to the rat SNpr.

Chronic flurazepam differentially regulates a behavioral effect of GABA agonists

Subsensitivity to gamma-aminobutyric acid (GABA) agonists was sought in rats treated 1 or 4 weeks with flurazepam (FZP). Sensitivity to GABA and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) was assessed by measuring contralateral rotation following unilateral microinjection of drug into the substantia nigra pars reticulata (SNpr). Immediately and 48 h after chronic treatment GABA, 200 micrograms or THIP, 60 ng was infused into SNpr. Immediately, but not 48 h after 1 week of FZP treatment, GABA subsensitivity was shown by a significantly reduced total number of contralateral turns and peak rotation rate. There was no change in the response to THIP after 1 week FZP treatment. Following 4 week FZP treatment, no subsensitivity to GABA or THIP was evident. Previous results showed subsensitivity to muscimol after 4, but not 1 week of FZP treatment. Since muscimol and THIP are not subject to uptake, there may be increased uptake of GABA after 1 week of FZP treatment, though it may not persist during continued treatment. Differential regulation of GABA agonist effects in SNpr may be related to their acting at differing GABAA receptor subpopulations, and variable responses of these subpopulations to chronic BZ treatment.

Changes in central GABAergic function following acute and repeated stress

1. The function of gamma-aminobutyric acid (GABA)ergic systems in response to acute and repeated stressful manipulations was evaluated in both the corpus striatum and frontal cerebral cortex of the rat. 2. In the corpus striatum the activity of the synthetic enzyme for GABA (glutamic acid decarboxylase, GAD) and the levels of GABA were reduced by acute immobilization stress (1 h). GABA turnover was reduced only by acute cold stress (3 h, 4 degrees C). 3. In the frontal cerebral cortex no changes were observed after acute stressful manipulations, but repeated stress (0.5 h immobilization per day for 14 days) enhanced both GAD activity and GABA turnover, and reduced GABA levels. 4. In conclusion, it would appear that the GABAergic system in the corpus striatum of the rat is most sensitive to acute stress and that the system in the frontal cerebral cortex area is preferentially responsive to chronic stress. It is speculated that the cortical GABAergic system is responsible for adaptive responses to the adverse conditions prevailing during chronic stress.

Behavioral measurement of benzodiazepine tolerance and GABAergic subsensitivity in the substantia nigra pars reticulata

Rotational behavior was elicited by unilateral microinjection of the benzodiazepine flurazepam, and the gamma-aminobutyric acid (GABA) agonist, muscimol, into the substantia nigra pars reticulata (SNpr). This response was used to quantitate benzodiazepine tolerance and GABAergic subsensitivity after chronic benzodiazepine treatment. Studies in naive rats established the dose requirements for inducing contralateral circling and demonstrated the reproducibility of the behavioral response as a measure of SNpr function. There was a large difference in potency between the two drugs for causing dose-related rotation. The response to microinjected flurazepam could be blocked by 16 mg/kg of the benzodiazepine antagonist, Ro15-1788. Tolerance to intranigral flurazepam (50 micrograms) was measured by a reduction in the turning response after a 1- or 4-week chronic flurazepam treatment. The time course for the reversal of tolerance after a 4-week benzodiazepine treatment correlates with the time course of the reversal of benzodiazepine receptor down-regulation in the SNpr. Subsensitivity of the GABAergic system was demonstrated by the decreased rotational response to muscimol (10 ng), confirming the idea that the GABAergic system is also functionally altered by chronic benzodiazepine treatment. The time course of the decreased sensitivity to muscimol does not coincide with the development and reversal of tolerance to the turning produced by flurazepam or with benzodiazepine receptor down-regulation. These data suggest differential regulation of SNpr sensitivity to benzodiazepine and GABA agonists following chronic benzodiazepine treatment and may provide a basis for differential tolerance; the development of tolerance to some but not other benzodiazepine actions.

Enhanced muscimol-induced behavioral responses after 6-OHDA lesions: relevance to susceptibility for self-mutilation behavior in neonatally lesioned rats

Adult rats lesioned with 6-hydroxydopamine (6-OHDA), either as neonates or as adults, demonstrated increased turning, compared to unlesioned controls, when muscimol was unilaterally microinjected into the substantia nigra reticulata (SNR). At the higher doses of muscimol, the lesioned rats were so intensely lateralized that circling was impeded. These data suggest a functional supersensitivity of receptors associated with GABA function in the SNR of 6-OHDA-lesioned rats. When 30 ng muscimol was administered bilaterally into the SNR, self-mutilation behavior (SMB) was observed in 2/11 of the control unlesioned rats, in 0/8 adult 6-OHDA-lesioned rats, and in 11/11 of the neonatally-lesioned rats tested. The ability of muscimol to produce SMB in the rats lesioned as neonates was dose related. Behavioral observations indicated that behaviors associated with SMB (self-biting and taffy pulling) were present in neonatal, but not adult lesioned rats. Behavioral responses to dopamine agonist administration were also different between rats lesioned as neonates and those lesioned as adults with 6-OHDA. These data support the view that lesions of dopaminergic neurons cause an increased functional responsiveness of receptors acted upon by muscimol in the SNR, and that the increased susceptibility for SMB in neonatally lesioned rats is determined by neurons distal to the GABA receptor complex in the SNR.

Modification of the antinociceptive effect of morphine by centrally administered diazepam and midazolam

1 Intracerebroventricular administration of diazepam or midazolam decreased the antinociceptive effect of morphine in rats as measured by the "tail flick" method. 2 Midazolam, injected into the periaqueductal grey matter (PAG) antagonized the analgesic effect of morphine. The action of midazolam was partially reversed by bicuculline. 3 These findings support the view that the effect of benzodiazephines on morphine antinociception may be mediated through gamma-aminobutyric acid receptors.

Acceleration by stress of dopamine synthesis and metabolism in prefrontal cortex: antagonism by diazepam

Using liquid chromatography and electrochemical detection (LCEC), we have measured the accumulation of 3,4-dihydroxyphenylalanine (DOPA) (after L-aromatic amino acid decarboxylase inhibition), dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the frontal cortex and in the corpus striatum of the rat. Mild-footshock stress increased frontal cortex DOPA accumulation, as well as DA and DOPAC, without changing the concentration of these substances in the corpus striatum. The increases in cortical DA synthesis and metabolism were antagonized by diazepam which, given alone, tended to decrease DOPA accumulation to a small degree. In addition, we have measured the indoles serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and the noradrenergic metabolite MHPG, none of which were altered by stress. The accumulation of 5-hydroxytryptophan (5-HTP) was also unaffected by stress but, like DOPA accumulation, was reduced to a small degree by diazepam. This study directly demonstrates a selective activation of frontal cortex catechol synthesis (in vivo tyrosine hydroxylation) by a mild stress, which did not significantly alter cortical noradrenergic or serotonergic metabolism.

Amino acid neurotransmitter utilization in discrete rat brain regions is correlated with conditioned emotional response

The content and utilization of amino acid neurotransmitters were evaluated in discrete brain areas of rats exposed to a conditioned emotional response (CER) procedure and in control groups which received either equivalent yoked shock history (shock only) or compound stimulus presentation (tone only). On test day, CER animals suppressed responding and exhibited anxious behavior after presentation of the CS, while shock only and tone only control groups, or CER animals which received an acute dose of diazepam prior to testing, did not suppress. Few changes were observed in the content of amino acids, suggesting that the behavioral manipulations were acting within normal physiological limits. On the other hand, numerous changes were observed in the utilization (turnover, metabolism) of the amino acid neurotransmitters. The effects of a history of shock presentation (shock only versus tone only) were persistent long after the conditioning sessions were terminated, and resulted in decreased turnover of the amino acids in many areas. CER conditioning-emotion (CER versus shock only) produced an increase in the turnover of aspartate and glutamate in many structures, while changes in GABA turnover were generally limited to decreases in limbic areas. If CER represents an animal model of anxiety, these observations may suggest roles for neurons which utilize amino acids in mediating or responding to emotional components of the paradigm.

The GABA postsynaptic membrane receptor-ionophore complex. Site of action of convulsant and anticonvulsant drugs

The function of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), has been implicated in the mode of action of many drugs which excite or depress the central nervous system. Many convulsant agents appear to block GABA action whereas anticonvulsants enhance GABA action. Some of these drug effects involve altered GABA-mediated synaptic transmission at the level of GABA biosynthesis, release from nerve endings, uptake into cells, and metabolic degradation. A greater number of agents of diverse classes appear to affect GABA action at the postsynaptic membrane, as determined from both electrophysiological and biochemical studies. The recently developed in vitro radioactive receptor binding assays have led to a wealth of new information about GABA action and its alteration by drugs. GABA inhibitory transmission involves the regulation, by GABA binding to its receptor site, of chloride ion channels. In this GABA receptor-ionophore system, other drug receptor sites, one for benzodiazepines and one for barbiturates/picrotoxinin (and related agents) appear to form a multicomponent complex. In this complex, the drugs binding to any of the three receptor categories are visualized to have an effect on GABA-associated chloride channel regulation. Available evidence suggests that the complex mediates many of the actions of numerous excitatory and depressant drugs showing a variety of pharmacological effects.

Effects of electrolytic and 6-hydroxydopamine lesions of the lateral hypothalamus on rotation evoked by electrical stimulation of the substantia nigra in rats

Contralateral rotation evoked by electrical stimulation of the left substantia nigra was studied in rats before and after electrolytic or 6-hydroxydopamine (6-OHDA) lesions of the lateral hypothalamus. Electrolytic lesions (2 mA DC, 15 sec) which produced mean ipsilateral striatal dopamine depletion of 58% significantly reduced the rotation at 2 h to 14 days postlesion. 6-OHDA (8 microgram in 4 microliter) which produced mean ipsilateral striatal dopamine depletion of 93% significantly increased the rotation at 3 to 14 days postlesion. Haloperidol 0.1 and 0.5 mg/kg i.p. partially reduced rotation in both control and lesioned rats in a dose-related manner. Control and lesioned rats showed no significant differences in haloperidol sensitivity. If stimulus induced rotation were mediated by activation of dopaminergic neurons, one would have expected lesion effects in the present experiments to parallel those on rotation caused by pharmacologically evoked release of dopamine. The lesions effects we obtained on stimulus induced rotation, however, parallel those on rotation evoked by the predominantly directly acting dopamine agonist, apomorphine, rather than those on rotation evoked by the indirect (presynaptic) action of amphetamine. We suggest that contralateral rotation evoked by electrical stimulation of the substantia nigra may reflect direct activation of neurons postsynaptic to the dopaminergic nigrostriatal neurons.

The striatonigral GABA pathway: functional and neurochemical characteristics in rats with unilateral striatal kainic acid lesions

Rats with unilateral striatal kainic acid (KA) lesions showed ipsilateral rotation to subcutaneous apomorphine, contralateral rotation to intranigral muscimol and reductions in striatal glutamic acid decarboxylase (GAD) activity and nigral gamma-aminobutyric acid (GABA) concentration. Rotational responses to apomorphine were highly correlated with nigral GABA depletions, and were a sensitive index of the functional integrity of striatonigral GABA neurons. Rotational responses to muscimol were also correlated with nigral GABA depletions, consistent with supersensitivity of denervated nigral GABA receptors. Striatal GAD was not correlated with either behavioural measure or with nigral GABA, and was a poor index of striatonigral function. These results are discussed in terms of (i) the functional role and adaptive capacity of striatonigral GABA neurons in linking the striatum with its effector mechanisms, (ii) parallels between parameters in GABA-dependent and DA-dependent rotational models and (iii) the status of the striatal KA lesion as a model for Huntington's disease and other extrapyramidal movement disorders.

A methodological weakness in the use of neuroleptic antagonism as a sole criterion for DAergic mediation of drug-induced behavioural effects

Contralateral rotational behavior following unilateral injections of the GABA agonist muscimol (100 ng) or the GABA analogue baclofen (1 microgram) into the zona reticulata of the rat substantia nigra was moderately attenuated by pretreatment with haloperidol (0.4 mg/kg i.p.), but not by destruction of ipsilateral dopamine (DA) neurons induced with 6-hydroxydopamine. Antagonism by neuroleptics can erroneously indicate DAergic mechanisms in the behavioural effects of drugs if used as a sole criterion.

Pharmacology of (+)- and (-)-baclofen: GABA-dependant rotational behaviour and [3H]GABA receptor binding studies

(+)- and (-)-baclofen were equiactive in inducing contralateral rotational behaviour following their unilateral injection into the zona reticulata of the rat substantia nigra, a proposed GABA-dependent rotational behaviour model, and in displacing the specific binding of [3H]GABA to frozen and thawed membrane preparations of rat brain. These non-stereospecific actions of baclofen at a population of cerebral GABA receptors contrast with stereospecific neuronal depressant effects probably mediated via presynaptic inhibition of the release of excitatory amino acid neurotransmitters. The present results indicate that baclofen may have two distinct mechanisms of action and this may account for the debate on the pharmacology of this compound.

Baclofen and muscimol: behavioural and neurochemical sequelae of unilateral intranigral administration and effects on 3H-GABA receptor binding

Log dose-response curves for induction of contralateral rotational behaviour in the rat by unilateral intranigral injections of the GABA agonist muscimol and the GABA analogue baclofen have been compared. Baclofen, 5--1000 ng, produced a maximal rotational response that was only 40% of that produced by 0.25--100 ng muscimol, and log dose-response curves failed to show parallelism. The behavioural effects of both drugs were only weakly antagonised by haloperidol and were not antagonised by 6-hydroxydopamine lesions of ipsilateral dopamine (DA) neurons, indicating that these responses were independent of DAergic mechanisms. The effects of baclofen were weakly antagonised by picrotoxin. Intranigral muscimol and baclofen substantially elevated striatal DA concentrations. While muscimol also substantially elevated striatal dihydroxyphenylacetic acid (DOPAC) but not homovanillic acid (HVA), bactofen did not significantly effect either DOPAC or HVA. Baclofein, GABA and muscimol displaced specific 3H-GABA binding in vitro with IC50's of 40 micron, 400 nM and 40 nM respectively. These results indicate that muscimol and baclofen do not act via a unitary GABAergic mechanism, but suggest that baclofen may be a partial GABA agonist, at least at nigral GABA receptors.