Alterations in a low affinity GABA recognition site following chronic benzodiazepine treatment

Isoflurane exacerbates electrically evoked seizures in amygdala-kindled rats during recovery

Neuroexcitatory effects of isoflurane during or following anesthesia are controversial, particularly in epileptic patients. In contrast, halothane is generally considered to be highly anticonvulsant. Kindling is an animal model of epilepsy suitable for studying the effects of anesthetic agents on the epileptic brain. Fully kindled, Sprague-Dawley rats were either untreated or received a 5 min exposure to isoflurane or halothane 30 min prior to a seizure and compared to seizures in the absence of prior anesthesia. Afterdischarge duration was assessed via electroencephalographs recorded from electrodes implanted in the basolateral amygdala and behavioral seizure stereotypy (stages I-V) was simultaneously recorded and analyzed using digital video for all seizures. Total seizure duration and clonus duration were significantly (P<0.05) increased 30 min after isoflurane but not halothane exposure relative to pre-treatment control. These results are the first to demonstrate that isoflurane exacerbates electrically evoked secondarily generalized seizures in fully kindled animals during recovery. These results also show that the kindling paradigm is useful for evaluating the mechanism of anesthetic agents that may be proconvulsant in epileptic subjects.

Influence of benzodiazepines on auditory perception

The aim of this study was to test for an influence of benzodiazepine (BZD) on various perceptual and/or cognitive auditory processes. Loudness, auditory selective attention, and the ability of subjects to form perceptual streams out of alternating tone sequences were tested. Nine subjects were tested before, 1, 3, 7, and 24 h after a single-dose oxazepam vs placebo administration in a crossover design. A sample of blood allows us to measure plasma oxazepam concentration. The results revealed a significant reduction in stream segregation expressed as d' scores 1 h after oxazepam intake in the test subjects. No significant change occurred across time in the same subjects when they were administrated a placebo in another session. Furthermore, oxazepam had no substantial and systematic influence either on auditory selective attention or on loudness perception. Altogether, these results suggest that the perceptual organization of sound sequences involves inhibitory neural mechanisms, which can be affected by BZDs. This outcome is consistent with existing models of auditory stream segregation and may be paralleled with earlier findings on the effect of BZDs on perceptual binding in the visual modality.

Behavioral analysis indicates benzodiazepine-tolerance mediated by the benzodiazepine binding-site at the GABA(A)-receptor

1. GABA(A)-receptor induced changes in locomotion and anxiety-like behaviors were studied in rats using an open-field and an elevated plus-maze. Acute and chronic doses of the benzodiazepine diazepam without and in combination with the GABA uptake inhibitor SKF-89976A were investigated. 2. Fifty-six male rats of the strain PVG/OlaHsd (PVG; 180-200 g body wt) were used to assess the influence of the benzodiazepine binding-site to the development of tolerance. Rats were divided into six groups: The first receiving saline (0.9%), the second and third diazepam (10.0 mg/kg) daily for 23 days with or without an acute challenge of 2.0 mg/kg diazepam. The fourth group received diazepam (10.0 mg/kg) daily and acutely SKF-89976A (15.0 mg/kg) plus diazepam and the fifth and sixth group received acute treatment with diazepam (2.0 mg/kg) or SKF-89976A (15.0 mg/kg). 3. Under chronic treatment with diazepam the animals became tolerant to acute doses of diazepam in activity and anxiety-related behaviors. Acute treatment with SKF-89976A increased exploration. Parameters expressing anxiolytic-like behaviors were increased, too, but not all of them significantly. In diazepam tolerant animals SKF-89976A produced anxiolytic-like behaviors 4. We conclude that the BZ- and not the GABA-binding site at the GABA(A)-receptor is involved in the development of BZ-tolerance.

GABA(A) receptor gene expression in rat cortex: differential effects of two chronic diazepam treatment regimes

Diazepam is widely prescribed as an anxiolytic but its therapeutic application is limited because with daily use tolerance develops to certain aspects of its pharmacological profile. We compared the effects of two dosing paradigms on GABA(A) receptor gene expression and benzodiazepine binding characteristics. Equivalent daily doses of 15 mg/kg/day diazepam were delivered either via constant infusion or daily subcutaneous injection for 14 days. The two distinct treatment regimes produced significantly different changes in GABA(A) receptor alpha4-, beta2-, beta3- and gamma1-subunit mRNA steady-state levels. Similar changes in the GABA enhancement of flunitrazepam binding and the BZ3/BZ2 subtype ratio determined ex vivo were produced, however, significant differences were found in [(3)H]-Ro 15-4513 binding between cortical tissue from diazepam injected animals compared with diazepam infused animals. Our data suggest that it is the diurnal fluctuations in receptor occupancy that are responsible for the different effects produced by these two dosing regimes.

The behavioural and neuronal effects of the chronic administration of benzodiazepine anxiolytic and hypnotic drugs

Benzodiazepine anxiolytic and hypnotic drugs are some of the most widely prescribed drugs in the Western world. Despite this fact, the mechanisms that underlie the development of tolerance to, and dependence upon, benzodiazepines are poorly understood. The aim of this review is to summarize and critically evaluate the experimental evidence relating to the chronic behavioural and neuronal effects of benzodiazepines. Behavioural studies in animals generally indicate that tolerance gradually develops to the muscle relaxant, ataxic, locomotor and anticonvulsant effects of benzodiazepines. The evidence relating to the development of tolerance to the anxiolytic effects of benzodiazepines is less clear. The literature on the possible mechanisms of benzodiazepine tolerance and dependence is large, highly complex and difficult to interpret. The effect of chronic benzodiazepine treatment varies enormously as a function of the benzodiazepine used and the treatment schedule employed. Many studies have demonstrated a down-regulation of benzodiazepine binding sites, although affinity is usually unchanged. The evidence relating to the number and affinity of GABAA binding sites is unclear. Some studies suggest that chronic benzodiazepine administration results in a reduction in the number of Cl- channels associated with the GABAA receptor complex, although it is not clear that the efficacy of the GABA binding site in operating the Cl- channel necessarily changes. There is, however, substantial evidence to support the hypothesis that chronic benzodiazepine treatment results in a reduction in the coupling between the GABAA and benzodiazepine binding sites (the "functional uncoupling hypothesis"). Although some electrophysiological studies suggest that chronic benzodiazepine treatment results in a subsensitivity to GABA, this effect seems to be highly area-specific.

Changes in benzodiazepine-GABA receptor coupling in an accumbens-habenula circuit after chronic diazepam treatment

1. The effects of subacute and of chronic diazepam treatment upon binding to the GABAA receptor have been examined by use of receptor autoradiography for determining flunitrazepam (FNZP) binding, GABA enhancement of FNZP binding. SR 95531 2-(3'-carboxy-2',propyl)-3-amino-6-p-methoxyphenylpyridazinium bromide) binding and GABA binding in parallel sections from rat brain. Prior to the autoradiographic procedures, a behavioural assessment of the rats was made in the elevated plus-maze test of anxiety. 2. Rats receiving diazepam either subacutely (3 days) or chronically (28 days) by both continuous release, from previously implanted subcutaneous silastic capsules, or by daily injection (5 mg kg-1) did not display changes in FNZP or GABA binding in any of the 47 brain structures analysed. Similarly, there were no significant effects of treatment upon mean total entries or on the open:total ratio for entries in the elevated plus-maze. 3. There were reductions in the GABA enhancement of FNZP binding in the nucleus accumbens and central grey after subacute diazepam treatment. This effect persisted in the nucleus accumbens after chronic treatment. Less marked effects occurred in the lateral habenula, dorsal raphe and substantia nigra pars compacta. In the dorsal tegmental nucleus, GABA enhancement of FNZP binding was enhanced after chronic treatment and this was accompanied by reductions in SR 95531 binding. Treatment did not otherwise affect SR 95531 binding, with the exception of the dorsal raphe where binding was decreased after subacute treatment. 4. In general, the patterns of binding produced by the two different treatment routes were very similar. However, SR 95531 binding was lower in certain hippocampal fields in the i.p. treated animals compared to the rats implanted with silastic capsules. 5. It is concluded that repeated administration of diazepam evokes changes in benzodiazepine and GABA receptor coupling, and to a lesser extent changes in low affinity GABA binding, in certain interrelated brain structures of which an accumbens-habenula circuit is a central feature. These changes occur soon after the initiation of diazepam treatment, suggesting that they are unlikely to account for tolerance to the anxiolytic effects of diazepam but may trigger and/or accompany other critical neurochemical events.

Discriminative stimulus effects of midazolam and abecarnil in rats treated chronically with diazepam or abecarnil

Abecarnil (ABC) is a beta-carboline that acts as an agonist at benzodiazepine (BZD) receptors. It possesses anxiolytic and anticonvulsant properties, but produces little sedation and is without muscle relaxant effects. To explain this unusual profile of activity, two hypotheses have been advanced: either 1) ABC acts as a partial agonist or 2) ABC acts as a full agonist, but only at a sub-population of BZD receptors. The present experiment used cross-tolerance profiles between BZDs and ABC to differentiate these hypotheses based upon predictions of receptor theory: tolerance produced to a full agonist should confer even greater cross-tolerance to a partial agonsit. Rats were trained in a three-choice drug discrimination procedure to detect the benzodiazepine, midazolam (MDZ, 1.0 mg/kg) from pentylenetetrazole (PTZ, 20 mg/kg) from saline. Tested acutely, MDZ and ABC substituted for MDZ with similar potencies. Following chronic treatment with the BZD-agonist diazepam (DZP; 20 mg/kg per 8 h for 7 days), both the MDZ and ABC dose-effect curves were significantly shifted to the right, and both drugs showed a comparable three-fold decrease in potency. The chronic administration of ABC (4.0 mg/kg per 8 h for 7 days) produced a different spectrum of results. No significant shift occurred in the MDZ dose-effect curve, but there was a significant seven-fold shift to the right of the ABC dose-effect curve. Throughout all test, PTZ-lever responding rarely occurred and did not account for more than 20% of lever selections for any individual test. These data support the hypothesis that ABC acts as a full agonist at a sub-population of BZD receptors, which mediate its substitution for MDZ.

Tolerance to the effects of diazepam, clonazepam and bretazenil on GABA-stimulated Cl- influx in flurazepam tolerant rats

The effect of chronic flurazepam treatment on the GABA (gamma-aminobutyric acid) receptor/chloride channel complex was studied using GABA-stimulated 36Cl- influx into brain microsacs, and its potentiation by diazepam, clonazepam and bretazenil. Rats were given flurazepam for 1 week, then microsacs were prepared from cerebral cortices of rats that were still receiving flurazepam, and from those that had stopped treatment 48 h earlier. Diazepam and clonazepam produced concentration-dependent increases in GABA-stimulated 36Cl- influx while bretazenil produced a much smaller effect, which did not reach statistical significance in the tissue from control rats. There was no significant change in the basal or 10 microM GABA-stimulated 36Cl- influx between control and treated groups. Tolerance was shown by a significantly reduced effect of diazepam and clonazepam to enhance GABA-stimulated 36Cl- influx in the tissue prepared from non-withdrawn rats. However, for both diazepam and clonazepam, there was no tolerance 48 h after chronic treatment. The results suggest that changes in the GABA receptor/Cl- channel complex on cerebral cortical neurons contribute to cross-tolerance from flurazepam to other benzodiazepines.

Influences of gender, gonadectomy 5 and estrous cycle on GABA/BZ receptors and benzodiazepine responses in rats

Benzodiazepines (BZ) and steroid hormone derivatives can potentiate the inhibitory actions of GABA through interactions with the GABAA/BZ/chloride ionophore complex. The present study examines whether the in vivo hormone milieu of rats modulates GABA/BZ receptors and/or benzodiazepine responses. The influences of gender, estrous cycle, and the diminution of steroid levels on GABA/BZ receptors and BZ anticonvulsant responses were tested by comparing these parameters in groups of intact male, intact female, orchidectomized, and ovariectomized rats. The hormonal milieu appears to modulate the GABA recognition site and possibly GABA-related responses in rats. This is evidenced by the decrease in cortical GABAA receptor affinity seen in females compared with other hormone groups and the gender-related difference observed in susceptibility to seizures induced by the GABA antagonist bicuculline. In cycling females, high circulating levels of progesterone were correlated with heightened seizure thresholds, suggesting that progestins serve a protective role in the control of seizure activity. Although a gender-related difference in cortical BZ binding affinity was observed, BZ receptor parameters in several other brain areas and BZ anticonvulsant responses were unaffected by physiological fluctuations in gonadal hormones.

Chronic benzodiazepine treatment of rats induces reduction of paired-pulse inhibition in CA1 region of in vitro hippocampus

Paired-pulse inhibition was studied extracellularly in in vitro hippocampal slices from rats sacrificed 48 h or 7 days after 1 week flurazepam (FZP) treatment. Population spikes and field excitatory postsynaptic potentials (EPSPs) were recorded with NaCl-containing glass micropipettes in the stratum pyramidale and stratum radiatum, respectively, of the CA1 region. Conditioning pulses were delivered by stimulating Shaffer collaterals (orthodromic) or the alveus (antidromic). Orthodromic test pulses were delivered with interpulse intervals of 10-200 ms. There was a significant reduction in paired-pulse inhibition in slices from treated vs control rats in both the orthodromic-orthodromic and antidromic-orthodromic paradigms. Reduced inhibition was evident 48 h, but not 7 days, after the end of FZP treatment. Furthermore, there was a significant prolongation of the half decay time of the field EPSP, without a significant change in the initial slope or maximum amplitude. The results may suggest an impairment of endogenous gamma-aminobutyric acid function in the hippocampus after chronic benzodiazepine (BZ) treatment and may provide a basis for a mechanism of BZ tolerance.

Muscimol-associated changes in local cerebral glucose use following chronic diazepam administration

Local cerebral glucose use (LCGU) was determined in parallel groups of conscious rats receiving muscimol (1.5 mg/kg i.v.) after either saline pretreatment (28 days i.p.), saline pretreatment (27 days i.p.) followed by a single dose of diazepam (5 mg/kg i.p.) 24 h prior to muscimol administration, or chronic diazepam pretreatment (5 mg/kg i.p. daily for 28 days). Acute administration of muscimol produced a significant reduction in LCGU in 25 out of 66 structures examined compared with vehicle-treated controls. The pattern of reductions was heterogeneous. Thalamic and most cortical areas showed reductions of the order of 30-45%, whereas more modest depressions of 15-20% were observed in some limbic structures (e.g. basolateral amygdala, anterior thalamic nuclei, nucleus accumbens, subiculum). This contrasts with the more extensive and homogeneous pattern of LGCU reductions (around 20%) produced by diazepam. Neither acute diazepam treatment the previous day nor chronic diazepam pretreatment altered the LGCU response to muscimol. These data suggest that high-affinity GABA receptor-mediated responses are unchanged by both acute and chronic benzodiazepine pretreatment. It would appear unlikely that alterations in these responses contribute to the mechanism of benzodiazepine tolerance.

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.

Metabolic anatomy of the focal epilepsy produced by cessation of chronic intracortical GABA infusion in the rat

Cessation of chronic (5 days), unilateral infusion of GABA into the somatomotor cortex of rats induces focal epileptic spikes which remain limited to the infused site and never evolve into generalized seizures. We have considered this finding as a new model of focal epilepsy and named it "GABA withdrawal syndrome". In the present study, we have measured local cerebral glucose utilization in order to map the cortical and subcortical regions involved in the GABA withdrawal syndrome. Local cerebral glucose utilization increased two- to three-fold in a 1-1.5 mm diameter area, involving all the cortical layers at the GABA-infusion site. This hypermetabolic area contained a central (1-2 mm diameter) hypometabolic zone showing neuronal depopulation in some animals. Except for the epileptic focus, the hemisphere ipsilateral to the infusion site was slightly hypometabolic. However, there was a large increase (three- to five-fold) in some ipsilateral thalamic nuclei (posterior oralis, ventralis postero-lateralis, centralis lateralis, ventralis lateralis and reticularis thalami nucleus). The local cerebral glucose utilization of the contralateral cortex and thalamus were unchanged. The present results confirm the focal nature of the epileptogenic syndrome produced by stopping chronic, intracortical GABA infusion. These results are markedly different from those described in the penicillin focal epilepsy model. Our data also show that specific ipsilateral thalamic relays may, by an as yet unknown mechanism, play a role in maintaining paroxysmal activity during the GABA withdrawal syndrome.

Long-term changes in sensitivity to GABA in dorsal raphe neurons following amygdala kindling

The present experiments were undertaken to evaluate GABA sensitivity in dorsal raphe neurons following amygdala-kindled seizures. Dorsal raphe neurons of amygdala-kindled rats exhibited significant subsensitivity to GABA as measured electrophysiologically 3 or 4 weeks after the last stage 5 seizure. Amygdala stimulation with currents which did not produce kindled seizures did not produce subsensitivity to GABA. The subsensitivity observed after kindling was equivalent in magnitude to that observed following chronic diazepam treatment. However, exposure of fully kindled rats to chronic diazepam did not further decrease the sensitivity of dorsal raphe neurons to GABA. Additionally, while subsensitivity to GABA was reversed by bath application of the benzodiazepine antagonist, Ro 15-1788, in chronic diazepam-treated rats, it had no effect on GABA subsensitivity in fully kindled rats. These findings suggest a decrease in GABA sensitivity within the dorsal raphe might reflect long-term neuronal changes associated with kindled seizures. These data also suggest that the decrease in GABA sensitivity of dorsal raphe neurons following chronic diazepam may involve different mechanisms from those observed after amygdala kindling.

Regionally specific alterations in the low-affinity GABAA receptor following perinatal exposure to diazepam

Alterations in a low affinity form of the GABAA receptor were examined with [3H]bicuculline methylchloride in the adult rat following perinatal exposure to diazepam. Perinatal exposure resulted in a significant reduction in [3H]bicuculline binding in the cingulate cortex. A significant decrease in the ability of GABA to displace bound [3H]bicuculline was observed only in the hypothalamus. The results suggest that the effects of perinatal exposure to diazepam are regionally specific and that benzodiazepine receptors and low affinity GABAA receptors are functionally linked during the perinatal period.

Modulation of GABA-stimulated Cl- flux by a benzodiazepine agonist and an 'inverse agonist' after chronic flurazepam treatment

Rats treated one week with flurazepam were killed while still on the drug or 48 h after termination of drug treatment. The brain 'microsac' preparation derived from the cerebral cortices was used for studying the GABA-stimulated chloride influx. There was no significant change in the basal or GABA-stimulated influx between control and treated groups. However, the effect of flunitrazepam to enhance 10 microM GABA-stimulated influx was significantly reduced, indicating tolerance. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3- carboxylate (DMCM), an 'inverse agonist' at benzodiazepine receptors, dose dependently inhibited 50 microM GABA-stimulated influx; chronic treatment did not alter the effect of DMCM. This study demonstrates that one week treatment with flurazepam produces tolerance to benzodiazepines without any change in the effect of GABA or DMCM. This indicates that GABA and benzodiazepine sites are differently modulated after chronic treatment with benzodiazepines. However, since both benzodiazepine and DMCM act on the same receptors it appears that the different 'domains' on the benzodiazepine receptor are differently altered during chronic treatment.

Relationship of GABAa receptor heterogeneity to regional differences in drug response

Molecular biological approaches to the GABAa receptor have resulted in new insights into the structure and pharmacology of this complex. It is known that the GABAa complex is a hetero-oligomer composed of multiple subunits which contain binding sites for the GABA, benzodiazepines and barbiturates. These subunits also contain regulatory sites for phosphorylation by intracellular kinases. There appear to be regional differences in the expression of the various subunits for the GABAa receptor complex. The functional significance of molecular heterogeneity is not yet known but it is expected that regional differences may result in pharmacologically diverse responses. Studies on the effects of chronic administration of diazepam have clearly delineated such regional differences. Chronic benzodiazepine administration results in the development of subsensitivity to the electrophysiological actions of GABA in the dorsal raphe, but not in GABA receptive neurons of the substantia nigra pars reticulata. Such data is consistent with regional heterogeneity in response to chronic benzodiazepine exposure. It is hoped that by understanding GABAa receptor heterogeneity, and its molecular basis, we can improve the existing receptor subtype specificity and pharmacology of the benzodiazepines.

Tolerance to diazepam and methyl-beta-carboline-3-carboxylate measured in substantia nigra of benzodiazepine tolerant rats

The spontaneous activity of neurons in the pars reticulata of substantia nigra (SNpr) was studied in chloral hydrate anesthetized rats. As a function of dose, intravenous diazepam decreased, and methyl-beta-carboline-3-carboxylate (beta CCM) increased discharge frequency. Two days after terminating a one week treatment with flurazepam (FZP), both diazepam and beta CCM showed decreased ability to alter SNpr neuronal activity. Neither residual FZP nor down-regulation of benzodiazepine receptors can account for these results. In contrast, behavioral testing revealed no change in the ability of i.v. beta CCM to cause convulsions, suggesting that sites other than the SNpr are of prime importance in expressing the convulsant actions of systemically injected beta CCM.

Steroid and barbiturate modulation of the GABAa receptor. Possible mechanisms

This review describes the modulation of the GABAa receptor by steroid hormones and barbiturates and proposes guidelines for further research. Having examined the complex organization of the GABAa receptor complex and the multiple allosteric interactions between its drug and transmitter/modulator binding sites, the possibility that conformational changes of the receptor molecule may explain most of its characteristics is explored. On the basis of considerable evidence, we propose that the GABAa receptor may adopt as many as five different conformations. However, the heterogeneity of central GABAa receptor binding cannot only be explained by different configurations of a single protein. It also has been shown that different GABAa receptor subtypes exist within different brain regions. These receptor subtypes may differ from each other in their subunit composition. By describing the GABAa receptor as a macromolecular complex that may adopt different conformations and whose subunit composition may vary, it becomes possible to understand the molecular mechanisms by which steroid hormones modulate the receptor. This has led to two models of hormone actions. A first model addresses the direct effects that steroids exert on the GABAa receptor and predicts that steroid hormones may cause the conformation of the receptor complex to change between active and inactive states. A second model, which addresses the observed heterogeneity of GABAa receptor binding within the brain, suggests that steroid hormones may change the expression of the different subunits of the receptor complex by acting at the genomic level. This review complements other recent reviews describing the modulation of the GABAa receptor (Olsen and Venter, 1986; Gee, 1988).

Relationship of agonist efficacy to changes in GABA sensitivity and anticonvulsant tolerance following chronic benzodiazepine ligand exposure

Benzodiazepine ligands of differing efficacy including the agonist, diazepam, the partial agonist, Ro 16-6028 and the antagonist, Ro 15-1788, were administered in vivo to rats continuously for 3 weeks. The magnitude of change in various measures of GABA sensitivity could be correlated directly with increasing agonist efficacy: maximal changes were seen following chronic treatment with diazepam, intermediate changes were seen following the partial agonist Ro 16-6028 and no changes were observed following chronic Ro 15-1788 administration. The magnitude of change could also be correlated with increasing potential for tolerance development to anticonvulsant efficacy following chronic exposure to these benzodiazepine ligands.

Regional GABA/benzodiazepine receptor/chloride channel coupling after acute and chronic benzodiazepine treatment

GABA/benzodiazepine coupling was evaluated in 8 regions of rat brain by the ability of GABA to stimulate 0.5 nM [3H]flunitrazepam binding. Rats were treated acutely with diazepam (p.o) or chronically with flurazepam, offered in the drinking water for 4 weeks, and compared to a pair-handled vehicle-treated control group. Regional variations in GABA/benzodiazepine coupling were found in control membranes. GABA increased benzodiazepine binding maximally (40%) in cerebellum and medulla, and least (25%) in olfactory bulb. A significant decrease in the effect of GABA was found in cortex of chronically treated rats immediately after, but not 2 days following treatment. The Emax for GABA stimulation of [3H]flunitrazepam binding was significantly increased in medulla after acute treatment but was not altered after acute or chronic treatment in other brain areas evaluated. Treatment had no effect on the ability of bicuculline to inhibit [3H]flunitrazepam binding in cortex. Benzodiazepine/Cl- coupling in cortex or hippocampus of acutely and chronically treated rats, evaluated by the ability of Cl- to stimulate specific [3H]flunitrazepam binding, was not changed. The results support the hypothesis that a functional uncoupling of the benzodiazepine recognition site from the GABA receptor in cortex, but not from the anion recognition site, may play a role in tolerance development.

Modification of ethanol effects by bicuculline: genotype-dependent responses and inheritance

Genetic influences on the interaction between ethanol (ETOH) and gamma-aminobutyric acid (GABA) neurotransmitter systems were evaluated with a survey of responses to coadministration of ETOH and a GABA antagonist, bicuculline, in a battery of inbred mouse strains. The selectively bred ETOH-sensitive Long-Sleep (LS) mice, the relatively ETOH-resistant Short-Sleep (SS) mice, and a genetically heterogeneous stock (GHS) were also evaluated. The effect of bicuculline on ETOH-induced sedation, hypothermia, and blood ethanol content upon recovery from sedation was assessed. Inheritance of these responses was also examined using F1 hybrids. The effect of bicuculline on ETOH-produced narcosis varied widely among stocks and included antagonism, potentiation, and no effect. Changes in ETOH-induced narcosis produced by bicuculline were accompanied by changes in blood ethanol concentrations consistent with an hypothesis of altered central nervous system sensitivity to ETOH. Knowledge of a strain's seizure susceptibility to the GABA antagonist or of its sensitivity to the hypnotic effects of ETOH were of no predictive value in estimating the outcome of coadministration studies, suggesting at least partially separate genetic influences on each phenotype. In cross-breeding studies there was commonly dominance toward a profile of bicuculline antagonism of ETOH narcosis but different patterns of dominance were observed for seizure susceptibility, again indicating separate genetic control. The results suggest considerable complexity of GABAergic involvement in genotype-dependent ETOH sensitivity.

Steroid modulation of the GABA/benzodiazepine receptor-linked chloride ionophore

Recent findings suggest that steroids with sedative-hypnotic properties interact specifically with the gamma-aminobutyric acidA/benzodiazepine receptor-chloride ionophore complex (GBRC). They show positive heterotropic cooperativity by allosterically enhancing the binding of GABA agonists and the clinically useful benzodiazepines (BZs) to their respective recognition sites. These steroids have stringent structural requirements for activity at the GBRC, with the essential requirements for high potency being a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring. Some of these steroids are naturally occurring metabolites of progesterone and deoxycorticosterone and have nanomolar potencies as potentiators of chloride channel conductance. These 3 alpha-hydroxylated, 5 alpha-reduced steroids do not act through any known sites on the GBRC. Thus, the exact site and mechanism of action remain to be determined. Together with the observation that physiological levels of these metabolites are sufficient to influence the function of the GBRC, the evidence clearly suggests a role for these steroids in the normal regulation of brain excitability by potentiating the postsynaptic effects of gamma-aminobutyric acid (GABA). Pharmacological studies of the GBRC-active steroids show that they possess anxiolytic and anticonvulsant activities. The potential therapeutic application of these steroids in the treatment of mood disorders and catamenial exacerbation of seizures associated with the menstrual cycle is discussed. Collectively, the evidence from the studies of these steroids imply that another mechanism by which the endocrine system influences brain function has been identified. Its characterization will provide important insight into how steroids modulate brain excitability under normal and pathophysiological states.

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.

The natural history of tolerance to the benzodiazepines

Dependence on benzodiazepines following continued use is by now a well-documented clinical phenomenon. Benzodiazepines differ in their dependence potential. The present studies were aimed at examining the possibility that differential rates of tolerance development might account for differences in dependence risk. Four studies are reported. The first three studies concerned normal subjects. The development of tolerance over a fifteen day period was demonstrated for three different benzodiazepines (ketazolam, lorazepam and triazolam) using two paradigms. Tolerance in terms of a reduction in effectiveness of a repeated given dose was most notable for the benzodiazepine with a medium elimination half-life (lorazepam) for physiological, behavioural and subjective measures. In the case of the drug with the longest elimination half-life (ketazolam) reduction in effectiveness could only be assumed to be occurring if account was taken of the steady increase in plasma concentrations of active metabolites. For this drug it seemed that the physiological measures were those most likely to demonstrate the development of tolerance. Although triazolam showed few significant drug effects on this paradigm (testing being 12 hours after ingestion of this short half-life benzodiazepine), tolerance was seen to develop on some subjective measures. Using an alternative method of testing tolerance, assessing responses to a diazepam challenge dose, a high degree of tolerance on two-thirds of the measures was observed in subjects when pretreated with the benzodiazepine with the most marked accumulation of active metabolites (ketazolam). The other two drugs also led to tolerance development on a range of measures; this was more marked for lorazepam than triazolam. Blunting of the growth hormone response to diazepam was the most sensitive and reliable method of detecting tolerance to the benzodiazepines. Symptoms on discontinuation of the two weeks' intake of the benzodiazepines were marked for all the drugs but unrelated to either the tolerance induced or the elimination half-life of the particular drug. A further clinical study revealed that tolerance persisted in a group of long-term benzodiazepine users for between four months and two years following complete abstinence from the drug. These patients appeared to be less affected by diazepam in terms of its commonly observed subjective effects, regardless of their original medication. These ex-long-term users of benzodiazepines were, however, more likely to manifest two specific types of effects--immediate 'symptom' reduction and exacerbation of 'withdrawal symptoms' over the subsequent week.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement of a diazepam withdrawal symptom by bicuculline and yohimbine

The role of the GABA system in producing a pentylenetetrazol-like interoceptive discriminative stimulus during withdrawal from diazepam was investigated in rats by determining the sensitivity of this system to GABAergic drugs before and after chronic treatment with diazepam. Food-restricted rats were trained to obtain a reward of food by responding on one lever following an injection of pentylenetetrazol (PTZ; 20 mg/kg) and the other lever following an injection of saline (1 ml/kg). After rats had acquired this discrimination, the effectiveness of Ro 15-1788, bicuculline and yohimbine to substitute for pentylenetetrazol was determined. Prior to chronic treatment with diazepam, rats selected the appropriate lever for saline after Ro 15-1788 and the appropriate lever for pentylenetetrazol after bicuculline (0.04-2.5 mg/kg) or yohimbine (0.16-5.0 mg/kg). Although the selection of the appropriate lever for pentylenetetrazol was dose-dependent, full substitution for pentylenetetrazol was not obtained with either drug as larger doses of bicuculline produced convulsions while the rats began to select the appropriate lever for saline after larger doses of yohimbine (bell-shaped curve). Diazepam blocked the pentylenetetrazol-like interoceptive discriminative stimulus for bicuculline. The rats were then injected with diazepam (80 mg/kg/8 hr) for 24 days. Upon termination of the administration of diazepam, the animals were tested for lever-selection following the administration of saline, Ro 15-1788 (10 mg/kg), bicuculline (0.32, 0.64 and 1.25 mg/kg) or yohimbine (0.16, 0.64 and 2.5 mg/kg). After saline, 33% of the rats selected the appropriate lever for pentylenetetrazol whereas selection of this lever was enhanced after Ro 15-1788, bicuculline or yohimbine.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA induces down-regulation of the benzodiazepine-GABA receptor complex in the rat cultured neurons

Cultured neurons from embryonic rat brain display central type benzodiazepine receptors characterized by high-affinity binding of [3H]flunitrazepam which is allosterically enhanced in the presence of gamma-aminobutyric acid (GABA). A 48 h treatment of the cultured neurons with 1 microM diazepam, 0.1 microM clonazepam or 0.1 microM beta-carboline ester derivatives did not change either Bmax or KD values of the [3H]flunitrazepam specific binding. A 48 h incubation in the presence of GABA (1 mM) or muscimol (0.1 mM) induced a 30% decrease of the Bmax value of [3H]flunitrazepam specific binding without change of the KD value. The down-regulation was dependent on GABA concentrations and temperature, and was partially inhibited by bicuculline but not by the benzodiazepine antagonist Ro 15-1788. The other subunits of the benzodiazepine-GABA-chloride channel receptor complex also seemed to be down-regulated by GABA since there was a decrease of the specific binding of [3H]muscimol and [35S]t-butylbicyclophosphorothionate (TBPS) to the GABAA and chloride channel sites respectively. The GABA-induced down-regulation of the GABA-benzodiazepine receptor seems to be selective since the specific binding of ligands to other receptors was not affected. Our results suggests that activation of the low-affinity GABA subunit which is involved in cellular electrophysiological responses, induced the receptor down-regulation.

Lorazepam and FG 7142 induce tolerance to the DMCM antagonistic effect of benzodiazepine receptor ligands

Mice were given chronic treatment with lorazepam 10 mg/kg PO or FG 7142 40 mg/kg IP once a day for 14 days. The pretreatments with lorazepam and FG 7142 did not change the sensitivity of the mice to the convulsant effect of DMCM. Lorazepam pretreated mice showed a significantly lower sensitivity to the anticonvulsant effects of the benzodiazepine (BZ) receptor ligands lorazepam, ZK 93423, ZK 91296, Ro 15-1788 and ZK 93426 administered acutely by the IP route when challenged with DMCM 24 hr after the last dose of lorazepam. FG 7142 pretreated mice showed a significantly lower sensitivity to the anticonvulsant effect of the two agonists lorazepam and ZK 93423 and to the antagonist Ro 15-1788, whereas the effects of ZK 91296 and ZK 93426 were left unchanged. The reduced DMCM antagonistic effects of the BZ receptor ligands may indicate that these ligands may either have lost some of their affinity to those BZ receptors being responsible for the DMCM-induced seizures or they may have lost some efficacy in allosterically inhibiting DMCM binding or as a third possibility may have lost efficacy at a BZ receptor site downstream to the seizure-inducing center in the brain.

Time course for development of anticonvulsant tolerance and GABAergic subsensitivity after chronic diazepam

The time courses for development of neuronal and behavioral tolerance to diazepam (DZ) were estimated in rats continuously exposed to low levels of DZ for 3, 7, 14 or 21 days. Microiontophoretic sensitivity of dorsal raphe neurons to gamma-aminobutyric acid (GABA) was initially facilitated after short-term exposure to DZ released from implanted capsules for up to 3 days but returned to control levels by 7 days postimplantation and continued to decrease thereafter. GABAergic sensitivity remained depressed for a minimum of 5 days following removal of DZ capsules. To obtain a behavioral measure of tolerance, the anticonvulsant activity of DZ against bicuculline-induced seizures was also assessed. Rats studied 3 days after capsule implantation showed a significant elevation in seizure threshold. Seizure liability returned to control levels ca. 7 days after chronic treatment was initiated. These results indicate that tolerance to anticonvulsant efficacy against bicuculline seizures are temporally related to the onset of reduced GABA sensitivity on dorsal raphe neurons during prolonged exposure to DZ.

Protracted treatment with diazepam increases the turnover of putative endogenous ligands for the benzodiazepine/beta-carboline recognition site

DBI (diazepam-binding inhibitor) is a putative neuromodulatory peptide isolated from rat brain that acts on gamma-aminobutyric acid-benzodiazepine-Cl- ionophore receptor complex inducing beta-carboline-like effects. We used a cDNA probe complementary to DBI mRNA and a specific antibody for rat DBI to study in rat brain how the dynamic state of DBI can be affected after protracted (three times a day for 10 days) treatment with diazepam and chlordiazepoxide by oral gavage. Both the content of DBI and DBI mRNA increased in the cerebellum and cerebral cortex but failed to change in the hippocampus and striatum of rats receiving this protracted benzodiazepine treatment. Acute treatment with diazepam did not affect the dynamic state of brain DBI. An antibody was raised against a biologically active octadecaneuropeptide (Gln-Ala-Thr-Val-Gly-Asp-Val-Asn-Thr-Asp-Arg-Pro-Gly-Leu-Leu-Asp-Leu-Lys ) derived from the tryptic digestion of DBI. The combined HPLC/RIA analysis of rat cerebellar extracts carried out with this antibody showed that multiple molecular forms of the octadecaneuropeptide-like reactivity are present and all of them are increased in rats receiving repeated daily injections of diazepam. It is inferred that tolerance to benzodiazepines is associated with an increase in the turnover rate of DBI, which may be responsible for the gamma-aminobutyric acid receptor desensitization that occurs after protracted benzodiazepine administration.

Chronic treatment with lorazepam and FG 7142 may change the effects of benzodiazepine receptor agonists, antagonists and inverse agonists by different mechanisms

Treatment of mice with lorazepam 10 mg/kg p.o. or FG 7142 40 mg/kg i.p. once a day for 14 days changed the effects of benzodiazepine (BZ) receptor ligands injected acutely on the threshold of pentylenetetrazol (PTZ)-induced seizures. The effects of the two pretreatments differed qualitatively as well as quantitatively. Lorazepam elicited a shift in the effects of all BZ receptor ligands tested, whereby the agonists lorazepam and ZK 93423 now acted like partial agonists given acutely, the partial agonist ZK 91296 acted like an antagonist and the antagonists Ro 15-1788 and ZK 93426 like partial inverse agonists. The proconvulsant effects of the partial inverse agonist FG 7142 and the full inverse agonist DMCM on the PTZ-induced seizures did not change. However, FG 7142 became a full inverse agonist i.e. became convulsant, and DMCM may have increased in potency as a convulsant. After FG 7142 pretreatment lorazepam and ZK 93423 behaved like partial agonists given acutely whereas there was no change in effect for ZK 91296, Ro 15-1788 and ZK 93426. FG 7142 became convulsant (i.e. kindling occurred) and the potency of DMCM as a convulsant was non-significantly increased, while their proconvulsant effects with respect to PTZ-induced seizures were not altered. The fact that the effects of the two very different pretreatments on the BZ receptor ligand continuum were in the same direction may be explainable by assuming two different mechanisms, both of which may involve the GABA receptors.

Periodic benzodiazepine antagonist administration prevents benzodiazepine withdrawal symptoms in primates

Daily administration of diazepam (1.5 or 6 mg/kg) in Rhesus monkeys results in the progressive development of physical dependence, as evidenced by Ro15-1788 (5 mg/kg i.m.) precipitated withdrawal symptoms including retching, vomiting, face and limb tremors. Every third day administration of the benzodiazepine antagonist, Ro15-1788, during a similar period of continuous diazepam exposure, significantly decreases withdrawal behaviors. During the course of diazepam exposure (with or without periodic Ro15-1788 administration) effects of chronic diazepam on spontaneously elicited sedative and active behaviors were not altered. It is postulated that physical dependence reverts to a drug naive state after each exposure to the benzodiazepine antagonist. This treatment may represent a possible therapeutic approach for preventing the (time dependent) development of physical dependence and the accompanying severe withdrawal symptoms.

The nullification by diazepam of haloperidol-induced increases in the level of striatal dopamine but not in the activity of glutamic acid decarboxylase

In the therapeutic management of neuroleptic-induced tardive dyskinesia, diazepam, baclofen or gamma-vinyl-gamma-aminobutyric acid have been advocated. It has been postulated, but not proven, that the beneficial effects of these agents in tardive dyskinesia may be mediated by enhancing GABAergic transmission. In this study, it is reported that, during a 3-day withdrawal period following daily administration of 3 mg/kg of haloperidol (i.p.) for 3 weeks, the activity of glutamic acid decarboxylase in the striatum increased from 72.6 +/- 7.8 to 92.5 +/- 10.2 nmol 14CO2/mg protein/hr, and the concentration of dopamine in the striatum increased from 7.87 +/- 0.23 to 8.86 +/- 0.38 micrograms/g wet tissue. Diazepam (5 mg/kg, i.p.), given during the withdrawal period from haloperidol was able to nullify the enhancement in the concentration of dopamine but not in the activity of glutamic acid decarboxylase in the striatum. The results of these studies are interpreted to indicate that the reported beneficial effects of diazepam and GABA-mimetic agents in ameliorating the symptoms of tardive dyskinesia may occur through a mechanism which does not necessarily link transmission involving both dopamine and GABA.

Continuous release of diazepam: electrophysiological, biochemical and behavioral consequences

Neuronal GABAergic sensitivity was assessed using electrophysiological, biochemical and behavioral techniques following the continuous release and maintenance of relatively constant brain levels of diazepam for greater than or equal to 21 days. Our studies indicate that long-term exposure to diazepam results in: (1) a decrease in iontophoretic sensitivity to GABA in the dorsal raphe nucleus, (2) an increase in the affinity of the GABA recognition site in brain tissue and (3) an increase in susceptibility to bicuculline-induced seizures in the intact animal. Since the decrease in GABAergic responsiveness was observed in the presence of measurable levels of diazepam, it was concluded that this subsensitivity phenomenon is associated with tolerance and not with withdrawal effects of the benzodiazepines.

Tolerance to the anticonvulsant action of benzodiazepines. Relationship to decreased receptor density

Tolerance to the anticonvulsant action of benzodiazepines was studied in rats that had been treated for 4 weeks with 100-150 mg/kg per day of flurazepam. Previous studies had shown that this treatment produced tolerance to motor impairment induced by benzodiazepines and also down-regulation of benzodiazepine receptors in brain, which was seen as a reduced number of binding sites with no change in binding affinity. In the present study, seizures were produced using pentylenetetrazol (PTZ). In rats that had been chronically treated with flurazepam, pretreatment with diazepam was significantly less effective in blocking pentylenetetrazol-induced seizures, thus indicating tolerance. This tolerance could not be explained by a change in sensitivity to pentylenetetrazol resulting from chronic treatment, nor by any differences in levels of active drug in the brain following doses of diazepam. Residual amounts of flurazepam and its active metabolites may have artifactually reduced the apparent degree of tolerance measured 12 hr after the end of chronic treatment, but not at later times. Tolerance to the antipentylenetetrazol action of diazepam was evident up to the fourth day following chronic treatment with flurazepam, but tolerance had largely disappeared a week after chronic treatment. The duration of tolerance was much longer than that reported for tolerance to motor impairment induced by benzodiazepines, and for down-regulation of receptors. These results suggest that different mechanisms or different neural systems must mediate tolerance to these different actions of benzodiazepines. Furthermore, an adaptive reduction in the number of benzodiazepine receptors does not seem to be a likely mechanism for tolerance to the anticonvulsant action of these drugs.

Spontaneous and RO 15-1788-induced reversal of subsensitivity to GABA following chronic benzodiazepines

Chronic daily injections of diazepam (5 mg/kg i.p.) for 21-30 days have previously been shown to cause a selective subsensitivity to microiontophoretically applied GABA in serotonergic dorsal raphe neurons in the rat. Following termination of chronic benzodiazepine treatment, GABAergic sensitivity remained depressed for up to 96 h even though pharmacologically active concentrations of diazepam and its active metabolites could no longer be detected in brain tissue. In contrast, a single injection of the specific benzodiazepine antagonist, RO 15-1788, given 22 h prior to electrophysiological recording, restored GABAergic sensitivity to the control range. Sensitivity of dorsal raphe neurons to serotonin was not altered by either chronic treatment with or withdrawal from diazepam. These results indicate that (1) RO 15-1788 can accelerate the time course of withdrawal and (2) administration of this benzodiazepine antagonist can induce a persistent change in GABAergic responsiveness.

Time course for development of benzodiazepine tolerance and physical dependence

Chronic benzodiazepine treatment elicits adaptive responses in the CNS, seen behaviorally as functional tolerance and physical dependence. Experiments are described in which a radioreceptor assay is used to follow benzodiazepine activity in CSF samples during daily flurazepam treatment of cats. Tolerance is evident even after the second dose, despite increasing CSF drug activity, showing a large and rapidly developing functional tolerance. Other studies are discussed which also show tolerance within 24 hours of initiating benzodiazepine treatment. In contrast, a spontaneous withdrawal syndrome is usually seen only after prolonged treatment with high doses. However, physical dependence can also be studied by precipitating abstinence with a benzodiazepine antagonist, such as Ro15-1788. Cats were treated daily with flurazepam, then Ro15-1788 was given and abstinence signs were recorded. Abstinence could be precipitated 24 hours after beginning treatment, and dependence was nearly maximal after 7 days. Dependence developed during treatment with as little as 0.5 mg/kg flurazepam, which is near threshold for any behavioral response. Chronic diazepam caused the same dependence as flurazepam. Thus, the development of tolerance and physical dependence both show a remarkably rapid adaptation of the CNS in response to benzodiazepines.

Discriminative stimulus properties of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), an inverse agonist at benzodiazepine receptors

Rats (N = 8) were trained to discriminate the stimulus properties of the potent benzodiazepine (BZ) receptor inverse agonist methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) from saline in a two-lever operant task. The initial training dose of DMCM was 0.4 mg/kg at which the discrimination developed slowly; increasing the dose to 0.8 mg/kg resulted in rapid acquisition. However, since convulsions eventually developed during further training (sensitization), the training dose was finally individualized below the convulsive threshold (0.4-0.7 mg/kg). The DMCM cue was mimicked by FG 7142 (10 mg/kg), a non-convulsant anxiogenic beta-carboline, by pentylenetrazol (20-30 mg/kg), and by the GABA antagonist bicuculline (2 mg/kg). The DMCM cue was not, or marginally, blocked by diazepam (2.5 mg/kg) or pentobarbital (10-15 mg/kg). Furthermore, the BZ receptor antagonists CGS 8216 (2.5 mg/kg), ZK 93426 (20 mg/kg), and Ro 15-1788 (20-80 mg/kg) also did not, or only marginally, block the DMCM cue. However, the receptor antagonists (alone) substituted for DMCM although Ro 15-1788 was less effective. The partial BZ receptor agonist ZK 91296 (25 mg/kg), which is structurally similar to DMCM, blocked completely the DMCM stimulus effect. THIP (4 mg/kg) did not block the DMCM cue. To explain these results, we suggest that the repeated DMCM treatment, necessary for maintaining the discrimination, shifts the balancing point ("set-point") for positive (i.e., BZ-like) agonist efficacy versus inverse agonist efficacy, towards inverse action. This hypothesis was supported by the finding of an enhanced ability of GABA to reduce 3H-DMCM binding to cortical neuronal membranes of animals treated chronically with DMCM in a regimen similar to that used to maintain the DMCM discrimination. Furthermore, this treatment did not affect baseline 3H-DMCM binding, baseline or GABA stimulated 3H-diazepam binding, or 35S-TBPS binding (to chloride channels).

Tolerance to the benzodiazepine diazepam in an animal model of anxiolytic activity

The antipunishment properties of diazepam (DZP) were investigated in mice treated acutely, or following nine daily treatments with either DZP (5 mg/kg, PO) or its vehicle. Acutely, or following chronic vehicle treatment, DZP produced a dose-related increase in activity punished by footshock. Following chronic DZP, test doses of DZP given 24 or 48 h following the last chronic treatment were no longer, or less effective in enhancing punished activity. Effects on unpunished activity were unaffected. In a study of the time course of tolerance development, tolerance was not seen after one or three daily treatments but was present after 6 days. Following establishment of tolerance by 9 days' treatment, the antipunishment activity of DZP reappeared after 8 days' withdrawal and was restored to acute levels after 16 days. Tolerance was not associated with changes in benzodiazepine (BZ) receptor affinity or numbers, but the ability of GABA to enhance BZ binding was increased. There was no change in the ability of DZP or the convulsant beta-carboline DMCM to modulate 35S-TBPS binding. The mechanism of tolerance to the antipunishment properties of DZP therefore remains unknown.