Anxiogenic effects in benzodiazepine withdrawal are linked to the development of tolerance

Blockade of the mineralocorticoid receptors in the medial prefrontal cortex prevents the acquisition of one-trial tolerance in mice

One-trial tolerance (OTT) is characterized by the lack of anxiolytic-like effects of benzodiazepines in animals submitted to a trial 2 in the elevated plus-maze (EPM) and is described to be influenced by learning mechanisms. Mineralocorticoid receptors (MR) in the infralimbic subregion (IL) of the medial prefrontal cortex (mPFC) are important modulators of emotional learning, but the MR involvement in the establishment of OTT remains unclear. We investigated the effects of intra-IL infusions of RU 28318 (an MR antagonist) on the OTT to the anxiolytic effects of midazolam (MDZ, GABA_A-benzodiazepine agonist) in mice exposed to a two-trial protocol in the EPM. First, mice were treated with saline or MDZ (2mgkg^-1, i.p.) 30minutes before trial 1 or 2 in the EPM, to characterize the OTT. To investigate the role of MR in the OTT, independent groups of mice received intra-IL infusions of vehicle or RU 28318 (5 or 10ng 0.1µL^-1) immediately before or after first trial in the EPM. Twenty-four hours later, the same mice received injections of saline or MDZ and were re-tested in the EPM. The MDZ decreased anxiety-like behaviors in trial 1, but the same anxiolytic-like effect was not observed in MDZ-mice prior to the second EPM test, confirming the OTT. Blockade of MR in the IL before, but not after, trial 1 restored the anxiolytic effects if MDZ administered in trial 2. These findings indicate that the MR in the IL-mPFC contributing to the OTT by mediating the acquisition, but not the consolidation of emotional learning.

Sex differences in diazepam effects and parvalbumin-positive GABA neurons in trait anxiety Long Evans rats

In clinical populations, prevalence rates for a number of anxiety disorders differ between males and females and gonadal hormones are thought to contribute to these differences. While these hormones have been shown to modulate the anxiolytic effects of the benzodiazepine agonist diazepam in some models, findings are inconsistent. Here, we tested for sex differences in response to anxiogenic stimuli following a 30-min diazepam (1.0mg/kg) pre-treatment in male and female rats showing high (HAn) and low (LAn) anxiety-like behavior on the elevated plus maze. Acute diazepam administration resulted in decreased anxiety-like behavior only in HAn males as demonstrated by a significant increase in percent open arm time in the elevated plus maze (EPM). Immunohistochemical analysis for parvalbumin (PV; a calcium-binding protein that selectively stains GABAergic neurons) in central amygdala (CeA), caudate putamen (CPu) and the hippocampus indicated the number of GABAergic interneurons in these areas differed across sex and anxiety trait. In the CPu, females had significantly more PV-immunoreactive (IR) cells than males, and LAn females had greater PV-IR neurons than HAn females. In the CeA, males displayed an increased number of PV-IR neurons compared to females, with no differences found between LAn and HAn. Further, trait differences were evident in the CA2 region of the hippocampus, regardless of sex. Taken together, these data suggest that gonadal hormones and trait anxiety may influence the sensitivity to the anti-anxiety effects of diazepam and these differences may be due in part to the distribution of GABA-containing interneurons.

βCCT, an antagonist selective for α(1)GABA(A) receptors, reverses diazepam withdrawal-induced anxiety in rats

The abrupt discontinuation of prolonged benzodiazepine treatment elicits a withdrawal syndrome with increased anxiety as a major symptom. The neural mechanisms underlying benzodiazepine physical dependence are still insufficiently understood. Flumazenil, the non-selective antagonist of the benzodiazepine binding site of GABA(A) receptors was capable of preventing and reversing the increased anxiety during benzodiazepine withdrawal in animals and humans in some, but not all studies. On the other hand, a number of data suggest that GABA(A) receptors containing α(1) subunits are critically involved in processes developing during prolonged use of benzodiazepines, such are tolerance to sedative effects, liability to physical dependence and addiction. Hence, we investigated in the elevated plus maze the level of anxiety 24 h following 21 days of diazepam treatment and the influence of flumazenil or a preferential α(1)-subunit selective antagonist βCCt on diazepam withdrawal syndrome in rats. Abrupt cessation of protracted once-daily intraperitoneal administration of 2 mg/kg diazepam induced a withdrawal syndrome, measured by increased anxiety-like behavior in the elevated plus maze 24 h after treatment cessation. Acute challenge with either flumazenil (10mg/kg) or βCCt (1.25, 5 and 20 mg/kg) alleviated the diazepam withdrawal-induced anxiety. Moreover, both antagonists induced an anxiolytic-like response close, though not identical, to that seen with acute administration of diazepam. These findings imply that the mechanism by which antagonism at GABA(A) receptors may reverse the withdrawal-induced anxiety involves the α(1) subunit and prompt further studies aimed at linking the changes in behavior with possible adaptive changes in subunit expression and function of GABA(A) receptors.

Animal tests of anxiety

Animal tests of anxiety are used to screen novel compounds for anxiolytic or anxiogenic activity, to investigate the neurobiology of anxiety, and to assess the impact of other occurrences such as exposure to predator odors or early rearing experiences. This unit presents protocols for the most commonly used animal tests of anxiety. The Geller-Seifter conflict test, the social interaction test, light/dark exploration, the elevated plus-maze, defensive burying, and the thirsty rat conflict. The protocols are described in terms of drug screening tests, but can be modified easily for other purposes.

Social separation and diazepam withdrawal increase anxiety in the elevated plus-maze and serotonin turnover in the median raphe and hippocampus

The present work aimed to evaluate the effects of social separation for 14 days (chronic stress) and of withdrawal from a 14-day treatment with diazepam (acute stress) on the exploratory behaviour of male rats in the elevated plus-maze and on serotonin (5-hydroxytryptamine) turnover in different brain structures. Social separation had an anxiogenic effect, evidenced by fewer entries into, and less time spent on the open arms of the elevated plus-maze. Separation also selectively increased 5-hydroxytryptamine turnover in the hippocampus and median raphe nucleus. Diazepam withdrawal had a similar anxiogenic effect in grouped animals and increased 5-hydroxytryptamine turnover in the same brain structures. Chronic treatment with imipramine during the 14 days of separation prevented the behavioural and neurochemical changes caused by social separation. It is suggested that the increase in anxiety determined by both acute and chronic stress is mediated by the activation of the median raphe nucleus-hippocampal 5-hydroxytryptamine pathway.

Animal tests of anxiety

Animal tests of anxiety are used to screen novel compounds for anxiolytic or anxiogenic activity, to investigate the neurobiology of anxiety, and to assess the impact of other occurrences such as exposure to predator odors or early rearing experiences. This unit presents protocols for the most commonly used animal tests of anxiety. The Geller-Seifter conflict test, the social interaction test, light/dark exploration, the elevated plus-maze, defensive burying, and the thirsty rat conflict. The protocols are described in terms of drug screening tests, but can be modified easily for other purposes.

The blockade of AMPA-kainate and NMDA receptors in the dorsal periaqueductal gray reduces the effects of diazepam withdrawal in rats

It is well established that the most persistent sign of withdrawal from chronic benzodiazepine use in humans is anxiety. In contrast to other types of drugs of abuse, the emergence of this anxiety does not seem to be linked directly to alterations in the levels of dopamine in the mesolimbic system. Some studies have proposed that fear-like behaviors elicited by benzodiazepine withdrawal could be the result either of alterations in the sensitivity of GABAA receptors or in the neuronal hyperexcitability that results from neuroadaptative responses to chronic treatment, probably mediated by glutamate. The increased fear-like behaviors induced by benzodiazepine withdrawal are similar to the defense reaction displayed by animals exposed to dangerous situations or submitted to electrical or chemical stimulation of the dorsal periaqueductal gray (dPAG), a key structure of the brain aversive system. However, the involvement of the dPAG in drug abuse has been investigated only in the context of the physical effects of drug dependence. Thus, in this study we investigated the effects of injections into the dPAG of the glutamic acid diethyl ester (GDEE) and 2-amino-7-phosphonoheptanoate (AP-7) (AMPA-kainate and NMDA receptors antagonists, respectively) on fear-like behaviors promoted by benzodiazepine withdrawal in rats submitted to aversive events (foot-shocks) immediately before chronic diazepam administration in a conditioning place-preference paradigm, using a light-dark box. Our results showed that inhibition of the glutamatergic neurotransmission in the dPAG reduces the consequence of the diazepam withdrawal in rats, implicating the excitatory amino acids of the dPAG in the modulation of the aversive state induced by benzodiazepine drugs withdrawal.

Identifying the genetic determinants of emotionality in humans; insights from rodents

The identification of human quantitative trait genes underlying variation in emotionality has proved to be both difficult and expensive. Whilst several QTL have been consistently identified and independently replicated across both phenotypes and populations, little progression has been made towards the identification of underlying genes or even variants. In this review we discuss the potential for using cross-species QTL concordance as a tool for QTL dissection in behaviour genetics, using an affect-related locus mapped to human chromosome 1 as an example.

Effects of nicotine and a cannabinoid receptor agonist on negative contrast: distinction between anxiety and disappointment?

RATIONALE

Animals trained to lick for a sucrose solution of a given incentive value that subsequently encounter an incentive downshift (i.e. 32-4% sucrose) display an exaggerated decrease in the amount consumed, relative to unshifted controls. This change has been classified as a successive negative contrast (SNC) effect. The emotional component to this robust behavioural change is dynamic and changes from post-shift day (PSD) 1 to 2. Anxiolytics block SNC, but the possible link between anxiety and SNC needs further exploration. Both nicotine and a cannabinoid receptor agonist have been reported to change anxiety and both have actions on the reward process, but their effects on SNC have not been investigated.

OBJECTIVES

To determine: (1) whether exposure to SNC evokes an anxiogenic response; (2) whether an anxiolytic dose of nicotine has the same effects on SNC as those of chlordiazepoxide; (3) the effects of a low (anxiolytic) and a high (anxiogenic) dose of the cannabinoid receptor agonist CP 55,940 on SNC.

METHODS

Two groups of animals were given access to high (32%) or low (4%) sucrose solutions for 5 min per day for 10 days. On PSD 1 and 2, the shifted group had access to a devalued incentive (from 32 to 4% sucrose) and the unshifted group remained at 4% sucrose. The volumes (ml) of sucrose solution consumed were measured pre-shift and on PSD 1 and 2. In experiment 1, immediately after SNC testing on PSD 1 and 2, the rats were tested in the social interaction and elevated plus-maze tests of anxiety. In experiment 2, the effects of chlordiazepoxide (5 and 7.5 mg/kg) and nicotine (0.1 mg/kg) were examined on PSD 1 and 2. In experiment 3, the effects of CP 55,940 (5 and 40 microg/kg) were examined on PSD 1 and 2.

RESULTS

There were no anxiogenic effects of shift in either test of anxiety on either test day. However, on PSD 1, the shifted group had significantly higher locomotor activity and spent a higher percentage of time on the open arms, perhaps reflecting search strategies. Nicotine was without significant effect on SNC on either test day. On PSD 1, chlordiazepoxide (5 mg/kg) and CP 55,940 (5 and 40 microg/kg, IP) blocked SNC. On PSD 2, both doses of chlordiazepoxide and the low, anxiolytic dose of CP 55,940 (5 microg/kg) blocked SNC, the high dose of CP 55,940 was without effect.

CONCLUSIONS

The pattern of results allows for the separation between effects on anxiety and SNC. The block of contrast on PSD 1 was independent of changes in anxiety, since both anxiolytic and anxiogenic drug doses were effective. It is suggested that this may provide an animal model of disappointment in which the cannabinoid system plays an important role. An anxiolytic action would seem to be a necessary, but not a sufficient, action to block SNC on PSD 2.

Elevated startle during withdrawal from acute morphine: a model of opiate withdrawal and anxiety

RATIONALE

An elevated startle response has been observed in humans and animals during withdrawal from multiple substances of abuse, a phenomenon thought to reflect the anxiogenic effects of withdrawal. Although anxiety is a common symptom of opiate withdrawal, few studies have examined the effects of morphine withdrawal on acoustic startle.

OBJECTIVE

To develop a procedure for assessing opiate dependence through measurement of the startle reflex in rats.

METHODS

The effects of opiate withdrawal on startle were evaluated using both spontaneous and naloxone-precipitated withdrawal from an acute dose of morphine. The ability of the treatment drugs clonidine and chlordiazepoxide to block withdrawal-induced increases in startle was also tested.

RESULTS

Spontaneous withdrawal from an injection of morphine sulfate produced a significant increase in acoustic startle 2 h (3.2 mg/kg) or 4 h (10 mg/kg) after drug administration. Morphine withdrawal (10 mg/kg morphine sulfate) precipitated by the opiate antagonist naloxone (2.5 mg/kg) also produced a significant increase in startle magnitude. This elevation of startle was blocked by both clonidine (35 microg/kg) and chlordiazepoxide (10 mg/kg).

CONCLUSIONS

These data demonstrate that both spontaneous and precipitated withdrawal from an acutely administered opiate produce anxiety-like effects on acoustic startle. This paradigm may be useful in the study of anxiety and the early mechanisms of drug dependence.

Anxiogenic effects of neurosteroid exposure: sex differences and altered GABAA receptor pharmacology in adult rats

Acute exposure to progesterone or its neurosteroid derivative allopregnanolone (3alpha,5alpha-THP) is anxiolytic, consistent with the GABA modulatory effects of 3alpha,5alpha-THP at the GABA(A) receptor. However, continuous exposure to progesterone increases anxiety in association with increased expression of the benzodiazepine-insensitive GABA(A) receptor alpha4 subunit. Furthermore, negative mood symptoms and altered GABA(A) receptor pharmacology in patients with premenstrual dysphoric disorder occur in the early luteal phase in association with peak circulating levels of progesterone and 3alpha,5alpha-THP. Because sex differences have been reported in steroid-regulated anxiety responses, the present study investigated the role of sex and development in the regulation of anxiety after short-term exposure to 3alpha,5alpha-THP. To this end, we compared the effects of hormone administration in adult male, adult female, and juvenile female rats. Increased anxiety in the elevated plus maze was evident in all groups after 48-h exposure to either 3alpha,5alpha-THP or progesterone. At this time point, alterations in the anxiolytic profile of benzodiazepine agonists and antagonists were also observed in both adult males and females in the elevated plus maze. However, sex differences in the acoustic startle response were observed after short-term hormone treatment such that only female rats displayed an increased response indicative of higher anxiety levels. These results suggest that although neurosteroid exposure may influence both the pharmacological properties of the GABA(A) receptor and the manifestation of anxiety in both sexes, the effects of neurosteroids may be modulated in a sex- and task-specific manner.

Tolerance to midazolam's anxiolytic effects after short-term nicotine treatment

In the social interaction test of anxiety, microinjections of midazolam (2-8 microg) into the dorsal hippocampus or dorsal raphé nucleus significantly increased the time spent in active social interaction, without changing locomotor activity, thus indicating specific anxiolytic effects. However, tolerance developed to these effects in rats that had been pre-treated for 6 days with (-)-nicotine (0.1 mg/kg/day; subcutaneous). Thus, cross-tolerance to the anxiolytic effects of midazolam develops rapidly following a short period of treatment with a low dose of nicotine, which contrasts with the more slowly developing tolerance (about 3 weeks) that develops after benzodiazepine treatment. Following 6 days of nicotine treatment there was a significant reduction in [(3)H]flunitrazepam binding at 2 and 10 nM in the hippocampus, but no change in the midbrain. The decrease in benzodiazepine binding could explain tolerance to the effects of midazolam when administered to the dorsal hippocampus, but other mechanisms, such as indirect effects on the serotonergic (5-HT) system, might be involved in tolerance to the effects of dorsal raphé nucleus administration.

Tolerance to nicotine's effects in the elevated plus-maze and increased anxiety during withdrawal

In the elevated plus-maze test of anxiety, nicotine (0.1 mg/kg sc; 30 min after injection) had a significant anxiogenic effect, shown by specific decreases in the percentage of time spent on the open arms and in the percentage of open-arm entries. Tolerance developed to this anxiogenic effect after 7 days of nicotine treatment (0.1 mg/kg/day). Five minutes after an acute injection, nicotine (0.1 mg/kg) was ineffective, but after 7 days of treatment a significant anxiolytic effect, shown by specific increases in the percentage of time spent on the open arms and in the percentage of open-arm entries, emerged. After 14 days of nicotine treatment, tolerance developed to this anxiolytic effect. There was a complete dissociation between the effects of nicotine on the measures of anxiety, and on the locomotor activity as measured by closed-arm entries. No changes in closed-arm entries were found after acute administration of nicotine, but rats tested 30 min after their 7th injection made significantly fewer, and those tested 5 min after their 14th injection made significantly more, entries than their respective controls. Rats that were tested after 24 h withdrawal from six daily nicotine injections showed a significant anxiogenic effect. A low dose of nicotine (5 ng) injected into the dorsal hippocampus was without effect in vehicle pretreated rats, but it was able to reverse the anxiogenic effect found after 24 h of withdrawal from 6 days of nicotine treatment.

Tolerance to anxiolytic- and antidepressant-like effects of a partial agonist of glycineB receptors

The present study examined effects of acute and repeated administration of 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist of glycineB receptors, in the conflict drinking test and the forced swim test in rats. Diazepam and imipramine were used, respectively, as reference drugs in those tests. In the conflict drinking test, acute administration of ACPC (200 mg/kg) increased fivefold the number of punished licks. A three- and fivefold increase in the number of punished licks was observed in rats treated repeatedly with ACPC (200 mg/kg daily; 14 days) and challenged with the same dose of the drug 24 h or 4 days later, respectively. A single injection of ACPC (400 mg/kg) reduced by 40% the immobility time in the forced swim test. In rats treated repeatedly with ACPC (400 mg/kg daily; 14 days) and challenged with the same dose 24 h or 4 days later, the drug either produced no significant effect or reduced the immobility time by 50%, respectively. On the other hand, no changes in anxiolytic- and antidepressant-like effects of chronically administered diazepam (10 mg/kg daily; 14 days) and imipramine (30 mg/kg daily; 14 days), respectively, were observed. The above results indicate that tolerance develops to the anxiolytic- and, particularly, to the antidepressant-like activity of ACPC.

Aggressive and social behavior after alprazolam withdrawal: experimental therapy with Ro 19-8022

The aim of the present study was to ascertain whether withdrawal from alprazolam can increase anxiety-like and aggressive behavior during intra species conflict in mice and, if so, whether a partial benzodiazepine agonist Ro 19-8022 is able to reverse these behavioral changes without untoward effects such as sedation. An experimental model consisting of interactions of pairs of singly-housed male mice with non-aggressive group-housed male mice was used. Alprazolam (1 mg/kg) was given orally twice daily for 8 days and once on the 9th day. When withdrawn from alprazolam (3 days after the last dose), mice reduced social investigation and increased the incidence of aggressive behavior in comparison to the pre-withdrawal level. However, the increase of aggression was moderate and occurred only in subjects with low pre-treatment levels of aggression. Ro 19-8022 (10 mg/kg) significantly antagonized the decrease of social behavior and the increase of aggression after alprazolam withdrawal without causing sedation or ataxia.

Tolerance to acute anxiolysis but no withdrawal anxiogenesis in mice treated chronically with 5-HT1A receptor antagonist, WAY 100635

Anxiolytic-like activity in the mouse elevated plus-maze has recently been demonstrated for a range of compounds varying in degree of selectivity as 5-HT1A receptor antagonists. As tolerance and dependence liability are among the major clinical disadvantages of benzodiazepine therapy, the present study examined the effects of acute drug challenge on the plus-maze profiles of mice following daily treatment for 20 days with saline, chlordiazepoxide (CDP; 10.0 mg/kg) or the selective 5-HT1A receptor antagonist, WAY 100635 (0.1-1.0 mg/kg). To assess the development of physical dependence (withdrawal anxiogenesis), the study incorporated independent groups of animals tested on the maze 24 h after the final dose. Challenge with CDP or WAY 100635 produced behavioural changes indicative of anxiety reduction in mice that had received daily handling/saline for 20 days, thereby demonstrating that the chronic injection regimen per se had not compromised the acute efficacy of either agent. The absence of a similar response to acute drug challenge in mice treated chronically with CDP or WAY 100635 suggested the development of tolerance to the acute anxiolytic effects of both compounds under present test conditions. Despite these observations, however, no signs of enhanced anxiety were evident 24 h following discontinuation of chronic treatment with either compound. In a further experiment, the absence of withdrawal anxiogenesis at 24 h was replicated and extended to discontinuation periods of 36 and 48 h for both drugs. Although present results show that tolerance develops to the acute anxiolytic effects of CDP and WAY 100635 in the murine plus-maze, they also suggest that enhanced anxiety is not an inevitable consequence of abrupt cessation of chronic treatment with either compound.

Lack of tolerance to the anxiolytic effect of diazepam and pentobarbital following chronic administration in perinatally undernourished rats

Adult female rats, undernourished at perinatal age, were evaluated for anxiolytic action in the plus-maze test after acute and chronic administration of diazepam (DZP) and pentobarbital (PTB). Deprived (D) rats chronically treated with vehicle showed an increased anxiety as compared with control (C) animals. A single intraperitoneal (i.p.) administration of DZP (1 mg/kg) or PTB (7.5 mg/kg) produced similar anticonflict effect in both C and D rats. Tolerance to the anxiolytic effect of DZP and PBT developed in C rats after a 15-day administration schedule, whereas no tolerance was observed in D animals. Drug disposition was not altered after chronic treatment either in C or in D rats. Gamma-aminobutyric acid (GABA)-mediated chloride uptake in microsacs of cerebral cortex of naive D rats was decreased as compared with naive C rats. After chronic DZP administration (1 mg/kg/day i.p. for 15 days), GABA-mediated 36Cl- influx in brain cortex microsacs of C rats did not change; however, GABA efficacy was increased in microsacs of D animals. In addition, chronic DZP treatment induced GABA-benzodiazepine uncoupling in brain cortex of C rats, but not in D animals, as assessed by chloride uptake in microsacs. Chronic PTB treatment (7.5 or 30 mg/kg/day i.p. for 15 days) did not modify GABA stimulation or GABA-PTB interaction in cortical microsacs of C or D rats.

An anxiolytic-like effect of ondansetron disappears in oxazepam-tolerant rats

In our experiments a drug from the group of 5-HT3 antagonists--ondansetron (OND)--has been used in rats developing tolerance to oxazepam (OXZ). After 7 days of oxazepam administration (5 mg/kg i.p.) a significant decrease in the anxiolytic behavior was observed in the Crawley test. In the rats already partly tolerant to oxazepam, an undiminished anxiolytic-like effect of ondansetron (single injection of 0.1 mg/kg i.p., seventh day) was observed. After 14 days of oxazepam administration its anxiolytic activity was even more diminished. A single injection of ondansetron 0.1 mg/kg restored the anxiolytic behavior: rise of BWT (black-white transition) and WSE (white square entrance). After 21 days the anxiolytic activity of oxazepam was totally abolished and the single injection of ondansetron did not restore the state of anxiolysis. The results show that the anxiolytic effects of ondansetron were not influenced in the first stages of tolerance development to oxazepam, but the drug was not able to produce an anxiolytic effect in the state of full tolerance to oxazepam (after 3 weeks).

Dorsal raphe and substantia nigra response to flumazenil in diazepam-dependent rats

Flumazenil (FLU; 25 micrograms) and DMSO-vehicle were focally injected (1 microliter) into the substantia nigra (SN) and the dorsal raphe nucleus (DR) in rats chronically implanted with silastic capsules containing diazepam (DZ; 540 mg/week). FLU precipitated an abstinence syndrome in the SN as indicated by a significant abstinence score, several abstinence signs and reduced total power of the fast frequency bands of the electroencephalogram (EEG) in the injections sites frontal cortex, (FC) and hippocampus (H). In contrast, FLU did not produce an abstinence syndrome in the DR, and its effect on the power of the EEG in DR, FC and H was not significantly different from that of the DMSO-vehicle. The data show regional heterogeneity in the response of the SN and the DR to chronic DZ treatment in terms of a focally precipitated abstinence syndrome.

Tolerance to the anticonflict effects of diazepam: importance of methodological considerations

The present study examines the effects of chronic diazepam treatment on conflict behavior in rats using the Geller-Seifter paradigm. A dose-response function for the effects of diazepam (DZ) on punished and unpunished responding was determined (0.0, 0.63, 1.25, 2.5, and 5.0 mg/kg DZ intraperitoneally) using five independent groups. The test doses of DZ produced an inverted U-shaped function where punished responding increased as a function of dose up to 2.5 mg/kg and then decreased at 5.0 mg/kg. All groups were then treated with 2 x 5 mg/kg DZ per day for 5 days. When the dose-response function was redetermined at 36 h post-chronic treatment, it was found that the function had shifted to the right, indicating tolerance. Because of the inverted U-shaped nature of the original function, tolerance was manifested as a decrease in responding on the ascending portion of the function and as an increase in responding on the dose (5 mg/kg) representing the descending side of the inverted U.

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.

Effect of chronic diazepam treatment on footshock-induced ultrasonic vocalization in adult male rats

Rats were tested in the footshock-induced ultrasonic vocalization model of anxiety. The ultrasounds were recorded 1-11 min after 10 inescapable 0.6 mA footshocks each of 1 sec. duration. Repeated administration of benzodiazepines in the clinic has been reported to be accompanied by development of tolerance and withdrawal anxiety. The present study examined whether the ultrasonic vocalization model could reflect these two side effects. Diazepam 4.6 or 8.8 mumol/ kg (1.3 or 2.5 mg/kg, subcutaneously) administered twice a day (8 a.m. and 4 p.m.) abolished the vocalization after acute administration and after 3 weeks of treatment. Hence, no tolerance developed to the anxiolytic effect of diazepam. When the rats were tested 24 and 48 hr after the last doses of diazepam there were no significant differences from the control group, i.e. no apparent withdrawal anxiety. Instead, the control groups developed tolerance to the shock regimen during the chronic experiments. This was examined further by daily testing of a group of naive rats for 13 days. The findings indicated that there is a limitation in number of test sessions before tolerance to the model develops. In conclusion, the results of the present study contribute to the many contradictory and by no mean unequivocally findings in the literature. It indicates that substantial prediction of anxiolytic effects as well as unwanted side effects cannot be made from one single test model.

Chronic bretazenil produces tolerance to chlordiazepoxide, midazolam, and abecarnil

The purpose of the present study was to determine if chronic treatment with a nonsedative benzodiazepine partial agonist would confer tolerance to the rate-decreasing effects of other benzodiazepine ligands in a fixed-interval procedure in rats. A separate group of rats was treated chronically with the sedative benzodiazepine full agonist, chlordiazepoxide, for comparison. It was hypothesized that tolerance would develop rapidly to chlordiazepoxide due to loss of reinforcement density at rate-decreasing doses and that there would probably be cross-tolerance to other rate-decreasing benzodiazepine ligands such as midazolam and abecarnil. Because bretazenil does not produce rate decreases, however, it was not expected that tolerance would be found to chlordiazepoxide, midazolam, or abecarnil. After 8-12 weeks of chronic treatment with either chlordiazepoxide or bretazenil, however (final dose of benzodiazepine = 30 mg/kg/day), tolerance was found to the rate-decreasing effects of chlordiazepoxide, midazolam, and abecarnil in both groups. It is concluded that such tolerance was most likely due to a saturation of benzodiazepine receptors by this high-affinity partial agonist.

Noise stress and the development of benzodiazepine dependence in the rat

Rats housed in conditions of noise stress were given daily injections of diazepam (4 mg/kg). Significant tolerance developed to the sedative effects within 5 days of treatment, as measured by head dipping and motor activity in the holeboard and by the number of closed arm entries in the plus-maze. These results are in agreement with other reports of rapid tolerance to sedative effects. However, in contrast to the usual finding of tolerance to anxiolytic actions after 2-3 weeks of treatment, in this study no tolerance developed after 23 days of treatment to diazepam's anxiolytic effects in the plus-maze. On withdrawal from the 23 days of diazepam treatment, there was no anxiogenic response in the plus-maze. Therefore, it seems that when chronic administration of diazepam is accompanied by chronic stress, tolerance does not occur to the anxiolytic effects, although it does develop to the sedative effects.

Flumazenil induces localised increases in glucose utilization during diazepam withdrawal in rats

The quantitative [14C]2-deoxyglucose autoradiographic technique has been employed to identify the neural circuits involved in diazepam withdrawal. Local cerebral glucose utilization (LCGU) was assessed in parallel groups of rats chronically treated with diazepam (5 mg/kg i.p., daily for 28 days), in rats that were withdrawn from chronic diazepam 24 h previously and in those that received flumazenil (5 mg/kg i.v.) immediately or 24 h after the last dose of diazepam. Two further groups received chronic vehicle or acute flumazenil (5 mg/kg i.v.). Rats withdrawn from diazepam 24 h previously did not produce changes in LCGU in the 51 structures examined compared with both control and chronic diazepam treated groups, suggesting that spontaneous withdrawal from small doses of diazepam does not evoke marked alterations in functional activity. In contrast, flumazenil-precipitated diazepam withdrawal produced a marked increase in glucose use in structures of the Papez circuit of emotion (mammillary body, anterior thalamus, cingulate cortex), together with increases in the septal nucleus, basolateral amygdala and nucleus accumbens. Less widespread increases in glucose use occurred in primary auditory and visual areas and in extrapyramidal areas. This pattern resembles that produced after acute FG-7142 administration (Brain Res., 475 (1988) 218-231). In rats receiving flumazenil 24 h after the last dose of diazepam there was a similar, but more restricted, pattern of change in LCGU. Flumazenil had no effect on LCGU in drug naive rats. Thus, flumazenil could only exert an effect upon LCGU in rats chronically treated with diazepam. These data provide functional neuroanatomical evidence for a withdrawal shift in the inverse agonist direction after chronic diazepam and suggest that flumazenil-precipitated withdrawal changes may merely be a reflection of this phenomenon.

The interplay of learning and anxiety in the elevated plus-maze

In naive rats benzodiazepines have clear anxiolytic effects in the elevated plus-maze, but in rats with previous plus-maze experience benzodiazepines are ineffective. This phenomenon does not depend on the drug state on trial 1 or on the inter-trial interval and generalises across mazes of different material; it is dependent on experience of the open arm. The phenomenon of "one-trial tolerance" to the anxiolytic effect of benzodiazepines is not seen in other animal tests; and there is no equivalent phenomenon of "one-trial withdrawal" or of tolerance to anxiogenic effects in the plus-maze. The phenomenon is not seen in unhandled rats, in rats given longer trials in the plus-maze, or in those given an amnesic treatment on trial 1. It is suggested that during the first 5 min in the elevated plus-maze the rats are acquiring a fear of heights and it is this phobic anxiety state that is measured during the second 5-min trial. Thus the nature of the anxiety generated by the maze is different on trials 1 and 2. The results of a factor analysis study confirm this conclusion.

The effect of the 5-HT3 receptor antagonist, RS-42358-197, in animal models of anxiety

The S-isomer of the novel 5-HT3 receptor antagonist RS-42358 ((S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1-H- benzo[de]isoquinolin-1-one, RS-42358-197) disinhibited behaviour in the mouse suppressed by the aversive situation of the light/dark test box. RS-42358-197 was effective at sub-ng/kg dose levels and the efficacy was maintained over a 100 million-fold dose range. In contrast, the R-isomer was ineffective at all doses studied. The S-isomer also disinhibited a suppressed behaviour in social interaction and elevated X-maze tests in the rat and reduced anxiety-related behaviours in a marmoset human threat test. RS-42358-197 prevented the exacerbation of the suppression of behaviour in the mouse light/dark test following withdrawal from treatment with alcohol, nicotine, cocaine and diazepam. Thus, the S-isomer of RS-42358 has a consistent non-sedating anxiolytic profile in rodent and primate models. It is exceptionally potent and a maintained efficacy at high doses distinguishes its actions from many other 5-HT3 receptor antagonists.

Neuropharmacology of a new potential anxiolytic compound, F 2692, 1-(3'-trifluoromethyl phenyl) 1, 4-dihydro 3-amino 4-oxo 6-methyl pyridazine. 2. Evaluation of its tolerance and dependence producing potential and of its effects on benzodiazepine withdrawal in the elevated plus-maze test in rats

After 21 days administration, diazepam (0.3-3 mg/kg/day) exhibited, 30 min after the last injection, tolerance to the sedative effect and "anxiolytic" activity as recorded in the elevated plus-maze test in rats. A dose-dependent increase of "anxiety" was also observed 24 h after withdrawal from 21 or 90 days of diazepam treatment. In contrast, under the same experimental conditions, F 2692 [1-(3'-trifluoromethyl phenyl) 1,4-dihydro 3-amino 4-oxo 6-methyl pyridazine] (3-30 mg/kg/day) exhibited no tolerance to either the sedative effect or the "anxiolytic" activity and showed no "anxiogenic rebound" response after withdrawal. Chronic diazepam pretreatment for 21 days modified neither the sedative effect nor the dose-dependent "anxiolytic" effect of F 2692. Furthermore, F 2692 could reverse the anxiogenic response after withdrawal from 21 days administration of diazepam. Finally, administration of diazepam for 3 weeks followed by a daily administration of F 2692 for a week induced no increase of "anxiety" 24 h after withdrawal.

The benzodiazepines: anxiolytic and withdrawal effects

Benzodiazepine withdrawal, spontaneous or precipitated by the receptor antagonist, flumazenil, produces anxiety that can be measured in animal models. Benzodiazepine inverse agonists also cause anxiety. Their convulsive effects increase after chronic agonist treatment, but they become anxiolytic. Decreases in GABAA receptor sensitivity occur after chronic benzodiazepine treatment. Flumazenil, given 24h prior to the measurements, prevented both the sensitivity changes and benzodiazepine tolerance in vivo. The anxiety and decreases in seizure threshold during withdrawal were also prevented. It has been suggested that flumazenil causes a prolonged 'resetting' of the benzodiazepine receptor complex. Acute flumazenil decreased anxiety-related behaviour during ethanol withdrawal. Concurrent chronic treatment with verapamil completely prevented anxiety following chronic benzodiazepine treatment.

Diazepam withdrawal responses measured in the social interaction test of anxiety and their reversal by baclofen

After 21 days of treatment with diazepam (0.5 or 2 mg/kg/day) rats were tolerant to the effects of diazepam to increase social interaction in the low light unfamiliar test condition of the social interaction test of anxiety. When they were tested 24 h after the last of 21 injections they showed significant decreases in social interaction, indicating an anxiogenic withdrawal response. However, the social interaction scores of rats tested 48 h after withdrawal from diazepam treatment were no longer different from those of the control group. The decreased social interaction, indicating increased anxiety, detected 24 h after withdrawal of diazepam (21 daily injections of 0.5 or 2 mg/kg), could be reversed by the usual daily diazepam dose (0.5 or 2 mg/kg, respectively) or by baclofen (0.5 or 1 mg/kg). Baclofen (2 mg/kg) was sedative in both control treated and diazepam-dependent rats, but was ineffective at reversing the decrease in social interaction seen after diazepam withdrawal. Possible sites of action mediating these effects of baclofen are discussed, and it is suggested that either post-synaptic GABAB sites in the hippocampus are involved or that the reversal of the decreased social interaction detected on withdrawal of diazepam treatment is due to a baclofen-mediated inhibition of 5-HT release in the hippocampus.

Ondansetron inhibits a behavioural consequence of withdrawing from drugs of abuse

The ability of the selective 5-HT3 receptor antagonist ondansetron to influence the behavioural consequences of withdrawal from chronic treatment with ethanol, nicotine or cocaine was investigated in the light/dark exploration test in the mouse and social interaction test in the rat. In both tests acute and chronic (7 days) treatments with ondansetron (0.01-1.0 microgram.kg-1 IP) disinhibited suppressed behaviour; withdrawal from chronic treatment (0.1 mg/kg IP b.i.d.) did not exacerbate the behavioural suppression. Chronic treatment for 14 days with ethanol (8% w/v in the drinking water), nicotine (0.1 mg/kg b.i.d.) or cocaine (1.0 mg/kg b.i.d.) released suppressed behaviour in the mouse and rat tests. Behavioural suppression was increased following withdrawal from ethanol, nicotine and cocaine. The administration of ondansetron (0.01 mg/kg IP b.i.d.) during the period of ethanol, nicotine and cocaine withdrawal prevented the exacerbation in suppressed behaviour. It is concluded that ondansetron potently reduces behavioural suppression during acute and chronic treatments in the rodent models, does not cause a rebound exacerbation of behavioural suppression following withdrawal, and is a highly effective inhibitor of the increased behavioural suppression following withdrawal from the drugs of abuse: ethanol, nicotine and cocaine.

The history of benzodiazepine dependence: a review of animal studies

This article provides a historical review of the animal literature relating to the development of tolerance to the behavioral effects of benzodiazepines, and the incidence of biochemical and behavioral changes that result from termination of benzodiazepine treatment (spontaneous withdrawal responses). It charts the slow emergence of a pertinent animal literature and highlights conclusions that were prevalent in 1963 (at the introduction of diazepam), 1973 (at the introduction of lorazepam), 1980 and the present day. For 25 years the animal literature has lagged behind the clinical literature, but recent studies into the neurochemical mechanisms of benzodiazepine dependence and possible treatments for withdrawal responses suggest that, at last, animal experiments may be about to make a substantial contribution.

Long-term diazepam treatment produces changes in cholecystokinin receptor binding in rat brain

This study examined the effect of chronic diazepam administration on central benzodiazepine and CCK-8 receptor binding in rat brain. After a two-week treatment with diazepam (5 mg/kg per day) tolerance developed towards the sedative but not towards the anxiolytic action of this drug as determined using elevated plus-maze and open field tests. The % entries the rats made onto open arms and % time the rats spent in open arms were markedly decreased 24 h after the last dose of diazepam, probably indicating withdrawal anxiety. There were no changes in [3H]flunitrazepam binding either 30 min or 24 h after the last diazepam dose. However, 30 min after the last diazepam administration the apparent number of sulphated [3H]CCK-8 binding sites was significantly increased in the primary olfactory cortex. Acute diazepam treatment (5 mg/kg) had no influence on [3H]flunitrazepam or sulphated [3H]CCK-8 binding in any brain region studied. Cessation of chronic diazepam treatment was followed after 24 h by an increase in the number of CCK-8 receptors in frontal cortex and hippocampus as compared to the vehicle group. These results demonstrate that certain alterations in CCK-8 receptor characteristics may be important in the anti-anxiety effect, tolerance, and withdrawal reaction reaction after benzodiazepine administration.

Chronic handling modifies the anxiolytic effect of diazepam in the elevated plus-maze

Rats treated for 3 days with diazepam demonstrated a clear anxiolytic action of the drug as assessed in the elevated plus-maze. However, in a chronic experiment, in which all rats were handled for 28 days, no anxiolytic effect of diazepam could be shown either in rats treated for 3 days or in those treated for 28 days. These results suggest that there is an interaction between handling and the anxiolytic effect of diazepam.

One-trial tolerance to the anxiolytic effects of chlordiazepoxide in the plus-maze

Chlordiazepoxide (CDP 7.5 mg/kg) had a significant anxiolytic effect in rats tested on the plus-maze for the first time. On a second trial the control scores did not change, but those of the CDP group did and they no longer differed from controls. Rats previously tested undrugged or after flumazenil (4 mg/kg) also failed to show an anxiolytic response to CDP. Thus this phenomenon of one-trial tolerance depended on prior experience with the plus-maze. It also depended on CDP acting at the benzodiazepine receptors on trial 2, since the joint administration of CDP and flumazenil on trial 2 reversed the phenomenon.

Anxiogenic-like action of caerulein, a CCK-8 receptor agonist, in the mouse: influence of acute and subchronic diazepam treatment

Effects of caerulein, a cholecystokinin octapeptide (CCK-8) receptor agonist, on exploratory activity of mice were investigated. Exploratory and locomotor activity of animals were measured using elevated plus-maze and open field tests. The systemic administration of caerulein at non-sedative doses (100 ng/kg-1 micrograms/kg i.p.) resulted in a significant decrease in the exploratory activity of mice. This effect was completely blocked by proglumide, a CCK-8 receptor. Acute treatment with low doses (0.1-0.75 mg/kg i.p.) of diazepam did not attenuate the anxiogenic-like effect of caerulein, but at more high doses of diazepam the coadministration depressed locomotor activity in mice. After subchronic diazepam treatment (2.5 mg/kg once a day, 10 days, i.p.) tolerance was developed toward the sedative effect of diazepam, and 72 h after withdrawal of the drug the animals showed increased anxiety in the plus-maze test. 30 min after the last injection procedure the anxiogenic-like effect of caerulein (500 ng/kg i.p.) on exploration was absent in both diazepam or vehicle groups. However, 72 h after the last pretreatment injection caerulein (500 ng/kg i.p.) reduced significantly the exploratory activity in control group, whereas it was inactive after diazepam withdrawal. The results obtained in this study support the hypothesis that endogenous CCK-8 an CCK-8 receptors are involved in the neurochemistry of anxiety and the anxiolytic action of benzodiazepine tranquillizers.

The psychopharmacology of 5-HT3 receptors

The review presents evidence that 5-HT3 receptors within the brain may contribute to the control of behavior. 5-HT3 receptor antagonists GR38032F, zacopride, ICS 205-930 and other agents are very potent in reducing mesolimbic dopamine hyperactivity caused by the injection of amphetamine or infusion of dopamine into the rat nucleus accumbens and amygdala, and the ventral striatum of the marmoset. Such actions are distinguished from those of neuroleptic agents by a failure to reduce normal levels of activity or to induce a rebound hyperactivity after discontinuation of treatment. Indeed, the 5-HT3 receptor antagonists can prevent the neuroleptic-induced rebound hyperactivity. Further evidence that 5-HT3 receptors moderate limbic dopamine function is shown by their ability to reduce both the behavioral hyperactivity and changes in limbic dopamine metabolism caused by DiMe-C7 injection into the ventral tegmental area. The 5-HT3 receptor antagonists also have an anxiolytic profile in the social interaction test in the rat, the light/dark exploration test in the mouse, the marmoset human threat test and behavioral observations in the cynomolgus monkey. They differ from the benzodiazepines by an absence of effect in the rat water lick conflict test and a withdrawal syndrome. Importantly, the 5-HT3 receptor antagonists are highly effective to prevent the behavioral syndrome following withdrawal from treatment with diazepam, nicotine, cocaine and alcohol. Intracerebral injection techniques in the mouse indicate that the dorsal raphe nucleus and amygdala may be important sites of 5-HT3 receptor antagonist action to inhibit aversive behavior. Studies with GR38032F indicate an additional effect in reducing alcohol consumption in the marmoset. The identification and distribution of 5-HT3 receptors in the brain using a number of 5-HT3 receptor ligands, [3H]65630, [3H]zacopride and [3H]ICS 205-930 correlates between studies, and the 5-HT3 recognition sites in cortical, limbic and other areas meet the criteria for 5-HT3 receptors to mediate the above behavioral effects. Thus the use of 5-HT3 receptor antagonists reveals an important role for 5-hydroxytryptamine in the control of disturbed behavior in the absence of effect on normal behavior. The profile of action of the 5-HT3 receptor antagonists has generated a major clinical interest in their potential use for schizophrenia, anxiety and in the control of drug abuse.

A theory of benzodiazepine dependence that can explain whether flumazenil will enhance or reverse the phenomena

Repeated administration of benzodiazepines (BDZs) produces dependence in man and animals and this is reflected in the phenomena of tolerance and withdrawal responses. In BDZ-dependent animals the BDZ-receptor antagonist flumazenil (Ro 15-1788) reverses the increased anxiety and decreased seizure threshold seen when benzodiazepine treatment is withdrawn. In contrast are reports that flumaenil enhances BDZ-withdrawal responses. Indirect influences on the direction of flumazenil's effects on anxiety are the duration and dose of BDZ treatment, whether tolerance has developed to its anxiolytic effect and whether there is an anxiogenic response on drug withdrawal. However, we conclude that the crucial factor is the anxiety level of the animal: when this is high flumazenil becomes anxiolytic; when this is low flumazenil is anxiogenic. These bidirectional effects of flumazenil can be seen in drug-naive and BDZ-dependent animals. We propose a theory of benzodiazepine dependence that can account for anxiogenic responses on drug withdrawal and for flumazenil's bidirectional effects; central to this theory is the assumption that flumazenil normalises the benzodiazepine receptor, returning it to a baseline state. Thus it is whether an animal's score lies above or below this baseline that will determine the direction of flumazenil's effect. The clinical implications of this theory are discussed. We suggest that during the development of benzodiazepine dependence, two independent adaptive biochemical mechanisms are triggered: one underlying the development of tolerance to the anxiolytic responses, the other underlying the incidence of increased anxiety on drug withdrawal. It is only changes in the latter that are induced by the administration of flumazenil.

Characterisation of the phenomenon of "one-trial tolerance" to the anxiolytic effect of chlordiazepoxide in the elevated plus-maze

In the elevated plus-maze test of anxiety the scores of control animals remain stable over repeated tests. However, a single prior exposure to the plus-maze renders an animal insensitive to the anxiolytic effects of chlordiazepoxide. This phenomenon of "one-trial tolerance" persisted even when the two trials were separated by as much as 2 weeks. It has previously been shown that the drug state of the animal on trial 1 is not important to the development of the phenomenon, but one-trial tolerance did not develop if a very high dose (75 mg/kg) of chlordiazepoxide was given on trial 1; it is suggested that this is due to the amnesic effects of the drug. The learning on trial 1 was not specific to a particular plus-maze and tolerance could be observed even when the maze on trial 1 was made from different material. The crucial experience on trial 1 was experience of an open arm of the maze. Whereas tolerance could be obtained as a result of a previous plus-maze experience, there was no evidence of an anxiogenic withdrawal response when rats were tested the following day undrugged. The phenomenon of one-trial tolerance is explained within our recently proposed two-factor theory of benzodiazepine dependence; it is suggested that one-trial tolerance provides a method for studying the mechanism underlying the development of tolerance to anxiolytic effects, independently from the mechanism underlying the development of withdrawal responses.

Local cerebral glucose utilization following subacute and chronic diazepam pretreatment: differential tolerance

Local cerebral glucose utilization (LCGU) was determined in parallel groups of conscious rats receiving diazepam (0.3 mg/kg i.v.) either acutely or following subacute (5 mg/kg i.p. daily for 3 days) or chronic (5 mg/kg i.p. daily for 28 days) diazepam pretreatment, using 2-deoxyglucose quantitative autoradiography. Acute administration of diazepam reduced LCGU in 44 of the 66 structures examined compared to vehicle-treated controls. These included limbic, cortical and extrapyramidal structures, and areas associated with sensory processing. These data are consistent with many brain regions being functionally involved in the diverse acute pharmacological effects of diazepam and with the widespread distribution of benzodiazepine receptors throughout the neuroaxis. Following subacute treatment, when animals were tolerant to the sedative effects of diazepam, glucose use remained depressed in the majority of areas studied. However, in the locus coeruleus, dorsal tegmental nucleus and most structures associated with auditory processing, tolerance to the depressant effect of diazepam upon glucose use had occurred suggesting the importance of these structures in the sedative effect of diazepam. The most striking feature of the patterns of LCGU after chronic diazepam treatment was that tolerance had occurred in the mammillary body and subiculum. However, glucose use remained depressed in hippocampal layers and in structures that provide input to the hippocampus (e.g. raphe nuclei). These data suggest that the outflow of neuronal activity from the hippocampus to the mammillary body via the subiculum is restored after chronic treatment, and may implicate these pathways in the anxiolytic action of diazepam. Overall, it would appear that different neuroanatomical substrates underlie the various pharmacological effects of diazepam and that there may be regional differences in tolerance mechanisms.

Behavioral effects in the mouse during and following withdrawal from ethanol ingestion and/or nicotine administration

The mechanism(s) by which ethanol or nicotine produces dependency and withdrawal symptoms during abstinence is poorly understood. In addition, it has been observed that a high correlation exists between ethanol intake and smoking. Therefore, studies were undertaken to evaluate aversion to the open arms of the elevated plus-maze and the modification of spontaneous locomotor activity during and following repeated ethanol and/or nicotine administration in mice. The ethanol plus nicotine treated animals increased time spent in the open arms of the maze during treatment relative to controls. Withdrawal from this combination treatment led to a rapid onset of intense aversion to the open arms of the maze and a concomitant reduction in locomotor activity which was greater than that produced by withdrawal from ethanol or nicotine treatment alone. The present results suggest that the combined effects of ethanol and nicotine reduced aversion to the open arms of the elevated plus-maze test system and may indicate an anti-aversive action. However, mice demonstrate an increased aversiveness to the open arms following sudden withdrawal of the combination treatment.

Evidence that the increased anxiety detected in the elevated plus-maze during chlordiazepoxide withdrawal is not due to enhanced noradrenergic activity

Rats displayed a reduction in the percentage of time spent on the open arms of the elevated plus-maze 24-30 hours after withdrawal from chronic chlordiazepoxide treatment (10 mg/kg/day IP for 4 weeks). This indicated an anxiogenic response in this test. This anxiogenic response was not significantly reversed by DL-propranolol (5 and 10 mg/kg IP) or clonidine (0.02 and 0.04 mg/kg IP). These results provide no evidence to suggest that the anxiogenic effects of chlordiazepoxide withdrawal are mediated by an increase in noradrenergic activity. The possible involvement of multiple transmitter systems in benzodiazepine withdrawal symptomology is discussed.

The effects of ondansetron (GR38032F) in rats and mice treated subchronically with diazepam

Using rat and mouse models of aversive behaviour, we have further investigated the properties of the 5-HT3 receptor antagonist ondansetron (GR38032F) that are relevant to its proposed use as an anxiolytic agent. Tolerance to the disinhibitory properties of diazepam was readily demonstrated in the social interaction test in the rat, but did not occur after subchronic treatment with ondansetron. In both the light/dark exploration test in mice and the social interaction test in rats, withdrawal from subchronic treatment with diazepam increased behavioural suppression, whereas this was not observed with ondansetron. The behavioural suppression and weight loss induced by either the withdrawal of diazepam or the administration of the benzodiazepine receptor antagonist, flumazenil, in animals treated subchronically with diazepam, was prevented or antagonised by diazepam or ondansetron. Buspirone was ineffective. It is concluded that, in rats and mice, tolerance to the disinhibitory effects of ondansetron does not occur, that withdrawal from subchronic treatment with ondansetron is not associated with any behavioural disturbances and that ondansetron is highly effective in preventing the behavioural suppression and weight loss following withdrawal from subchronic diazepam treatment. These data suggest that ondansetron may have major therapeutic advantages over currently available anxiolytic agents, particularly in patients who have previously received prolonged benzodiazepine therapy.

The actions of nicotine and cocaine in a mouse model of anxiety

The acute administration of nicotine (0.01-1.0 mg/kg IP) to the mouse increased the time spent and rearings and line crossings in the aversive brightly illuminated white area of a two compartment white/black test box, with a corresponding decrease in the black. This profile of change was maintained during twice daily administration (0.1 mg/kg IP) for 14 days. Eight to 96 hr following withdrawal of nicotine (14-day treatment), the behavioural profile was reversed to a preference for the black area: by 240 hr values had returned to control levels. In contrast to the effects of nicotine, an acute injection of cocaine (0.1-10 mg/kg IP) exacerbated the aversive response to the white area. However, similarly to nicotine, the administration of cocaine (1.0 mg/kg IP) twice daily for 14 days reduced the aversion to the white area and exacerbated the response following cocaine withdrawal. The effects of nicotine and cocaine to reduce and enhance responsiveness to the aversive properties of the white area are discussed in terms of an anxiolytic and anxiogenic response and the possibility of a serotonergic involvement.

Flumazenil prevents the development of chlordiazepoxide withdrawal in rats tested in the social interaction test of anxiety

Rats were chronically treated with chlordiazepoxide (CDP 10 mg/kg/day) or vehicle for 27 days. Twenty-four hours after their last dose, they received flumazenil (4 mg/kg) or vehicle and were tested in the social interaction test, in a low-light, familiar arena. CDP withdrawal significantly reduced the time spent in social interaction compared with controls, indicating an anxiogenic withdrawal response. This was completely reversed by flumazenil. A second group received CDP for 27 days and, in addition, received a single dose of flumazenil (4 mg/kg) 6 days before testing. Flumazenil prevented the development of the anxiogenic withdrawal response in these rats.

Withdrawal, tolerance and sensitization after a single dose of lorazepam

Forty-eight hours after a single dose of lorazepam (0.25 mg/kg), there was tolerance to the lorazepam-induced reduction of locomotor activity in the holeboard; but no tolerance to the reductions in exploratory head-dipping or rearing. Mice tested undrugged at this time showed significant hyperactivity and increased rearing, indicating withdrawal responses, but no change in head-dipping. In the elevated plus-maze, no tolerance could be detected to the effects of lorazepam (0.25 mg/kg) when the mice were tested 48 hr after an initial dose; in fact, there was a trend towards enhanced effects in this group. When mice were tested undrugged 24, 48 or 72 hr after a single dose of lorazepam there was an increase in the % time spent on the open arms, compared with controls, that reached significance for the 24 hr group. This indicates a sensitization to the anxiolytic effects of lorazepam, as assessed in the plus-maze. These results demonstrate long-lasting effects of even a single dose of lorazepam.

Reversal of increased anxiety during benzodiazepine withdrawal: evidence for an anxiogenic endogenous ligand for the benzodiazepine receptor

Rats were injected daily for 21 days with water or with chlordiazepoxide hydrochloride (10 mg/kg) and then tested in the elevated plus-maze 24-30 hr after the last of their chronic injections. At this time, rats withdrawn from chlordiazepoxide showed a significant decrease in the % of time spent on the open arms, compared with controls, thus indicating enhanced anxiety. The benzodiazepine antagonist, flumazenil (Ro 15-1788, 4 mg/kg, IP 20 min before test) significantly (p less than 0.01) reversed this withdrawal anxiety, and was without effect in the control group. The partial inverse agonist, FG 7142 (5 mg/kg IP 30 min before test) had no significant effect on the withdrawal anxiety. Possible mechanisms underlying the enhanced anxiety displayed by rats in withdrawal from chlordiazepoxide are discussed. It is concluded that a likely explanation is that chronic benzodiazepine treatment leads to an increased production and release of an endogenous ligand for the benzodiazepine receptor, with inverse agonist properties.