Are there changes in sensitivity to 5-HT3 receptor ligands following chronic diazepam treatment?

Administration of 1-(3-chlorophenyl)-biguanide (mCPB), a 5-HT₃ receptor agonist (1 and 10 mg/kg IP), was found to be significantly anxiogenic in vehicle treated rats tested in the plus-maze, while having no significant effect in rats withdrawn for 24 h from 21 days diazepam treatment (2 mg/kg/day), suggesting a decreased agonist action at 5-HT₃ receptors following withdrawal from chronic diazepam treatment. In the social interaction test, diazepam withdrawn rats showed a significant decrease in social interaction when compared to the chronic vehicle treated group. This anxiogenic response was reversed by low doses of zacopride (0.0001–0.01 mg/kg IP); in the vehicle treated animals 0.1 mg/kg was significantly anxiogenic. The overall pattern of results with zacopride is explained by suggesting that the anxiogenic effects of high doses of zacopride are detectable at low levels of 5-HT function and are due to an agonist action of the S-isomer in the rat at 5-HT₃ receptors. The anxiolytic action of low doses is attributed to the R-isomer acting at the R-zacopride binding site and is enhanced in conditions of high 5-HT function, e.g. in the diazepam withdrawn rats. If this hypothesis is correct, then we would predict the R-isomer alone would be more effective in reversing the anxiogenic effects of diazepam withdrawal than the racemate, used here.     

Actions and some interactions of 5-HT1A ligands in the elevated X-maze and effects of dorsal raphe lesions

Effects of 5-HT_1A agonists and partial agonists on open/total arm entry ratio (OTR) have been examined in the elevated X-maze anxiety model. 8-OH-DPAT (0.05–0.2 mg/kg), RU 24969 (0.5–2.0 mg/kg) and BAY R 1521 (0.1–1.2 mg/kg) produced dose-dependent reductions in OTR, signifying anxiogenic effects. Buspirone reduced OTR only at doses (0.25–5.0 mg/kg) decreasing total entries; gepirone (0.1–5.0 mg/kg) was inactive. Ipsapirone (0.25–5.0 mg/kg) increased OTR and at 1.0 mg/kg antagonised the anxiogenic action of 8-OH-D-PAT, RU 24969 and BAY R 1531. Gepirone (2.5 mg/kg) failed to antagonise 8-OH-DPAT, but the dose was limited by its effect on total entries. The anxiogenic effect of a low dose of 8-OH-DPAT was also prevented byp-chlorophenylalanine (p-CPA) pretreatment and reversed to anxiolytic by 5,7-dihydroxytryptamine lesions of dorsal raphe, which spared median raphe. These lesions also abolished the anxiolytic effect of ipsapirone without affecting the anxiogenic response to yohimbine. This study provides preliminary evidence that 8-OH-DPAT may be capable of acting as an agonist and ipsapirone as an antagonist at a presynaptic site related to dorsal raphe which is separate from the site of action of yohimbine. 5-HT_1A agonists and partial agonists may have multiple sites and/or mechanisms of action in the elevated X-maze.     

The pharmacology of VA21B7: an atypical 5-HT3 receptor antagonist with anxiolytic-like properties in animal models

VA21B7 (3-[2-(4-piperonylpiperazinyl) indolyl] carboxaldehyde) was synthesized as a potential 5-HT₃ receptor antagonist. Even though VA21B7 showed a higher affinity towards 5-HT₃ receptors as compared to other receptors studied, it was not a potent 5-HT₃ receptor antagonist either in the periphery or in the brain. In a simple animal model of anxiety such as the two-compartment box in mice, a remarkable anxiolytic-like effect was found at doses of 2–500 µg/kg IP and also at low oral doses, in the microgram range. These drug doses did not produce any significant effect on spontaneous motor activity of mice. The anxiolytic profile of VA21B7 was further explored using other models of anxiety in rats such as the elevated plus-maze and punished-drinking. VA21B7 was compared with standard 5-HT₃ receptor antagonists such as ondansetron, tropisetron and granisetron, with the 5-HT_1A agent buspirone and with diazepam. In the plus-maze, VA21B7 showed an anxiolytic-like profile after doses of 0.25–0.5 mg/kg IP or 2–4 mg/kg PO which did not modify the number of total entries into the open and closed arms of the maze. Diazepam, granisetron and tropisetron were also effective in this test but not ondansetron and buspirone. VA21B7 was also able to release suppressed behaviour in the punished-drinking test. The dose-response curve was bell-shaped with a peak at 2–4 mg/kg. At variance with other studies, 5-HT₃ receptor antagonists also increased the number of shocks taken in this test and the dose-response curve was also bell-shaped. VA21B7 was not anticonvulsant like diazepam, its anxiolytic action in the light/dark test was not flumazenil-sensitive and there was no rebound anxiogenic effect on withdrawal from chronic VA21B7 treatment for 15 consecutive days. Moreover, VA21B7 was not amnesic like the benzodiazepines but low doses of 2–4 mg/kg reduced the memory deficits induced in rats by scopolamine. Much higher doses were necessary to decrease spontaneous motor activity in rats. Since VA21B7 appears to be well tolerated in rodents at high doses, we think that it is of potential interest as an anxiolytic in humans.     

Effect of established and putative anxiolytics on extracellular 5-HT and 5-HIAA in the ventral hippocampus of rats during behaviour on the elevated X-maze

One of the proposed mechanisms of action for the anxiolytic effects of the benzodiazepines is via a decrease in central serotonergic neurotransmission. The aim of this study was to combine in vivo microdialysis in the rat with behaviour on the elevated X-maze to determine changes in 5-HT release in the ventral hippocampus with concomitant measurement of behaviour. Twenty minutes exposure to the elevated X-maze resulted in an increase in extracellular 5-HT in the ventral hippocampus with no change in extracellular 5-HIAA. Restricting the rat to either the open or the closed arms produced an increase in extracellular 5-HT, however the increase in 5-HT when restricted to the open arms was not significantly greater than that on the closed arms. Forty minutes pretreatment with diazepam (2.5 mg kg^–1 IP) significantly inhibited the increase in extracellular 5-HT in the ventral hippocampus and had an anxiolytic profile over 5 min and 20 min exposures of the rats to the X-maze. Diazepam had no effect on basal 5-HT levels before exposure to the X-maze but reduced extracellular 5-HT levels when the animal was returned to the holding cage. Forty minutes pretreatment with the 5-HT_1A receptor partial agonist ipsapirone (1 mg kg^–1 IP) significantly inhibited the increase in extracellular 5-HT in the ventral hippocampus but did not produce behaviour different from vehicle controls after 5 or 20 min periods on the X-maze. Ipsapirone had no effect on basal 5-HT levels before exposure to the X-maze but reduced extracellular 5-HT levels when the animal was returned to the holding cage. Forty minutes pretreatment with the novel anxiolytic F2692 (10 mg kg^–1 IP) significantly inhibited the increase in extracellular 5-HT in the ventral hippocampus and had an anxiolytic profile over the 5 min but not the 20 min period when the rat was on the X-maze. F2692 reduced basal extracellular 5-HT levels both 20 min before exposure to the X-maze and when the animal was returned to the holding cage. The results are discussed based on the effects of these compounds on basal and elevated extracellular 5-HT levels and their behavioural profile on the X-maze.     

Increased 5-HT release mediates the anxiogenic response during benzodiazepine withdrawal: a review of supporting neurochemical and behavioural evidence

This paper reviews the biochemical and behavioural evidence that the increased anxiety that occurs during benzodiazepine withdrawal is caused by increased 5-HT activity. In hippocampal slices taken from rats withdrawn for 24 h from chronic diazepam treatment (2 mg/kg/day for 21 days) there was a significant increase in K⁺-evoked release of [³H]5-HT and in⁴⁵Ca²⁺ uptake and both of these changes were reversed by the GABA_B agonist, baclofen. Baclofen also reversed the anxiogenic response that is detected on withdrawal from chronic diazepam treatment. Other drugs that reduce 5-HT function (tianeptine which increases 5-HT uptake; buspirone, a 5-HT_1A receptor agonist/partial agonist; zacopride, a 5-HT₃ receptor antagonist) also reversed this anxiogenic response. Finally, we present data from a group of rats that did not develop tolerance to the anxiolytic effects of diazepam (2 mg/kg), even after 5 weeks treatment. This group failed to show an anxiogenic response on withdrawal from diazepam, nor was there an increase in hippocampal 5-HT release. We discuss the extent to which increased hippocampal 5-HT release can be causally linked to the increased anxiety during benzodiazepine withdrawal.     

Withdrawal syndrome following subchronic treatment with anxiolytic agents

The acute administration of diazepam (0.1-2.5 mg/kg IP), sulpiride (0.5-20 mg/kg IP) and tiapride (0.5-40 mg/kg IP) to the mouse enhanced exploratory activity (rearings/line crossings) in the brightly illuminated white area of a two compartment white/black anxiety test box, with a corresponding decrease in the black, indicating an anxiolytic action. This profile of change was maintained during a twice daily administration for 7 days with diazepam (2.5 and 10 mg/kg), sulpiride (5 and 20 mg/kg) and tiapride (10 and 40 mg/kg). However, 8 and 48 hr following withdrawal of diazepam, the profile of exploratory behaviour was reversed to a preference for the black area: by 96 hr values for behaviour had returned to control levels. In contrast, an anxiolytic profile of action was maintained 8 and 48 hr following the withdrawal of sulpiride and tiapride, the values returning to control levels after 96 hr. It is concluded that a sub-chronic treatment with diazepam, sulpiride and tiapride induces an anxiolytic profile of action in the mouse model, that an anxiogenic profile follows the abrupt withdrawal of diazepam but that this is not recorded following the abrupt withdrawal of sulpiride and tiapride.     

Comparison of acute and repeated treatment with the 5-HT1A receptor ligands 8-OH-DPAT and ipsapirone in animal models of anxiety and depression

The present study compared the 5-HT_1A receptor ligands 8-OH-DPAT and ipsapirone with diazpepam and imipramine in the shock induced ultrasonic vocalization anxiety test and the forced swimming depression test in the rat. Acutely, 8-OH-DPAT induced anxiolytic and antidepressive effects (ED₅₀: 0.12 and 1.4 mg/kg, i.p., respectively), whereas ipsapirone induced anxiolytic (ED₅₀: 0.6 mg/kg) and moderate antidepressive effects (33% at 3-10 mg/kg). Virtually no tolerance developed for the anxiolytic effects after 2 weeks of treatment with 0.03-1 mg/kg 8-OH-DPAT or 0.1-10 mg/kg ipsapirone (i.p., b.i.d.), with 10 mg/kg/day ipsapirone (s.c., mini-pumps), or with 1.5 μg/rat/hr 8-OH-DPAT (local infusion in the dorsal raphe nucleus, mini-pumps). However, some tolerance developed for the antidepressive effects of 8-OH-DPAT (ED₅₀: 0.6, 1.4, 2.5 and >3 mg/kg, after 2 weeks of pretreatment with vehicle, 0.3, 1, and 3 mg/kg 8-OH-DPAT, respectively, i.p., b.i.d.). In the case of ipsapirone, the dose-effect curve in the forced swimming test was shifted to the left after 2 weeks of pretreatment with ipsapirone (0.3-10 mg/kg, i.p., b.i.d.). Acutely, diazepam induced an anxiolytic effect (ED₅₀: 3.6 mg/kg, i.p.), but failed to induce an antidepressive effect; whereas imipramine induced an antidepressive effect (ED₅₀: 20.5 mg/kg) and a moderate anxiolytic effect (max. efficacy: 47% at 30 mg/kg). Upon repeated administration (2 weeks), diazepam (5 mg/kg) showed t0olerance for its anxiolytic effects and weak antidepressive effects emerged, whereas imipramine (20 mg/kg) showed weak sensitization for both effects. It is concluded that (a) with all compounds, tolerance, as well as sensitization can be observed, depending on the behavioral test, the dose and the type of compound; and (b) compared with the other compounds tested, relatively low doses of 5-HT_1A drugs offer the most attractive profile of mixed anxiolytic/antidepressive activity. © 1993 wiley-Liss, Inc.     

Acute and chronic effects of gepirone and fluoxetine in rats tested in the elevated plus-maze: an ethological analysis

The potential role of 5-hydroxytryptamine (5-HT) in anxiety has been the subject of much research, most of it addressed to the hypothesis that 5-HT promotes anxiety and, therefore, that drugs that reduce 5-HT functions will be effective anxiolytic agents in human anxiety disorders. However, the effects of serotoninergic drugs in different behavioral paradigms have been inconsistent. These inconsistencies have been particularly well illustrated in the elevated plus-maze. In the present study we provided an ethopharmacological analysis (in addition to conventional measures) of the behavior of rats in the elevated plus-maze with transparent walls after acute and chronic treatments with gepirone, an agonist of 5-HT1A receptors, and fluoxetine, a selective inhibitor of serotonin reuptake. Although gepirone has been used to treat anxiety, fluoxetine is a mainstay in the treatment of depression. Acute treatment with gepirone (1, 3, 5.6, and 10 mg/kg, IP) produced an anxiogenic profile with increased risk assessment behaviors (e.g., flat-back approach) and decreased behavioral measures that are inversely related to "anxiety" (e.g., head dipping and end-arm activity). In contrast, chronic gepirone (10 mg/kg day, PO) produced an opposite effect showing an anxiolytic profile that is consistent with the clinical use of this drug, which shows efficacy after 2-4 weeks of treatment. Acute fluoxetine (5.6 and 10 mg/kg, IP) also produced an anxiogenic profile with reduced head dipping and end-arm activity. On the other hand, chronic fluoxetine (10 mg/kg day, PO) had no effect on any of the behavioral measures. These data demonstrate: (a) the anxiogenic and anxiolytic effects of acute and chronic gepirone, respectively, corroborate with the observed effects of these treatments in the clinic; (b) similarly, the anxiogenic effects of acute fluoxetine observed here have also been reported in clinical studies with 5-HT reuptake blockers. This class of compounds has not been systematically used as anxiolytic; (c) the elevated plus-maze with transparent walls shows good sensitivity for evaluating serotonergic drugs with anxiogenic and anxiolytic profile.     

An evaluation of the elevated plus-maze test using the novel anxiolytic buspirone

Previous work suggests that the elevated plusmaze test of anxiety is insensitive to the anxiolytic effects of the novel anxiolytic buspirone, which shows an anxiogenic-like profile in this test. This paper examines some of the possible reasons for this and the role that buspirone's agonist activity at 5-HT_1A receptors plays in this effect. A variety of 5-HT_1A receptor agonists (p-aminophenylethylm-trifluromethylphenyl piperazine, (+)- and (-)-MDL 72832) showed similar activity to buspirone, as did the related compound ipsapirone. (-)-MDL 72832 was more potent than (+)-MDL 72832, in keeping with its stereoselective action at 5-HT_1A receptors. The ₂-adrenoceptor antagonist properties of 1-pyrimidinyl piperazine, a metabolite of buspirone, did not appear to be relevant to this action of buspirone as neither it nor idazoxan showed an anxiogenic-like profile. Neither chronic treatment with buspirone (1 mg/kg SC twice a day for 16 days) nor depletion of 5-HT withp-chlorophenylalanine changed the anxiogenic-like activity of buspirone in the elevated plus-maze test. These results suggest that an agonist action at postsynaptic 5-HT_1A receptors mediates the anxiogenic-like effects of buspirone in the elevated plus-maze test and that this test may either be insensitive to certain classes of anxiolytics or is measuring something unrelated to human anxiety states.     

Effects of 5-HT1A receptor agonists,partial agonists and a silent antagonist on the performance of the conditioned emotional response test in the rat

In the present study, the effects of 5-HT_1A receptor ligands with varying degrees of intrinsic activity at the 5-HT_1A receptor were examined in the conditioned emotional response (CER) test and their effects compared to those of the benzodiazepine receptor agonists, diazepam and chlordiazepoxide. Diazepam (3.0 mg/kg) and chlordiazepoxide (3.0 mg/kg), and the 5-HT_1A receptor partial agonists, ipsapirone (10.0 mg/kg) and gepirone (3.0 mg/kg), alleviated conditioned suppression of lever pressing. The 5-HT_1A receptor partial agonist, buspirone (0.1–1.0 mg/kg), the 5-HT_1A receptor agonist, 8-OH-DPAT (0.01– 0.10 mg/kg), and the 5-HT_1A receptor antagonist, WAY-100635 (0.03–3.0 mg/kg), had no effects on conditioned fear. Neither enhancing the level of food deprivation nor pretreatment with the amnesic agent scopolamine induced anxiolytic-like effects in the present CER test. The anxiolytic-like effects of ipsapirone in this test were completely reversed by WAY-100635. These results indicate that 5-HT_1A agonist, but not antagonist actions, induce an anxiolytic effect in the CER test in rats. Received: 13 March 1996/Final version: 8 July 1996     

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.     

The role of 5-HT receptor subtypes in the anxiolytic effects of selective serotonin reuptake inhibitors in the rat ultrasonic vocalization test

 We evaluated whether the anxiolytic effects of selective serotonin reuptake inhibitors (SSRIs) in the rat ultrasonic vocalization (USV) test are preferentially mediated by (indirect) activation of 5-HT_1A, 5-HT_1B/1D, 5-HT_2A, 5-HT₃ or 5-HT₄ receptors. The SSRIs, paroxetine (ED₅₀ in mg/kg, IP: 6.9), citalopram (6.5), fluvoxamine (11.7) and fluoxetine (>30), dose dependently reduced shock-induced USV. The effects of paroxetine (3.0 mg/kg, IP) were not blocked by the selective 5-HT_1A receptor antagonist, WAY-100635 (3.0 mg/kg, IP), the 5-HT_1B/1D receptor antagonist, GR 127935 (30 mg/kg, IP), the nonselective 5-HT_2A receptor antagonists, ritanserin (3.0 mg/kg, IP) and ketanserin (1.0 mg/kg, IP), the 5-HT₃ receptor antagonist, ondansetron (0.1 mg/kg, IP), or the 5-HT₄ receptor antagonist, GR 125487D (3.0 mg/kg, SC). In contrast, the selective 5-HT_2A receptor antagonist, MDL 100,907 (0.1 mg/kg, IP), completely prevented the paroxetine-induced reduction of USV. Under similar conditions, WAY-100635 blocked the anxiolytic-like effects of the selective 5-HT_1A receptor agonist, 8-OH-DPAT [(±)-8-hydroxy-2-(di-n-propylamino)tetralin, 1.0 mg/kg, IP], and ritanserin, ketanserin, and MDL 100,907 blocked the anxiolytic-like effects of the mixed 5-HT_2A/2C receptor agonist, DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, 3.0 mg/kg, IP]. WAY-100635 (1.0 mg/kg, IP) in combination with ritanserin (3.0 mg/kg, IP), but not ondansetron (0.1 mg/kg, IP), GR 125487D (3.0 mg/ kg, SC), or GR 127935 (30 mg/kg, IP), attenuated the USV reducing effects of paroxetine. Although the results suggest that selective stimulation of 5-HT_1A and 5-HT_2A receptors produces a decrease of USV, we postulate that only 5-HT_2A receptors play a pivotal role in the effects of SSRIs in this model of anxiety. Received: 19 May 1997 / Final version: 21 July 1997     

Buspirone,gepirone, ipsapirone,and zalospirone have distinct effects on the differential-reinforcement-of-low-rate 72-s schedule when compared with 5-HTP and diazepam

The effects of four serotonin (5-HT)-1A compounds (buspirone, gepirone, ipsapirone and zalospirone) were compared with 5-hydroxytryptophan (5-HTP) [a 5-HT precursor with antidepressant (AD) efficacy], and diazepam (a benzodiazepine anxiolytic), on a differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule. Past research has shown that AD and anxiolytic compounds each have distinct effects on the DRL 72-s interresponse time (IRT) distribution profile. In the present paper, the profile of the IRT distribution was quantitatively characterized by three metrics: burst ratio, peak location and peak area. 5-HTP shifted the IRT distribution peak toward longer IRT durations, increased reinforcement rate and decreased response rate. The profile of the IRT distribution was not disrupted by 5-HTP. Diazepam disrupted the IRT distribution and increased bursting. In general, the arylpiperazine, 5-HT_1A compounds increased reinforcement rate, decreased response rate and disrupted the profile of the IRT distribution. The effects of the four arylpiperazine 5-HT_1A compounds on the IRT distribution profile were different from the AD profile of 5-HTP and the benzodiazepine anxiolytic profile of diazepam. Disruption of the IRT distribution by buspirone, gepirone, ipsapirone and zalospirone may result from decreased 5-HT transmission mediated by the presynaptic, somatodendritic 5-HT_1A receptor.     

Influence of 5-HT3 receptor antagonists and the indirect 5-HT agonist,dexfenfluramine, on heroin self-administration in rats

The purpose of the present study was to examine the effects of the 5-HT₃ antagonists ondansetron and MDL72222, and the 5-HT releaser and reuptake inhibitor dexfenfluramine, on intravenous heroin self-administration by Wistar rats. Using separate squads of animals, two separate schedules of heroin reinforcement were used; a relatively low dose (0.03 mg/kg per infusion) made available under a FR5 schedule for 1 h each day, and a moderate heroin dose (0.1 mg/kg per infusion) available under a FR1 schedule for 2 h each day. Following the acquisition of stable levels of responding across days, both naloxone pretreatment (0.25 mg/kg SC) and halving the heroin infusion dose produced increases in operant responding for heroin at each concentration. Neither ondansetron (0.01–1 mg/kg SC) nor MDL72222 (0.1–3 mg/kg SC) pretreatment influenced heroin self-administration. Chronic treatment (5 day) of ondansetron (0.01–0.1 mg/kg) was similarly ineffective. However, dexfenfluramine (0.5–2.5 mg/kg IP) consistently reduced heroin self-administration at doses producing only modest decreases in food responding. These findings are in contrast to place conditioning studies, which show that 5-HT₃ antagonists but not indirect 5-HT agonists block a morphine-induced place preference. Reasons for such discrepancies remain to be determined.     

Evidence for the involvement of the 5-HT1A receptor in the anxiolytic action of indorenate and ipsapirone

The systemic injection of diazepam (0.5 and 1.0 mg/kg), indorenate (5 and 10 mg/kg) and ipsapirone (2.5 and 5.0 mg/kg) reduced anxiety as tested in an exploratory avoidance model in mice. The injection of pindolol (2.0 mg/kg), alprenolol (5.0 mg/kg) or methiotepin (0.25 mg/kg) effectively prevented the anxiolytic action of indorenate and ipsapirone. The combined treatment of the antagonists with indorenate or ipsapirone did not reduce the motor activity, therefore suggesting that the inhibition of exploratory behaviour, after such combinations, was not mediated through a general motor impairment. Neither diazepam nor indorenate alone modified the motor activity; ipsapirone (5 and 2.5 mg/kg), however, reduced ambulation. This reduction was also prevented by administering pindolol, alprenolol or methiotepin. These observations suggest that the anxiolytic actions of indorenate and ipsapirone are mediated via stimulation of the 5-HT1A like receptor subtype.     

Differential effects of diazepam on anxiety in streptozotocin induced diabetic and non-diabetic rats

 The anxiolytic activity of diazepam (DZP) (0.25–1 mg/kg) was investigated in streptozotocin (STZ)-induced diabetic adult Charles Foster albino rats of either sex. Diabetes was induced by injecting STZ IP (50 mg/kg; in citrate buffer, pH 4.5). Experiments were performed 72 h later. The rats were subjected to various anxiety paradigms, including the open-field exploratory behaviour, elevated plus maze and elevated zero maze behaviours and the social interaction tests. In addition, rat brain tribulin activity was also assessed as a biochemical marker of anxiety. The results indicate that diabetic rats showed significantly more anxiogenic activity in comparison to non-diabetic rats on open-field, elevated plus maze, zero maze and social interaction tests. In diabetic rats, brain tribulin activity (MAO-A inhibitory component) was significantly increased. DZP dose dependently produced anxiolytic activity on the various behavioural parameters in non-diabetic rats. DZP (0.5 and 1 mg/kg) partially reversed the anxiogenic behaviour of STZ diabetic rats in elevated plus maze and zero maze tests. However, in open field behaviour and social interaction tests significant anxiolytic activity was observed only at a higher dose of DZP (1 mg/kg). The findings indicate that STZ-induced diabetic rats exhibited augmented anxiety on various experimental paradigms and that the anxiolytic effect of diazepam was less marked in diabetic rats as compared to their euglycaemic counterparts. Received: 5 May 1997 / Final version: 14 August 1997     

Blockade of cocaine sensitization and tolerance by the co-administration of odansetron, a 5-HT3 receptor antagonist, and cocaine

The present experiment evaluated the ability of the 5-HT₃ antagonist, ondansetron, administered during chronic cocaine administration, to block the development of sensitization and tolerance induced by the intermittent or continuous administration of cocaine, respectively. Rats were pretreated with 40 mg/kg per day cocaine for 14 days by either SC injections or osmotic minipumps, or 0.9% saline, administered by SC injection. During this chronic (cocaine) treatment, all rats received a daily SC injection of 0–1.0 mg/kg ondansetron. The rats were then withdrawn from the pretreatment regimen for 7 days. On day 7 of withdrawal from the cocaine pretreatment all subjects received a 15.0 mg/kg IP cocaine challenge, and their behavior was then rated according to the modified Ellinwood and Balster scale for 60 min. The results indicated that daily injections of ondansetron had no consistent or significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron with cocaine significantly blocked the development of sensitization with an inverted U-shape dose-response curve. In the continuous cocaine group ondansetron injections also attenuated the development of behavioral tolerance. The results therefore indicate that 5-HT₃ receptor stimulation during continuous and intermittent cocaine administration is an important link in the development of behavioral tolerance and sensitization. Received: 1 July 1996/Final version: 25 September 1996     

BRL 46470A: a highly potent,selective and long acting 5-HT3 receptor antagonist with anxiolytic-like properties

The novel 5-HT₃ antagonist, BRL 46470A (endo-N-(8-methyl-8-azabicyclo [3.2.1]oct-3-yl)2,3-dihydro-3,3 dimethyl-indole-1-carboxamide, hydrochloride), has been investigated in a series of in vitro and in vivo tests, including the effect of the drug in models of anxiolysis. In classical tests for 5-HT₃ receptor antagonism, BRL 46470A was shown to antagonise 5-HT₃ mediated responses in the guinea-pig ileum [pA₂ 8.3±0.5, slope 0.98±0.20, mean±SEM (5)], the rabbit isolated heart (pA₂ 10.1±0.1, slope 1.2±0.2,n=5) and the rat Bezold-Jarisch model (ID₅₀ 0.7 µg/kg IV±0.1,n=8), with a long duration of action (>3 h). BRL 46470A selectively displaced [³H]-BRL 43694 from 5-HT₃ binding sites in rat brain membranes (K_i 0.32 nM±0.04,n=4) without displacing (at concentrations greater than 1 µM) a wide variety of ligands binding to other neurotransmitter receptors, opioid receptors and to neurotransmitter gated ion channel complexes. In vivo, BRL 46470A showed anxiolytic-like activity in two animal models predictive of antianxiety effects-elevated X-maze and social interaction in rats. In both models, BRL 46470A showed significant activity over a wide dose-range following both oral (0.0001–0.1 mg/kg PO) and systemic administration. The unique level of potency of BRL 46470A was apparent in the rat social interaction test and was shown to be 100 fold more potent than the 5-HT₃ receptor antagonist ondansetron, with no evidence of a bell-shaped dose response curve over 4 orders of magnitude. BRL 46470A (0.0001 and 0.01 mg/kg SC) also reduced the anxiogenic effects of m-CPP (1-(3-chlorophenyl) piperazine) in the rat elevated X-maze. BRL 46470A is therefore a chemically novel potent and selective 5-HT₃ receptor antagonist with anxiolytic potential and a long duration of action in animal studies.     

Interaction of GABA and serotonin in the anxiolytic action of diazepam and serotonergic anxiolytics.

The general purpose of the present study was to analyze the possible interactions between the GABA-benzodiazepine and the serotonergic (5-HT) systems in the anxiolytic action of diazepam and the 5-HT1A agonists, ipsapirone, indorenate, and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The effect of the benzodiazepine receptor antagonist, flumazenil (10.0 mg/kg), on the anxiolytic action of ipsapirone (5.0 mg/kg), indorenate (5.0 mg/kg), and 8-OH-DPAT (0.125 mg/kg) was examined on the avoidance exploratory behavior paradigm in mice. The effect of the 5-HT1 blockers, methiotepin (0.31 mg/kg), pindolol (3.1 mg/kg), and alprenolol (5.0 mg/kg), on the anxiolytic action of diazepam (0.5 mg/kg) was also studied. In the last part of this work, the putative potentiation between diazepam (0.25 mg/kg) and each of the serotonergic anxiolytics was investigated. The antianxiety effect of diazepam, ipsapirone, indorenate, and 8-OH-DPAT was prevented by flumazenil. The serotonergic/beta-blocker, alprenolol, partially antagonized the diazepam effect. Finally, a potentiation of suboptimal doses of diazepam and ipsapirone, but not with indorenate or 8-OH-DPAT, was observed. The findings suggest an interaction between both systems on the anxiolytic action of diazepam and the 5-HT1A agonists.     

Species differences in the mechanism through which the serotonergic agonists indorenate and ipsapirone produce their anxiolytic action

The effect of three anxiolytic drugs, indorenate, ipsapirone and diazepam, on the burying behaviour of rats and mice was studied. All three drugs induced a reduction in burying behaviour interpreted as a reduction in anxiety. However, a species difference in the diazepam sensitivity was found: rats showed a clear effect after 1.0 mg/kg while already at 0.25 mg/kg an action was observed in mice. The serotonergic anxiolytics produced similar responses at similar doses (2.5–5.0 mg/kg) in both species. The serotonergic antagonists, pindolol (3.1 mg/kg), alprenolol (5.0 mg/kg) and methiotepin (0.31 mg/kg), induced a slight reduction in the time spent burying but effectively counteracted the anxiolytic action of the serotonergic agonists in mice but not in rats. By contrast, in rats, the beta blocker, practolol (0.5 mg/kg), was the only drug effective in preventing the anxiolytic actions of ipsapirone. The combined treatment of indorenate and methiotepin resulted in an impairment of motor coordination and ambulatory behaviour in both species studied, thereby suggesting that the lack of effect of such combination was mediated by altering the motor behaviour. Finally, the reduction in ambulatory behaviour in mice produced by ipsapirone was effectively prevented by the antagonists methiotepin, pindolol and alprenolol indicating the involvement of a serotonergic receptor in this effect. From these results it is concluded that a different mechanism underlies the anxiolytic actions of indorenate and ipsapirone in mice and rats.