Actions of 5-HT1 ligands on excitatory synaptic transmission in the hippocampus of alert rats.

1. The effects of 5-hydroxytryptamine1 (5-HT1) ligands on excitatory synaptic transmission were examined in the stratum radiatum of the CA1 region of the dorsal hippocampus of alert, gently restrained, rats. 2. 5-HT produced a dose-dependent reduction in the amplitude of the electrically evoked population excitatory postsynaptic potential (e.p.s.p.) when injected directly into the hippocampus via a cannula (dose producing 50% maximum inhibition, ED50 = 0.46 microgram). 3. Direct intrahippocampal (i.h.) application of buspirone (ED50 = 0.29 microgram), gepirone (1 microgram), ipsapirone (1 microgram), BMY 7378 (0.1 microgram) and 5-carboxamidotryptamine (5-CT, 0.02 microgram) mimicked the inhibitory effect of 5-HT. 4. Systemic injection of buspirone (ED50 = 0.88 mg kg-1, i.p.), BMY 7378 (0.01 mg kg-1, i.p.) and RU 24969 (1 mg kg-1, s.c.) also had an inhibitory effect on the amplitude of the e.p.s.p. 5. Injection of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 2 micrograms) and spiroxatrine (1 microgram) i.h. alone had no effect on the e.p.s.p. amplitude but prevented the inhibitory effect of 5-HT. 6. Systemic injection (i.p.) of methysergide (5 mg kg-1) and spiroxatrine (1 mg kg-1) antagonized the inhibitory effect of buspirone whereas pretreatment with ketanserin (1 mg kg-1), trifluoperazine (1 mg kg-1) and idazoxan (1 mg kg-1) had no effect on the response to buspirone. 7. It is concluded that hippocampal synaptic transmission is highly sensitive to the agonist and antagonist properties of 5-HT1 ligands in the alert rat.     

Blockade of the discriminative stimulus effects of d-amphetamine in rhesus monkeys with serotonin 5-HT(1A) agonists

Rhesus monkeys (n = 4) were trained in a two-lever drug discrimination paradigm to discriminate d-amphetamine (AMPH; 0.56 or 1.0mg/kg, i.g., 60min pre-session) from saline. Lever pressing was maintained under a discrete-trials shock avoidance schedule of reinforcement (30 trials/day, 30-s intertrial interval, fixed-ratio 1). Before test sessions, in which responding on either lever avoided shock, the monkeys were infused (i.g.) with saline or various doses of AMPH (0.1-1.7mg/kg), alone or in combination with intramuscular injections of the serotonin (5-HT(1A)) agonists buspirone (0.3-1.0mg/kg), gepirone (0.56-3.0mg/kg), 8-hydroxy-2(di-n-propylamino) tetralin (8-OH-DPAT; 0.01-0.1mg/kg), the dopamine D2 antagonist raclopride (0.03 and 0.1mg/kg), or pentobarbital (10mg/kg). Doses of the 5-HT(1A) agonists and raclopride, which occasioned only saline-lever responding when tested alone, shifted the AMPH dose-response function 0.5 to 1.0 log unit to the right in all monkeys, while pentobarbital had no effect. For the 5-HT(1A) agonists, the order of potency for attenuating the AMPH discriminative stimulus was 8-OH-DPAT>buspirone>gepirone, similar to their binding affinities at 5-HT(1A) receptors. Raclopride was equipotent to 8-OH-DPAT. In addition, although the effect of buspirone and gepirone could be surmounted by increasing the dose of AMPH, the effect of 8-OH-DPAT, the 5-HT(1A) agonist with the highest intrinsic activity (efficacy), was insurmountable in two of the three monkeys tested. These results suggest that the discriminative stimulus effect of AMPH can be modulated by stimulation of 5-HT(1A) receptors to an extent comparable with that seen with a D2 antagonist. Further, these results suggest that 5-HT(1A) agonists may attenuate the subjective effects of psychomotor stimulants in humans.     

Modulatory role of 5-HT1B receptors in the discriminative signal of amphetamine in the conditioned taste aversion paradigm

Drugs of abuse, such as amphetamine (AMPH), share the ability to activate the mesolimbic dopamine (DA) system. The behavioral effects of AMPH are largely mediated by increased DA neurotransmission in the nucleus accumbens. However, there is evidence that serotonin (5-hydroxytryptamine - 5-HT) systems may regulate forebrain DA function. We examined the role of 5-HT1B receptors on the discriminative stimulus properties of AMPH using conditioned taste aversion (CTA) as the drug discrimination procedure. Male Wistar rats were deprived of water and trained in the CTA procedure. They received the administration of AMPH (1.0 mg/kg) before a 10 min period of access to saccharin solution and followed by an injection of LiCl; on alternate days, rats received saline before and after the access to saccharin solution. In generalization and combination tests, the training dose of AMPH was substituted by 5-HT1B receptor ligands RU24969 (5-HT1B agonist: 0.1, 0.3 and 1.0 mg/kg), CP94253 (5-HT1B agonist: 1.0, 3.0 and 5.6 mg/kg) and GR127935 (5-HT1B antagonist: 0.3, 1.0 and 3.0 mg/kg) or a combination of RU24969 (0.1, 0.3 and 1.0 mg/kg), CP94253 (1.0, 3.0 and 5.6 mg/kg) or GR127935 (0.3, 1.0 and 3.0 mg/kg) with AMPH (0.3 mg/kg) or GR127935 (0.3, 1.0 and 3.0 mg/kg) and CP94253 (5.6 mg/kg) with AMPH (0.3 mg/kg). The results showed that 5-HT1B agonists RU24969 and CP94253 produced partial generalization of 48% and 60%, respectively, and the 5-HT1B antagonist GR127935 neither substituted for AMPH nor affected the discriminative cue of AMPH; however, when RU24969 or CP94253 were administrated in combination with AMPH, they increased the discriminative cue of AMPH. This effect was reversed by the administration of 5-HT1B antagonist GR127935. These data suggest that 5-HT1B receptors play a modulatory role in the discriminative cue of AMPH.     

Further investigation of the in vivo pharmacological properties of the putative 5-HT1A antagonist, BMY 7378

The present study examined the actions of the putative 5-HT1A antagonist BMY 7378 on central pre- and postsynaptic 5-HT1A function in the rat in vivo. Unlike the direct acting 5-HT1A agonist 8-hydroxy-2-(di-n-pro-pylamino)tetralin (8-OH-DPAT), BMY 7378 (0.25-5 mg/kg s.c.) did not induce the full postsynaptically mediated 5-HT behavioural syndrome (forepaw treading, head weaving, flat body posture hindlimb abduction). Indeed, the maximal 5-HT behavioural syndrome scores of BMY 7378 were about 10% of those for 8-OH-DPAT. Following pretreatment, however, BMY 7378 dose dependently (0.25-5 mg/kg s.c.) reduced to undetectable levels forepaw treading and head weaving induced by 8-OH-DPAT (0.75 mg/kg s.c.). BMY 7378 also inhibited stereotypy and locomotor activity induced by 0.5 mg/kg apomorphine although this effect was only statistically significant at the highest dose tested (5 mg/kg). In contrast to its apparent 5-HT1A antagonist properties in the behavioural experiments, BMY 7378 caused a marked and dose-dependent (0.01-1.0 mg/kg s.c.) decrease of 5-HT release in ventral hippocampus of the anaesthetized rat as detected by brain microdialysis. This effect of BMY 7378 had a similar onset and duration of action but with slightly reduced efficacy compared to that previously described for 8-OH-DPAT. As with 8-OH-DPAT, the inhibitory effect of BMY 7378 on 5-HT release was attenuated by pretreatment with the 5-HT1 receptor/beta-adrenoceptor antagonist pindolol (8 mg/kg s.c.) but not its counterpart propranolol (20 mg/kg s.c.). Pretreatment with a combination of the beta 1- and beta 2-adrenoceptor antagonists metoprolol (4 mg/kg s.c.) and ICI 118 551 (4 mg/kg s.c.), respectively, did not alter the 5-HT response to BMY 7378. From these data we conclude that BMY 7378 is a mixed agonist/antagonist at central 5-HT1A receptors.     

Receptor reserve for 5-hydroxytryptamine1A-mediated inhibition of serotonin synthesis: possible relationship to anxiolytic properties of 5-hydroxytryptamine1A agonists

The irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was used to determine the relationship between receptor occupancy and response at central 5-hydroxytryptamine1A (5-HT1A) serotonin receptors mediating the inhibition of serotonin synthesis in rat cortex and hippocampus. Rats were treated with vehicle or EEDQ (2 or 6 mg/kg) and 24 hr later dose-response curves were constructed for inhibition of 5-hydroxytrytophan (5-HTP) accumulation (after decarboxylase inhibition with NSD-1015) by the selective 5-HT1A agonists 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) (0.01-3 mg/kg), buspirone (0.1-7.5 mg/kg), and ipsapirone (0.1-6.25 mg/kg) and the 5-HT1A agonist/antagonist BMY 7378 (0.015-5 mg/kg). In vehicle-pretreated rats, a similar maximal inhibition of 5-HT synthesis (range, 52-59%) was observed in both brain areas with 8-OH-DPAT, buspirone, and ipsapirone. These three agonists were also more potent in reducing 5-HTP accumulation in the cortex than in the hippocampus (ED50, 8-OH-DPAT, 14 and 30 microgram/kg; buspirone, 0.42 and 0.63 mg/kg; ipsapirone, 0.44 and 1.26 mg/kg, respectively). In the cortex, EEDQ treatment shifted the dose-response curves for 8-OH-DPAT, buspirone, and ipsapirone 8.6-, 2.0-, and 2.8-fold to the right, respectively. Corresponding rightward shifts in the hippocampus were smaller, 6.0-, 1.6-, and 2.1-fold, respectively. The EEDQ-induced shifts in the dose-response curves were accompanied by reductions in maximal response. In contrast, whereas the maximal inhibition of cortical 5-HTP accumulation by BMY 7378 (55%) was similar to that obtained with the agonists, maximal response in the hippocampus was much smaller (32%). Furthermore, in both brain regions EEDQ reduced the maximal response to BMY 7378 without shifting the dose-response curves. Analysis of the data by the double-reciprocal method of Furchgott, followed by calculation of fractional receptor occupancy for each dose of agonist, revealed a nonlinear relationship between receptor occupancy and response for 8-OH-DPAT, buspirone, and ipsapirone in both brain regions, demonstrating the presence of a large receptor reserve. For BMY 7378, in contrast, linear relationships were obtained. Because 5-HT1A receptor-mediated regulation of 5-HT synthesis appears to be mediated by somatodendritic autoreceptors on 5-HT neurons in the midbrain raphé nuclei, the results suggest that these autoreceptors possess a large receptor reserve for agonists. The relevance of these findings for the mechanism of action of nonbenzodiazepine anxiolytics is discussed.     

Involvement of 5-HT1A activity in the discriminative stimulus effects of imipramine.

Pigeons were trained to discriminate the tricyclic antidepressant imipramine (3.0 or 5.6 mg/kg) from saline. The selective 5-HT1A agonist 8-OH-DPAT (0.03-1.0 mg/kg) resulted in dose-dependent increases in responding on the key correlated with imipramine administration. Doses of 8-OH-DPAT from 0.3 to 1.0 mg/kg substituted completely for imipramine. NAN-190 (0.3-3.0 mg/kg), a putative 5-HT1A antagonist with affinity for both 5-HT1A and alpha 1 receptors, blocked the discriminative stimulus effects of imipramine and resulted in saline-key responding. The discriminative stimulus effects of imipramine were also blocked by administration of the alpha 1-adrenoceptor antagonist prazosin, suggesting a dual mediation of imipramine through both 5-HT1A and alpha 1-adrenoreceptor systems. Although antidepressants have not been used frequently as stimuli in drug discrimination studies, it may be possible to arrive at a more complete understanding of their neurochemical and behavioral effects using this procedure.     

Discriminative stimulus properties of indorenate, a 5-HT1A, 5-HT1B and 5-HT2C agonist: a study in rats

Indorenate (INDO), initially described as an antihypertensive agent, also has some effects on behaviour, with anxiolytic and anorectic actions being reported. The aim of the present experiment was to examine the activity of INDO at the behavioural level at various serotonin (5-hydroxytryptamine, 5-HT) receptor sites by comparing its stimulus properties with those of other 5-HT receptor agonists and by examining its interactions with some 5-HT antagonists. Rats were trained to discriminate between 10.0 mg/kg INDO (administered intraperitoneally (90 min before the start of the session) from saline. A Fixed Ratio 10 (FR10) schedule of reinforcement was in effect in each drug condition. During generalization test sessions, the discrimination index (DI, responses to drug lever/responses to drug + saline lever) was calculated from the responses emitted before the first reinforcer of the session. DI was a function of the dose of INDO employed. Generalization to the discriminative stimulus properties of INDO was observed with the 5-HT1A receptor agonist 8-OH-DPAT (1.0 mg/kg produced 90% generalization) and the 5-HT(1B/2C) receptor agonist 1-(3-trifluoromethylphenyl) piperazine (TFMPP) (3.0 mg/kg produced up to 75% generalization). Yohimbine (5.6 mg/kg), buspirone (1.0 mg/kg), 6-chloro-2-(1-piperaziny)pyrazine (1.0 mg/kg) and m-chlorophenylpiperazine (mCPP) (1.0 mg/kg) induced a DI of 70%, 50% and 48% and 55%, respectively. In generalization tests, ritanserin (0.01-1.0 mg/kg) induced saline-like responding. NAN-190 (3.0 mg/kg), a 5-HT1A receptor antagonist, was able to reduce the DI of INDO to 50%. Although the 5-HT(2C/2A) receptor antagonists cinanserin (10.0 mg/kg) and metergoline (0.3 mg/kg) were able to reduce the stimulus properties of INDO to 60% and 30%, respectively, only ritanserin (1.0 mg/kg) reduced the stimulus properties of INDO to 25% with a clear dose-response relationship. The results suggest that INDO acts as an agonist at 5-HT1A receptor sites, but its activity at 5-HT(1B/2C) receptor sites also contributes to its discriminative function.     

Effects of chlordiazepoxide,buspirone, and serotonin receptor agonists and antagonists on responses of squirrel monkeys maintained under second-order schedules of intramuscular cocaine injection or food presentation

Lever pressing of squirrel monkeys was maintained under second-order schedules of either food presentation or cocaine injection. The first response after 3 min produced a 2 sec change in the color of a visual stimulus; the tenth stimulus presentation was followed by either an i.m. injection of cocaine (0.3, 1.0, or 2.0 mg/kg) or the delivery of food. The benzodiazepine chlordiazepoxide (0.3-5.6 mg/kg) increased responding maintained by food at doses that decreased cocaine-maintained responding. In contrast, buspirone, a novel nonbenzodiazepine anxiolytic (0.001-0.03 mg/kg), its analog gepirone (0.003-0.03 mg/kg), and the N serotonin 1A (5-HT_1A) agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT; 0.0003-0.001 mg/kg) increased cocaine-maintained responding at doses that decreased responding maintained by food. The inverse agonist at benzodiazepine receptors β-carboline-3-carboxylic acid ethyl ester (βCCE) only decreased response rates irrespective of the maintaining event. m-Chlorophenylpiperazine (mCPP), an agonist at 5-HT_1b receptors, increased responding maintained by food while only decreasing cocaine maitained responding (0.001-0.03 mg/kg), whereas metergoline (0.1-0.3 mg/kg), a serotonin antagonist with affinity for both 5-HT₁ and 5-HT₂ receptors, produced large response rate increases in both groups of monkeys. Administration of the 5-HT₂ antagonist ketanserin (0.03-3.0 mg/kg), the 5-HT_1A compound spiroxatrine (0.0003-0.03 mg/kg), or the nonselective 5-HT agonist quipazine (0.003-1.0 mg/kg) resulted in dose-dependent response rate decreases in both groups. These results further differentiate buspirone and related compounds from typical anxiolytics and suggest a differential involvement of 5-HTIA receptors in behaviors maintained by cocaine or food presentation.     

Blockade of potentiated startle responding in rats by 5-hydroxytryptamine1A receptor ligands

A potentiated whole-body startle response was produced in rats by pairing intense acoustic stimuli with a light (conditional stimulus) formerly presented contiguously with electric shock. Administration of the selective serotonin (5-HT) 1A ligand 8-hydroxy-2-(di-n-propylamino)tetralin (0.125 and 0.5 mg/kg) blocked the potentiation of startle induced by the conditional stimulus. 1-[3-Chlorophenyl]piperazine, a non-anxiolytic 5-HT1B/1C agonist, did not block potentiated startle, even at a dose (1.0 mg/kg) that induced significant overall decreases in startle amplitude. The non-benzodiazepine anxiolytics buspirone (1.25-5.0 mg/kg), gepirone (3.0-10.0 mg/kg) and the related 5-HT1A ligand ipsapirone (1.0-10.0 mg/kg) blocked potentiated startle, though effects of the non-selective serotonin antagonist methysergide (0.3-10.0 mg/kg) fell short of significance. These data support a role for the 5-HT1A binding site in the anti-anxiety effects of buspirone and related compounds over a range of behavioral procedures.     

Serotonin 5-HT1A receptor-mediated hypothermia in mice: absence of spare receptors and rapid induction of tolerance.

The mixed 5-hydroxytryptamine1A (5-HT1A) receptor agonist/antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol-[4.5]- decane-7,9-dione (BMY 7378) (5 mg/kg) did not significantly depress body temperature, but pretreatment with BMY 7378 blocked hypothermia induced by the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In contrast, another partial 5-HT1A agonist, pindolol (10 mg/kg), slightly but significantly depressed body temperature by itself but did not attenuate hypothermia elicited by 8-OH-DPAT. Attempts to identify the synaptic locus of the receptor were unsuccessful because depletion of central serotonin (5-HT) by treatment with para-chlorophenylalanine (PCPA; 3 x 150 mg/kg) did not alter the hypothermic response to 8-OH-DPAT. Partial, irreversible 5-HT1A receptor inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (1 mg/kg) reduced the maximal hypothermic effect of 8-OH-DPAT (to 53% of control) without altering its ED50 (0.96 mg/kg). Analysis of the data indicated a linear relationship between 5-HT1A receptor occupancy and hypothermic response, that is, absence of receptor reserve. When groups of mice were treated with each of five different doses of 8-OH-DPAT (0.04, 0.16, 0.63, 2.5, and 10 mg/kg) 48 h apart, there was a significant reduction in hypothermic response after the second injection, but only at the three highest doses. The results demonstrate that 8-OH-DPAT-induced hypothermia in mice is mediated by a 5-HT1A receptor whose synaptic localization is uncertain but that has no receptor reserve. In addition, tolerance is observed after only a single agonist treatment.     

Behavioral and neurochemical effects of the serotonin (5-HT)1A receptor ligand spiroxatrine

The effects of spiroxatrine, a putative antagonist with selectivity for the serotonin (5-HT)_1A receptor, were compared with compounds believed to function as agonists at the 5-HT_1A receptor. Schedule-controlled responding of pigeons was maintained under a multiple 30-response fixed-ratio (FR), 3-min fixed-interval (FI) schedule or under a schedule in which responding was suppressed by electric shock (conflict procedure). Under the multiple schedule, spiroxatrine (0.3–1.0 mg/kg) decreased FR responding but did not affect FI responding; responding was decreased in both schedule components at 3.0 mg/kg. When administered alone, buspirone, a compound believed to produce its anxiolytic effects through 5-HT_1A agonist actions, produced effects similar to those of spiroxatrine; in combination, the two drugs produced greater effects than when either was administered alone. As with 5-HT_1A agonists such as buspirone and 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) in the pigeon, spiroxatrine (0.01–1.0 mg/kg) increased punished responding. Spiroxatrine and buspirone were potent inhibitors of [³H]8-OH-DPAT binding to pigeon cerebral membranes with IC₅₀ values in the nM range. Neurochemical analyses of metabolite changes produced by spiroxatrine in pigeon cerebrospinal fluid showed buspirone-like effects, with increases in MHPG, DOPAC and HVA at doses that decreased 5-HIAA levels. Spiroxatrine dose-dependently blocked the behavioral effects of the dopamine agonist piribedil indicating that, like buspirone, it also is a potent dopamine antagonist. Spiroxatrine most likely functions as an agonist at the 5-HT_1A receptor. As with buspirone, however, spiroxatrine has a prominent dopamine antagonist component.     

Serotonin involvement in the discriminative stimulus effects of mu and kappa opioids in rats

The role of serotonin (5-HT) in the discriminative stimulus effects of opioids was examined using a two-lever, food-reinforced drug discrimination procedure. The effects of the 5-HT(1A) full agonist 8-OH-DPAT, the 5-HT(1A) partial agonist buspirone and the 5-HT(2) antagonist ketanserin were evaluated in rats trained to discriminate the mu opioid agonist morphine, or the kappa opioid agonist U50, 488 from saline. In rats trained to discriminate 5.6mg/kg of morphine from saline, morphine dose-dependently substituted (produced >/= 80% morphine-appropriate responding) for the morphine stimulus. In contrast, U50,488, 8-OH-DPAT and ketanserin did not substitute for morphine, and buspirone produced only a small degree of substitution (approx. 40% morphine-appropriate responding). When administered in combination with morphine, 8-OH-DPAT, but not buspirone and ketanserin, attenuated the discriminative stimulus effects of higher doses of morphine. In rats trained to discriminate 5.6mg/kg of U50, 488 from saline, U50, 488 dose-dependently substituted for the U50, 488 stimulus. When administered alone, 8-OH-DPAT and buspirone partially substituted (produced between 40% and 79% U50, 488-appropriate responding) for the U50,488 stimulus, whereas morphine and ketanserin did not substitute for U50,488. The opioid antagonist naltrexone failed to antagonize the effects of 8-OH-DPAT and buspirone suggesting that the effects of these drugs in U50,488-trained rats were not mediated by opioid receptors. When administered in combination with U50,488, 8-OH-DPAT, but not buspirone or ketanserin, attenuated the discriminative stimulus effects of the training dose of U50,488. These results suggest that the 5-HT system is involved in the discriminative stimulus effects of both morphine and U50,488, although the exact nature of this 5-HT involvement is not clear.     

S 14671: a novel naphthylpiperazine 5-HT1A agonist of high efficacy and exceptional in vivo potency.

The novel, naphthylpiperazine 5-HT1A agonist, S 14671 (4-[(thenoyl-2)aminoethyl]-1-(7-methoxynaphtylpiperazine], displayed very high affinity for 5-HT1A binding sites (pKi = 9.3) as compared to the serotonin (5-HT)1A agonists, 8-OH-DPAT (9.2) and (+)-flesinoxan (8.7) and the 5-HT1A partial agonists, buspirone (7.9) and BMY 7378 (8.8). In vivo, S 14671 induced the typical 5-HT1A agonist-induced responses of hypothermia and spontaneous tail-flicks at doses as low as greater than or equal to 5 micrograms/kg s.c. and greater than or equal to 40 micrograms/kg s.c., respectively. In each test, it was about 10-fold more potent than 8-OH-DPAT and 100-fold more potent than (+)-flesinoxan and buspirone. The actions of S 14671 could be blocked by BMY 7378 and the 5-HT1A receptor antagonist, (-)-alprenolol, but not by the 5-HT1C/2 receptor antagonist, ritanserin, nor the 5-HT3 receptor antagonist, ICS 205930. Thus, S 14671 is a novel 5-HT1A ligand of high efficacy and exceptional in vivo potency.     

The role of alpha(1)-adrenoceptors and 5-HT(1A) receptors in the control of the micturition reflex in male anaesthetized rats

1. The effects of the alpha(1)-adrenoceptor antagonists doxazosin (0.1 -- 2 mg kg(-1)), RS-100329 (alpha(1A); 0.01 -- 1 mg kg(-1)), RS-513815 (Ro 151-3815, alpha(1B); 0.3 -- 3 mg kg(-1)) and BMY 7378 (alpha(1D); 0.1 -- 1 mg kg(-1)), the 5-HT(1A) receptor agonist, 8-OH-DPAT (0.03 -- 0.3 mg kg(-1)) and antagonist WAY-100635 (0.03 -- 0.3 mg kg(-1)) were investigated (i.v.) on the 'micturition reflex' in the urethane anaesthetized male rat. 2. Reflex-evoked urethra contractions were most sensitive to the inhibitory action of RS-100329, followed by doxazosin, BMY 7378 and WAY-100635 and then RS-513815. The maximum inhibition was 66, 63, 54, 46 and 22% at doses of 0.3, 0.5, 0.3, 0.3 and 3 mg kg(-1) respectively. 3. BMY 7378 and 8-OH-DPAT decreased, while WAY-100635 increased, the pressure threshold to induce bladder contraction. WAY-100635 (0.01 mg kg(-1)) blocked the effects of BMY 7378 (1 mg kg(-1)) on bladder pressure and volume threshold. 4. Doxazosin, RS-100329 and BMY 7378 had a similar potency in inducing a fall in arterial blood pressure while WAY-100635 only caused a fall at the highest dose. 5. Therefore, reflex-evoked urethral contraction involves the activation of alpha(1A/1D)-adrenoceptors, as BMY 7378 and RS-100329 are similarly potent in attenuating this effect. The ability of WAY-100635 to attenuate this contraction may suggest that 5-HT(1A) receptors are also involved. However, as this inhibition occurred at the highest dose of WAY-100635, which also caused a fall in arterial blood pressure; this effect is considered to be due to blockade of alpha(1)-adrenoceptors not 5-HT(1A) receptors. Nevertheless the initiation of the 'micturition reflex' involves the activation of 5-HT(1A) receptors.     

Evaluation of the discriminative stimulus effects of the novel sedative-hypnotic CL 284,846

CL 284,846,N-[3-(3-cyanopyrazolo[1, 5-a]pyrimidin-7-yl)phenyl)]-N-ethylacetamide, is a novel non-benzodiazepine sedative-hypnotic with benzodiazepine-like sedative effects, but with less apparent liability for accompanying undesired side effects. In an effort to further characterize its pharmacological activity, CL 284,846 (3.0 mg/kg, IP, 30 min pretreatment) was established as a discriminative stimulus (DS) in rats (n=7). CL 284,846 (0.3–10.0 mg/kg) showed a dose-related increase in drug-appropriate responding up to the training dose and a dose-related decrease in response rate. The benzodiazepine agonist triazolam (0.1–1.0 mg/kg), the benzodiazepine partial agonist Ro 17-1812 (0.3–3.0 mg/kg) and the triazolopyridazine CL 218,872 (1.0–3.0 mg/kg) substituted for CL 284,846 in all rats, whereas the imidazopyridines zolpidem (3.0–10.0 mg/kg) and alpidem (10.0–30.0 mg/kg), the benzodiazepine partial agonist bretazenil (0.03–10.0 mg/kg) and the novel putative anxiolytic CL 273,547 (10.0–56.0 mg/kg) substituted in most, but not all, rats. Ro 17-1812, bretazenil, and CL 218,872 had no effect on response rate while the other drugs showed a concomitant decrease in rate. The 5-HT_1A agonist buspirone (1.0–10.0 mg/kg) and the barbiturate pentobarbital (3.0–17.0 mg/kg) failed to substitute for CL 284,846 up to rate-decreasing doses. The benzodiazepine antagonist flumazenil (3.0–10.0 mg/kg) blocked the DS effects of CL 284,846 in most rats with no effect on response rate. Taken together, these results suggest that the DS effects of CL 284,846 are mediated via benzodiazepine receptors; however, the DS profile of CL 284,846 remains distinct from both benzodiazepine and non-benzodiazepine sedative-hypnotic drugs.     

Cocaine-induced suppression of renin secretion is partially mediated by serotonergic mechanisms.

Acute cocaine reduces renin secretion. To determine whether serotonergic neurons mediate this effect, male Sprague-Dawley rats received the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (75 micrograms/side, ICV) 2 weeks prior to cocaine injections (3.75-15 mg/kg, IP). 5-HT lesions attenuated the cocaine-induced reduction of plasma renin concentration (PRC), suggesting a partial 5-HT role. To determine which receptors mediate this response, rats were pretreated with the partial 5-HT1A agonist 8-[2-[4-(2-methoxyphenyl)-l-piperazinyl]ethyl]-8-azaspirol-[4,5]- decane-7,9-dione (BMY 7378) (1 mg/kg, SC), the 5-HT1C/5-HT2 antagonist ritanserin (0.1 mg/kg, SC), or the alpha 2/5-HT1A antagonist yohimbine (1 mg/kg, SC) prior to cocaine. None of the antagonists altered the cocaine-induced suppression of PRC, although BMY 7378 and yohimbine elevated PRC. The data suggest that cocaine's effect is partially mediated by a serotonergic mechanism, but do not support a role for 5-HT1A receptors, 5-HT2/5-HT1C receptors, or alpha 2-adrenoceptors in mediating the suppressive effect of cocaine on renin secretion.     

The effect of 5-HT1A receptor agonists on locomotor activity in the guinea-pig.

1. The present study examined the effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, ipsapirone and buspirone, all agonists at the 5-HT1A receptor, on the locomotor activity of guinea-pigs. The effects of these drugs were contrasted with those of the non-selective 5-HT agonist, 5-methoxy-N,N-dimethyl tryptamine (5-MeO-DMT) and the dopamine D2 antagonist, raclopride. 2. 8-OH-DPAT, flesinoxan and 5-MeO-DMT markedly increased the locomotor activity of naive, unhabituated guinea-pigs in a dose-dependent manner. Buspirone also did so, although to a lesser extent and for a shorter time. The doses at which this effect was seen were higher than those normally employed in rats. Ipsapirone and raclopride had no significant effects on locomotor activity. 3. The locomotor activity increasing effect of 1.0 mg kg-1 8-OH-DPAT was blocked by the selective 5-HT1A antagonist (S)-UH-301 (3.0 and 10.0 mg kg-1), but not by (-)-alprenolol (15.0 mg kg-1). Ipsapirone (30.0 mg kg-1) and raclopride (3.0 mg kg-1) antagonized 8-OH-DPAT-induced locomotor activity but only to a small extent. The 5-HT reuptake inhibitor, zimelidine (10.0 mg kg-1) had no effect. 4. The effect of the 5-HT1A agonists in the guinea-pig contrasts with the effects of 8-OH-DPAT on the locomotor activity of unhabituated rats and mice tested in the same apparatus, but are similar to the effects of 8-OH-DPAT on habituated rats, which show a low baseline of activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dose-dependent discriminative stimulus properties of 8-OH-DPAT

Separate groups of rats were trained to discriminate either 0.1mg/kg (low dose; L) or 2.5mg/kg (high dose; H) of 8-OH-DPAT from saline, in a standard operant task. Both cues were found to be dose, time and route dependent and generalized completely to the 5-HT(1A) agonists ipsapirone and flesinoxan. Buspirone substituted completely for 8-OH-DPAT in L and partially in H, whereas the 5-HT(1A/1B) receptor agonist eltoprazine substituted completely for 8-OH-DPAT in H but only partially in L. The 5-HT(1A/1B) receptor agonist RU24969, the 5-HT(1B/2C/1A) receptor agonist TFMPP and the 5-HT reuptake blocker fluvoxamine did not completely mimic the effect of 8-OH-DPAT in either L or H and the 5-HT(1A) mixed agonists/antagonists BMY 7378 and NAN-190 produced partial generalization in L, but no generalization in H. In antagonism tests, NAN-190 and BMY 7378 only partially blocked the 8-OH-DPAT cue in both groups. The non-selective 5-HT receptor antagonist methysergide did not completely block the 8-OH-DPAT cue in in L or H. However, in generalization studies, it completely mimicked the 8-OH-DPAT cue in L and produced partial generalization in H. The beta-adrenergic/5-HT(1A/1B) receptor antagonist pindolol completely blocked the 8-OH-DPAT cue in L and H and did not mimic the 8-OH-DPAT cue in either condition. The alpha(2)-adrenoceptor blocker yohimbine substituted fully for the 8-OH-DPAT cue in L and partially in H. Idazoxan did not substitute for the cue of 8-OH-DPAT in H, but produced nearly 80% generalization in L. The dopamine receptor antagonist pimozide neither blocked nor mimicked the cue of 8-OH-DPAT in either group. A number of other drugs (i.e. m-CPP, S(-)-propranolol, DOI, ketanserin, clonidine and apomorphine) were only tested in H. S(-)-Propranolol blocked the 8-OH-DPAT cue but the other compounds produced neither stimulus generalization nor antagonism. The present study demonstrates that the cues produced by the low and the high training dose of 8-OH-DPAT are quantitatively different and mediated by the agonistic activity of 8-OH-DPAT at 5-HT(1A) receptors. Although the results suggest that the 8-OH-DPAT cue (both L and H) is mediated via postsynaptic 5-HT(1A) receptors, the involvement of presynaptic 5-HT(1A) receptors cannot yet be ruled out.     

The putative 5-HT1A antagonist BMY 7378 blocks 8-OH-DPAT-induced changes in local cerebral glucose utilization in the conscious rat.

It has previously been shown that the 5-HT1A agonist, 8-OH-DPAT, caused discrete changes in cerebral glucose utilization in the rat, as assessed by quantitative 2-deoxyglucose autoradiography. Here, the effect of the putative 5-HT1A antagonist, BMY 7378, on regional cerebral glucose utilization was examined, when injected alone and in rats treated with 8-OH-DPAT. In control rats, BMY 7378 (5 mg/kg, s.c.) markedly increased glucose utilization in the lateral habenular nucleus and moderately reduced glucose utilization in the hippocampal formation. Pretreatment with BMY 7378 (5 mg/kg) significantly attenuated the reductions in glucose utilization in the hippocampus, entorhinal, piriform and cingulate cortex, induced by 8-OH-DPAT (0.25 mg/kg). The 8-OH-DPAT-induced increase in glucose utilization in the copula pyramis, that is putatively associated with the appearance of the 5-HT behavioural syndrome, was also blocked by BMY 7378, as was the behavioural syndrome. In summary, BMY 7378 produced few of the discrete changes in cerebral glucose utilization that are seen with 8-OH-DPAT. However, many of the changes induced by 8-OH-DPAT were reversed by BMY 7378. These data are consistent with the hypothesis that the effects of 8-OH-DPAT on regional cerebral glucose utilization are mediated by 5-HT1A receptors.     

Serotonergic involvement in the inhibitory effects of repeated buspirone treatment on synaptic transmission in the hippocampus

The effects of acute and long-term treatment with buspirone on synaptic transmission in the hippocampus were compared in alert rats with chronic indwelling electrodes and cannula. Buspirone produced a transient dose-dependent reduction in the amplitude of the excitatory postsynaptic potential (EPSP) when acutely injected either systemically (0.3-3.0 mg/kg i.p.) or directly into the hippocampus (0.1-1.0 microgram i.h.). Whereas acute application of 0.5 mg/kg i.p. produced a 20% reduction which reversed within 2 h, during long-term treatment with this relatively low dose there was a gradual reduction of the baseline EPSP amplitude which reached a maximum (40%) between days 7-14 and which did not reverse completely until 72 h after the last injection. Intrahippocampal injection of either buspirone or 5-hydroxytryptamine did not have any additional effect during the period of baseline reduction. The 5-HT1A receptor antagonist spiroxatrine (1 mg/kg i.p.) produced a transient reversal of the effect of chronic buspirone. It is concluded that the chronic inhibitory effect of buspirone is probably an extension of its acute action on 5-HT1A receptors in the hippocampus.