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

The antidepressant-like effect and proposed mechanism of action of TPN672MA, a novel serotonin-dopamine receptor modulator for the treatment of schizophrenia

TPN672MA, an innovative antipsychotic drug candidate currently in clinical trials, acts as a dopamine D2/D3 receptor partial agonist, serotonin 5-HT1A receptor agonist, and serotonin 5-HT2A receptor antagonist. Preclinical investigations have demonstrated its potential in treating the core symptoms of schizophrenia. The present study highlights TPN672MA's significant antidepressant-like effects in classical behavioral models, such as the chronic social defeat stress paradigm. The pronounced 5-HT1A receptor agonism and D2/D3 receptor partial agonism of TPN672MA likely contribute to its therapeutic effects in depression. Additionally, TPN672MA's antidepressant-like efficacy may be linked to its ability to enhance the expression levels of brain-derived neurotrophic factor (BDNF) and postsynaptic density protein-95 (PSD95) in the hippocampus. Furthermore, TPN672MA displayed a more rapid onset of antidepressant-like action. In conclusion, TPN672MA represents a promising new drug candidate for the treatment of symptoms of schizophrenia and depression.

Collaborative Virtual Screening Identifies a 2-Aryl-4-aminoquinazoline Series with Efficacy in an In Vivo Model of Trypanosoma cruzi Infection

Probing multiple proprietary pharmaceutical libraries in parallel via virtual screening allowed rapid expansion of the structure-activity relationship (SAR) around hit compounds with moderate efficacy against Trypanosoma cruzi, the causative agent of Chagas Disease. A potency-improving scaffold hop, followed by elaboration of the SAR via design guided by the output of the phenotypic virtual screening efforts, identified two promising hit compounds 54 and 85, which were profiled further in pharmacokinetic studies and in an in vivo model of T. cruzi infection. Compound 85 demonstrated clear reduction of parasitemia in the in vivo setting, confirming the interest in this series of 2-(pyridin-2-yl)quinazolines as potential anti-trypanosome treatments.

Effects of repeated treatment with the 5-HT1A and 5-HT1B agonists (R)-( +)-8-hydroxy-DPAT and CP-94253 on the locomotor activity and axillary temperatures of preweanling rats: evidence of tolerance and behavioral sensitization

RATIONALE

Drugs that stimulate 5-HT_1A/1B receptors produce both tolerance and behavioral sensitization in adult rats and mice, yet it is unknown whether the same types of plasticity are evident during earlier ontogenetic periods.

OBJECTIVE

The purpose of this study was to determine whether repeated treatment with selective 5-HT_1A and/or 5-HT_1B agonists cause tolerance and/or sensitization in preweanling rats.

METHODS

In Experiments 1 and 2, male and female preweanling rats were given a single pretreatment injection of saline, the 5-HT_1A agonist (R)-( +)-8-hydroxy-DPAT (8-OH-DPAT), or the 5-HT_1B agonist CP-94253 on PD 20. After 48 h, rats received a challenge injection of 8-OH-DPAT or CP-94253, respectively. In Experiment 3, rats were pretreated with saline or DPAT + CP on PD 20 and challenged with the same drug cocktail on PD 22. In Experiment 4, the tolerance- or sensitization-inducing properties of 8-OH-DPAT, CP-94253, or DPAT + CP were tested after a 4-day pretreatment regimen on PD 17-20.

RESULTS

On the first pretreatment day, 8-OH-DPAT, CP-94253, and DPAT + CP increased locomotion and caused hypothermia. Repeated treatment with 8-OH-DPAT (2 or 8 mg/kg) or DPAT + CP caused locomotor sensitization in preweanling rats. In contrast, tolerance to the hypothermic effects of 8-OH-DPAT (8 mg/kg), CP-94253 (5-20 mg/kg), or DPAT + CP was evident after repeated drug treatment.

CONCLUSIONS

During the preweanling period, a single injection of a selective 5-HT_1A or 5-HT_1B agonist is capable of producing drug-induced plasticity. A pretreatment administration of 8-OH-DPAT causes both tolerance (hypothermia) and behavioral sensitization (locomotor activity) in preweanling rats, whereas repeated CP-94253 treatment results in tolerance.

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Anticonvulsant action of indazole

Here we report that indazole is characterized as a potential anticonvulsant, inhibiting pentylenetetrazole-, electroshock- and strychnine-induced convulsions in mice (ED50's: 39.9, 43.2 and 82.4 mg/kg, respectively) but not bicuculline- and picrotoxin-induced convulsions. The median toxic dose (TD(50)) of indazole was 52.3 mg/kg by the minimal motor impairment test. Therefore, nontoxic doses produced anticonvulsant activity against pentylenetetrazole- and electroshock-induced seizures. Indazole (50 mg/kg) had no effect on spontaneous activity but induced hypothermia. It also inhibited the metabolism of dopamine and 5-hydroxytryptamine in the brain in vivo and the activities of monoamine oxidase A and B in vitro, with IC(50) values of 20.6 μM and 16.3 μM, respectively. However, these inhibitory effects do not account for the anticonvulsant activity because treatment with typical monoamine oxidase inhibitors such as pargyline or tranylcypromine did not completely reproduce the anticonvulsant activity of indazole. In the animal seizure models tested, the anticonvulsant profile of indazole most resembled that of gabapentin and somewhat resembled those of the AMPA/kainate antagonist NBQX and the sodium channel inhibitor phenytoin, but differed from that of benzodiazepine. The isobolographic analyses showed that the interactive mode of indazole with gabapentin, NBQX or phenytoin is additive. These results suggest that indazole has anticonvulsant activity and multiple mechanisms.

Relationship between Rgs2 gene expression level and anxiety and depression-like behaviour in a mutant mouse model: serotonergic involvement

RGS2 is a member of a family of proteins that negatively modulate G-protein coupled receptor transmission. Variations in the RGS2 gene were found to be associated in humans with anxious and depressive phenotypes. We sought to study the relationship of Rgs2 expression level to depression and anxiety-like behavioural features, sociability and brain 5-HT1A and 5-HT1B receptor expression. We studied male mice carrying a mutation that causes lower Rgs2 gene expression, employing mice heterozygous (Het) or homozygous (Hom) for this mutation, or wild-type (WT). Mice were subjected to behavioural tests reflecting depressive-like behaviour [forced swim test (FST), novelty suppressed feeding test (NSFT)], elevated plus maze (EPM) for evaluation of anxiety levels and the three-chamber sociability test. The possible involvement of raphe nucleus 5-HT1A receptors in these behavioural features was examined by 8-OH-DPAT-induced hypothermia. Expression levels of 5-HT1A and 5-HT1B receptors in the cortex, raphe nucleus and hypothalamus were compared among mice of the different Rgs2 genotype groups. NSFT results demonstrated that Hom mice showed more depressive-like features than Rgs2 Het and WT mice. A trend for such a relationship was also suggested by the FST results. EPM and sociability test results showed Hom and Het mice to be more anxious and less sociable than WT mice. In addition Hom and Het mice were characterized by lower basal body temperature and demonstrated less 8-OH-DPAT-induced hypothermia than WT mice. Finally, Hom and Het mice had significantly lower 5-HT1A and 5-HT1B receptor expression levels in the raphe than WT mice. Our findings demonstrate a relationship between Rgs2 gene expression level and a propensity for anxious and depressive-like behaviour and reduced social interaction that may involve changes in serotonergic receptor expression.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

CB1 knockout mice display impaired functionality of 5-HT1A and 5-HT2A/C receptors

Interaction between brain endocannabinoid (EC) and serotonin (5-HT) systems was investigated by examining 5-HT-dependent behavioral and biochemical responses in CB(1) receptor knockout mice. CB(1) knockout animals exhibited a significant reduction in the induction of head twitches and paw tremor by the 5-HT(2A/C) receptor selective agonist (+/-) DOI, as well as a reduced hypothermic response following administration of the 5-HT(1A) receptor agonist (+/-)-8-OH-DPAT. Additionally, exposure to the tail suspension test induced enhanced despair responses in CB(1) knockout mice. However, the tricyclic antidepressant imipramine and the 5-HT selective reuptake inhibitor fluoxetine induced similar decreases in the time of immobility in the tail suspension test in CB(1) receptor knockout and wild-type mice. No differences were found between both genotypes with regard to 5-HT(2A) receptor and 5-HT(1A) receptors levels, measured by autoradiography in different brain areas. However, a significant decrease in the ability of both, the 5-HT(1A) receptor agonist (+/-)-8-OH-DPAT and the 5-HT(2A/C) receptor agonist (-)DOI, to stimulate [(35)S]GTPgammaS binding was detected in the hippocampal CA(1) area and fronto-parietal cortex of CB(1) receptor knockout mice, respectively. This study provides evidence that CB(1) receptors are involved in the regulation of serotonergic responses mediated by 5-HT(2A/C) and 5-HT(1A) receptors, and suggests that a reduced coupling of 5-HT(1A) and 5-HT(2A) receptors to G proteins might be involved in these effects.

Hypothermic responses to 8-OH-DPAT in the Ts65Dn mouse model of Down syndrome

Recently, we have demonstrated that potassium channels containing G-protein-activated potassium channel 2 (GIRK2) subunits play a significant role in hypothermia induced by several neurotransmitter receptor agonists, including the serotonin (5-HT)1A/5-HT7 receptor agonist 8-OH-DPAT [R-(+)-8-hydroxy-2-(di-n-propylamino) tetralin]. The GIRK2 gene is located in human chromosome 21 (its mouse ortholog, Girk2, is in mouse chromosome 16). Down syndrome is produced by the trisomy of chromosome 21. Here, we used quantitative radiotelemetry to investigatehypothermic responses to 8-OH-DPAT in the Down syndrome mouse model Ts65Dn (which carries an extra chromosomal 16 segment containing Girk2). Our results indicate that, in relation to euploid controls, Ts65Dn mice display significantly increased hypothermic responses to 8-OH-DPAT. This finding may be relevant to the understanding of previously reported differences in serotoninergic neurotransmission in persons with Down syndrome.

G-protein-gated potassium (GIRK) channels containing the GIRK2 subunit are control hubs for pharmacologically induced hypothermic responses

Hypothermic responses of rodents to the peripheral or intraventricular injection of many individual neurotransmitter receptor agonists have been well documented. Because many hypothermia-inducing agonists are also known to activate G-protein-gated potassium (GIRK) channels, we investigated the hypothermic response to several of these agents on Girk2 null mutant mice. Core body temperatures were measured through radiotelemetry, and animals were maintained in special temperature-regulated chambers to ensure the accuracy of the measurements. The resulting data indicate that the activation of GIRK2-containing potassium channels plays a significant role in hypothermia induced by the activation of serotonergic (5-HT(1A)), GABAergic (GABA(B)), muscarinic (m2), adenosine (A1), and mu, delta, and kappa opioid receptors. These channels also are involved in the alcohol-induced hypothermic response. These results have implications for the understanding of pharmacologically induced hypothermia and thermoregulatory mechanisms.

5-HT1A receptor-mediated regulation of mitogen-activated protein kinase phosphorylation in rat brain

Mitogen-activated protein kinases (MAPKs), a family of signal transduction mediators important in a host of cellular activities, include the extracellular signal-regulated kinases Erk1 and Erk2. We determined whether 5-HT(1A) receptors activate Erk1/2 in rat brain in vivo, as they do in recombinant cell lines. In contrast to the effect in cells, the 5-HT(1A) receptor agonist 8-hydroxy-N,N-diproylaminotetralin (8-OH-DPAT) dose- and time-dependently decreased basal levels of phosphorylated Erk1/2 (phospho-Erk1/2) in rat hippocampus (ED(50) approximately 0.1 mg/kg, maximum approximately 90%) without altering total Erk1/2. The effects were kinase-specific, as 8-OH-DPAT did not modify phosphorylated or total levels of the MAPKs c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 MAPK. Moreover, 8-OH-DPAT did not modify phospho-Erk1/2 in striatum or frontal cortex. The effect of 8-OH-DPAT was blocked by pretreatment with the selective 5-HT(1A) receptor antagonists N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635), 1-(2-methoxyphenyl)-4-(4-[2-phthalimido]butyl)piperazine (NAN-190) and 4-fluoro-N-(2-[4-(2-methoxyphenyl)1-piperazinyl]ethyl)-N-(2-pyridinyl)benzamide dihydrochloride (p-MPPF), but not by the weak partial agonist/antagonist 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8-azaspiro(4.5)decane-7,9-dione dihydrochloride (BMY 7378). Other 5-HT(1A) receptor agonists (buspirone, gepirone and ipsapirone) also reduced phospho-Erk1/2 levels in hippocampus. 8-OH-DPAT also reduced the levels of the upstream activator of Erk1/2, phosphorylated extracellular signal-regulated kinase kinase (phospho-MEK1/2), and at least one potential downstream target, the nuclear transcription factor phospho-Elk-1. The region- and kinase-specific effects suggest that the Erk1/2 signal transduction cascade is likely an important differential mediator of 5-HT(1A) receptor-regulated events in the central nervous system.

Are there gender differences in the temperature profile of mice after acute antidepressant administration and exposure to two animal models of depression?

Numerous studies have reported gender differences in the rates of depression in humans, but few behavioural observations of antidepressant drug effects have been investigated in female mice. The forced swimming test (FST) is widely used as a predictor of antidepressant activity in rodents, as is the tail suspension test (TST), where immobility is objectively measured and in this last test, no hypothermia is induced by immersion in cold water. The present study investigated gender differences in the temperature profile of mice after acute antidepressant administration (imipramine and paroxetine) and exposure to two animal models of depression. Imipramine and paroxetine were active at 32 mg/kg in male mice in the FST, whereas they were active at 8, 16 and 32 mg/kg in female mice. In the TST, for both antidepressants immobility duration was reduced at a dose of 16 and 32 mg/kg in male mice and at 32 mg/kg in female mice. No significant difference was observed between male and female mice for immobility duration. Imipramine administration, but not paroxetine, decreased the temperature at the higher dose (32 mg/kg) in male and female mice in the FST. The body temperature was reduced in male and female mice for all treatment groups after FST challenge. Imipramine (16 and 32 mg/kg in male and 32 mg/kg in female mice), paroxetine (4, 16 and 32 mg/kg in male and 4 to 32 mg/kg in female mice) attenuated the reduction in temperature due to the FST. In the TST, imipramine tends to decrease the temperature in male and female mice, even though only imipramine at a dose of 32 mg/kg in female mice significantly decreases the temperature. Paroxetine had no effect on temperature. The TST enhanced the body temperature in male and female mice. In mice, there was no difference between the sexes after imipramine or paroxetine administration in the FST and TST. Both tests can be used to predict the activity of antidepressants as the decrease or enhancement of temperature is not correlated with a reduction in immobility duration.

Selective breeding of 5-HT(1A) receptor-mediated responses: application to emotion and receptor action

Rat lines that were selectively bred for high (high DPAT-sensitive, HDS) or low (low DPAT-sensitive, LDS) hypothermic responses to the specific 5-HT(1A) receptor agonist, 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT), differ in receptor binding and certain behaviors related to anxiety and depression. After reviewing this literature, the present communication summarizes new experiments designed to clarify and extend the nature of the pharmacological and biochemical differences between the lines. A challenge with the 5-HT(2) receptor agonist, DOI, produced similar degrees of head shakes and skin crawls in the HDS and LDS rats, suggesting similar sensitivity of 5-HT(2A) and 5-HT(2C) receptors. In contrast, DOI-induced flat body posture (FBP), which has been linked to 5-HT(1A) receptor stimulation, was observed more readily in the HDS rats. The HDS and LDS rats exhibited similar degrees of increase in 8-OH-DPAT-stimulated [35S]GTPgammaS binding in several brain regions. This result suggests that the dramatic differences in hypothermia in HDS and LDS rats cannot be related to 5-HT(1A) receptor-mediated action on G proteins. Overall, these findings indicate that the selective breeding for 5-HT(1A)-mediated hypothermia has been fairly selective, and that differences in emotionally relevant behaviors between these two rat lines can strongly be associated with an unidentified component of the 5-HT(1A) receptor signaling pathway.

Inactivation of 5-HT(1A) receptors in hippocampal and cortical homogenates

5-HT(1A) receptor function can be assessed in rat hippocampal and cortical membrane preparations as agonist-stimulated [35S]GTPgammaS binding. Membranes were preincubated in vitro with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). R(+)-8-hydroxy-2-(di-n-propylamino)tetralin [R(+)-8-OH-DPAT]-stimulated [35S]GTPgammaS binding and [3H]8-OH-DPAT binding assays were used to assess 5-HT(1A) receptor function and density, respectively. EEDQ decreased both R(+)-8-OH-DPAT-stimulated [35S]GTPgammaS and [3H]8-OH-DPAT binding in hippocampal and cortical membranes. The E(max) but not the EC(50) of R(+)-8-OH-DPAT to stimulate [35S]GTPgammaS binding was decreased by EEDQ in both preparations. Additionally, the IC(50) for EEDQ to reduce R(+)-8-OH-DPAT-stimulated [35S]GTPgammaS and [3H]8-OH-DPAT binding was the same for both brain regions in both assays. In contrast to EEDQ alone, agonist-stimulated [35S]GTPgammaS binding was not reduced in hippocampal membranes preincubated with EEDQ and the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl- cyclohexanecarboxamide maleate (WAY 100,635), suggesting that EEDQ acts directly on the receptor. Due to parallel reductions in receptor density and maximal functional response, it is concluded that there is little or no reserve for 5-HT(1A) receptor coupling to G(alpha) in these preparations. In addition, the sensitivity of hippocampal and cortical 5-HT(1A) receptors to inactivation by EEDQ in vitro is the same.

Serotonin receptor 1A knockout: an animal model of anxiety-related disorder

To investigate the contribution of individual serotonin (5-hydroxytryptamine; 5-HT) receptors to mood control, we have used homologous recombination to generate mice lacking specific serotonergic receptor subtypes. In the present report, we demonstrate that mice without 5-HT1A receptors display decreased exploratory activity and increased fear of aversive environments (open or elevated spaces). 5-HT1A knockout mice also exhibited a decreased immobility in the forced swim test, an effect commonly associated with antidepressant treatment. Although 5-HT1A receptors are involved in controlling the activity of serotonergic neurons, 5-HT1A knockout mice had normal levels of 5-HT and 5-hydroxyindoleacetic acid, possibly because of an up-regulation of 5-HT1B autoreceptors. Heterozygote 5-HT1A mutants expressed approximately one-half of wild-type receptor density and displayed intermediate phenotypes in most behavioral tests. These results demonstrate that 5-HT1A receptors are involved in the modulation of exploratory and fear-related behaviors and suggest that reductions in 5-HT1A receptor density due to genetic defects or environmental stressors might result in heightened anxiety.

Electrophysiological and neurochemical evidence that pindolol has agonist properties at the 5-HT1A autoreceptor in vivo

1. It has been hypothesized that 5-HT1A autoreceptor antagonists may enhance the therapeutic efficacy of SSRIs and other antidepressants. Although early clinical trials with the beta-adrenoceptor/5-HT1 ligand, pindolol, were promising, the results of recent more extensive trials have been contradictory. Here we investigated the actions of pindolol at the 5-HT1A autoreceptor by measuring its effect on 5-HT neuronal activity and release in the anaesthetized rat. 2. Pindolol inhibited the electrical activity of 5-HT neurones in the dorsal raphe nucleus (DRN). This effect was observed in the majority of neurones tested (10/16), was dose-related (0.2-1.0 mg kg(-1), i.v.), and was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.), in 6/7 cases tested. 3. Pindolol also inhibited 5-HT neuronal activity when applied microiontophoretically into the DRN in 9/10 neurones tested. This effect of pindolol was current-dependent and blocked by co-application of WAY 100635 (3/3 neurones tested). 4. In microdialysis experiments. pindolol caused a dose-related (0.8 and 4 mg kg(-1), i.v.) fall in 5-HT levels in dialysates from the frontal cortex (under conditions where the perfusion medium contained 1 microM citalopram). In rats pretreated with WAY 100635 (0.1 mg kg(-1), i.v.), pindolol (4 mg kg(-1), i.v.) did not decrease, but rather increased 5-HT levels. 5. We conclude that, under the experimental conditions used in this study, pindolol displays agonist effects at the 5-HT1A autoreceptor. These data are relevant to previous and ongoing clinical trials of pindolol in depression which are based on the rationale that the drug is an effective 5-HT1A autoreceptor antagonist.

Effects of para-chlorophenylalanine (pCPA) on the bush baby auditory brainstem response

Click-evoked auditory brainstem responses were recorded in a prosimian primate, the bush baby (Otolemur garnettii), before and after depletion of serotonin (by systemic injection of para-chlorophenylalanine; pCPA) and up to 20 days after discontinuing pCPA injections (during the recovery of serotonin). Biphasic 100 micros clicks were presented at five repetition rates (13.2, 33.2, 53.2, 73.2, and 93.2 clicks/s; RATE) and sound pressure levels (SPL) were varied in 10 dB steps from 120-60 dB SPL peak equivalent. Absolute latencies of vertex-positive peaks I, III, IV, and V were measured from click onset. The latencies from each wave were statistically analyzed with a two-way analysis of variance using either RATE or SPL (but not both) and TIME AFTER pCPA as independent variables. Prior to pCPA, brainstem response latencies increased as a function of both decreasing SPL and increasing RATE. After pCPA, these normal increases in wave latency increased even more, particularly in response to high click rates. After pCPA was discontinued, measurements taken at weekly intervals indicated that latencies decreased after 1 week, increased to the highest values recorded after 2 weeks, and returned to normal after 3 weeks. These dynamic changes were interpreted to be the result of postsynaptic receptor up-regulation during the 10 days of continuous pCPA administration. These results suggest that serotonin plays an important role in sensory processing at the cellular level and, tonically, facilitates the auditory brainstem response to sound.

Anxiolytic effects of flesinoxan in the stress-induced hyperthermia paradigm in singly-housed mice are 5-HT1A receptor mediated

In the stress-induced hyperthermia paradigm in singly-housed male mice, two sequential rectal temperature measurements reveal the basal temperature (T1) and, 10 min later, an enhanced body temperature (T2), due to the stress of the first rectal measurement. The difference T2 - T1 (deltaT) is the stress-induced hyperthermia and putatively reflects a stress-induced anxiogenic response. The full 5-HT1A receptor agonist flesinoxan ((+)-enantiomer), its (-)-enantiomer and the racemic mixture reduced stress-induced hyperthermia effects, indicating putative anxiolytic properties. The ratio of their potencies to reduce stress-induced hyperthermia was similar to their potency in receptor binding affinities for 5-HT1A receptors, supporting that the anti-hyperthermia effects are mediated by the 5-HT1A receptor. This was further substantiated when the 5-HT1A receptor antagonists WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane carboxamine trihydrochloride) and DU 125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide, monomesylate) both were able to antagonize the anti-stress-induced hyperthermia effects of flesinoxan. The stress-induced hyperthermia paradigm in singly-housed mice represents a simple and robust paradigm to measure putative anxiolytic effects of drugs.

Brain 5-HT1A receptor autoradiography and hypothermic responses in rats bred for differences in 8-OH-DPAT sensitivity

Three rat lines were selectively bred for high (HDS), random (RDS), or low (LDS) hypothermic responses to the specific 5-HT1A receptor agonist 8-OH-DPAT. Forty-five minutes after 8-OH-DPAT administration (0.5 mg/kg), body temperatures dropped 3-5 degrees C in HDS rats, yet this dose produced only about 1.2 degrees C and 0.7 degree C drops in RDS and LDS rats, respectively. To investigate the relationship of body temperature of 5-HT1A receptor binding sites, autoradiographic analyses of [3H]8-OH-DPAT binding to 5-HT1A receptors in brains of these rats were conducted. Significant differences in binding were found in specific limbic cortical projection sites, with the HDS line having the greatest density of sites. Body temperature responses correlated significantly with [3H]8-OH-DPAT receptor binding in only a few areas of frontal cortex. Binding in many other brain regions, including the anterior and posterior hypothalami (regions long associated with body temperature regulation) and the raphe showed no significant differences among the lines. [3H]Ketanserin binding to cortical 5-HT2 receptors did not differ among the lines, except in the cingulate and superficial frontal cortices where HDS exhibited higher binding. These data suggest that differences in 5-HT1A receptor number may contribute to the exaggerated hypothermic response to 8-OH-DPAT in HDS rats. These studies also suggest that genetic regulation of receptor density may be brain region specific which should encourage future studies of the mechanisms of 5-HT1A receptor activity in brain and the action of drugs affecting this receptor.

A single pretreatment with 8-OH-DPAT reduces behavioral indices of serotonin 1A receptor activation in ovariectomized rats

The effects of prior treatment with 8-OH-DPAT on 8-OH-DPAT-induced eating behavior, hypothermia, flat body posture and forepaw treading were examined in ovariectomized rats. Twenty-four hours after a single pretreatment with 0.25 mg/kg of the drug, the eating behavior and flat body posture induced by 8-OH-DPAT were reduced relative to that seen following the first treatment with the drug. In contrast, within 24 hr of the prior drug treatment, the drug's effects on forepaw treading and rectal temperature were unchanged. However, 7 days after prior treatment with 8-OH-DPAT, eating behavior, flat body posture and hypothermia induced by the drug were suppressed. Only forepaw treading remained unchanged. These findings are discussed in reference to similar studies of 5-HT1A receptor agonist-induced modulation of behaviors in male rats.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

The critical role of nucleus accumbens dopamine systems in the mediation of fixed interval schedule-controlled operant behavior

Microinjections of the irreversible dopamine antagonist EEDQ (N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline), into nucleus accumbens but not into dorsal striatum, markedly decreased response rates on a fixed interval schedule of reinforcement. These preferential effects in nucleus accumbens could be prevented by selectively activating either D1 or D2 receptors. These data demonstrate a critical role for mesolimbic dopamine systems in the mediation of fixed interval schedule-controlled operant behavior.

Effects of alpha 2-adrenoceptor antagonists and imidazoline2-receptor ligands on neuronal damage in global ischaemia in the rat

1. In the present study the neuroprotective effects of 3 mg/kg idazoxan, an alpha 2-adrenoceptor antagonist and imidazoline2-receptor (I2-receptor) ligand, 3 mg/kg methoxyidazoxan, a specific alpha 2-adrenoceptor antagonist, and 0.6 and 3 mg/kg BU224, a selective I2-receptor ligand, were evaluated following 10 min of global ischaemia in rats. 2. Neuronal cell counts in the CA1 region of the hippocampus 8 days postischaemia indicated 46-96% cell loss compared with control (P < 0.001) and a 320% increase in [3H]-PK11195 binding (P < 0.001) used as a marker of gliosis. No significant neuroprotective effect could be detected on these markers of neuronal damage in the active treatment groups. In a subset of idazoxan-treated rats, neuronal loss and gliosis was minimal. 3. Mean body temperature over 3 h postischaemia was lower in idazoxan-treated rats than in the other treatment groups (P < 0.001) and there was a correlation between mean body temperature and cell counts (P < 0.01) and mean body temperature and gliosis in this group (P = 0.057). 4. These results indicate that at the doses used neither BU224 nor methoxyidazoxan are neuroprotective in this ischaemia model and they raise the possibility that any neuroprotective effect of idazoxan may be related to hypothermic effects of the drug.

A comparison of the effects of 8-OH-DPAT pretreatment of different behavioural responses to 8-OH-DPAT

The effects of daily pretreatments with the prototypical 5-HT1A receptor agonist 8-hydroxy-(di-n-propylamino) tetralin (8-OH-DPAT) (1.0 mg/kg s.c.) on behavioural responses to challenge by 8-OH-DPAT (0.5 mg/kg s.c.) due to activation of 5-HT1A receptors were determined. The responses had strikingly different susceptibilities to pretreatment. These were not explicable by different effects on pre- and postsynaptic responses. Thus, two components of the 5-HT syndrome due to action at postsynaptic sites (i.e. flat body posture and reciprocal forepaw treading) were substantially attenuated 1 day after a single pretreatment with 8-OH.DPAT, but the tail-flick response, though due to action at postsynaptic 5-HT1A sites, was completely unimpaired by 14 pretreatments while the hypothermic response which also probably involves postsynaptic sites showed progressively increased attenuation on 14 pretreatments. 8-OH-DPAT-induced hyperphagia which depends on activation of presynaptic sites was unimpaired by the pretreatment schedule. The results are discussed in relation to receptor reserve, second messenger changes and effects at NMDA receptors. They imply a need for caution in the use of chronic effects of 5-HTergic drugs on specific 5-HT1A receptor-dependent responses as indices of mechanisms for the therapeutic actions of the drugs.

5-HT1A receptor agonists: recent developments and controversial issues

During the last decade, serotonin (5-HT)1A receptors have been a major target for neurobiological research and drug development. 5-HT1A receptors have been cloned and a variety of selective agonists, such as the aminotetraline 8-OH-DPAT and the pyrimidinylpiperazine ipsapirone, have become available. Demonstrations of apparent intrinsic activity of these ligands at 5-HT1A receptors, however, depend highly on the particular assay system. This may be due to the possible existence of receptor subtypes and to assay (or brain region)-dependent differences in receptor reserve and the nature of receptor-effector coupling. Nevertheless, the apparent intrinsic activity of 8-OH-DPAT seems to be higher (although possibly not yet maximal) than that of the pyrimidinylpiperazines. In the brain, 5-HT1A receptors are located presynaptically as somatodendritic receptors on 5-HT neurons and postsynaptically in particular limbic and cortical regions. Although it is generally accepted that presynaptic 5-HT1A receptors control 5-HT neuronal activity, recent evidence suggests an additional role of postsynaptic 5-HT1A receptors in cortex as part of a negative feedback loop. Anxiolytic and antidepressive properties of selective 5-HT1A receptor agonists have now been confirmed by clinical studies. Although it is well established that the latter properties depend on the agonistic activity of these compounds, the optimal level of intrinsic activity is still a matter of debate and may be dependent on the clinical indication. Such compounds may also have antiaggressive effects, and possibly anticraving effects (manifested by their alcohol intake-reducing effects in dependent animals), but the specificity of these so-called anti-impulsivity effects is still controversial and not yet tested clinically. Anticataleptic, antiemetic and neuroprotective properties have been demonstrated in different species. Behavioral studies on the mechanisms underlying the anxiolytic and antidepressive effects have examined the relative contribution of pre- and postsynaptic 5-HT1A receptors by means of local cerebral application and lesion techniques. Most evidence points towards a critical involvement of presynaptic receptors in the anxiolytic effects of 5-HT1A receptor agonists (although a possible contribution of postsynaptic receptors cannot be excluded). With regard to the antidepressive properties, a case can be made for the reverse; i.e., a strong involvement of postsynaptic receptors and a questionable contribution of presynaptic receptors. However, as the therapeutic effects of those 5-HT1A receptor (partial) agonists which have been tested clinically require repeated administration, attention has been directed increasingly towards chronic studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions between 5-hydroxytryptamine receptor subtypes: is a disturbed receptor balance contributing to the symptomatology of depression in humans?

The purpose of this review is to describe the consequences of antidepressant treatment on the behaviour of rodents after activation of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes. In a summary table, the involvement of 5-HT receptors in inducing behavioural changes are described. It is emphasized that these effects are not always only exclusively linked to serotonergic functions nor that they are only initiated by central 5-HT receptors. Hereafter, the complex mutual inhibitory effects of 5-HT receptor subtype-mediated processes are discussed by interpreting effects of antagonists and describing the different effects of low and high doses of mixed 5-HT1C/5-HT2 receptor agonists. Mutual influences are seen particularly with 5-HT1A, 5-HT1C and 5-HT2, but not with 5-HT1B, 5-HT1D or 5-HT3 receptor-mediated effects. It is shown that the behavioural consequences of 5-HT1A, 5-HT1C and 5-HT2 receptor stimulation may be changed by brain lesions or chronic treatment with drugs. Among these drugs are the antidepressants. Finally, 5-HT receptor function in depressed patients is discussed, and the hypothesis is proposed that an important function of antidepressants is to restore a disturbed balance between 5-HT1A, 5-HT1C and 5-HT2 receptors in depressed patients.

5-HT 1D receptors mediate SKF 99101H-induced hypothermia in the guinea pig

The selective, brain penetrant, 5-HT(1D) receptor agonist SKF 99101H (10-30 mg/kg i.p.) caused a dose-related fall in rectal temperature in guinea pigs which lasted longer than 2 h. Sumatriptan (1.0-100 mg/kg i.p.), a selective 5-HT(1D) agonist which does not penetrate the brain, did not produce hypothermia, suggesting that peripheral mechanisms are not critically involved in the response. The hypothermia induced by SKF 99101H (30 mg/kg i.p.) was dose-dependently blocked by the 5-HT(1D) receptor antagonists GR 127935 (0.01-1 mg/kg i.p.) and GR 125743 (0.01-3 mg/kg i.p.), confirming the role of 5-HT(1D) receptors. Mianserin (0.3-10.0 mg/kg i.p.) and granisetron (0.1-3.0 mg/kg i.p.) were inactive, suggesting that 5-HT(2A/2B/2C) or 5-HT( 3) receptors play no significant role in the generation of the hypothermic response. Nor was the hypothermia reversed by prazosin (0.03-1.0 mg/kg i.p.), idazoxan (0.03-1.0 mg/kg i.p.) or scopolamine (0.01-0.3 mg/kg i.p.), thereby excluding mediation by α(1)- and α(2)-adrenoceptors and muscarinic receptors. WAY 100635 (0.1-1.0 mg/kg) significantly potentiated the effect of SKF 99101H. The antagonists, when given alone, had no effect on body temperature, with the exception of prazosin (0.1 and 1.0 mg/kg). Three days of treatment with parachloroamphetamine (30 mg/kg i.p.) depleted forebrain 5-HT by ∼ 75% in frontal cortex, hypothalamus, hippocampus and striatum, but failed to alter the hypothermic response to SKF 99101H. The hypothermia is, therefore, unlikely to be mediated by 5-HT(1D) receptors located on 5-HT neurons. SKF 99101H-induced hypothermia in the guinea pig may serve as a useful model for investigation of centrally acting 5-HT( 1D) receptor antagonists.

Stress-induced hyperthermia in mice: a methodological study

When the rectal temperature of group-housed mice is measured sequentially, the temperature of the last measured mouse is higher than that of the first mouse. This phenomenon is called stress-induced hyperthermia (SIH). We varied several experimental parameters to elucidate the mechanism behind this SIH. SIH was stable and found by all technicians performing the experiments. The large intertechnician difference in the mean rectal temperature could be eliminated by training in an identical fixation and handling technique. SIH was both independent of the number of handling days preceding the experiment and of the number of disturbances (0, 1, 2, or 5) implied on the mice per minute. The percentage of hyperthermic mice in 10-mice cages increased when the time interval between the individual measurements increased from 1 to 2, 5 or 10 min. In all groups the maximum increase was reached after an interval of approximately 10 min. SIH of mouse 10 returned time dependently in approximately 60 min to basal temperature. When SIH was tested on 2 or 5 successive days no tolerance developed. When animals were reused after 7 or 14 days SIH did not differ from day 1, implying that animals can be reused. When the number of mice was decreased from 10 to 5 mice per cage, the SIH of vehicle-treated mice was slightly lower in 5-mice cages compared to 10-mice cages. The blocking effects on SIH by anxiolytics was also less clear in 5-mice cages compared to 10-mice cages.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of the dependence potential of the potent high-efficacy 5-HT1A agonist S-14506 in rats

This study assessed the dependence potential of S-14506 [ 1- [ 2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxy napthyl) piperazine], a novel, potent 5-HT(1A) full agonist with anxiolytic and antidepressant actions in animal models. The dependence potential of S-14506 was compared with that of the benzodiazepine (BZ) chlordiazepoxide (CDP). BZ withdrawal caused weight loss, aphagia and hyperthermia after chronic b.i.d. treatment for 21 days. None of these withdrawal effects were seen after similar b.i.d. S-14506 treatment at high doses. However, the acute pharmacological actions of CDP and S-14506 differed on a number of indices. Specifically, CDP increased food intake and body weight, whilst S-14506 decreased these measures, possibly due to the induction of the serotonin syndrome. Of particular interest was the observation that S-14506 induced marked hypothermia, to which complete tolerance developed very rapidly (after only 1 day). The observation of marked, rapid tolerance to S-14506-induced hypothermia, in conjunction with the absence of withdrawal hyperthermia after prolonged chronic treatment at high doses, suggests that tolerance to this effect of S-14506 can be dissociated from dependence. Collectively, the data reported suggest that the full 5-HT(1A) agonist S-14506 is devoid of dependence potential, other human and animal studies having previously suggested that partial 5-HT(1A) agonists typically induce no, or minimal, dependence.