Sublingual Dexmedetomidine for the Treatment of Acute Agitation in Adults With Schizophrenia or Schizoaffective Disorder: A Randomized Placebo-Controlled Trial

Objective: Determine if sublingual dexmedetomidine, a selective α₂ adrenergic receptor agonist, reduces symptoms of acute agitation associated with schizophrenia or schizoaffective disorder. Methods: This phase 3, randomized, double-blind, placebo-controlled study was conducted in adults diagnosed with schizophrenia or schizoaffective disorder per the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria. The study was conducted at 15 US sites between January 23, 2020, and May 8, 2020. Participants were randomized to sublingual dexmedetomidine 180 μg, 120 μg, or matching placebo. The primary efficacy endpoint was mean change from baseline in the Positive and Negative Syndrome Scale-Excited Component (PEC) total score at 2 hours postdose. Results: Altogether, 380 participants (mean age 45.6 years, 63.4% identifying as male, 77.9% identifying as Black or African American) were randomized; 380 (100%) self-administered study medication, and 372 (97.9%) completed the study. The mean PEC total score at baseline (17.6) indicated mild to moderate agitation. At 2 hours postdose, the least squares mean changes (SE) from baseline were -10.3 (0.4) for sublingual dexmedetomidine 180 μg, -8.5 (0.4) for 120 μg, and -4.8 (0.4) for placebo. Least squares mean differences (97.5% confidence intervals) in the sublingual dexmedetomidine groups were -5.5 (-6.7 to -4.3) for 180 μg and -3.7 (-4.9 to -2.5) for 120 μg (both P < .001 vs placebo). The most commonly encountered adverse events with dexmedetomidine (incidence ≥ 5% and ≥ 2× rate observed with placebo) were somnolence, dry mouth, and hypotension for the 120 μg dose, and somnolence, dizziness, orthostatic hypotension, and oral hypoesthesia for the 180 μg dose. Conclusions: Treatment with sublingual dexmedetomidine 180 μg or 120 μg was more efficacious than placebo in reducing acute agitation associated with schizophrenia as measured by PEC scores at 2 hours postdose. Trial Registration: ClinicalTrials.gov identifier: NCT04268303.

Effect of Sublingual Dexmedetomidine vs Placebo on Acute Agitation Associated With Bipolar Disorder: A Randomized Clinical Trial

IMPORTANCE

Acute agitation is common in patients with bipolar disorder and requires urgent management to relieve distress and to prevent escalation to aggressive behavior.

OBJECTIVE

To evaluate the effect of orally absorbed, sublingual dexmedetomidine, a selective α2A-adrenergic receptor agonist on symptoms of acute agitation in patients with bipolar disorder.

DESIGN, SETTING, AND PARTICIPANTS

Phase 3, randomized, double-blind, placebo-controlled trial conducted in 15 sites in the US with enrollment between February 24, 2020, and April 27, 2020, and final follow-up on May 21, 2020. A total of 380 adults with bipolar I or II disorder were randomized and 362 completed the study.

INTERVENTIONS

Participants were randomized to 3 groups: sublingual dexmedetomidine 180 μg (n = 127), sublingual dexmedetomidine 120 μg (n = 127), or placebo (n = 126).

MAIN OUTCOMES AND MEASURES

The primary efficacy end point was the mean change from baseline at 2 hours for the Positive and Negative Syndrome Scale-Excited Component (PEC) total score. The range of possible total scores is 5 (absence of agitation) to 35 (extremely severe). The secondary end point was the earliest time of a statistically significant change in PEC total score from baseline for the drug vs placebo. On the primary efficacy end point, to account for multiplicity associated with comparing 2 sublingual dexmedetomidine doses with placebo, the 2-sided significance level for each dose vs placebo was set at .025.

RESULTS

Of 380 patients randomized (mean age, 45.6 years; 54.8% women; and 56.1% Black individuals), 378 (99.5%) self-administered the study medication and completed the study. Baseline agitation was mild to moderate, with an overall mean PEC total score of 18.0. Two hours after taking the medication, the mean changes from baseline in PEC total score were -10.4 for sublingual dexmedetomidine 180 μg, -9.0 for sublingual dexmedetomidine 120 μg, and -4.9 for placebo. Least-square mean differences from placebo in the sublingual dexmedetomidine groups at 2 hours were -5.4 (97.5% CI, -6.6 to -4.2) for 180 μg and -4.1 (97.5% CI, -5.3 to -2.9) for 120 μg (both doses P < .001 vs placebo). Treatment effects began 20 minutes after taking the medication among patients in the sublingual dexmedetomidine groups (least-square mean difference for 180 μg, -1.1 [97.5% CI, -2.0 to -0.2]; P = .007; for 120 μg, -1.0 [97.5% CI, -1.9 to -0.1]; P = .009). Adverse events occurred in 35.7% of patients taking 180 μg of dexmedetomidine, 34.9% taking 120 μg, and 17.5% taking placebo. The most common adverse events (≥5%) in the respective 180 μg, 120 μg, and placebo groups were somnolence (21.4% and 20.6% vs 4.8%); dry mouth (4.8% and 7.1% vs 0.8%); hypotension (6.3% and 4.8% vs 0%); and dizziness (5.6% and 5.6% vs 0.8%).

CONCLUSIONS AND RELEVANCE

Among patients with mild to moderate agitation associated with bipolar disorder, treatment with a sublingual film formulation of dexmedetomidine 120 μg or 180 μg, compared with placebo, resulted in significantly greater reduction in the agitation score at 2 hours. Further research is needed to understand the spectrum of patients for whom this treatment would be effective and feasible and to better understand the clinical importance of the observed effect size.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT04276883.

6-Hydroxybuspirone Is a Major Active Metabolite of Buspirone: Assessment of Pharmacokinetics and 5-Hydroxytryptamine1AReceptor Occupancy in Rats

The pharmacokinetics and in vivo potency of 6-hydroxybuspirone (6-OH-buspirone), a major metabolite of buspirone, were investigated. The plasma clearance (47.3 +/- 3.5 ml/min/kg), volume of distribution (2.6 +/- 0.3 l/kg), and half-life (1.2 +/- 0.2 h) of 6-OH-buspirone in rats were similar to those for buspirone. Bioavailability was higher for 6-OH-buspirone (19%) compared with that for buspirone (1.4%). After intravenous infusions to steady-state levels in plasma, 6-OH-buspirone and buspirone increased 5-hydroxytryptamine (HT)(1A) receptor occupancy in a concentration-dependent manner with EC(50) values of 1.0 +/- 0.3 and 0.38 +/- 0.06 microM in the dorsal raphe and 4.0 +/- 0.6 and 1.5 +/- 0.3 microM in the hippocampus, respectively. Both compounds appeared to be approximately 4-fold more potent in occupying presynaptic 5-HT(1A) receptors in the dorsal raphe than the postsynaptic receptors in the hippocampus. Oral dosing of buspirone in rats resulted in exposures (area under the concentration-time profile) of 6-OH-buspirone and 1-(2-pyrimidinyl)-piperazine (1-PP), another major metabolite of buspirone, that were approximately 12 (6-OH-buspirone)- and 49 (1-PP)-fold higher than the exposure of the parent compound. As a whole, these preclinical data suggest that 6-OH-buspirone probably contributes to the clinical efficacy of buspirone as an anxiolytic agent.

Interaction of the novel antipsychotic aripiprazole with 5-HT1A and 5-HT 2A receptors: functional receptor-binding and in vivo electrophysiological studies

BACKGROUND

Aripiprazole (7-{4-[4-(2,3-dichlorophenyl)-1-piperazinyl]butoxy}-3,4-dihydro-2(1H)-quinolinone) is a novel antipsychotic with a mechanism of action that differs from current typical and atypical antipsychotics. Aripiprazole interacts with a range of receptors, including serotonin [5-hydroxytryptamine (5-HT)] and dopamine receptors.

MATERIALS AND METHODS

This study examined aripiprazole's interactions with 5-HT systems in vitro and in vivo to further clarify its pharmacologic properties.

RESULTS

Aripiprazole produced increases in [(35)S]GTPgammaS binding to rat hippocampal membranes. Its potency (pEC(50) = 7.2) was similar to that of ziprasidone (7.1) and greater than that of 5-HT (6.7) and buspirone (6.4), a 5-HT(1A)-receptor partial agonist, whereas its intrinsic activity was similar to that of ziprasidone and buspirone. The stimulatory effect of aripiprazole was blocked by WAY-100635, a 5-HT(1A)-receptor antagonist. In in vivo electrophysiology studies, aripiprazole produced a dose-related reduction in the firing rate of 5-HT-containing dorsal raphe neurons in rats, which was both prevented and reversed by WAY-100635 administration. Aripiprazole showed a high affinity for human 5-HT(1A) receptors (K (i) = 4.2 nM) using parietal cortex membrane preparations. In membranes from cells expressing human recombinant receptors, aripiprazole bound with high affinity to 5-HT(2A) receptors (K (i) = 3.4 nM), moderate affinity to 5-HT(2C) (K (i) = 15 nM) and 5-HT(7) (K (i) = 39 nM) receptors, and low affinity to 5-HT(6) receptors (K (i) = 214 nM) and 5-HT transporter (K (i) = 98 nM). In addition, aripiprazole potently blocked 5-HT(2A)-receptor-mediated increases in intracellular Ca(2+) levels in a rat pituitary cell line (IC(50) = 11 nM).

DISCUSSION

These results support a partial agonist activity for aripiprazole at 5-HT(1A) receptors in vitro and in vivo, and suggest important interactions with other 5-HT-receptor subtypes. This receptor activity profile may contribute to the antipsychotic activity of aripiprazole in humans.

The novel antipsychotic aripiprazole is a partial agonist at short and long isoforms of D2 receptors linked to the regulation of adenylyl cyclase activity and prolactin release

Aripiprazole is a novel antipsychotic with a unique mechanism of action, which differs from currently marketed typical and atypical antipsychotics. Aripiprazole has been shown to be a partial agonist at the D(2) family of dopamine (DA) receptors in biochemical and pharmacological studies. To demonstrate aripiprazole's action as a partial D(2) agonist in pituitary cells at the molecular level, we retrovirally transduced the short (D(2S)) and the long (D(2L)) form of the human DA D(2) receptor gene into a rat pituitary cell line, GH4C1. [(3)H]-raclopride saturation binding analyses revealed a B(max) value approximately four-fold higher at D(2S) receptor-expressing GH4C1 cells than at D(2L) receptor-expressing GH4C1 cells, while a K(d) value was similar. Aripiprazole inhibited forskolin-stimulated release of prolactin in both D(2S) and D(2L) receptor-expressing GH4C1 cells, whereas the maximal inhibition of prolactin release was less than that of DA. Similarly, aripiprazole partially inhibited forskolin-induced cAMP accumulation in both D(2) receptor-expressing cells. Aripiprazole antagonized the suppression attained by DA (10(-7) M) in both D(2) receptor-expressing cells and, at the maximal blockade of cAMP, yielded residual cAMP levels equal to those produced by aripiprazole alone. These results indicate that aripiprazole acts as a partial agonist at both D(2S) and D(2L) receptors expressed in GH4C1 cells. These data may explain, at least in part, the observations that aripiprazole shows a novel antipsychotic activity with minimal potential for adverse events including no significant increase of serum prolactin levels in clinical studies.

Indanyl piperazines as melatonergic MT2 selective agents

Optimization of a benzyl piperazine pharmacophore produced N-acyl-4-indanyl-piperazines that bind with high affinity to melatonergic MT(2) receptors. (R)-4-(2,3-dihydro-6-methoxy-1H-inden-1-yl)-N-ethyl-1-piperazine-carboxamide fumarate (13) is a water soluble, selective MT(2) agonist, which produces advances in circadian phase in rats at doses of 1-56 mg/kg that are no different from those of melatonin at 1 mg/kg. Unlike melatonin, 13 produced only weak contractile effects in rat tail artery.

Development of a presynaptic 5-HT1A antagonist

A new 5-HT(1A) silent antagonist 14 (5-HT(1A) IC(50)=2.2 nM) antagonizes the effects of agonists on reciprocal forepaw treading behavior, on neuronal firing in the rat dorsal raphé, and on 5-HT(1A) release in the raphé and hippocampus. While 14 alone was inactive in the social interaction paradigm, it completely reversed the social interaction activity of the serotonergic compounds (buspirone, 1, and 2).

Aripiprazole, a novel antipsychotic, is a high-affinity partial agonist at human dopamine D2 receptors

Aripiprazole is the first next-generation atypical antipsychotic with a mechanism of action that differs from currently marketed typical and atypical antipsychotics. Aripiprazole displays properties of an agonist and antagonist in animal models of dopaminergic hypoactivity and hyperactivity, respectively. This study examined the interactions of aripiprazole with a single population of human D2 receptors to clarify further its pharmacologic properties. In membranes prepared from Chinese hamster ovary cells that express recombinant D2L receptors, aripiprazole bound with high affinity to both the G protein-coupled and uncoupled states of receptors. Aripiprazole potently activated D2 receptor-mediated inhibition of cAMP accumulation. Partial receptor inactivation using the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) significantly reduced the maximum effect of aripiprazole on inhibition of cAMP accumulation. This effect was seen with concentrations of EEDQ that did not alter the maximal inhibitory effect of dopamine. Consistent with the expected effects of a partial agonist, increasing concentrations of aripiprazole blocked the action of dopamine with maximal blockade equal to the agonist effect of aripiprazole alone. The efficacy of aripiprazole relative to that of dopamine varied from 25% in cells that lacked spare receptors for dopamine to 90% in cells with receptor reserve. These results, together with previous studies demonstrating partial agonist activity at serotonin 5-hydroxytryptamine (5-HT)1A receptors and antagonist activity at 5-HT2A receptors, support the identification of aripiprazole as a dopamine-serotonin system stabilizer. The receptor activity profile may underlie the unique activity of aripiprazole in animals and its antipsychotic activity in humans.

Synthesis and biological characterization of .alpha.-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinebutanol and analogs as potential atypical antipsychotic agents

A series of 1-(pyrimidin-2-yl)piperazine derivatives were prepared and evaluated in receptor binding assays and in in vivo behavioral paradigms as potential atypical antipsychotic agents. Compound 16 (BMS 181100 (formerly BMY 14802)) emerged as the lead compound from within the series on the basis of its good activity and duration of action in the inhibition of both conditioned avoidance responding and apomorphine-induced stereotopy in the rat. Compound 16 not only failed to induce catalepsy in the rat but was quite effective in reversing the cataleptic effect of neuroleptic agents, thus indicating a low propensity for causing extrapyramidal side effects. In comparison to reference antipsychotic agents, 16 appeared to be less sedating and was relatively weaker in causing muscle incoordination. The compound was essentially inactive in binding to dopamine D2 receptors and its chronic administration to rats did not result in dopamine receptor supersensitivity. It exhibited modest to weak affinity for 5-HT1A and alpha 1 receptors but was found to be a fairly potent ligand for sigma binding sites (IC50 vs (+)-[3H]-3-PPP = 112 nM). Although the resolved enantiomers of racemic 16 did not show dramatic differences from racemate or from each other in most tests, the R(+) enantiomer was up to 11-fold more potent than its antipode in binding to sigma sites. Several studies have indicated that 16 may be a limbic-selective agent which may modulate dopaminergic activity by an indirect mechanism. The compound has been selected for clinical evaluation in the treatment of psychosis.

Differential effects of coadministration of fluoxetine and WAY-100635 on serotonergic neurotransmission in vivo: sensitivity to sequence of injections

Serotonin 5-HT(1A) receptor antagonists potentiate the effects of serotonin reuptake inhibitors on extracellular serotonin levels in a variety of brain regions. These effects are quite variable, however, with reports indicating potentiations of anywhere from 100-1900%. One factor that might impact the magnitude of such potentiations is the timing of administration of the two agents; reports in which the reuptake inhibitor is given prior to the serotonin receptor antagonist consistently report larger potentiations than reports in which the antagonist is given first. To test this relationship directly, microdialysis and electrophysiology studies were performed to assess the magnitude of increase in extracellular serotonin and changes in cellular activity produced by the serotonin reuptake inhibitor fluoxetine and the 5-HT(1A) receptor antagonist WAY-100635 under various dosing regimens. In microdialysis studies, when WAY-100635 (0.5 mg/kg s.c.) was administered 80 min after fluoxetine (10 mg/kg i.p.) the increase in serotonin was more than twice that observed when the compounds were coadministered. In electrophysiology studies in vivo, WAY-100635 reversed the depression of cell firing produced by fluoxetine when administered 30 min after fluoxetine, but when the two compounds were coadministered, a depression in firing rate was observed comparable to that produced by fluoxetine alone. In contrast, slice recording studies showed that WAY-100635 blocked the effects of fluoxetine regardless of the order of administration. These results indicate that fluoxetine and WAY-100635 can interact in a fashion not predicted by the currently accepted model. It is likely that neuronal circuitry outside of the raphe nuclei underlies this relationship.

BMS-181885, a 5-HT1B/1D receptor ligand, in experimental models predictive of antimigraine activity and coronary side-effect potential

Many acutely acting antimigraine drugs have the ability to constrict porcine arteriovenous anastomoses as well as the human isolated coronary artery. These two experimental models seem to serve as indicators, respectively, for the therapeutic and coronary side-effect potential of the compounds. Using these two models, we have investigated the effects of BMS-181885 (3-[3-[4-(5-methoxy-4-pyrimidyl)-1-piperazinyl]propyl]-5-(1,2-dioxo-4-me thyl-3-cyclobuten-3-yl)amino-1H-indole), a 5-HT1B/1D receptor ligand. In anaesthetised pigs, BMS-181885 (10, 30, 100 and 300 microg kg(-1)) decreased the total carotid blood flow and conduction, exclusively at the expense of the arteriovenous anastomotic fraction as the capillary fraction did in fact increase. The highest dose (300 microg kg(-1)) produced a reduction of 52+/-6% from the baseline arteriovenous anastomotic flow. When carotid haemodynamic changes after a single 100 microg kg(-1)dose of BMS-181885 or sumatriptan were studied at different time-points, BMS-188185 had a longer duration of action. Both BMS-181885 (pD2:7.9+/-0.1; Emax:9+/-3% of the contraction to 100 mM K+) and sumatriptan (pD2:6.3+/-0.1; Emax:28+/-8% of the contraction to 100 mM K+) contracted the human isolated coronary artery. The above results suggest that (i) the longer-lasting vasoconstrictor action of BMS-181885 on porcine carotid arteriovenous anastomoses may be related to its reported slow dissociation from 5-HT1B/1D receptor, and (ii) BMS-181885 should be able to abort migraine headaches in patients. It will be interesting to find out whether these properties are clinically important so that the drug exhibits less headache recurrence and coronary side-effects than sumatriptan.

Phase shifting of circadian rhythms and depression of neuronal activity in the rat suprachiasmatic nucleus by neuropeptide Y: mediation by different receptor subtypes

Neuropeptide Y (NPY) has been implicated in the phase shifting of circadian rhythms in the hypothalamic suprachiasmatic nucleus (SCN). Using long-term, multiple-neuron recordings, we examined the direct effects and phase-shifting properties of NPY application in rat SCN slices in vitro (n = 453). Application of NPY and peptide YY to SCN slices at circadian time (CT) 7.5-8.5 produced concentration-dependent, reversible inhibition of cell firing and a subsequent significant phase advance. Several lines of evidence indicated that these two effects of NPY were mediated by different receptors. NPY-induced inhibition and phase shifting had different concentration-response relationships and very different phase-response relationships. NPY-induced phase advances, but not inhibition, were blocked by the GABAA antagonist bicuculline, suggesting that NPY-mediated modulation of GABA may be an underlying mechanism whereby NPY phase shifts the circadian clock. Application of the Y2 receptor agonists NPY 13-36 and (Cys2,8-aminooctanoic acid5,24,D-Cys27)-NPY advanced the peak of the circadian rhythm but did not inhibit cell firing. The Y1 and Y5 agonist [Leu31,Pro34]-NPY evoked a substantial inhibition of discharge but did not generate a phase shift. NPY-induced inhibition was not blocked by the specific Y1 antagonist BIBP-3226; the antagonist also had no effect on the timing of the peak of the circadian rhythm. Application of the Y5 agonist [D-Trp32]-NPY produced only direct neuronal inhibition. These are the first data to indicate that at least two functional populations of NPY receptors exist in the SCN, distinguishable on the basis of pharmacology, each mediating a different physiological response to NPY application.

Effects of avitriptan, a new 5-HT 1B/1D receptor agonist, in experimental models predictive of antimigraine activity and coronary side-effect potential

Several acutely acting antimigraine drugs, including ergotamine and sumatriptan, have the ability to constrict porcine arteriovenous anastomoses as well as the human isolated coronary artery. These two experimental models seem to serve as indicators, respectively, for the therapeutic and coronary side-effect potential of the compounds. Using these two models, we have now investigated the effects of avitriptan (BMS-180048; 3-[3-[4-(5-methoxy-4-pyrimidinyl)-l-piperazinyl[propyl]-N-methyl-l H-indole-5-methane-sulfonamide monofumarate), a new 5-HT 1B/1D receptor agonist. In anaesthetized pigs, avitriptan (10, 30, 100 and 300 micrograms.kg-1) decreased the total carotid blood flow by exclusively decreasing arteriovenous anastomotic blood flow; capillary blood flow was increased. The mean +/- SEM i.v. dose of avitriptan eliciting a 50% decrease (ED50) in the porcine carotid arteriovenous anastomotic blood flow was calculated to be 76 +/- 23 micrograms.kg-1 (132 +/- 40 nmol.kg-1) and the highest dose (300 micrograms.kg-1) produced a 72 +/- 4% reduction. In recent comparative experiments (DeVries et al. 1996), the mean +/- SEM ED50 (i.v.) of sumatriptan in decreasing carotid arteriovenous anastomotic blood flow was 63 +/- 17 micrograms.kg-1 (158 +/- 43 nmol.kg-1), with a reduction of 76 +/- 4% by 300 micrograms.kg-1, i.v. Both avitriptan (pD2; 7.39 +/- 0.09; Emax: 13.0 +/- 4.5% of the contraction to 100 mM K+) and sumatriptan (pD2: 6.33 +/- 0.09; Emax: 15.5 +/- 2.3% of the contraction to 100 mM K+) contracted the human isolated coronary artery. The above results suggest that avitriptan should be able to abort migraine headaches in patients, but may exhibit sumatriptan-like effects on coronary arteries. Initial clinical studies have demonstrated the therapeutic action of the drug in acute migraine.

5-HT1A receptor agonist effects of BMY-14802 on serotonin release in dorsal raphe and hippocampus

BMY-14802 (BMS-181100; alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine butanol monohydrochloride) is a sigma receptor antagonist with potential antipsychotic activity. BMY-14802 also binds to 5-HT1A receptors and is able to inhibit the firing of dorsal raphe serotonergic neurons, suggesting that this compound has 5-HT1A receptor agonist properties in vivo. In the present study, we used in vivo microdialysis to study the effects of BMY-14802 on extracellular serotonin (5-hydroxytryptamine), 5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the dorsal raphe and ventral hippocampus in the awake rat. Systemic injections of 5-20 mg/kg BMY-14802 induced a simultaneous dose-dependent decrease in 5-HT and markedly increased the dopamine metabolite, HVA concentrations in dialysates from dorsal raphe and hippocampus. Extracellular concentrations of the 5-HT metabolite, 5-HIAA decreased only after 20 mg/kg BMY-14802. The 5-HT decreases in dorsal raphe and hippocampus produced by BMY-14802 were completely antagonized by pretreatment with 1.0 mg/kg of the specific 5-HT1A antagonist, WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). These data indicate that BMY-14802 decreases dorsal raphe and hippocampal release of 5-HT by interaction with somatodendritic 5-HT1A receptors in the raphe nuclei and suggest that this compound is a potential anxiolytic.

Lack of 5-hydroxytryptamine1A-mediated inhibition of adenylyl cyclase in dorsal raphe of male and female rats

In the rat hippocampus, 5-hydroxytryptamine (5-HT)1A receptors couple to two independent effector mechanisms, the inhibition of adenylyl cyclase activity and the opening of a K+ channel. In the dorsal raphe, 5-HT1A receptors also open K+ channels; however, coupling to adenylyl cyclase has not been demonstrated. In this study, the selective 5-HT1A agonists (+/-)- 8-hydroxy-2-(di-n-propylamino)tetralin, (R+)-8-hydroxy-2-(di-n-propylamino)tetralin and dipropyl-5-carboxamidotryptamine, did not inhibit forskolin-stimulated adenylyl cyclase (FSAC) activity in raphe region homogenates, although these drugs were efficacious in hippocampal homogenates. Other 5-HT1A agonists, NAN-190, BMY-7378, buspirone and gepirone, were also ineffective in raphe region homogenates. Estrogen-treatment of ovariectomized female rats, which is known to enhance 5-HT1A-mediated inhibition of FSAC in the hippocampus, did not promote the action of 5-HT1A agonists. Nor did activation of 5-HT1A receptors stimulate basal adenylyl cyclase activity in raphe homogenates as it does in the hippocampus. FSAC activity was inhibited in raphe region homogenates by activation of adenosine A1 or gamma-aminobutyric acidB receptors or by direct activation of the inhibitor G-protein, Gi, with guanyl-5'-6'-imidodiphosphate, indicating that the raphe homogenates have the biochemical machinery for inhibition of FSAC. High affinity binding studies showed that, in raphe homogenates, 5-HT1A receptors were expressed at a density comparable to that of adenosine A1 receptors and that they were coupled to G-proteins. It should be noted that our failure to observe 5-HT1A-mediated inhibition of adenylyl cyclase in the raphe does not prove that such coupling does not exist. However, a lack of 5-HT1A receptor coupling to adenylyl cyclase in the raphe would support contentions that coupling of the 5-HT1A receptor to adenylyl cyclase may be independent of its coupling to the K+ channel and that there may be distinct differences between pre- and postsynaptic 5-HT1A receptor systems.

Serotonin (5-HT) release in the dorsal raphé and ventral hippocampus: raphé control of somatodendritic and terminal 5-HT release

Somatodendritic and terminal release of serotonin (5-HT) was investigated by simultaneously measuring extracellular concentrations of 5-HT, 5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the dorsal raphé and ventral hippocampus in freely moving rats. Perfusion of tetrodotoxin (TTX, 1 microM and 10 microM) into the dorsal raphé simultaneously decreased dorsal raphé and hippocampal 5-HT release. However, following TTX perfusion into the hippocampus (10 microM), hippocampal 5-HT release was profoundly reduced but dorsal raphé 5-HT remained unchanged. Systemic injections with 5-HT1A agonist, buspirone (1.0-5.0 mg/kg, i.p.) decreased 5-HT and 5-HIAA and increased HVA concentrations in the dorsal raphé and in the hippocampus. The decreases in the raphé and hippocampal 5-HT induced by systemic buspirone were antagonized in rats pretreated with 1.0 mM (-) pindolol, locally perfused into the dorsal raphé. Local dorsal raphé perfusion of (-) pindolol alone (0.01-1.0 mM) increased dorsal raphé 5-HT and concomitantly induced a small increase in hippocampal 5-HT. Buspirone perfusion into the dorsal raphé did not change (10 nM, 100 nM), or produced a small increase (1.0 mM) in raphé 5-HT, without changing hippocampal 5-HT. These data provide evidence that 5-HT release in the dorsal raphé is dependent on the opening of fast activated sodium channels and that dorsal raphé 5-HT1A receptors control somatodendritic and hippocampal 5-HT release

Pharmacology and neurochemistry of nefazodone, a novel antidepressant drug

Nefazodone is a new antidepressant drug with a pharmacologic profile distinct from that of the tricyclic, monoamine oxidase inhibitor, and serotonin selective reuptake inhibitor antidepressants. Nefazodone was initially discovered for its ability to block 5-HT2A receptors and its reduced potency as an alpha 1-adrenergic blocker. It was later shown to inhibit both serotonin and norepinephrine uptake in vitro, attributes which most likely impart its clinical efficacy and which differentiate nefazodone from its chemical predecessor trazodone. The combination of these two mechanisms may ultimately result in a facilitation of 5-HT1A-mediated neurotransmission, which may be beneficial for treating symptoms of depression as evidenced by recent clinical findings. In addition, the preclinical profile of nefazodone demonstrates that it has decreased anticholinergic and antihistaminic activity relative to traditional agents. Clinical findings to date are consistent with these observations.

Differential sensitivity of 3H-agonist binding to pre- and postsynaptic 5-HT1A receptors in bovine brain

1. The full and weak partial 5-HT1A agonist ligands [3H]-8-OH-DPAT and [3H]-BMY-7378 were used to characterize the binding parameters of pre- and postsynaptic 5-HT1A binding sites in bovine dorsal raphe and hippocampal membranes, respectively. The Kd and Bmax values for the individual radioligands were indistinguisable across the regions tested, as were the Ki values generated by a series of agents acting at 5-hydroxytryptamine (5-HT) receptors. 2. The concentration-dependent allosteric attenuation of [3H]-8-OH-DPAT and [3H]-BMY-7378 binding produced by the nonhydrolyzable guanyl nucleotide, Gpp(NH)p, generated similar IC50 values within a particular region; however, these were significantly different between regions. While the maximal attenuation of [3H]-8-OH-DPAT and [3H]-BMY-7378 binding was similar in dorsal raphe, Gpp(NH)p produced a significantly greater attenuation of [3H]-8-OH-DPAT binding in hippocampal membranes when compared to [3H]-BMY-7378. The maximal attenuation of [3H]-8-OH-DPAT binding by Gpp(NHp) in hippocampus was also significantly greater than that seen with either radioligand in dorsal raphe. 3. Although exposure to Gpp(NH)p had no effect on the affinity constants of either radioligand in either region, it produced a concentration-dependent reduction in the maximal number of binding sites for both radioligands in both regions. While the percentage reduction in Bmax values were similar for both radioligands in the dorsal raphe, Gpp(NH)p reduced the Bmax of [3H]-8-OH-DPAT in hippocampus significantly more than that of [3H]-BMY-7378. 4. These results suggest that while pre- and postsynaptic 5-HT1A receptors may share similar pharmacological recognition properties, a region-dependent difference in the coupling of the 5-HT1A receptor to G-proteins may exist.

Characterization of human 5-HT1 receptors expressed in Sf9 insect cells

Four human 5-HT receptor subtypes (5-HT1A, 5-HT1D alpha, 5-HT1D beta and 5-HT1E) have been expressed in Sf9 insect cells. All four human 5-hydroxytryptamine receptors produced by Sf9 cells had the expected pharmacological properties. Surprisingly, levels of expression of these receptors were relatively low (1-5 pmol/mg protein). High affinity agonist binding to the four 5-hydroxytryptamine receptors was reduced to different extents by guanine nucleotides and/or NaCl. This suggests that the nature of receptor-G protein coupling and/or the predominant conformational state of the receptors in Sf9 cell membranes varies among the different receptors. Activation of all four receptors inhibited forskolin-stimulated cAMP formation in intact Sf9 cells. Expression of 5-hydroxytryptamine receptors in Sf9 cells should be useful for purification of these receptors, for studies of post-translational modification and for pharmaceutical screening.

Cloning and characterization of the rat 5-HT5B receptor. Evidence that the 5-HT5B receptor couples to a G protein in mammalian cell membranes

A gene encoding a novel G protein-coupled 5-hydroxytryptamine (5-HT) receptor, termed 5-HT5B, was cloned. The ligand binding profile of this receptor is distinct from that of other cloned 5-HT receptors. The 5-HT5B receptor couples to a G protein in COS1 cell membranes; however, activation of the 5-HT5B receptor does not appear to alter either cAMP accumulation or phosphoinositide turnover in a variety of fibroblast cell lines. In the rat brain, 5-HT5B gene expression occurs predominantly in the medial habenulae and hippocampal CA1 cells of the adult. Little expression is seen during embryonic development.

[3H]-5-carboxamidotryptamine labels 5-HT1D binding sites in bovine substantia nigra

1. [3H]-5-hydroxytryptamine (5-HT) has been shown to radiolabel at least five types of 5-HT binding sites in mammalian brain tissue, 5-HT1A, 5-HT1C, 5-HT1D and 5-HT1D and 5-HT1E (Frazer et al., 1990). Selective masking of 5-HT1A and 5-HT1C receptors, has uncovered binding sites which display both high (5-HT1D) and low (5-HT1E) affinity for 5-carboxamidotryptamine (5-CT). By utilizing [3H]-5-CT we have eliminated a portion of the complex binding (5-HT1E) seen when [3H]-5-HT is used as a radioligand. 2. [3H]-5-CT binding to 5-HT1D sites in bovine substantia nigra was rapid, reversible and saturable, displaying high affinity (Kd = 0.38 +/- 0.04 nM) and low non-specific binding (> 90% specific binding). 3. In bovine substantia nigra, [3H]-5-CT labelled an equivalent number of binding sites to [3H]-5-CT (403 +/- 18 and 362 +/- 20 fmol mg-1 protein, respectively) and binding was sensitive to guanine nucleotides. 4. A linear correlation (r2 = 0.99) existed between the potency of compounds to displace [3H]-5-HT and [3H]-5-CT in bovine substantia nigra. 5. Therefore, [3H]-5-CT is a novel radioligand for the examination of 5-HT1-like binding sites, which under proper experimental conditions can be used to radiolabel selectively 5-HT-1D-like binding sites.

Comparative effects of chronic 8-OH-DPAT, gepirone and ipsapirone treatment on the sensitivity of somatodendritic 5-HT1A autoreceptors

Chronic treatment with the full 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), either by twice daily subcutaneous injection (0.2 or 2.0 mg/kg) or continuous subcutaneous administration via osmotic minipumps (0.4 or 4.0 mg/kg/day), for 14 days, had no effect on the dose-response curves for inhibition of 5-hydroxytryptophan (5-HTP) accumulation in rat cortex or hippocampus by 8-OH-DPAT or the partial 5-HT1A agonist BMY 7378. In contrast, chronic treatment with the nonbenzodiazepine putative anxiolytic gepirone via osmotic minipumps (20 mg/kg/day) resulted in a small but significant rightward shift (1.8-fold) in the dose-response curve to 8-OH-DPAT challenge in the cortex and a slightly larger shift (2.4-fold) in the hippocampus. Similarly, chronic treatment with another putative anxiolytic, ipsapirone, administered via twice daily subcutaneous injections (20 mg/kg), also resulted in a rightward shift (2.6-fold) in the dose-response curve to 8-OH-DPAT in the cortex and a slightly smaller shift (2.3-fold) in the hippocampus. Neither drug, however, decreased the maximal response. The present results are consistent with the suggestion that the clinical anxiolytic effects of gepirone and ipsapirone, and not of 8-OH-DPAT, may be related to their ability to desensitize somatodendritic 5-HT1A autoreceptors; other potential mechanisms are discussed.

Allosteric interactions between the binding sites of receptor agonists and guanine nucleotides: a comparative study of the 5-hydroxytryptamine1A and adenosine A1 receptor systems in rat hippocampal membranes

The ternary complex formed between agonist, receptor and guanine nucleotide binding protein and its destabilization by guanine nucleotides (GN) was utilized to study early events in signal transduction, by characterizing the allosteric interactions between agonist and GN binding to the receptor/guanine nucleotide binding protein, G complex for adenosine A1 and 5-hydroxytryptamine1A receptors. The functional interaction between the ternary complex and GTP was examined by assaying adenylyl cyclase activity. Binding of a full adenosine A1 agonist ([3H]-R-(-)-N6-(2-phenylisopropyl)adenosine), and a full [(+-)-[3H]-8-hydroxydipropylaminotetralin] and partial ([3H]-8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8- azaspirol[4.5]-decane-7,9-dione) 5-hydroxytryptamine1A agonist was examined in relation to the binding of GN. The amount of ternary complex formed depended upon receptor type and drug relative efficacy. The ratio between the drug's EC50 value (adenylyl cyclase) and dissociation constant (binding) was also receptor and drug relative efficacy dependent. 5'-Guanylylimidodiphosphate (100 microM) caused an approximately 50% decrease in the Bmax for all drugs without affecting Kd values. 5'-Guanylylimidodiphosphate and guanosine 5'-O-(3-thiotriphosphate) attenuated [3H]-agonist binding in a concentration-dependent and saturable manner, with IC50 values increased 2- to 6-fold with increasing receptor occupancy. IC50 values were approximately one-tenth lower at the 5-hydroxytryptamine1A receptor than adenosine A1 receptor; similar values were obtained for inhibition of (+-)-[3H]-8-hydroxydipropylaminotetralin and [3H]-8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8- azaspirol[4.5]-decane-7,9-dione binding, suggesting an independence of agonist efficacy. We propose that the stabilization of the ternary complex by hormone binding, measured by Bmax values, is related to drug-relative efficacy, thus the amount of ternary complex available for destabilization by GN is greater for the more efficacious agonist. This is translated into greater relative efficacy observed in the maximal inhibition of adenylyl cyclase.

Lack of apparent receptor reserve at postsynaptic 5-hydroxytryptamine1A receptors negatively coupled to adenylyl cyclase activity in rat hippocampal membranes

Previous studies have demonstrated the existence of a large receptor reserve for agonists at somatodendritic 5-hydroxytryptamine1A (5-HT1A) serotonin receptors in the raphe nuclei of the rat. 5-HT1A agonists with anxiolytic properties (e.g., buspirone, gepirone, and ipsapirone) display full intrinsic activity at these receptors but are partial agonists at postsynaptic 5-HT1A receptors, which suggests that the latter sites may be devoid of a receptor reserve. In the present studies, this was directly determined by examining the relationship between receptor occupancy and response at postsynaptic 5-HT1A receptors, in rat hippocampus, mediating the inhibition of forskolin-stimulated adenylyl cyclase activity, using the method of partial irreversible receptor inactivation. Rats were treated with vehicle or the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), and 24 hr later hippocampi were removed for saturation analysis of [3H]8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) binding to 5-HT1A receptors or for adenylyl cyclase assays. EEDQ (1 and 6 mg/kg) dose-dependently reduced the maximal number of [3H]8-OH-DPAT binding sites by 68.5 and 80%, respectively, without altering the Kd. Concentration-response curves were generated for inhibition of forskolin-stimulated adenylyl cyclase activity by 5-HT and the selective 5-HT1A agonist N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT). EEDQ treatment dose-dependently reduced the maximal inhibitory effect of 5-HT [percentage of inhibition: control, 23.6; EEDQ (1 mg/kg), 13.4; EEDQ (6 mg/kg), 8.9], without altering either the slope factor (1.01) or the EC50 (96.4 nM). Analogous results were obtained with DP-5-CT [percentage of maximal inhibition: control, 24.1; EEDQ (1 mg/kg), 15.2; EEDQ (6 mg/kg), 10.7), again without changes in slope factor (0.89) or EC50 (9.9 nM). Analysis of double-reciprocal plots of equieffective concentrations of agonist, followed by calculation of fractional receptor occupancy, revealed a linear relationship between receptor occupancy and response for both 5-HT and DP-5-CT (i.e., an absence of receptor reserve). The receptor specificity of the effect of EEDQ was demonstrated in two ways. First, it was shown that pretreatment of rats with the selective 5-HT1A partial agonist BMY 7378 (10 mg/kg) before EEDQ afforded substantial protection (about 75%) against loss of the inhibitory effect of DP-5-CT on forskolin-stimulated adenylyl cyclase activity. Second, EEDQ did not alter the inhibition of forskolin-stimulated adenylyl cyclase activity induced by the adenosine A1 receptor agonist phenylisopropyladenosine (PIA).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of encainide, ODE, MODE, and flecainide on ADP/5-HT induced platelet aggregation and in the anesthetized dog coronary artery stenosis-occlusion model of intravascular thrombosis

Encainide is a class 1C antiarrhythmic agent that is indicated for the treatment of life-threatening arrhythmias, such as sustained ventricular tachycardia. Furthermore, encainide possesses a moderate degree of antiserotonin activity, which was quantitated in this present study by determining displacement of [3H]spiperone binding from rat cortical 5-HT2 binding sites. The Ki for encainide in this model was 66.1 nM, compared to 2.6 nM for ketanserin. Two encainide metabolites, ODE and MODE, were also active, but were weaker than encainide. Additionally, these agents were found to inhibit platelet aggregation induced in vitro in human platelet-rich plasma by the combination of ADP and serotonin. In view of the fact that serotonin is one of a variety of humoral factors capable of activating blood platelets and has been recently implicated as playing a role in certain thrombotic syndromes, encainide, along with its two principal human metabolites, ODE and MODE, and another class 1C antiarrhythmic, flecainide, were evaluated in an in vivo model of intravascular thrombosis. Intraduodenal doses of 1 mg/kg of either encainide, ODE, or MODE significantly inhibited thrombosis in a canine model of coronary artery stenosis-occlusion.

Effect of chronic administration of antidepressant drugs on 5-HT2-mediated behavior in the rat following noradrenergic or serotonergic denervation

Chronic (14 day) administration of several pharmacologically-distinct antidepressant drugs resulted in marked reductions in the serotonin2 (5-HT2)-mediated quipazine-induced head shake response which were accompanied by significant reductions in the density of cortical beta-adrenergic and 5-HT2 binding sites. Noradrenergic (DSP4[N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine]-induced) and serotonergic (5,7-DHT[5,7-dihydroxytryptamine]-induced) lesions either attenuated or blocked antidepressant-induced reductions in 5-HT2-mediated behavior. DSP4- and 5,7-DHT lesions did not alter the down-regulation of 5-HT2 binding sites produced by imipramine, desipramine, phenelzine or iprindole. To a large extent, the antagonism of antidepressant-induced reductions in 5-HT2-mediated behavior was coincident with the blockade of down-regulation of beta-adrenergic binding sites by both noradrenergic and serotonergic denervation. The functional interrelationship of 5-HT2 and beta-adrenergic receptors suggested by the present findings may provide insight into a common mechanism underlying the action of pharmacologically-distinct antidepressant drugs.

Unique modulation of central 5-HT2 receptor binding sites and 5-HT2 receptor-mediated behavior by continuous gepirone treatment

The effect of continuous treatment with the selective 5-HT1A agonist gepirone upon 5-HT2-mediated behavior and cortical 5-HT2 receptor binding sites was examined in naive rats or rats receiving noradrenergic (DSP4) or serotonergic (5,7-DHT) lesions. Continuous administration of gepirone in non-lesioned rats for 3, 7, or 14 days enhanced the head shake response to the 5-HT agonist quipazine. This enhancement of 5-HT2-mediated behavior occurred despite concomitant down-regulation of cortical 5-HT2 binding sites. However, 28 days of gepirone administration significantly reduced behavioral responsiveness to quipazine. The gepirone-induced facilitation of 5-HT2-mediated behavior observed after 7 days of continuous treatment was blocked in both DSP4 and 5,7-DHT-lesioned rats. However, both noradrenergic and serotonergic denervation failed to modify the down-regulation of 5-HT2 receptor binding sites produced by continuous gepirone administration. These results suggest that the curious dissociation of behavioral and biochemical indices of 5-HT2 receptor function produced by continuous gepirone treatment may be the result of a dual yet separate action of the drug on central presynaptic noradrenergic and serotonergic mechanisms and postsynaptic 5-HT receptors. Furthermore, the postsynaptic action of gepirone which reduces the maximal number of cortical 5-HT2 receptor binding sites may be the result of gepirone's agonist action at postsynaptic 5-HT1A receptors.

8-OH-DPAT and buspirone analogs inhibit the ketanserin-sensitive quipazine-induced head shake response in rats

Behavioral syndromes mediated by serotonergic mechanisms may reflect interactions between distinct effects initiated by specific 5-HT receptors, such as the 5-HT1A and the 5-HT2 receptor. This hypothesis was tested by examining the effect of various 5-HT1A agonists on the 5-HT2 receptor-mediated quipazine-induced head shake response in rats. Subcutaneous administration of 8-OH-DPAT, buspirone, gepirone, and ipsapirone produced a dose-dependent inhibition of the ketanserin-sensitive quipazine-induced head shake response. These effects were produced by doses of agonists which did not induce reciprocal forepaw treading. Furthermore, pretreatment with a partial 5-HT1A agonist (+/-)pindolol blocked the inhibitory effects of 8-OH-DPAT to the level of inhibition produced by (+/-)pindolol itself. These results suggest that stimulation of central 5-HT1A receptors can modulate the expression of a central 5-HT2 receptor-mediated behavior.

Effects of imipramine and serotonin-2 agonists and antagonists on serotonin-2 and beta-adrenergic receptors following noradrenergic or serotonergic denervation

The effects of chronic (14 day) administration of the tricyclic antidepressant imipramine, the serotonin-2 (5-HT2) antagonist ketanserin, and the serotonin agonist quipazine on 5-HT2 receptor binding parameters and 5-HT2-mediated behavior were examined in rats with or without prior serotonergic denervation [via 5,7-dihydroxytryptamine (5,7-DHT)] or noradrenergic denervation [via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)]. Chronic administration of imipramine, ketanserin, or quipazine produced a marked reduction in the number of 5-HT2 binding sites which was accompanied by reductions in the 5-HT2-mediated quipazine-induced head shake response. In animals receiving DSP4 or 5,7-DHT lesions and continuous vehicle treatment, beta-adrenergic receptor binding sites were significantly up-regulated while 5-HT2 receptor binding sites did not change. Imipramine normalized the lesion-induced increases in beta-adrenergic binding observed in DSP4 and 5,7-DHT-lesioned rats but failed to down-regulate beta-adrenergic binding sites below non-lesioned control levels. Chronic imipramine, ketanserin, and quipazine reduced quipazine-induced head shakes and down-regulated 5-HT2 binding sites in rats with noradrenergic denervation. While imipramine, ketanserin, and quipazine all down-regulated 5-HT2 binding sites in animals with serotonergic denervation, only imipramine's ability to reduce quipazine-induced head shakes was attenuated in 5,7-DHT-lesioned rats. The present results suggest that imipramine-induced down-regulation of 5-HT2 receptors may not involve presynaptic 5-HT mechanisms, and imipramine-induced alterations in 5-HT2 sensitivity as reflected in the quipazine-induced head shake may, in part, be influenced by beta-adrenergic receptors.

Noradrenergic denervation alters serotonin2-mediated behavior but not serotonin2 receptor number in rats: modulatory role of beta adrenergic receptors

Recent behavioral evidence suggests that enhancement of noradrenergic neurotransmission may alter the functional sensitivity of serotonin2 (5-HT2) receptors in the central nervous system. The present studies have examined the effects of two types of noradrenergic denervation [neurotoxic: via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment; and pharmacologic: via chronic beta adrenergic receptor blockade] on the 5-HT2-mediated head shake response and cortical beta adrenergic and 5-HT2 receptor number in the rat. No changes in quipazine-induced head shakes were observed 3 days after DSP4 lesion. However, the frequency of head shakes was significantly enhanced 10 days after DSP4 treatment in the presence of a 39% up-regulation of beta adrenergic receptors. Pretreatment with propranolol 10 days after DSP4 lesion selectively antagonized the enhancement of the behavioral response to quipazine without altering base-line response rate, whereas pretreatment with the 5-HT2 antagonist ketanserin totally blocked head shakes in both control and DSP4-treated rats. Pharmacologic denervation achieved by continuous (14 day) administration of the beta adrenergic antagonist propranolol also resulted in a potentiation of the head shake response (274% of control) and an upregulation of beta adrenergic receptors (44%). Conversely, continuous treatment with the beta adrenergic agonist clenbuterol resulted in a marked reduction in head shakes (36% of control) with a concomitant 29% down-regulation of beta adrenergic receptors. 5-HT2 receptor binding was not modified by either DSP4 lesion or continuous administration of beta adrenergic agonists or antagonists. These studies demonstrate that changes in cortical beta adrenergic receptor density may modify 5-HT2-mediated behavior in a manner that is independent of changes in 5-HT2 receptor number.

Reduction in cortical 5HT2 receptor sensitivity after continuous gepirone treatment

Continuous administration (28 days) of gepirone, a potential anxiolytic, decreased rat cortical serotonin type 2 (5HT2) receptor density by 17 percent and reduced the frequency of head shakes induced by the serotonin agonist quipazine. These data support the results of previous studies confirming gepirone's involvement with brain serotonin systems, and suggest that in addition to its preclinical anxiolytic profile, gepirone exhibits pharmacological properties common to clinically effective antidepressants.

Effect of immobilization stress on rat pineal beta-adrenergic receptor-mediated function

The results of this investigation indicate that stress produced by immobilization alters rat pineal function. Chronic stress reduced the density of pineal beta-adrenergic receptors and the activities of the intracellular enzyme serotonin N-acetyltransferase (NAT), its product N-acetylserotonin (NAS), and the pineal hormone melatonin, which was measured during the dark phase of the diurnal lighting cycle. Removal of the adrenal medulla did not prevent the reduction of pineal beta-adrenergic receptor binding sites that is observed after chronic stress. Acute immobilization stress suppressed the dark-induced elevations of pineal NAT activity and NAS levels 10 h after the stress session without altering pineal beta-adrenergic receptor binding. Although the precise mechanisms responsible for these effects are not completely clear, the results indicate that they are related to changes in sympathetic neuronal activity and not mediated by stress-induced elevations in plasma catecholamines.

Antidepressant drugs with varying pharmacological profiles alter rat pineal beta adrenergic-mediated function

The effects of acute and chronic administrations of two antidepressant drugs with differing pharmacological profiles on pineal beta adrenergic receptor-mediated functions were examined in the rat. Animals were treated with control powdered food or with either imipramine or iprindole-containing diets (0.067%, w/w) for various time intervals. Animals were sacrificed during different phases of the light/dark cycle and pineal [3H]dihydroalprenolol (DHA) binding, N-acetyltransferase (NAT) activity, N-acetylserotonin (NAS) and melatonin levels were measured. Plasma drugs and metabolite concentrations were also assessed. A 3-day treatment with imipramine resulted in an unchanged pineal [3H] DHA binding and an increase in pineal serotonin (5-HT), NAS and melatonin. A comparable treatment with iprindole did not alter any pineal measures. Three weeks of imipramine treatment resulted in therapeutic plasma drug and metabolite concentrations and elicited a reduction (31%) in the density of pineal [3H] DHA binding. This treatment, in addition, suppressed the dark-induced activation of the intracellular enzyme NAT (38%) and the concentrations of NAS (25%) and melatonin (23%) without altering pineal 5-HT rhythm. No apparent shift was observed in the NAT, NAS and melatonin rhythms. Chronic treatment with the atypical antidepressant iprindole for 3 weeks resulted in-plasma iprindole concentrations of 76 ng/ml and a significant reduction (24%) in pineal 5-HT levels 5 hr into the dark phase of the light/dark cycle. Pineal NAT activity and NAS and melatonin content were not significantly reduced by this treatment. However, 4 weeks of iprindole ingestion produced plasma drug concentrations of 141 ng/ml and significantly reduced pineal [3H] DHA binding density (18%) without changing binding affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic lithium treatment on rat pineal rhythms: N-acetyltransferase, N-acetylserotonin and melatonin

Chronic lithium treatment in rats suppressed the peak activity of pineal N-acetyltransferase as well as the maximal concentrations of pineal N-acetylserotonin and melatonin which occur during the dark phase of the light/dark cycle. These changes were not related to changes in the precursor indoles tryptophan, 5-hydroxytryptophan and serotonin, which were unaltered by lithium treatment. The change in N-acetyltransferase activity appears in part to be due to a decreased Vmax of the enzyme, as assessed using tryptamine as the substrate. Lithium also suppressed pineal activity during the light phase of the diurnal lighting cycle as indicated by a markedly reduced cyclic AMP response to L-isoproterenol. In addition, lithium treatment elicited a 1- to 3-hr delay in peak pineal N-acetyltransferase activity. These effects may result from a lithium-induced desensitization of pineal beta adrenergic receptors, from a direct effect of the ion on the suprachiasmatic nucleus and/or from an action on a site distal to this hypothalamus nucleus.

The effect of chronic lithium treatment on rat pineal N-acetyltransferase rhythm

Lithium chloride administration to rats for 5 weeks caused a significant decrease in dark-induced activity of rat pineal N-acetyltransferase (E.C.2.3.1.5.). This effect is not observed after 3 days or 3 weeks of treatment. Furthermore, chronic lithium treatment suppressed the amplitude and may have delayed the peak of the diurnal cycle of N-acetyltransferase activity. In vitro, various concentrations of lithium chloride (2, 4 and 10 microM) did not affect N-acetyltransferase activity. Beta adrenergic receptor binding studies with [3H]dihydroalprenolol indicated a decrease in number of pineal beta adrenergic receptors in rats treated chronically with lithium. These results are consistent with a lithium-induced subsensitivity of pineal beta adrenergic receptors and may explain the effect of lithium on pineal-mediated cyclic behaviors.