Di-n-propylamino)tetralin Research Articles

Specific binding and characteristics of geissoschizine methyl ether, an indole alkaloid of Uncaria Hook, in the rat brain

Ethnopharmacological relevanceGeissoschizine methyl ether (GM) is an indole alkaloid that is a component of Uncaria Hook, and has been identified as the active component responsible for the anti-aggressive effects of the Uncaria Hook-containing traditional Japanese medicine, yokukansan. Recently, GM was shown to reach the brain by crossing the blood–brain barrier in rats following the oral administration of yokukansan. This finding suggested that there may be specific binding sites for GM in the brain. Here we show evidence that tritium-labeled GM ([3H]GM) binds specifically to several brain areas of rats. Materials and methodsMale rats were used. [3H]GM was synthesized from a demethylated derivative of GM. Specific binding sites of [3H]GM on brain sections were determined by quantitative autoradiography, and maximum binding densities (Bmax) and dissociation constants (Kd) were calculated. Several chemical compounds were used to clarify the molecules that recognize [3H]GM in the completion-binding assay. Emulsion microautoradiography was also performed to identify the cells that bind [3H]GM. ResultsSpecific binding of [3H]GM was observed in the frontal cortex, including the prefrontal cortical region (e.g., prelimbic cortex (PrL)), hippocampus, caudate putamen, amygdala, central medial thalamic nucleus, dorsal raphe nucleus (DR), and cerebellum. Bmax ranged between 0.65 and 8.79pmol/mg tissue, and Kd was between 35.0 and 232.6nM. Specific binding with relatively high affinity (Kd less than 62nM) was dense in the frontal cortical region, moderate in the DR, and sparse in the cerebellum. The specific binding of [3H]GM in the PrL was significantly replaced by the serotonin 1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT), 5-HT2A receptor antagonist ketanserin, 5-HT2B receptor agonist BW 723C86, 5-HT2C receptor agonist RO60-0175, adrenergic α2A receptor antagonist yohimbine, L-type Ca2+ channel blocker verapamil, and μ-opioid receptor antagonist naloxone. Similar results were obtained in the frontal cortex and DR, but not in the cerebellum. Microautoradiography revealed that [3H]GM signals were distributed throughout the frontal cortex, which included neuron-like large cells. ConclusionThese results demonstrate that specific binding sites for GM exist in rat brain tissue, and suggest that the pharmacological actions of GM are mainly associated with 5-HT receptors in the frontal cortex and DR. These results provide an insight into the neuropharmacology of GM and GM-containing herbal medicines.

The broad-spectrum antiemetic effects ETI-385 result from stimulation of 5-HT1A and 5-HT1D receptors.

In the present study, we describe how a nonstoichiometric ratio of the isomers of 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT) produce a broad-spectrum of antiemetic effects in cats and shrews. Determination of the receptor profile of the isomers and testing them separately in cats revealed superior antiemetic effects but severe defensive behavior with the R isomer compared with the S isomer. Differing ratios yielded the best results with the 1:8 (R-S) ratio producing a drug more potent than DPAT and with negligible defensive behavior side effects. Studies with selective 5-HT1D ligands led to the conclusion that this site contributes antiemetic efficacy but is not related to defensive behavior, which is most likely a consequence of 5-HT7 receptor activation. ETI-385 was effective in preventing emetic responses to provocative motion, drugs acting at the chemical trigger zone and cisplatin in both cats and shrews. The results support a clinical trial of this drug for antiemetic effects.

Pronociceptive effect of 5-HT1A receptor agonist on visceral pain involves spinal N-methyl-d-aspartate (NMDA) receptor

The functional role of serotonergic 5-HT1A receptors in the modulation of visceral pain is controversial. The objective of this study was to systematically examine the mechanism and site of action of a selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (DPAT) on visceral pain. In the behavioral model of visceral pain, systemic injection (5–250μg/kg) of DPAT produced a significant increase in the viscero-motor response (VMR) to colorectal distension (CRD) and this effect was blocked by the selective 5-HT1A receptor antagonist WAY-100135 (5mg/kg, s.c.). Similarly, intrathecal (i.t.) injection (5μmol) of DPAT into the lumbo-sacral (L6–S1) spinal cord produced a significant increase in VMR. The administration of N-methyl d-aspartate (NMDA) receptor antagonist AP5 (50μg/kg) prior to DPAT injection completely blocked the pronociceptive effect of DPAT. Similarly, DPAT failed to increase VMR in rats chronically treated with NR1 subunit-targeted antisense oligonucleotide (ON), whereas the drug increased VMR in rats treated with mismatched-ON. Chronic i.t. injection of allylglycine (AG), a γ-amino decarboxylase (GAD) enzyme inhibitor, produced significant increase in VMRs, suggesting that the inhibition of GABA synthesis produces pronociception. In AG-treated rats, i.t. injection of DPAT failed to further increase in VMR, suggesting that the DPAT action is linked to GABA release. Similarly, WAY-100135 failed to attenuate VMR in AG-treated rats, suggesting that unlike DPAT, AG action is not via the activation of 5-HT1A receptors. In electrophysiology experiments, DPAT (50μg/kg) significantly increased the responses of spinal neurons to CRD, but did not influence the mechanotransduction property of CRD-sensitive pelvic nerve afferent fibers. The effect of DPAT on spinal neurons remained unaffected when tested in spinal-transected (C1–C2) rats. These results indicate that the 5-HT1A receptor agonist DPAT produces pronociceptive effects, primarily via the activation of presynaptic 5-HT1A receptors in GABAergic neuron to restrict GABA release and thereby disinhibits the excitatory glutamatergic neurons in the spinal cord.

Pre‐Botzinger Complex (PBC) Plays An Important Role in the Hyperventilatory Response to Intravenous Injection of 5HT1AR Agonist in Anesthetized Rats

Systemic administration of 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (DPAT), a 5HT1AR agonist, causes a hyperventilation via elevating fR in animals. It also prevents peripheral and central mu‐opioid receptor‐induced apnea (Zhuang et al, EB 2010; Sahibzada et al, JPET 2000). Because the PBC containing 5HT1ARs is critical in respiratory rhythm, we examined whether local 5HT1ARs are involved in the evoked hyperventilation. In anesthetized rats, the cardiorespiratory responses to systematic DPAT challenge (100 μg/kg, iv) were performed 1 hr before and 5 min after bilateral microinjection of WAY‐100635, a selective 5HT1AR antagonist (20 mM, 10 nl), into the PBC via a ventral approach. Our results showed that: 1) DPAT doubled VE primarily via elevating fR associated with a hypotension (−34%) and little change in HR, which recovered ~60 min later; 2) microinjection of WAY‐100635 into the PBC did not alter baseline cardiorespiratory variables, but strikingly attenuated (50% reduction) the DPAT‐induced hyperventilation and shortened the recovery period to ~30 min with the hypotension unchanged; and 3) the same microinjection made 1 mm rostral to the PBC or vehicle microinjected into the PBC failed to affect this evoked hyperventilation. Our data suggest that systemic DPAT challenge causes the hyperventilation largely through acting on PBC 5HT1ARs (Supported by HL 107462 and ALA RG‐191095‐N).

Medullary serotonergic neurones modulate the ventilatory response to hypercapnia, but not hypoxia in conscious rats

Serotonergic neurones in the mammalian medullary raphe region (MRR) have been implicated in central chemoreception and the modulation of the ventilatory response to hypercapnia, and may also be involved in the ventilatory response to hypoxia. In this study, we ask whether ventilatory responses across arousal states are affected when the 5-hydroxytryptamine 1A receptor (5-HT1A) agonist (R)-(+)-8-hydroxy-2(di-n-propylamino)tetralin (DPAT) is microdialysed into the MRR of the unanaesthetized adult rat. Microdialysis of 1, 10 and 30 mM DPAT into the MRR significantly decreased absolute ventilation values(VE) during 7% CO2 breathing by 21%, 19% and 30%, respectively, in wakefulness compared to artificial cerebrospinal fluid (aCSF) microdialysis, due to decreases in tidal volume (VT) and not in frequency (f), similar to what occurred during non-rapid eye movement (NREM) sleep. The concentration-dependence of the hypercapnic ventilatory effect might be due to differences in tissue distribution of DPAT. DPAT (30 mM) changed room air breathing pattern by increasing f and decreasing VT. As evidenced by a sham control group, repeated experimentation and microdialysis of aCSF alone had no effect on the ventilatory response to 7% CO2 during wakefulness or sleep. Unlike during hypercapnia, microdialysis of 30 mM DPAT into the MRR did not change the ventilatory response to 10% O2. Additionally, 10 and 30 mM DPAT MRR microdialysis decreased body temperature, and 30 mM DPAT increased the percentage of experimental time in wakefulness. We conclude that serotonergic activity in the MRR plays a role in the ventilatory response to hypercapnia, but not to hypoxia, and that MRR 5-HT1A receptors are also involved in thermoregulation and arousal.

Characterization of a novel effect of serotonin 5-HT 1A and 5-HT 2A receptors: increasing cGMP levels in rat frontal cortex

Elucidating the mechanisms of action of hallucinogens has become an increasingly important area of research as their abuse has grown in recent years. Although serotonin receptors appear to play a role in the behavioral effects of the phenethylamine and indoleamine hallucinogens, the signaling pathways activated by these agents are unclear. Here it is shown that administration of serotonin (5-hydroxytryptamine, 5-HT) increased cyclic guanosine monophosphate (cGMP) production in frontal cortical slices of rat brain. The effect of 5-HT was greater than that of N-methyl-D-aspartate (NMDA), a stimulant of cGMP formation in the central nervous system. The 5-HT 2A/2C receptor phenethylamine agonist, 2,5-dimethoxy-4-methylamphetamine (DOM), increased cGMP content in the slices. Additionally 8-hydroxy-2-(di- n-propylamino)tetralin (DPAT), a 5- HT1A/7 receptor agonist also increased cGMP production. Stimulation of cGMP formation by DOM was prevented by a 5-HT 2A/2C receptor antagonist, pirenperone, as well as by a 5-HT 2A receptor selective antagonist, MDL100907. A 5-HT 2C receptor antagonist, SB242084, did not block the effect of DOM. Stimulation of cGMP production by DPAT was blocked by the 5-HT 1A receptor antagonist, WAY100635. Stimulation of cGMP formation by serotonin could be prevented by pirenperone orWAY100635. In summary, activation of serotonin 5-HT 1A and 5-HT 2A receptors increase brain cGMP levels.

Serotonin phase-shifts the mouse suprachiasmatic circadian clock in vitro

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) receives multiple afferent signals that could potentially modulate its phase. One input, the serotonin (5-HT) projection from the raphe nuclei, has been extensively investigated in rats and hamsters, yet its role(s) in modulating circadian clock phase remains controversial. To expand our investigation of 5-HT modulation of the SCN clock, we investigated the phase-shifting effects of 5-HT and its agonist, (+)8-hydroxy-2-(di- n-propylamino)tetralin (DPAT), when applied to mouse SCN brain slices. 5-HT induced 2–3 h phase advances when applied during subjective day, while non-significant phase shifts were seen after 5-HT application at other times. These phase shifts were completely blocked by the 5-HT antagonist, metergoline. DPAT also induced phase shifts when applied during mid-subjective day, and this effect appeared dose-dependent. Together, these results demonstrate that the mouse SCN, like that of the rat, is directly sensitive to in vitro phase-resetting by 5-HT.

Effects of restraint stress and serotonin on macronutrient selection: a rat model of stress-induced anorexia.

The present study investigated the effects of brief (20 min), acute (2 hr) and chronic restraint stress (2 hr/day for five days) at the time of dark onset on macronutrient selection in female Wistar rats. The role of 5-HT1A receptors in nutrient intake was also examined in a dose-response study (100-700 microg/kg body weight) of 8-hydroxy-2-(di-n-propylamino) tetralin (DPAT) in an acute restraint model. A total of 200 rats aged 6-12 months were tested. In all cases, the restraint stress took the form of confinement within tight-fitting cylinder tubes. In the brief and acute paradigms, the intake of pure carbohydrate (CHO), protein (PRO), and fat was measured for 40 min after a single exposure to stress; in the chronic model, nutrient intake was assessed for 40 min only after the final restraint stress session on day 5. In the DPAT paradigm, intraperitoneal injections of the 5-HT1A agonist were given prior to acute restraint and a 40 min session of nutrient self-selection. Statistical analysis using the t-test for independent samples revealed that neither PRO nor total intake was significantly altered by restraint in the brief, acute or chronic restraint experiments, whereas CHO consumption was suppressed by acute (p=0.02) and chronic restraint (p=0.021), and fat intake was suppressed by brief (p=0.010), acute (p=0.002) and chronic restraint (p=0.001). In the DPAT paradigm, acute restraint stress suppressed CHO (p=0.0001), fat (p=0.000001) and total intake (p=0.003). These effects were not reversed by DPAT, and the administration of 300 pg/kg actually further reduced fat intake. In conclusion, fat intake is more sensitive to the suppressive effects of restraint stress than CHO intake, whereas protein intake is unaffected. Furthermore, 5-HT1A receptors do not seem to play a role in the effects of acute restraint on CHO and fat intake.

Acute Stress-Induced Facilitation of the Hypothalamic-Pituitary-Adrenal Axis: Evidence for the Roles of Stressor Duration and Serotonin

Stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis stimulates the release of both facilitatory and inhibitory components. We proposed that the transient removal of the inhibitory component, corticosterone, during a stressor would leave the HPA axis in a state of hyper-responsiveness (facilitated state). Consistent with this expectation, we have previously observed that aminoglutethimide (AG)-induced removal of corticosterone during an immobilization stressor resulted in the hypersecretion of both ACTH and corticosterone to a subsequent stressor. In the present study we determined the effect of stressor duration on the magnitude of facilitation. AG plus a 10-min immobilization (IMM₁₀) stress on day 1 resulted in facilitation of the HPA axis. This was reflected in higher ACTH and corticosterone responses to an injection stress on day 2 as compared to appropriate control rats. AG plus a 60-min immobilization (IMM₆₀) stress on day 1 resulted in significantly greater facilitation as compared to the AG+IMM₁₀ pretreatment. It is apparent that facilitation of the HPA axis is dependent on the duration of stress. Stress can alter plasma corticosterone-binding globulin levels and AG administration can cause accumulation of the corticosterone biosynthetic precursor, adrenal cholesterol. In order to rule out these peripheral reasons for the hypersecretion of ACTH and corticosterone in our paradigm, we measured the plasma free fraction of corticosterone and adrenal mitochondrial cholesterol levels on day 2 after different pretreatments on day 1. AG+IMM₆₀ pretreatment caused a significant increase in the plasma free fraction of corticosterone. Hypersecretion of ACTH and corticosterone in this group, despite an enhanced feedback signal, suggests central loci for the origin of facilitation. Also, AG treatment on day 1 did not result in accumulation of free or esterified adrenal cholesterol levels on day 2, and therefore cannot account for the hypersecretion of corticosterone. In our final study we attempted to determine if serotonin released during the first stressor is partially responsible for stress-induced facilitation of the HPA axis. We administered 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT), a 5HT_1A agonist, either alone or in conjunction with stress and examined the effects of these pretreatments on the magnitude of facilitation. Interestingly, DPAT administered in lieu of stress produced facilitation similar in magnitude to that produced by IMM₁₀. DPAT adminsitered in conjunction with IMM₁₀ augmented stress-induced facilitation. Our results suggest that stress-induced facilitation of the HPA axis is associated with the release of serotonin during stress.

Dose-dependent effects of buspirone on behavior and cerebral glucose metabolism in rats

In this study we compared the effects of the anxiolytic buspirone on behavior and regional cerebral metabolic rates for glucose (rCMRglc) with those of the reference serotonin (5-HT) 1A agonist 8-hydroxy-2(di- N-propylamino)tetralin (DPAT). Behavioral effects were assessed by scoring the 5-HT syndrome. rCMRglc was measured in 56 brain regions by using the quantitative autoradiographic [ 14C]2-deoxyglucose technique, at 10 min after i.p. injection of DPAT (1 mg/kg) or buspirone (0.4, 4 and 40 mg/kg) in awake male Fischer-344 rats. Whereas DPAT produced an intense 5-HT syndrome, buspirone had no behavioral effect. A low dose (0.4 mg/kg) of buspirone reduced rCMRglc in 18 brain areas (32%), more markedly in limbic areas and raphe nuclei. These were the only rCMRglc effects buspirone had in common with the potent 5-HT 1A agonist DPAT and suggest that low dose buspirone activates preferentially 5-HT 1A receptors. Hence, this receptor subtype may mediate buspirone functional effects on the limbic system and, given the role of these brain areas in mood control, possibly buspirone therapeutic actions. High doses (4 and 40 mg/kg) of buspirone produced widespread rCMRglc decreases in 46 (82%) and 44 (79%) of the areas studied and increased rCMRglc in one brain area, the lateral habenula, that was not affected by DPAT or a low dose of buspirone. The topographic distribution and direction of rCMRglc changes by high doses of buspirone differ from those produced by the 5-HT 1A agonist DPAT. Instead these changes resemble the rCMRglc effects of dopaminergic D 2 antagonists like haloperidol and are consistent with some pharmacological and binding properties of buspirone. In summary, this study suggests that buspirone produces dual, dose-dependent rCMRglc effects: (i) at a low dose rCMRglc reductions in limbic areas and raphe nuclei, probably due to preferential activation of 5-HT 1A receptors, and (ii) at higher doses widespread rCMRglc reductions along with a rCMRglc increase in the lateral habenula resulting from dopamine D 2 receptor blockade.

Antagonism of the stimulus effects of yohimbine and 8-hydroxydipropylaminotetralin

Stimulus control was established in rats using either 8-hydroxy-2-[di- n-propylamino]tetralin (DPAT) (0.2 mg/kg) or yohimbine (3 mg/kg). Tests were then conducted with purported antagonist at 5-hydroxytryptamine 1A (5-HT 1A) receptors. Drugs studied were NAN-190, [+/−]-pindolol, and [−]-alprenolol. In addition, each drug was characterized in terms of its affinity for 5- HT 1 A and α 2-adrenoceptors by means of radioligand binding techniques. None of the antagonists tested provided complete blockade of the stimulus effects of either DPAT or yohimbine. However, [+/−]-pindolol produced a statiscally significant intermediate degree of antagonism of both DPAT and yohimbine. The affinities of DPAT, yohimbine, and NAN-190 for the 5-HT 1A and α 2-adrenergic receptors, respectively, were sufficiently high to lead to some ambiguity of interpretation of the behavioral data. However, the results with [+/−]-pindolol, which has high affinity for the 5-HT 1A receptor (34 nM) and negligible affinity for the α 2-adrenoceptor (24,600 nM), indicate that a significant component of yohimbine-induced stimulus control is mediated by the 5-HT 1A receptor.

Chronic treatment with meta-chlorophenylpiperazine (m-CPP) alters behavioral and cerebral metabolic responses to the serotonin agonists m-CPP and quipazine but not 8-hydroxy-2(di-N-propylamino)tetralin.

The effects of the serotonin (5-HT) agonists meta-chlorophenylpiperazine (m-CPP), quipazine and 8-hydroxy-2(di-n-propylamino)tetralin (DPAT) on behavior and on regional cerebral metabolic rates for glucose (rCMRglc) were measured in control rats or in rats pretreated for 2 weeks with continuous infusion of saline or m-CPP (2.5 mg/kg/day, subcutaneously). rCMRglc was measured in 71 brain regions, using the quantitative autoradiographic [14C]2-deoxy-D-glucose technique, at 15 min after acute administration of m-CPP 2.5 mg/kg, 60 min after quipazine 20 mg/kg, or 10 min after DPAT 1 mg/kg. Behavioral effects were assessed for m-CPP with an activity monitor, for quipazine by counting head shakes and for DPAT by scoring the serotonin syndrome. Chronic m-CPP pretreatment produced tolerance to hypolocomotion induced by acute m-CPP and to head shakes caused by acute quipazine, but did not alter the serotonin syndrome produced by DPAT. m-CPP 2.5 mg/kg IP produced widespread rCMRglc reductions in control rats but failed to modify rCMRglc in any region after chronic m-CPP pretreatment. Quipazine increased rCMRglc in 4 regions in control rats, but reduced rCMRglc in 14 brain areas of chronically m-CPP-pretreated animals. DPAT altered rCMRglc to the same degree in control (25 regions affected) and in chronically m-CPP-pretreated rats (28 regions affected). Reduced behavioral and metabolic effects of acute m-CPP in chronically m-CPP-pretreated rats were not due to pharmacokinetic alterations. These results demonstrate that chronic administration of m-CPP produces behavioral and metabolic tolerance to acute administration of m-CPP, but not of DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of 5-hydroxytryptamine 1A receptors increases the affinity of galanin receptors in di- and telencephalic areas of the rat

Since galanin in vitro selectively increases the K D value of 5-HT 1A receptors without altering the binding of 5-HT 1B or 5-HT 2 receptors, we have studied whether 5-HT 1A receptor activation in turn may affect galanin binding in the ventral di- and telencephalon and the substantia nigra of the rat. As analyzed by autoradiography, the binding of 125I-galanin was increased by about 55% in the presence of 3–30 nM of 8-OH-2-( di-npropylamino)-tetralin (DPAT) in the paraventricular thalamic nucleus, the nucleus reuniens and rhomboideus, the zona incerta, the medial and the lateral hypothalamus, and the medial and the lateral amygdaloid area, but not in the pars compacta of the substantia nigra, which lacks 5-HT 1A binding sites. DPAT (10 nM) reduced the IC 50 values of galanin at 125I-galanin binding sites by approximately 55% within all the analyzed di- and telencephalic regions. The overall increase in B O values was 50 ± 11%. Using the filter wipe technique in cryostat sections at Bregma -2.8 mm covering all the brain regions at this level, DPAT (10 nM) decreased the IC 50 values of galanin from 21.6 ± 1.1nM (control) to 15.5 ± 0.9nM, and increased the B O values by 19.4 ± 4.1%. In membrane preparations from the ventral di- and telencephalon, DPAT decreased the IC 50 values of galanin binding sites by 20 ± 3% at 100 nM of DPAT. This effect could be completely blocked by the specific 5-HT 1A receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-pthalimido)butyl]piperazine. GTP (0.1 nM) produced a 17 ± 5% increase in the IC 50 value of galanin and a 23 ± 4% decrease in the B O value of 125I_galanin binding sites. However, DPAT (100 nM) was still able to decrease the IC 50 values of galanin in the presence of GTP ( -8 ± 3%;control-10 ± 3%). The B max value of 125I-galanin binding was not affected by DPAT. The increased affinity of galanin binding sites by DPAT seems to reflect a G-protein-independent intramembrane receptor-receptor interaction between 5-HT 1A and galanin receptors. This interaction may represent an intramembrane inhibitory feed-back mechanism of 5-HT 1A receptor sensitivity, and may be important both under normal conditions and in 5-HT-mediated mental disorders.

Conditioned reinforcing effects of 8-hydroxy-2-(di- N-propylamino) tetralin: Involvement of 5-hydroxytryptamine 1 A and D1 dopamine receptors

A conditioned place preference paradigm was used to examine the motivational effects of the putative 5-hydroxytryptamine 1A (5-HT 1 A ) receptor agonist 8-hydroxy-2-(di- n-propyl amino)tetralin (DPAT). Rats exhibited marked preferences for an environment paired with the s.c. injection of DPAT (0.1–0.25 mg/kg): vehicle injections were without effect. Pretreatment with the 5-HT 1A/D 2 dopamine (DA) receptor antagonist spiperone or the D 1 DA receptor antagonist SCH-23390 abolished the DPAT-induced preferences. The D 2 antagonist sulpiride was without effect. These data demonstrate that DPAT is an appetitive reinforcer and that this effect results from an interaction with 5-HT 1 A and D 1 DA receptors. They further suggest that 5HT 1 A agonists may have potential for abuse.

Receptor subtypes mediating facilitation by serotonin of excitability of spinal motoneurons

Serotonin receptor ligands, with differential affinity for subtypes of serotonin (5-HT) receptors, were administered intravenously or iontophoretically to urethane-anesthetized rats and the effects of these compounds on glutamate-evoked firing of spinal motoneurons were tested. The excitability of spinal motoneurons was markedly enhanced after intravenous administration of the selective 5-HT1 A ligand 8-hydroxy-2-(di- n-propylamino) tetralin (DPAT) in rats with acute spinal transections at C 1. However, local application of DPAT, directly into the ventral horn by microiontophoresis, inhibited the glutamate-evoked firing of motoneurons in direct contrast to the facilitatory effects of iontophoretically applied 5-HT. The DPAT-induced inhibition may have been nonspecific, since it was not antagonized by methysergide. Other 5-HT agonists, with relatively selective affinity for 5-HT 1B, 5-HT 1C and 5-HT 2 receptors, increased the excitability of spinal motoneurons when applied iontophoretically or intravenously. The excitatory effect of iontophoretically applied 5-HT was antagonized by the nonselective 5-HT antagonist, methysergide and by ketanserin and ritanserin, which have relatively selective affinity for 5-HT 1C and 5-HT 2 receptors. These results indicate that 5-HT 1A receptors do not mediate facilitation of excitability of motoneurons produced by local application of 5-HT directly into the vicinity of the motoneurons. However, the marked increase in firing of motoneurons that was caused by intravenous administration of DPAT in spinal transected rats, suggests that 5-HT 1A receptors in the spinal cord may participate in 5-HT-induced enhancement of somatomotor outflow, at sites presynaptic to the moto-neurons. The iontophoretic results suggest that 5-HT 1B, 5-HT 1C and 5-HT 2 receptors may all play a role in facilitation of the excitability of spinal motoneurons by locally applied 5-HT. Differentiation between these subtypes of receptor awaits the development of more completely selective agonists and antagonists.

Spiroxatrine augments fluoxetine-induced reduction of ethanol intake by the P line of rats

The present study was undertaken to determine if spiroxatrine, a reported 5-HT 1A antagonist, could block the attenuating effects of fluoxetine (a 5-HT uptake inhibitor) on voluntary ethanol intake by the selectively bred alcohol-preferring P line of rats. Fluoxetine (10 mg/kg, IP) significantly reduced the intake of 10% ethanol by P rats approximately 50% during the 4-hour period of alcohol availability. Spiroxatrine (4 mg/kg, IP) was without effect on ethanol intake when given alone. However, when given 5 minutes before fluoxetine (10 mg/kg, IP), this dose of spiroxatrine augmented the reduction of ethanol intake to approximately 15% of control values after 4 hours. Similar experiments conducted with 1 mg/kg (IP) 8-hydroxy-2(di-N-propylamino) tetralin (DPAT) demonstrated that this 5-HT 1A agonist also enhanced the attenuating effects of fluoxetine on ethanol intake. Likewise, spiroxatrine augmented the DPAT reduction of alcohol intake. Spiroxatrine enhanced the effect of DPAT and fluoxetine on food intake as it did on ethanol intake. The results suggest that spiroxatrine behaved as a partial agonist and/or modulator and not as an antagonist at 5-HT 1A receptors under the present experimental conditions.

A component analysis of the effects of DPAT on male rat sexual behavior

Male rat sexual behavior was examined in a variety of tests following a single acute IP injection of the serotonin1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT). The objective was to determine the effects of 5-HT1A receptor stimulation on the components of sexual behavior (arousal/motivation, erection and ejaculation) using this prototypical 5-HT1A ligand. In the ex copula genital reflex test, DPAT dramatically inhibited ejaculation and the display of penile erections. When examined in the mating behavior test, DPAT caused a significant reduction in intromission frequency (IF), ejaculation latency (EL), intercopulatory intervals (ICI) and postejaculatory interval (PEI). The decrease in IF and EL indicated a lowering of the behavioral-ejaculatory threshold, while the reduction in ICI and PEI indicated a stimulation of two aspects of sexual arousal. Further tests, however, revealed complexities. Using the mounting test (with genital anesthesia) as an assessment of sexual arousal, no effect of DPAT was found. Collection and examination of the coagulated ejaculates resulting from mating (copulatory plugs) provided evidence of an impairment in ejaculation, as a result of DPAT treatment. The data suggests that 5-HT1A receptor stimulation lowers the behavioral-ejaculatory threshold despite inhibition of ejaculation in and ex copula; as well as stimulating specific aspects of sexual arousal. It remains to be determined whether the effects on arousal were either due to nonselectivity as currently believed or because 5-HT1A receptor stimulation affects alpha 2-adrenoceptor activity by some as yet undefined mechanism; and as a result modulates sexual arousal.

Isapirone is a partial agonist at 5-hydroxytryptamine 1A (5-HT 1A) receptors in the rat hippocampus: electrophysiological evidence

Using iontophoretic techniques we observed that in vivo isapirone (TVX Q 7821), a selective ligand for the 5-HT 1A binding site, at low ejection currents (5–30 nA) antagonised 5-HT and 8-hydroxy-2-(di-n-propylamino) tetralin (DPAT)-induced suppression of hippocampal unit activity, with little effect on baseline firing rate itself. At higher ejection currents (20–100 nA) or following prolonged application, isapirone inhibited unit firing. Responses to GABA were unaffected by isapirone. These data demonstrate that isapirone is a 5-HT 1A receptor antagonist with partial agonist properties on 5-HT sensitive neurones in the rat hippocampus.

Selective interaction of novel anxiolytics with 5-hydroxytryptamine 1A receptors

Radioligand binding studies were used to analyze the interactions of two novel anxiolytics, buspirone and TVX Q 7821, with a series of 10 neuronal membrane receptor sites. Buspirone ( IC 50 = 24 n m ) and TVX Q 7821 ( IC 50 = 9.5 n m ) display the highest affinity for 5-hydroxytryptamine 1A (5-HT 1A) binding sites labeled by 3H-8-hydroxy-2-(di- n-propylamino) tetralin (DPAT). By contrast, buspirone is 16-fold weaker at dopamine (D 2) receptors ( IC 50 = 380 n m ), whereas TVX Q 7821 is 6-fold less potent at alpha-adrenergic 1 sites ( IC 50 = 58 n m ). At the other receptors studied, buspirone and TVX Q 7821 had similar pharmacological profiles. Both agents display moderate affinity for histamine (H 1), alpha-adrenergic 2, and 5-HT 2 binding sites. The drugs are essentially inactive at 5-HT 1B, calcium channel antagonist, muscarinic cholinergic, and benzodiazepine receptors. These results suggest that the anxiolytic effects of buspirone and TVX Q 7821 may be mediated by central 5-HT 1A receptors.

Effect of 8-hydroxy-2-(di-n-propylamino) tetralin on rat prolactin secretion.

8-OH- DPAT (8-hydroxy-2-[di-n-propylamino] tetralin) is a novel aminotetralin derivative which has been proposed to be a serotonin (5-HT) agonist devoid of dopamine agonist effects. We now report that the administration of 8-OH- DPAT , like known 5-HT agonists, produced a rapid elevation of serum prolactin concentrations in male rats. The prolactin response to 8-OH- DPAT , like that induced by other 5-HT agonists, was greatly potentiated in animals pretreated with the tryptophan hydroxylase inhibitor, para-chlorophenylalanine. However, the 8-OH- DPAT -induced elevation of serum prolactin concentrations in untreated rats was not dose-dependent and was modest in magnitude compared to that produced by known 5-HT agonists. In contrast to the stimulatory effects of 8-OH- DPAT on prolactin secretion in vivo 8-OH- DPAT suppressed the secretion of prolactin from anterior pituitary tissue in vitro, and this effect was blocked by haloperidol. The results of the present study are supportive of the view that 8-OH- DPAT has dopamine agonist, as well as 5-HT agonist, properties.