Evidence for selective serotonergic receptor involvement in p-chloroamphetamine-induced antinociception

Summary Administration of the serotonin (5-HT) releasing compound p-chloroamphetamine (PCA; 2.5 mg/kg) induced potent analgesia in rats tested with the hot plate method. The analgesia was prevented by pretreatment with either of the 5-HT uptake inhibitors alaproclate (20 mg/kg) or fluoxetine (10 mg/kg). Taking into account that the noradrenergic uptake inhibitor desipramine in previous experiments failed to interfere with the effect of PCA, these results demonstrate that PCA selectively acts on 5-HT terminals. The analgesia was attenuated by administration of the 5-HT antagonists methiothepin (0.125–0.5 mg/kg) and danitracen (0.25–2.5 mg/kg) but not by a series of other 5-HT receptor antagonists or antagonists acting on noradrenergic, dopaminergic, GABAergic, histaminergic or muscarinic receptors. It is concluded that the analgesic effect of PCA is mediated via stimulation of a type of 5-HT receptors possibly belonging to the 5-HT-1 class. Further studies are, however, needed in order to firmly establish the relationship to any particular sub-type of 5-HT receptor as characterized in in vitro binding studies.     

Evidence for the involvement of central serotonin in mechanism of domestication of silver foxes

Silver foxes selected for more than 30 years for tame behavior and displaying no defensive reaction to human contact were shown to have a higher serotonin level in midbrain and hypothalamus, and a higher 5-hydroxyindole acetic acid (5-HIAA) content in midbrain, hypothalamus and hippocampus in comparison to nonselected wild silver foxes bred in captivity over the same time span. Tryptophan hydroxylase (TPH) activity in midbrain and hypothalamus in domesticated foxes was increased as compared with their aggressive/defensive counterparts. Monoamine oxidase type A (MAO A) activity was decreased with an increased K_m and unchanged V_max in domesticated foxes. No changes in specific [³H]ketanserin or [³H]8-OH-DPAT binding in frontal cortex was revealed. A reduced density (B_max) of 5HT_1A receptors in hypothalamic membranes in domesticated foxes was shown. It is suggested that the brain serotonergic system is involved in the mechanism of domestication converting wild aggressive/defensive animals into tame ones.     

Evidence for a peripheral effect of serotonin or metabolites in 5-hydroxytryptophan-induced hypothermia

The effects of l-5-hydroxytryptophan (l-5-HTP) on the body temperatures of restrained rats were studied alone and following pretreatment with the central and peripheral decarboxylase inhibitor, benserazide (R04-4602), the peripheral decarboxylase inhibitor, carbidopa (MK-486), or the peripheral serotonergic antagonist, xylamidine tosylate. l-5-Hydroxytryptophan alone (25–200 mg/kg, i.p.) decreased body temperature in a dose-dependent manner. Pretreatment (1 or 4 hr) with benserazide (25–400 mg/kg) produced a dose- and time-dependent blockade of the hypothermic effects of 25 mg/kg and 200 mg/kg of l-5-HTP. Pretreatment with carbidopa (12.5–50 mg/kg) or xylamidine tosylate (1 and 3mg/kg) also produced dose- and time-dependent antagonism of the hypothermic effects of 25 mg/kg of l-5-HTP. Serotonin (5-HT) creatinine sulfate (5–20 mg/kg, i.p.), produced a dose-related hypothermic effect similar to l-5-HTP. Pretreatment with xylamidine tosylate (1 and 3 mg/kg) produced a dose- and time-dependent blockade of the temperature decrease produced by 5 mg/kg of 5-HT. These results suggest that the hypothermic effect produced by systemic l-5-HTP administration in the rat is entirely the result of an effect of serotonin (or metabolites) outside the blood-brain barrier.     

The involvement of brain serotonin in the clonidine-induced hypothermia in rats

The effects of manipulation with brain serotonergic (5HT) activity on clonidine-induced hypothermia in rats were investigated. Lesion to the median raphe nucleus as well as p-chlorophenylalanine pretreatment significantly potentiated the temperature decrease after 0.2 mg/kg of clonidine. Pretreatment with a 5HT releasing agent--fenfluramine, or a 5HT receptor agonist--m-chlorophenylpiperazine (mCPP) antagonized the effect of clonidine. Additionally, both fenfluramine and mCPP given alone produced an elevation in body temperature. These results are discussed in terms of an involvement of the 5HT neuronal system in clonidine hypothermia, and the dissociation of the role of 5HT system in clonidine-induced behavioral and vegetative effects is suggested.     

Utility of selective serotonin reuptake inhibitors in premature ejaculation

The introduction of selective serotonin reuptake inhibitors (SSRIs) has revolutionized our understanding of the treatment of premature ejaculation. Lifelong premature ejaculation may be a neurobiological phenomenon, namely part of a biological variability of the intravaginal ejaculation latency time in men. Animal studies support this view, and an animal model for premature and delayed ejaculation has recently been developed. It is proposed that drug treatment of premature ejaculation should consist of 5-hydroxytryptamine (5-HT)2c receptor stimulation and/or 5-HT1A receptor inhibition. A meta-analysis of 35 daily treatment studies with selective serotonin reuptake inhibitors (SSRIs) and clomipramine demonstrated comparable efficacy of clomipramine with the SSRIs sertraline and fluoxetine in delaying ejaculation, whereas the efficacy of the SSRI paroxetine was greater than all other SSRIs and clomipramine. It is postulated that acute treatment with SSRIs, including those with short half-lives, will not produce an ejaculation delay equivalent to that induced by daily treatment of SSRIs.     

Evidence for the involvement of bradykinin in chemically-evoked cystitis in anaesthetized rats

Summary The effect of Hoe 140, a potent bradykinin B2 receptor antagonist, on the micturition reflex and detrusor hyperreflexia induced by chemical cystitis has been investigated in anaesthetized rats. Hoe 140 (1–100 nmol/kg i. v.) produced a dose-dependent blockade of the contraction of the rat urinary bladder induced by i. v. administration of bradykinin (100 nmol/kg) without affecting the response produced by the selective tachykinin NK-1 receptor agonist, [Sar⁹] substance P (SP) sulfone (1 nmol/kg i. v.). At doses which produce selective and long-lasting blockade of bradykinin receptors in the urinary bladder, Hoe 140 did not modify urodynamic parameters in normal rats.Intravesical instillation of xylene in female rats decreased bladder capacity and increased micturition frequency. These effects also occurred in rats pretreated with capsaicin as adults. Hoe 140 did not modify xylene-induced cystitis. Intraperitoneal administration of cyclophosphamide (150 mg/kg, 48 h before) decreased bladder capacity and increased micturition frequency. These effects of cyclophosphamide were abolished in rats pretreated with capsaicin as adults. Hoe 140 increased bladder capacity and decreased micturition frequency in rats pretreated with cyclophosphamide.Addition of bradykinin (10 µmol/l) to the medium in the superfused rat urinary bladder preparation evoked a prompt increase in the outflow of calcitonin gene-related peptide like immunoreactivity (CGRP-LI). Hoe 140 (3 µmol/l) inhibited (by about 50%) the CGRP-LI outflow stimulated by bradykinin.These findings demonstrate the participation of bradykinin, through 132 receptors, in the genesis of detrusor hyperreflexia during cyclophosphamide-induced cystitis. Capsaicin-sensitive primary afferent neurons are a likely target for Hoe 140 action in this model of experimental cystitis, as exemplified by its ability to prevent CGRP-LI outflow by bradykinin.Correspondence to C. A. Maggi at the above address     

Central serotonin involvement in the elaboration of the startle reaction in rats

The present investigation provides evidence for serotonergic involvement in the inhibition of the acoustic startle reaction which results from the presentation of neutral stimuli (prepulses) shortly before reflex elicitation. While the specific serotonin (5-HT) synthesis inhibitor p-Chlorophenylalanine did not affect the response, a large amine-depleting dose of reserpine enhanced the reaction elicited under stable (control) stimulus conditions (no prepulse delivered). Selective replacement of 5-HT by administration of the l-amino acid precursor and a monoamine oxidase inhibitor further enhanced control startle amplitude and also eliminated prepulse inhibition. Chemical assays indicated especially large increases in 5-HT levels in the cortex and brainstem. Administration of the 5-HT precursor in the non-reserpinized rat both increased control startle levels and also interfered with prepulse inhibition although not as completely as in the reserpinized animal. The results indicate that 5-HT has a facilitatory effect on the reflex and suggest that a catecholaminergic system is also involved in determining reflex amplitude.     

Tryptophan pretreatment augmentation of p-chloroamphetamine-induced serotonin and dopamine release and reduction of long-term neurotoxicity

The impact of tryptophan (TRP) pretreatment on the neurochemical effects of p-chloroamphetamine (PCA) was investigated. The neurotoxic effects of PCA on serotonin (5-HT) neurons, the acute effects of PCA on extracellular 5-HT and dopamine (DA), and the displacement by PCA of whole blood 5-HT were examined. TRP pretreatment (400 mg/kg of the methyl ester) significantly reduced the long-term (1 week) decrease in tissue 5-HT resulting from PCA (2 mg/kg, i.p., of the hydrochloride salt) in the prefrontal cortex and striatum, but not in the dorsal hippocampus. Microdialysis studies in awake animals showed that this pretreatment regimen resulted in augmented PCA-induced increases in extracellular 5-HT (4-fold) and DA (2-fold). TRP pretreatment also resulted in increased displacement of 5-HT from whole blood. The implications of these results toward possible mechanisms of action of PCA-induced neurotoxicity are discussed.     

Evidence for an involvement of D1 and D2 dopamine receptors in mediating nicotine-induced hyperactivity in rats

Previous studies have suggested that repeated exposure of rats to the drug or to the experimental environment is necessary to observe nicotine-induced locomotor stimulation. In the present study the role of habituation to the experimental environment on the stimulant effect of nicotine in rats was examined. In addition, the role of dopamine receptors in mediating nicotine-induced locomotor stimulation was investigated by examining the effects of selective D1 and D2 dopamine receptor antagonists on activity induced by nicotine. Locomotor activity was assessed in male Sprague-Dawley rats tested in photocell cages. Nicotine (1.0 mg/kg) caused a significant increase in locomotor activity in rats that were habituated to the test environment, but had only a weak and delayed stimulant action in rats that were unfamiliar with the test environment. The stimulant action of nicotine was blocked by the central nicotinic antagonist mecamylamine but not by the peripheral nicotinic blocker hexamethonium, indicating that the response is probably mediated by central nicotinic receptors. Nicotine-induced hyperactivity was blocked by the selective D1 antagonist SCH 23390, the selective D2 antagonist raclopride and the D1/D2 antagonist fluphenazine. Pretreatment with the D2 agonist PHNO enhanced nicotine-induced hyperactivity, whereas the D1 agonist SKF 38393 had no effect. The results indicate that acute nicotine injection induces a pronounced hyperactivity in rats habituated to the test environment. The effect appears to be mediated by central nicotine receptors, possibly located on dopaminergic neurons, and also requires the activation of both D1 and D2 dopamine receptors.     

Modulation by peripheral serotonin of the threshold for sexual reflexes in female rats.

Using immunohistochemical techniques, a large number of serotonergic paracrine cells were identified in the urethral mucosa of the female rat. The functional significance of these cells was investigated. A model for the study of sexual climax in the anesthetized, acutely spinalized female rat was used. In this model, distension of the urethra with saline elicits a stereotyped coordinated genital response which closely resembles the neuromuscular concomitants of sexual climax. Addition of serotonin to the urethral perfusate (10(-4)-10(-7) M) caused a dose-dependent decrease in the mechanical threshold necessary to elicit the climax-like response. This effect was blocked by a specific 5-HT3 receptor antagonist. The physiological implications of these findings are discussed.     

Possible involvement of peripheral serotonin 5-HT3 receptors in fluvoxamine-induced emesis in Suncus murinus

Selective serotonin reuptake inhibitors fluvoxamine and fluoxetine, as well as serotonin (5-HT), induced vomiting in Suncus murinus (a house musk shrew). Fluvoxamine- and fluoxetine-induced vomiting gradually decreased with their repeated administration. Vomiting induced by serotonin also decreased with repeated treatment with serotonin. In these shrews, fluvoxamine-induced vomiting was partially inhibited. Fluvoxamine might induce vomiting, at least partially, by indirectly activating peripheral 5-HT(3) receptors, since serotonin has been reported to induce vomiting by activating peripheral 5-HT(3) receptors and granisetron, a 5-HT(3) antagonist, partially suppressed fluvoxamine-induced vomiting in our previous finding. In addition, fluvoxamine-induced vomiting was impaired more effectively using a step-wise dose-up schedule of fluvoxamine than a fixed high-dose schedule. Therefore, a careful dosing strategy starting with a low dose might be effective for avoiding emesis associated with the clinical use of fluvoxamine.     

Evidence for multiple opiate receptor involvement in different phencyclidine-induced unconditioned behaviors in rats

Rats were used for comparing the behavioral response profiles of phencyclidine (PCP) and d,1-N-allylnormetazocine (NANM), two drugs that are proposed to exert their effects through the PCP/sigma receptor. Phencyclidine (1.0–5.0 mg/kg) and NANM (2.5–10.0 mg/kg) induced dose-related increases in locomotion, sniffing, repetitive head movements, non-object directed mouth movements, and ataxia. Both drugs also increased food and water consumption during the latter portion of the drug response. Ingestive behaviors induced by PCP (2.5 mg/kg), as with eating and drinking stimulated by the mu-opiate morphine (2.0 mg/kg), were blocked by a relatively low dose of the opiate antagonist naloxone (0.5 mg/kg). Multiple injections of PCP (2.5 mg/kg for 4 days) or NANM (10.0 mg/kg for 4 days) augmented several measures of behavioral activation, including horizontal locomotion, rearing, and nonfocused sniffing, but did not significantly change stereotyped behaviors or ataxia. Reciprocal cross-sensitization of locomotor activation is indicated by the finding that the response to a challenge injection of PCP (2.5 mg/kg) or to NANM (10.0 mg/kg) after 4 days of treatment with the other drug closely resembled the enhanced locomotor response observed after the chronic treatment. Phencyclidine and NANM thus appear to exert many of their effects on unconditioned behavior through common mechanisms, including interaction with sigma receptors. In addition, these findings are consistent with previous suggestions that a muopiate receptor system may modulate some effects of PCP.     

Quinidine-digoxin interaction: Evidence for involvement of an extrarenal mechanism

Summary The influence of quinidine 750 mg per day for one week on serum digoxin concentration (SDC) was evaluated in digitalized anuric patients on chronic haemodialysis. During quinidine administration the SDC increased markedly, from 0.84±0.37 to 1.58±0.72 ng/ml (p<0.01), a comparable effect to that reported previously in patients with normal renal function. Neither in vitro nor in vivo did quinidine alter the serum protein binding of digoxin. The increase in SDC in anuric patients indicates a decrease in the extrarenal clearance of digoxin, which means that mechanisms other than of renal origin are also involved in the interaction of quinidine and digoxin. There was great interindividual variability in the extent of the quinidine-induced rise in SDC. Regardless of the state of renal function, careful monitoring of digitalized patients seems mandatory once quinidine treatment is initiated.     

The involvement of nigral serotonin innervation in the control of punishment-induced behavioral inhibition in rats

In rats, serotonergic innervation of the substantia nigra plays a role in the control of experimentally-elicited anxiety: punishment-induced inhibition is lessened following bilateral intra-nigral infusion of 5,7-dihydroxytryptamine (2 μg; 0.5 μl). A significant correlation (0.62) is found between the loss of nigral, but not hippocampal, tryptophan hydroxylase activity and the release of behavior in two situations of shock-induced suppression of responding. Likewise, infusion of this neurotoxin (1 μg; 0.4 μl) into the nucleus raphé dorsalis causes an attenuation of punishment-induced suppression. These findings suggest an involvement of serotonergic raphé-nigral neurons in experimentally-elicited anxiety.     

Evidence for noradrenergic involvement in naloxone-induced stimulation of luteinizing hormone release in prepubertal female rats

The putatively selective dopamine autoreceptor agonists TL-99 and 3-PPP were compared with apomorphine for the production of contraversive turning in the 6-hydroxydopamine-lesioned rat. Although less potent than apomorphine, 3-PPP produced dose-related contralateral turning. The contralateral turning produced by TL-99 plateaued at the 3 mg/kg i.p. dose level. Yohimbine significantly enhanced the TL-99-induced turning, whereas it failed to modify the 3-PPP turning. The results suggest that the alpha 2-adrenergic properties of TL-99 at doses of greater than 3.0 mg/kg masked its dopaminergic effects. Hence, 3-PPP is clearly the more selective agent for DA receptors.     

Evidence for an antagonistic action of tabernanthine on hypoxia-induced changes in brain serotonin levels

Summary The effects of tabernanthine on serotonin (5-HT) levels were determined in several brain areas of rats exposed to various simulated altitudes (1800, 5200, 7000 m). The 5-HT synthesis inhibitor, para-chlorophenylalanine (PCPA), was used to dissociate the effects occurring at synthesis and release levels. Tabernanthine antagonized the decrease in hypothalamic 5-HT levels induced by a 7000 m hypoxia and also suppressed the decrease in PCPA-induced depletion observed at 5200 and 7000 m in the hypothalamus, the striatum and the rest of the brain. It was assumed that tabernanthine stimulates different steps of 5-HT metabolism. These effects, revealed by hypoxia, are related to other peripheral and central properties of this drug.     

Evidence for a peripheral component in the sympatholytic effect of clonidine in rats.

In an attempt to assess separately the peripheral and central effects of clonidine on cardiovascular parameters and plasma catecholamine levels, the selective alpha 2-adrenoceptor antagonist idazoxan (RX 781094) was given either intravenously (i.v.) or intracerebroventricularly (i.c.v.) to anaesthetized rats before administration of intravenous clonidine. Plasma noradrenaline and plasma growth hormone concentrations were used as indices of peripheral sympathetic nervous activity and central alpha-adrenoceptor stimulation, respectively. Peripheral and central administration of idazoxan antagonized the cardiovascular responses to i.v. clonidine, 5 micrograms kg-1. However, idazoxan was more effective against the hypotension than the bradycardia induced by clonidine. Idazoxan 300 micrograms kg-1 i.v. and 50 micrograms i.c.v. prevented clonidine-induced falls in plasma noradrenaline and adrenaline. The results suggest that 50 micrograms idazoxan i.c.v. caused some blockade of peripheral as well as central alpha 2-adrenoceptors. Idazoxan, 10 micrograms i.c.v., caused similar inhibition of the hypotensive response to clonidine as 300 micrograms kg-1 i.v. and 50 micrograms i.c.v. but did not significantly inhibit the clonidine-induced fall in plasma noradrenaline concentration. Animals pretreated with i.v. or i.c.v. idazoxan had significantly lower levels of plasma growth hormone than vehicle-treated rats. Idazoxan 10 micrograms and 50 micrograms i.c.v. suppressed growth hormone secretion to the same extent. These results suggest that stimulation of peripheral, prejunctional alpha 2-adrenoceptors in anaesthetized rats may contribute to the fall in plasma catecholamines produced by i.v. clonidine, and confirm that the hypotensive effect is centrally mediated.     

Adrenergic-induced hyperglycemia in anaesthetized rats: involvement of peripheral alpha 2-adrenoceptors

The selective alpha 2-adrenoceptor agonist, UK 14.304, induces a time- and dose-dependent hyperglycemic response in the anaesthetized rat. This hyperglycemia seems to be mediated through the pancreas because lesions of the pancreatic beta-cells induced with streptozotocin completely abolished this hyperglycemic response to UK 14.304 while ganglionic blockade by chlorisondamine did not affect the response. The non-selective agonist, adrenaline, induces a similar hyperglycemic response which is antagonized selectivity by the alpha 2-adrenoceptor antagonist, idazoxan. Combined alpha 1- and beta-adrenoceptor blockade resulted in an increase in the alpha 2-adrenoceptor-mediated hyperglycemic response to adrenaline. Since adrenaline is known not to cross the blood-brain barrier it is concluded that alpha 2-adrenoceptor stimulation induces hyperglycemic responses through a peripheral effect that involves postsynaptic alpha 2-adrenoceptors in the pancreatic beta-cell which are linked to the inhibition of insulin release.