Activation of 5-HT2 receptors induces glycogenolysis in the rat brain

The effect of 5-HT(2) receptor activation on brain glycogen and the extracellular concentration of glucose was investigated in the present study. An injection of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2 mg/kg, i.p.) or mescaline (10 mg/kg, i.p.) at an ambient temperature of 29 degrees C produced a 35-45% decrease in brain glycogen that persisted for at least 2 h. DOI also increased the extracellular concentration of glucose in the striatum by 60%. Maintenance of rats at 22 degrees C significantly attenuated DOI-induced glycogenolysis, as well as DOI-induced hyperthermia, and the increase in the extracellular concentration of glucose in the striatum. DOI-induced hyperthermia, glycogenolysis and increase in the extracellular concentration of glucose also were attenuated in rats treated with the 5-HT(2) receptor antagonist, 6-methyl-1-(methylethyl)-ergoline-8beta-carboxylic acid 2-hydroxy-1-methylpropyl ester maleate (LY-53,857) (3 mg/kg, ip). These results support the conclusion that 5-HT(2) receptor activation promotes glycogenolysis and that hyperthermia exerts a prominent role in this process.     

Activation of D2-like receptors induces sympathetic climactic-like responses in male and female anaesthetised rats

In anaesthetised male rats an intravenous (i.v.) injection of p-chloroamphetamine (PCA) produced a specific patterned bursting response in the sympathetic vas deferens nerve (VDN) that corresponds to ejaculation. In the present, study selective dopamine agonists and antagonists were used to investigate whether dopaminergic mechanisms influence the generation of this ejaculatory-related response. Administration of a mixed D(1/2) receptor agonist (0.1-1.0 mg kg(-1) apomorphine i.v.) also evoked the characteristic bursting pattern responses in the VDN. Similar, but fewer, burst pattern responses could also be evoked by a selective D(2/3) receptor agonist (0.1-2.0 mg kg(-1) piribedil). Responses to 1.0 mg kg(-1) apomorphine were blocked by pretreatments with either 0.5 mg kg(-1) remoxipride (D(2) receptor antagonist) or 0.5 mg kg(-1) nafadotride (D(3) receptor antagonist), suggesting that D(2)-like receptors were involved. Responses could not be evoked by i.v. injections of apomorphine (1.0 mg kg(-1)) in anaesthetised male rats with a midthoracic spinal section, indicating that activation of D(2)-like receptors at supraspinal sites leads to an increase in the excitability of the lumbosacral pattern generator for ejaculation. In anaesthetised female rats a similar patterned bursting response occurred in the uterine nerve (UN) in response to apomorphine (0.5-2.0 mg kg(-1) i.v.). Thus a common neural mechanism may regulate sexual climactic reflexes in both sexes.     

Action of botulinum toxin on transmission from sympathetic nerves to the vas deferens

Botulinum toxin (Type A) depressed or abolished transmission from postganglionic nerves to smooth muscle of isolated preparations of guinea-pig and mouse vas deferens. The time course of blockade was 2 to 6 times slower than that observed with the same concentration of the same batch of toxin on the rat diaphragm. Spontaneous excitatory junction potentials were still observed after 7 h exposure, indicating that the smooth muscle membrane was still able to respond to noradrenaline. Depression of transmission is probably due to a presynaptic action of unknown nature.     

Blockade of 5-HT2 receptors with sarpogrelate uncovers 5-HT7 receptors inhibiting the tachycardic sympathetic drive in pithed rats

Our group has previously shown in pithed rats that the cardiac sympathetic drive, which produces tachycardic responses, is inhibited by 5-HT via the activation of prejunctional 5-HT_1B/1D/5 receptors. Interestingly, when 5-HT₂ receptors are chronically blocked with sarpogrelate, the additional role of cardiac sympatho-inhibitory 5-HT_1F receptors is unmasked. Although 5-HT₂ receptors mediate tachycardia in rats, and the chronic blockade of 5-HT₂ receptors unmasked 5-HT₇ receptors mediating cardiac vagal inhibition, the role of 5-HT₇ receptors in the modulation of the cardiac sympathetic tone remains virtually unexplored. On this basis, male Wistar rats were pretreated during 14 days with sarpogrelate (a 5-HT₂ receptor antagonist) in drinking water (30 mg/kg/day; sarpogrelate-pretreated group) or equivalent volumes of drinking water (control group). Subsequently, the rats were pithed to produce increases in heart rate by either electrical preganglionic spinal (C₇-T₁) stimulation of the cardiac sympathetic drive or iv administration of exogenous noradrenaline. The iv continuous infusion of AS-19 (a 5-HT₇ receptor agonist; 10 µg/kg/min) (i) inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline only in sarpogrelate-pretreated rats. This inhibition was completely reversed by SB258719 (a selective 5-HT₇ receptor antagonist; 1 mg/kg, iv) or glibenclamide (an ATP-sensitive K⁺ channel blocker; 20 mg/kg, iv). These results suggest that chronic 5-HT₂ receptor blockade uncovers a cardiac sympatho-inhibitory mechanism mediated by 5-HT₇ receptors, involving a hyperpolarization due to the opening of ATP-sensitive K⁺ channels. Thus, these findings support the role of 5-HT₇ receptors in the modulation of the cardiac sympathetic neurotransmission.     

Agonists at presynaptic receptors on sympathetic nerves differentially affect two phases of the contractile response in the rat vas deferens

A series of adrenoceptor agonists were investigated for their prejunctional effects on field stimulated rat vas deferens. Tissues were stimulated in 10 s trains of impulses, frequency 10 Hz, every 100 s. This produced a biphasic response comprising an initial twitch followed by a prolonged, plateau phase of contraction. The order of potency for a series of alpha 2-agonists against the twitch phase of contraction was UK14304 greater than clonidine greater than noradrenaline = alpha-methyl noradrenaline greater than B-HT920. The same order of potency was observed against the plateau phase, but approximately 10 fold higher concentrations of agonist were needed. Surprisingly, B-HT920 was inactive against the plateau phase of contraction. Characteristic differences in the slopes and maximum responses of the dose-response curves to the imidazolines (UK14304 and clonidine) and the beta-phenethylamines (noradrenaline and alpha-methyl noradrenaline) were seen against both phases of contraction. It is concluded that the two phases of contraction are influenced by activation of two distinct heterogeneous populations of prejunctional alpha 2-adrenoceptors.     

Physiological relevance of constitutive activity of 5-HT2A and 5-HT2C receptors

It is generally accepted that seven-transmembrane receptors have the capacity to regulate cellular signaling systems in the absence of occupancy by a ligand (i.e. the receptors display constitutive activity). Drugs can increase (agonists), decrease (inverse agonists) or not change (antagonists) receptor activity towards a cellular effector. Moreover, some drugs (protean ligands) have multiple pharmacological properties (e.g. agonism towards one response and inverse agonism towards another response coupled to the same receptor and measured from the same cells, simultaneously). In this article, we describe response-dependent constitutive activity and ligand pharmacology for 5-HT2A and 5-HT2C receptors in vitro. Moreover, we provide evidence that 5-HT2A and 5-HT2C receptor constitutive activity is physiologically relevant in vivo and suggest that strong consideration should be given to the impact of constitutive receptor activity on disease and the therapeutic potential of inverse agonism.     

Central administration of 5-HT activates 5-HT1A receptors to cause sympathoexcitation and 5-HT2/5-HT1C receptors to release vasopressin in anaesthetized rats.

1. The effects of intracerebroventricular injections to the right lateral ventricle (i.c.v.) of 5-hydroxytryptamine (5-HT, 40 and 120 nmol kg-1), N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT; 3 nmol kg-1), 5-carboxamidotryptamine (5-CT; 3 nmol kg-1), 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT; 3, 40 and 120 nmol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 40 and 120 nmol kg-1) on renal sympathetic nerve activity, blood pressure, heart rate and phrenic nerve activity were investigated in normotensive rats anaesthetized with alpha-chloralose. 2. 5-HT caused a long lasting pressor response which was associated with an initial bradycardia and renal sympathoinhibition followed by a tachycardia and renal sympathoexcitation. Pretreatment with the 5-HT2/5-HT1C receptor antagonists, cinanserin (300 nmol kg-1, i.c.v.) or LY 53857 (300 nmol kg-1, i.c.v.) reversed the initial bradycardia and sympathoinhibition to tachycardia and sympathoexcitation. Combined pretreatment with LY 53857 (300 nmol kg-1, i.c.v.) and the 5-HT1A antagonist, spiroxatrine (300 nmol kg-1, i.c.v.), blocked the effects of 5-HT on all the above variables. 3. Pretreatment with the vasopressin V1-receptor antagonist, beta-mercapto-beta,beta-cyclopentamethylene-propionyl1, O-Me-Tyr2, Arg8-vasopressin [(d(CH2)5Tyr(Me)AVP, 10 micrograms kg-1, i.v.] did not affect the magnitude but reduced the duration of the pressor response produced by i.c.v. 5-HT and reversed the initial bradycardia and renal sympathoinhibition to tachycardia and sympathoexcitation. 4. 1-(2,5-Di-methoxy-4-iodophenyl)-2-aminopropane (DOI) caused a pressor effect which was associated with a bradycardia and sympathoinhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory presynaptic 5-hydroxytryptamine (5-HT) receptors on the sympathetic nerves of the human saphenous vein

Superfused strips of the human saphenous vein preincubated with 3H-noradrenaline were used to investigate the influences of serotonin (5-HT) receptor agonists and antagonists on the electrically evoked tritium overflow. 5-HT and the preferential 5-HT1A receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] concentration-dependently inhibited the evoked 3H overflow. The evoked 3H overflow was not affected by 0.1 or 1 mumol/l TVX Q 7821 (2-(4-[4-(2-pyrimidinyl)-1-piperazinyl]-butyl)-1,2-benzoisothiazol -3(2H)one-1,1-dioxide), which selectively binds to 5-HT1A sites; TVX Q 7821 10 mumol/l produced an increase in overflow. The inhibitory effect of 5-HT on the impulse-evoked 3H overflow was abolished by the nonselective 5-HT receptor antagonist metitepin, but was not attenuated by propranolol. Metitepin also abolished the inhibitory effect of 8-OH-DPAT on evoked 3H overflow, whereas the 5-HT2 receptor antagonist ketanserin was inactive in this respect. There was also no antagonism of the effect of 8-OH-DPAT by the alpha 2-adrenoceptor antagonist rauwolscine or the dopamine receptor antagonist flupenthixol. These results suggest that both 5-HT and 8-OH-DPAT inhibit noradrenaline release by activating inhibitory 5-HT receptors on the sympathetic nerves of the human saphenous vein. These receptors possess similarities to 5-HT1 recognition sites, but a further subclassification is not yet possible on the basis of the available data.     

Differences in agonist-independent activity of 5-HT2A and 5-HT2C receptors revealed by heterologous expression

5-Hydroxytryptamine 5-HT_2A and 5-HT_2C receptors share many properties, including a common ability to stimulate phospholipase C. Traditionally, this activation was thought to be initiated only after agonist binding, in accordance with the ternary complex model of receptor function. Recently, though, the 5-HT_2C receptor was shown to deviate from this tenet by spontaneously isomerizing into the active receptor state, thereby activating G proteins in the absence of agonist. To determine if 5-HT_2A receptors share this property of constitutive activity, 5-HT_2A and 5-HT_2C receptor function was evaluated in transiently transfected NIH 3T3 fibroblasts. In 3T3 cells expressing 5-HT_2C receptors, agonist-independent phosphatidyl inositol hydrolysis was substantially elevated relative to mock-transfected cells. In contrast, expression of the 5-HT_2A receptor at the same density caused only a marginal increase in basal signaling. Control experiments in the current and previous papers establish that basal activity does not reflect contaminating serotonin. In addition, the magnitude of serotonin-induced signaling was the same in cells expressing either receptor, suggesting that the intrinsic ability of the two receptors to couple to G proteins is comparable. These data indicate that the 5-HT_2A receptor has a much lower intrinsic ability to spontaneously adopt or maintain the active receptor conformation than does the closely related 5-HT_2C receptor.

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Received: 25 August 1998 / Accepted: 30 October 1998     

Ontogenesis of autonomic receptors and AChE activity in the rat vas deferens

The study was undertaken to obtain some information about the development of the autonomic receptors and the AChE activity in the rat vas deferens. The results suggest that the adrenoceptors were fully developed at birth. The M1-ACh receptors were developed before the M2-ACh receptors. The AChE activity developed before the ACh muscarinic receptors of the rat vas deferens.     

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein via presynaptic 5-HT receptors similar to the 5-HT1D subtype

Summary The human saphenous vein preincubated with [³H]noradrenaline was used to determine the pharmacological properties of the release-inhibiting presynaptic serotonin (5-HT) receptor on the sympathetic nerves. The overflow of tritium evoked by transmural electrical stimulation (2 Hz) was concentration-dependently inhibited by drugs known to stimulate 5-HT receptors in the following rank order: oxymetazoline 5-HT 5-carboxamidotryptamine = 5-methoxytryptamine = sumatriptan > tryptamine > N,N(CH₃)₂-5-HT = yohimbine = 8-hydroxy-2-(di-n-propylamino)-tetraline. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT_1B and 5-HT_1D binding sites, but not with those for 5-HT_1A or 5-HT_1C binding sites. 5-Aminotryptamine, methysergide, ipsapirone, cyanopindolol, SDZ 21009 and metergoline dit not produce a significant inhibition. Metitepine and methysergide antagonized the inhibitory effect of 5-HT, whereas spiroxatrine, propranolol, ketanserin and ICS 205-930 did not.These data exclude the idea that the inhibitory presynaptic 5-HT receptor on the sympathetic nerves belongs to the 5-HT₂ and 5-HT₃ receptor class; the pattern of agonist potencies suggests that the receptor is very similar to the 5-HT_1D receptor subtype. Send offprint requests to M. Gothert at the above address     

Activation of the brain 5-HT2C receptors causes hypolocomotion without anxiogenic-like cardiovascular adjustments in mice

The present study evaluated whether hypolocomotion elicited by subcutaneous administration of the non-specific 5-HT/preferential 5-HT(2C) receptor agonist mCPP during novelty exposure was due to an enhanced anxiety-like state. The effects of mCPP on exploratory behavior during exposure to a new environment (novelty) were studied in male C57BL/6N mice. Subcutaneous injection of mCPP (1 and 3mg/kg) and the preferential 5-HT(2C) receptor agonist MK212 (0.7 and 1mg/kg) induced hypolocomotion during novelty exposure. The selective 5-HT(2C) receptor antagonist SB242084 (0.3mg/kg) reversed the mCPP-induced hypolocomotion into hyperlocomotion. In contrast, MK212 induced hypolocomotion that was blocked by SB242084, indicating a specific 5-HT(2C) receptor involvement. When injected intracerebroventricularly, mCPP (30microg) elicited hypolocomotion, whereas the same dose mildly increased locomotion when injected into the dorsal hippocampus. Since anxiety affects autonomic functions, effects of mCPP on cardiovascular function were studied by radio-telemetry in the home cage of unrestrained mice. Subcutaneous injection of mCPP (3mg/kg) had no significant effect on heart rate and mean arterial blood pressure. In summary, in view of lack of autonomic effects, and the lack of hypoactivity upon forebrain stimulation, the hypolocomotion induced by systemic mCPP cannot be explained by an enhanced anxiety-like state.     

Role of 5-HT(2) receptors in the tryptamine-induced 5-HT syndrome in rats

We distinguished the functions of the different 5-hydroxytryptamine-2 (5-HT(2)) receptor (5-HT(2)R) subtypes in the tryptamine-induced 5-HT syndrome in rats using (1) the 5-HT(2A)R antagonist R93274 (N-[(3-p-fluorophenyl-1-propyl)-4-methyl-4-piperidinyl]-4-amino-5-iodo-2-methoxybenzamide), the 5-HT(2A/C)R antagonist R99647 (2-(dimethylaminomethyl)2,3,3a,8-tetrahydrodibenzo[c,f]isoxazolo[2,3-a]azepine), the 5-HT(2B/C)R antagonist SB-242084 (6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline), and several 5-HT(2)R antagonists (ketanserin, risperidone, pipamperone and mianserin); and (2) chronic 5-HT(2)R activation by 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM). In contrast to SB-242084, the selective 5-HT(2A)R antagonist R93274 as well as the non-selective 5-HT(2A)R antagonists (R99647, ketanserin, risperidone, pipamperone and mianserin) significantly inhibited tryptamine-induced forepaw treading and tremors, and reversed peripherally mediated cyanosis into hyperaemia; only the 5-HT(2A/C)R antagonists R99647 and mianserin inhibited the tryptamine-induced hunched back. Intermittent DOM administration (intravenously every 48 h for 12 days) did not change the centrally mediated tryptamine-induced forepaw treading, tremors and hunched back at 1, 4 or 7 days after the last DOM pretreatment. The DOM-induced head twitch response, measured immediately after every DOM injection, was not affected. In contrast, peripherally mediated cyanosis was reversed into hyperaemia in 75, 11 and 20% of all pretreated rats at 1, 4 and 7 days, respectively, after the last DOM administration. Taken together, these finding suggest that central 5-HT(2A)Rs mediate tryptamine-induced forepaw treading and tremors, that peripheral 5-HT Rs mediate tryptamine-induced cyanosis, and that 5-HT(2A)Rs mediate tryptamine-induced hunched back. Peripheral 5-HT(2C)Rs are more sensitive to desensitization after intermittent treatment with an agonist than central 5-HT(2A)Rs.     

Potentiation of sympathetic neuromuscular transmission mediated by muscarinic receptors in guinea pig isolated vas deferens

In guinea-pig isolated vasa deferentia, purinergic neurogenic contractions and responses to applied adenosine 5-triphosphate (ATP) were potentiated by carbachol; responses to adrenergic transmission and applied noradrenaline were not. Following blockade of P2 receptors and -adrenoceptors, the residual neurogenic response was massively potentiated by carbachol, suggesting the presence of a non-purinergic, non-adrenergic component. In the presence of guanethidine, carbachol had no significant effect, indicating that sympathetic transmission was the only element involved. Use of oxotremorine and selective muscarinic receptor antagonists suggested that the potentiating effect of carbachol and oxotremorine was mediated via M3 muscarinic receptors without involvement of nicotinic receptors.     

5-HT2C receptors are involved in the discriminative stimulus effects of citalopram in rats

 Rats were trained on a fixed ratio 10, food-reinforced schedule to recognize a discriminative stimulus (DS) elicited by the selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram (2.5 mg/kg, IP). The preferential, high efficacy agonist at 5-HT_2C receptors, Ro60-0175, dose-dependently generalized to citalopram with an ED₅₀ of 0.3 mg/kg, IP. Further, the selective 5-HT_2C receptor antagonist, SB242,084, dose-dependently (ED₅₀=0.1 mg/kg, IP) blocked the citalopram DS. These data suggest that 5-HT_2C receptors are involved in the DS properties of the SSRI, citalopram, in rats. They do not, however, exclude a potential role of other 5-HT receptor types. Received: 15 October 1998 / Final version: 10 December 1998     

Pharmacology of delta-opioid receptors in the hamster vas deferens

Electrically evoked contractions of the hamster isolated vas deferens are inhibited only by opioid drugs which have agonist activity at delta-opioid receptors. Opioids which are mu-, kappa- or sigma-selective were either inactive or were antagonists. The compound beta-funaltrexamine, which irreversibly blocks mu- and delta-opioid receptors, caused a flattening of the dose-response curve and a reduced maximum inhibition available to delta-opioid agonists. Analysis of the curves by the double-reciprocal null method enabled the affinity of these agonists at delta-opioid receptors to be calculated.     

Effects of blockade of 5-HT2 receptors and activation of 5-HT1A receptors on the exploratory activity of rats in the elevated plus-maze

Acute administration of gepirone, a 5-HT_1A agonist, caused a dose dependent (1–10 mg/kg, IP) reduction in the locomotor activity (open and closed arms) of rats tested in the elevated plus-maze. However, rats housed in individual cages and submitted to chronic treatment with gepirone (10 mg/kg PO) showed a marked increase in the percentages of number and time spent in the open arms as compared to controls. These results are compatible with the idea that the antiaversive effect due to long-term treatment with 5-HT_1A agonists is the result of a progressive desensitization of the somatodendritic 5-HT autoreceptor with the consequent recovery of firing rate of 5-HT neurons along with an activation of normosensitive postsynaptic 5-HT neurons. Ketanserin caused a biphasic effects on the exploratory behavior of rats in the plus-maze. The lower dose (0.5 mg/kg) decreased the aversion to the open arms and the higher dose (1.0 mg/kg) caused an unspecific decrease in the overall activity of the animals. Ketanserin is supposed to have antagonistic action on 5-HT₂ and on -adrenergic receptors. As prazosin (0.5–1.0 mg/kg), an -adrenergic receptor blocker, did not present any significant effect in the present work it is suggested that the effects of the lower dose of ketanserin was due to its high antagonistic action on 5-HT₂ receptors.