Alpha 2 receptors mediate the antinociceptive action of 8-OH-DPAT in the hot-plate test in mice

Actions of the prototypical 5-HT1A receptor agonist 8-OH-DPAT at human alpha2-adrenoceptors: (+)8-OH-DPAT, but not (-)8-OH-DPAT is an alpha2B subtype preferential agonist

8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] is the prototypical agonist at serotonin 5-HT1A receptors; however, activity at other targets contributes to the functional effects of the compound as well. We examined the properties of 8-OH-DPAT and its enantiomers at recombinant human (h)alpha2-adrenoceptor subtypes, using a panel of radioligand binding and functional tests. In competition binding experiments using [3H]-RX821002, about 10-fold selectivity of (+)8-OH-DPAT for the halpha2B subtype (pKi about 7) over halpha2A- and halpha2C-adrenoceptors was observed. In contrast, the S(-) enantiomer of 8-OH-DPAT showed similar weak affinities for the three receptor subtypes (pKis<6). The binding affinity of (+)8-OH-DPAT at the halpha2B- and the halpha2A-adrenoceptor was found sensitive to GTPgammaS, a receptor/G protein-uncoupling agent, indicating agonist properties of the drug. Furthermore, using [35S]GTPgammaS binding determination at CHO-halpha2B or CHO-halpha2A cell membranes and G protein coupled inwardly rectifying potassium (GIRK) current recordings in Xenopus oocytes expressing halpha2B, partial agonist activity of (+)8-OH-DPAT at the respective receptors was confirmed in these two different functional assays. Potency of (+)8-OH-DPAT for stimulation of [35S]GTPgammaS incorporation was lower at the halpha2A- than at the halpha2B-adrenoceptor, consistent with binding affinities. Thus, (+)8-OH-DPAT and, as a consequence, racemic (+/-)8-OH-DPAT are partial agonists at halpha2-adrenoceptors with selectivity for the halpha2B subtype, a property that might contribute to the effects of the compound described in native systems.

Pharmacological profiles of alpha 2 adrenergic receptor agonists identified using genetically altered mice and isobolographic analysis

Endogenous, descending noradrenergic fibers impose analgesic control over spinal afferent circuitry mediating the rostrad transmission of pain signals. These fibers target alpha 2 adrenergic receptors (alpha(2)ARs) on both primary afferent terminals and secondary neurons, and their activation mediates substantial inhibitory control over this transmission, rivaling that of opioid receptors which share a similar pattern of distribution. The terminals of primary afferent nociceptive neurons and secondary spinal dorsal horn neurons express alpha(2A)AR and alpha(2C)AR subtypes, respectively. Spinal delivery of these agents serves to reduce their side effects, which are mediated largely at supraspinal sites, by concentrating the drugs at the spinal level. Targeting these spinal alpha(2)ARs with one of five selective therapeutic agonists, clonidine, dexmedetomidine, brimonidine, ST91 and moxonidine, produces significant antinociception that can work in concert with opioid agonists to yield synergistic antinociception. Application of several genetically altered mouse lines had facilitated identification of the primary receptor subtypes that likely mediate the antinociceptive effects of these agents. This review provides first an anatomical description of the localization of the three subtypes in the central nervous system, second a detailed account of the pharmacological history of each of the six primary agonists, and finally a comprehensive report of the specific interactions of other GPCR agonists with each of the six principal alpha(2)AR agonists featured.

The acute and chronic administration of (±)-8-hydroxy-2-(di-n-propylamino)tetralin significantly alters the activity of spontaneously active midbrain dopamine neurons in rats: An in vivo electrophysiological study

This study examined the effect of the acute and chronic systemic administration of (+/-)-8-Hydroxy-2-(Di-n-propylamino)Tetralin(8-OH-DPAT) on the number and firing pattern of spontaneously active dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and substantia nigra pars compacta (SNC or A9) in anesthetized male rats. These parameters were measured using extracellular in vivo electrophysiology. A single s.c. injection of 0.01, 0.1, or 1 mg/kg of 8-OH-DPAT did not significantly alter the number of spontaneously active SNC DA neurons compared to vehicle-treated animals (controls). The acute administration of 0.01 or 0.1 mg/kg of 8-OH-DPAT did not significantly alter, whereas the 1 mg/kg dose significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The acute administration of 8-OH-DPAT significantly increased the percentage of VTA DA neurons firing in a bursting pattern. In contrast, there was a significant decrease in the percentage of SNC DA neurons firing in a bursting pattern following the acute administration of 8-OH-DPAT. The number of spontaneously active SNC DA neurons was not significantly altered by the chronic s.c. administration of 8-OH-DPAT (0.01, 0.1, or 1 mg/kg s.c.) as compared to controls. However, the chronic s.c. administration of all doses of 8-OH-DPAT significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The i.v. administration of (+)-apomorphine (50 microg/kg) did not reverse the 8-OH-DPAT-induced decrease in the number of spontaneously active VTA DA neurons, suggesting that this effect is unlikely due to depolarization blockade. The percentage of VTA DA neurons exhibiting burst firing was significantly increased by 0.01 and 0.1 mg/kg, but significantly decreased by 1 mg/kg of 8-OH-DPAT. Overall, the systemic administration of 8-OH-DPAT preferentially affects the activity of spontaneously active A10 DA neurons in rats.

Role of 5-HT1A and 5-HT1B receptors in the antinociceptive effect of tramadol

Tramadol, (1RS,2RS)-2-[(dimethylamine)-methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an atypical centrally acting analgesic agent with relatively weak opioid receptor affinity and which, like some antidepressants, is able to inhibit the reuptake of serotonin (5-hydroxytryptamine, 5-HT) in the raphe nucleus. We have previously demonstrated that pindolol, a beta-adrenoceptor blocker/5-hydroxytryptamine(1A/1B) receptor antagonist, enhanced tramadol antinociception and that the selective 5-HT1A agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reduced it. These effects were related to the negative feedback control that regulates raphe region neurones. The current study examines the ability of the selective antagonist at somatodendritic 5-HT1A receptors, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY100635, 0.8 mg/kg), the selective antagonist at terminal 5-HT1B receptors, N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB216641, 0.1-0.8 mg/kg) and the selective agonist at 5-HT1B receptors, 1,4-tDihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b] pyridin-5-one (CP93129, 0.2-0.4 mg/kg), to modify the antinociceptive effect of 4-64 mg/kg of tramadol in the hot plate test in mice. The results show that 0.8 mg/kg of WAY100635 enhanced antinociceptive effect of tramadol while neither agonism nor antagonism at the 5-HT1B receptor modifies it significantly at the doses tested. These results account for involvement of the somatodendritic 5-HT1A receptors in the analgesic effect of tramadol and support the supraspinal interaction of serotonin and the opioid system in the regulation of pain.

8-OH-DPAT acts on both 5-HT1A and 5-HT7 receptors to induce hypothermia in rodents

Studies using selective drugs and knockout mice have demonstrated that the 5-HT(7) receptor plays an instrumental role in serotonin-induced hypothermia. There is also evidence supporting an involvement of the 5-HT(1A) receptor, although mainly from studies using 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT(1A/7) receptor agonist. Here we studied the effects of 8-OH-DPAT and selective antagonists for the 5-HT(1A) and 5-HT(7) receptors on body temperature in rats, wild-type (5-HT(7)(+/+)) mice and knockout (5-HT(7)(-/-)) mice. At lower doses (0.3-0.6 mg/kg, i.p.), 8-OH-DPAT decreased body temperature in 5-HT(7)(+/+) mice but not in 5-HT(7)(-/-) mice. At a higher dose (1 mg/kg, i.p.) 8-OH-DPAT induced hypothermia in both 5-HT(7)(-/-) and 5-HT(7)(+/+) mice. The 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide (WAY-100135) (10 mg/kg, i.p.) inhibited the effect of 8-OH-DPAT at all doses in rats and mice. In 5-HT(7)(+/+) mice the selective 5-HT(7) receptor antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970) (10 mg/kg, i.p.) fully inhibited the hypothermia induced by 0.3 mg/kg 8-OH-DPAT, but not that of higher doses. In rats, SB-269970 caused a 60% inhibition of the hypothermia induced by 0.3 mg/kg 8-OH-DPAT. Thus, both 5-HT(7) and 5-HT(1A) receptors are involved in a complex manner in thermoregulation, with the 5-HT(7) receptor being more important at lower, possibly more physiological, concentrations.

Radioligand binding analysis of knockout mice reveals 5-hydroxytryptamine7 receptor distribution and uncovers 8-hydroxy-2-(di-n-propylamino)tetralin interaction with α2 adrenergic receptors

In the present autoradiographic study, we took advantage of 5-hydroxytryptamine(7) (5-HT(7)) receptor knockout mice to analyze the brain distribution of 5-HT(7) receptor binding sites using [(3)H]5-carboxamidotryptamine (5-CT; a 5-HT(1A/1B/1D/5/7) receptor ligand) and [(3)H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT(1A/7) receptor ligand). Low to moderate densities of [(3)H]5-CT (2 nM) binding sites insensitive to pindolol (10 microM, for 5-HT(1A/1B) receptor blockade) and GR-127935 (1 microM; for 5-HT(1D) receptor blockade) were observed in wild-type mice (mainly in thalamus and hypothalamus) but not in 5-HT(7) receptor knockout mice. Surprisingly, moderate to high densities of [(3)H]8-OH-DPAT (10 nM) binding sites insensitive to pindolol (10 microM) remained in 5-HT(7) receptor knockout mouse brain. These non-5-HT(1A), non-5-HT(7) binding sites were found to be adrenergic alpha(2A) receptor binding sites. In alpha(2A) receptor knockout mice low to moderate densities of [(3)H]8-OH-DPAT binding sites insensitive to pindolol but sensitive to the selective 5-HT(7) receptor antagonist SB-269970 (300 nM) were observed mainly in thalamus and hypothalamus. Therefore, in addition to 5-HT(1A) and 5-HT(7) binding sites, [(3)H]8-OH-DPAT also binds to alpha(2A) receptor binding sites in wild-type mouse brain. [(3)H]8-OH-DPAT (in the presence of pindolol and 1 microM RX-821002 for alpha(2) receptor blockade) and [(3)H]5-CT (in the presence of pindolol and GR-127935) bind to a similar receptor binding population corresponding to 5-HT(7) binding sites. Detailed anatomical mapping of 5-HT(7) receptor binding sites in wild-type mouse brain was then performed using both radioligands in the presence of suitable pharmacological agents for non-5-HT(7) receptor binding sites blockade. The mapping revealed binding sites consistent with the mRNA distribution with the highest densities found in anterior thalamic nuclei.

Anti-parkinsonian agents procyclidine and ethopropazine alleviate thermal hyperalgesia in neuropathic rats

Procyclidine and ethopropazine, widely used as anti-parkinsonian agents because of their anti-cholinergic action, are also known to have NMDA antagonist properties. Unlike other NMDA antagonists, these agents-because of their anti-cholinergic action-are devoid of neurotoxic side effects. In the present study, we used a sciatic nerve ligation model that produces a hyperalgesic (neuropathic pain) state in adult rats to evaluate the ability of procyclidine or ethopropazine, either alone or in combination with an alpha(2) adrenergic agonist, to ameliorate neuropathic pain. We found that both procyclidine and ethopropazine alleviated thermal hyperalgesia in a dose dependent manner; when a marginally effective dose of these agents was combined with an ineffective dose of an alpha(2) adrenergic agonist (clonidine or guanabenz), the combination therapy provided effective and long-lasting relief from neuropathic pain. In addition, the combination therapy was free from neurotoxic or behavioral side effects, and hyperactivity, a side effect associated with procyclidine monotherapy, was counteracted by clonidine.

Adrenergic pharmacology: focus on the central nervous system

Norepinephrine and epinephrine are involved in the control of several important functions of the central nervous system (CNS), including sleep, arousal, mood, appetite, and autonomic outflow. Catecholamines control these functions through activation of a family of adrenergic receptors (ARs). The ARs are divided into three subfamilies (alpha1, alpha2, and beta) based on their pharmacologic properties, signaling mechanisms, and structure. ARs in the CNS are targets for several therapeutic agents used in the treatment of depression, obesity, hypertension, and other diseases. Not much is known, however, about the role of specific AR subtypes in the actions of these drugs. In this paper, we provide an overview of adrenergic pharmacology in the CNS, focusing on the pharmacologic properties of subtype-selective AR agonists and antagonists, the accessibility of these drugs to the CNS, and the distribution of ARs in different areas of the brain.

Alpha(2) adrenoceptor agonists as potential analgesic agents. 1. (Imidazolylmethyl)oxazoles and -thiazoles

A series of (imidazolylmethyl)oxazoles and -thiazoles were prepared and evaluated as alpha(2) adrenoceptor agonists. These compounds were also tested in in vivo paradigms that are predictive of analgesic activity. Variations in both the imidazole and thiazole portions of the molecule were investigated. Some of the more potent compounds such as 22, 26, 45, and 53 displayed alpha(2) receptor binding in the 10-20 nM range and also had significant antinociceptive activity in the mouse abdominal irritant test (MAIT).

MKC-242, a novel 5-HT1A receptor agonist, facilitates cortical acetylcholine release by a mechanism different from that of 8-OH-DPAT in awake rats

We have previously reported that 5-¿3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy¿-1,3-be nzodioxole (MKC-242), a potent and selective serotonin (5-HT)1A receptor agonist, exerts anxiolytic- and antidepressant-like effects in animal models and that the antidepressant-like effect may be mediated by postsynaptic 5-HT1A receptors. The present study, using a microdialysis technique, was undertaken to characterize in vivo the effect of MKC-242 on cholinergic neurons. Subcutaneous injection of MKC-242 (0.5-1.0 mg/kg), like the typical 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), increased extracellular acetylcholine (ACh) levels in the rat cerebral cortex. The increase in ACh release by MKC-242 was also observed in the hippocampus. The effect of MKC-242 on cortical ACh release was attenuated by pretreatment with the 5-HT1A receptor antagonists (10 mg/kg, s.c.) propranolol and N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropana mide. The increase in cortical ACh release by MKC-242 was blocked by lesion of serotonergic neurons with 5,7-dihydroxytryptamine, whereas that by 8-OH-DPAT was not. Lesion of noradrenergic neurons with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine did not affect the MKC-242-induced increase in ACh release. These results suggest that systemic injection of MKC-242 facilitates in vivo ACh release via an activation of somadendritic 5-HT1A autoreceptors, and that MKC-242 and 8-OH-DPAT affect cholinergic neurons in the rat cerebral cortex via different mechanisms.

Enhancement and depression of spinal reflexes by 8-hydroxy-2-(di-n-propylamino)tetralin in the decerebrated and spinalized rabbit: involvement of 5-HT1A- and non-5-HT1A-receptors

1. In decerebrated, spinalized and paralyzed rabbits, intravenous administration of the 5-HT1A-receptor agonists (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 3-300 nmol kg(-1), cumulative) and flesinoxan (22-2200 nmol kg(-1), cumulative) significantly increased the short latency reflex evoked in gastrocnemius medialis motoneurones by electrical stimulation of all myelinated afferents (Abeta and Adelta fibres) of the sural nerve. Reflexes increased to median values of 198% (inter-quartile range (IQR) 148-473%) and 296% (IQR 254-522%) of pre-drug values with the highest doses of 8-OH-DPAT and flesinoxan, respectively. The enhancement of reflexes induced by 5-HT1A-receptor agonists was not reversed by the selective 5-HT1A-receptor antagonist (S)WAY-100135 (2.05 micromol kg[-1]). 2. The effects of 8-OH-DPAT were tested after pretreatment with (S)WAY-100135 (2.05 micromol kg[-1]), its more potent analogue WAY-100635 (185 nmol kg[-1]), and the 5-HT2/5-HT1D-/5-HT7-receptor ligand ritanserin (1.67 micromol kg[-1]). 8-OH-DPAT (300 nmol kg(-1) single dose) significantly increased gastrocnemius reflex responses in the presence of (S)WAY-100135 and WAY-100635, to median values of 260% (IQR 171-295%) and 165% (IQR 136-170%) of pre-drug levels, respectively. These values were not significantly different from each other, or from the effects of 8-OH-DPAT given alone. When 8-OH-DPAT was given after ritanserin, reflexes were a median of 102% (IQR 76-148%) of pre-drug values: i.e. there was no significant increase in responses. Neither WAY-100635 nor ritanserin had any effects on reflexes per se. 3. WAY-100635 (185 nmol kg[-1]) and ritanserin (1.67 micromol kg[-1]) were given after 8-OH-DPAT (300 nmol kg[-1]). The agonist increased reflexes to a median value of 184% (IQR 135-289%), after which WAY-100635 significantly reduced responses to 165% (IQR 130-254%) and ritanserin further decreased reflexes to a median of 107% (IQR 100-154%) of pre-drug levels, i.e. not significantly different from controls. 4. Previous studies have shown that reflexes evoked by large myelinated axons tend to be suppressed, rather than enhanced, by 5-HT1A-receptor agonists. When tested against reflexes evoked by stimulation of the sural nerve at strengths between 1.5 and 2.5 times threshold, 8-OH-DPAT (3-300 nmol kg(-1), cumulative) and flesinoxan (22-2200 nmol kg(-1), cumulative) significantly reduced gastrocnemius responses to median values of 36% (IQR 15-75%) and 17% (IQR 12-38%) of pre-drug levels, respectively. This inhibition was fully reversed by (S)WAY-100135 (2.05 micromol kg[-1]). 5. These data show that drugs that are agonists at 5-HT1A-receptors increase polysynaptic spinal reflexes evoked by moderate to high stimulus intensities and depress responses to very low intensity stimuli. The inhibitory effects of these drugs were mediated through 5-HT1A-receptors as they were abolished by a selective antagonist for these sites. However, the facilitatory effects of 8-OH-DPAT could be completely blocked only by a combination of ritanserin, which has no significant affinity for 5-HT1A-receptors, with WAY-100635. It appears that the enhancement of reflexes by 8-OH-DPAT arises from a combined action at 5-HT1A-receptors and other, ritanserin-sensitive, sites which could be 5-HT1D- or 5-HT7-receptors.

Effects of K+ channel blockers and openers on antinociception induced by agonists of 5-HT1A receptors

The modulation by K+ channel-acting drugs of the antinociceptive effect of several 5-HT1A receptor agonists was examined with the hot plate test in mice. All the 5-HT1A receptor agonists tested induced dose-dependent antinociception, the order of potency being (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) > buspirone > or = lesopitron > or = tandospirone. The blockers of ATP-sensitive K+ channels (KATP) gliquidone and glipizide (1-4 and 16-64 micrograms/mouse i.c.v., respectively) reduced the antinociceptive effect of 8-OH-DPAT, whereas cromakalim (32-64 micrograms/mouse i.c.v.), an opener of KATP channels, enhanced the effect. In contrast, 4-aminopyridine (25-250 ng/mouse i.c.v.) and tetraethylammonium (10-20 micrograms/mouse i.c.v.), which antagonize several non-ATP-dependent K+ conductances, were inactive. The same results were found with other agonists of 5-HT1A receptors (lesopitron, buspirone and tandospirone): gliquidone inhibited whereas cromakalim increased their antinociceptive effects. None of the K+ channel-acting drugs modified the binding of [3H]8-OH-DPAT to hippocampal membranes, whereas all the 5-HT1A receptor agonists displaced the ligand. These results suggest that ATP-sensitive K+ conductances are involved in the antinociception induced by agonists of 5-HT1A receptors.

Interactions of descending serotonergic systems with other neurotransmitters in the modulation of nociception

The effects of 5-hydroxytryptamine (5-HT) and ligands selective for particular 5-HT receptor subtypes on the transmission of nociceptive information in the spinal cord are complex. In these studies, we have focused on their interactions with two endogenous mediators of pain suppression, noradrenaline (NA) and adenosine. Spinal antinociception by 5-HT is blocked by alpha-adrenoreceptor antagonists and depletion of endogenous NA by 6-hydroxydopamine, while it is potentiated by blockade of NA reuptake with desipramine. These observations provide evidence for a 5-HT receptor-mediated increase in the release of NA from the spinal cord. This action appears to be due to activation of a 5-HT1-like receptor as it is mimicked by some 5-HT1 receptor ligands (mCPP, TFMPP and 5-Me-O-DMT), but not by DOI (5-HT2) or 2-Me-5-HT (5-HT3). An additional component of 5-HT action is via release of adenosine. Antinociception by 5-HT is blocked by the adenosine receptor antagonist 8-phenyltheophylline, and 5-HT has been shown to release adenosine from the spinal cord in in vitro and in vivo paradigms. Methylxanthine-sensitive antinociception is seen with some 5-HT1 receptor ligands (CGS 12066B, mCPP), but not with others or with DOI or 2-Me-5-HT. Further characterization of the 5-HT receptor subtype involved in adenosine release will require the use of additional approaches.

Distribution of alpha 2-adrenergic binding sites in the parabrachial complex of the rat

The present study describes the distribution of alpha 2-adrenoceptors in the parabrachial and Kölliker-Fuse nucleus of the rat by employing the tritium-labeled alpha 2-receptor antagonist rauwolscine ([3H]-RAUW) as a ligand. The [3H]-RAUW binding was densitometrically quantified in five nuclei of the parabrachial (PB) complex in serial coronal sections. We found that cytoarchitectonically and anatomically distinct nuclei of the PB complex exhibit different numbers of [3H]-RAUW-binding sites. The largest number of binding sites was observed over the external lateral PB and caudally over the waist area of the PB. Lower numbers of binding sites were found in the remaining lateral PB nuclei, followed by the medial PB and the Kölliker-Fuse nucleus. In addition we disclosed that the internal lateral PB contains a very low number of binding sites while the external medial PB is marked by dense [3H]-RAUW binding. Also, the affinities of the binding sites differed between the PB areas. High affinities were observed in the external lateral PB, the remaining lateral PB nuclei and in the waist area of the PB, while the medial PB and the Kölliker-Fuse nucleus exhibited only low affinities for the ligand. Furthermore, saturation curves demonstrated non-linear profiles, indicating the presence of more than one population of binding sites in the PB nuclei for the radioligand. Our data demonstrate that the PB exhibits a distinct distribution of alpha 2-adrenergic binding sites. These correlate well with the cytoarchitectonically defined nuclei of the PB complex and with the pattern of ascending axons from the medial nucleus of the solitary tract and the area postrema terminating in the PB. Since a large number of these projection neurons utilize adrenaline or noradrenaline as their transmitters, we conclude that solitary-parabrachial neurotransmission to the forebrain is, at least in part, mediated via alpha 2-adrenoceptors.

Pro- and antinociceptive actions of serotonin (5-HT)1A agonists and antagonists in rodents: relationship to algesiometric paradigm

In mice injected with formalin into the hindpaw, the 5-HT1A receptor agonists, 8-OH-DPAT and flesinoxan, equipotently inhibited the early phase (EP) and late phase (LP) of licking. At higher doses, they provoked ataxia and inhibited the writhing elicited by intra-abdominal acetic acid. The antagonists, (-)-alprenolol, (-)-tertatolol, WAY-100,135 and S 15931 were more potent against the LP than the EP. They also inhibited writhing, and only at very high doses did they elicit ataxia. In rats, 8-OH-DPAT and flesinoxan increased the current required to elicit vocalisation upon electrical stimulation of the tail. The action of 8-OH-DPAT was blocked by WAY-100,135, which, like other antagonists, was inactive alone. Interestingly, a low dose of 8-OH-DPAT partially inhibited the antinociceptive action of the mu-opioid agonist, morphine, the action of which was dose-dependently facilitated by (-)-alprenolol and S 15931. Administered s.c., 8-OH-DPAT elicited spontaneous tail-flicks (STFs) in rats: these were abolished by WAY-100,135, (-)-tertatolol, (-)-alprenolol and S 15931. STFs were also eliminated by s.c. or i.t. administration of the alpha 2-adrenergic receptor agonist, clonidine, the GABAA agonist, muscimol or the GABAB agonist, baclofen. The mu-opioid, morphine, blocked STFs only at high doses and the kappa-opioid agonists, U 50,488 and U 69,593, even at supra-ataxic doses, were inactive. Antagonists at neurokinin (NK)1 (RP 67580), NK2 (SR 48,968) and bradykinin (BK)2 (Hoe 140) receptors, as well as aspirin, did not block STFs, though indomethacin was effective. Antagonists at the glycine B site coupled to the NMDA receptor, L 687,414, L 701,324 and (+)-HA966, blocked STFs. Furthermore, (+)-HA 966 and the competitive NMDA receptor antagonist, CPP, were active upon i.t. administration. STFs were also blocked by s.c. or i.t. administration of the AMPA antagonists, YM 900 and NBQX. In conclusion, the influence of 5-HT1A ligands upon nociception is dependent upon the algesiometric paradigm. Intriguingly, modulation of 5-HT1A receptor-mediated STFs reveals parallels to neuropathic pain.

Normal distribution of alpha 2-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 2 (alpha 2)-adrenoceptors along cervical, thoracic, lumbar, and sacral segments of the spinal cord of normal rats has been studied by quantitative autoradiography using the specific alpha 2-antagonist [3H]rauwolscine as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 2-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 2-adrenoceptor densities revealed the presence of these receptors throughout the whole gray matter with a preferential location in the superficial dorsal horn. This pattern was the same at all rostro-caudal levels of the cord and appeared very well correlated with the distribution of NA terminals revealed by immunohistochemistry, particularly in the superficial layers of the dorsal horn. After total transection of the spinal cord (caudally to the section) and 6-OHDA-induced lesion, an increase of alpha 2-adrenoceptor density was mainly observed within the distal dorsal horn thus evidencing supersensitivity in this area, while modifications were not detectable in other regions of the spinal gray matter, except at the lumbar level where other dorsal, central, and intermediate zones were significantly enriched.

Role of 5-HT1A receptors in the antinociceptive action of 8-hydroxy-2-(di-n- propylamino)tetralin in the rat

The role of 5-HT1A receptors in the antinociceptive action of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was investigated by using the shock titration test in rats. A subcutaneous injection of 300 micrograms/kg 8-OH-DPAT significantly raised the threshold for flinching, jumping and vocalization whereas 100 micrograms/kg only inhibited the flinch response. l-Propranolol and (+)-[N-tert-butyl-3-4-(2-methoxyphenyl)piperazin-1-yl-2-phenyl propanamide dihydrochloride], (+)-WAY100135, two antagonists at 5-HT1A receptors at 10 mg/kg s.c. antagonized the effect of 300 micrograms/kg 8-OH-DPAT on all measures. The effect of 300 micrograms/kg 8-OH-DPAT on the three measures was unmodified in rats which had received 150 micrograms 5,7-dihydroxytryptamine intracerebroventricularly 10 days before testing. The results suggest that 8-OH-DPAT inhibits nociceptive responses by stimulating postsynaptic 5-HT1A receptors.

Serotonin and pain: evidence that activation of 5-HT1A receptors does not elicit antinociception against noxious thermal, mechanical and chemical stimuli in mice

In this study, we examined whether activation of 5-HT1A receptors elicits antinociception in response to acute noxious chemical, thermal and mechanical stimuli in mice. In the writhing test, both agonists (e.g., 8-OH-DPAT, S 14671 and WY 50,324) and partial agonists (e.g., buspirone and gepirone) elicited a pronounced antinociception. However, antagonists (e.g., (-)-alprenolol and WAY 100,135) also induced antinociception and, at lower (inactive) doses, failed to modify the action of agonists. In addition, the separation between doses required for induction of antinociception as compared to those required for induction of ataxia (in the rotarod test) was variable and low for both agonists (median: 1.9) and partial agonists (median: 1.3), although it was somewhat greater for antagonists (> or = 3.3). In the hot-plate test, only certain agonists (e.g., 8-OH-DPAT) and partial agonists (e.g., gepirone) elicited antinociception and their actions were not attenuated by 5-HT1A antagonists which, themselves, were inactive in this paradigm. The 5-HT1C/2 antagonist, ritanserin, the 5-HT3 antagonist, ondansetron, the dopamine D2 receptor antagonist, raclopride, and the alpha 1-adrenoceptor antagonist, prazosin, were also ineffective in modifying the antinociception evoked by 5-HT1A agonists and partial agonists in the hot-plate test. In contrast, their actions were strongly attenuated by the alpha 2-adrenoceptor antagonist, idazoxan. In the tail-flick tests to noxious heat and noxious pressure, 5-HT1A receptor agonists, partial agonists and antagonists generally failed to induce antinociception. Moreover, modulation of stimulus intensity (from very weak to very intense) did not reveal any influence upon the latency to respond. In conclusion, in the writhing test, the data provide no evidence for a specific antinociceptive effect of the activation of 5-HT1A receptors. Further, in the hot-plate test, for those 5-HT1A agonists and partial agonists which induce antinociception, alpha 2-adrenoceptors rather than 5-HT1A receptors are implicated in their actions. Finally, in reflexive tests, irrespective of stimulus quality or intensity, 5-HT1A agonists and partial agonists do not mediate antinociception. These data suggest that the activation of 5-HT1A receptors does not, under these conditions of acute noxious stimulation, elicit antinociception.

Normal distribution of alpha-1-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 1 (alpha 1)-adrenoceptors along the different segments of the spinal cord (cervical, thoracic, lumbar, and sacral) of normal rats has been studied by quantitative autoradiography using the specific alpha 1-antagonist [3H]Prazosin as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 1-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 1-adrenoceptor densities revealed a widespread distribution of these receptors along all segments of the spinal cord with a similar pattern in the various subregions of gray matter studied. This distribution of alpha 1-adrenoceptors was quite well correlated with the distribution of NA terminals, when referring to previous descriptions by immunohistochemistry. After 6-OHDA lesion, as well as caudally to the transection, a significant increase of alpha 1-adrenoceptor densities was observed in all spinal subregions thus evidencing supersensitivity. These results suggest that NA may act in the spinal cord, at least partly, via alpha 1-adrenoceptors and that the expression of these receptors could be influenced by NA dysfunction, as demonstrated here through the effects observed in lesioned animals.

Evidence that an α2A-adrenoceptor subtype mediates antinociception in mice

In the hot-plate test in mice, the antinociceptive action of the alpha 2-adrenoceptor agonist, UK 14,304, was abolished by the alpha 2-adrenoceptor antagonist, idazoxan, the potent alpha 2A-adrenoceptor antagonist, RX 821002 and the preferential alpha 2A-adrenoceptor antagonist, BRL 44408. In contrast, the preferential alpha 2B- (and alpha 2C)-adrenoceptor ligands ('antagonists'), ARC-239, BRL 41992 and prazosin were inactive. The preferential alpha 2A-adrenoceptor partial agonist, guanfacine, partially inhibited UK 14,304-induced antinociception. Further, guanfacine BRL 44408 reversibly elicited submaximal antinociception. It is concluded that alpha 2A-adrenoceptors mediate antinociception in mice.

Desipramine, administered chronically, influences 5-hydroxytryptamine1A-receptors, as measured by behavioral tests and receptor binding in rats

The 5-hydroxytryptamine1A (5-HT1A) receptor subtype seems to be of importance in the pathogenesis of depression and in the mode of action of antidepressants. In this study, behavioural experiments were performed in rats after oral administration of desipramine for 18-20 days, followed by an acute injection of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), either systemically or intrathecally. Chronic administration of desipramine prolonged the behavioural 5-HT syndrome in the animals injected systemically with 8-OH-DPAT. Treatment with desipramine was also found to potentiate and prolong the antinociceptive effect of an acute injection, systemically or intrathecally, of 8-OH-DPAT in the increasing temperature hot plate test. After systemic administration of 8-OH-DPAT, the colonic temperature was lowered similarly in the desipramine-treated group and in controls, whereas an intrathecal injection of 8-OH-DPAT resulted in a fall in the colonic temperature in the desipramine-treated group only. In vitro receptor binding studies, using [3H]8-OH-DPAT as the ligand, showed a statistically significant reduction of Kd and Bmax in the frontal cortex and of Kd in the spinal cord, after treatment with desipramine. No changes of Kd and Bmax were found in the hippocampus after this treatment. Thus, desipramine, administered chronically, resulted in a functional up-regulation of the 5-HT1A-receptors, both spinally and supraspinally, whereas in the in vitro receptor binding, a slight down-regulation or no change was found. It seems therefore that the results of in vitro receptor binding studies do not necessarily reflect the functional state of the neuronal system.(ABSTRACT TRUNCATED AT 250 WORDS)

Supraspinal and spinal monoamine-modified function and the expression of opioid antinociception

Opioids are highly valuable clinical agents for the treatment of pain which are thought to act both at the spinal and supraspinal level. During the course of their actions, they have complex interactions with monoamine systems. These include 5-hydroxytryptamine (5-HT) and noradrenaline (NA), so this topic is discussed using these two transmitter systems, their locations and receptor sub-types, as prime candidates for modulating nociceptive and antinociceptive processes. Several classes of 5-HT receptors, as well as α(2)-adrenoceptors, appear to be clearly involved in antinociception and the functions of systems carrying these receptors may be modified using psychotropic agents. In particular, some antidepressants may acutely augment opioid antinociception and this property may be exploited to delay the onset of opioid tolerance in the sub-acute situation.