Phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex

Alpha- and beta- adrenergic receptors regulate inflammatory responses to acute and chronic sleep fragmentation in mice

Sleep is a recuperative process, and its dysregulation has cognitive, metabolic, and immunological effects that are largely deleterious to human health. Epidemiological and empirical studies have suggested that sleep fragmentation (SF) as result of obstructive sleep apnea (OSA) and other sleep abnormalities leads to pronounced inflammatory responses, which are influenced by the sympathetic nervous system (SNS). However, the underlying molecular mechanisms contributing to SNS regulation of SF-induced inflammation are not fully understood. To assess the effects of the SNS upon inflammatory responses to SF, C57BL/6j female mice were placed in automated SF chambers with horizontally moving bars across the bottom of each cage at specified intervals to disrupt sleep. Mice were first subjected to either control (no bar movement), acute sleep fragmentation (ASF), or chronic sleep fragmentation (CSF) on a 12:12-h light/dark schedule. ASF involved a bar sweep every 120 s for 24 h, whereas CSF involved a bar sweep every 120 s for 12 h (during 12 L; resting period) over a period of 4 weeks. After exposure to these conditions, mice received an intraperitoneal injection of either phentolamine (5 mg/kg BW; an α-adrenergic receptor blocker), propranolol (5 mg/kg BW; a β-adrenergic receptor blocker), or vehicle (saline). Serum corticosterone concentration, brain and peripheral cytokine (IL1β, TNFα, and TGFβ) mRNA expression, and body mass were assessed. ASF and CSF significantly elevated serum corticosterone concentrations as well as cytokine mRNA expression levels compared with controls, and mice subjected to CSF had decreased body mass relative to controls. Mice subjected to CSF and treated with phentolamine or propranolol had a greater propensity for a decrease in cytokine gene expression compared with ASF, but effects were tissue-specific. Taken together, these results suggest that both α- and β-adrenergic receptors contribute to the SNS mediation of inflammatory responses, and adrenergic antagonists may effectively mitigate tissue-specific SF-mediated inflammation.

Central α-adrenoceptors contribute to mustard oil-induced central sensitization in the rat medullary dorsal horn

Our previous studies have demonstrated that application of the inflammatory irritant mustard oil (MO) to the tooth pulp produces trigeminal central sensitization that includes increases in mechanoreceptive field size and responses to noxious stimuli and decrease in activation threshold in brainstem nociceptive neurons of trigeminal subnucleus caudalis (the medullary dorsal horn, MDH). The aim of the present study was to test if central noradrenergic processes are involved in the central sensitization of MDH neurons and if α1-adrenoceptors or α2-adrenoceptors or both are involved. In urethane/α-chloralose-anesthetized rats, the activity of extracellularly recorded and functionally identified single nociceptive neurons in the MDH was studied. Continuous intrathecal (i.t.) superfusion of the adrenergic modulator guanethidine and α-adrenoceptor blocker phentolamine or selective α1-adrenoceptor antagonist prazosin over the medulla strongly attenuated all three MO-induced parameters of central sensitization in the MDH nociceptive neurons, compared to phosphate-buffered saline (as vehicle control). In contrast, i.t. superfusion of the selective α2-adrenoceptor antagonist yohimbine had little effect on the mechanoreceptive field expansion and the decreased mechanical activation threshold, and indeed facilitated responses to noxious stimuli of sensitized nociceptive neurons. Superfusion of each of the four chemicals alone did not affect baseline nociceptive neuronal properties. These findings provide the first documentation of the involvement of central noradrenergic processes in MDH in the development of the central sensitization, and that α1- and α2-adrenoceptors may be differentially involved.

The transmitter release pattern of serotonin axons in rabbit brain cortex slices during short pulse trains

The release of serotonin was studied in superfused rabbit brain cortex slices that had been preincubated with [(3)H]serotonin. The slices were stimulated by single electrical pulses or by trains of 4 pulses delivered at 0.011, 1 or 100 Hz. The overflow of tritium elicited by these stimuli was calcium-dependent and tetrodoxin- and Mg(2+)-sensitive. When the superfusion medium contained nitroquipazine or fluvoxamine, the overflow elicited by 4 pulses at 0.011 or 1 Hz was about 2-fold, whereas the overflow elicited by 4 pulses/100 Hz was about 3.7-fold the single pulse-evoked overflow. Metitepin increased little the overflow of tritium elicited by single pulses or 4 pulses/100 Hz, but increased by up to about 150% the overflow evoked by 4 pulses at 1 or 0.011 Hz. Unlabelled serotonin inhibited the evoked overflow in a similar manner, irrespective of the stimulation conditions. When the superfusion medium did not contain serotonin uptake inhibitors, the overflow elicited by a single pulse was too low to be reliably measured. Metitepin increased only slightly the overflow at 4 pulses/0.011 Hz or 4 pulses/100 Hz but increased by up to about 160% the overflow at 4 pulses/1 Hz. The results indicate that the release of serotonin elicited by single pulses as well as by 4 pulses/100 Hz is subject to only a small tonic, stimulation-independent presynaptic autoinhibition, and under these conditions the three pulses following the first one at intervals of 10 ms release about the same amount of transmitter as does pulse No. 1. In contrast, stimulation-dependent presynaptic autoinhibition develops in trains of 4 pulses delivered at 0.011 Hz (only when serotonin re-uptake is blocked) or 1 Hz so that there is a marked fall in the release elicited by each pulse in the course of the train. The total release elicited by such short, autoinhibited trains is dominated by the large response to pulse No. 1. It seems possible that more presynaptic autoreceptors are available for exogenous agonists than for released serotonin.

Serotonergic activity of HP 184: does spontaneous release have a role?

Examination of HP 184, [N-n-propyl)-N-(3-fluoro-4-pyridinyl) -1H-3-methylindodel-1-amine hydrochloride], in a variety of tests for serotonergic activity revealed some unique properties of this compound. We report here that 100 microM HP 184 enhanced spontaneous release of [3H]serotonin (5-HT) from rat hippocampal slices. This release was independent of the uptake carrier. In vivo assays confirmed that HP 184 (20 mg/kg, i.p.) lacked significant interactions at the norepinephrine (NE) or 5-HT uptake carrier itself. Notably, HP 184 (15 mg/kg, i.p.) reduced drinking behavior in schedule-induced polydipsic (SIP) rats. We previously reported that some selective 5-HT reuptake inhibitors decrease SIP 30-40% after a 14-21 day treatment. In the current study, HP 184 decreased SIP beginning with the first treatment, and this reduction (30%) was maintained for 28 days. We further investigated HP 184 and serotonin metabolite levels. One hour after i.p. administration of 30 mg/kg HP 184, the ratio of whole brain 5-hydroxyindolacetic acid (5-HIAA) to 5-HT was increased, suggesting serotonergic activation. Under these conditions, the brain:plasma ratio of HP 184 was approximately 2:1, with brain concentrations of 1.6 micrograms/gram. We speculate that the spontaneous release effects of HP 184 may be responsible for the behavioral effects observed.

Subtype determination of presynaptic alpha 2-autoreceptors in the rabbit pulmonary artery and human saphenous vein

The pharmacological properties of the presynaptic a2-autoreceptors mediating inhibition of noradrenaline release were investigated in human saphenous vein and rabbit pulmonary artery. Segments of these blood vessels were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing uptake1 and uptake2 blockers. The potencies of a2-adrenoceptor antagonists in facilitating (pEC40) the electrically (2 Hz) evoked tritium overflow were determined. The order of potency and potency ratios of a2-adrenoceptor antagonists obtained in our experiments were compared with the corresponding order of affinity and affinity ratios from radioligand binding studies in tissues and cells expressing only one of the alpha 2-adrenoceptor subtypes. In the rabbit pulmonary artery, oxymetazoline was a highly potent agonist at presynaptic a2-adrenoceptors, as reflected by its ability to inhibit at low concentrations the electrically evoked tritium overflow. However, in the human saphenous vein oxymetazoline behaved as a partial agonist, which, in interaction experiments with the a2-adrenoceptor agonist B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), exhibited high potency in antagonizing the inhibitory effect of the latter drug on tritium overflow. Prazosin given alone at concentrations up to 1 mumol/l did not affect tritium overflow. The data obtained with oxymetazoline and prazosin make it very improbable that the a2-autoreceptors on the sympathetic nerves in both tissues are of the a2B- or a2C-subtype. In both blood vessels, rauwolscine given alone was highly potent in facilitating the electrically evoked overflow. In agreement with this, rauwolscine exhibited high potency in antagonizing the inhibitory effect of oxymetazoline on tritium overflow in the rabbit pulmonary artery and of B-HT 920 in the human saphenous vein. The ratio phentolamine/rauwolscine calculated from their potencies in increasing the electrically evoked tritium overflow was also used to discriminate between the various a2-adrenoceptor subtypes. Comparison of this potency ratio with the corresponding affinity ratios for a2-adrenergic binding sites on HT 29 cells, human platelets, bovine pineal gland, rat submaxillary gland, and cell lines transfected with the human a2 genes indicates that in the rabbit pulmonary artery and human saphenous vein the pharmacological characteristics of the autoreceptors conform best to those of a2A-adrenoceptors. Finally, in both blood vessels the potencies of the antagonists BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), rauwolscine, corynanthine, phentolamine, idazoxan, SKF 104078 (6-chloro-9-[(3-methyl-2-butenyl) oxyl]-3-methyl-1H-2,3,4,5-tetrahydro-3-benzazepine), and/or tolazoline in facilitating evoked noradrenaline release was determined. The potencies of these drugs which can discriminate between a2A- and a2D-adrenoceptors (but not between these and a2B/2C-adrenoceptors) were correlated significantly with their affinities for a2A, but not a2D, sites in radioligand binding studies. In conclusion, the present results suggest that the sympathetic nerves of the human saphenous vein and rabbit pulmonary artery are endowed with a2-autoreceptors of the a2A subtype.

In vivo electrophysiological evidence for tonic activation by endogenous noradrenaline of alpha 2-adrenoceptors on 5-hydroxytryptamine terminals in the rat hippocampus

The activation of alpha 2-adrenergic heteroreceptors was studied by comparing the effectiveness of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of CA3 dorsal hippocampus pyramidal neurons prior to, and following, the intravenous administration of noradrenergic agents. Desipramine (2 mg/kg), a selective noradrenaline reuptake blocker, reduced the efficacy of the stimulation; this effect was reversed by the alpha 2-adrenoceptor antagonists yohimbine (0.5 mg/kg) and (-)mianserin (0.5 mg/kg), but not by idazoxan (0.5 mg/kg), an adrenoceptor antagonist with preferential affinity for the imidazoline recognition sites. Low doses of the alpha 2-adrenoceptor agonist clonidine (2 and 10 micrograms/kg) enhanced the efficacy of the stimulation, while high doses (100 and 400 micrograms/kg) reduced it. These incremental and decremental effects of clonidine were reversed by 0.1 and 1 mg/kg of yohimbine, respectively. The enhancing effect of the low dose of clonidine (10 micrograms/kg) was abolished in rats pretreated with the noradrenaline neurotoxin 6-hydroxydopamine. However, the inhibitory effect of a high dose of clonidine (100 micrograms/kg) was unaltered by this pretreatment. These results indicate that low doses of clonidine preferentially activate alpha 2-adrenergic autoreceptors on the noradrenaline neurons resulting in a reduction of the tonic inhibitory effect of endogenous noradrenaline on 5-HT neurotransmission, while higher doses of clonidine would decrease 5-HT neurotransmission through the direct activation of alpha 2-adrenergic heteroreceptors on 5-HT terminals. Furthermore, the selective alpha 2-adrenergic heteroreceptors antagonist (-)mianserin (0.5 mg/kg) increased by itself the efficacy of 5-HT neurotransmission, an effect not observed with yohimbine and idazoxan.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of repeated electroconvulsive shocks on serotonergic neurons

The hypothermia induced by the serotonin (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was attenuated in rats that had received a course of six electroconvulsive shocks (ECS) over a two-week period. The firing activity of dorsal raphe 5-HT neurons, as well as their responsiveness to microiontophoretic applications of 5-HT and 8-OH-DPAT, was unaltered in ECS-treated rats. The electrically evoked overflow of [3H]5-HT from preloaded slices of guinea pig hypothalamus was unchanged after the same ECS treatment. The concentration-effect curves of the 5-HT autoreceptor agonist 5-carboxyamidotryptamine (0.1-100 nM) were similar in slices prepared from control and ECS-treated guinea pigs. In addition, the reduction in the evoked [3H]5-HT overflow obtained by increasing the stimulation frequency from 1 to 5 Hz, which is due to a greater activation of terminal 5-HT autoreceptors at the higher frequency, was not altered by the ECS treatment. The enhancing effects of the 5-HT autoreceptor antagonist methiothepin (0.1-1 microM) and of the 5-HT3 agonist 2-methyl-5-HT (0.1-1 microM) on the evoked [3H]5-HT overflow were unaltered by the ECS treatment. These results thus indicate that repeated ECS attenuates the 8-OH-DPAT-induced hypothermia in rats, as previously reported, but does not affect the firing activity of 5-HT neurons and the sensitivity of their somatodendritic 5-HT1A autoreceptors in the dorsal raphe. The function of 5-HT terminals in the guinea pig hypothalamus was also unaffected by repeated ECS. In conclusion, repeated ECS does not affect the function of 5-HT neurons at the cell body and nerve terminal.

Quantitative evaluation of the autoinhibitory feedback of release of 5-HT in the caudate nucleus of the rabbit where an endogenous tone on α2-adrenoceptors does not exist

Slices of caudate nucleus of the rabbit were preincubated with [3H]serotonin [( 3H]5-HT) in the presence of nomifensine, then superfused and twice stimulated electrically. The stimulation-evoked overflow of tritium, representing action potential-induced, exocytotic release of 5-HT, was inhibited concentration-dependently by 5-HT receptor ligands, the potencies of which were compatible with the assumption of a 5-HT1D-like autoreceptor. The inhibitor of the uptake of 5-HT, 6-nitroquipazine, markedly changed the shape of the concentration-response curve of the 5-HT autoreceptor agonist, 5-carboxamido-tryptamine (5-COHT). The maximum effects of the concentration-response curves of 5-COHT and of exogenous 5-HT became more pronounced in the additional presence of the 5-HT autoreceptor antagonists, metitepin or metergoline. Nonlinear regression analysis of these curves was used to estimate the pKd-value of endogenous 5-HT and the 5-HT biophase concentration at the autoreceptor, in the absence and in the presence of 6-nitroquipazine and in the additional presence of metitepin or metergoline. This revealed a highly operative autoinhibitory feedback, via a 5-HT1D type autoreceptor of release of 5-HT in tissue from the caudate nucleus. Also the inhibition by the alpha 2-adrenoceptor agonists, clonidine and UK-14,304, of release of 5-HT was concentration-dependent. There was neither an enhancement of release by rauwolscine, being a 5-HT autoreceptor agonist and alpha 2-adrenoceptor antagonist, in the presence of metitepin, or by the alpha-adrenoceptor antagonist, phentolamine (10(-6) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin uptake blockers influence serotonin autoreceptors by increasing the biophase concentration of serotonin and not through a "molecular link"

The mechanism of the attenuation, by serotonin uptake blockers, of the release-inhibiting effect of exogenous serotonin autoreceptor agonists was studied in rabbit brain cortex and rat hypothalamus slices. The slices were preincubated with 3H-serotonin and then superfused and stimulated electrically. In rabbit brain slices stimulated by trains of 4 pulses at 100 Hz, 5-carboxamidotryptamine and 5-methoxytryptamine reduced the evoked overflow of tritium, and their concentration-response curves were not changed by any of three serotonin uptake inhibitors, namely citalopram, fluvoxamine and 6-nitroquipazine. In contrast, when the slices were stimulated by trains of 10 pulses at 0.033 Hz, fluvoxamine shifted the concentration-response curve of 5-methoxytryptamine to the right. Experiments with the autoreceptor antagonist metitepine indicated that little, if any, endogenous autoinhibitory tone developed in the course of trains of 4 pulses/100 Hz, irrespective of the absence or presence of uptake inhibitors, as well as during trains of 10 pulses/0.033 Hz in the absence of uptake inhibitors, whereas marked autoinhibition developed when 10 pulses/0.033 Hz were applied in the presence of fluvoxamine. In rat hypothalamic slices stimulated by trains of 4 pulses at 100 Hz, citalopram also failed to change the concentration-response curve of 5-methoxytryptamine. These results indicate that serotonin uptake blockers attenuate the effect of exogenous autoreceptor agonists by an increase in the biophase concentration of released serotonin and, hence, in endogenous autoinhibitory tone, and not by some direct "molecular link" unrelated to the biophase concentration of released serotonin.

Mutual interaction between presynaptic alpha 2-adrenoceptors and 5-HT1B receptors on the sympathetic nerve terminals of the rat inferior vena cava

Segments of the rat vena cava preincubated with [3H]noradrenaline and superfused with [3H]noradrenaline-free solution containing desipramine and corticosterone were stimulated electrically (standard parameters: 150 mA, 0.3 ms, 0.66 Hz; duration 6 min). In some experiments the stimulation parameters were modified in order to obtain similar absolute release values despite the presence of an alpha-adrenoceptor agonist or antagonist or of 5-hydroxytryptamine (5-HT). In a first set of experiments, the vascular segments were first exposed to an alpha-adrenoceptor ligand, which was kept present throughout the remainder of superfusion, and then to 5-HT. The release-inhibiting effect of 5-HT was attenuated by the alpha 2-adrenoceptor agonists clonidine and B-HT 920 whereas it was enhanced by the alpha-adrenoceptor antagonists phentolamine and idazoxan. The alpha 1-adrenoceptor antagonist prazosin did not change the 5-HT-induced inhibition of noradrenaline (NA) release. In a second set of experiments, 5-HT was administered first and kept present in the superfusion fluid for the remainder of the experiment. In the presence of 5-HT, the overflow-inhibiting effects of B-HT 920 and clonidine and the overflow-enhancing effect of idazoxan were reduced. The results demonstrate that activation of one kind of receptor decreased the inhibition of noradrenaline release produced by activation of the other. These effects were not the consequence of the change of release per se induced by the interacting drugs, since they also occurred when the release was adjusted to similar levels by modification of the stimulation parameters.(ABSTRACT TRUNCATED AT 250 WORDS)