Involvement of alpha-receptors in clonidine-induced inhibition of transmitter release from central monoamine neurones

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

[Antidepressants - stimulators for the release of norepinephrine and serotonin (history of creation, study of neurochemical effects and classification)]

The history of the creation and putting into practice of antidepressants and experimental agents - blockers of α₂-adrenergic receptors and serotonin 5-HT₂-receptors is described. The author analyzes the history of development of mianserin, mirtazapine and other drugs and their position in the classification of antidepressants. On the basis of a generalization of historical facts, the rationality of assigning mianserin, mirtazapine, and possibly other compounds similar in chemical structure and mechanism of action to one neurochemical group and its designation by the term 'stimulators of the release of norepinephrine and (presumably) serotonin' is determined.

Noradrenaline Release from Locus Coeruleus Terminals in the Hippocampus Enhances Excitation-Spike Coupling in CA1 Pyramidal Neurons Via β-Adrenoceptors

Release of the neuromodulator noradrenaline signals salience during wakefulness, flagging novel or important experiences to reconfigure information processing and memory representations in the hippocampus. Noradrenaline is therefore expected to enhance hippocampal responses to synaptic input; however, noradrenergic agonists have been found to have mixed and sometimes contradictory effects on Schaffer collateral synapses and the resulting CA1 output. Here, we examine the effects of endogenous, optogenetically driven noradrenaline release on synaptic transmission and spike output in mouse hippocampal CA1 pyramidal neurons. We show that endogenous noradrenaline release enhances the probability of CA1 pyramidal neuron spiking without altering feedforward excitatory or inhibitory synaptic inputs in the Schaffer collateral pathway. β-adrenoceptors mediate this enhancement of excitation-spike coupling by reducing the charge required to initiate action potentials, consistent with noradrenergic modulation of voltage-gated potassium channels. Furthermore, we find the likely effective concentration of endogenously released noradrenaline is sub-micromolar. Surprisingly, although comparable concentrations of exogenous noradrenaline cause robust depression of slow afterhyperpolarization currents, endogenous release of noradrenaline does not, indicating that endogenous noradrenaline release is targeted to specific cellular locations. These findings provide a mechanism by which targeted endogenous release of noradrenaline can enhance information transfer in the hippocampus in response to salient events.

Baclofen as a Potential Analgesic Agent

It is proposed that analgesia might be achieved by pharmacological interruption of sensory input to the C.N.S. at the level of the spinal cord. On the basis of early reports that baclofen mimicked presynaptic inhibitory mechanisms in the spinal cord, baclofen was tested for potential analgesic activity in the mouse hot-plate test. Baclofen appeared to have some antinocisponsive activity in its own right and it potentiated the antinocisponsive effect of morphine. This observation has been confirmed and extended by several authors and the antinocisponsive activity appears to reside in the 1-isomer. Both spinal and supraspinal sites have been suggested for the locus of action in the C.N.S., whilst the underlying cellular mechanism may be inhibition of neurotransmitter release (via a bicuculline-refractory GABA receptor) or antagonism of the post-synaptic actions of substance P as a neurotransmitter.

Baclofen is compared with two other antinocisponsive agents (morphine and clonidine) in terms of its effects on release of neurotransmitters. Consideration is given to the possibility that a further characteristic is shared by these compounds, namely that following chronic administration, abrupt cessation of therapy might be accompanied by withdrawal or ‘rebound’ effects.

Atomoxetine modulates spontaneous and sensory-evoked discharge of locus coeruleus noradrenergic neurons

Atomoxetine (ATM) is a potent norepinephrine (NE) uptake inhibitor and increases both NE and dopamine synaptic levels in prefrontal cortex, where it is thought to exert its beneficial effects on attention and impulsivity. At the behavioral level, ATM has been shown to cause improvements on the measures of executive functions, such as response inhibition, working memory and attentional set shifting across different species. However, the exact mechanism of action for ATM's effects on cognition is still not clear. One possible target for the cognitive enhancing effects of ATM is the noradrenergic locus coeruleus (LC), the only source of NE to key forebrain areas such as cerebral cortex and hippocampus. Although it is known that ATM increases NE availability overall by blocking reuptake of NE, the effects of this agent on impulse activity of LC neurons have not been reported. Here, the effect of ATM (0.1-1 mg/kg, ip) on NE-LC neurons was investigated by recording extracellular activity of LC neurons in isoflurane-anesthetized rats. ATM caused a significant decrease of the tonic activity of LC single-units, although leaving intact the sensory-evoked excitatory component of LC phasic response. Moreover, the magnitude of the inhibitory component of LC response to paw stimulation was increased after 1 mg/kg of ATM and its duration was prolonged at 0.3 mg/kg. Together, these effects of ATM produced an increase in the phasic-to-tonic ratio of LC phasic response to sensory stimulation. ATM also modulated the average sensory-evoked local field potential (LFP) and spike-field coherence in LC depending on the dose tested. The lower dose (0.1 mg/kg) significantly decreased early positive and negative components of the sensory-evoked LFP response. Higher doses (0.3-1 mg/kg) initially increased and then decreased the amplitude of components of the evoked fields, whereas the spike-field coherence was enhanced by 1 mg/kg ATM across frequency bands. Finally, coherence between LC fields and EEG signals was generally increased by 1 mg/kg ATM, whereas 0.1 and 0.3 mg/kg respectively decreased and increased coherence values in specific frequency bands. Taken together these results suggest that ATM effects on LC neuronal activity are dose-dependent, with different doses affecting different aspects of LC firing. This modulation of activity of LC-NE neurons may play a role in the cognitive effects of ATM. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Enhancement of serotonergic and noradrenergic neurotransmission in the rat hippocampus by sustained administration of bupropion

RATIONALE

Previous studies reported that bupropion, an effective antidepressant, exerts modulatory actions on serotonin (5-HT) and norepinephrine (NE) neurons.

OBJECTIVES

This study examined effects of bupropion administration on 5-HT and NE neurotransmission in hippocampus.

METHODS

Electrophysiological recordings were obtained from anesthetized Sprague-Dawley rats. Subcutaneously implanted minipumps delivered saline or bupropion (30 mg/kg/day) for 2 and 14 days.

RESULTS

Although sustained bupropion administration did not alter the sensitivity of 5-HT(1A) and α₂-adrenergic receptors, the tonic activation of postsynaptic 5-HT(1A) receptors by endogenous 5-HT was enhanced in 14-day bupropion-treated rats to a greater extent than in the 2-day and control rats, as revealed by the greater disinhibitory action of the 5-HT(1A) antagonist WAY-100635 on hippocampus pyramidal neurons. The function of terminal 5-HT(1B) autoreceptors was not changed as determined by the unaltered effectiveness of different frequencies of stimulation of the ascending 5-HT fibers. The function of α₂-adrenergic receptors on 5-HT terminals was, however, diminished, as indicated by the lesser effect of the α₂-adrenoceptor agonist clonidine. Tonic activation of postsynaptic α₂- and α₁-adrenoceptors by endogenous NE was also increased in 14-day bupropion-treated rats, as indicated by the greater effect of the α₂- and α₁-adrenoceptor antagonists idazoxan and prazosin, respectively, on pyramidal firing. The function of terminal α₂-adrenergic autoreceptors was attenuated since increasing frequency of stimulation of the ascending NE pathway produced a lesser degree of suppression of pyramidal neurons in rats administered bupropion than the control.

CONCLUSION

Enhancement of 5-HT and NE transmissions in hippocampus by prolonged bupropion may account for its effectiveness in major depression.

Addition of the alpha2-antagonist yohimbine to fluoxetine: effects on rate of antidepressant response

Electrophysiological studies suggest that alpha2-adrenoceptors profoundly affect monoaminergic neurotransmission by enhancing noradrenergic tone and serotonergic firing rates. Recent reports suggest that alpha2-antagonism may hasten and improve the response to antidepressant medications. To test this hypothesis, a randomized double-blind controlled trial was undertaken to determine if the combination of an alpha2-antagonist (yohimbine) with a selective serotonin reuptake agent (SSRI) (fluoxetine) results in more rapid onset of antidepressant action than an SSRI agent alone. In all, 50 subjects with a DSM-IV diagnosis of major depressive disorder confirmed by SCID interview were randomly assigned to receive either fluoxetine 20 mg plus placebo (F/P) or fluxetine 20 mg plus a titrated dose of yohimbine (F/Y). The yohimbine dose was titrated based on blood pressure changes over the treatment period, in a blind-preserving manner. Hamilton depression scale ratings (HDRS) and clinical global impression (CGI) ratings were obtained weekly over a period of 6 weeks. The rate of achieving categorical positive responses was significantly more rapid in the F/Y group compared to the F/P group using both the HDRS and the CGI scales as outcome measures in a survival analysis using a log-rank test (chi2(1) = 5.86, p = 0.016 and chi2(1) = 5.29, p = 0.021, respectively). At the last observed visit, 18 (69%) of the 26 F/Y subjects met the response criteria for CGI compared to 10 (42%) of 24 F/P subjects. Using the HDRS criteria, 17 (65%) of 26 F/Y subject vs 10 (42%) of 24 F/P subjects were responders. The addition of the alpha2-antagonist yohimbine to fluoxetine appears to hasten the antidepressant response. There is also a trend suggesting an increased percentage of responders to the combined treatment at the end of the 6-week trial.

Depression as a spreading adjustment disorder of monoaminergic neurons: a case for primary implication of the locus coeruleus

A model for the pathophysiology of depression is discussed in the context of other existing theories. The classic monoamine theory of depression suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. More recent elaborations of the classic theory also implicitly include this postulate, other theories of depression frequently prefer to depart from the monoamine-based model altogether. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity, and concomitantly, exaggerated responses to acute increases in the presynaptic firing rate and transmitter release. It is proposed that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus (LC). Dysregulation of the LC projection activities may lead in turn to dysregulation of serotonergic and dopaminergic neurotransmission. Failure of the LC function could explain the basic impairments in the processing of novel information, intensive processing of irrational beliefs, and anxiety. Concomitant impairments in the serotonergic neurotransmission may contribute to the mood changes and reduction in the mesotelencephalic dopaminergic activity to loss of motivation, and anhedonia. Dysregulation of CRF and other neuropeptides such as neuropeptide Y, galanin and substance P may reinforce the LC dysfunction and thus further weaken the adaptivity to stressful stimuli.

Pharmacological evidence for inhibition of ACTH secretion by a central adrenergic system in the dog

To obtain more information about the transmitter involved in catecholaminergic inhibition of ACTH secretion, the site of this inhibition, and the receptors involved, the secretion of ACTH was studied in pentobarbital-anesthetized dogs that were surgically stressed and treated with drugs which modify central catecholaminergic transmission. The index of ACTH secretion was adrenal venous output of corticoid hormones. Intravenous L-dopa inhibited ACTH secretion, and this inhibition was not modified by blockade of peripheral decarboxylation of L-dopa with carbidopa. Intravenous or centrally administered clonidine inhibited stress-induced ACTH secretion, whereas centrally administered apomorphine did not. When given into the third ventricle, phenoxybenzamine (but not phentolamine) blocked the inhibitory effect of L-dopa and clonidine. Pimozide had no effect. L-propranolol caused a small but significant decrease in stress-induced ACTH secretion. Intraventricular procaine blocked the stress response. The data support the conclusion that the site of catecholaminergic inhibition of ACTH secretion is central, 'inside the blood-brain barrier', instead of the pituitary or the median eminence. They indicate that dopamine is not the mediator involved, and suggest that it is probably norepinephrine, although epinephrine is not ruled out. The receptor on which the released catecholamines act, presumably on the surface of the cells that secrete the hypothalamic hormone that regulates ACTH secretion, appears to be a type of alpha-adrenergic receptor.

Alpha2-adrenoceptors modulating neuronal serotonin release: a study in alpha2-adrenoceptor subtype-deficient mice

1. The release-inhibiting alpha2-adrenoceptors of cerebral serotoninergic axons were studied in mice. Slices of the hippocampus or the occipito-parietal cortex from NMRI mice, from mice lacking the alpha2A/D-, the alpha2B-, the alpha2C- or both the alpha2A/D- and the alpha2C-adrenoceptor, and from mice sharing the genetic background of the receptor-deficient animals (WT) were preincubated with [3H]-serotonin and then superfused and stimulated electrically, in most experiments by trains of 8 pulses at 100 Hz. 2. The concentration-response curves of the alpha2-adrenoceptor agonist medetomidine were virtually identical in hippocampal slices from NMRI and WT mice, with maximally 70% inhibition and an EC50 of about 2 nM. In hippocampal slices from NMRI mice, phentolamine and rauwolscine were equipotent antagonists against medetomidine. 3. The effect of medetomidine was greatly reduced, with maximally 20% inhibition, in hippocampal slices from alpha2A/D-adrenoceptor-deficient mice; was slightly reduced, with maximally 59% inhibition, in hippocampal slices from alpha2C-adrenoceptor-deficient mice; was not changed in hippocampal slices from alpha2B-adrenoceptor-deficient mice; and was abolished in hippocampal slices from mice lacking both the alpha2A/D- and the alpha2C-adrenoceptor. 3. Similar results were obtained in: (i) occipito-parietal slices from NMRI and alpha2A/D-adrenoceptor-deficient mice and (ii) hippocampal slices that were preincubated with [3H]-serotonin in the presence of oxaprotiline to rule out cross-labelling of noradrenergic axons. 5. The serotoninergic axons of the mouse brain possess both alpha2A/D-heteroreceptors, which predominate, and alpha2C-heteroreceptors but lack alpha2B-adrenoceptors. The situation resembles the coexistence of alpha2A/D- and alpha2C-autoreceptors but lack of alpha2B-autoreceptors at the noradrenergic axons of mice.

Modulation of 5-hydroxytryptamine release in hippocampal slices of rats: effects of fimbria-fornix lesions on 5-HT1B-autoreceptor and alpha2-heteroreceptor function

Fimbria-fornix lesions disrupt important parts of serotonergic and noradrenergic hippocampal afferents and elicit sprouting of sympathetic fibers from the superior cervical ganglion. Since 5-hydroxytryptamine (5-HT) release in the hippocampus is modulated by 5-HT1B auto- and alpha2-heteroreceptors, we investigated whether such lesions may alter these presynaptic mechanisms. Hippocampal slices of sham-operated (SHAM) and fimbria-fornix-lesioned (LES) rats (14 months after surgery) were preincubated with [3H]5-HT, superfused continuously, and stimulated electrically using two stimulation conditions: either (a) 360 pulses 3 Hz, or (b) 20 pulses 100 Hz (2 ms, 28 mA, 4 V/chamber). The amount of [3H]5-HT taken up by slices from LES rats was significantly reduced, whereas the evoked 5-HT release (in percent of tissue-3H) was unchanged compared to that of SHAM rats. The 5-HT1B agonist CP 93,129 or the alpha2-agonist UK 14,304 reduced the evoked 5-HT release more potently in slices from LES rats, but only using stimulation condition (a), which permits inhibition by endogenously released transmitters. In LES rats, the facilitatory effect of the 5-HT antagonist metitepine was weaker, whereas that of the alpha2-antagonist idazoxane was more pronounced than in SHAM rats. In LES rats, hippocampal 5-HT content was reduced to about 45% of SHAM levels, whereas that of noradrenaline was increased by about 30% (high-performance liquid chromatography). We conclude: (1) despite LES-induced changes in tissue levels of endogenous ligands, there is no down- or upregulation of 5-HT1B-autoreceptors or alpha2-heteroreceptors on serotonergic neurons in the denervated rat hippocampus. (2) The reduced endogenous autoinhibition (by 5-HT) seems to be compensated for by an increased heteroinhibition (by noradrenaline).

Distribution of alpha 1a-, alpha 1b- and alpha 1d-adrenergic receptor mRNA in the rat brain and spinal cord

The technique of in situ hybridization with specific ribonucleotide probes was used to determine the distribution patterns of mRNA encoding the alpha 1a-, alpha 1b- and alpha 1d-adrenoceptor (AR) subtypes in rat brain and spinal cord. The expression pattern of alpha 1a-AR mRNA has not been reported previously, and was found to be widespread throughout the rat central nervous system. High levels were found in regions of the olfactory system, several hypothalamic nuclei, and regions of the brainstem and spinal cord, particularly in areas related to motor function. Regions expressing moderate levels of mRNA for this receptor were the septum, bed nucleus of the stria terminalis, cerebral cortex, amygdala, cerebellum and pineal gland. Low expression levels were detected in the hippocampal formation. Most nuclei in the basal ganglia and thalamus expressed extremely low or undetectable levels of alpha 1a-AR mRNA. The expression patterns of the alpha 1b- and alpha 1d-AR mRNAs were similar to those described using oligonucleotide probes in earlier studies. High expression of alpha 1b-AR mRNA was noted in the pineal gland, most thalamic nuclei, lateral nucleus of the amygdala and dorsal and median raphe nuclei. Moderate expression levels were noted throughout the cerebral cortex, and in some olfactory, septal, and brainstem regions. The distribution of alpha 1d-AR mRNA was the most discrete of the three receptors examined. Expression was strong in the olfactory bulb, cerebral cortex, hippocampus, reticular thalamic nucleus, regions of the amygdala, motor nuclei of the brainstem, inferior olivary complex and spinal cord. Comparison of the distributions of the alpha 1a-, alpha 1b- and alpha 1d-AR mRNA suggests unique functional roles for each of these receptors.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Clonidine-induced emesis: a multitransmitter pathway concept

The emetic effect of clonidine injected into the cerebral ventricles through chronically implanted cannulae was investigated in unanaesthetized cats. Clonidine (0.1-300 micrograms) induced dose-dependent and shortlasting emesis. The emesis induced by the supramaximal dose of clonidine (100 micrograms) was not abolished after the ablation of area postrema. Both the alpha 2 adrenoceptor blocking agent idazoxan and the mixed alpha 1 and alpha 2 adrenoceptor antagonist phenoxybenzamine, injected intracerebroventricularly, attenuated or abolished the emesis induced by clonidine (100 micrograms). On the other hand, the alpha 2 adrenoceptor blocking agent yohimbine, the alpha 1 adrenoceptor blocking drug prazosin and the non-selective beta-adrenoceptor antagonist propranolol, injected into the cerebral ventricles, had no significant effect on clonidine-induced emesis. The antimuscarinic drug atropine injected into the cerebral ventricles prevented the clonidine-induced emesis in a dose-dependent manner. The dopamine antagonist chlorpromazine, the 5-hydroxytryptamine blocking agent methysergide and the histamine H1 and H2 receptor antagonists, antazoline and cimetidine, injected intracerebroventricularly reduced or abolished the emesis produced by clonidine. The ganglionic blocking substance mecamylamine and the opioid antagonist naloxone, all injected into the cerebral ventricles, had no significant effect on clonidine-induced emesis. In cats pretreated with the intracerebroventricular competitive inhibitor of the synthesis of catecholamines, alpha-methyl-p-tyrosine, as well as with the inhibitor of acetylcholine synthesis hemicholinium-3, the emesis caused by clonidine was depressed or abolished. The clonidine-induced emesis was also abolished when catecholamine stores were depleted by intracerebroventricular reserpine. However, the clonidine-induced emesis was not significantly changed when 5-hydroxylryptaminergic nerve terminals were damaged by 5,6-dihydroxytryptamine. It follows, therefore, that cholinergic and noradrenergic mechanisms are of basic importance for the emetic action of clonidine. With regard to receptors, the emesis induced by clonidine injected into the cerebral ventricles, is mediated at least in part through alpha-adrenoceptors, muscarinic cholinoceptors, 5-hydroxytryptamine receptors and H1 and H2 histamine receptors. These receptors appear to be located mostly presynaptically and they transmit emetic impulses to neurones integrating them into emesis. However, the direct effect of clonidine on postsynaptic receptors cannot be excluded, particularly when muscarinic and 5-hydroxytryptamine receptors are implicated. Taken together, these results point to the existence of a multitransmitter pathway/s outside the area postrema, subserving the central regulation of emesis.

Changes in noradrenergic sensitivity to tumor necrosis factor-alpha in brains of rats administered clonidine

Tumor necrosis factor-alpha (TNF alpha) and the imidazoline clonidine modulate norepinephrine (NE) release from noradrenergic nerve terminals in the central nervous system. The present study demonstrates an intrinsic association between presynaptic alpha 2-adrenergic receptor sensitivity and TNF alpha responsiveness in governing this NE release. Superfusion and electrical field stimulation were applied to a series of rat hippocampal brain slices in order to study the regulation of [3H]-NE release. The alpha 2-adrenergic agonist clonidine and the cytokine TNF alpha concentration-dependently inhibit [3H]-NE release; whereas, the alpha 2-adrenergic antagonist idazoxan potentiates [3H]-NE release. The fractional release of [3H]-NE during field stimulation of control hippocampal slices was decreased by the addition of TNF alpha in a concentration-dependent manner, an effect which was potentiated by the alpha 2-adrenergic antagonist idazoxan; whereas, TNF alpha attenuated the concentration-dependent potentiating effect of idazoxan. Furthermore, constitutive TNF alpha, demonstrated to be present in several brain areas, was significantly decreased following administration of the alpha 2-adrenergic agonist clonidine (0.6 mg/kg, i.p., twice daily) to rats for either 1 or 14 days, without a change in TNF alpha mRNA accumulation. We next investigated whether the presynaptic sensitivity to TNF alpha was changed after clonidine administration to rats. TNF alpha enhanced, rather than inhibited, [3H]-NE release after 1 day of clonidine administration, while a suppressed sensitivity to TNF alpha was observed in the hippocampus after 14 days of clonidine administration. In addition, in the presence of idazoxan, TNF alpha potentiation of [3H]-NE release after 1 day clonidine administration was reversed to a decreased inhibition as compared to control slices exposed to idazoxan. Therefore, the temporary reversal in the presynaptic TNF alpha response after 1 day of clonidine administration illustrates a mechanism of action for its persistent antihypertensive effect, its transient sedative and antihyperpathic effects, and its acute ability to promote antidepressants. These results demonstrate a novel role for an immune mediator in the central nervous system, and demonstrates that presynaptic TNF alpha responsiveness is intimately associated with adrenergic receptor sensitivity.

The effects of clonidine on blood pressure, catecholamine and growth hormone release in hypogonadal men is preserved and not influenced by testosterone replacement therapy

It has been demonstrated that castration impairs the hypotensive effect of clonidine in rat as well as its GH-releasing activity while testosterone replacement restores to normal the effects of alpha-2 adrenoceptor activation. Thus, these data point to main role of the gonadal steroid testosterone in modulating the effects of alpha-2 adrenergic activation on blood pressure, catecholamine and GH release in animal. Aim of the present study was to verify the activity of clonidine on blood pressure, catecholamine and GH release in human male hypogonadism before and after testosterone replacement. To this goal, 14 hypogonadal men (HP, age 33.8 +/- 2.9 yr; BMI < 25 kg/m2; 8 with hypergonadotropic and 6 with hypogonadotropic hypogonadism) received clonidine administration (CLON, 300 micrograms po at 0 min) before and after 3 months of testosterone replacement (testosterone propionate depot, 250 mg i.m. every 21 days). Ten normal adult volunteers (NS, age 31.5 +/- 1.9 yr; BMI < 25 kg/m2) were studied as control group. In all subjects, before and after clonidine administration, systolic and diastolic blood pressure (SBP and DBP), pulse rate (PR), norepinephrine (NE), epinephrine (E) and GH levels were recorded. In HP basal testosterone levels were lower than those in NS (1.25 +/- 0.3 vs 7.34 +/- 1.5 ng/ml, p < 0.05) and were restored to normal by hormonal replacement (6.91 +/- 1.3 ng/mL) in HP, both SBP and DBP as well as PR were normal in basal conditions and were not modified by testosterone replacement. Both before and during testosterone CLON lowered SBP, DBP and PR in HP to the same extent observed in NS. In HP, basal NE levels were lower than those in NS (0.85 +/- 0.15 vs 1.28 +/- 0.19 nmol/l, p < 0.05) and were restored to normal during testosterone replacement (1.25 +/- 0.13 nmol/l). On the other hand, basal E levels in HP were similar to those in NS (179 +/- 42 vs 197 +/- 38 pmol/l) and were not modified by testosterone therapy (167 +/- 28 pmol/l). In HP, both before and during testosterone replacement, CLON reduced NE (0.44 +/- 0.10 and 0.58 +/- 0.07 nmol/l) levels to the same levels recorded in NS (0.68 +/- 0.08 nmol/l). Basal GH and IGF-I levels in HP (1.15 +/- 0.5 and 234 +/- 42 micrograms/l, respectively) were similar to those in NS (1.18 +/- 0.4 and 221 +/- 38 micrograms/l, respectively) and were not modified by testosterone (1.35 +/- 0.6 and 256 +/- 32 micrograms/l, respectively). CLON administration induced a clear GH response in HP (F = 37; p < 0.001) which overlapped with that recorded in NS and was not modified by testosterone (F = 1.7; P = NS). Our present findings demonstrate that, differently from in animal, in man testosterone has no role in modulating the effects of alpha-2 adrenergic activation by clonidine on blood pressure, catecholamine and GH release. On the other hand, our data suggest the existence in male hypogonadism of a reduced basal noradrenergic activity which is restored by testosterone replacement.

Depression as a spreading neuronal adjustment disorder

The outlines of a theory of the pathophysiology of depression are presented. The classic monoamine theory of depression as well as its more recent elaborations suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity and, concomitantly, exaggerated responses to acute increases in presynaptic firing rate and transmitter release. We propose that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus, which in turn leads to dysregulation of 5-HT-ergic and dopaminergic neurotransmission.

Changes in the regulation of 5-hydroxytryptamine release by alpha2-adrenoceptors in the rat hippocampus after long-term desipramine treatment

In vivo microdialysis was used to measure the effects of long-term treatment of rats with desipramine upon the regulation by alpha2-adrenoceptors of serotonin (5-hydroxytryptamine, 5-HT) release from the serotonergic neurons in the hippocampus. Rats were injected with saline or desipramine, 10 mg/kg, i.p., every 12 h for 14 days. When added to the perfusion solution, brimonidine, an alpha2-adrenoceptor agonist, significantly inhibited the K+-evoked release of 5-HT in the hippocampi of saline-treated, control rats. This action of brimonidine was prevented by pretreating the rats with idazoxan, an alpha2-adrenoceptor antagonist. Long-term desipramine treatment significantly reduced the inhibitory effect of brimonidine upon the K+-evoked 5-HT release. With long-term administration of desipramine, noradrenaline content in the hippocampi was significantly decreased as compared with that of the control rats, whereas the basal noradrenaline concentration in the dialysate was significantly increased. On the other hand, both the 5-HT content of the hippocampus and the basal 5-HT concentration in the dialysate were significantly increased. The present study suggests that long-term administration of desipramine causes a functional subsensitivity of the presynaptic alpha2-adrenoceptors that regulate serotonergic neuronal function in the rat hippocampus. It also supports the concept that changes in the sensitivity of alpha2-adrenoceptors that regulate neurotransmitter release play an important role in the mechanism of antidepressant drug action.

Pharmacological characterization of alpha 2-adrenoceptor regulated serotonin release in the rat hippocampus

The purpose of the present study was to confirm the functional regulation by alpha 2-adrenoceptors of the release of serotonin (5-HT) from the rat hippocampus in vivo. Under several pharmacological conditions, extracellular levels of 5-HT were estimated by assaying its concentrations in the perfusate by high performance liquid chromatography with electrochemical detection. Extracellular 5-HT in the hippocampus was reduced by tetrodotoxin (10 microM) co-perfusion, but increased by perfusion of a selective 5-HT re-uptake inhibitor, fluoxetine (10 microM). Addition of potassium (K+, 120 mM) to the perfusion fluid evoked an approximately 3-fold increase in 5-HT release. When the alpha 2-adrenoceptor agonist UK14,304 (0.1-10 microM) was added to the perfusion solution, the K(+)-evoked 5-HT release was significantly inhibited in a concentration-dependent manner. This inhibitory action of UK14,304 was reversed by pretreatment with an alpha 2-adrenoceptor antagonist, idazoxan (5 mg/kg, i.p.). In rats which were catecholaminergically denervated with 6-hydroxydopamine, UK14,304 (10 microM) still inhibited the K(+)-evoked 5-HT release. Treatment with pertussis toxin (PTX) did not alter the K(+)-evoked release of 5-HT but abolished the inhibitory effect of UK14,304. These findings suggest that 5-HT release is functionally modulated via alpha 2-adrenoceptors located on the serotonergic nerve terminals in the rat hippocampus and furthermore, the possibility that the inhibitory of alpha 2-adrenoceptors is linked to G-proteins which are substrates of PTX.

Receptor binding and functional evidence suggest that postsynaptic alpha 2-adrenoceptors in rat brain are of the alpha 2D subtype

This study has determined the subtype(s) of postsynaptic alpha 2-adrenoceptors in rat brain. This question has been addressed by using two separate approaches, i.e. ligand displacement of [3H]2-(2-methoxy)-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX 821002) from membranes prepared from rat cortex after noradrenergic denervation and, secondly, by antagonism of clonidine-induced mydriasis. After rats had been lesioned using N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4; 100 mg/kg i.p., 30 min after zimeldine 10 mg/kg i.p.), noradrenaline was undetectable in the cortex 3 days later. Displacement of [3H]RX 821002 with a range of agonists and antagonists which distinguish between the known alpha 2-adrenoceptor subtypes (alpha 2A-2D) yielded pKi values which correlated very well with reported values for the alpha 2D-adrenoceptor (r = 0.929; P < 0.001), but not the alpha 2A (r = 0.450; P = 0.192), alpha 2B (r = 0.280, P = 0.434) or alpha 2C (r = 0.283; P = 0.460) subtypes. Similarly, the potencies of various alpha 2-adrenoceptor antagonists to inhibit clonidine (0.03 mg/kg i.p.)-induced mydriasis in conscious rats correlated strongly with their pKi values for alpha 2D-adrenoceptors (r = 0.899; P = 0.015) but not alpha 2A-(r = 0.369; P = 0.472), alpha 2B-(r = -0.224; P = 0.670) or alpha 2C-adrenoceptors (r = 0.253; P = 0.584). These data are, therefore, consistent and argue strongly that postsynaptic alpha 2-adrenoceptors in the rat cortex and Edinger-Westphal nucleus are of the alpha 2D subtype.

Differential inhibition of serotonin release by 5-HT and NA reuptake blockers after systemic administration

The inhibition of serotonin (5-HT) release produced by antidepressants varying in relative selectivity for blocking uptake of 5-HT and noradrenaline (NA) was compared. Release was measured by microdialysis in anesthetized rats with nerve terminal 5-HT uptake inhibited by local infusion of citalopram (1 microM) through a dialysis probe in hippocampus. With 5-HT uptake first blocked in hippocampus, systemic injection of uptake inhibitors produced decreases in dialysate 5-HT, presumably due to autoreceptor stimulation in the raphe. The largest decreases (about 60-70%) in 5-HT were produced by the selective 5-HT uptake inhibitors sertraline, paroxetine and citalopram. Nonselective blockers caused less suppression of release. Thus, the maximum decrease in 5-HT was 35% after clomipramine, a less selective 5-HT uptake inhibitor, and < or = 30% after the nonselective 5-HT/NA uptake blockers imipramine and amitriptyline, 5-HT was not decreased after maprotiline, a selective NA uptake blocker. Pretreatment with (+)WAY100135 to block 5-HT1A autoreceptors, abolished the inhibition of 5-HT release produced by systemic sertraline, clomipramine and imipramine. One explanation for the difference between selective and nonselective inhibitors with respect to central 5-HT release, is the excitatory effect of (alpha 1) adrenergic receptor stimulation on 5-HT neuronal discharge. However, pretreatment with alpha-methyl-p-tyrosine to deplete NA, did not influence the inhibition of 5-HT release produced by imipramine.

Quantification of presynaptic alpha 2-adrenoceptors in rat brain after short-term DSP-4 lesioning

The relative numbers of pre- and postsynaptic alpha 2-adrenoceptors were determined in various rat brain regions after short-term DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) lesioning. For these studies, rats pretreated with zimeldine (10 mg/kg i.p.) were injected with DSP-4 (100 mg/kg i.p.) and were killed either 3 or 15 days later. At the 3 day time-point, DSP-4 treatment produced marked reductions in the noradrenaline content of the cortex (93%), hippocampus (89%), hypothalamus (83%) and cerebellum (92%) with no change in the levels of dopamine or 5-HT. This treatment also decreased the number of alpha 2-adrenoceptors labelled with [3H]idazoxan in the cortex (20%), hippocampus (18%), cerebellum (24%) and hypothalamus (39%). Fifteen days after DSP-4 lesioning, the marked reductions of noradrenaline were sustained in the cortex, hippocampus and cerebellum, but there was a considerable reversal of the effect of DSP-4 in the hypothalamus. At this time-point, the decrease in alpha 2-adrenoceptors was attenuated in cortex (4%) and cerebellum (0%) and their number was increased in hippocampus (8%) and hypothalamus (7%). Together, the data argue that presynaptic alpha 2-adrenoceptors comprise approximately 20% of the total alpha 2-adrenoceptor population in the cortex, hippocampus and cerebellum, but about 40% of it in the hypothalamus. Furthermore, they also demonstrate that although the number of presynaptic alpha 2-adrenoceptors in rat brain can be determined by the reduction of radioligand-receptor binding shortly after DSP-4 lesioning, this effect is rapidly masked by receptor proliferation in response to noradrenergic denervation.

Distributions of mRNAs for alpha-2 adrenergic receptor subtypes in rat brain: an in situ hybridization study

Selective 35S-labeled oligonucleotide probes were designed to sequences of the rat alpha-2A (RG20), alpha-2B (RNG), and alpha-2C (RG10) adrenoreceptor mRNAs for use in in situ hybridization experiments on sections of unfixed rat brain, spinal cord and kidney. After hybridized sections were exposed to film or dipped in autoradiographic emulsion, specific and selective labeling patterns characteristic for each probe and region of the central nervous system were observed. Alpha-2A mRNA labeling was most pronounced in neurons in layer six of the cerebral cortex, hypothalamic paraventricular nucleus, reticular thalamic nucleus, pontine nuclei, locus coeruleus, vestibular nuclei, trapezoid nuclei, deep cerebellar nuclei, nucleus tractus solitarii, ventrolateral medullary reticular formation, and the intermediolateral cell column of the thoracic spinal cord. In some of these locations, the receptor mRNA, in all probability, is present in noradrenaline and perhaps adrenaline neurons. The alpha-2B probe, which primarily labels the kidney, gave only a very light signal in the thalamus in the central nervous system after extended exposure times. Alpha-2C mRNA labeling was primarily observed in the olfactory bulb, cerebral cortex, islands of Calleja, striatum, hippocampal formation, cerebellar cortex, and dorsal root ganglia. Labeling patterns disappeared when excess unlabeled probes were added to their respective radiolabeled probes, or when sense probes were employed. When a hybrid antisense probe homologous to all three alpha-2 probes was used, labeling patterns also disappeared. The present study therefore justifies the pharmacological subclassification of alpha-2 receptors by providing anatomical evidence for specific and selective cell groups in the rat central nervous system containing mRNA for three alpha-2 receptor subtypes.

Development of monoamine systems after neonatal anoxia in rats

Neurochemical and morphological effects of neonatal anoxia on monoamine systems were studied after 100% N2 exposure for 25 min at 30 h postnatally (postnatal day 2-P2). At 20 min after anoxia, reductions of tissue levels of cerebellar noradrenaline (NA) and striatal dopamine (DA) and metabolites were seen, while 5-hydroxyindoleacetic acid (5-HIAA) was increased in cortex and cerebellum. At P7, NA increased in cerebellum, while serotonin (5-HT) and 5-HIAA decreased in cortex and cerebellum. At P21, increased hippocampal NA and striatal homovanillic acid (HVA) were found, while striatal 5-HT decreased and 5-HIAA increased in striatum and hippocampus. At P60, striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-HIAA levels were found to be enhanced. No effects were seen on 5-HT, tyrosine hydroxylase, or DARPP-32 immunostaining in cortex, hippocampus, and striatum. Thus, the neonatal anoxia induced both acute and persistent neurochemical abnormalities in monoamine systems that were not accompanied by morphological changes detectable with the methods used. The monoamine alterations found could be critically connected to the behavioral disturbances observed in rats after neonatal anoxia. The findings may also be of relevance to dysfunctions seen in humans after perinatal oxygen deficiency, e.g., the attention deficit hyperactivity disorder syndrome.

Effect of clonidine on the release of serotonin from the rat hippocampus as measured by microdialysis

The purpose of the present study is to clarify the effect of clonidine on the release of serotonin from the rat hippocampus in vivo. For this purpose, endogenous serotonin release was measured by brain microdialysis. Potassium-evoked serotonin release from the hippocampus of freely moving rats was significantly inhibited when clonidine (10(-5) M) was added to the perfusion solution, while the 5-hydroxyindoleacetic acid output remained unchanged. In catecholaminergically denervated rats, clonidine (10(-5) M) also inhibited the potassium-evoked serotonin release from the hippocampus and the 5-hydroxyindoleacetic acid output was unaffected by clonidine. These results suggest that the inhibitory effect of clonidine on serotonin release from the hippocampus might reflect the activation of alpha 2-adrenoceptors which are localized on the serotonergic nerve terminals.

Role of cholinergic and adrenergic pathways of the medial septal area in the control of water intake and renal excretion in rats

In this study, we investigated an interaction between noradrenergic and cholinergic pathways of the medial septal area (MSA) on the control of water intake and urinary electrolyte excretion by means of injection of their respective agonists. Noradrenaline (a nonspecific alpha-adrenergic agonist) and clonidine (an alpha 2-adrenergic agonist), but not phenylephrine (an alpha 1-adrenergic agonist), induced natriuresis and kaliuresis. alpha-Adrenergic activation had no effect on the natriuresis and kaliuresis induced by carbachol (a cholinergic agonist) and it inhibited the antinatriuresis and antikaliuresis induced by isoproterenol (a beta-adrenergic agonist). Interactions related to volume excretion are complex, alpha-Adrenergic activation induced a mild diuresis and inhibited the antidiuresis induced by isoproterenol, but phenylephrine combined with carbachol induced antidiuresis. The water intake induced by carbachol was inhibited by clonidine and noradrenaline, but not phenylephrine. These results show an asymmetry in the interaction between alpha-adrenergic and cholinergic receptors concerning water intake and electrolyte excretion.

?2-Adrenoceptor modulation of rat ventral hippocampal 5-hydroxytryptamine release in vivo

The putative existence of a functional alpha 2-adrenoceptor-mediated modulation of 5-HT release in vivo from serotonergic neuronal terminals in the ventral hippocampus was investigated using intracerebral microdialysis in chloral hydrate-anaesthetised rats. The alpha 2-adrenoceptor agonist clonidine (0.01-0.3 mg/kg, SC) dose-dependently decreased the 5-HT output. The response to clonidine was antagonized by systemic or local administration of the alpha 2-adrenoceptor antagonist idazoxan (0.1 mg/kg, SC, or 10 mumol/l, via the dialysis perfusion medium). Similarly, the 5-HT release-suppressing response to the thiazole alpha 2-adrenoceptor agonist jingsongling (0.1 mg/kg, SC) was blocked by idazoxan (0.1 mg/kg, SC). The mixed beta-adrenoceptor/5-HT1 receptor antagonist pindolol (8.0 mg/kg, SC) did not affect the clonidine-induced reduction of 5-HT release. Tyrosine hydroxylase inhibition by means of alpha-methyl-para-tyrosine (alpha-MT; 250 mg/kg, IP) caused a drastic reduction (greater than 80%) in dialysate 3,4-dihydroxyphenyl acetic acid (DOPAC) output but did not affect the 5-HT output per se over 3 h post-injection. Nor did the alpha-MT pretreatment prevent, but instead significantly enhanced, the 5-HT release-suppressing effect of clonidine. The results demonstrate that the release of 5-HT from serotonergic nerve terminals in rat ventral hippocampus in vivo is modulated by alpha 2-adrenoceptors, probably both by heteroreceptors on the axon terminals of the serotonergic neurones and by other alpha 2-adrenoceptor sites situated pre- and/or postsynaptic to the noradrenergic terminals. Our results also suggest that while functionally operative, these sites may receive little physiological tone, at least in chloral hydrate-anaesthetised rats.

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.

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.

Involvement of adenylate cyclase and protein kinase C in the alpha 2-adrenoceptor-mediated inhibition of noradrenaline and 5-hydroxytryptamine release in rat hypothalamic slices

In superfused rat hypothalamic slices prelabelled with [3H]-noradrenaline, the alpha 2-adrenoceptor agonist UK 14304 inhibited in a concentration-dependent manner the electrically-evoked release of tritium. This inhibition was antagonized by the alpha 2-adrenoceptor blocking agent idazoxan, which by itself increased the electrically-evoked tritium overflow. Exposure to forskolin, an adenylate cyclase activator, increased the electrically-evoked release of [3H]-noradrenaline. In the presence of forskolin (1 mumol/l), both the inhibitory effect of UK 14304 and the increasing effect of idazoxan on the electrically-evoked release of [3H]-noradrenaline were less pronounced than in the absence of the adenylate cyclase activator. Exposure to forskolin and to the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine shifted to the right the concentration-effect curve for UK 14304 in a similar manner as that observed in the presence of forskolin alone. Exposure to phorbol-12,13-dibutyrate (0.01-10 mumol/l), a drug which activates protein kinase C, increased the electrically-evoked release of [3H]-noradrenaline. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mumol/l), the concentration effect curve for UK 14304 on tritium overflow was significantly shifted to the right. The increasing effect of idazoxan on tritium overflow was significantly less pronounced in the presence of 1 mumol/l phorbol-12,13-dibutyrate. In superfused rat hypothalamic slices prelabelled with [3H]-5-hydroxytryptamine, the alpha 2-adrenoceptor agonist UK 14304 significantly inhibited the electrically-evoked release of tritium. Exposure to forskolin increased in a concentration-dependent manner [3H]-5-hydroxytryptamine overflow, but did not modify the UK 14304-mediated inhibition. Exposure to 3-isobutyl-1-methylxanthine enhanced the electrically-evoked release of [3H]-5-hydroxytryptamine. In the presence of both forskolin (1 mumol/l) and 3-isobutyl-1-methylxanthine (1 mmol/l), the concentration-response curve for UK 14304 was significantly shifted to the right. Exposure to phorbol-12,13-dibutyrate (0.01-10 mumol/l) enhanced in a concentration-dependent manner the electrically-evoked overflow of [3H]-5-hydroxytryptamine. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mumol/l), UK 14304 was significantly less potent to inhibit tritium release than in the absence of the protein kinase C activator. It is concluded that both cyclic AMP and phosphoinositide turnover are involved in the modulation of noradrenaline and 5-hydroxytryptamine release by presynaptic alpha 2-adrenoceptors in rat hypothalamic slices. However, these interactions do not represent definitive proof for a cause-effect relationship for the second messengers mediating the alpha 2-adrenoceptor induced inhibition of transmitter release either as autoreceptor or as heteroreceptor.

Phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex

Possible antagonist effects of phentolamine at presynaptic serotonin autoreceptors were studied in slices of the occipito-parietal cortices of the rabbit and the rat. The slices were preincubated with 3H-serotonin and then superfused and stimulated electrically with single pulses or pulse trains. Nitroquipazine 1 mumol/l, a compound that inhibits the high affinity neuronal uptake of serotonin, was present in the superfusion medium in all one pulse-experiments as well as in experiments in which the effect of unlabelled serotonin was examined. In rabbit cortical slices, unlabelled serotonin reduced the single pulse-evoked overflow of tritium. Its concentration-response curve was not changed by the selective alpha 2-adrenoceptor antagonist idazoxan 1 mumol/l but was shifted to the right by phentolamine 1 and 10 mumol/l. Phentolamine 10 mumol/l also shifted to the right the concentration-inhibition curve of the selective 5-HT1-receptor agonist 5-carboxamidotryptamine. When the slices were stimulated by trains of 30 pulses at 3 Hz, phentolamine 1 and 10 mumol/l but not 0.1 mumol/l increased the evoked overflow of tritium, the maximal increase amounting to 178%; its effect was enhanced in the presence of nitroquipazine 1 mumol/l plus idazoxan 10 mumol/l (a drug combination that, when given alone, slightly increased the evoked overflow of tritium). The serotonin receptor antagonist metitepin at concentrations of 0.01-1 mumol/l also increased the overflow of tritium elicited by 30 pulses/3 Hz, the maximal increase amounting to 280%; its effect was potentiated in the presence of nitroquipazine 1 mumol/l plus idazoxan 10 mumol/l but was abolished or almost abolished in the presence of nitroquipazine 1 mumol/l plus phentolamine 10 mumol/l (a drug combination that, given alone, greatly increased the evoked overflow of tritium). When slices were stimulated by trains of 360 pulses at 3 Hz, there was no apparent antagonism of phentolamine 10 mumol/l against the inhibitory effect of unlabelled serotonin. In rat brain cortex slices, unlabelled serotonin reduced the overflow of tritium elicited by 4 pulses delivered at 100 Hz. Again, phentolamine 10 mumol/l shifted the concentration-response curve to the right. It is concluded that phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex with pA2 values of 6.44 and 5.95, respectively. Previous failures to detect the antagonistic effect against exogenous agonists were probably due to stimulation conditions that led to marked endogenous autoinhibition of serotonin release.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of intravenous cocaine are partially attenuated by haloperidol

We examined the effect of the dopaminergic blocking agent, haloperidol, on the subjective and physiologic response to cocaine in cocaine-using volunteers. Five subjects received cocaine (40 mg) or placebo administered intravenously 20 min following pretreatment with haloperidol (8 mg) or placebo intramuscularly in a randomized double-blind study design. Haloperidol pretreatment attenuated cocaine-induced increases in systolic and diastolic blood pressure but not heart rate. Pretreatment with haloperidol reduced subject ratings of pleasant sensations but had no effect on drug "rush." Haloperidol (8 mg) has a small and limited effect on the subjective response to cocaine when given 20 min before cocaine.

Inhibition of ischemia-induced brain catecholamine alterations by clonidine

The effect of clonidine, an alpha 2-agonist, on ischemia-induced alterations in brain catecholamine and metabolite levels was studied in Mongolian gerbils subjected to 180 min of unilateral cerebral ischemia. The gerbils were randomly assigned to four treatment groups: sham-operated or unilateral carotid lesion; each pretreated with clonidine 0.4 mg/kg IP, or untreated. All animals were neurologically assessed and categorized as asymptomatic, neurological deficit or seizure activity at the time of sacrifice. Hemispheric levels of noradrenaline (NA), dopamine (DA), homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured using high pressure liquid chromatography with electrochemical detection. No changes from control were found in animals that remained asymptomatic regardless of treatment. In untreated gerbils that exhibited neurological deficits, marked reductions in both NA and DA and increases in HVA occurred in the ischemic hemisphere. These alterations were greater in gerbils that developed seizures during the observation period. Ischemic animals pretreated with clonidine did not show any significant alterations in catecholamine or metabolite levels from clonidine-treated, sham-operated controls in spite of the presence of neurological deficits. Although significant reductions in NA and DA still occurred in pretreated animals that developed seizures, the changes were markedly less than in untreated gerbils. These results indicate that alpha 2-adrenoceptor stimulation is an effective approach for inhibition of ischemia-induced brain catecholamine alterations, and thus may provide a useful method for assessing the role of catecholamine release in the production of acute ischemic neuronal damage.

Release and modulation of release of serotonin in rabbit superior colliculus

The release of previously incorporated [3H]serotonin and its presynaptic modulation were studied in slices of rabbit superior colliculus. Electrical stimulation at frequencies of 0.017-3 Hz greatly increased the outflow of tritiated compounds; this response was almost abolished by tetrodotoxin and in a low calcium medium. Unlabelled serotonin, when added in the presence of nitroquipazine, an inhibitor of high-affinity neuronal serotonin uptake, reduced the electrically evoked overflow of tritium, an effect antagonized by metitepin. Given alone, metitepin caused an increase. The evoked overflow was also decreased by clonidine, and the effect of clonidine was counteracted by phentolamine. Phentolamine itself increased the overflow response. However, this was probably not due to antagonism against an inhibitory effect of endogenous noradrenaline because, first, the selective alpha 2-adrenoceptor antagonist idazoxan did not share with phentolamine the overflow-enhancing effect, second, phentolamine continued to increase the overflow after noradrenergic axons had been destroyed by 6-hydroxydopamine, and third, the facilitatory effects of metitepin and phentolamine were not additive. Phentolamine, like metitepin, antagonized the presynaptic inhibitory effect of serotonin, indicating that it may increase the evoked overflow of tritium by blocking serotonin receptors rather than alpha-adrenoceptors. Ethylketocyclazocine decrease the electrically evoked overflow, and its effect was prevented by naloxone: peptides selective for opioid mu- or delta-receptors caused no change. Nicotine increased the basal outflow of tritium (in the absence of electrical stimulation); the increase was attenuated by hexamethonium and low calcium medium. No or minimal changes in tritium outflow were obtained with beta-adrenoceptor, dopamine receptor, muscarine receptor and GABA receptor ligands or with substance P and glutamate. In conjunction with our previous studies, these results indicate that serotonin is a neurotransmitter in the superior colliculus. Its release is modulated through presynaptic autoreceptors (probably 5-HT1), alpha 2-adrenoceptors, opioid kappa-receptors and nicotine receptors, of which only the autoreceptors receive an endogenous input, at least under the experimental conditions chosen. Each of the three groups of collicular monoamine axons that we have studied recently (cholinergic, noradrenergic, serotoninergic) possesses a specific pattern of presynaptic, release-modulating receptors. A physiological role seems likely only for the alpha 2-autoreceptors at the noradrenergic and the 5-HT1-autoreceptors at the serotoninergic axons.

Intravenous cocaine: psychiatric effects, biological mechanisms

Volunteer addicts were administered iv loading doses of cocaine, followed by 4-hr cocaine infusions that maintained steady-state conditions. The loading doses were followed by the "rush" and "high" subjective effects that users typically experience; cocaine infusions maintained the experience of drug "high", but not "rush". In a subsequent experiment, haloperidol pretreatment did not alter cocaine "rush" but partially attenuated cocaine "high." During cocaine infusions, we also noted suspicious and paranoid behavior, which were blindly rated by nurses. During one of the infusion conditions, the degree of suspiciousness observed was related to the amount of cocaine previously administered. Although cardiovascular responses to cocaine were marked, we found no alterations in plasma catecholamines following cocaine administrations. Baseline homovanillic acid (HVA) levels, however, were related to the degree of suspiciousness observed following cocaine dosing. The potential contributions of dopaminergic systems and physiological sensitization to the development of the psychiatric toxicity of cocaine are discussed.

Additive effects of clonidine and antidepressant drugs in the mouse forced-swimming test

In the mouse forced-swimming model, dose-dependent reversal of immobility was induced by the alpha-agonist clonidine given IP 30 min before testing. In addition, three preferential inhibitors of 5-HT uptake (citalopram, indalpine and fluvoxamine) had similar activity in the dose range 8-16 mg/kg as did the 5-HT1 agonist 8-OH-DPAT (1-4 mg/kg). Pretreatment with alpha-methyl-paratyrosine (100 mg/kg) did not prevent clonidine (1 mg/kg) action, suggesting that there was mediation by alpha post-junctional receptors. The effect of clonidine was unaltered by prazosin (2 mg/kg) and reversed by yohimbine (4 mg/kg) and 5-MeODMT (1 mg/kg), whereas it was potentiated by reserpine (2.5 mg/kg), methysergide (2 mg/kg) and ketanserin (8 mg/kg). Moreover, an ineffective dose of clonidine (0.06 mg/kg at 45 min pre-testing) made active subthreshold doses of various antidepressants (given at 30 min pre-testing): imipramine (4 mg/kg), amitriptyline (1 mg/kg), maprotiline (8 mg/kg), citalopram (2 mg/kg), indalpine, fluvoxamine and mianserin (4 mg/kg), viloxazine (2 mg/kg). Similar interactions were found with iprindole and nialamide (32 mg/kg), which were inactive alone up to 64 mg/kg, and 8-OH-DPAT (0.5 mg/kg) but not with major and minor tranquillizers. It is suggested that one effect of antidepressants might be the triggering of different relationships between alpha-2 and 5-HT mechanisms.

Evidence against a functional link between noradrenaline uptake mechanisms and presynaptic alpha-2 adrenoceptors

Slices prepared from rat cerebral cortex were labelled with 3H-noradrenaline and superfused. Electrical field stimulation was carried out 15 min (S1) and 45 min (S2) after the start of collection of 5-min samples using 4 pulses delivered at 100 Hz. Drugs acting at alpha 2-adrenoceptors were added 20 min before S2, and their effects were evaluated using the S2/S1-ratio. The alpha 2-adrenoceptor antagonists idazoxan (1 mumol/l) and rauwolscine (1 mumol/l) failed to increase stimulation-evoked overflow of radioactivity in the absence or presence of the noradrenaline reuptake inhibitor desipramine (1 mumol/l). This indicates that the duration of electrical stimulation was too short to allow development of alpha 2-adrenoceptor-mediated autoinhibition by released noradrenaline. The effect of clonidine (3-1000 nmol/l) on stimulation-evoked overflow of radioactivity was tested in the absence and presence of three different reuptake inhibitors (desipramine, 1 mumol/l; maprotiline, 1 mumol/l; cocaine, 10 mumol/l). The analysis yielded identical concentration-response curves under all conditions. These results argue against an action of inhibitors of neuronal reuptake of noradrenaline at the presynaptic alpha 2-adrenoceptor and against the concept of a functional link between uptake site and receptor.

Sedative action of the alpha 2-agonist medetomidine in cats

Medetomidine, a novel alpha 2-agonist drug intended for small animal sedation, was injected intramuscularly at dose rates of 0.02, 0.06 and 0.18 mg/kg. Xylazine (3.0 mg/kg) and saline were used for comparison. The five treatments were tested in a Latin square design in five cats. Treatments differed significantly in three-way analysis of variance, medetomidine inducing an increase in drowsiness with a corresponding decrease in both aroused waking and sleep determined by polygraphical criteria. The duration of effect was dose-dependent. The effect of 0.18 mg/kg medetomidine was comparable to 3.0 mg/kg of xylazine. The drugs also induced bradycardia.

Reversal of opioid-induced muscular rigidity in rats: evidence for alpha-2 adrenergic involvement

Compounds from several different pharmacological classes were tested for their ability to reverse the muscular rigidity induced by an intravenous dose of fentanyl that also caused loss of the righting reflex (LOR). Opioid antagonists reversed the entire syndrome--LOR and rigidity but, generally, rigidity could be reversed nonspecifically by doses of compounds that caused LOR by themselves (e.g., CNS depressants). Muscle relaxants and agonists of histamine, which appeared to be acting peripherally, were also effective. On the other hand, serotonergic drugs and dopamine agonists were not. However, dopaminergic antagonists with adrenolytic activity (i.e., chlorpromazine, haloperidol) reversed rigidity, whereas sulpiride did not. Moreover, rigidity reversed by neuroleptics could be restored by piperoxane, an alpha-2 adrenergic antagonist. In addition, clonidine and other alpha-2 agonists selectively reversed only rigidity following systemic or central administration at doses several orders of magnitude lower than other compounds tested. It is proposed that opioid-induced rigidity is reversed by inhibition of sympathoadrenal outflow which can be accomplished selectively, centrally, by alpha-2 agonists.

Attenuation of isobutylmethylxanthine-induced suppression of operant behavior by pretreatment of rats with clonidine

Administration of 3-isobutyl-1-methylxanthine (IBMX) to rats performing a FR30 operant for food reinforcement produces a dose-dependent suppression of behavior. Operant behavior suppressed by 5 mg IBMX/kg is attenuated by pretreatment, 30 min before the operant session, with the alpha 2 adrenergic agonist clonidine (5-30 micrograms/kg). Clonidine itself causes a dose-dependent reduction in FR30 responding prior to the administration of IBMX. However, doses of clonidine which also suppressed responding were not more effective than lower doses in attenuating the suppression of operant behavior caused by IBMX, perhaps due to postsynaptic or nonspecific actions of clonidine. Methylxanthines, alone or in combination with the opiate antagonist naloxone, produce signs of opiate withdrawal. This quasi-morphine withdrawal syndrome may be useful in studies of either the development or expression of opiate withdrawal. Since clonidine attenuates the rate-suppressant effect of IBMX, it is likely that a significant component of IBMX's behavioral effects are due to increases in NE neurotransmission. These results are similar to those obtained with true opiate withdrawal in rats, strengthening the idea that suppression of operant behavior by IBMX involves mechanisms in common with opiate withdrawal. It may be a useful way of objectively studying the expression of the withdrawal syndrome in the absence of opiates and/or a way of determining if a drug can selectively block withdrawal.

Differential effects of clonidine, B-HT 933 and B-HT 920 in immature rat pups

The effects of the alpha-adrenergic agonist clonidine were compared with two experimental hypotensive drugs, B-HT 920 and B-HT 933, in 10-day-old rat pups. Clonidine induced the expected dose-dependent (0.1-1.0 mg/kg) motor activation and wall-climbing syndrome typical at this age. B-HT 933, thought to be a more selective alpha 2-agonist than clonidine, elicited locomotor activity and wall-climbing only at the highest dose used (50 mg/kg). The high dose of B-HT 933 necessary to begin to mimic the effects of clonidine, a finding consistent with some studies using B-HT 933 in adults, suggests that the wall-climbing syndrome is mediated by receptors which have a low affinity for B-HT 933. In striking contrast, B-HT 920, a presynaptic dopamine agonist in mature rats, produced a very different behavioral profile. B-HT 920 induced long periods of sniffing accompanied by locomotion at low doses (peak at 0.12 mg/kg) and ataxic locomotion and poorly coordinated wall-climbing at high doses (30-50 mg/kg). Experiment 2 demonstrated that the active sniffing evoked by low doses of B-HT 920 was dose-dependently blocked by haloperidol (0.035-1.0 mg/kg). These findings of behavioral effects in 10-day-old rats suggest that B-HT 920 stimulates dopaminergic receptors in immature rats, presumably located on postsynaptic neurons. We propose that B-HT 920 and B-HT 933 also may be differentiated in terms of the time of onset of functional development of dopaminergic and noradrenergic autoreceptors, respectively.

Quantitative autoradiography of alpha 2 agonist binding sites in the spontaneously hypertensive rat brain

The relative binding of the alpha 2 agonist [3H]para-aminoclonidine ([3H]PAC) was examined in discrete regions of the spontaneously hypertensive (SHR) and WKY normotensive rat brains by quantitative autoradiography. Substantially less binding was noted in various brain nuclei known to influence cardiovascular function when comparing the SHR to WKY rat brains. These areas included the nucleus of the solitary tract, dorsal motor nucleus of the vagus, locus coeruleus and paraventricular nucleus of the hypothalamus. This difference in binding was also noted in the bed nucleus of the stria terminalis and the nucleus reuniens of the thalamus. Other brain areas which are not believed to be directly involved in central cardiovascular control such as the cerebral cortex, dorsal lateral geniculate and hippocampus exhibited no significant difference in binding between the two strains. Scatchard analysis of [3H]PAC binding indicated that the difference in binding may be due to a selective loss of the high affinity alpha 2 agonist sites in the SHR. These results indicate that a selective loss of alpha 2 receptors in discrete brain cardiovascular control regions of the SHR may contribute to the elevated blood pressure seen in this strain.

Enhancement of clonidine-induced analgesia by lesions induced with spinal and intracerebroventricular administration of 5,7-dihydroxytryptamine

The role of serotonin (5-HT) in analgesia induced by clonidine was examined by determining the effect of intraspinal (i.s.) and intracerebroventricular (i.c.v.) injections of 5,7-dihydroxytryptamine (5,7-DHT) on analgesia produced by clonidine in the tail-flick and hot plate tests. Depletion of amines was verified by high performance liquid chromatography analysis. Intraspinal injections of 5,7-DHT potentiated the action of clonidine in both tests for analgesia and caused depletion of 5-HT in the spinal cord. Intracerebroventricularly injected 5,7-DHT also increased the action of clonidine and depleted 5-HT in brain as well as in the spinal cord. In the groups given intracerebroventricular injections, there appeared to be a biphasic increase in the action of the clonidine. Significant hyperalgesia from pretreatment with neurotoxin was observed only on a limited number of occasions. The present results indicate that 5-HT mechanisms in the CNS are important mediators of the analgesic action of clonidine. Interactions between clonidine and 5-HT systems at both spinal and supraspinal sites are considered.

Role of stress in the exacerbation of chronic illness: effects of clonidine administration on blood pressure and plasma norepinephrine, cortisol, growth hormone and prolactin concentrations

Systolic blood pressure (SBP), diastolic blood pressure (DBP), and plasma norepinephrine (NE), cortisol (CRT), growth hormone (GH) and prolactin (PRL) were studied before and after clonidine (2.5 micrograms/kg i.m.) administration in 193 chronic severely ill patients and 193 normal subjects matched by age and sex. During exacerbation periods (positive manifestations of impairment and progressive disease), the patients showed higher NE, CRT and DBP than the normals or when they were investigated during non-exacerbation periods (92 of the 193). Clonidine induced sharp, marked reductions of NE, CRT and DBP, plus a sudden increase of GH, in all the patients during exacerbation periods. Non-significant reductions of NE, CRT and DBP were observed in normals and in patients during non-exacerbation periods. On the other hand, the GH increase registered during exacerbation periods was of an order of magnitude higher than that registered in normals and in patients during non-exacerbation periods. Significant reduction of SBP was registered both in normals and patients (exacerbation and non-exacerbation periods). Some tendency to PRL lowering was observed during exacerbation periods only. A high positive correlation between NE and DBP (pre- and post-clonidine values) was obtained during exacerbation periods in patients, but not in normals or during non-exacerbation periods in the patients. Similarly, a close negative correlation was obtained between CRT and GH (postclonidine values) during exacerbation periods, but not in normals or during non-exacerbation periods. No significant correlation was found between NE and SBP in any group of subjects. The clonidine-induced changes in GH and CRT observed in the patients during exacerbation periods were in striking contrast to the absence of these changes in depressed patients. This finding is consistent with the low rate of depression (6.7%) registered among our patients during exacerbation periods. The high plasma NE and CRT levels registered in chronic severely ill patients during exacerbation periods reflect a central and peripheral sympathetic hyperactivity, accompanied by an overactivity of the pituitary--adrenocortical axis. The strong reduction of DBP, NE and CRT, along with the sharp and great increase of GH, might be useful as indicators in assessing the exacerbation and progression of severe chronic illnesses.