Functional identification of 5HT receptor subtypes

Serotonin and the sleep/wake cycle: special emphasis on microdialysis studies

Several areas in the brainstem and forebrain are important for the modulation and expression of the sleep/wake cycle. Even if the first observations of biochemical events in relation to sleep were made only 40 years ago, it is now well established that several neurotransmitters, neuropeptides, and neurohormones are involved in the modulation of the sleep/wake cycle. Serotonin has been known for many years to play a role in the modulation of sleep, however, it is still very controversial how and where serotonin may operate this modulation. Early studies suggested that serotonin is necessary to obtain and maintain behavioral sleep (permissive role on sleep). However, more recent microdialysis experiments provide evidence that the level of serotonin during W is higher in most cortical and subcortical areas receiving serotonergic projections. In this view the level of extracellular serotonin would be consistent with the pattern of discharge of the DRN serotonergic neurons which show the highest firing rate during W, followed by a decrease in slow wave sleep and by virtual electrical silence during REM sleep. This suggests that during waking serotonin may complement the action of noradrenaline and acetylcholine in promoting cortical responsiveness and participate to the inhibition of REM-sleep effector neurons in the brainstem (inhibitory role on REM sleep). The apparent inconsistency between an inhibitory and a facilitatory role played by serotonin on sleep has at least two possible explanations. On the one hand serotonergic modulation on the sleep/wake cycle takes place through a multitude of post-synaptic receptors which mediate different or even opposite responses; on the other hand the achievement of a behavioral state depends on the complex interaction between the serotonergic and other neurotransmitter systems. The main aim of this commentary is to review the role of brain serotonin in relation to the sleep/wake cycle. In particular we highlight the importance of microdialysis for on-line monitoring of the level of serotonin in different areas of the brain across the sleep/wake cycle.

Cloning, sequence, and expression of kynureninase from Pseudomonas fluorescens

We have cloned the gene encoding kynureninase from Pseudomonas fluorescens using a restriction site polymerase chain reaction technique (RS-PCR) (G. Sarkar, R. T. Turner, and M. E. Bolander PCR Methods Appl. 2, 318-322, 1993) and expressed the enzyme in Escherichia coli DH5a F'. The kynureninase gene has an open reading frame (ORF) of 1251 base pairs that codes for a protein of 416 amino acids with a calculated molecular weight of 45,906. The protein purified from P. fluorescens has N-terminal threonine and an observed molecular weight of 45,787 by electrospray mass spectrometry, suggesting that the N-terminal methionine is removed by posttranslational processing. The complete gene was obtained by PCR and inserted into pTZ18U. The resultant plasmid was used to transform E. coli DH5alpha F', and these cells overexpressed kynureninase to about 37% of total soluble protein. The isolated recombinant protein has molecular weight and Km values identical to those of the native protein from P. fluorescens. The amino acid sequence exhibits 29% identity with those of rat and human kynureninases and 32% identity with the amino acid sequence translated from a Saccharomyces cerevisiae ORF. Alignment of the four sequences shows a highly conserved region which corresponds to the pyridoxal-5'-phosphate (PLP) binding site of rat kynureninase. Based on this alignment, we predict that Lys227 and Asp212 in P. fluorescens kynureninase are involved in pyridoxal-5'-phosphate binding. P. fluorescens kynureninase also exhibits significant homology to the nifS gene product, cysteine desulfurase, and to eucaryotic serine/pyruvate aminotransferases, suggesting that it is a member of subgroup IV of the aminotransferase family of PLP-dependent enzymes.

Similarity of the effects of training and application of serotonin on the electrical activity of live hippocampal slices from rats

A pronounced sensitization phase was observed on low-frequency stimulation of hippocampus slices from rats decapitated immediately after training to a conditioned bilateral escape reflex. The same effect was recorded in slices from control animals after brief application of serotonin. Serotonin had significantly smaller effect on the electrical activity of trained animals. Changes in the efficiency of the serotoninergic input to hippocampus neurons are suggested as one of the factors eliciting low-frequency facilitation in trained rats.

Alcohol drinking in rats injected ICV with 6-OHDA: effect of 8-OHDPAT and tropisetron (ICS 205930)

6-Hydroxydopamine (6-OHDA) was administered ICV to Wistar male rats. Lesioned animals displayed lower preference for ethanol (ETOH) than sham-operated rats. Among 6-OHDA lesioned rats only 9% became high-preferring whereas 20% of sham-operated animals became high-preferring ones. Both tropisetron (the antagonist of 5-HT3 receptors) and 8-OHDPAT (the 5-HT1A receptor agonist) reduced ETOH drinking in high-preferring rats. However, in 6-OHDA lesioned rats the effect of tropisetron was reduced although 8-OHDA retained its effect on ETOH consumption. These results suggest that brain DA neurons are involved in tropisetron action but are not responsible for antipreference effect of 8-OHDPAT.

Alcohol drinking in rats treated with 5,7-dihydroxytryptamine: effect of 8-OH-DPAT and tropisetron (ICS 205-930)

5,7-Dihydroxytryptamine (5,7-DHT) was administered ICV to Wistar male rats. Lesioned rats displayed higher preference for ETOH than sham-lesioned animals. Among 5,7-DHT-pretreated rats 38% became high-preferring, while only 22% of sham-lesioned rats displayed this behavioural pattern (p < 0.05). Both 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; the agonist of serotonin 5-HT1A receptors) and tropisetron (ICS 205-930, the antagonist of 5-HT3 receptors) reduced ETOH consumption in high-preferring, sham-lesioned rats. However, in 5,7-DHT rats the effect of 8-OH-DPAT was completely abolished, while tropisetron retained its antipreference activity. Therefore, it seems that 5-HT1A autoreceptors are critically involved in 8-OH-DPAT action, while 5-HT3 receptor sites responsible for tropisetron action are located beyond the 5-HT system.

Effects of the selective 5-HT1B agonist, CGS 12066B, on sleep/waking stages and EEG power spectrum in rats

Sleep/waking stages and EEG power spectra were studied in rats for 8 h following intraperitoneal administration of CGS 12066B, a selective 5-HT1B agonist. Waking was increased and rapid eye movement (REM) sleep decreased in a dose-dependent manner. Total slow-wave sleep (TSWS) was reduced, but only in the first 2 h period. The latencies to REM sleep and stable sleep were increased dose-dependently. The drug also induced profound behavioural changes that may account for some of the sleep/waking changes. EEG power densities in waking and TSWS were reduced dose-dependently from 7 to 20 Hz after CGS 12066B, suggesting a tendency towards general deactivation. The increase in waking together with a general deactivation suggest complex effects of CGS 12066B on the sleep/waking axis.

Prenatal exposure to cocaine: effects on aggression in Sprague-Dawley rats

Social/aggressive behavior in adult rat offspring (beginning at postnatal Day 180) prenatally exposed to saline, cocaine, or amfonelic acid (AFA) was examined. Pregnant rats received injections of 15 mg/kg of cocaine, or 0.9% saline twice daily, s.c., or on 2 consecutive days at 4-day intervals, or 1.5 mg/kg amfonelic acid daily throughout gestational Days 1-20. Frequency, duration, and latency of 11 social/aggressive behaviors were recorded for two 15-min sessions during which a smaller male intruder replaced an ovariectomized female in the resident's home cage. Subjects received a s.c. saline injection before Session 1 and 2.0 mg/kg of gepirone, a 5HT1a partial agonist, prior to Session 2. Prenatal cocaine treatment resulted in alterations of aggressive behavior. Aggressive behavior was reduced by gepirone in all groups but to a lesser extent in the AFA group.

The 5-HT1A antagonist (-)-alprenolol fails to modify sleep or zimeldine-induced sleep-waking effects in rats

Sleep and waking in rats were studied for 8 h following administration of a selective 5-hydroxytryptamine (5-HT) reuptake inhibitor (zimeldine), a putative 5-HT1A antagonist (L(-)-alprenolol hydrogene tartrate monohydrate [(-)-alprenolol]) and a combination of (-)-alprenolol and zimeldine. Consistent with earlier findings, zimeldine gave a biphasic effect on sleep and waking. Waking was increased during the first 3 h, followed by a small decrease. Deep slow-wave sleep (SWS-2) showed the opposite trend. An initial decrease in SWS-2 was followed by an increase after around 3 h. Rapid eye movement sleep was markedly suppressed and latencies to sleep increased after zimeldine. (-)-Alprenolol had no effects on the different sleep and waking stages or latencies to sleep. The 5-HT1A antagonist also failed to modify the effects of zimeldine administration. The behavioral syndrome induced by a selective 5-HT1A agonist [8-hydroxy-2-(di-n-propyl-amino)-tetralin (8-OH-DPAT)] was clearly antagonized by administration of (-)-alprenolol, indicating that (-)-alprenolol was an efficient 5-HT1A blocker. The data indicate that the sleep-waking effects of zimeldine cannot easily be explained by stimulation of 5-HT1A receptors.

The neuroanatomical basis of anxiety

This review details the neural systems that are important in anxiety-related behaviours. In particular, the role of the amygdaloid complex, Papez circuit, septohippocampal formation and raphe nuclei are described and discussed. Evidence is gathered from a variety of experimental approaches. These include behavioural assessment of anxiety in animals after intracerebral injection of pharmacological agents and following lesions of discrete brain nuclei and selective neurotransmitter pathways. Further evidence is provided by functional brain mapping studies applied to animals and humans. It is proposed that the neural systems recruited in different experimental conditions of anxiety may differ, supporting the notion that clinical anxiety exists in several forms. This has implications for the identification of new anxiolytic treatments. In particular, the findings suggest that approaches aimed at identifying new anxiolytic agents must take into account both the distribution of receptors for the drug and the neuronal systems activated by the experimental protocol.

Effects of idazoxan on dorsal raphe 5-hydroxytryptamine neuronal function

The effects of the alpha 2-adrenoceptor antagonist idazoxan on 5-hydroxytryptamine (5-HT) neuronal firing and release have been investigated. Idazoxan, administered i.v. (10 micrograms/kg and 0.5 mg/kg) increased dorsal raphe nucleus (DRN)-5-HT neuronal firing rate in a dose-dependent fashion. At the higher dose, a voltammetric study revealed increases in extracellular 5-HT and 5-hydroxyindole acetic acid (5-HIAA) levels, there was no effect with the lower dose. Intra-raphe administration of idazoxan (1 ng) also elevated the firing rate of 5-HT neurones in the dorsal raphe, suggesting that idazoxan may produce the increase in firing by a direct effect in the DRN. However, microiontophoretic application of idazoxan did not increase the firing rate of 5-HT neurones in the DRN. Thus the increase in the firing rate of 5-HT neurones in the DRN observed with systemic and local administration of idazoxan is probably not due to a direct action of idazoxan on the 5-HT neurone. Possibly the idazoxan acted at alpha 2-adrenoceptors located on noradrenergic terminals thus stimulating noradrenaline release and consequently increased 5-HT activity. Chronic administration of idazoxan (0.8 mg/kg per h for 14 days), using osmotic mini-pumps, caused an elevation in basal firing rate and an attenuation of the inhibitory response of DRN 5-HT neurones to the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (10 micrograms/kg i.v.). This finding suggests that chronic infusion with idazoxan leads to desensitisation of the 5-HT1A somatodendritic autoreceptor.

Acute and chronic effects of the atypical antidepressant, mianserin on brain noradrenergic neurons

Corticotropin-releasing factor (CRF), which may serve as a neurotransmitter in the noradrenergic nucleus, locus coeruleus (LC), has been postulated to be hypersecreted in depression. The present study was designed to test the hypothesis that antidepressants interfere with CRF putative neurotransmission in the LC. The acute and chronic effects of the atypical antidepressant mianserin on LC spontaneous discharge, LC sensory-evoked discharge, LC activation by a stressor which requires endogenous CRF, and LC activation by ICV CRF were characterized in halothane-anesthetized rats. Acute IV administration of mianserin (0.0001-1.0 mg/kg) increased LC spontaneous discharge and decreased LC discharge evoked by repeated sciatic nerve stimulation in a dose-dependent manner. Additionally, mianserin (0.1 mg/kg) inhibited LC activation by hemodynamic stress (IV infusion of nitroprusside) and by ICV administration of CRF (3.0 micrograms). In rats chronically administered mianserin LC spontaneous and sensory-evoked discharge rates, and LC activation by CRF were similar to those of untreated rats or rats chronically administered saline. Moreover, acute IV administration of mianserin (0.1 mg/kg) to rats chronically treated with mianserin was less effective in altering LC spontaneous and sensory-evoked discharge. In contrast, LC activation by hemodynamic stress was still greatly attenuated in rats chronically administered mianserin. This is similar to the previously reported effect produced by chronic administration of the antidepressant, desmethylimipramine. The present results demonstrate that acute administration of low doses of mianserin attenuates LC activation by a variety of stimuli and suggest that tolerance develops with chronic administration to some of the effects of mianserin on LC discharge characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

L-5-HTP facilitates the electrically stimulated flexor reflex in pithed rats: evidence for 5-HT2-receptor mediation

Different serotonin (5-HT) receptor agonists were tested on the electrically stimulated flexor reflex in pithed rats. The 5-HT2 receptor agonist, (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) [+/-)DOI), the mixed 5-HT1/5-HT2 receptor agonist, quipazine, and the 5-HT precursor, l-5-HTP, showed agonistic activity upon intravenous injection while 5-HT was without effect. A combination of the peripheral decarboxylase inhibitor, Ro 4-4602 (benzerazide), the specific 5-HT-uptake inhibitor, citalopram, and l-5-HTP induced a prolonged (greater than 3 h) increase of the flexor reflex in pithed rats. Different compounds were tested for an inhibitory effect against this l-5-HTP-induced flexor reflex. The 5-HT2 antagonists (ketanserin, methergoline and methiothepin) were potent antagonists. (-)Alprenolol (5-HT1A and 5-HT1B receptor antagonist) and the 5-HT3-receptor antagonist, ICS 205-930, were without an antagonistic effect. The inhibitory potencies in the reflex model (l-5-HTP, citalopram and Ro 4-4602) were significantly correlated (r = 0.83, P less than 0.01, r2 = 0.69) with the potencies to inhibit l-5-HTP-induced head twitches and quipazine-induced head twitches (r = 0.81, P less than 0.01, r2 = 0.66). There was less correlation (r = 0.75, P less than 0.01, r2 = 0.56) with the affinities for 5-HT2 receptors in vitro. There was no significant correlation between inhibitory potencies in the reflex model and affinities for dopamine (DA) D-2 receptors or alpha 1-adrenoceptors (r2 = 0.13 and 0.14, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a selective 5-HT2 agonist, DOI, on 5-HT neuronal firing in the dorsal raphe nucleus and 5-HT release and metabolism in the frontal cortex

Systemic administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (50 and 100 micrograms kg-1, i.v.) inhibited dorsal raphe neuronal firing. DOI (100 micrograms kg-1, i.v.) also produced a decrease in extracellular 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex measured by microdialysis. However, local administration of DOI into the frontal cortex produced no change in extracellular 5-HT and 5-HIAA at any dose given (1, 10 and 100ng). The results demonstrate that DOI is a potent inhibitor of 5-HT neuronal firing and terminal release and that the effects on release are not mediated by an action within the terminal region. The site of action and the receptor involved in the inhibition remains to be determined.

Drugs affecting serotonin neurons

Advances in serotonin pharmacology, the development of drugs that intervene at specific sites to modify serotonergic function, have accompanied advances in the understanding of physiologic roles of serotonin present in neurons and elsewhere and of serotonin receptors that are widely distributed in brain and many peripheral tissues. The pharmacologic advances have sometimes been stimulated by developments in serotonin physiology, such as the recognition of multiple serotonin receptor subtypes, and in other cases have been a major factor in providing new insights into physiologic roles of serotonin. Drugs that modify serotonin function have a variety of therapeutic applications currently and many more potential therapeutic uses to be explored in the future. Having drugs that act with high specificity or selectivity on particular enzymes in serotonin biosynthesis, on particular serotonin receptors, or at other sites such as uptake carriers for serotonin not only offers the hope of improved clinical therapy in diseases caused by abnormal serotonergic function or in which alteration of serotonergic function can alleviate symptoms, but also provides valuable pharmacologic tools for learning more about serotonin physiology and probing the functional status of serotonergic systems. The next few years promise to yield important new serotonergic drugs.