Description of an indolaminergic cell component in the cat locus coeruleus: a fluorescence histochemical and radioautographic study

The role of serotonin in respiratory function and dysfunction

Serotonin (5-HT) is a neuromodulator-transmitter influencing global brain function. Past and present findings illustrate a prominent role for 5-HT in the modulation of ponto-medullary autonomic circuits. 5-HT is also involved in the control of neurotrophic processes during pre- and postnatal development of neural circuits. The functional implications of 5-HT are particularly illustrated in the alterations to the serotonergic system, as seen in a wide range of neurological disorders. This article reviews the role of 5-HT in the development and control of respiratory networks in the ponto-medullary brainstem. The review further examines the role of 5-HT in breathing disorders occurring at different stages of life, in particular, the neonatal neurodevelopmental diseases such as Rett, sudden infant death and Prader-Willi syndromes, adult diseases such as sleep apnoea and mental illness linked to neurodegeneration.

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Monoaminergic systems in the brainstem and spinal cord of the turtlePseudemys scripta elegansas revealed by antibodies against serotonin and tyrosine hydroxylase

With the aim of gaining more insight into the monoaminergic regulation of spinal motor systems in the turtle, we have studied the distribution of 5-HT (5-HTir) and tyrosine hydroxylase immunoreactivity (THir) in the brainstem and spinal cord of Pseudemys scripta elegans. 5-HTir cell bodies were located in the midline in nucleus raphe inferior, nucleus raphe superior, and laterally in nuclei reticularis superior and inferior and nucleus reticularis isthmi. THir cell bodies were located in the commissural nucleus, nucleus tractus solitarii, the locus coeruleus-subcoeruleus complex, nuclei reticularis superior and inferior, the pretectal area, and substantia nigra. 5-HTir and THir tracts were found in lateral and ventral bundles superficially in the brainstem. 5-HTir fibers in the spinal cord were located in a large dorsolateral and a smaller ventrolateral tract. In the gray matter, a high concentration of 5-HTir fibers were observed in areas I-IV and in the lateral motor column of cervical and lumbar enlargements. Areas V-VIII and area X were less intensively innervated, with the lowest fibre concentration in areas VII-VIII and area X. Throughout the spinal cord, THir nerve fibres were located in the same areas but with a lower density. Small bipolar 5-HTir and THir cell bodies were found ventromedially to the central canal especially in cervical and lumbosacral segments. Large THir cells were found in area IX in the caudal sacral and coccygeal spinal cord. THir cerebrospinal fluid-contacting cells were also found in the most caudal part of the brainstem and the upper cervical spinal cord. The well developed spinal 5-HT system and the less developed THir system provides an anatomical explanation for the monoaminergic modulation of turtle motoneuron membrane properties, which has been observed in electrophysiological experiments.

Parasympathetic control of the heart. III. Neuropeptide Y-immunoreactive nerve terminals synapse on three populations of negative chronotropic vagal preganglionic neurons

The vagal postganglionic control of cardiac rate is mediated by two intracardiac ganglia, i.e., the sinoatrial (SA) and posterior atrial (PA) ganglia. Nothing is known about the vagal preganglionic neurons (VPNs) that innervate the PA ganglion or about the neurochemical anatomy of central afferents that innervate these VPNs. These issues were examined using light microscopic retrograde labeling methods and dual-labeling electron microscopic histochemical and immunocytochemical methods. VPNs projecting to the PA ganglion are found in a narrow column exclusively in the ventrolateral nucleus ambiguus (NA-VL). These neurons are relatively large (37.6 +/- 2.7 microm by 21.3 +/- 3.4 microm) with abundant cytoplasm and intracellular organelles, rare somatic and dendritic spines, round uninvaginated nuclei, and myelinated axons. Previous physiological data indicated that microinjections of neuropeptide Y (NPY) into the NA-VL cause negative chronotropic effects. The present morphological data demonstrate that NPY-immunoreactive nerve terminals formed 18 +/- 4% of the axodendritic or axosomatic synapses and close appositions on VPNs projecting to the PA ganglion. Three approximately equal populations of VPNs in the NA-VL were retrogradely labeled from the SA and PA ganglia. One population each projects to the SA ganglion, the PA ganglion, or to both the SA and PA ganglia. Therefore, there are both shared and independent pathways involved in the vagal preganglionic controls of cardiac rate. These data are consistent with the hypothesis that the central and peripheral parasympathetic controls of cardiac rate are coordinated by multiple potentially redundant and/or interacting pathways and mechanisms.

Alpha-adrenoceptor-mediated modulation of 5-HT2 receptor agonist induced impulsive responding in a 5-choice serial reaction time task

The activation of 5-HT2A receptors has been shown to enhance the probability of premature responding, regarded as a form of motor impulsive behaviour. At the behavioural level, the interaction of alpha-adrenoceptors and 5-HT2 receptors has been linked to head twitch behaviour, regarded as an experimental model of compulsive behaviour. The aim was to determine whether the probability of premature responding induced by an excess activation of 5-HT2A receptors can be modulated by the blockade of alpha1- or alpha2- adrenoceptors. In the experiments, the 5-choice serial reaction time task was used to measure attention and response control of the rats. The experiments assessed the effects of (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) 0.1-0.2 mg/kg subcutaneously, a 5-HT2A/2C agonist, and prazosin, an alpha1-adrenoceptor antagonist, alone or in combination, on the performance of rats. In an additional experiment to examine the possible role of the alpha2-adrenoceptors, a potent, selective and specific alpha2-adrenoceptor antagonist, atipamezole, was given alone or in combination with DOI. Results showed that DOI increased the probability of premature responses, but it did not affect the choice accuracy. Prazosin (0.1 or 0.3 mg/kg, subcutaneously), given on its own had no effects on probability of responding prematurely, but prazosin (0.3 mg/kg.) was able to attenuate the DOI-induced responding. Atipamezole (0.1 mg/kg, s.c.) did not attenuate the effect of DOI on probability of premature responding. When given at lower doses, DOI (0.03 mg/kg) and atipamezole (0.03 mg/kg) synergistically increased the probability of premature responding, whereas a higher dose of atipamezole (0.3 mg/kg) on its own increased the probability of responding prematurely, but this effect was not additive to that of 0.1 mg/kg DOI. These data indicate that 5-HT2 receptor activation enhances impulsive responding and this effect can be diminished by the blockade of alpha1-adrenoceptors. Atipamezole, an alpha2-antagonist, enhances the probability of premature responding and shares the mechanism of action with the 5-HT2 agonist in this respect. These results provide evidence for an interaction between the serotonergic 5-HT2 receptors and alpha-adrenoceptors in the modulation of response control to the motor impulsivity type of behaviour (premature responding) in addition to that of compulsory behaviour (head shakes) found previously.

Nitric oxide as modulator of neuronal function

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Influence of excitatory amino acids on basal and sensory stimuli-induced release of 5-HT in the locus coeruleus

1. The interactions between 5-hydroxytryptaminergic neurones and excitatory amino acid utilizing neurones were studied in the locus coeruleus of conscious, freely moving rats. The locus coeruleus was superfused with artificial cerebrospinal fluid through a push-pull cannula and 5-hydroxytryptamine (5-HT) was determined in the superfusate that was continuously collected in time periods of 10 min. 2. Superfusion of the locus coeruleus with the NMDA receptor antagonist AP5 (10 microM), kynurenic acid (1 mM), or the AMPA/kainate receptor antagonist DNQX (10 microM) reduced the 5-HT release in the locus coeruleus. 3. Superfusion with the agonists NMDA (50 microM), kainic acid (50 microM) or AMPA (10 microM) enhanced the release rate of 5-HT. AP5 (10 microM) blocked the stimulant effect of NMDA, while tetrodotoxin (1 microM) failed to influence the NMDA-induced release of 5-HT. In the presence of 10 microM DNQX, the releasing effect of 50 microM kainic acid was abolished. 4. Pain elicited by tail pinch, as well as noise-induced stress, increased the release of 5-HT. Superfusion of the locus coeruleus with 10 microM AP5 reduced the tail pinch-induced 5-HT release. AP5 (10 microM) did not affect the noise-induced release of 5-HT which was reduced, when the locus coeruleus was superfused simultaneously with this concentration of AP5 and 1 microM kynurenic acid. DNQX (10 mM) failed to influence the release of 5-HT induced by tail pinch or noise. 5. The findings suggest that 5-hydroxytryptaminergic neurones of the locus coeruleus are tonically modulated by excitatory amino acids via NMDA and AMPA/kainate receptors. The release of 5-HT elicited by tail pinch and noise is mediated to a considerable extent through endogenous excitatory amino acids acting on NMDA receptors, while AMPA/kainate receptors are not involved in this process.

Release of serotonin in the rat locus coeruleus: effects of cardiovascular, stressful and noxious stimuli

To investigate the function of serotonergic neurons within the locus coeruleus, this brain nucleus of conscious, freely moving rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the extracellular concentration of serotonin was determined in the superfusate. Serotonin release was increased by depolarization with veratridine (5 microM) or 80 mM K+, while superfusion with tetrodotoxin (1 microM) or systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin substantially diminished the release rate of serotonin in the locus coeruleus. The pressor response to intravenous infusion of noradrenaline (4 micrograms/kg/min) was associated with a pronounced increase in the release rate of serotonin. Superfusion of the locus coeruleus with tetrodotoxin (1 microM) abolished the increase in serotonin release evoked by the pressor response. A fall of blood pressure produced by intravenous administration of nitroprusside (150 micrograms/kg/min) or chlorisondamine (3 mg/kg) diminished the release rate of serotonin. Immobilization, noise (95 dB) or tail pinch increased the release of serotonin in the locus coeruleus and slightly elevated blood pressure. Chlorisondamine abolished the rise in blood pressure elicited by tail pinch without influencing the increased serotonin release. Tail pinch-induced serotonin release was abolished by superfusion with tetrodotoxin. The findings demonstrate that neuronal serotonin release in the locus coeruleus responds to cardiovascular and sensory stimuli, suggesting a function of serotonergic neurons in central blood pressure regulation, as well as in the modulation of locus coeruleus activity by stress and noxious stimuli.

Distribution of serotonin-immunoreactivity in the brain of the pigeon (Columba livia)

The distribution of serotonin (5-HT)-containing perikarya, fibers and terminals in the brain of the pigeon (Columba livia) was investigated, using immunohistochemical and immunofluorescence methods combined with retrograde axonal transport. Twenty-one different groups of 5-HT immunoreactive (IR) cells were identified, 2 of which were localized at the hypothalamic level (periventricular organ, infundibular recess) and 19 at the tegmental-mesencephalic and rhombencephalic levels. Ten of the cell groups were situated within the region of the midline from the isthmic to the posterior rhombencephalic level and constituted the raphe system (nucleus annularis, decussatio brachium conjunctivum, area ventralis, external border of the nucleus interpeduncularis, zona peri-nervus oculomotorius, zona perifasciculus longitudinalis medialis, zona inter-flm, nucleus linearis caudalis, nucleus raphe superior pars ventralis, nucleus raphe inferior). The 9 other cell populations belonged to the lateral group and extended from the posterior mesencephalic tegmentum to the caudal rhombencephalon [formatio reticularis mesencephali, nucleus ventrolateralis tegmenti, ectopic area (Ec) of the nucleus isthmo-opticus (NIO), nucleus subceruleus, nucleus ceruleus, nucleus reticularis pontis caudalis, nucleus vestibularis medialis, nucleus reticularis parvocellularis and nucleus reticularis magnocellularis]. Combining the retrograde axonal transport of rhodamine beta-isothiocyanate (RITC) after intraocular injection and immunohistofluorescence (fluoresceine isothiocyanate: FITC/5-HT) showed the centrifugal neurons (NIO, Ec) to be immunonegative. Serotonin-IR fibers and terminals were found to be very broadly distributed within the brain and were particularly prominent in several structures of the telencephalon (archistriatum pars dorsalis, nucleus taeniae, area parahippocampalis, septum), diencephalon (nuclei preopticus medianus, magnocellularis, nucleus geniculatus lateralis pars ventralis, nucleus triangularis, nucleus pretectalis), mesencephalon-rhombencephalon (superficial layers of the optic tectum, nucleus ectomamillaris, nucleus isthmo-opticus and in most of the cranial nerve nuclei). Comparing the present results with those of previous studies in birds suggests some major serotonin-containing pathways in the avian brain and clarifies the possible origin of the serotonin innervation of some parts of the brain. Moreover, comparing our results in birds with those obtained in other vertebrate species shows that the organization of the serotoninergic system in many regions of the avian brain is much like that found in reptiles and mammals.

In vivo-synthesized radioactively labelled alpha-methyl serotonin as a selective tracer for visualization of brain serotonin neurons

To investigate the use of alpha-[3H]methyl tryptophan (alpha-[3H]MTrp) as a tracer for the in vivo study of brain serotonergic neurons, we examined whether alpha-[3H]MTrp and its metabolite alpha-[3H]methyl serotonin (alpha-[3H]M5-HT) selectively label serotonergic neurons and whether once accumulated in these neurons, the radioactive metabolite behaves like endogenous serotonin. Rats received a systemic injection of 1-5 mCi of alpha-[3H]MTrp and 24 h later their brains were immediately removed or fixed by perfusion before removal. Tissue sections in which serotonergic neurons had been immunostained for 5-HT or its synthesizing enzyme, tryptophan hydroxylase, were processed for radioautography at the light and electron microscopic level. In another group of rats, the release of radioactivity from different brain areas was studied both under basal and depolarizing conditions. In the dorsal raphe nucleus, the light microscopic examination revealed almost complete colocalization between serotonergic neurons and those that accumulated radioactivity, with a heterogeneity in the content of alpha-[3H]M5-HT among the various cells. At the ultrastructural level, immunoidentified serotonergic perikarya and dendritic processes in the dorsal raphe nucleus, as well as nerve terminals in the cerebral cortex were also found to contain alpha-[3H]M5-HT. Under basal conditions, radioactivity was released from the brainstem raphe region and from projection areas such as the striatum and hippocampus. The basal output of alpha-[3H]M5-HT increased approximately twofold after a depolarizing 50 mM KCl solution was added to the perfusion fluid. These findings suggest that newly synthesized alpha-[3H]M5-HT can be released both at somatodendritic and terminal sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tryptophan depletion on responses to yohimbine in healthy human subjects

There is considerable evidence that both the norepinephrine (NE) and serotonin (5-HT) systems are involved in the regulation of human anxiety and fear responses. To assess the modulating effects of central 5-HT levels on NE function, 11 healthy human subjects were studied with placebo-controlled challenge tests involving tryptophan depletion followed by administration of the alpha-2-adrenergic antagonist yohimbine 0.4 mg/kg IV. Five of the 11 subjects reported a marked increase in feelings of nervousness (> or = 25 mm on a 100 mm analog scale) following the combination test, while 1/11 had this response to yohimbine alone. No subjects had an increase in nervousness during other control tests. The increase in nervousness after the tryptophan depletion-yohimbine test was statistically significant for the whole group, but there were no other unique changes in behavioral, physiologic or biochemical (MHPG, cortisol) variables with this test. These data are discussed in terms of possible functional interactions between the 5-HT and NE neurotransmitter systems.

Origin of serotoninergic afferents to the hypoglossal nucleus in the rat

The hypoglossal nucleus contains serotonin and several different serotonin receptors, and serotonin is present in fibers and terminals contacting hypoglossal motoneurons. Serotonin alters the excitability of hypoglossal motoneurons, and may influence hypoglossal motoneuron activity in a variety of physiological processes. Since the hypoglossal nucleus contains no serotoninergic somata, the present study sought to identify the sources of serotoninergic afferents to the hypoglossal nucleus. Fluorogold was injected into the hypoglossal nucleus and serotoninergic immunofluorescence was utilized in a dual-fluorescence technique to identify the sources of serotoninergic afferents to the hypoglossal nucleus. The results demonstrate that most serotoninergic afferents to the hypoglossal nucleus originate from the nuclei raphe pallidus and obscurus, while fewer originate from the nucleus raphe magnus and the parapyramidal region. Other regions of the medial tegmental field and the pons that contain both serotoninergic neurons and neuronal afferents to the hypoglossal nucleus contain no double-labeled neurons.

Effects of the serotonin reuptake inhibitor fluvoxamine on yohimbine-induced anxiety in panic disorder

To assess the effects of the selective serotonin reuptake blocker fluvoxamine on noradrenergic function in patients with panic disorder, an intravenous yohimbine challenge test was administered to eight patients with panic disorder before and after 8 weeks of fluvoxamine treatment and to a parallel group of eight patients treated with placebo. Fluvoxamine treatment reduced yohimbine-induced anxiety while placebo treatment had no effect on this variable. Both fluvoxamine and placebo treatment had little effect on biochemical or physiologic responses to yohimbine.

Raphespinal and reticulospinal axon collaterals to the hypoglossal nucleus in the rat

Neurons in the medial tegmental field project directly to spinal somatic motoneurons and to cranial motoneuron pools such as the hypoglossal nucleus. The axons of these neurons may be highly collateralized, projecting to multiple levels of the spinal cord and to many diverse regions at different levels of the neuraxis. We employed a double fluorescent retrograde tracer technique to examine whether medial tegmental neurons that project to the spinal cord also project to the hypoglossal nucleus. Injections of Diamidino Yellow into the hypoglossal nucleus and Fast Blue into the spinal cord produced large numbers of double labeled neurons in the medial tegmental field, particularly in the caudal raphe nuclei and adjacent ventromedial reticular formation. In these structures the number of neurons projecting to both the hypoglossal nucleus and the spinal cord was equivalent to the number of neurons projecting to multiple levels of the spinal cord observed in control animals. Fewer neurons projecting to both the hypoglossal nucleus and the spinal cord were observed in several other nuclei and subregions of the medial tegmental field, while almost no such neurons were observed in the lateral tegmental field or other pontomedullary structures. These results demonstrate that neurons of the caudal raphe nuclei and adjacent ventromedial reticular formation project to both the spinal cord and the hypoglossal nucleus, and support the concept that the diffuse projections to motoneuron pools from the medial tegmental field globally modulate both spinal and cranial somatic motoneuron excitability.

Functional interrelationship of serotonin and norepinephrine: cortisol response to MCPP and DMI in patients with panic disorder, patients with depression, and normal control subjects

The relationship between norepinephrine (NE) and serotonin (5-hydroxytryptamine; 5HT) functioning was explored in a neuroendocrine challenge paradigm. Ten normal control subjects, 17 patients with major depression, and 22 patients with panic disorder volunteered to participate in this study. Each subject received a challenge with meta-chlorophenylpiperazine (MCPP; 0.25 mg/kg, p.o.), a 5HT agonist, and desmethylimipramine (DMI; 75 mg, i.m.), an indirect NE agonist, in randomized order. The peak-minus-baseline cortisol response to MCPP was used as an indicator of 5HT function, and cortisol response at 75 minutes-minus-baseline to DMI was used as an indicator of NE function. The cortisol responses to DMI and MCPP were found to be highly negatively correlated in the total sample, in particular in the patients with major depression and panic disorder. This finding suggests that the functions (or dysfunctions) of the NE and 5HT systems may not be separate as is usually believed, and that the NE and 5HT disturbances observed in major depression and panic disorder may not be independent. Rather, there may be a joint disturbance of NE-5HT in these disorders.

The serotoninergic system of the brain of the lamprey, Lampetra fluviatilis: an evolutionary perspective

The distribution of serotonin(5HT)-immunoreactive cell bodies, nerve fibers and terminals was investigated by light microscopy in the lamprey Lampetra fluviatilis. Twenty-three distinct groups of 5HT neuronal somata were identified from diencephalic to rhombencephalic levels in the brain. The diencephalon contained a subependymal population of immunoreactive cells in contact with the cerebrospinal fluid (CSF), which could be subdivided into five separate groups situated in the hypothalamus and ventral thalamus; five additional groups of immunoreactive diencephalic neurons, situated in the dorsal thalamus and thalamo-pretectum, which were not in contact with the CSF, were also identified. In the midbrain, in addition to a few labelled neurons in the optic tectum, two structures containing immunoreactive cells were identified in the tegmentum mesencephali. None of these 5HT cells corresponded to the retinopetal neurons which are situated in the same region. A very large number of 5HT neurons were observed in the hindbrain which could be divided into seven groups in the isthmus rhombencephali and a further three in the rhombencephalon proper. Immunoreactive fibers and terminals were widely distributed throughout the neuraxis. In the telencephalon two 5HT fibers assemblies, lateral and medial, could be identified which terminated in both pallial and subpallial structures. The richest serotoninergic innervation in the telencephalon was found in the lateral portion of the primordium hippocampi and the medial part of the corpus striatum. In the diencephalon, the distribution of immunoreactive fibers and terminals was heterogeneous, being most pronounced in the lateral hypothalamic area and in the infundibulum. The densest arborization of fibers in the mesencephalon was found in the stratum fibrosum et cellulare externum of the optic tectum, a major site of retinal projection, and in the nucleus interpeduncularis mesencephali as well as in the oculomotor nuclei. The rhombencephalon is richly endowed with serotoninergic fibers and terminals, many labelled arborizations being found in the nuclei isthmi rhombencephali and around the nucleus motorius nervi trigemini. Comparative analysis of the serotoninergic systems of petromyzontiforms and gnathostomes indicates that the evolution of this system involves a progressive elimination of the rostral immunoreactive cells and an increasing complexity of the caudal population of serotoninergic neurons.

m-Chlorophenylpiperazine as a probe of serotonin function

m-Chlorophenylpiperazine (mCPP) is the most extensively used probe of serotonin function in psychiatry. This article reviews its in vitro and in vivo properties in animals, normal human subjects, and psychiatric patients. mCPP is a safe, reliable, direct 5-hydroxytryptamine (5HT) agonist, which may be used to evaluate 5HT receptor sensitivity. It causes a consistent, dose-dependent elevation of ACTH, cortisol, and prolactin levels in both animals and humans, as well as increased body temperature in man. It also causes a variety of behavioral effects, depending on the population studied. These effects are probably 5HT receptor-related, although specific 5HT receptor subtype mechanisms have not yet been established. mCPP may be considered an important addition to armamentarium of 5HT receptor probes, which is especially useful until more selective 5HT receptor agonists have been tested.

Noradrenergic locus coeruleus neurons: their distant connections and their relationship to neighboring (including cholinergic and GABAergic) neurons of the central gray and reticular formation

Noradrenergic LC neurons appear to be relatively unique in the brain, being unsurpassed in the divergence and ubiquity of their projections through the central nervous system. In this regard, they share certain characteristics with peripheral noradrenaline neurons of the sympathetic nervous system. As such they would be assumed to play a very general role in modulating the activity of large populations of neurons in multiple, functionally diverse systems. Like other periventricular and reticular neurons, they have the potential to receive afferent information from multiple sources via long dendrites, upon which the majority of their inputs from brainstem and forebrain may arrive. They appear closely related to the cholinergic neurons of the laterodorsal tegmental nucleus, their neighbors that are located medial and rostral to them within the periventricular gray and that have similarly oriented and positioned long dendrites that would allow reception of similar afferent input as the LC neurons and also possibly interaction with the LC neurons. As evidenced by input to the noradrenergic cell bodies in the compact portion of the nucleus, a moderate GABAergic innervation, that may derive in part from local neurons, could have a potent influence on the activity of the cells. Periventricular GABAergic cells could also serve as intermediaries to other afferent input, from a distance, terminating in the periventricular region or from local neurons such as the cholinergic cells of the laterodorsal tegmental nucleus.

An immunocytochemical study using the PAP method for tyrosine hydroxylase and serotonin in alternate sections, and in situ hybridization to detect tryptophan hydroxylase mRNA in the rat's locus ceruleus

In situ hybridization (LARRSON and HOUGAARD 1990), using the APAAP complex, shows that many small and medium-sized neurons with signals of tryptophan hydroxylase mRNA are uniformly scattered throughout the rat's locus ceruleus, and that a few extrinsic neurons just ventro-medial to it also shows the hybridization signals. The specificity of this technique has been established by Northern blot hybridization. Synthesis of serotonin in intrinsic neurons is indicated not only by the results obtained from in situ hybridization, but also by the fact that, in distribution, masked serotonin cells, immunocytochemically revealed after pargyline and 5-hydroxytryptophan loading, correspond to the neurons showing mRNA hybridization signals. Identification of the same neurons in adjacent cryostat sections, immunostained alternately for serotonin or tyrosine hydroxylase after loading, provides evidence for the coexistence of serotonin and noradrenaline in a single neuron of this center. A few extrinsic, non-specific indoleamine cells located just ventro-medial to the center may be related to the lateralization of the raphe's neurons. The expression of tryptophan hydroxylase in CNS appears to be restricted to the specific regions such as the locus ceruleus and raphe's nuclei.

Pontospinal transmitters and their distribution

The dorsolateral pontine tegmentum of the cat is known to contain a large population of catecholaminergic neurons. Additionally, several studies have also shown the presence of other neurochemicals (acetylcholine, enkephalin, neuropeptide Y, serotonin, somatostatin and substance P). In this study, we have employed retrograde transport of horseradish peroxidase in combination with immunocytochemistry to determine the locations of pontospinal neurons which contain catecholamine, enkephalin, neuropeptide Y, and serotonin. Furthermore, we have combined the retrograde transport of Fast Blue and immunofluorescence histochemistry to determine whether enkephalin-containing neurons are catecholaminergic. All pontospinal neurons, irrespective of the neurochemical content, were observed in the ventral and lateral parts of the dorsolateral pontine tegmentum at coronal levels P1.8-P4.0. These neurons were located in the nuclei locus coeruleus alpha and subcoeruleus and the Kölliker-Fuse nucleus. A high concentration of these neurons was evident in the Kölliker-Fuse nucleus when compared to the nuclei locus coeruleus alpha and subcoeruleus. Quantitative data have revealed that enkephalin is contained in a large proportion of the pontospinal catecholaminergic neurons (75%). The observations suggest that catecholaminergic neurons may contain one or more putative peptide neurotransmitters.

Aromatic L-amino acid decarboxylase-immunohistochemistry in the cat lower brainstem and midbrain

By indirect immunohistochemistry, the present study examined the distribution of neuronal structures in the cat medulla oblongata, pons, and midbrain, showing immunoreactivity to aromatic L-amino acid decarboxylase (AADC), which catalyzes the conversion of L-3, 4-dihydroxyphenylalanine (L-DOPA) to dopamine, and 5-hydroxytryptophan to serotonin (5HT). With simultaneous and serial double immunostaining techniques, immunoreactivity to this enzyme was demonstrated in most of the catecholaminergic and serotonergic neurons. We could also demonstrate AADC-IR cell bodies that do not contain tyrosine hydroxylase (TH-) or 5HT-immunoreactivity (called "D-type cells") outside such monoaminergic cell systems. At the medullo-spinal junction, very small D-type cells were found within and beneath the ependymal layer of the 10th area of Rexed surrounding the central canal. D-type cells were localized in the caudal reticular formation, nucleus of the solitary tract, a dorsal aspect of the lateral parabrachial nucleus, and pretectal areas as have been reported in the rat. Furthermore, the present study describes, in the cat brainstem, new additional D-type cell groups that have not been reported in the rat. Dense or loose clusters of D-type cells were localized in the external edge of the laminar trigeminal nucleus, dorsal motor nucleus of the vagus, external cuneate nucleus, nucleus praepositus hypoglossi, central, pontine, and periaqueductal gray, superficial layer of the superior colliculus, and area medial to the retroflexus. D-type cells were loosely clustered in the lateral part of the central tegmental field dorsal to the substantia nigra, extending dorsally in the medial division of the posterior complex of the thalamus and medial side of the brachium of the inferior colliculus. They extended farther rostrodorsally along the medial side of the nucleus limitans and joined with the pretectal cell group. Almost all these cells were very small and ovoid to round with 1-2 short processes with the exception of dorsal motor vagal cells. AADC-IR axons were clearly identified in the vagal efferent nerves, longitudinal medullary pathway, dorsal tegmental bundle rostral to the locus coeruleus. Serotonergic axons were identified not only in the central tegmentum field and lateral side of the central superior nucleus, but also in the ventral surface of the medulla oblongata. We describe principal densely stained fiber plexuses in the cat brainstem. The findings of the present study provide a morphological basis for neurons that decarboxylate endogenous and exogenous L-DOPA, 5HTP, and other aromatic L-amino acids.

The distribution of serotonin immunoreactivity in the rat locus ceruleus after intraventricular injections of either 5,6- or 5,7-dihydroxytryptamine with special reference to serotonin synthesis

The localization of serotonin-immunoreactivity (5-HT-IR) in the locus ceruleus (LC) of rats was studied by the peroxidase-anti-peroxidase method using a purified antibody obtained from a rabbit. Antibody production was performed according to the method of Grota and Brown (1974). The antibody was applied to serial cryostat sections with alternate counterstaining by cresyl violet, after intraventricular injections of 5,6-dihydroxytryptamine or 5,7-dihydroxytryptamine prior to treatment with pargyline and a precursor of 5-HT. The majority of LC neurons were immunopositive, and more than half of all LC neurons clearly showed 5-HT-IR. Although core cells were the most predominant, all types of neurons were immunopositive, and randomly scattered throughout the LC. The uptake inhibitor, Lilly 110140, administered in sufficient amounts prior to an injection of pargyline, did not reduce 5-HT-IR within the LC. The results suggest that LC neurons receive 5-hydroxytryptophan (5-HTP) through an afferent vascular-neuronal channel and/or by diffusion from blood capillaries much more than 5-HT itself. We consider from these results that all types of LC neurons throughout the nucleus are masked 5-HT cells, and that the majority of LC neurons utilize blood-borne 5-HTP as an immediate precursor for intraneuronal 5-HT synthesis.

An immunocytochemical study on the GABA-ergic and serotonin-ergic neurons in rat locus ceruleus with special reference to possible existence of the masked indoleamine cells

Alternate consecutive cryostat sections of the rat locus ceruleus (LC) were immunostained for either GABA or serotonin by the unlabelled peroxidase-antiperoxidase method. By identifying the same neurons in adjacent sections of this series, we confirmed that the GABA- and serotonin-ergic neurons are different cell groups. Serial sections of the LC were alternately stained either by the immunocytochemical method for serotonin or with cresyl violet for neuroanatomical observations, under normal and various experimental conditions using pharmacological manipulations. By using both a monoamine oxidase (MAO) inhibitor (pargyline) and a serotonin precursor, 5-hydroxytryptophan rather than L-tryptophan, with or without an inhibitor of serotonin synthesis, p-chlorophenylalanine, we have demonstrated that many LC neurons have the capacity to accumulate serotonin and not other indoleamines. It is suggested that the LC has numerous masked indoleamine cells that contain large amounts of MAO in the cytoplasm, so that under physiological conditions, the serotoninimmunoreactivity of these cells is masked.

Cerebellar cortical and nuclear afferents from the feline locus coeruleus complex

The origin and distribution of cerebellar cortical and nuclear afferents from the locus coeruleus complex (the nucleus locus coeruleus, the nucleus subcoeruleus, the medial and lateral subdivisions of the parabrachial nucleus and the Kölliker-Fuse nucleus) have been studied by means of retrograde transport of the wheat germ agglutinin-horseradish peroxidase complex in the cat. Cerebellar cortical depositions of the tracer were made by pressure injections, while nuclear depositions were made by implanting the tracer in crystalline form. The projection is bilateral with an ipsilateral preponderance. It reaches all the cerebellar nuclei as well as vermal, intermediate and lateral parts of the cerebellar cortex. The highest cell counts were made after tracer depositions in vermal and intermediate parts of the cerebellum. The projection in the cat has a more widespread origin than previously reported. It originates mainly within the nucleus locus coeruleus and the parabrachial nucleus (especially in its lateral subdivision), but retrogradely labelled neurons were also found in nucleus subcoeruleus, the Kölliker-Fuse nucleus and the A4 cell group. The cells of origin were of different shapes, but usually had a maximum diameter of the cell body between 15 and 30 micron. Cerebellar efferent axons passing through the locus coeruleus complex were anterogradely labelled following implants in all the cerebellar nuclei, but definite terminal labelling was not observed within the nuclei of the locus coeruleus complex.

Control from the brainstem of synchrony of discharge between gamma motoneurones in the cat

An assessment has been made of the effect of partial and complete section of the spinal cord on the discharges of gamma motoneurones to hind limb muscles in the decerebrated cat. The degree to which the discharges of pairs of individual gamma motoneurones exhibit short-term synchrony and the variability in interspike intervals of the discharge in individual neurones was measured. Variability of discharge was assessed as coefficient of variation of interspike intervals and degree of synchronization assessed from cross correlation analysis. The discharges of gamma motoneurones in the decerebrated cat with intact spinal cord are regular (low coefficient of variation) and show no tendency to synchrony for up to 24 h following decerebration. Section of the more medial part of the dorsolateral funiculus, on either side of the thoracic spinal cord, resulted in a less regular discharge and the development of short-term synchrony between gamma motoneurones. A dependence of synchrony on variability of discharge was established. The synchrony and increased variability in the spinal cat persisted for at least 24 h. Both were markedly reduced following administration of the precursors of monoamines (either L-Dopa or 5-HTP). We conclude that a bilateral, monoaminergic pathway descending in the dorsolateral funiculus from the brainstem controls synchrony of gamma motoneurone discharge in the decerebrated cat. The possibility is discussed that synchrony of discharge between alpha motoneurones may be controlled by a similar pathway.

Coerulospinal influence on recurrent inhibition of spinal motonuclei innervating antagonistic hindleg muscles of the cat

The locus coeruleus's (LC's) effect on recurrent inhibition of gastrocnemius-soleus (GS) and common peroneal (CP) monosynaptic reflexes (MSRs) was demonstrated to exceed the concomitant facilitation, indicating the independency of LC's disinhibition and facilitation measures in this study. In contrast, the disinhibition effect correlated closely with the recurrently inhibited MSRs. The disinhibition phenomenon was also accompanied by progressive delay and diminution in the Renshaw cell field potential. Hence, the recovery of recurrently inhibited MSRs was probably due, in part at least, to the LC's inhibition of the related Renshaw cell activity. Furthermore, the site-specific, discordant changes in the disinhibition of GS, compared with CP MSRs, as revealed by tracking studies imply that representations of these antagonistic motonuclei may occupy different LC loci. Accordingly, the nonuniform disinhibition may be due to the activation of discrete aggregates of LC neurons which are responsible predominantly in controlling the recurrent inhibitory pathway belonging to one or the other of the antagonistic motonuclei. These findings support a differential LC inhibitory control of Renshaw cell activity, releasing the related motoneurones for the Ia synaptic transmission - a disinhibitory process that is crucial for the LC's independent control of the recurrent circuit of antagonistics extensor and flexor motoneurons.

Locus coeruleus neurons projecting to the forebrain and the spinal cord in the cat

The intranuclear organization of the cat locus coeruleus neurons was investigated anatomo-physiologically. The locus coeruleus neurons project to the forebrain through the dorsal noradrenergic bundle and to the spinal cord. Horseradish peroxidase, a retrograde tracer, was pressure-injected into either the dorsal noradrenergic bundle or the ventrolateral funiculus of the high cervical cord (C1-C2). The cats (n = 12) were killed after a 2- or 3-day survival period. The frontal sections (100 micron) throughout the locus coeruleus were observed by light microscope after carrying out the diaminobenzidine reaction. The labeled locus coeruleus neurons were located predominantly in the rostral locus coeruleus proper and locus coeruleus alpha when horseradish peroxidase was injected into the dorsal noradrenergic bundle, whereas they were predominantly located in the caudal locus coeruleus alpha and subcoeruleus when horseradish peroxidase was injected into the spinal cord. In the electrophysiological experiments, cats (n = 30) were anesthetized with alpha-chloralose and two stimulating electrodes were placed stereotaxically in the dorsal noradrenergic bundle and the ipsilateral ventrolateral funiculus of the high cervical cord (C1-C2), respectively. Monophasic square-wave pulses (2.5 Hz, 100 microsecond duration, 800 microA) were delivered. A recording glass electrode, filled with 2 M NaCl saturated with Fast Green, was placed in the locus coeruleus. Neurons with different conduction velocities, which were evoked by the antidromic stimulation of the dorsal noradrenergic bundle and spinal cord, were verified in the locus coeruleus and the adjacent areas. The slow conductive neurons with a conduction velocity of less than 1 m/s had a slow firing rate (1.6 +/- 0.9/s). They were located predominantly in the rostral locus coeruleus proper and locus coeruleus alpha by the dorsal noradrenergic bundle stimulation. From the anatomical and electrophysiological experimental results, it was concluded that the conduction velocities of the horseradish peroxidase-labeled neurons observed in locus coeruleus proper and locus coeruleus alpha were mostly slow and less than 1 m/s. Most of the slow conductive neurons were considered to be noradrenergic. Neurons evoked antidromically by both the dorsal noradrenergic bundle and spinal cord stimulation were not observed.

Immunocytochemical localization of serotonin-containing neurons in the myelencephalon, brainstem and diencephalon of the sheep

Using immunocytochemistry, morphological characteristics and distribution of serotonin-containing neurons and fibers of the sheep myelencephalon, brainstem and diencephalon were studied, employing highly specific antibodies to serotonin. The immunocytochemical procedure described here allowed the visualization of endogenous, and thus presumably physiological, pools of serotonin, because no pharmacological treatments (colchicine, inhibitors of monoamine oxidase or 5-hydroxytryptophan) were used to increase the endogenous amount of antigen. The distribution of serotonin cell bodies observed in the study is in agreement with that described by other authors in the rat using a similar method. The present work also shows more numerous groups than the formaldehyde-induced fluorescence method, because five additional groups were revealed, designated S1 to S5. Compared with those in the rat, sheep serotonergic structures exhibit striking specific characteristics: (1) greater scattering of cell bodies within the different groups visualized, (2) absence of group B4 and hypothalamic groups, (3) only a weak serotonergic innervation of the suprachiasmatic nuclei area.

Immunohistochemistry of tryptophan hydroxylase in the rat brain

An antiserum raised against tryptophan tetrahydropterine oxygen oxidoreductase was used to examine in rat brain the immunohistochemical localization of this rate-limiting enzyme catalysing the biosynthesis of serotonin. Tryptophan tetrahydropterine oxygen oxidoreductase was detected in numerous nerve cell bodies, proximal dendrites and axon varicosities or terminals corresponding to those of serotonin neurons as judged by their anatomical distribution and concomitant immunoreactivity to an antiserum against serotonin. In hypothalamus, a serotonin-containing nerve cell group previously visualized in the pars ventralis of the nucleus dorsomedialis by radioautography after serotonin uptake, and by serotonin immunohistochemistry after tryptamine loading, remained tryptophan tetrahydropterine oxygen oxidoreductase-unreactive even in rats treated with colchicine. On the other hand, a small group of tryptophan tetrahydropterine oxygen oxidoreductase-positive cells was identified in the rostrolateral portion of nucleus dorsomedialis, which could play a part in the intrinsic serotonin innervation of hypothalamus. There was no overlap between tryptophan tetrahydropterine oxygen oxidoreductase immunostaining and the cellular distribution of N-acetyl serotonin as reported in earlier studies. It is therefore likely that the synthesis of N-acetyl serotonin from tryptophan does not take place in N-acetyl serotonin-containing neurons.

Immunohistochemical localization of monoamine oxidase-B in the cat brain: clues to understanding N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity

The immunohistochemical localization of monoamine oxidase-B in normal cat brain was examined. The enzyme was localized in both neural and nonneural elements of the cat brain. Neurons in the hypothalamus (lateral, dorsal, ventromedial, dorsomedial, and supraoptic nuclei), raphe system, dorsal tegmental nucleus, locus ceruleus, Kölliker-Fuse nucleus, dorsal parabrachial region, and central tegmental field were positive. No substantia nigra pars compacta, retrorubral, or ventral tegmental neurons stained positively. Glial cells (astrocytes) stained positively for monoamine oxidase-B in many regions of the central nervous system, however, there was a significantly greater number of monoamine oxidase-B-positive glial cells in the substantia nigra pars compacta than in other adjacent dopaminergic regions. Because nigra compacta neurons are specifically damaged by the neurotoxin MPTP and because the toxicity of the drug is expressed only in the presence of monoamine oxidase-B, it is possible that the preferential loss of substantia nigra pars compacta neurons in the cat brain may be related to the regional and cellular localization of monoamine oxidase-B.

Distribution of acetylcholine and catecholamine neurons in the cat brainstem: a choline acetyltransferase and tyrosine hydroxylase immunohistochemical study

The distribution of acetylcholine neurons in the brainstem of the cat was studied by choline acetyltransferase (ChAT) immunohistochemistry and compared to that of catecholamine neurons examined in the same or adjacent sections by tyrosine hydroxylase (TH) immunohistochemistry. The largest group of ChAT-positive (+) neurons was located in the lateral pontomesencephalic tegmentum within the pedunculopontine tegmental nucleus and the laterodorsal tegmental nucleus rostrally and within the parabrachial nuclei and locus coeruleus nucleus more caudally. TH+ neurons were found to be coextensive and intermingled with ChAT+ neurons in the dorsolateral pontomesencephalic tegmentum, where the number of ChAT+ cells (approximately 18,500) exceeded that of the TH+ cells (approximately 12,000). In the caudal pons, scattered ChAT+ neurons were situated in the ventrolateral tegmentum together with TH+ neurons. In the medulla, numerous ChAT+ cells were located in the lateral tegmental field, where they extended in a radial column from the dorsal motor nucleus of the vagus to the ventrolateral tegmentum around the facial and ambiguus nuclei, occupying the position of preganglionic parasympathetic neurons of the 7th, 9th, and 10th cranial nerves. TH+ cells were also present in this field. Neurons within the general visceral, special visceral, and somatic motor cranial nerve nuclei were all immunoreactive to ChAT. Scattered ChAT+ neurons were also present within the medullary gigantocellular and magnocellular tegmental fields together with a small number of TH+ neurons. Other groups of ChAT+ cells were identified within the periolivary nuclei, parabigeminal nucleus, prepositus hypoglossi nucleus, and the medial and inferior vestibular nuclei. Acetylcholine neurons thus constitute a heterogeneous population of cells in the brainstem, which in addition to including the somatic and visceral efferent systems, comprises many other discrete systems and represents an important component of the brainstem reticular formation. The proximity to and interdigitation with catecholamine neurons within these systems may be of important functional significance.

Microinjected clonidine inhibits noradrenergic neurons of the locus coeruleus in freely moving cats

Microinjection of the alpha 2-adrenoceptor agonist, clonidine (CLON; 1.0 microgram/0.1 microliter) effected a virtually complete suppression of the spontaneous activity of noradrenergic neurons of the locus coeruleus in freely moving cats. This effect lasted for approximately 90 min and was reversible by systemic administration of the alpha 2-adrenoceptor antagonist, yohimbine. In contrast, CLON had no consistent effect on the activity of neighboring non-noradrenergic neurons. These results provide additional evidence for the direct inhibition of central noradrenergic neurons by CLON by demonstrating such effects independent of anesthesia or the behavioral effects of systemic drug administration. More generally, these findings demonstrate the usefulness of a technique in which small amounts of drug can be applied in small volumes to produce a neuropharmacologically specific effect upon locally recorded neurons in behaving animals.

Inhibition of dorsal-horn cell responses by stimulation of the Kölliker-Fuse nucleus

The Kölliker-Fuse nucleus (KF) in the dorsolateral pons has been shown to be the major source of catecholamine innervation of the spinal cord. This has important implications in terms of pain control mechanisms, since catecholamine-mediated mechanisms are essential for the expression of opiate and other varieties of antinociception. This study examines the effects of KF stimulation on responses of dorsal-horn cells to innocuous and noxious cutaneous stimuli in anesthetized cats. Stimulation of the KF potently inhibits the responses of dorsal-horn cells to both noxious and innocuous stimuli. The threshold for the inhibitory effect is significantly lower for responses to noxious stimuli as opposed to innocuous stimuli. The inhibitory effect is specific to the stimulus site, as evidenced by a marked decrease in the effect following small changes in the position of the stimulating electrode in the brain stem. The latency of the effects indicates a bulbospinal conduction velocity of 4 to 5 m/sec, which is much slower than usual reticulospinal effects and is consistent with a catecholamine-mediated system. The dependence of KF-spinal inhibition on intact biogenic amines was tested by depleting the animals of these amines with reserpine pretreatment. Depletion of biogenic amines resulted in a significant decrease in the KF spinal inhibitory effects, suggesting their dependence on intact noradrenergic stores. The results of these studies are consistent with the idea that the KF-spinal system plays an important noradrenergic-dependent role in the brain-stem modulation of spinal processing of noxious, potentially painful stimuli.