Immunohistochemical studies of cholecystokininlike peptides and their relation to 5-HT, CGRP, and bombesin immunoreactivities in the brainstem and spinal cord of lampreys

Cholecystokinin in the central nervous system of the sea lamprey Petromyzon marinus: precursor identification and neuroanatomical relationships with other neuronal signalling systems

Cholecystokinin (CCK) is a neuropeptide that modulates processes such as digestion, satiety, and anxiety. CCK-type peptides have been characterized in jawed vertebrates and invertebrates, but little is known about CCK-type signalling in the most ancient group of vertebrates, the agnathans. Here, we have cloned and sequenced a cDNA encoding a sea lamprey (Petromyzon marinus L.) CCK-type precursor (PmCCK), which contains a CCK-type octapeptide sequence (PmCCK-8) that is highly similar to gnathostome CCKs. Using mRNA in situ hybridization, the distribution of PmCCK-expressing neurons was mapped in the CNS of P. marinus. This revealed PmCCK-expressing neurons in the hypothalamus, posterior tubercle, prethalamus, nucleus of the medial longitudinal fasciculus, midbrain tegmentum, isthmus, rhombencephalic reticular formation, and the putative nucleus of the solitary tract. Some PmCCK-expressing neuronal populations were only observed in adults, revealing important differences with larvae. We generated an antiserum to PmCCK-8 to enable immunohistochemical analysis of CCK expression, which revealed that GABA or glutamate, but not serotonin, tyrosine hydroxylase or neuropeptide Y, is co-expressed in some PmCCK-8-immunoreactive (ir) neurons. Importantly, this is the first demonstration of co-localization of GABA and CCK in neurons of a non-mammalian vertebrate. We also characterized extensive cholecystokinergic fibre systems of the CNS, including innervation of habenular subnuclei. A conspicuous PmCCK-8-ir tract ascending in the lateral rhombencephalon selectively innervates a glutamatergic population in the dorsal isthmic grey. Interestingly, this tract is reminiscent of the secondary gustatory/visceral tract of teleosts. In conclusion, this study provides important new information on the evolution of the cholecystokinergic system in vertebrates.

The Lamprey Pallium Provides a Blueprint of the Mammalian Layered Cortex

The basic architecture of the mammalian neocortex is remarkably similar across species. Pallial structures in the reptilian brain are considered amniote precursors of mammalian neocortex, whereas pallia of anamniotes ("lower" vertebrates) have been deemed largely insignificant with respect to homology. Here, we examine the cytoarchitecture of the lateral pallium in the lamprey, the phylogenetically oldest group of extant vertebrates. We reveal a three-layered structure with similar excitatory cell types as in the mammalian cortex and GABAergic interneurons. The ventral parts are sensory areas receiving monosynaptic thalamic input that can be activated from the optic nerve, whereas the dorsal parts contain motor areas with efferent projections to the brainstem, receiving oligosynaptic thalamic input. Both regions receive monosynaptic olfactory input. This three-layered "primordial" lamprey lateral pallium has evolved most features of the three-layered reptilian cortices and is thereby a precursor of the six-layered "neo" cortex with a long-standing evolutionary precedent (some 500 million years ago).

The Pharmacology of Vertebrate Spinal Central Pattern Generators

Central pattern generators are networks of neurons capable of generating an output pattern of spike activity in a relatively stereotyped, rhythmic pattern that has been found to underlie vital functions like respiration and locomotion. The central pattern generator for locomotion in vertebrates seems to share some basic building blocks. Activation and excitation of activity is driven by descending, sensory, and intraspinal glutamatergic neurons. NMDA receptor activation may also lead to the activation of oscillatory properties in individual neurons that depend on an array of ion channels situated in those neurons. Coordination across joints or the midline of the animal is driven primarily by glycinergic inhibition. In addition to these processes, numerous modulatory mechanisms alter the function of the central pattern generator. These include metabotropic amino acid receptors activated by rhythmic release of glutamate and GABA as well as monoamines, ACh, and peptides. Function and stability of the central pattern generator is also critically dependent on the array of ion channels found in neurons that compose these oscillators, including Ca2+and voltage-gated K+channels and Ca2+channels.

Modulation of calcium currents and membrane properties by substance P in the lamprey spinal cord

Substance P is endogenously released within the locomotor network of the adult lamprey, accelerates the burst frequency of fictive locomotion, and reduces the reciprocal inhibition. Previous studies have shown that dopamine, serotonin, and GABA regulate calcium channels, which control neurotransmitter release, action potential duration, and slow afterhyperpolarization (sAHP). Here we examine the effect of substance P on calcium channels in motoneurons and commissural interneurons using whole cell patch clamp in the lamprey spinal cord. This study analyzed the effects of substance P on calcium currents activated in voltage clamp. We examined the calcium-dependent sAHP in current clamp, to determine the involvement of three calcium channel subtypes modulated by substance P. The effects of substance P on membrane potential and during N-methyl-d-aspartic acid (NMDA) induced oscillations were also analyzed. Depolarizing voltage steps induced inward calcium currents. Substance P reduced the currents carried by calcium by 61% in commissural interneurons and by 31% in motoneurons. Using specific calcium channel antagonists, we show that substance P reduces the sAHP primarily by inhibiting N-type (CaV2.2) channels. Substance P depolarized both motoneurons and commissural interneurons, and we present evidence that this occurs due to an increased input resistance. We also explored the effects of substance P on NMDA-induced oscillations in tetrodotoxin and found it caused a frequency increase. Thus the reduction of calcium entry by substance P and the accompanying decrease of the sAHP amplitude, combined with substance P potentiation of currents activated by NMDA, may both contribute to the increase in fictive locomotion frequency.

Glutamatergic neuronal populations in the brainstem of the sea lamprey, Petromyzon marinus: an in situ hybridization and immunocytochemical study

Glutamate is the major excitatory neurotransmitter in vertebrates, and glutamatergic cells probably represent a majority of neurons in the brain. Physiological studies have demonstrated a wide presence of excitatory (glutamatergic) neurons in lampreys. The present in situ hybridization study with probes for the lamprey vesicular glutamate transporter (VGLUT) provides an anatomical basis for the general distribution and precise localization of glutamatergic neurons in the sea lamprey brainstem. Most glutamatergic neurons were found within the periventricular gray layer throughout the brainstem, with the following regions being of particular interest: the optic tectum, torus semicircularis, isthmus, dorsal and medial nuclei of the octavolateral area, dorsal column nucleus, solitary tract nucleus, motoneurons, and reticular formation. The reticular population revealed a high degree of cellular heterogeneity including small, medium-sized, large, and giant glutamatergic neurons. We also combined glutamate immunohistochemistry with neuronal tract-tracing methods or γ-aminobutyric acid (GABA) immunohistochemistry to better characterize the glutamatergic populations. Injection of Neurobiotin into the spinal cord revealed that retrogradely labeled small and medium-sized cells of some reticulospinal-projecting groups were often glutamate-immunoreactive, mostly in the hindbrain. In contrast, the large and giant glutamatergic reticulospinal perikarya mostly lacked glutamate immunoreactivity. These results indicate that glutamate immunoreactivity did not reveal the entire set of glutamatergic populations. Some spinal-projecting octaval populations lacked both VGLUT and glutamate. As regards GABA and glutamate, their distribution was largely complementary, but colocalization of glutamate and GABA was observed in some small neurons, suggesting that glutamate immunohistochemistry might also detect non-glutamatergic cells or neurons that co-release both GABA and glutamate.

Specialized brain regions and sensory inputs that control locomotion in leeches

Locomotor systems are often controlled by specialized cephalic neurons and undergo modulation by sensory inputs. In many species, dedicated brain regions initiate and maintain behavior and set the duration and frequency of the locomotor episode. In the leech, removing the entire head brain enhances swimming, but the individual roles of its components, the supra- and subesophageal ganglia, in the control of locomotion are unknown. Here we describe the influence of these two structures and that of the tail brain on rhythmic swimming in isolated nerve cord preparations and in nearly intact leeches suspended in an aqueous, "swim-enhancing" environment. We found that, in isolated preparations, swim episode duration and swim burst frequency are greatly increased when the supraesophageal ganglion is removed, but the subesophageal ganglion is intact. The prolonged swim durations observed with the anterior-most ganglion removed were abolished by removal of the tail ganglion. Experiments on the nearly intact leeches show that, in these preparations, the subesophageal ganglion acts to decrease cycle period but, unexpectedly, also decreases swim duration. These results suggest that the supraesophageal ganglion is the primary structure that constrains leech swimming; however, the control of swim duration in the leech is complex, especially in the intact animal.

Neuronal control of swimming behavior: comparison of vertebrate and invertebrate model systems

Swimming movements in the leech and lamprey are highly analogous, and lack homology. Thus, similarities in mechanisms must arise from convergent evolution rather than from common ancestry. Despite over 40 years of parallel investigations into this annelid and primitive vertebrate, a close comparison of the approaches and results of this research is lacking. The present review evaluates the neural mechanisms underlying swimming in these two animals and describes the many similarities that provide intriguing examples of convergent evolution. Specifically, we discuss swim initiation, maintenance and termination, isolated nervous system preparations, neural-circuitry, central oscillators, intersegmental coupling, phase lags, cycle periods and sensory feedback. Comparative studies between species highlight mechanisms that optimize behavior and allow us a broader understanding of nervous system function.

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.

Serotonergic neuron system in the spinal cord of the gar Lepisosteus oculatus (Lepisosteiformes, Osteichthyes) with special regard to the juxtameningeal serotonergic plexus as a paracrine site

Immunohistochemical and electron microscopic studies were carried out to elucidate the structure of the serotonergic neuron system in the spinal cord of the spotted gar, Lepisosteus oculatus, a nonteleost actinopterygian. Serotonin-immunoreactive (5HT-IR) cell bodies and fibers were widely distributed in the spinal cord, constituting an intrinsic neuron system. This system comprised three anatomical cell groups in different portions of the spinal cord, i.e., the rostromedial cell group, the paired ventrolateral cell groups, and the ventral superficial cell group. The rostromedial cell group included cerebrospinal fluid-contacting neurons with intraventricular processes. The immunostained fibers projecting from all three of these cell groups ran in various directions, mainly ventrally and ventrolaterally, and partly gave rise to a dense plexus at the ventrolateral surface of the spinal cord. Immunoelectron microscopy of the relevant portion demonstrated many varicose fibers containing 5HT-immunopositive vesicles. Conventional electron microscopy of the plexus showed that the constituent varicose fibers were unmyelinated and frequently made a direct contact with the basement membrane contiguous to the leptomeniges (meninx primitiva). There, exocytotic figures of cytoplasmic vesicles were demonstrated, suggesting that 5HT may be secreted, in a paracrine way, into the extraspinal space. This specialized area in the gar spinal cord may be referred to as the juxtameningeal serotonergic plexus.

Ontogeny of 5-HT neurons in the brainstem of the lamprey, Petromyzon marinus

This study examined the spatial and temporal distribution of serotonin-immunoreactive (5-HT-ir) neurons in the brainstem of Petromyzon marinus at three developmental stages, larval, postmetamorphic, and reproductive. Computer-assisted 3-D reconstructions were made of the three main 5-HT-ir neuron groups. The rostralmost brainstem group was located near the posterior commissure, the second group at the isthmus, and the third group in the bulbar area. For each of those groups, the distribution of the 5-HT-ir neurons was very similar in the three developmental stages examined, suggesting that the 5-HT system is relatively mature early in larval animals. The soma of 5-HT-ir neurons increased in size and their dendritic fields increased in complexity with development. Furthermore, the number of 5-HT-ir neurons in each group increased significantly from the larval to the reproductive stage. To determine whether this was due to the genesis of 5-HT neurons, bromodeoxyuridine (BrdU) was injected into larval, metamorphosing, and postmetamorphic lampreys. These experiments revealed a few neurons colocalizing BrdU and 5-HT in metamorphosing animals. Taken together, the present results suggest that 5-HT neurons increase in number during maturation and that neurogenesis could, at least partially, contribute to the appearance of new 5-HT cells at different developmental stages.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

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.

Distribution of thyrotropin-releasing hormone immunoreactivity in the brain of larval and adult sea lampreys, Petromyzon marinus L

This study investigated the distribution of thyrotropin-releasing hormone-immunoreactive (TRHir) neurons and fibers in the brain and retina of lampreys. Our results in the brains of large larvae and upstream-migrating adults of the sea lamprey showed the presence of TRHir neurons mainly in the preoptic region and the hypothalamus. A few TRHir neurons were also found in the striatum. The number and staining intensity of TRHir neurons increased from larval stages to adulthood, and the distribution of TRHir populations was wider in adults. The TRHir fibers were more easily traced in adults. Some TRHir fibers entered the neurohypophysis, although most fibers coursed in the different regions of the brain, mostly in the basal region, from the forebrain to the hindbrain. The presence of TRHir stellate cells was observed in the adenohypophysis. In the retina of adult lampreys, but not in that of larvae, TRHir amacrine cells are present.

Neurotensin-like Peptides in the CNS of Lampreys: Chromatographic Characterization and Immunohistochemical Localization with Reference to Aminergic Markers

Neurotensin (NT)-like peptides in the CNS of the lamprey Lampetra fluviatilis were studied by radioimmunoassay (C-terminal specific NT antiserum), reverse-phase HPLC and immunohistochemistry. Multiple peaks of NT-immunoreactive (-ir) material were observed upon HPLC, of which a major peak eluted in the position of bovine NT. Immunofluorescence histochemistry showed that a monoclonal antibody recognizing the N-terminal (1 - 11) fragment of NT, as well as two polyclonal NT antisera labelled a large number of cell bodies in the periventricular area of hypothalamus, including the postinfundibular commissural nucleus and the ventral and dorsal hypothalamic nuclei. Additional groups of NT-ir cells were observed in the preoptic nucleus, the postoptic commissural nucleus, the mesencephalic tegmentum (L.fluviatilis), and in the spinal cord (L.fluviatilis and Ichtyomyzon unicuspis). Dense NT-ir fibre plexuses were present in the caudal hypothalamus, corpus striatum, ventral mesencephalon, and in the dorsal horn and lateral margin of the spinal cord. At the ultrastructural level the lateral spinal margin showed NT-ir terminal structures, which in most cases were not associated with synaptic specializations, although occasional synaptic contacts with unlabelled elements were found. The relation between NT-ir and monoamine-containing cells was examined with immunofluorescence double-staining, using antisera to tyrosine hydroxylase (TH), 5-hydroxytryptamine (5-HT), and histamine respectively. In the periventricular nuclei of hypothalamus numerous TH-, 5-HT-, as well as histamine-ir cells were located in close association with NT-ir cells, but none of the aminergic markers could be detected within NT-ir neurons. The chemical properties as well as the anatomical distribution of lamprey NT-like peptides show several similarities with those present in mammals, suggesting that NT-containing neuronal systems in the CNS developed early in vertebrate phylogeny.

Group III mGluR-mediated depression of sensory synaptic transmission

The effect of L-AP4, a group III mGluR agonist, on sensory synaptic transmission in the lamprey spinal cord has been analyzed. Paired recordings were made between cutaneous mechanosensory neurons (dorsal cells) and postsynaptic spinobulbar giant interneurons. L-AP4 reduced the monosynaptic dorsal cell-evoked EPSP, but at concentrations higher (200-500 microM) than those necessary to depress reticulospinal axon-evoked EPSPs. Stimulation of the dorsal column, which contains dorsal cell axons and the axons of putative nociceptive and theromosensory axons, elicited compound EPSPs that were consistently depressed by L-AP4. Sensory inputs in the lamprey are thus inhibited by group III mGluRs.

The effects of serotonin on functionally diverse isolated lamprey spinal cord neurons

The experiments reported here showed that application of serotonin (5-hydroxytryptamine, 5-HT) (100 microM) did not induce any significant current through the membranes of any of the spinal neurons studied (n = 62). At the same time, the membranes of most motoneurons and interneurons (15 of 18) underwent slight depolarization (2-6 mV) in the presence of 5-HT, which was not accompanied by any change in the input resistance of the cells. Depolarization to 10-20 mV occurred in some cells (3 of 18) of these functional groups, this being accompanied by 20-60% decreases in input resistance. The same concentration of 5-HT induced transient low-amplitude depolarization of most sensory spinal neurons (dorsal sensory cells), this changing smoothly to long-term hyperpolarization by 2-7 mV. The input resistance of the cell membranes in these cases showed no significant change (n = 8). Data were obtained which provided a better understanding of the mechanism by which 5-HT modulates the activity of spinal neurons. Thus, 5-HT facilitates chemoreceptive currents induced by application of NMDA to motoneurons and interneurons, while the NMDA responses of dorsal sensory cells were decreased by 5-HT. 5-HT affected the post-spike afterresponses of neurons. 5-HT significantly decreased the amplitude of afterhyperpolarization arising at the end of the descending phase of action potentials in motoneurons and interneurons and increased the amplitude of afterdepolarization in these types of cells. In sensory spinal neurons, 5-HT had no great effect on post-spike afterresponses. The results obtained here support the suggestion that 5-HT significantly modulates the activity of spinal neurons of different functional types. 5-HT facilitates excitation induced by subthreshold depolarization in motoneurons and some interneurons, facilitating the generation of rhythmic discharges by decreasing afterhyperpolarization. In sensory cells, 5-HT enhances inhibition due to hyperpolarization, suppressing NMDA currents. The differences in the effects of 5-HT on functionally diverse neurons are presumed to be associated with the combination of different types of 5-HT receptors on the membranes of these types of spinal neurons.

Gating and braking of short- and long-term modulatory effects by interactions between colocalized neuromodulators

Spinal locomotor networks in the lamprey are modulated by tachykinin neuropeptides. A single 10 min application of the tachykinin substance P evokes a short-term ( approximately 1 hr) presynaptic facilitation of glutamate release and the postsynaptic potentiation of NMDA responses. The latter effect induces a long-term (>24 hr) protein synthesis-dependent increase in the frequency of network activity. Tachykinins are contained in a ventromedial spinal plexus into which the medial dendrites of network neurons project. Neurons in this plexus also contain colocalized dopamine and 5-HT. Here, dynamic plasticity evoked by modulator interactions has been examined by investigating the effects of 5-HT and dopamine on specific cellular, synaptic, and network effects of substance P. Preapplied 5-HT blocked the substance P-mediated increase in the network burst frequency and the potentiation of NMDA-evoked cellular responses that underlies its induction. 5-HT also blocked the presynaptic facilitation of glutamatergic synaptic transmission by substance P. The presynaptic, but not postsynaptic, effect of 5-HT was reduced by the protein phosphatase 2B inhibitor cypermethrin. Dopamine did not directly modulate the effects of substance P. However, it reduced the presynaptic interactive effect of 5-HT and thus gated the presynaptic potentiation of glutamatergic inputs by substance P. However, the substance P-mediated potentiation of NMDA responses was not gated by dopamine, and thus the long-term network modulation was not induced. Neuromodulator effects and their interactions can thus be modulated. By selecting components from the modulatory repertoire of substance P, these interactions evoke dynamic changes in short- and long-term synaptic and network plasticity.

The intrinsic function of a motor system--from ion channels to networks and behavior

The forebrain, brainstem and spinal cord contribution to the control of locomotion is reviewed in this article. The lamprey is used as an experimental model since it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre- and postsynaptic modulation. This experimental model is bridging the gap between the molecular and cellular level to the network and behavioral level.

Phenotypical segregation among female rat hypothalamic gonadotropin-releasing hormone neurons as revealed by the sexually dimorphic coexpression of cholecystokinin and neurotensin

The neuroendocrine control of the gonad is exerted primarily by the gonadotropin-releasing hormone neurons located in the septum and the hypothalamus. Despite their sexually dimorphic activity, tonic in males and phasic in females, these neurons have not appeared qualitatively different between sexes in intrinsic organization or chemical phenotype. Here, by using multiple-label immunocytochemistry, it is demonstrated that the phenotype of gonadotropin-releasing hormone neurons is sex specific. In females only, 54.5% of them co-expressed cholecystokinin immunoreactivity and 29.4% additionally expressed neurotensin immunoreactivity. These multipeptidergic neurons were observed in the hypothalamus but not in the septum. During postnatal development, cholecystokinin and neurotensin immunoreactivities were first detected in gonadotropin-releasing hormone-containing axons of the median eminence at vaginal opening, suggesting an involvement of the neuropeptides in peri-ovulatory events. This peptidergic phenotype was not apparent in females ovariectomized as adults but was reinstated by estradiol treatment. In adult males, the testicle does not control this phenotype because orchidectomized adults did not display it, whatever the post-operative delay (one to five weeks) or substitutive chronic steroid treatment (testosterone or estradiol). The testicle may, however, masculinize the phenotype neonatally because estradiol or testosterone treatment in adulthood induced an expression of cholecystokinin immunoreactivity in gonadotropin-releasing hormone-containing axons of the median eminence in both males and females that were gonadectomized at birth. This procedure, however, failed to significantly induce an expression of neurotensin immunoreactivity, suggesting a role of the postnatal ovary on this element of the chemistry of gonadotropin-releasing hormone neurons.Thus, the gonad permanently organizes the gonadotropin-releasing hormone neuronal population, resulting, at least in females, in a mosaic of phenotypically distinct, functional subunits.

Presynaptic and interactive peptidergic modulation of reticulospinal synaptic inputs in the lamprey

The modulatory effects of neuropeptides on descending inputs to the spinal cord have been examined by making paired recordings from reticulospinal axons and spinal neurons in the lamprey. Four peptides were examined; peptide YY (PYY) and cholecystokinin (CCK), which are contained in brain stem reticulospinal neurons, and calcitonin-gene-related peptide (CGRP) and neuropeptide Y (NPY), which are contained in primary afferents and sensory interneurons, respectively. Each of the peptides reduced the amplitude of monosynaptic reticulospinal-evoked excitatory postsynaptic potentials (EPSPs). The modulation appeared to be presynaptic, because postsynaptic input resistance and membrane potential, the amplitude of the electrical component of the EPSP, postsynaptic responses to glutamate, and spontaneous miniature EPSP amplitudes were unaffected. In addition, none of the peptides affected the pattern of N-methyl-D-aspartate (NMDA)-evoked locomotor activity in the isolated spinal cord. Potential interactions between the peptides were also examined. The "brain stem peptides" CCK and PYY had additive inhibitory effects on reticulospinal inputs, as did the "sensory peptides" CGRP and NPY. Brain stem peptides also had additive inhibitory effects when applied with sensory peptides. However, sensory peptides increased or failed to affect the amplitude of reticulospinal inputs in the presence of the brain stem peptides. These interactive effects also appear to be mediated presynaptically. The functional consequence of the peptidergic modulation was investigated by examining spinal ventral root responses elicited by brain stem stimulation. CCK and CGRP both reduced ventral root responses, although in interaction both increased the response. These results thus suggest that neuropeptides presynaptically influence the descending activation of spinal locomotor networks, and that they can have additive or novel interactive effects depending on the peptides examined and the order of their application.

Serotonergic systems in the spinal cord of the amphibian urodele Pleurodeles waltl

The role of the monoamine serotonin (5-HT) in modulating the neural networks underlying axial locomotor movements was studied in an adult amphibian urodele, Pleurodeles waltl. 5-HT was applied to an in vitro brainstem-spinal cord preparation of P. waltl, which displayed fictive axial locomotor patterns following bath application of N-methyl-D-aspartate (5 microM) with D-serine (10 microM). Our results showed that 5-HT (1-25 microM) produces a reversible increase in the cycle duration and the duration of rhythmic bursting activity recorded extracellularly from ventral roots innervating the axial musculature. When applied alone, 5-HT does not trigger axial locomotor activity. The distribution pattern of 5-HT immunoreactive (5-HT-ir) cells along the spinal cord was investigated both in intact and in chronic spinal animals. The number of 5-HT-ir cell bodies is higher at brachial levels and decreases through crural levels. Sparse oval or fusiform 5-HT-ir somata are present within the gray matter, just ventrolateral to the central canal. Longitudinal fibers were detected throughout the entire white matter, except in the medial part of the dorsal funiculi. Two columns of intensely labeled and profusely branching thick and thin fibers associated with numerous varicosities run continuously along the ventrolateral surface of the spinal cord. Three weeks following full spinal cord transection at the level of the second spinal root, all longitudinal processes had disappeared, indicating their supraspinal origin, whereas the ventrolateral plexes remained, suggesting that they originated from intraspinal 5-HT-ir cell bodies. Our data showing that spinal 5-HT is organized according to a rostrocaudal gradient suggest that the 5-HT systems of P. waltl are not related to the presence of limb motor pools but more likely are related to axial central pattern generators (CPGs) networks down the length of the spinal cord. The possible involvement of these two sources (descending vs. intraspinal) of 5-HT innervation in the modulation of the axial CPGs is discussed.

Neuropeptide-mediated facilitation and inhibition of sensory inputs and spinal cord reflexes in the lamprey

The effects of neuromodulators present in the dorsal horn [tachykinins, neuropeptide Y (NPY), bombesin, and GABAB agonists] were studied on reflex responses evoked by cutaneous stimulation in the lamprey. Reflex responses were elicited in an isolated spinal cord preparation by electrical stimulation of the attached tail fin. To be able to separate modulator-induced effects at the sensory level from that at the motor or premotor level, the spinal cord was separated into three pools with Vaseline barriers. The caudal pool contained the tail fin. Neuromodulators were added to this pool to modulate sensory inputs evoked by tail fin stimulation. The middle pool contained high divalent cation or low calcium Ringer to block polysynaptic transmission and thus limit the input to the rostral pool to that from ascending axons that project through the middle pool. Ascending inputs and reflex responses were monitored by making intracellular recordings from motor neurons and extracellular recordings from ventral roots in the rostral pool. The tachykinin neuropeptide substance P, which has previously been shown to potentiate sensory input at the cellular and synaptic levels, facilitated tail fin-evoked synaptic inputs to neurons in the rostral pool and concentration dependently facilitated rostral ventral root activity. Substance P also facilitated the modulatory effects of tail fin stimulation on ongoing locomotor activity in the rostral pool. In contrast, NPY and the GABAB receptor agonist baclofen, both of which have presynaptic inhibitory effects on sensory afferents, reduced the strength of ascending inputs and rostral ventral root responses. We also examined the effects of the neuropeptide bombesin, which is present in sensory axons, at the cellular, synaptic, and reflex levels. As with substance P, bombesin increased tail fin stimulation-evoked inputs and ventral root responses in the rostral pool. These effects were associated with the increased excitability of slowly adapting mechanosensory neurons and the potentiation of glutamatergic synaptic inputs to spinobulbar neurons. These results show the possible behavioral relevance of neuropeptide-mediated modulation of sensory inputs at the cellular and synaptic levels. Given that the types and locations of neuropeptides in the dorsal spinal cord of the lamprey show strong homologies to that of higher vertebrates, these results are presumably relevant to other vertebrate systems.

Serotonin-immunoreactive structures in the central nervous system of the garfish Lepisosteus productus (Semionotiformes, Osteichthyes)

Serotonin-immunoreactive (5HT-IR) neurons were investigated in the brain and rostral (cervical) spinal cord of the garfish, a non-teleost actinopterygian. The diencephalon contained a prominent 5HT-IR cell group consisting mainly of liquor-contacting neurons in the hypothalamic periventricular wall. Their ependymofugal processes formed thick bundles or patches within the hypothalamus and then arborized profusely. Other groups of 5HT-IR cells were found in the dorsal thalamus and in the caudal cortex of the hypothalamic inferior lobe. In the caudal brainstem, 5HT-IR cells were predominant in the raphe region. The spinal 5HT-IR cells were scattered rostrocaudally in the ventromedial zone. The structure corresponding to the submeningeal serotonergic system in the ventral spinal cord of the lamprey and stingray was demonstrated also in the garfish.

Vertebrate locomotion--a lamprey perspective

The forebrain, brain stem, and spinal cord contribution to the control of locomotion is reviewed in this chapter. The lamprey is used as an experimental model because it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre- and postsynaptic modulation. Neuropeptides target different cellular and synaptic mechanisms and cause long-lasting changes (> 24 h) in network function.

Substance P modulates sensory action potentials in the lamprey via a protein kinase C-mediated reduction of a 4-aminopyridine-sensitive potassium conductance

We have examined the effects of the tachykinin substance P on the action potential of lamprey mechanosensory dorsal cells. Substance P increased the spike duration and reduced the afterhyperpolarization. These effects were mimicked by stimulation of the dorsal root, which contains tachykinin-like immunoreactive fibres. The tachykinin antagonist spantide II blocked the effects of both substance P and dorsal root stimulation. The spike broadening was voltage-dependent, and was due to the reduction of a 4-aminopyridine-sensitive potassium conductance. The spike broadening was mimicked by G-protein activators and blocked by the G-protein inhibitor GDPbetaS. Pertussis toxin did not block the effects of substance P. The spike broadening was blocked by the protein kinase C and cAMP-dependent protein kinase inhibitor H7, and by the specific protein kinase C antagonist chelerythrine, but not by the cAMP and cGMP-dependent protein kinase inhibitor H8. The phorbol ester phorbol 12,13-dibutyrate mimicked and blocked the effects of substance P, supporting the role of protein kinase C in the spike modulation. The adenylate cyclase activator forskolin and the cAMP agonist SpcAMPs mimicked but did not block the effects of substance P on the spike duration, suggesting that protein kinase A also modulates the dorsal cell action potential, but that substance P acts independently of this pathway. Substance P also increased the excitability of the dorsal cells. This effect was blocked by 4-AP, PDBu and chelerythrine, but not by H8, suggesting that the increase in excitability shares the same intracellular and effector pathways as the spike broadening.

5-HT inhibits calcium current and synaptic transmission from sensory neurons in lamprey

In the lamprey spinal cord, 5-hydroxytryptamine (5-HT) immunoreactivity (ir) is present in the ventromedial plexus originating from intraspinal neurons, ventrolateral column arising from the brainstem, and dorsal column. The latter 5-HT system originates from small dorsal root ganglion neurons. Combined Lucifer yellow intracellular labeling of the intraspinal sensory neurons, dorsal cells, and 5-HT immunohistochemistry showed close appositions between 5-HT-ir fibers and dorsal cell axons. Application of 5-HT depressed monosynaptic EPSPs evoked in giant interneurons by stimulation of single dorsal cells, dorsal roots, or dorsal column without any detectable change in the input resistance of postsynaptic neurons. Furthermore, the amplitude of AMPA-evoked depolarizations in giant interneurons was unaffected by 5-HT. The lack of postsynaptic effects of 5-HT indicates that the decrease of the amplitude of sensory monosynaptic EPSPs by 5-HT is mediated by presynaptic mechanisms. The inhibition of monosynaptic EPSPs by 5-HT was not counteracted by an antagonist of 5-HT1A receptors. 5-HT also reduced the amplitude of the calcium current recorded in isolated dorsal cells and slowed down its kinetics. The inhibition of calcium channels could represent the mechanism mediating the depression of synaptic transmission at the axonal level. These results show that activation of 5-HT receptors on dorsal cell axons as well as on other sensory neurons mediates inhibition of sensory synaptic transmission to giant interneurons. In intact animals, 5-HT could be released from small 5-HT neurons in dorsal root ganglia, which thus may underlie direct sensory-sensory interactions.

Rostrocaudal distribution of 5-HT innervation in the lamprey spinal cord and differential effects of 5-HT on fictive locomotion

5-hydroxytryptamine (5-HT) is known to modulate the locomotion generator network in the lamprey spinal cord, but little is known about the pattern of 5-HT innervation along the spinal cord. The distribution of 5-HT-immunoreactive (5-HT-ir) cells and fibers, as well as the effects of 5-HT on the locomotor network in the rostral and caudal parts of the spinal cord were compared in two lamprey species, Lampetra fluviatilis and Petromyzon marinus. Intraspinal 5-HT cells form a very dense ventromedial plexus in which the dendrites of neurons forming the locomotor network are distributed. The number of 5-HT cells and varicosities in this plexus decreases in the fin area (segments 70-90), and then increases somewhat in the most caudal segments. The descending 5-HT fibers from the rhombencephalon are located in the lateral and ventral columns, and their numbers gradually decrease to around 50% in the tail part of the spinal cord. In contrast, the number of 5-HT-ir axons in the dorsal column remains the same along the spinal cord. Bath application of both N-methyl-D-aspartic acid (NMDA, 20-250 microM) and D-glutamate (250-1000 microM) was used to induce fictive locomotion in the isolated spinal cord. Bath application of 5-HT (1 microM) reduced the burst frequency in the presence of NMDA. The 5-HT effect was, however, significantly greater in the rostral as compared to the caudal part. With D-glutamate, the 5-HT effects was instead more pronounced in the caudal spinal cord. To account for this difference in 5-HT effects on NMDA- and D-glutamate-induced fictive locomotion, the cellular effect of D-glutamate was further investigated. It activates not only NMDA, but also alpha amino-3-hydroxy-5-methyl-4-isoxyl propionate (AMPA)/kainate and metabotropic glutamate receptors. In contrast to NMDA, D-glutamate did not elicit tetrodotoxin (TTX)-resistant membrane potential oscillations. This difference in action between NMDA (selective NMDA receptor agonist) and D-glutamate (mixed agonist) may partially account for the differences in effect of 5-HT on the locomotor pattern.

Synaptic and nonsynaptic monoaminergic neuron systems in the lamprey spinal cord

In the lamprey spinal cord, dopamine- (DA) and 5-hydroxytryptamine-(5-HT) containing cells appear to play an important role in controlling the firing properties of motoneurons and interneurons and, thereby, in modulating the efferent motor pattern. To determine the detailed morphology and synaptic connectivity of the intraspinal DA and 5-HT systems in Lampetra fluviatilis and Ichthyomyzon unicuspis, DA and 5-HT antisera were used in light and electron microscopic immunocytochemical experiments. Two main groups of labeled cells were distinguished: DA-containing liquor-contacting (LC) cells distributed along the central canal, and 5-HT+DA-containing multipolar cells located near the midline ventral to the central canal. Both types were synaptically connected with other neuronal elements. The DA-immunoreactive LC cells, which extended a ciliated process into the central canal, received symmetrical synapses from unlabeled terminals containing small synaptic vesicles. The distal process of the LC cells could be traced to the lateral cell column, to the ventral aspect of the dorsal column, or to the ventromedial area. Ultrastructural analysis of DA fibers in these regions showed the presence of labeled terminals containing numerous small synaptic vesicles and a few dense-core vesicles. These terminals formed symmetrical synapses with unlabeled cell bodies and dendrites, with GABA-immunopositive LC cells, and with the multipolar DA+5-HT cells. The multipolar DA+5-HT cells also received input from unlabeled synapses. Intracellular recording from these cells showed that they received excitatory postsynaptic potentials in response to stimulation of fibers in the ventromedial tracts and dorsal roots. The terminals of the multipolar DA+5-HT neurons in the ventromedial spinal cord contained numerous dense-core vesicles and small synaptic vesicles, but no synaptic specializations could be detected. In addition, a small number of larger DA-immunoreactive cells were observed in the lateral cell column at rostral levels. The lamprey spinal cord thus contains distinct populations of synaptically interconnected monoaminergic neurons. Dopamine-containing LC cells synapse onto DA+5-HT-containing multipolar cells, in addition to GABAergic LC cells and unidentified spinal neurons. In contrast, the multipolar cells appear to exert their influence by nonsynaptic mechanisms.

Distribution of galanin-like immunoreactive elements in the brain of the adult lamprey Lampetra fluviatilis

Galanin is a brain-gut peptide present in the central nervous system of vertebrates and invertebrates. The distribution of galanin-like immunoreactive perikarya and fibers in the brain of the river lamprey Lampetra fluviatilis (Agnatha) has been studied immunocytochemically by using antisera against rat and porcine galanin. Galanin-like immunoreactive perikarya were seen in the telencephalon and mediobasal diencephalon. In the telencephalon, they were present in the nucleus olfactorius anterior, nucleus basalis, and especially, in the nucleus commissurae anterioris. The diencephalon contained most of the immunoreactive neurons. They were located in the nucleus commissurae praeinfundibularis, nucleus ventralis hypothalami, nucleus commissurae postinfundibularis, nucleus ventralis thalami, and nucleus dorsalis thalami pars medius. Most of the galanin-like immunoreactive infundibular neurons showed apical processes contacting the cerebrospinal fluid. Immunoreactive fibers and terminals were widely distributed throughout the neuraxis. In the telencephalon, the richest galaninergic innervation was found in the nucleus olfactorius anterior, lobus subhippocampalis, corpus striatum, and around the nucleus septi and the nucleus praeopticus. In the diencephalon, the highest density of galanin-like immunoreactive fibers was seen in the nucleus commissurae postopticae, nucleus commissurae praeinfundibularis, nucleus ventralis hypothalami, nucleus dorsalis hypothalami, and neurohypophysis. In the mesencephalon and rhombencephalon, the distribution of immunoreactive fibers was heterogeneous, being most pronounced in a region between the nucleus nervi oculomotorii and the nucleus interpeduncularis mesencephali, in the nucleus isthmi, and in the raphe region. A subependymal plexus of immunoreactive fibers was found throughout the ventricular system. The distribution of immunoreactive neurons and fibers was similar to that of teleosts but different to those of other vertebrate groups. The possible hypophysiotropic and neuroregulatory roles of galanin are discussed.

5-HT innervation of reticulospinal neurons and other brainstem structures in lamprey

In order to determine if reticulospinal neurons involved in the control of locomotion and responsive to exogenously applied 5-hydroxytryptamine (5-HT) are innervated by fibers that contain serotonin, the serotoninergic innervation of reticulospinal neurons, identified by retrograde labeling with fluorescein-conjugated dextran-amine (FDA), was investigated by immunohistochemistry in the lamprey brainstem. A widespread distribution of 5-HT immunoreactive (5-HT-ir) fibers was seen within the basal plate of the brainstem, an area containing reticulospinal somata and dendritic aborizations. Numerous 5-HT varicose fibers were found in close relation to large reticulospinal cell bodies, particularly in the middle and anterior rhombencephalic reticular nuclei (MRRN and ARRN). Some of these reticulospinal somata were surrounded by a very dense pericellular 5-HT innervation. 5-HT-ir fibers were also seen in other brain structures that are known to influence reticulospinal neurons such as the rhombencephalic alar plate containing sensory relay interneurons, cranial nerves (III-X), cerebellum, and tectum. These findings suggest that, as in the spinal cord, motor behavior controlled by reticulospinal neurons may be subject to a serotoninergic modulation.

Immunohistochemical analysis of the relation between 5-hydroxytryptamine- and neuropeptide-immunoreactive elements in the spinal cord of an amphibian (Xenopus laevis)

In mammals, a large proportion of the bulbospinal 5-hydroxytryptamine (5-HT) neurons also contain neuropeptides, such as substance P (SP) and galanin (GAL). To examine whether a similar coexistence occurs in an amphibian, an immunofluorescence double-labelling technique was employed on sections of the Xenopus laevis spinal cord. Antisera raised against SP, GAL, enkephalin (ENK), corticotropin-releasing factor (CRF), calcitonin gene-related peptide (CGRP), and cholecystokinin (CCK) produced a labelling of fibers at all rostrocaudal levels of the spinal cord, with the highest fiber densities for SP and ENK and intermediate densities for GAL, CCK, and CGRP, while CRF-immunoreactive fibers were barely detectable in intact animals. 5-HT-immunoreactive fibers were widely distributed in the spinal cord, and they often occurred in the vicinity of different types of peptide-immunoreactive fibers. However, no coexistence between 5-HT and the different peptide immunoreactivities could be detected, although SP and GAL immunoreactivities were sometimes found to be colocalized in the same fiber. Similar negative results were obtained when 5-HT+SP- and 5-HT+GAL-labelled sections were examined in single focal planes with a confocal microscope. After a spinal transection, (survival period 6 weeks to 4 months), almost all 5-HT-immunoreactive fibers below the lesion were lost, and a build-up of immunoreactive material occurred in fibers just rostral to the cut. In contrast, no significant loss of peptide-immunoreactive fibers occurred, although some swollen SP-, GAL-, ENK-, CRF-, and CCK-immunoreactive fibers were present rostral to the cut. The distribution of swollen peptide-immunoreactive fibers did not overlap with that of the swollen 5-HT-immunoreactive fibers. Although negative immunohistochemical data must be interpreted with caution, in conjunction with previous studies (Brodin et al. [1988] J. Comp. Neurol. 271:1-18; Sakamoto and Atsumi [1991] Cell Tissue Res. 264:221-230), the present results indicate that bulbospinal 5-HT neurons in nonmammalian vertebrates cocontain neuropeptides to a lesser extent than in mammals.

Localization of cholecystokinin-like peptide in neuroendocrine cells of mammalian lungs: a light and electron microscopic immunohistochemical study

We report immunohistochemical localization of cholecystokinin (CCK)-like immunoreactivity at the light and electron microscopy (EM) level in pulmonary neuroendocrine (NE) cells of human and other mammals (monkey, rabbit, rat, hamster, pig, dog and lamb). In addition, immunolocalization of CCK-like peptide was compared with that of bombesin (predominant peptide in human lung) and serotonin (an amine found in NE cells of most species). While CCK-like and serotonin-like immunoreactivity were identified in both solitary NE cells and NE cell clusters (neuroepithelial bodies, NEB) of all species studied, bombesin-like immunoreactive NE cells were found in human and monkey lungs only. The distribution and intensity of immunostaining for CCK-like peptide varied between species with some showing relatively high levels of expression (e.g., monkey, piglet, dog and lamb), others intermediate (human, rabbit) or weak immunostaining (rat, hamster). At the EM level, CCK-like immunoreactivity was localized in dense-core vesicles (DCV), the expected site of peptide storage. Using a double immunolabeling technique, CCK and serotonin were colocalized in some, but not all DCV. The potential role of CCK in the lung (or for other pulmonary peptides) may include a variety of functions such as modulation of bronchial or vascular tone, growth factor-like and/or hormonal effects.

Subcellular distribution of serotonin in the lamprey spinal cord

The subcellular distribution of serotonin (5-hydroxytryptamine; 5-HT) in the lamprey (Ichtyomyzon unicuspis, Lampetra fluviatilis) spinal cord was investigated by using ultracentrifugation on continuous density gradients combined with an electron microscopic analysis of the gradients and of immunostained tissue. Endogenous 5-HT was analyzed by high-performance liquid chromatography with electrochemical detection. After differential centrifugation, the highest levels of 5-HT were found in the particulate fractions. After ultracentrifugation of lysed synaptosomal fractions on continuous sucrose gradients and the subsequent sedimentation of the individual fractions, 5-HT showed a biphasic distribution in the gradient. The two peaks corresponded to 0.30-0.40 M and 0.85-1.05 M sucrose. Electron microscopy of intact tissue showed that some of the boutons were strongly immunoreactive to 5-HT with dense precipitates over large granular vesicles. The area around these large vesicles, however, also showed reaction product. Large granular vesicles could be clearly distinguished in the immunostained axonal varicosities. In tissue not processed for 5-HT immunoreactivity it was seen that the varicosities contained not only large dense-cored vesicles, but also small agranular vesicles. An electron microscopical analysis of the subcellular fractions revealed that the fraction corresponding to the "light" 5-HT peak contained numerous vesicular structures, which in most cases were electron lucent. In the "heavy" fractions, nerve ending particles containing vesicles of various sizes were observed. The results suggest that 5-HT in the lamprey spinal cord may be distributed in more than one subcellular compartment which, apart from the cytosol, possibly corresponds to small and large synaptic vesicles.

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.

Gut hormones in cyclostomes

The current state of knowledge about regulatory peptides in endocrine cells and nerves of the alimentary canal of lampreys and hagfishes is reviewed. Cyclostomes have a wide range of peptides similar immunochemically to those of higher vertebrates. They include, in the endocrine cells of the intestine, peptides resembling glucagon, gastrin/cholecystokinin, peptide YY (pancreatic polypeptide/neuropeptide Y), substance P, vasoactive intestinal peptide, somatostatin and, in the larval stages at least, insulin. The enteric nerves of some lamprey species contain peptides resembling bombesin (gastrin-releasing peptide) and calcitonin gene-related peptide, as well as serotonin. The occurrence of other peptides is less well documented.Little is known of the molecular structure or the biological roles of the enteric peptides in cyclostomes. Extraction, purification, sequencing and physiological experiments are greatly needed.

5-hydroxytryptamine immunoreactive varicosities in the lamprey spinal cord have no synaptic specializations--an ultrastructural study

The distribution and fine structure of 5-hydroxytryptamine (5-HT) immunoreactive cell bodies and axonal varicosities have been studied in the lamprey spinal cord, using the peroxidase-antiperoxidase (PAP) immunohistochemical technique and subsequent analysis of ultrathin serial sections. Immunostained cell bodies were found in the ventral spinal cord close to the central canal. Immunostained varicosities were found throughout the spinal cord with the highest density in the ventromedial plexus and the dorsal horn. Only large granular vesicles could be clearly distinguished in immunostained cell bodies and varicosities, but it was concluded based on a comparison with unstained normal tissue that these boutons also contained small, pleomorphic agranular vesicles. Immunoreactive varicosities were studied in the ventromedial plexus, the dorsal horn, the dorsal column, the dorsolateral and ventrolateral funiculi and the grey matter. No morphological differences could be observed between varicosities in the different loci. The varicosities were in no case seen to make synaptic contact with surrounding neuronal elements, even when followed through serial sections. Consequently, 5-HT released from boutons in all parts of the spinal cord could be expected to act on 5-HT receptors located on nearby as well as distant receptors.

Distribution of histaminergic neurons in the brain of the lamprey Lampetra fluviatilis as revealed by histamine-immunohistochemistry

An antiserum against conjugated histamine was used to study the distribution of histaminergic neurons in the CNS of the lamprey Lampetra fluviatilis. Numerous histamine-immunoreactive cell bodies were detected in the dorsal and ventral hypothalamic nuclei and in the adjacent postinfundibular commissural nucleus. Histamine-immunoreactive fibers of high density were present in the ventral hypothalamus, and fibers could also be traced dorsally from the hypothalamus to the corpus striatum and septal nucleus where they appeared to terminate in dense plexuses. Another, smaller group of histamine-immunoreactive perikarya was observed in the border area between mesencephalon and rhombencephalon, near the caudal pole of the mesencephalic reticular nucleus. Sparsely distributed histamine-immunoreactive fibers were present in the ventral mesencephalon. The distribution of histaminergic neurons in cyclostomes, which diverged very early from the main vertebrate line, shows similarities with the corresponding systems in the CNS of amphibians and mammals, which suggests that histaminergic neuronal systems are phylogenetically old and have been conserved during evolution.

Three types of GABA-immunoreactive cells in the lamprey spinal cord

Polyclonal antisera raised against conjugated GABA were used to study the distribution of GABAergic neurons in the spinal cords of lampreys (Lampetra fluviatilis and Ichtyomyzon unicuspis) using immunofluorescence and peroxidase-antiperoxidase techniques. Three morphologically distinct types of GABA-immunoreactive (GABA-ir) cell bodies were observed, multipolar neurons in the lateral grey cell column, apparently bipolar cells in the ventral aspect of the dorsal horn, and small liquor-contacting cells surrounding the central canal. A high density of immunoreactive fibers of spinal origin were present in the lateral and ventral funiculi, whereas the dorsal column had a relatively low density. Dense GABA-ir plexuses were situated in the lateral spinal margin, and in the dorsal part of the dorsal horn. A chronic lesion of the rostral spinal cord did not result in any observable loss of GABA-ir fibers below or above the lesion, suggesting that the 3 types of segmental GABA-ir neurons are the main sources of the GABAergic innervation of the lamprey spinal cord.

Increase in endogenous 5-hydroxytryptamine levels modulates the central network underlying locomotion in the lamprey spinal cord

To investigate the effects of an endogenous release of serotonin (5-HT) in the lamprey spinal cord, in vitro, the 5-HT uptake-blocker citalopram (1-10 microM) was added to the bathing solution. Samples taken from the physiological solution through high-performance liquid chromatography (HPLC) showed that 5-HT was released from the spinal cord. To study the effect of this endogenous release of 5-HT on the spinal network generating locomotion, 'fictive locomotion' was induced by bath application of N-methyl-D-aspartate (NMDA, 100 microM). It elicited a steady locomotor rhythm between 0.2 and 2.5 Hz. The effects of citalopram were the following: (1) the locomotor frequency slowed down, (2) the intensity of the ventral root bursts was increased and (3) the intersegmental phase lag was prolonged. The effects of citalopram and thus presumably of an endogenous release of 5-HT were similar to what has previously been observed during bath application of 5-HT. In the latter case the conditions are different, since all 5-HT receptors regardless of their location in relation to the 5-HT-containing boutons will be affected in a similar way.

Phasic modulation of reticulospinal neurones during fictive locomotion and other types of spinal motor activity in lamprey

The intracellular activity of different types of reticulospinal neurones was studied during fictive locomotion and other types of spinal motor activity in an in vitro preparation of the lamprey brainstem-spinal cord. The examined neurones included large Müller cells of the rhombencephalic and mesencephalic reticular formation, the Mauthner cell, and neurones in the posterior rhombencephalic reticular nucleus with different sizes and conduction velocities. During bouts of fictive swimming initiated spontaneously or by stimulation of the trigeminal nerve or spinal cord, the Müller cells were depolarized and fired action potentials. Bulbar Müller cells in addition showed a phasic modulation of membrane potential with excitation in phase with ipsilateral motoneurones of the rostral spinal cord. The Mauthner cell was depolarized in phase with contralateral motoneurones. Many neurones in the posterior rhombencephalic reticular nucleus showed modulation in phase with ipsilateral motoneurones during fictive swimming. Such oscillations were observed in both fast-conducting neurones, located mainly in the medial part of the nucleus, and slower conducting cells with a more lateral distribution. All examined reticulospinal neurones showed a strong coupling also with other types of spinal motor activity, such as slow alternating bursting and synchronous bilateral ventral root bursts, but the reticulospinal activity had no correlation with respiratory activity recorded from the Xth nerve. The consequences of a phasic reticulospinal activity during locomotion are discussed.

Further evidence for excitatory amino acid transmission in lamprey reticulospinal neurons: selective retrograde labeling with (3H)D-aspartate

The distribution of radiolabeled neurons in the brain stem of Lampetra fluviatilis was studied following unilateral injections of (3H)D-aspartate in the rostral spinal cord. After survival periods of 1-3 days, labeled perikarya were present within and nearby the posterior, middle, and anterior rhombencephalic reticular nuclei and in the mesencephalic reticular nucleus. The highest number of (3H)D-aspartate labeled cell bodies were present in the posterior rhombencephalic reticular nucleus. The labeled reticulospinal neurons were distributed mainly ipsilateral to the injection site and included the giant Müller cells as well as medium-sized and small neurons. Contralateral labeling occurred in cell bodies scattered along the lateral margin of the rhombencephalic reticular formation, the most rostral of these contralaterally projecting neurons being the Mauthner cell. The (3H)D-aspartate labeling correlates with previous electrophysiological studies showing that lamprey reticulospinal neurons utilize excitatory amino acid transmission.