Ultrastructural Characterization of Adrenocorticotrope Hormone (ACTH) Immunoreactive Fibres in the Mesencephalic Central Grey Substance of the Rat

Human cerebrospinal fluid increases the excitability of pyramidal neurons in the in vitro brain slice

KEY POINTS

The cerebrospinal fluid contains numerous neuromodulators at ambient levels but whether, and how, they affect the activity of central neurons is unknown. This study provides experimental evidence that human cerebrospinal fluid (hCSF) increases the excitability of hippocampal and neocortical pyramidal neurons. Hippocampal CA1 pyramidal neurons in hCSF displayed lowered firing thresholds, depolarized resting membrane potentials and reduced input resistance, mimicking properties of pyramidal neurons recorded in vivo. The excitability-increasing effect of hCSF on CA1 pyramidal neurons was entirely occluded by intracellular application of GTPγS, suggesting that neuromodulatory effects were mediated by G-protein coupled receptors. These results indicate that the CSF promotes spontaneous excitatory neuronal activity, and may help to explain observed differences in the activity of pyramidal neurons recorded in vivo and in vitro. The composition of brain extracellular fluid is shaped by a continuous exchange of substances between the cerebrospinal fluid (CSF) and interstitial fluid. The CSF is known to contain a wide range of endogenous neuromodulatory substances, but their collective influence on neuronal activity has been poorly investigated. We show here that replacing artificial CSF (aCSF), routinely used for perfusion of brain slices in vitro, with human CSF (hCSF) powerfully boosts spontaneous firing of CA1, CA3 and layer 5 pyramidal neurons in the rat brain slice. CA1 pyramidal neurons in hCSF display lowered firing thresholds, more depolarized resting membrane potentials and reduced input resistance, mimicking properties of pyramidal neurons recorded in vivo. The increased excitability of CA1 pyramidal neurons was completely occluded by intracellular application of GTPγS, suggesting that endogenous neuromodulators in hCSF act on G-protein coupled receptors to enhance excitability. We found no increase in spontaneous inhibitory synaptic transmission by hCSF, indicating a differential effect on glutamatergic and GABAergic neurons. Our findings highlight a previously unknown function of the CSF in promoting spontaneous excitatory activity, and may help to explain differences observed in the activity of pyramidal neurons recorded in vivo and in vitro.

Unraveling the central proopiomelanocortin neural circuits

Central proopiomelanocortin (POMC) neurons form a potent anorexigenic network, but our understanding of the integration of this hypothalamic circuit throughout the central nervous system (CNS) remains incomplete. POMC neurons extend projections along the rostrocaudal axis of the brain, and can signal with both POMC-derived peptides and fast amino acid neurotransmitters. Although recent experimental advances in circuit-level manipulation have been applied to POMC neurons, many pivotal questions still remain: how and where do POMC neurons integrate metabolic information? Under what conditions do POMC neurons release bioactive molecules throughout the CNS? Are GABA and glutamate or neuropeptides released from POMC neurons more crucial for modulating feeding and metabolism? Resolving the exact stoichiometry of signals evoked from POMC neurons under different metabolic conditions therefore remains an ongoing endeavor. In this review, we analyze the anatomical atlas of this network juxtaposed to the physiological signaling of POMC neurons both in vitro and in vivo. We also consider novel genetic tools to further characterize the function of the POMC circuit in vivo. Our goal is to synthesize a global view of the POMC network, and to highlight gaps that require further research to expand our knowledge on how these neurons modulate energy balance.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

The regulation of brain states by neuroactive substances distributed via the cerebrospinal fluid; a review

The cerebrospinal fluid (CSF) system provides nutrients to and removes waste products from the brain. Recent findings suggest, however, that in addition, the CSF contains message molecules in the form of actively released neuroactive substances. The concentrations of these vary between locations, suggesting they are important for the changes in brain activity that underlie different brain states, and induce different sensory input and behavioral output relationships.The cranial CSF displays a rapid caudally-directed ventricular flow followed by a slower rostrally-directed subarachnoid flow (mainly towards the cribriform plate and from there into the nasal lymphatics). Thus, many brain areas are exposed to and can be influenced by substances contained in the CSF. In this review we discuss the production and flow of the CSF, including the mechanisms involved in the regulation of its composition. In addition, the available evidence for the release of neuropeptides and other neuroactive substances into the CSF is reviewed, with particular attention to the selective effects of these on distant downstream receptive brain areas. As a conclusion we suggest that (1) the flowing CSF is involved in more than just nutrient and waste control, but is also used as a broadcasting system consisting of coordinated messages to a variety of nearby and distant brain areas; (2) this special form of volume transmission underlies changes in behavioral states.

Immunoelectron microscopic study of beta-endorphinergic synaptic innervation of GABAergic neurons in the dorsal raphe nucleus

Using a preembedding double immunoreactive technique by immunostaining with antirat beta-endorphin and antisynthetic glutamic acid decarboxylase antisera sequentially, the synaptic relationships between beta-endorphinergic neuronal fibers and GABAergic neurons in the dorsal raphe nucleus of the rat were examined at the ultrastructural level. Although both beta-endorphin-like immunoreactive fibers and glutamic acid decarboxylase-like immunoreactive neurons can be found in the mediodorsal and medioventral parts of the dorsal raphe nucleus, the synapses between them were found only in the mediodorsal part. Most of the beta-endorphin-like immunoreactive neuronal fibers contained many dense-cored vesicles. The synapses made by beta-endorphin-like immunoreactive neuronal axon terminals on glutamic acid decarboxylase-like immunoreactive neurons were both symmetrical and asymmetrical, with the latter predominant, especially in the axo-dendritic synapses. Perikarya with beta-endorphin-like immunoreactivity were found only in the ventrobasal hypothalamus. These findings suggest the possibility that the beta-endorphin-producing neurons in the ventrobasal hypothalamus could influence GABAergic neurons in the dorsal raphe nucleus directly by synaptic relationships.

The emergence of the volume transmission concept

Interneuronal communication in the central nervous system (CNS) have always been of basic importance for theories on the cerebral morphofunctional architecture. Our group has proposed that intercellular communication in the brain can be grouped into 2 broad classes based on some general features of the transmission: wiring (WT) and volume (VT) transmission. WT occurs via a relatively constrained cellular chain (wire), while VT consists of 3-dimensional diffusion of signals in the extracellular fluid (ECF) for distances larger than the synaptic cleft. Both morphological and functional evidence indicates that dopamine (DA) synapses in striatum are 'open' synapses, i.e., synapses which favor diffusion of the transmitter into the surrounding ECF and observations are compatible with the view that DA varicosities can synthesize, store and release DA for VT. The DAergic mesostriatal transmission has, therefore, been examined by several groups to give experimental support to VT. Moreover, due to its minor structural requirements, VT may become prevalent under some pathological conditions, e. g. Parkinson's disease. In animal models of DAergic pathway degeneration, it has been shown that a compensatory activation of surviving DA terminals may lead to a preferential potentiation of VT. WT and VT favor different and complementary types of computation. VT is markedly slower and less safe than WT, but has minor spatial constraints and allows the reach of a large number of targets. Models of neuronal systems integrating classical neuronal circuits and diffusible signals begin to show how WT and VT may interact in the neural tissue.

Wiring and volume transmission in the central nervous system: the concept of closed and open synapses

During the past two decades, several revisions of the concepts underlying interneuronal communication in the central nervous system (CNS) have been advanced. Our group has proposed to classify intercellular communication in the CNS under two general frames: 'wiring' (WT) and 'volume' transmission (VT). WT is characterized by a single 'transmission channel' made by cellular (neuronal or glial) structures and with a region of discontinuity not larger than a synaptic cleft. VT is characterized by the diffusion from a cell source (neuronal or glial) of chemical and electrical signals in the extracellular fluid (ECF) for a distance larger than the synaptic cleft Based on morphological and functional characteristics, and in light of the distinction proposed, six main modes of intercellular communication can be recognized in the CNS: gap-junction, membrane juxtaposition, and closed synapse (which represent WT-type modes of communication); open synapse, paracrine transmission and endocrine-like transmission (which represent VT-type modes of communication). Closed and open synapses are distinguished on the basis of the sealing of the signal within or the leakage of the signal outside the synapse Intra-synaptic restriction or extra-synaptic diffusion of transmitters are insured by a number of anatomical arrangements (e.g. glial ensheathment of synapse, size of the synaptic cleft) and functional mechanisms (e.g. density and location of transmitter re-uptake sites and metabolic enzymes). Some central synapses can switch from closed to open state and vice versa, e.g. by changing the amount of transmitter released. Finally, a synapse containing several transmitters can work as an open synapse for one transmitter and as a closed synapse for another.

Intercellular communication in the brain: wiring versus volume transmission

During the past two decades several revisions of the concepts underlying interneuronal communication in the central nervous system have been advanced. We propose here to classify communicational phenomena between cells of the central neural tissue under two general frames: "wiring" and "volume" transmission. "Wiring" transmission is defined as intercellular communication occurring through a well-defined connecting structure. Thus, wiring transmission is characterized by the presence of physically identifiable communication channels within the neuronal and/or glial cell network. It includes synaptic transmission but also other types of intercellular communication through a connecting structure (e.g., gap junctions). "Volume" transmission is characterized by signal diffusion in a three-dimensional fashion within the brain extracellular fluid. Thus, multiple, structurally often not well characterized extracellular pathways connect intercommunicating cells. Volume transmission includes short- (but larger than synaptic cleft, i.e. about 20 nm) and long-distance diffusion of signals through the extracellular and cerebrospinal fluid. It must be underlined that the definitions of wiring and volume transmission focus on the modality of transmission and are neutral with respect to the source and target of the transmission, as well as type of informational substance transmitted. Therefore, any cell present in the neural tissue (neurons, astroglia, microglia, ependyma, tanycytes, etc.) can be a source or a target of wiring and volume transmission. In this paper we discuss the basic definitions and some distinctive characteristics of the two types of transmission. In addition, we review the evidence for different types of intercellular communication besides synaptic transmission in the central nervous system during phylogeny, and in vertebrates in physiological and pathological conditions.

Immunoelectron microscopy of beta-endorphinergic synaptic innervation of nitric oxide synthase immunoreactive neurons in the dorsal raphe nucleus

On the basis of the comparing of the distribution of beta-endorphin-like immunoreactive neuronal fibres and nitric oxide synthase-like immunoreactive neurons in the dorsal raphe nucleus, the synapses between the two immunocytochemically identified neurons were studied with a modified DAB-silver-gold intensification double immunostaining technique at the electron microscopic level. Although both of them can be found in the mediodorsal and medioventral parts of the dorsal raphe nucleus, the synapses between them could only be found in the mediodorsal part. The majority of the beta-endorphin-like immunoreactive neuronal fibers contained many dense-cored vesicles. The synapses made by beta-endorphin-like immunoreactive neuronal axon terminals on nitric oxide synthase-like immunoreactive neurons were both symmetrical and asymmetrical with the former predominant, especially in the axo-dendritic ones. beta-Endorphin-like immunoreactive perikarya could only be found in the ventrobasal hypothalamus. These findings suggest the possibility that the beta-endorphin- producing neurons in the ventrobasal hypothalamus could influence nitric oxide synthase-containing neurons in the dorsal raphe nucleus by synaptic relations.

Immunocytochemical study of the CLIP/ACTH-immunoreactive nerve fibres in the dorsal raphe nucleus of the rat

The injection of corticotrophin-like intermediate lobe peptide (CLIP) in the lateral ventricle or the dorsal raphe nucleus (DRN) of the rat is followed by a significant increase in the amount of paradoxical sleep. In the DRN, CLIP would act through a somatic and/or dendritic release of serotonin. To establish the anatomical basis of these effects, the nerve fibres immunoreactive for CLIP/ACTH were labelled and their fine anatomical relationships with the neuronal elements of the DRN were studied at the ultrastructural level. Half of the profiles of labelled varicosities were 'free' in the neuropile, the other half was in close contact with dendrites, either shafts or spines. It was interesting to note that few contacts with dendritic shafts were under the form of a synapse, whereas a large number of the contacts with spines were synaptic. The chemical nature of these dendritic targets remains to be determined but preliminary results indicate that they are partly serotoninergic.

Efferent connections of the hypothalamic "grooming area" in the rat

The efferent connections of the hypothalamic area, where grooming can be elicited by local electrical stimulation or injection of various substances, were studied using iontophoretic injections of Phaseolus vulgaris leucoagglutinin. This hypothalamic "grooming area" consists of parts of the hypothalamic paraventricular nucleus and of the dorsal hypothalamic area. The specificity of these efferents for the hypothalamic "grooming area" was investigated by comparison with efferents of hypothalamic sites adjacent to this area. In addition, the distribution of oxytocinergic fibres was studied, since oxytocinergic neurons are present in the hypothalamic "grooming area" and oxytocin is possibly involved in grooming behaviour. The efferents of the hypothalamic "grooming area" as well as of hypothalamic sites surrounding this area and the oxytocinergic fibres studied do not form well determined bundles, but rather spread out throughout the hypothalamus. Clusters of fibres could be traced rostrally and caudally, forming diffuse fibre "streams". Three rostral, two thalamic and three caudal fibre "streams" have been distinguished along which efferent fibres innervate different brain areas. The many varicosities on labelled fibres "en passant" suggest that hypothalamic fibres are able to influence many parts of the brain along their way. The anterior periventricular area, the median preoptic nucleus, the ventral tegmental area and nucleus of the solitary tract were found to be more or less specifically innervated by hypothalamic "grooming area" fibres and oxytocinergic fibres. Other brain areas, like the septum, the medial amygdaloid nucleus, the central gray and the paraventricular nucleus of the thalamus were found to receive efferent projections from the hypothalamic "grooming area" and hypothalamic loci outside this area, as well as from the oxytocinergic system. Within the septum and the mesencephalic central gray, differences in the spatial organization of terminating fibres from the hypothalamic "grooming area" and hypothalamic "non-grooming" sites have been found. Fibres from the grooming area clustered in the ventral part of the lateral septal nucleus, while fibres from surrounding hypothalamic loci innervated other parts of that brain area. In the central gray, fibres from the hypothalamic "grooming area" clustered in rostrodorsal and caudoventral parts. A number of brain areas, that are innervated by hypothalamic "grooming area" fibres and oxytocinergic fibres, like central gray, ventral tegmental area and the noradrenergic A5 area, have been reported previously to be involved in grooming behaviour. It is concluded from the present findings, that the hypothalamic "grooming area" has preferential connections with a number of brain sites, not shared with hypothalamic projections from outside the "grooming area".(ABSTRACT TRUNCATED AT 400 WORDS)

Redistribution of B-50/growth-associated protein 43 during differentiation and maturation of rat hippocampal neurons in vitro

Morphologically polarized hippocampal neurons, grown in culture for two days, contain immunoreactivity of the growth-associated protein B-50 along the plasma membrane of both dendrites and axons. In mature hippocampal neurons, both in vitro and in vivo, B-50 is located in the axon. In order to assess at which stage during neuronal differentiation B-50 is selectively located in the axon, an immuno-light and electron-microscopic study was performed on rat hippocampal neurons developing in vitro. B-50 immunofluorescence was detected in the axon, dendrites and soma of two-day-old polarized neurons. Simultaneously, microtubule-associated protein 2, a marker specific to dendritic microtubules, was predominantly found in the soma, the short dendritic processes and at the base of axonal growth cones. In hippocampal neurons cultured beyond seven days in vitro, microtubule-associated protein 2 immunofluorescence is restricted to the cell soma and dendrites. The spatial distribution of B-50, however, varies. In solitary neurons maturing without interneuronal contacts, B-50 immunofluorescence is observed in axons and in the dendrosomatic domain characterized by the presence of microtubule-associated protein 2. In contrast, in high-density cell cultures B-50 immunofluorescence is absent in the cell body and dendrites, but punctate in axons running along the dendrites. Electron microscopy was carried out on hippocampal neurons of eight to 21 days in vitro to study the process of redistribution of B-50 at the subcellular level. In neurons of eight days in vitro with prominent synapses, B-50 immunoreactivity is significantly elevated at the axonal plasma membrane compared to the plasma membrane of the dendrites and the soma. In neurons from the same culture without synapses, B-50 immunoreactivity is distributed rather densely along the plasma membrane of the soma, dendrites, and on the axonal plasma membrane. A similar B-50 distribution is observed in mature neurons cultured at low cell density without interneuronal cell contacts, for 15 days in vitro. In high-density cell cultures of 21 days in vitro, B-50 is virtually absent at the plasma membrane of the soma and dendrites, and heterogenously distributed along the plasma membrane of axon and axonal varicosities. Our results indicate that selective sorting of B-50 into axons occurs after initial morphological polarization of hippocampal neurons and is correlated with the formation of synapses and with the cessation of dendritic outgrowth.

Ultrastructure of the periaqueductal grey matter of the rat: an electron microscopical and horseradish peroxidase study

The neurons of the mesencephalic periaqueductal grey substance (PAG) in the rat are small and medium sized. The cells are frequently located in small clusters, without interdigitating glial elements and may be connected by direct membrane appositions or by gap junctions. The inner zone of the PAG is cell poor. In many cases, the cytoplasm of the cells is filled with extensive rough endoplasmic reticulum, free ribosomes, Golgi apparatus, and large lysosome-like granules. The nuclei show large indentations. The cells have a high nucleus-cytoplasm ratio. The neuropil is very extensive and particularly rich in large numbers of small unmyelinated axons, dendrites, axonal varicosities, and synaptic connections. Myelinated fibres are relatively scarce. The orientation of the fibres was studied in transverse and horizontal sections, in combination with HRP track tracing experiments. It appeared that throughout the PAG most of the fibres were orientated longitudinally. Quantitation showed that most fibres were present in the inner zones of the PAG. Moreover, the diameter of the fibres adjacent to the aqueduct was smaller than that of the fibres in the peripheral parts of the PAG. The thin unmyelinated fibres made extensive synaptic connections within the PAG. Many synaptic varicosities were found in the neuropil of the PAG. There were four types of synaptic varicosities, characterized by different populations of clear and dense-core secretory granules and by the different morphology of the synaptic specializations. In general, the different types of varicosity were homogeneously distributed in the different parts of the PAG. Electron dense secretory granules, when present, were located at some distance from the synaptic junction. Serial sections revealed varicosities which contained only dense-core secretory granules, without synaptic specializations. The dendrites of PAG neurons generally lacked synaptic spines. Many dendrites, particularly those of neurons located in the peripheral parts of the PAG, were directed toward the aqueduct. The present study shows that the PAG is a very complex brain area. The crisscrossing of axons and dendrites with synaptic connections at considerable distances from the cell bodies render it very difficult to unravel the relationships between the possible sources and destinations of ongoing information. This structure complicates the search for relationships between the functional organization and the cytoarchitectural borders in the PAG area.

Distribution of the pro-opiomelanocortin-immunoreactive axons in relation to the serotoninergic neurons in the dorsal raphe nucleus of the rat

The anatomical relationships between pro-opiomelanocortin-containing axons and serotonin neurons in the nucleus raphe dorsalis (NRD) of the rat were examined at the light microscope level with antibodies against CLIP (corticotropin-like intermediate lobe peptide), alpha-MSH (alpha-melanocyte-stimulating hormone) and serotonin. Sequential double labeling was performed with either immunofluorescence or peroxidase-antiperoxidase techniques. It was observed that the network of POMC-immunoreactive axons displayed a gradient of decreasing density from rostral to caudal levels and from dorsal to ventral parts or the NRD. The examples of close proximity between immunoreactive axons and serotonin cell bodies or dendrites were rather scarce. On the whole, the immunoreactive fibers seemed to run quasi-independently of the serotonin neurons.

Grooming induced by intrahypothalamic injection of ACTH in the rat: comparison with grooming induced by intrahypothalamic electrical stimulation and i.c.v. injection of ACTH

Intracerebroventricular (i.c.v.) injection of adrenocorticotropic hormone (ACTH) elicits grooming in the rat, but the neural organization of this response is still obscure. Electrical stimulation (EHS) in an area around the hypothalamic paraventricular nucleus (PVH) also elicits grooming. This hypothalamic area contains many ACTH-immunoreactive fibres. Injection of ACTH1-24 (0.3 microgram/0.3 microliters) in the same area elicits intense grooming responses in the rat. Latency, intensity and precise patterning of the grooming response are dependent upon the exact site of injection. Comparison of grooming responses elicited by EHS, ACTH injected i.c.v. and ACTH injected in the PVH reveals that these are slightly dissimilar. This may provide clues as to the brain mechanisms involved in the organization of the different components of grooming. EHS does not elicits scratching and even reduces 'spontaneous' scratching. Also, EHS-elicited grooming is characterized by short pauses. The time-course of appearance of yawning differs between ACTH-PVH and ACTH-i.c.v. injections. Excited locomotion elicited only by ACTH-i.c.v. is apparently caused by ACTH-sensitive systems outside the PVH. The results suggest that the ACTH-containing part of the hypothalamus around the PVH is crucially involved in the organization of grooming behaviour. We believe that at this level in the brain, the subroutines of grooming, scratching and yawning are integrated into one skin maintenance behaviour.

Ultrastructural localization of adrenocorticotrophic hormone and the phosphoprotein B-50/growth-associated protein 43 in freeze-substituted, Lowicryl HM20-embedded mesencephalic central gray substance of the rat

Previous studies have shown that the endogenous phosphorylation of the neuron-specific protein B-50 in isolated synaptic plasma membranes is inhibited by adrenocorticotrophic hormone(1-24). The aim of this study is to examine if there is a specific neuroanatomical interaction of adrenocorticotrophic hormone and B-50 in the mesencephalic central gray substance of the rat. With light microscopy, high B-50 immunoreactivity was detected throughout the mesencephalic central gray substance, overlapping with those areas where adrenocorticotrophic hormone-immunoreactive fibres were present. To study the ultrastructural localization of B-50 and adrenocorticotrophic hormone, we employed a method of immunogold labelling on ultrathin sections of freeze-substituted and Lowicryl HM20-embedded fixed brain tissue. This offered optimal morphological preservation together with high retention of antigenicity. At the electron microscopic level, adrenocorticotrophic hormone immunoreactivity was detected in dense-core secretory granules present in non-junctional regions of axoinal varicosities. This suggests a non-synaptic release of adrenocorticotrophic hormone from the axons. Using double immunolabelling techniques we showed that in adrenocorticotrophic hormone-innervated areas of the mesencephalic central gray substance B-50 immunoreactivity was present at plasma membranes of all unmyelinated axons and axonal varicosities and virtually absent in dendrites. The result on B-50 localization agrees well with previous studies in the hippocampus [Van Lookeren Campagne et al. 1990 J. Neurocytol. 19, 948-961] and in the pyramidal tract [Gorgels et al. 1989 J. Neurosci. 9, 3861-3869] of the rat and suggests that in the mature rat central nervous system, B-50 expression in axons is a general phenomenon. For the adrenocorticotrophic hormone-innervated areas, we discuss the proposal that non-synaptically released adrenocorticotrophic hormone modulates B-50 phosphorylation in axons and axon terminals.