Serotonin axons in the supra- and subependymal plexuses and in the leptomeninges; their roles in local alterations of cerebrospinal fluid and vasomotor activity

Identification of a novel perifornical-hypothalamic-area-projecting serotonergic system that inhibits innate panic and conditioned fear responses

The serotonin (5-HT) system is heavily implicated in the regulation of anxiety and trauma-related disorders such as panic disorder and post-traumatic stress disorder, respectively. However, the neural mechanisms of how serotonergic neurotransmission regulates innate panic and fear brain networks are poorly understood. Our earlier studies have identified that orexin (OX)/glutamate neurons within the perifornical hypothalamic area (PFA) play a critical role in adaptive and pathological panic and fear. While site-specific and electrophysiological studies have shown that intracranial injection and bath application of 5-HT inhibits PFA neurons via 5-HT1a receptors, they largely ignore circuit-specific neurotransmission and its physiological properties that occur in vivo. Here, we investigate the role of raphe nuclei 5-HT inputs into the PFA in panic and fear behaviors. We initially confirmed that photostimulation of glutamatergic neurons in the PFA of rats produces robust cardioexcitation and flight/aversive behaviors resembling panic-like responses. Using the retrograde tracer cholera toxin B, we determined that the PFA receives discrete innervation of serotonergic neurons clustered in the lateral wings of the dorsal (lwDRN) and in the median (MRN) raphe nuclei. Selective lesions of these serotonergic projections with saporin toxin resulted in similar panic-like responses during the suffocation-related CO2 challenge and increased freezing to fear-conditioning paradigm. Conversely, selective stimulation of serotonergic fibers in the PFA attenuated both flight/escape behaviors and cardioexcitation responses elicited by the CO2 challenge and induced conditioned place preference. The data here support the hypothesis that PFA projecting 5-HT neurons in the lwDRN/MRN represents a panic/fear-off circuit and may also play a role in reward behavior.

History of research concerning the ependyma: a view from inside the human brain

The history of research concerning ependymal cells is reviewed. Cilia were identified along the surface of the cerebral ventricles c1835. Numerous anatomical and histopathological studies in the late 1800's showed irregularities in the ependymal surface that were thought to be indicative of specific pathologies such as syphilis; this was subsequently disproven. The evolution of thoughts about functions of cilia, the possible role of ependyma in the brain-cerebrospinal fluid barrier, and the relationship of ependyma to the subventricular zone germinal cells is discussed. How advances in light and electron microscopy and cell culture contributed to our understanding of the ependyma is described. Discoveries of the supraependymal serotoninergic axon network and supraependymal macrophages are recounted. Finally, the consequences of loss of ependymal cells from different regions of the central nervous system are considered.

An adult-stage transcriptional program for survival of serotonergic connectivity

Neurons must function for decades of life, but how these non-dividing cells are preserved is poorly understood. Using mouse serotonin (5-HT) neurons as a model, we report an adult-stage transcriptional program specialized to ensure the preservation of neuronal connectivity. We uncover a switch in Lmx1b and Pet1 transcription factor function from controlling embryonic axonal growth to sustaining a transcriptomic signature of 5-HT connectivity comprising functionally diverse synaptic and axonal genes. Adult-stage deficiency of Lmx1b and Pet1 causes slowly progressing degeneration of 5-HT synapses and axons, increased susceptibility of 5-HT axons to neurotoxic injury, and abnormal stress responses. Axon degeneration occurs in a die back pattern and is accompanied by accumulation of α-synuclein and amyloid precursor protein in spheroids and mitochondrial fragmentation without cell body loss. Our findings suggest that neuronal connectivity is transcriptionally protected by maintenance of connectivity transcriptomes; progressive decay of such transcriptomes may contribute to age-related diseases of brain circuitry.

Lasmiditan and 5-Hydroxytryptamine in the rat trigeminal system; expression, release and interactions with 5-HT1 receptors

Background

5-Hydroxytryptamine (5-HT) receptors 1B, 1D and 1F have key roles in migraine pharmacotherapy. Selective agonists targeting these receptors, such as triptans and ditans, are effective in aborting acute migraine attacks and inhibit the in vivo release of calcitonin gene-related peptide (CGRP) in human and animal models. The study aimed to examine the localization, genetic expression and functional aspects of 5- HT_1B/1D/1F receptors in the trigeminal system in order to further understand the molecular sites of action of triptans (5-HT_1B/1D) and ditans (5-HT_1F).

Methods

Utilizing immunohistochemistry, the localization of 5-HT and of 5-HT_1B/1D/1F receptors was examined in rat trigeminal ganglion (TG) and combined with quantitative polymerase chain reaction to quantify the level of expression for 5-HT_1B/1D/1F receptors in the TG. The functional role of these receptors was examined ex vivo with a capsaicin/potassium induced 5-HT and CGRP release.

Results

5-HT immunoreactivity (ir) was observed in a minority of CGRP negative C-fibres, most neuron somas and faintly in A-fibres and Schwann cell neurolemma. 5-HT_1B/1D receptors were expressed in the TG, while the 5-HT_1F receptor displayed a weak ir. The 5-HT_1D receptor co-localized with receptor activity-modifying protein 1 (RAMP1) in Aδ-fibres in the TG, while 5-HT_1B-ir was weakly expressed and 5-HT_1F-ir was not detected in these fibres. None of the 5-HT₁ receptors co-localized with CGRP-ir in C-fibres.

5-HT_1D receptor mRNA was the most prominently expressed, followed by the 5-HT_1B receptor and lastly the 5-HT_1F receptor. The 5-HT_1B and 5-HT_1D receptor antagonist, GR127935, could reverse the inhibitory effect of Lasmiditan (a selective 5-HT_1F receptor agonist) on CGRP release in the soma-rich TG but not in soma-poor TG or dura mater. 5-HT release in the soma-rich TG, and 5-HT content in the baseline samples, negatively correlated with CGRP levels, showing for the first time a physiological role for 5-HT induced inhibition.

Conclusion

This study reveals the presence of a subgroup of C-fibres that store 5-HT. The data shows high expression of 5-HT_1B/1D receptors and suggests that the 5-HT_1F receptor is a relatively unlikely target in the rat TG. Furthermore, Lasmiditan works as a partial agonist on 5-HT_1B/1D receptors in clinically relevant dose regiments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01394-z.

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

Emergence and Developmental Roles of the Cerebrospinal Fluid System

We summarize recent work illuminating how cerebrospinal fluid (CSF) regulates brain function. More than a protective fluid cushion and sink for waste, the CSF is an integral CNS component with dynamic and diverse roles emerging in parallel with the developing CNS. This review examines the current understanding about early CSF and its maturation and roles during CNS development and discusses open questions in the field. We focus on developmental changes in the ventricular system and CSF sources (including neural progenitors and choroid plexus). We also discuss concepts related to the development of fluid dynamics including flow, perivascular transport, drainage, and barriers.

Release of insulin produced by the choroid plexis is regulated by serotonergic signaling

The choroid plexus (ChP) is a highly vascularized tissue found in the brain ventricles, with an apical epithelial cell layer surrounding fenestrated capillaries. It is responsible for the production of most of the cerebrospinal fluid (CSF) in the ventricular system, subarachnoid space, and central canal of the spinal cord, while also constituting the blood-CSF barrier (BCSFB). In addition, epithelial cells of the ChP (EChP) synthesize neurotrophic factors and other signaling molecules that are released into the CSF. Here, we show that insulin is produced in EChP of mice and humans, and its expression and release are regulated by serotonin. Insulin mRNA and immune-reactive protein, including C-peptide, are present in EChP, as detected by several experimental approaches, and appear in much higher levels than any other brain region. Moreover, insulin is produced in primary cultured mouse EChP, and its release, albeit Ca2+ sensitive, is not regulated by glucose. Instead, activation of the 5HT2C receptor by serotonin treatment led to activation of IP3-sensitive channels and Ca2+ mobilization from intracellular storage, leading to insulin secretion. In vivo depletion of brain serotonin in the dorsal raphe nucleus negatively affected insulin expression in the ChP, suggesting an endogenous modulation of ChP insulin by serotonin. Here, we show for the first time to our knowledge that insulin is produced by EChP in the brain, and its release is modulated at least by serotonin but not glucose.

Critical appraisal of pathology transmission in the α-synuclein fibril model of Lewy body disorders

Lewy body disorders are characterized by the emergence of α-synucleinopathy in many parts of the central and peripheral nervous systems, including in the telencephalon. Dense α-synuclein+ pathology appears in regio inferior of the hippocampus in both Parkinson's disease and dementia with Lewy bodies and may disturb cognitive function. The preformed α-synuclein fibril model of Parkinson's disease is growing in use, given its potential for seeding the self-propagating spread of α-synucleinopathy throughout the mammalian brain. Although it is often assumed that the spread occurs through neuroanatomical connections, this is generally not examined vis-à-vis the uptake and transport of tract-tracers infused at precisely the same stereotaxic coordinates. As the neuronal connections of the hippocampus are historically well defined, we examined the first-order spread of α-synucleinopathy three months following fibril infusions centered in the mouse regio inferior (CA2+CA3), and contrasted this to retrograde and anterograde transport of the established tract-tracers FluoroGold and biotinylated dextran amines (BDA). Massive hippocampal α-synucleinopathy was insufficient to elicit memory deficits or loss of cells and synaptic markers in this model of early disease processes. However, dense α-synuclein+ inclusions in the fascia dentata were negatively correlated with memory capacity. A modest compensatory increase in synaptophysin was evident in the stratum radiatum of cornu Ammonis in fibril-infused animals, and synaptophysin expression correlated inversely with memory function in fibril but not PBS-infused mice. No changes in synapsin I/II expression were observed. The spread of α-synucleinopathy was somewhat, but not entirely consistent with FluoroGold and BDA axonal transport, suggesting that variables other than innervation density also contribute to the materialization of α-synucleinopathy. For example, layer II entorhinal neurons of the perforant pathway exhibited somal α-synuclein+ inclusions as well as retrogradely labeled FluoroGold+ somata. However, some afferent brain regions displayed dense retrograde FluoroGold label and no α-synuclein+ inclusions (e.g. medial septum/diagonal band), supporting the selective vulnerability hypothesis. The pattern of inclusions on the contralateral side was consistent with specific spread through commissural connections (e.g. stratum pyramidale of CA3), but again, not all commissural projections exhibited α-synucleinopathy (e.g. hilar mossy cells). The topographical extent of inclusions is displayed here in high-resolution images that afford viewers a rich opportunity to dissect the potential spread of pathology through neural circuitry. Finally, the results of this expository study were leveraged to highlight the challenges and limitations of working with preformed α-synuclein fibrils.

Alzheimer's Disease: Characterization of the Brain Sites of the Initial Tau Cytoskeletal Pathology Will Improve the Success of Novel Immunological Anti-Tau Treatment Approaches

Alzheimer's disease (AD) represents the most frequent neurodegenerative disease of the human brain worldwide. Currently practiced treatment strategies for AD only include some less effective symptomatic therapeutic interventions, which unable to counteract the disease course of AD. New therapeutic attempts aimed to prevent, reduce, or remove the extracellular depositions of the amyloid-β protein did not elicit beneficial effects on cognitive deficits or functional decline of AD. In view of the failure of these amyloid-β-based therapeutic trials and the close correlation between the brain pathology of the cytoskeletal tau protein and clinical AD symptoms, therapeutic attention has since shifted to the tau cytoskeletal protein as a novel drug target. The abnormal hyperphosphorylation and intraneuronal aggregation of this protein are early events in the evolution of the AD-related neurofibrillary pathology, and the brain spread of the AD-related tau aggregation pathology may possibly follow a corruptive protein templating and seeding-like mechanism according to the prion hypothesis. Accordingly, immunotherapeutic targeting of the tau aggregation pathology during the very early pre-tangle phase is currently considered to represent an effective and promising therapeutic approach for AD. Recent studies have shown that the initial immunoreactive tau aggregation pathology already prevails in several subcortical regions in the absence of any cytoskeletal changes in the cerebral cortex. Thus, it may be hypothesized that the subcortical brain regions represent the "port of entry" for the pathogenetic agent from which the disease ascends anterogradely as an "interconnectivity pathology".

Distribution of serotonin 5-HT1A-binding sites in the brainstem and the hypothalamus, and their roles in 5-HT-induced sleep and ingestive behaviors in rock pigeons (Columba livia)

Serotonin 1A receptors (5-HT1ARs), which are widely distributed in the mammalian brain, participate in cognitive and emotional functions. In birds, 5-HT1ARs are expressed in prosencephalic areas involved in visual and cognitive functions. Diverse evidence supports 5-HT1AR-mediated 5-HT-induced ingestive and sleep behaviors in birds. Here, we describe the distribution of 5-HT1ARs in the hypothalamus and brainstem of birds, analyze their potential roles in sleep and ingestive behaviors, and attempt to determine the involvement of auto-/hetero-5-HT1ARs in these behaviors. In 6 pigeons, the anatomical distribution of [(3)H]8-OH-DPAT binding in the rostral brainstem and hypothalamus was examined. Ingestive/sleep behaviors were recorded (1h) in 16 pigeons pretreated with MM77 (a heterosynaptic 5-HT1AR antagonist; 23 or 69 nmol) for 20 min, followed by intracerebroventricular ICV injection of 5-HT (N:8; 150 nmol), 8-OH-DPAT (DPAT, a 5-HT1A,7R agonist, 30 nmol N:8) or vehicle. 5-HT- and DPAT-induced sleep and ingestive behaviors, brainstem 5-HT neuronal density and brain 5-HT content were examined in 12 pigeons, pretreated by ICV with the 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (N:6/group). The distribution of brainstem and diencephalic c-Fos immunoreactivity after ICV injection of 5-HT, DPAT or vehicle (N:5/group) into birds provided with or denied access to water is also described. 5-HT1ARs are concentrated in the brainstem 5-HTergic areas and throughout the periventricular hypothalamus, preoptic nuclei and circumventricular organs. 5-HT and DPAT produced a complex c-Fos expression pattern in the 5-HT1AR-enriched preoptic hypothalamus and the circumventricular organs, which are related to drinking and sleep regulation, but modestly affected c-Fos expression in 5-HTergic neurons. The 5-HT-induced ingestivebehaviors and the 5-HT- and DPAT-induced sleep behaviors were reduced by MM77 pretreatment. 5,7-DHT increased sleep per se, decreased tryptophan hydroxylase expression in the raphe nuclei and decreased prosencephalic 5-HT release but failed to affect 5-HT- or DPAT-induced drinking or sleep behavior. 5-HT- and DPAT-induced ingestive and sleep behaviors in pigeons appear to be mediated by heterosynaptic and/or non-somatodendritic presynaptic 5-HT1ARs localized to periventricular diencephalic circuits.

Serotonin metabolites in the cerebrospinal fluid in sudden infant death syndrome

Forensic biomarkers are needed in sudden infant death syndrome (SIDS) to help identify this group among other sudden unexpected deaths in infancy. Previously, we reported multiple serotonergic (5-HT) abnormalities in nuclei of the medulla oblongata that help mediate protective responses to homeostatic stressors. As a first step toward their assessment as forensic biomarkers of medullary pathology, here we test the hypothesis that 5-HT-related measures are abnormal in the cerebrospinal fluid (CSF) of SIDS infants compared with those of autopsy controls. Levels of CSF 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), the degradative products of 5-HT and dopamine, respectively, were measured by high-performance liquid chromatography in 52 SIDS and 29 non-SIDS autopsy cases. Tryptophan (Trp) and tyrosine (Tyr), the substrates of 5-HT and dopamine, respectively, were also measured. There were no significant differences in 5-HIAA, Trp, HVA, or Tyr levels between the SIDS and non-SIDS groups. These data preclude the use of 5-HIAA, HVA, Trp, or Tyr measurements as CSF autopsy biomarkers of 5-HT medullary pathology in infants who have died suddenly and unexpectedly. They do, however, provide important information about monoaminergic measurements in human CSF at autopsy and their developmental profile in infancy that is applicable to multiple pediatric disorders beyond SIDS.

Axonal control of the adult neural stem cell niche

The ventricular-subventricular zone (V-SVZ) is an extensive germinal niche containing neural stem cells (NSCs) in the walls of the lateral ventricles of the adult brain. How the adult brain's neural activity influences the behavior of adult NSCs remains largely unknown. We show that serotonergic (5HT) axons originating from a small group of neurons in the raphe form an extensive plexus on most of the ventricular walls. Electron microscopy revealed intimate contacts between 5HT axons and NSCs (B1) or ependymal cells (E1) and these cells were labeled by a transsynaptic viral tracer injected into the raphe. B1 cells express the 5HT receptors 2C and 5A. Electrophysiology showed that activation of these receptors in B1 cells induced small inward currents. Intraventricular infusion of 5HT2C agonist or antagonist increased or decreased V-SVZ proliferation, respectively. These results indicate that supraependymal 5HT axons directly interact with NSCs to regulate neurogenesis via 5HT2C.

Day-night differences in neural activation in histaminergic and serotonergic areas with putative projections to the cerebrospinal fluid in a diurnal brain

In nocturnal rodents, brain areas that promote wakefulness have a circadian pattern of neural activation that mirrors the sleep/wake cycle, with more neural activation during the active phase than during the rest phase. To investigate whether differences in temporal patterns of neural activity in wake-promoting regions contribute to differences in daily patterns of wakefulness between nocturnal and diurnal species, we assessed Fos expression patterns in the tuberomammillary (TMM), supramammillary (SUM), and raphe nuclei of male grass rats maintained in a 12:12 h light-dark cycle. Day-night profiles of Fos expression were observed in the ventral and dorsal TMM, in the SUM, and in specific subpopulations of the raphe, including serotonergic cells, with higher Fos expression during the day than during the night. Next, to explore whether the cerebrospinal fluid is an avenue used by the TMM and raphe in the regulation of target areas, we injected the retrograde tracer cholera toxin subunit beta (CTB) into the ventricular system of male grass rats. While CTB labeling was scarce in the TMM and other hypothalamic areas including the suprachiasmatic nucleus, which contains the main circadian pacemaker, a dense cluster of CTB-positive neurons was evident in the caudal dorsal raphe, and the majority of these neurons appeared to be serotonergic. Since these findings are in agreement with reports for nocturnal rodents, our results suggest that the evolution of diurnality did not involve a change in the overall distribution of neuronal connections between systems that support wakefulness and their target areas, but produced a complete temporal reversal in the functioning of those systems.

Innervation of ventricular and periventricular brain compartments

Synaptic transmission is divided into two broad categories on the basis of the distance over which neurotransmitters travel. Wiring transmission is the release of transmitter into synaptic clefts in close apposition to receptors. Volume transmission is the release of transmitters or modulators over varying distances before interacting with receptors. One case of volume transmission over potentially long distances involves release into cerebrospinal fluid (CSF). The CSF contains neuroactive substances that affect brain function and range in size from small molecule transmitters to peptides and large proteins. CSF-contacting neurons are a well-known and universal feature of non-mammalian vertebrates, but only supra- and subependymal serotonergic plexuses are a commonly studied feature in mammals. The origin of most other neuroactive substances in CSF is unknown. In order to determine which brain regions communicate with CSF, we describe the distribution of retrograde neuronal labeling in the rat brain following ventricular injection of Cholera toxin, ß subunit (CTß), a tracer frequently used in brain circuit analysis. Within 15 to 30 min following intraventricular injection, there is only diffuse, non-specific staining adjacent to the ventricular surface. Within 2 to 10 days, however, there is extensive labeling of neuronal perikarya in specific nuclear groups in the telencephalon, thalamus, hypothalamus and brainstem, many at a considerable distance from the ventricles. These observations support the view that ventricular CSF is a significant channel for volume transmission and identifies those brain regions most likely to be involved in this process.

New concepts of molecular communication among neurons

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Receptors Involved in the Modulation of 5-Hydroxytryptamine Release in Bovine Cerebral Arteries

The uptake of [3H]5-hydroxytryptamine (5-HT) in bovine cerebral arteries was reduced by cocaine (1 μm), ouabain (100 μm), pretreatment with 6-hydroxydopamine (6-OHDA) (1·46 Mm, 10 min) and metitepine (1 μm). Electrically-stimulated tritium release was decreased by tetrodotoxin (0·8 μm), Ca-free medium, denervation with 6-OHDA (1·46 Mm, 10 min), 5-HT (10 μm), noradrenaline (1 μm) and the agonist of α2-adrenoceptors B-HT 920 (0·1 and 1 μm), enhanced by metitepine (1 μm, antagonists of presynaptic 5-HT1 receptors) and rauwolscine (1 μm, antagonist at α2-adrenoceptors, and also of 5-HT,1d receptors) and not affected by ketanserin (1 μm, antagonist of 5-HT2 receptors), methysergide (0·1 μm, antagonist of 5-HT1 and 5-HT2 receptors) and phentolamine (1 and 3 μm antagonist of α-adrenoceptors and less potent of 5-HT1 receptors). The inhibitory action of 10 μm 5-HT was partially reversed by phentolamine (3 μm) and cocaine (1 μm) and completely reversed by both metitepine (1 μm) and rauwolscine (1 μm). Ketanserin (1 μm), methysergide (0·1 μm) or phentolamine (1 μm) had no effect. Rauwolscine (1 μm) antagonized the inhibition induced by both noradrenaline (1 μm) and B-HT 920 (0·1 and 1 μm). 5-HT induced tritium release which was inhibited by cocaine (an antagonist of 5-HT3 receptors) and denervation with 6-OHDA. These results suggest that 5-HT is mainly accumulated in adrenergic nerve endings, that evoked [3H]5-HT release is modulated by 5-HT1-like receptors, but the participation of α2-adrenoceptors cannot be discounted, or more probably both types of receptors have features in common, and evoked [3H]5-HT release elicited by 5-HT may be partially mediated by activation of 5-HT3 receptors.

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.

The dorsal raphe nucleus—From silver stainings to a role in depression

Over a hundred years ago, Santiago Ramón y Cajal used a new staining method developed by Camillo Golgi to visualize, among many other structures, what we today call the dorsal raphe nucleus (DRN) of the midbrain. Over the years, the DRN has emerged as a multifunctional and multitransmitter nucleus, which modulates or influences many CNS processes. It is a phylogenetically old brain area, whose projections reach out to a large number of regions and nuclei of the CNS, particularly in the forebrain. Several DRN-related discoveries are tightly connected with important events in the history of neuroscience, for example the invention of new histological methods, the discovery of new neurotransmitter systems and the link between neurotransmitter function and mood disorders. One of the main reasons for the wide current interest in the DRN is the nucleus' involvement in depression. This involvement is particularly attributable to the main transmitter of the DRN, serotonin. Starting with a historical perspective, this essay describes the morphology, ascending projections and multitransmitter nature of the DRN, and stresses its role as a key target for depression research.

Modulators in concert for cognition: modulator interactions in the prefrontal cortex

Research on the regulation and function of ascending noradrenergic, dopaminergic, serotonergic, and cholinergic systems has focused on the organization and function of individual systems. In contrast, evidence describing co-activation and interactions between multiple neuromodulatory systems has remained scarce. However, commonalities in the anatomical organization of these systems and overlapping evidence concerning the post-synaptic effects of neuromodulators strongly suggest that these systems are recruited in concert; they influence each other and simultaneously modulate their target circuits. Therefore, evidence on the regulatory and functional interactions between these systems is considered essential for revealing the role of neuromodulators. This postulate extends to contemporary neurobiological hypotheses of major neuropsychiatric disorders. These hypotheses have focused largely on aberrations in the integrity or regulation of individual ascending modulatory systems, with little regard for the likely possibility that dysregulation in multiple ascending neuromodulatory systems and their interactions contribute essentially to the symptoms of these disorders. This review will paradigmatically focus on neuromodulator interactions in the PFC and be further constrained by an additional focus on their role in cognitive functions. Recent evidence indicates that individual neuromodulators, in addition to their general state-setting or gating functions, encode specific cognitive operations, further substantiating the importance of research concerning the parallel recruitment of neuromodulator systems and interactions between these systems.

Serotonin in Trigeminal Ganglia of Female Rodents: Relevance to Menstrual Migraine

OBJECTIVES

We examined changes in the serotonin system across the estrous cycle in trigeminal ganglia of female rodents to determine which components are present and which are regulated by the variations in levels of ovarian steroids that occur during the estrous cycle.

BACKGROUND

Migraine is 2-3 times more prevalent in women than in men and attacks are often timed with the menstrual cycle, suggesting a mechanistic link with ovarian steroids. Serotonin has been implicated in the pathogenesis of migraine, and the effectiveness of triptans, selective 5HT-1B/D/F agonists, has provided further support for this concept. It is not known whether serotonin, its rate-limiting enzyme tryptophan hydroxylase (TPH), or its receptors are regulated by ovarian steroids in trigeminal ganglia.

METHODS

We used reverse transcription-polymerase chain reaction to examine gene expression in cycling mice, Western blots to examine protein expression, double-labeling immunohistochemistry using markers of nociceptors and nonnociceptors and confocal microscopy to identify specific types of neurons, and primary tissue culture to examine effects of estrogen on trigeminal neurons in vitro.

RESULTS

In C57/BL6 mice mRNA levels of TPH-1, the rate-limiting enzyme in serotonin synthesis, were over 2-fold higher and protein levels were 1.4-fold higher at proestrus, the high estrogen stage of the cycle than at diestrus, the low estrogen stage. TPH protein also was present in primary trigeminal cultures obtained from female Sprague-Dawley rats, but levels were not affected by 24-hour treatment with physiological levels (10(-9) M) of 17beta-estradiol. Gene expression of 5HT-1B and 5HT-1D receptors in trigeminal ganglia was not regulated by the estrous cycle. Serotonin was present in trigeminal neurons containing CGRP, a potent vasoactive neuropeptide, peripherin, an intermediate filament present in neurons with unmyelinated axons, neurofilament H, which is present in neurons with myelinated axons, and in neurons binding IB4, a marker of nonpeptidergic nociceptors. Serotonin was also present in neurons containing 5HT-1B. The serotonin-positive population was significantly larger in diameter than the serotonin-negative population. Conclusions.-Expression of the rate-limiting enzyme required for serotonin synthesis is regulated during the natural estrous cycle, and serotonin is present in larger trigeminal neurons of all the major subtypes. Colocalization of serotonin with 5HT-1B suggests that this receptor functions as an autoreceptor to regulate serotonin release. Cyclical changes in serotonin levels in trigeminal ganglia could contribute to the pathogenesis of menstrual migraine.

Local inhibition of organic cation transporters increases extracellular serotonin in the medial hypothalamus

In the rat dorsomedial hypothalamus (DMH), serotonin (5-HT) concentrations are altered rapidly in response to acute stressors. The mechanism for rapid changes in 5-HT concentrations in the DMH is not clear. We hypothesize that the mechanism involves corticosteroid-induced alterations in the uptake of 5-HT from extracellular fluid through the action of corticosterone-sensitive organic cation transporters (OCTs). To determine if OCTs affect the clearance of 5-HT from the extracellular fluid compartment within the medial hypothalamus (MH), the OCT blocker, decynium 22 (0, 10, 30, or 100 microM), was perfused into the MH via a microdialysis probe, and dialysate 5-HT concentrations were measured at 20 min intervals. In addition, home cage behavior was measured both before and after drug administration. Inhibition of OCTs in the MH resulted in a reversible dose-dependent increase in extracellular 5-HT concentration. Increases in extracellular 5-HT concentrations were associated with increases in grooming behavior in rats treated with the highest concentration of decynium 22. No other behavioral responses were observed following administration of any concentration of decynium 22. These findings are consistent with the hypothesis that OCTs in the MH play an important role in the regulation of serotonergic neurotransmission and specific behavioral responses. Because the MH plays an important role in the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli, these data lead to new questions regarding the role of interactions between corticosterone and corticosterone-sensitive OCTs in stress-induced 5-HT accumulation within the MH as well as the physiological and behavioral consequences of these interactions.

Serotonin 5-HT2C receptor-mediated phosphoinositide hydrolysis in rat choroid plexus after fluoxetine and citalopram treatments

Selective serotonin reuptake inhibitors (SSRIs) bind directly to various neurotransmitter receptors. The clinical effects of SSRIs appear gradually during weeks of treatment, suggesting a role for adaptive changes in neurotransmitter receptors. Most clinically used antidepressants, e.g. fluoxetine, bind to 5-HT2C receptors. When administered chronically, many antidepressants elicit adaptive regulation of 5-HT2C receptors. The present study was conducted in order to determine the effects of acute and chronic fluoxetine and citalopram treatments on the density and function of 5-HT2C receptors in the rat choroid plexus. Acute and chronic treatments followed by phosphoinositide (PI) hydrolysis assays and quantitative receptor autoradiography were performed. Acute (single-dose) treatment with neither drug significantly affected basal or 5-HT-stimulated PI hydrolysis, but acute citalopram (20 mg/kg) treatment increased both agonist and antagonist binding to 5-HT(2C) receptors. Chronic (14 days) citalopram treatment (20 mg/kg) increased the maximal PI hydrolysis response by 40%, but fluoxetine lacked this effect. The present data suggest that sensitisation of 5-HT2C receptor-mediated intracellular signal transduction may play a role in the effects of citalopram. In contrast, fluoxetine treatment does not functionally sensitise 5-HT2C receptors. Thus, functional 5-HT2C receptor sensitisation is not a common effect of antidepressants, but the differential effects may explain some of the pharmacodynamic differences seen with these drugs, especially upon repeated administration.

Transcription of SCO-spondin in the subcommissural organ: evidence for down-regulation mediated by serotonin

The subcommissural organ (SCO) is a brain gland located in the roof of the third ventricle that releases glycoproteins into the cerebrospinal fluid, where they form a structure known as Reissner's fiber (RF). On the basis of SCO-spondin sequence (the major RF glycoprotein) and experimental findings, the SCO has been implicated in central nervous system development; however, its function(s) after birth remain unclear. There is evidence suggesting that SCO activity in adult animals may be regulated by serotonin (5HT). The use of an anti-5HT serum showed that the bovine SCO is heterogeneously innervated with most part being poorly innervated, whereas the rat SCO is richly innervated throughout. Antibodies against serotonin receptor subtype 2A rendered a strong immunoreaction at the ventricular cell pole of the bovine SCO cells and revealed the expected polypeptides in blots of fresh and organ-cultured bovine SCO. Analyses of organ-cultured bovine SCO treated with 5HT revealed a twofold decrease of both SCO-spondin mRNA level and immunoreactive RF glycoproteins, whereas no effect on release of RF glycoproteins into the culture medium was detected. Rats subjected to pharmacological depletion of 5HT exhibited an SCO-spondin mRNA level twofold higher than untreated rats. These results indicate that 5HT down-regulates SCO-spondin biosynthesis but apparently not its release, and suggest that 5HT may exert the effect on the SCO via the cerebrospinal fluid.

Persistent changes in the intrinsic excitability of rat deep cerebellar nuclear neurones induced by EPSP or IPSP bursts

The deep cerebellar nuclei (DCN) are the major output of the cerebellum, and have been proposed as a site of memory storage for certain forms of motor learning. Microelectrode and whole-cell patch recordings were performed on DCN neurones in acute slices of juvenile rat cerebellum. DCN neurones display tonic and bursting basal firing patterns. In tonically firing neurones, a stimulus consisting of EPSP bursts produced a brief increase in dendritic Ca(2+) concentration and a persistent increase in the number of spikes elicited by a depolarizing test pulse, along with a decrease in spike threshold. In intrinsically bursting DCN neurones, EPSP bursts induced an increase in the number of depolarization-evoked spikes in some neurones, but in others produced a change to a more tonic firing pattern. Application of IPSP bursts evoked a large number of rebound spikes and an associated dendritic Ca(2+) transient, which also produced a persistent increase in the number of spikes elicited by a test pulse. Intracellular perfusion of the Ca(2+) chelator BAPTA prevented the increase in intrinsic excitability. Thus, rapid changes in intrinsic excitability in the DCN may be driven by bursts of both EPSPs and IPSPs, and may result in persistent changes to both firing frequency and pattern.

In vitro increase in intracellular calcium concentrations induced by low or high extracellular glucose levels in ependymocytes and serotonergic neurons of the rat lower brainstem

Pancreatic glucokinase (GK)-like immunoreactivities are located in ependymocytes and serotonergic neurons of the rat brain. The present study investigated in vitro changes in intracellular calcium concentrations ([Ca(2+)](i)) in response to low (2 mm) or high (20 mm) extracellular glucose concentrations in isolated cells from the wall of the central canal (CC), raphe obscurus nucleus (ROb), ventromedial hypothalamus (VMH), and lateral hypothalamic area (LHA) in male rats. An increase in [Ca(2+)](i) was found in cells from the CC (21.1% or 9.8% of ependymocytes), ROb (10.9% or 14.5% of serotonergic neurons), VMH (7.8% and 25.2% of neurons), and LHA (20% or 15.7% of neurons), when extracellular glucose levels were changed from 10 to either 2 or 20 mm, respectively. Most of the ependymocytes and serotonergic neurons responding to the glucose changes were immunoreactive to the anti-GK in the CC (96.8% for low glucose and 100% for high glucose) and ROb (100% for low and high glucose). The [Ca(2+)](i) increase was blocked with calcium-free medium or L-type calcium channel blocker. Cells with an increase in [Ca(2+)](i) in response to low glucose did not respond to high glucose and vice versa. Inhibition of GK activity with acute alloxan treatment blocked low or high glucose-induced [Ca(2+)](i) increases in most GK-immunoreactive cells from the CC or ROb. The glucose-sensitive [Ca(2+)](i) increase in neurons of the VMH and LHA was also alloxan-sensitive, but no cells taken from the VMH and LHA were immunoreactive to the antibody used. The present study further indicates that ependymocytes of the CC and serotonergic neurons in the ROb are also sensitive to the changes in extracellular glucose in a GK-dependent manner, but that the subtype of GK in these cells could be different from that in the VMH and LHA.

Organization of projections from the raphe nuclei to the vestibular nuclei in rats

Previous anatomic and electrophysiological evidence suggests that serotonin modulates processing in the vestibular nuclei. This study examined the organization of projections from serotonergic raphe nuclei to the vestibular nuclei in rats. The distribution of serotonergic axons in the vestibular nuclei was visualized immunohistochemically in rat brain slices using antisera directed against the serotonin transporter. The density of serotonin transporter-immunopositive fibers is greatest in the superior vestibular nucleus and the medial vestibular nucleus, especially along the border of the fourth ventricle; it declines in more lateral and caudal regions of the vestibular nuclear complex. After unilateral iontophoretic injections of Fluoro-Gold into the vestibular nuclei, retrogradely labeled neurons were found in the dorsal raphe nucleus (including the dorsomedial, ventromedial and lateral subdivisions) and nucleus raphe obscurus, and to a minor extent in nucleus raphe pallidus and nucleus raphe magnus. The combination of retrograde tracing with serotonin immunohistofluorescence in additional experiments revealed that the vestibular nuclei receive both serotonergic and non-serotonergic projections from raphe nuclei. Tracer injections in densely innervated regions (especially the medial and superior vestibular nuclei) were associated with the largest numbers of Fluoro-Gold-labeled cells. Differences were observed in the termination patterns of projections from the individual raphe nuclei. Thus, the dorsal raphe nucleus sends projections that terminate predominantly in the rostral and medial aspects of the vestibular nuclear complex, while nucleus raphe obscurus projects relatively uniformly throughout the vestibular nuclei. Based on the topographical organization of raphe input to the vestibular nuclei, it appears that dense projections from raphe nuclei are colocalized with terminal fields of flocculo-nodular lobe and uvula Purkinje cells. It is hypothesized that raphe-vestibular connections are organized to selectively modulate processing in regions of the vestibular nuclear complex that receive input from specific cerebellar zones. This represents a potential mechanism whereby motor activity and behavioral arousal could influence the activity of cerebellovestibular circuits.

Subdural engraftment of serotonergic neurons following spinal hemisection restores spinal serotonin, downregulates serotonin transporter, and increases BDNF tissue content in rat

Spinal hemisection injury at T13 results in development of permanent mechanical allodynia and thermal hyperalgesia due to interruption and subsequent loss of descending inhibitory modulators such as serotonin (5-HT) and its transporter (5-HT(T)). We hypothesize that lumbar transplantation of non-mitotic cells that tonically secrete 5-HT and brain-derived neurotrophic factor (BDNF) will restore alterations in 5-HT and 5-HT(T) systems within the spinal dorsal horn. We used an immortalized rat neuronal cell line derived from E13 raphe (RN46A-B14) which is shown to secrete 5-HT and BDNF in vitro and in vivo. Three groups (n=35) of 30 day old male Sprague-Dawley rats were spinally hemisected at T13 and 28 days later received either lumbar RN46A-V1 control empty-vector (n=15) or RN46A-B14 (n=15) intrathecal grafts, or no transplant. Twenty-eight days following transplantation, animals were perfused and tissue examined for changes in 5-HT, 5-HT(T), and BDNF at the site of transplantation or at lumbar enlargements (L5). Immunohistochemistry revealed that RN46A-B14, but not RN46A-V1 cells, increased 5-HT tissue staining at L5 in the dorsal white matter as well as in superficial dorsal horn laminae I and II on both ipsilateral and contralateral sides, results confirmed by ELISA. Transplantation of RN46A-B14 cells significantly reduced ipsilateral 5-HT(T), upregulated after injury. Significantly increased levels of BDNF were also observed after RN46A-B14 transplantation but were not localized to particular spinal laminae. These results are consistent with recovery of locomotor function and reductions in chronic pain behaviors observed behaviorally after RN46A-B14 transplantation and supports the pragmatic application of cell-based therapies in correcting damaged circuitry after spinal cord injury.

Nerve fibers innervating the cranial and spinal meninges: morphology of nerve fiber terminals and their structural integration

Pachymeninx and leptomeninx of cranial cavity and spine are considerably different in their collagenous fiber texture, cellular composition, vascularization, and innervation. The majority of meningeal nerve fibers terminate as free nerve endings whereas encapsulated and lamellated nerve terminals additionally occur in higher vertebrates including man. With respect to nerve fiber classification, arborization pattern, topography, and organization of the microenvironment at the termination site afferent and efferent nerve terminals are differentiated. Only the dura mater and the pial subcompartment of the leptomeninx possess the morphological prerequisites for neurogenic inflammation. In the current review, the results of morphological studies regarding the meningeal innervation including the sites of CSF (cerebrospinal fluid) production and absorption are discussed with emphasis on their structure-function relationships.

Subcommissural organ, cerebrospinal fluid circulation, and hydrocephalus

Under normal physiological conditions the cerebrospinal fluid (CSF) is secreted continuously, although this secretion undergoes circadian variations. Mechanisms operating at the vascular side of the choroidal cells involve a sympathetic and a cholinergic innervation, with the former inhibiting and the latter stimulating CSF secretion. There are also regulatory mechanisms operating at the ventricular side of the choroidal cells, where receptors for monoamines such as dopamine, serotonin, and melatonin, and for neuropeptides such as vasopressin, atrial natriuretic hormone, and angiotensin II, have been identified. These compounds, that are normally present in the CSF, participate in the regulation of CSF secretion. Although the mechanisms responsible for the CSF circulation are not fully understood, several factors are known to play a role. There is evidence that the subcommissural organ (SCO)--Reissner's fiber (RF) complex is one of the factors involved in the CSF circulation. In mammals, the predominant route of escape of CSF into blood is through the arachnoid villi. In lower vertebrates, the dilatation of the distal end of the central canal, known as terminal ventricle or ampulla caudalis, represents the main site of CSF escape into blood. Both the function and the ultrastructural arrangement of the ampulla caudalis suggest that it may be the ancestor structure of the mammalian arachnoid villi. RF-glycoproteins reaching the ampulla caudalis might play a role in the formation and maintenance of the route communicating the CSF and blood compartments. The SCO-RF complex may participate, under physiological conditions, in the circulation and reabsorption of CSF. Under pathological conditions, the SCO appears to be involved in the pathogeneses of congenital hydrocephalus. Changes in the SCO have been described in all species developing congenital hydrocephalus. In these reports, the important question whether the changes occurring in the SCO precede hydrocephalus, or are a consequence of the hydrocephalic state, has not been clarified. Recently, evidence has been obtained indicating that a primary defect of the SCO-RF complex may lead to hydrocephalus. Thus, a primary and selective immunoneutralization of the SCO-RF complex during the fetal and early postnatal life leads to absence of RF, aqueductal stenosis, increased CSF concentration of monoamines, and a moderate but sustained hydrocephalus.

Intracellular pathways regulating ciliary beating of rat brain ependymal cells

1. The mammalian brain ventricles are lined with ciliated ependymal cells. As yet little is known about the mechanisms by which neurotransmitters regulate cilia beat frequency (CBF). 2. Application of 5-HT to ependymal cells in cultured rat brainstem slices caused CBF to increase. 5-HT had an EC50 of 30 microM and at 100 microM attained a near-maximal CBF increase of 52.7 +/- 4.1 % (mean +/- s.d.) (n = 8). 3. Bathing slices in Ca2+-free solution markedly reduced the 5-HT-mediated increase in CBF. Fluorescence measurements revealed that 5-HT caused a marked transient elevation in cytosolic Ca2+ ([Ca2+]c) that then slowly decreased to a plateau level. Analysis showed that the [Ca2+]c transient was due to release of Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive stores; the plateau was probably due to extracellular Ca2+ influx through Ca2+ release-activated Ca2+ (CRAC) channels. 4. Application of ATP caused a sustained decrease in CBF. ATP had an EC50 of about 50 microM and 100 microM ATP resulted in a maximal 57.5 +/- 6.5 % (n = 12) decrease in CBF. The ATP-induced decrease in CBF was unaffected by lowering extracellular [Ca2+], and no changes in [Ca2+]c were observed. Exposure of ependymal cells to forskolin caused a decrease in CBF. Ciliated ependymal cells loaded with caged cAMP exhibited a 54.3 +/- 7.5 % (n = 9) decrease in CBF following uncaging. These results suggest that ATP reduces CBF by a Ca2+-independent cAMP-mediated pathway. 5. Application of 5-HT and adenosine-5'-O-3-thiotriphosphate (ATP-gamma-S) to acutely isolated ciliated ependymal cells resulted in CBF responses similar to those of ependymal cells in cultured slices suggesting that these neurotransmitters act directly on these cells. 6. The opposite response of ciliated ependymal cells to 5-HT and ATP provides a novel mechanism for their active involvement in central nervous system signalling.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

Serotonergic systems, their phylogeny and ontogeny have been thoroughly described up to the ultrastructural level, thanks to the multiplicity of methodological approaches. They have often been referred to as a 'Rosetta stone', as several features first described for serotonin neurons or paraneurons have been then extended to other neurotransmitter systems: coexistence with neuropeptides or even a canonical neurotransmitter (GABA), volume transmission, regrowth after lesioning, and characterization of multiple receptor subtypes. This review deals with the contributions of neuroanatomical approaches for studying serotoninergic systems, and focuses on recent advances concerning the topological relationships between serotonergic innervation, receptors and target cells. This aspect is particularly important with regard to the possibility for serotonin to act through classical synaptic transmission and/or non-junctional transmission. Serotonin then can selectively regulate different neuronal systems through the activation of distinct receptor subtypes, which in turn can be linked to different transduction pathways. Neurocytochemical approaches constitute unique tools to analyse both anatomical and functional characteristics of complex neuronal systems.

Projection patterns from the raphe nuclear complex to the ependymal wall of the ventricular system in the rat

The goal of the present study was to characterize the anatomical and neurochemical relationships that the raphe nuclear complex maintains with respect to lateralized and centralized components of the ventricular system. From this investigation, we 1) determined the ipsilateral vs. contralateral distribution of raphe efferents to the ependymal wall of the lateral ventricle, 2) assessed the degree of collateralization of individual ependymal projection neurons to other sites along the ventricular path, 3) compared the topography of raphe neurons that project to the ventricular lining as well as the lumen of the fourth and lateral ventricles, and 4) evaluated the neurochemical identity of raphe neurons that innervate the ventricular system. After tracer injections into the lateral ventricle, labeled cells were distributed evenly on both sides of the midline in the dorsomedial subregion of the intermediate dorsal raphe nucleus. Further rostrally, labeled cells were clustered bilaterally above the medial longitudinal fasciculi and extended into the median raphe nucleus. Injections that involved the ependymal wall of the lateral ventricle resulted in prominent ipsilateral labeling within the dorsal raphe nucleus, just ventral to the cerebral aqueduct. Most of the labeled cells in this latter group had collateral projections to other sites along the ventricular path. Most of the ventricle projection cells contained serotonin but not nicotinamide adenine dinucleotide phosphate diaphorase. These findings indicate that the raphe nuclear complex is topographically organized with respect to the ventricular system. Selected subsets of serotoninergic dorsal raphe neurons may influence discrete segments of the ventricular system independently as well as coordinate functions throughout the system through axon collaterals to other sites along the ventricular neuraxis.

Loss of intraventricular fluid melatonin can explain the neuropathology of Alzheimer's disease

Loss of intraventricular fluid melatonin explains why Alzheimer's disease neuropathology has a laminar, regional and neural-system-specific pattern, which includes the entorhinal cortex, CA1 of the hippocampus, the dorsal raphe nucleus and the locus ceruleus. High metabolic activity may be part of the neuronal vulnerability. Without elevated levels of ventricular fluid melatonin, the hydroxyl radical damages the mitochondria of the most active neurons, causing apoptosis, with resultant failure of the memory system. General brain atrophy follows in the absence of new memory formation.

Serotonergic innervation of the supraependymal neuronal complex of the golden hamster

A ganglion-like system of intraventricular neuronal perikarya and processes known as the supraependymal neuronal complex (SENC) is located on the floor of the third ventricle of the golden hamster. Serotonergic neuronal elements were identified in the SENC by immunocytochemical techniques. Serotonin (5-hydroxytryptamine, 5-HT) was not detected within supraependymal neuronal perikarya. However, serotonergic nerve fibers were found to project via the median eminence both onto the surface and into the neuropil of the SENC. The SENC is thought to be involved in neuroendocrine regulation since it innervates the pituitary gland. The results of the present study suggest that serotonin plays a role in SENC/pituitary gland interaction.

Expression of serotonin transporter mRNA in rat brain: presence in neuronal and non-neuronal cells and effect of paroxetine

The expression of serotonin transporter mRNA in rat brain was-examined by in situ hybridisation. Hybridisation was observed in cells of the known serotonergic nuclei and no other neuronal populations. It was also associated with ependymal cells of the aqueduct which may indicate a specialisation of this part of the ventricular system in anatomical and neurophysiological terms. The effect of the selective serotonin reuptake inhibitor paroxetine on neuronal expression of the serotonin transporter mRNA was examined. Quantitation at the cellular level in the dorsal and median raphe nuclei was carried out by analysis of the mean number of silver grains per cell in autoradiographed sections. No significant change (P > 0.1) in serotonin transporter mRNA expression was observed following 21 day administration of paroxetine (5 mg/kg per day).