Transneuronal transport of herpes simplex virus from the cervical vagus to brain neurons with axonal inputs to central vagal sensory nuclei in the rat

Non-neuronal cholinergic system in the heart influences its homeostasis and an extra-cardiac site, the blood-brain barrier

The non-neuronal cholinergic system of the cardiovascular system has recently gained attention because of its origin. The final product of this system is acetylcholine (ACh) not derived from the parasympathetic nervous system but from cardiomyocytes, endothelial cells, and immune cells. Accordingly, it is defined as an ACh synthesis system by non-neuronal cells. This system plays a dispensable role in the heart and cardiomyocytes, which is confirmed by pharmacological and genetic studies using murine models, such as models with the deletion of vesicular ACh transporter gene and modulation of the choline acetyltransferase (ChAT) gene. In these models, this system sustained the physiological function of the heart, prevented the development of cardiac hypertrophy, and negatively regulated the cardiac metabolism and reactive oxygen species production, resulting in sustained cardiac homeostasis. Further, it regulated extra-cardiac organs, as revealed by heart-specific ChAT transgenic (hChAT tg) mice. They showed enhanced functions of the blood-brain barrier (BBB), indicating that the augmented system influences the BBB through the vagus nerve. Therefore, the non-neuronal cardiac cholinergic system indirectly influences brain function. This mini-review summarizes the critical cardiac phenotypes of hChAT tg mice and focuses on the effect of the system on BBB functions. We discuss the possibility that a cholinergic signal or vagus nerve influences the expression of BBB component proteins to consolidate the barrier, leading to the downregulation of inflammatory responses in the brain, and the modulation of cardiac dysfunction-related effects on the brain. This also discusses the possible interventions using the non-neuronal cardiac cholinergic system.

Cognitive heterogeneity in Parkinson's disease: A mechanistic view

Cognitive impairment occurs in most individuals with Parkinson's disease (PD), exacting a high toll on patients, their caregivers, and the healthcare system. In this review, we begin by summarizing the current clinical landscape surrounding cognition in PD. We then discuss how cognitive impairment and dementia may develop in PD based on the spread of the pathological protein alpha-synuclein (aSyn) from neurons in brainstem regions to those in the cortical regions of the brain responsible for higher cognitive functions, as first proposed in the Braak hypothesis. We appraise the Braak hypothesis from molecular (conformations of aSyn), cell biological (cell-to-cell spread of pathological aSyn), and organ-level (region-to-region spread of aSyn pathology at the whole brain level) viewpoints. Finally, we argue that individual host factors may be the most poorly understood aspect of this pathological process, accounting for substantial heterogeneity in the pattern and pace of cognitive decline in PD.

Dexamethasone attenuates the modulatory effect of the insular cortex on the baroreflex in anesthetized rat

The arterial baroreflex (BR) is an important neural mechanism for the stabilization of arterial pressure (AP). It is known that the insular cortex (IC) and other parts of the central autonomic network (CAN) are able to modulate the BR arc, altering baroreflex sensitivity (BRS). In addition, the sensitivity of the BR changes under the influence of hormones, in particular glucocorticoids (GC). It has been suggested that GC may influence BRS by altering the ability of the IC to modulate the BR. This hypothesis has been tested in experiments on rats anesthetized with urethane. It was found that microelectrostimulation of the visceral area in the left IC causes a short-term drop in AP, which is accompanied by bradycardia, and impairs BRS. The synthetic GC dexamethasone (DEX) did not significantly affect the magnitude of depressor responses but increased BRS and impaired the effect of IC stimulation on the BR. The results obtained confirm the hypothesis put forward and suggest that GC can attenuate the inhibitory effects of the IC on the BR arc, thereby enhancing the sensitivity of the BR.

Neuron-to-neuron α-synuclein propagation in vivo is independent of neuronal injury

INTRODUCTION

Interneuronal propagation of α-synuclein has been demonstrated in a variety of experimental models and may be involved in disease progression during the course of human synucleinopathies. The aim of this study was to assess the role that neuronal injury or, vice versa, cell integrity could have in facilitating interneuronal α-synuclein transfer and consequent protein spreading in an in vivo animal model.

RESULTS

Viral vectors carrying the DNA for human α-synuclein were injected into the rat vagus nerve to trigger protein overexpression in the medulla oblongata and consequent spreading of human α-synuclein toward pons, midbrain and forebrain. Two vector preparations sharing the same viral construct were manufactured using identical procedures with the exception of methods for their purification. They were also injected at concentrations that induced comparable levels of α-synuclein transduction/overexpression in the medulla oblongata. α-Synuclein load was associated with damage (at 6 weeks post injection) and death (at 12 weeks) of medullary neurons after treatment with only one of the two vector preparations. Of note, neuronal injury and degeneration was accompanied by a substantial reduction of caudo-rostral propagation of human α-synuclein.

CONCLUSIONS

Interneuronal α-synuclein transfer, which underlies protein spreading from the medulla oblongata to more rostral brain regions in this rat model, is not a mere consequence of passive release from damaged or dead neurons. Neuronal injury and degeneration did not exacerbate α-synuclein propagation. In fact, data suggest that cell-to-cell passage of α-synuclein may be particularly efficient between intact, relatively healthy neurons.

The effect of mouse strain on herpes simplex virus type 1 (HSV-1) infection of the central nervous system (CNS)

BACKGROUND

Mice infected with HSV-1 can develop lethal encephalitis or virus induced CNS demyelination. Multiple factors affect outcome including route of infection, virus and mouse strain. When infected with a sub-lethal dose of HSV-1 strain 2 via the oral mucosa, susceptible SJL/J, A/J, and PL/J mice develop demyelinating lesions throughout the brain. In contrast, lesions are restricted to the brainstem (BST) in moderately resistant BALB/c mice and are absent in resistant BL/6 mice. The reasons for the strain differences are unknown.

METHODS

In this study, we combine histology, immunohistochemistry, and in-situ hybridization to investigate the relationship between virus and the development of lesions during the early stage (< 24 days PI) of demyelination in different strains of mice.

RESULTS

Initially, viral DNA and antigen positive cells appear sequentially in non-contiguous areas throughout the brains of BALB/c, SJL/J, A/J, and PL/J mice but are restricted to an area of the BST of BL/6 mice. In SJL/J, A/J, and PL/J mice, this is followed by the development of 'focal' areas of virus infected neuronal and non-neuronal cells throughout the brain. The 'focal' areas follow a hierarchical order and co-localize with developing demyelinating lesions. When antigen is cleared, viral DNA positive cells can remain in areas of demyelination; consistent with a latent infection. In contrast, 'focal' areas are restricted to the BST of BALB/c mice and do not occur in BL/6 mice.

CONCLUSIONS

The results of this study indicate that susceptible mouse strains, infected with HSV-1 via the oral mucosa, develop CNS demyelination during the first 24 days PI in several stages. These include: the initial spread of virus and infection of cells in non-contiguous areas throughout the brain, the development of 'focal' areas of virus infected neuronal and non-neuronal cells, the co-localization of 'focal' areas with developing demyelinating lesions, and latent infection in a number of the lesions. In contrast, the limited demyelination that develops in BALB/c and the lack of demyelination in BL/6 mice correlates with the limited or lack of 'focal' areas of virus infected neuronal and non-neuronal cells in these two strains.

Glutamatergic neurotransmission in the nucleus tractus solitarii: structural and functional characteristics

Glutamate is the main excitatory transmitter in the central nervous system. As such, it plays a major role in transmitting and processing visceral sensory information within the nucleus tractus solitarii (NTS). Here, we review current knowledge on NTS glutamatergic transmission. We describe the main organizational features of NTS glutamatergic synapses as determined by work performed during the last decade using antibodies against glutamate receptors and transporters proteins. In light of these recent neuronatomical findings, we discuss some functional properties of developing and adult NTS glutamatergic synapses.

Parkinson's disease: a dual-hit hypothesis

Accumulating evidence suggests that sporadic Parkinson's disease has a long prodromal period during which several non-motor features develop, in particular, impairment of olfaction, vagal dysfunction and sleep disorder. Early sites of Lewy pathology are the olfactory bulb and enteric plexus of the stomach. We propose that a neurotropic pathogen, probably viral, enters the brain via two routes: (i) nasal, with anterograde progression into the temporal lobe; and (ii) gastric, secondary to swallowing of nasal secretions in saliva. These secretions might contain a neurotropic pathogen that, after penetration of the epithelial lining, could enter axons of the Meissner's plexus and, via transsynaptic transmission, reach the preganglionic parasympathetic motor neurones of the vagus nerve. This would allow retrograde transport into the medulla and, from here, into the pons and midbrain until the substantia nigra is reached and typical aspects of disease commence. Evidence for this theory from the perspective of olfactory and autonomic dysfunction is reviewed, and the possible routes of pathogenic invasion are considered. It is concluded that the most parsimonious explanation for the initial events of sporadic Parkinson's disease is pathogenic access to the brain through the stomach and nose - hence the term 'dual-hit'.

Ventral medial prefrontal cortex and cardiovagal control in conscious humans

The autonomic nervous system plays a critical role in regulating the cardiovascular responses to mental and physical stress. Recent neuroimaging studies have demonstrated that sympathetic outflow to the heart is modulated by the activity of the anterior cingulate cortex (ACC). However, the cortical modulation of cardiovagal activity is still unclear in humans. The present study used functional MRI to investigate the cortical network involved in cardiovagal control. Seventeen healthy individuals performed graded handgrip exercise while heart rate (HR) and cortical activity were recorded. Muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP) and HR were measured while participants repeated the same protocol in a parallel experiment session. The handgrip exercise elevated HR and MAP without concurrent elevations in MSNA supporting earlier conclusions that the cardiovascular responses are mainly modulated by vagal withdrawal. The imaging data showed activation in the insular cortex, thalamus, parietal cortices and cerebellum during the exercise period. Consistently across all the participants, the HR response correlated with the deactivation in the ventral medial prefrontal cortex (vMPFC), which has substantial anatomical connection with the subcortical autonomic structures. The deactivation of the vMPFC was independent of the motor control and was observed commonly in both left and right hand exercise. Stronger vMPFC deactivation was observed when participants completed a higher intensity exercise that elicited a larger HR response. Our findings support the hypothesis that the vMPFC is involved in modulating the vagal efferent outflow to the heart and the suppression of its activity elevates cardiovascular arousal in conscious humans.

Vesicular glutamate transporters type 1 and 2 expression in axon terminals of the rat nucleus of the solitary tract

The nucleus of the solitary tract is the site of termination of primary afferent fibers running in the facial, glossopharyngeal and vagus nerves. The present study was performed to map the distribution of glutamatergic axons terminals in the rat nucleus of the solitary tract using immunodetection of vesicular glutamate transporter 1 and vesicular glutamate transporter 2. The two vesicular glutamate transporters were differentially distributed among nucleus of the solitary tract subdivisions. Vesicular glutamate transporter 1 immunoreactivity was mostly found in the lateral part of the nucleus (ventrolateral, interstitial and intermediate subdivisions) whereas vesicular glutamate transporter 2 labeling was distributed throughout the nucleus of the solitary tract. Electron microscope examination indicated that vesicular glutamate transporter immunoreactivity was localized in axon terminals filled with round synaptic vesicles. After injection of cholera toxin B subunit in sensory ganglia, anterograde labeling was found in vesicular glutamate transporter 1, as well as vesicular glutamate transporter 2-immunoreactive boutons. Double immunolabeling experiments allowed distinctions between terminals expressing either vesicular glutamate transporter 1 or vesicular glutamate transporter 2 or both vesicular glutamate transporter 1 and vesicular glutamate transporter 2 immunoreactivities. The latter population, expressing both transporters immunolabeling, completely disappeared after deafferentation induced by removal of sensory ganglia. This study indicates that vesicular glutamate transporter content identifies three different subpopulations of glutamatergic boutons in the nucleus of the solitary tract and provides definitive evidence that primary afferent neurons contribute glutamatergic terminals to the nucleus of the solitary tract.

Supraspinal connections of the reproductive organs: structural and functional aspects

Gonadal functions are governed by the hypothalamo-hypophyseal system. Other organs of the reproduction tract are under the regulatory action of gonadal steroids. In the past two decades several data have been accumulated on the involvement of fine-tuning control mechanisms which include autocrine and paracrine effects of biologically active substances produced locally and the regulatory action of nerves innervating the organs of the system. Recent studies using the viral transsynaptic technique have revealed cell groups in the central nervous system that are transneuronally connected with the male and female reproductive organs. This review summarizes neuromorphological data on the supraspinal innervation of reproductive organs and the functional significance of these brain areas in the control of reproduction.

Anatomy and function of sensory hepatic nerves

Vagal and spinal afferent innervation of the portal hepatic area has not been studied as thoroughly as the innervation of other important organs. It is generally agreed that unlike noradrenergic sympathetic efferent nerve fibers, sensory nerve fibers of either vagal or dorsal root/spinal origin do not directly innervate hepatocytes, but are restricted to the stroma surrounding triades of hepatic vasculature and bile ducts, and to extrahepatic portions of the portal vein and bile ducts. For vagal afferent innervation, retrograde and anterograde tracing studies in the rat have clearly shown that only a minor portion of the common hepatic branch innervates the liver area, while the major portion descends in the gastroduodenal branch toward duodenum, pancreas, and pylorus. Hepatic paraganglia, bile ducts, and portal vein receive the densest vagal afferent innervation. Calretinin may be a relatively specific marker for vagal afferent innervation of the portal-hepatic space. Calcitonin gene-related peptide (CGRP) is a specific marker for dorsal root afferents, and CGRP-immunoreactive fibers are mainly present near the intrahepatic vascular bundles and bile ducts, and in the same extrahepatic compartments that contain vagal afferents. Because of the specific anatomical organization of hepatic nerves, selective hepatic denervation, whether selective for the vagal or sympathetic division, or for efferents and afferents, is nearly impossible. Great caution is therefore necessary when interpreting functional outcomes of so-called specific hepatic denervation studies.

Prolactin-releasing peptide affects gastric motor function in rat by modulating synaptic transmission in the dorsal vagal complex

Prolactin-releasing peptide (PrRP) is a recently discovered neuropeptide implicated in the central control of feeding behaviour and autonomic homeostasis. PrRP-containing neurones and PrRP receptor mRNA are found in abundance in the caudal portion of the nucleus tractus solitarius (NTS), an area which together with the dorsal motor nucleus of the vagus (DMV) comprises an integrated structure, the dorsal vagal complex (DVC) that processes visceral afferent signals from and provides parasympathetic motor innervation to the gastrointestinal tract. In this study, microinjection experiments were conducted in vivo in combination with whole-cell recording from neurones in rat medullary slices to test the hypothesis that PrRP plays a role in the central control of gastric motor function, acting within the DVC to modulate the activity of preganglionic vagal motor neurones that supply the stomach. Microinjection of PrRP (0.2 pmol (20 nl)(-1)) into the DMV at the level of the area postrema (+0.2 to +0.6 mm from the calamus scriptorius, CS) markedly stimulated gastric contractions and increased intragastric pressure (IGP). Conversely, administration of peptide into the DMV at sites caudal to the obex (0.0 to -0.3 mm from the CS) decreased IGP and reduced phasic contractions. These effects occurred without change in mean arterial pressure and were abolished by ipsilateral vagotomy, indicating mediation via a vagal-dependent mechanism(s). The pattern of gastric motor responses evoked by PrRP mimicked that produced by administration of L-glutamate at the same sites, and both the effects of L-glutamate and PrRP were abolished following local administration of NMDA and non-NMDA-type glutamate receptor antagonists. On the other hand, microinjection of PrRP into the medial or comissural nucleus of the solitary tract (mNTS and comNTS, respectively) resulted in less robust changes in IGP in a smaller percentage of animals, accompanied by marked alterations in arterial pressure. Superfusion of brain slices with PrRP (100-300 nm) produced a small depolarization and increased spontaneous firing in 10 of 30 retrogradely labelled gastric-projecting DMV neurones. The excitatory effects were blocked by administration of TTX (2 mum) or specific glutamate receptor antagonists, indicating that they resulted from interactions of PrRP at a presynaptic site. Congruent with this, PrRP increased the amplitude of excitatory postsynaptic currents (EPSCs, 154 +/- 33%, 12 of 25 neurones) evoked by electrical stimulation in mNTS or comNTS. In addition, administration of PrRP decreased the paired-pulse ratio of EPSCs evoked by two identical stimuli delivered 100 ms apart (from 0.95 +/- 0.08 to 0.71 +/- 0.11, P < 0.05), whereas it did not affect the amplitude of inward currents evoked by exogenous application of L-glutamate to the slice. The frequency, but not amplitude of spontaneous EPSCs and action potential-independent miniature EPSCs was also increased by administration of PrRP, suggesting that the peptide was acting at least in part at receptors on presynaptic nerve terminals to enhance glutamatergic transmission. In recordings obtained from a separate group of slices, we did not observe any direct effects of PrRP on spontaneous discharge or postsynaptic excitability in either mNTS or comNTS neurones (n = 31). These data indicate that PrRP may act within the DVC to regulate gastric motor function by modulating the efficacy of conventional excitatory synaptic inputs from the NTS onto gastric-projecting vagal motor neurones.

Novel tracing paradigms--genetically engineered herpesviruses as tools for mapping functional circuits within the CNS: present status and future prospects

The mammalian CNS is composed of an extremely complex meshwork of highly ordered interconnections among billions of neurons. To understand the diverse functions of this neuronal network we need to differentiate between functionally related and nonrelated elements. A powerful labeling method for defining intricate neural circuits is based on the utilization of neurotropic herpesviruses, including pseudorabies virus and herpes simplex virus type 1. The recent development of genetically engineered tracing viruses can open the way toward the conception of novel tract-tracing paradigms. These new-generation tracing viruses may facilitate the clarification of problems, which were inaccessible to earlier approaches. This article first presents a concise review of the general aspects of neuroanatomical tracing protocols. Subsequently, it discusses the molecular biology of alpha-herpesviruses, and the genetic manipulation and gene expression techniques that are utilized for the construction of virus-based tracers. Finally, it describes the current utilization of genetically modified herpesviruses for circuit analysis, and the future directions in their potential applications.

Anatomic relationships of the human nucleus of the solitary tract in the medulla oblongata: a DiI labeling study

The nucleus of the solitary tract (nTS) is a major site of brainstem control of vital functions (e.g., cardiovascular reflexes and respiration). We examined anatomic relationships of the human nucleus of the solitary tract, using a bidirectional lipophilic fluorescent tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in 10 postmortem human fetal midgestational medullae oblongatae. Labeling by diffusion of DiI from the nucleus of the solitary tract included: (1) neuropil of all future subdivisions of the nucleus of the solitary tract ipsilateral to the DiI crystal; (2) stellate cells in the caudal raphe at the junction of the nucleus raphe pallidus and the arcuate nucleus at the ventral medullary surface, as well as single fibers along the caudal raphe and the arcuate nucleus; (3) cells and fibers in other medullary areas related to autonomic and respiratory control, including the dorsal motor nucleus of the vagus, nucleus ambiguus complex/ventral respiratory group, rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM), and medullary reticular formation. The pattern of connections of the nucleus of the solitary tract already established by midgestation in the human fetus is consistent with the pattern previously demonstrated in adult experimental animals. A major finding of the study is that of the stellate cells at the junction of nucleus raphe pallidus and the arcuate nucleus at the ventral medullary surface, which project to the nucleus of the solitary tract, and could be homologous to chemosensitive serotonergic neurons at the midline ventral medullary surface of experimental animals. This connection between the ventral caudal raphe and the nucleus of the solitary tract may participate in chemoreception and central regulation of cardiorespiratory reflexes during human perinatal development; it is, therefore, relevant to the study of sudden infant death syndrome (SIDS).

Role of ventrolateral periaqueductal gray neurons in the behavioral and cardiovascular responses to contextual conditioned fear and poststress recovery

We have previously shown that conditioned fear to context increases Fos expression in the caudal ventrolateral region of the periaqueductal gray in the rat. To understand the reason for this activation and its role in the expression of the contextual fear response, the ventrolateral periaqueductal gray was temporarily blocked with bilateral microinjections (0.4 microl) of the GABA agonist muscimol (0.2 mM) or the glutamate antagonist kynurenic acid (0.1 M). Cardiovascular changes and activity were recorded by radio-telemetry and the microinjections were made immediately before testing the conditioned response in the aversive context. Muscimol and kynurenic acid had the same effects: when compared to saline controls, freezing immobility and ultrasonic vocalizations were reduced and replaced by marked locomotor activity, and the increase in heart rate was enhanced; however, the increase in arterial blood pressure remained the same. Interesting changes were also observed when animals were returned to the safe context of their home box after fear (recovery). Basically, the recovery response was either prevented or delayed: instead of returning to resting immobility, the rats remained agitated in their home box with a moderately elevated activity, heart rate and blood pressure. However, the effect of ventrolateral periaqueductal gray blockade on heart rate, arterial pressure and activity did not appear to be specific to the fear response or its recovery because they were also observed in animals returned to the safe context of their home box immediately after injection. The later response was also a recovery response from the milder stress of handling and the injection procedure.We discuss the results by arguing that the ventrolateral periaqueductal gray is involved in the immobility component of both the fear response and poststress recovery responses. To explain our interpretation we consider the findings in relation to the classic descending defence-arousal system and the hyporeactive-hypotensive immobility pattern that has been attributed to the ventrolateral periaqueductal gray. We propose that there is a dual activation of the defence-arousal system and of the ventrolateral periaqueductal gray during fear, with the ventrolateral periaqueductal gray acting as a brake on the defence-arousal system. The role of this brake is to impose immobility and hold off active defence responses such as fight and flight. The result of this combination of arousal and immobility is a hyperreactive freezing immobility associated with ultrasonic vocalizations, and a pressor response accompanied with a slow rise in heart rate. Basically, the animal is tense and ready for action but temporarily immobilised. The ventrolateral periaqueductal gray also acts to impose immobility during recovery; however, this is without coactivation of the defence-arousal system. The result is a return to resting immobility, associated with a return to baseline blood pressure and heart rate. This is an active process that insures a faster and complete return to rest. We conclude that the ventrolateral periaqueductal gray is an immobility center involved not only in the fear response but also in poststress recovery responses.

Supraspinal connections of the ovary: structural and functional aspects

This review summarizes our recent studies using the viral transneuronal tracing technique to identify sites in the central nervous system (CNS) that are connected with the ovary. A neurotropic virus (pseudorabies virus) was injected into the ovary and various times after the inoculation the spinal cord and brain were examined for virus-infected neurons identified by immunocytochemistry. Such neurons could be detected in well-defined cell groups of the spinal cord (intermediolateral cell column), brain stem (vagal nuclei, area postrema, parapyramidal nucleus, caudal raphe nuclei, A1, A5, A7 noradrenergic cell groups, locus coeruleus, Barrington's nucleus, periaqueductal gray), hypothalamus (paraventricular nucleus, anterior hypothalamus, arcuate nucleus, zona incerta), and, at longer survival time, in some telencephalic structures (amygdala, bed nucleus of the stria terminalis). These findings provided the first neuromorphological evidence for the existence of a multisynaptic neuronal pathway between the brain and the ovary presumably involved in the neuronal control of the organ. The observations indicate that there is a significant overlap of CNS structures connected with the ovary, the testis, other organs and organ systems, suggesting similar neuronal circuitries of the autonomic nervous system innervating the different organs. The known descending neuronal connections between the CNS structures labeled from the ovary by the viral transneuronal tracing technique and the findings suggesting a pituitary independent interplay between certain cerebral structures such as the hypothalamus, the amygdala, and the ovary are also summarized in this review.

Tracing functionally identified neurones in a multisynaptic pathway in the hamster and rat using herpes simplex virus expressing green fluorescent protein

Using a genetically modified herpes simplex virus encoding green fluorescent protein we sought to establish if this viral modification could be used in transneuronal tracing studies of the sympathetic nervous system. The herpes simplex virus encoding green fluorescent protein was injected into the adrenal medulla of three hamsters and six rats. After a suitable survival period, neurones in the sympathetic intermediolateral cell column of the thoracolumbar spinal cord, rostral ventral medulla and paraventricular nucleus of the hypothalamus were clearly identified by the presence of a green fluorescence in the cytoplasm of the neurones of both species. Thus, herpes simplex virus encoding green fluorescent protein labelled chains of sympathetic neurones in the hamster and rat and therefore has the potential to be used in transneuronal tracing studies of autonomic pathways in these species.

Glucose effects on gastric motility and tone evoked from the rat dorsal vagal complex

1. To examine the effects of glucose on the central components of the vago-vagal reflex control of gastric function, we performed both in vivo and in vitro experiments on neurones in the medial nucleus of the tractus solitarius (mNTS) and in the dorsal motor nucleus of the vagus (DMV). 2. In the in vivo anaesthetized rat preparation, unilateral microinjection of D-glucose (10 or 50 mM (60 nl)(-1)) in mNTS produced inhibition of gastric motility and an increase in intragastric pressure. D-glucose had no effect in the DMV. 3. In the in vitro rat brainstem slice preparation, whole-cell recordings of DMV neurones showed that increasing the glucose concentration of the perfusion solution from 5 mM to 15 or 30 mM produced outward currents of 35 +/- 5 pA (n = 7) and 51 +/- 10 pA (n = 11), respectively. These were blocked by tetrodotoxin and picrotoxin, indicating that glucose was acting indirectly to cause the release of GABA. Decreasing the glucose concentration of the perfusing solution by one-half produced an inward current of 36 +/- 5 pA (n = 7). 4. Stimulation of the NTS evoked inhibitory postsynaptic currents (IPSCs) in DMV neurones. The amplitude of the evoked IPSCs was positively correlated with glucose concentration. Perfusion with the ATP-sensitive K(+) (K(ATP)) channel opener diazoxide mimicked the effect of reduced glucose, while perfusion with the K(ATP) channel blocker glibenclamide mimicked the effects of increased glucose. 5. Our data indicate that glucose had no direct excitatory effect on DMV neurones, but DMV neurones appear to be affected by an action of glucose on cell bodies of mNTS neurones via effects on an ATP-sensitive potassium channel.

Elucidation of neuronal circuitry: protocol(s) combining intracellular labeling, neuroanatomical tracing and immunocytochemical methodologies

We describe a protocol combining either intracellular biotinamide staining or anterograde biotinylated dextran amine (BDA) tracing with retrograde horseradish peroxidase (HRP) labeling and immunocytochemistry in order to map physiologically identified neuronal pathways. Presynaptic neurons including their boutons are labeled by either intracellular injection of biotinamide or extracellular injection of BDA while postsynaptic neurons are labeled with HRP via retrograde transport. Tissues are first processed to detect HRP using a tetramethylbenzidine and sodium-tungstate method. Biotinamide or BDA staining is then visualized using an ABC-diaminobenzidine-Ni method and finally the tissue is immunocytochemically stained using choline acetyltransferase (ChAT) or parvalbumin antibodies and a peroxidase-anti-peroxidase method. After processing, biotinamide, BDA, HRP and immunocytochemical staining can readily be distinguished by differences in the size, color and texture of their reaction products. We have utilized this methodology to explore synaptic relationships between trigeminal primary afferent neurons and brainstem projection and motoneurons at both the light and electron microscopic levels. This multiple labeling methodology could be readily adapted to characterize the physiological, morphological and neurochemical properties of other neuronal pathways.

Anatomic relationships of the human nucleus paragigantocellularis lateralis: a DiI labeling study

The nucleus paragigantocellularis lateralis (PGL) is located in the rostral ventrolateral medulla (RVLM), a brainstem region that regulates homeostatic functions, such as blood pressure and cardiovascular reflexes, respiration. central chemosensitivity and pain. In the present study, we examined anatomic relationships of the human nucleus paragigantocellularis lateralis using a bidirectional lipophilic fluorescent tracer, 1,1'-dioctadecyl-3,3.3',3'-tetramethylindocarbocyanine perchlorate (DiI), in nine postmortem human fetal midgestational brainstems. The areas which were labeled by diffusion of DiI from the nucleus paragigantocellularis lateralis included the arcuate nucleus (ARC) of the medulla, caudal raphe (nucleus raphe obscurus and pallidus), hilum and amiculum of the inferior olive, bilateral "reticular formation" (including the nucleus paragigantocellularis lateralis, nucleus gigantocellular-is and the intermediate reticular zone (IRZ)). vestibular and cochlear nuclei, cells and fibers at the floor of the fourth ventricle with morphologic features of tanycytes, parabrachial nuclei (PBN), medial lemniscus, lateral lemniscus, inferior cerebellar peduncle and cerebellar white matter, central tegmental tract, and the capsule of the red nucleus. This pattern of DiI labeling bears many similarities with the pattern of connections of the nucleus paragigantocellularis lateralis previously demonstrated by tract-tracing methods in experimental animals, and is consistent with the role of the nucleus paragigantocellularis lateralis in central regulation of homeostatic functions. In contrast to the animal studies, however, we did not demonstrate connections of the nucleus paragigantocellularis lateralis with the nucleus of the tractus solitarius (nTS) (only connections with the rostral subdivision were examined), locus coeruleus, or the periaqueductal gray (PAG) in the human midgestational brainstem. In our previous studies, six medullary areas showed reduced serotonin receptor binding in a subset of victims of sudden infant death syndrome (SIDS). The present study demonstrated DiI labeling in all of these six areas, suggesting that they are interconnected.

Dorsal and ventral medullary catecholamine cell groups contribute differentially to systemic interleukin-1beta-induced hypothalamic pituitary adrenal axis responses

Medial parvocellular paraventricular corticotropin-releasing hormone (mPVN CRH) cells are critical in generating hypothalamic-pituitary-adrenal (HPA) axis responses to systemic interleukin-1beta (IL-1beta). However, although it is understood that catecholamine inputs are important in initiating mPVN CRH cell responses to IL-1beta, the contributions of distinct brainstem catecholamine cell groups are not known. We examined the role of nucleus tractus solitarius (NTS) and ventrolateral medulla (VLM) catecholamine cells in the activation of mPVN CRH, hypothalamic oxytocin (OT) and central amygdala cells in response to IL-1beta (1 microg/kg, i.a.). Immunolabelling for the expression of c-fos was used as a marker of neuronal activation in combination with appropriate cytoplasmic phenotypic markers. First we confirmed that PVN 6-hydroxydopamine lesions, which selectively depleted catecholaminergic terminals, significantly reduced IL-1beta-induced mPVN CRH cell activation. The contribution of VLM (A1/C1 cells) versus NTS (A2 cells) catecholamine cells to mPVN CRH cell responses was then examined by placing ibotenic acid lesions in either the VLM or NTS. The precise positioning of these lesions was guided by prior retrograde tracing studies in which we mapped the location of IL-1beta-activated VLM and NTS cells that project to the mPVN. Both VLM and NTS lesions reduced the mPVN CRH and OT cell responses to IL-1beta. Unlike VLM lesions, NTS lesions also suppressed the recruitment of central amygdala neurons. These studies provide novel evidence that both the NTS and VLM catecholamine cells have important, but differential, contributions to the generation of IL-1beta-induced HPA axis responses.

Central nervous system structures labelled from the testis using the transsynaptic viral tracing technique

In the present study, the transneuronal transport of neurotrophic virus technique was used to identify cell groups of the spinal cord and the brain that are transsynaptically connected with the testis. Pseudorabies virus was injected into the testis and after survival times of 3-6 days, the spinal cord and brain were processed immunocytochemically using a polyclonal antibody against the virus. Virus-infected perikarya were detected in the preganglionic neurones of the spinal cord (T10-L1, L5-S1) and in certain cell groups and areas of the brain stem, the hypothalamus and the telencephalon. In the brain stem, the cell groups and areas in which labelled neurones were present included, among others, the nucleus of the solitary tract, the caudal raphe nuclei, the locus coeruleus and the periaqueductal grey of the mesencephalon. In the hypothalamus, virus infected perikarya were observed in the paraventricular nucleus and in certain other cell groups. Telencephalic structures containing labelled neurones included the preoptic area, the bed nucleus of the stria terminalis, the central amygdala and the insular cortex. These data identify a multisynaptic circuit of neurones in the spinal cord and in the brain which may be involved in the control of testicular functions.

CNS structures presumably involved in vagal control of ovarian function

The contribution of the vagus nerve to viral transneuronal labeling of brain structures from the ovaries demonstrated recently by us was investigated. Unilateral vagotomy was performed prior to ipsilateral intraovarian virus injection. Virus-infected neurons were visualized by immunostaining. In vagotomized rats such neurons were detected only in certain cell groups of the brain (parapyramidal nucleus, A(1), A(5) cell group, caudal raphe nuclei, hypothalamic paraventricular nucleus, lateral hypothalamus). Vagotomy interfered with labeling of several structures that were labeled in controls, including area postrema, nucleus of the solitary tract, dorsal vagal complex, nucleus ambiguus, A(7) cell group, Barrington's nucleus, locus coeruleus, periaqueductal gray, dorsal hypothalamus. Findings provide a morphological basis to study the functional significance of brain structures presumably involved in the control of ovarian function and acting via the vagus or the sympathetic nerves.

Lidocaine blockade of amygdala output in fear-conditioned rats reduces Fos expression in the ventrolateral periaqueductal gray

We showed recently that conditioned fear to context induces Fos expression in the ventrolateral periaqueductal gray [Neuroscience (1997) 78, 165-177]. Neurons in this region are thought to play an important role in the expression of freezing during conditioned fear. To test the possibility that this activation comes directly from the amygdala, we looked at changes in Fos expression after a unilateral blockade of the ventral amygdalofugal pathway with lidocaine. The pathway contains fibres originating from the central nucleus of the amygdala that project directly and mainly ipsilaterally to the ventrolateral periaqueductal gray. Conditioned fear was evoked by re-exposing rats to the same box in which they had previously received electric footshocks. The test re-exposure was preceded by a unilateral microinjection of lidocaine (2%, 0.5-1 microl; n = 20) or saline (n = 14). Lidocaine was also tested in non-conditioned animals (n = 13). The results show that, when lidocaine was microinjected in the medial part of the central nucleus of the amygdala or along the ventral amygdalofugal pathway of conditioned rats, fear-induced Fos expression in the ventrolateral periaqueductal gray was reduced on the side ipsilateral to the injection (up to 37% reduction in comparison to the contralateral side). Ipsilateral reductions were also observed with saline, but they were weaker (maximum of 27% reduction). Fos expression remained low on both sides in the non-fear-conditioned animals injected with lidocaine. Finally, although freezing was only partly reduced in the conditioned animals unilaterally injected with lidocaine, it was significantly correlated to the ipsilateral reduction in Fos expression. This study provides direct evidence that the projection from the central nucleus of the amygdala to the ventrolateral periaqueductal gray is activated during fear and that it contributes to the Fos response of the ventrolateral periaqueductal gray.

Nitrergic stimulation of the locus coeruleus modulates blood pressure and heart rate in the anaesthetized rat

To investigate whether nitric oxide is involved in the cardiovascular responses mediated via the locus coeruleus, the effects of microinjections of L-arginine and L-glutamate into the locus coeruleus on blood pressure and heart rate were investigated in sodium pentobarbitone-anaesthetized rats. Unilateral microinjection of L-arginine (25, 50 nmol) elicited dose-related depressor (-17 +/- 4, -25 +/- 4 mmHg) and bradycardic (13 +/- 3, 24 +/- 6 b.p.m.) effects. Furthermore, these effects were attenuated by prior local microinjection of N(G)-nitro-L-arginine (40 nmol). Peripheral muscarinic receptor blockade with atropine methyl nitrate (1 mg/kg, i.v.) attenuated the bradycardic but not the depressor responses to L-arginine. L-Glutamate (2 nmol) microinjections also mediated depressor (-27 +/- 6 mmHg) and bradycardic (53 +/- 23 b.p.m.) effects that were attenuated by microinjections of dizocilpine maleate (1 nmol) into the locus coeruleus. In addition, pretreatment with N(G)-nitro-L-arginine (40 nmol) also significantly attenuated the depressor response elicited by L-glutamate. These results suggest that nitrergic and glutamatergic pathways are operative within the locus coeruleus to modulate cardiovascular function, and also that a functional interaction may exist between the nitrergic and glutamatergic systems within the rat locus coeruleus.

Herpes simplex virus as a transneuronal tracer

Determining the connections of neural systems is critical for determining how they function. In this review, we focus on the use of HSV-1 and HSV-2 as transneuronal tracers. Using HSV to examine neural circuits is technically simple. HSV is injected into the area of interest, and after several days, the animals are perfused and processed for immunohistochemistry with antibodies to HSV proteins. Variables which influence HSV infection include species of host, age of host, titre of virus, strain of virus and phenotype of infected cell. The choice of strain of HSV is critically important. Several strains of HSV-1 and HSV-2 have been utilized for purposes of transneuronal tract-tracing. HSV has been used successfully to study neuronal circuitry in a variety of different neuroanatomical systems including the somatosensory, olfactory, visual, motor, autonomic and limbic systems.

Viruses as transneuronal tracers for defining neural circuits

Live viruses can be used as tools to label chains of neurons and thus to define functionally connected CNS circuits. This review summarizes the background and general principles involved in using the viral tracing technology. An attenuated form of a pig herpes virus, known as the Bartha's K strain of pseudorabies virus, has proven to be a useful type of virus for the analysis of CNS systems in the rat. The properties of this virus and the evidence for its specificity in causing trans-synaptic infections is discussed.

Neuronal labeling in the rat brain and spinal cord from the ovary using viral transneuronal tracing technique

In the present investigations the viral transneuronal labeling method, which is able to reveal hierarchial chains of central nervous system (CNS) neurons, was applied to identify sites in the CNS connected with the ovary and presumably involved in the control of ovarian functions. Pseudorabies virus was injected into the ovaries of rats and a few days later (at various times after the injection) the spinal cord and brain were examined for virus-infected neurons from the ovary. The virus-labeled nerve cells were identified by immunocytochemistry using polyclonal antiviral antibody. Virus-labeled neurons were detected both in the spinal cord and the brain. In the spinal cord such elements were observed in the intermediolateral cell column, in the dorsal horn close to the marginal zone and in the central autonomic nucleus. In the medulla oblongata and pons, neurons of several nuclei and cell groups (area postrema, nucleus of the solitary tract, dorsal vagal complex, nucleus ambiguus, paragigantocellular nucleus, parapyramidal nucleus, A1, A5 and A7 cell groups, caudal raphe nuclei, locus ceruleus, subceruleus nucleus, Barrington's nucleus, Kölliker-Fuse nucleus) were found to be transneuronally labeled. In the mesencephalon, the ventrolateral part of the periaqueductal gray matter contained virus-labeled neurons. In the diencephalon, a very intensive cell body labeling was observed in the hypothalamic paraventricular nucleus and a few virus-infected neurons could be detected in the lateral and dorsal hypothalamus, in the arcuate nucleus, zona incerta, perifornical area and in the anterior hypothalamus. Concerning the telencephalic structures, virus-labeled cells were found in the bed nucleus of the stria terminalis and in the central amygdala nucleus. These findings provide the first neuromorphological evidence for the existence of a multisynaptic neuronal pathway between the ovary and the CNS, and give a detailed account of the structures involved in this pathway.

Efferent projection from the rostral ventrolateral medulla to the area postrema in rats

The rostral ventrolateral medulla (RVLM) is a region of the brain primarily involved in cardiovascular control. It receives information from several areas of the brainstem, among which the area postrema (AP) and the nucleus of the solitary tract (NTS). The medial subnuclei of the solitary tract (TS) project towards the RVLM, providing cardiopulmonary information, and the AP serves information about circulatory hormones. Although the efferent pathways are well known, it is not the case for the connections from the RVLM towards the AP and the NTS. The present study was designed to examine the efferent connections from the RVLM onto the dorsal structures of the medulla: quantitatively by means of anatomical techniques, and functionally by means of electrophysiological techniques. Morphologically, Biocytin or Biotinylated dextran amine microinjections into the RVLM were followed by labelling of many fibres running towards the bulbar dorsomedial structures, with some pathways lying in the AP itself, or located in its caudal vicinity. Conversely, when microinjections of Fast Blue (FB) were made into the AP, FB-labelled cells could be observed within the RVLM. Electrophysiologically, single shock stimulation carried on AP allowed identification of axonal fibres issuing from somata located into the cardiovascular neuronal pool in the RVLM. From these results, we can assume: (1) the existence of dense efferent projection from RVLM to aspects of the dorsal vagal complex, including the AP and, among this dense projection, (2) the existence of some fibres terminating in, or crossing through the AP, and identified as conveying baroreceptor-related information, in the rat.

Neuronal connections between the auricular skin and the sympathetic pre- and postganglionic neurons of the dog as studied by using pseudorabies virus

Pseudorabies virus (PrV) as a neuronal tracer was microinjected into the concave surface of the puppy's left pinna to establish the morphological basis of somato-visceral linkage. The virus infected neurons were detected by FITC conjugated with polyclonal swine anti-PrV serum. Labelled neurons were localized in: (1) the trigeminal, geniculate and superior vagal ganglia; (2) the subnucleus caudalis of the spinal trigeminal nucleus; (3) the intermediolateral column (IML) of the thoracolumbar segments and (4) the sympathetic chain ganglia. Present results suggest that when injected into the peripheral nerves, PrV was retrogradely transported to the nerve cell bodies located in the respective sensory ganglia. From the first order sensory neurons, the virus would self-replicate and was transported trans-synaptically via the brainstem nuclei and IML to reach the neurons in the sympathetic ganglia.

Facilitation of memory by peripheral administration of substance P

This report summarizes a recent series of experiments dealing with the effect of peripheral (i.p.) administration of SP on the learning of avoidance and habituation tasks. In summary, the results from these studies show that peripheral post-training SP administration in rats enhances memory in a dose- and time-dependent way. The effect of substance P on retention was observed across tasks with different response requirements and in the absence of explicit punishment. The memory-enhancing effects are long-lasting, until 21 days post-training, and are mediated, at least in part, via interactions with the endogenous opioid system. The mnemotropic effects of peripherally administered SP are sensitive to the functional integrity of the vagus, suggesting that the vagus nerve may be one pathway by which systemic SP influences memory storage processes in the brain. Furthermore, the data indicated that these effects seemed to be encoded by different SP sequences, the N-terminal SP1-7, but not the C-terminal hepta- and hexapeptide sequences being responsible for the memory-promoting effects. Taken together, these studies strongly suggest that SP may be considered to have memory-promoting effects.

Oral inoculation with herpes simplex virus type 1 infects enteric neuron and mucosal nerve fibers within the gastrointestinal tract in mice

Herpes simplex virus type 1 (HSV-1) is commonly encountered first during childhood as an oral infection. After this initial infection resolves, the virus remains in a latent form within innervating sensory ganglia for the life of the host. We have previously shown, using a murine model, that HSV-1 placed within the lumen of the esophagus gains access to nerves within the gut wall and establishes a latent infection in sensory ganglia (nodose ganglia) of the tenth cranial nerve (R. M. Gesser, T. Valyi-Nagy, S. M. Altschuler, and N. W. Fraser, J. Gen. Virol. 75:2379-2386, 1994). Peripheral processes of neurons in these ganglia travel through the vagus nerve and function as primary sensory receptors in most of the gastrointestinal tract, relaying information from the gut wall and mucosal surface to secondary neurons within the brain stem. In the work described here, we further examined the spread of HSV-1 through the enteric nervous system after oral inoculation. By immunohistochemistry, HSV-1 was found to infect myenteric ganglia in Auerbach's plexus between the inner and outer muscle layers of the gut wall, submucosal ganglia (Meisner's plexus), and periglandular ganglion plexuses surrounding submucosal glands. Virus-infected nerve fibers were also seen projecting through the mucosal layer to interact directly with surface epithelial cells. These intramucosal nerve fibers may be a conduit by which intraluminal virus is able to gain access to the enteric nervous system from the gastrointestinal lumen.

Transneuronal transport of intracellularly injected biotinamide in primary afferent axons

Transneuronal transport of biotinamide was observed following intracellular injection of biotinamide into rat jaw-muscle spindle afferent axons. Microelectrodes were advanced into the mesencephalic nucleus of the trigeminal nerve where jaw-muscle spindle afferent axons were identified by their increased firing during stretching of the jaw-elevator muscles. Biotinamide (Neurobiotin) was then injected into individual axons and the animals were maintained under anesthesia for 2-6 h. The animals were then killed via an overdose of anesthetic and the brainstem was processed histochemically. Biotinamide-filled axon collaterals and terminals were readily visible in the trigeminal motor nucleus, the trigeminal sensory nuclei, and adjacent reticular formation. In addition to these intracellularly stained axons, two to five neurons per animal (total of 36 in eight rats) were observed with a homogeneous gray reaction product distributed throughout their somata, proximal, and secondary dendrites. These neurons ranged in size from small (8-20 mu m, n - 26) to medium-sized (<30 mu m, n = 10) and were closely apposed by numerous (up to 20) biotinamide-stained spindle afferent boutons. Most of these neurons (n = 22) were located in the dorsomedial portion of the spinal trigeminal subnucleus interpolaris (Vi) 2.5-4.5 mm caudal to the intra-axonal injection site. Electron microscopic analysis in two rats suggests that the transneuronal biotinamide labeling occurred predominantly through asymmetric, axodendritic synapses between biotinamide-filled axon terminals and Vi neuronal dendrites. Although recent in vitro studies have reported that biotinamide permeates through gap junctions, in this study we found no evidence of biotinamide traversing the gap junctions which exist between trigeminal mesencephalic nucleus (Vme) neuronal somata. These results demonstrate that biotinamide can occasionally be transneuronally transported presumably via synapses; further information is needed to explain the seemingly sporadic nature of this transport.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Transneuronal labeling in hamster brainstem following lingual injections with herpes simplex virus-1

Brainstem projections to hypoglossal motoneurons innervating the intrinsic and extrinsic muscles of the tongue were determined using the transneuronal transfer of Herpes simplex virus-1. Injections of Herpes simplex virus-1 into the intrinsic muscles of the anterior tongue, the geniohyoid and styloglossus muscles each produced specific patterns of label within the hypoglossal nucleus that corresponded closely to the distributions of retrogradely labeled neurons produced by similar injections of horseradish peroxidase. With relatively short survival times, Herpes simplex virus-1 injections further labeled neurons in both the brainstem reticular formation lateral to the hypoglossal nucleus and in the nucleus of the solitary tract. Intrinsic lingual muscles injections of Herpes simplex virus-1 labeled reticular formation neurons distributed laterally along the entire anterior-posterior length of hypoglossal nucleus. In contrast, labeled reticular formation neurons in the immediate vicinity of the hypoglossal nucleus following extrinsic muscles injections, were located lateral to intermediate and anterior levels of hypoglossal nucleus. Thus, despite extensive areas of overlap, there was evidence for a differential distribution of pre-hypoglossal reticular formation neurons along the anterior-posterior axis associated with different lingual injections. Most of the overlap occurred anteriorly, at a level where the nucleus of the solitary tract abuts the fourth ventricle. The potential importance of this area is lingual integrative function was further suggested by camera lucida reconstructions that showed overlapping dendritic fields of labeled neurons in the reticular formation and nucleus of the solitary tract. The dendritic fields of other labeled neurons located more rostral and lateral in the reticular formation sometimes extended into the rostral (gustatory) nucleus of the solitary tract and spinal trigeminal nuclei, suggesting possible multisynaptic pathways through which tactile and gustatory information might influence hypoglossal nucleus. Not all injections of Herpes simplex virus-1 produced label in the hypoglossal nucleus. Some injections into the anterior tongue labeled neurons in the reticular formation near the exiting facial nerve, a region containing populations of preganglionic parasympathetic neurons. Other injections, particularly into the extrinsic lingual muscles, labeled brainstem neurons associated with the sympathetic nervous system, e.g. nuclei raphe magnus and pallidus, the rostral ventrolateral reticular formation, and neurons in the A5 region. These patterns of labeled neurons within the brainstem are suggestive of a differential autonomic innervation of the intrinsic and extrinsic muscles of the tongue.

Specificity of rabies virus as a transneuronal tracer of motor networks: transfer from hypoglossal motoneurons to connected second-order and higher order central nervous system cell groups

The specificity of transneuronal transfer of rabies virus [challenge virus standard (CVS) strain] was evaluated in a well-characterized neuronal network, i.e., retrograde infection of hypoglossal motoneurons and transneuronal transfer to connected (second-order) brainstem neurons. The distribution of the virus in the central nervous system was studied immunohistochemically at sequential intervals after unilateral inoculation into the hypoglossal nerve. The extent of transneuronal transfer of rabies virus was strictly time dependent and was distinguished in five stages. At 1 day postinoculation, labelling involved only hypoglossal motoneurons (stage 1). Retrograde transneuronal transfer occurred from 2.0-2.5 days postinoculation (stage 2). In stages 2-4, different groups of second-order neurons were labelled sequentially, depending on the strength of their input to the hypoglossal nucleus. In stages 4 and 5, labelling extended to several cortical and subcortical cell groups, which can be regarded as higher order because they are known to control tongue movements and/or to provide input to hypoglossal-projecting cell groups. The pattern of transneuronal transfer of rabies virus resembles that of alpha-herpesviruses with regard to the nonsynchronous labelling of different groups of second-order neurons and the transfer to higher order neurons. In striking contrast to alpha-herpesviruses, the transneuronal transfer of rabies is not accompanied by neuronal degeneration. Moreover, local spread of rabies from infected neurons and axons to adjoining glial cells, neurons, or fibers of passage does not occur. The results show that rabies virus is a very efficient transneuronal tracer. Results also provide a new insight into the organization of cortical and subcortical higher order neurons that mediate descending control of tongue movements indirectly via hypoglossal-projecting neurons.

Herpes simplex virus induces Fos expression in rat brainstem neurons

After the injection of Herpes Simplex Virus type 1 into the rat cervical vagus nerve, transneuronally labelled virus-containing neurons and glial cells were present in the medulla oblongata. Fos-containing nuclei were present in the same regions of the brain. A double-labelling procedure revealed that most of the virus-positive neurons also contained Fos-positive nuclei. Appearance of HSV1 antigen within the CNS is associated with Fos expression in neurons and glial cells.

Excitatory amino acid projections to the nucleus of the solitary tract in the rat: a retrograde transport study utilizing D-[3H]aspartate and [3H]GABA

Afferents to the nucleus tractus solitarius utilizing excitatory amino acid transmitters were described in rat brain by autoradiography following microinfusion and retrograde transport of D-[3H]aspartate. Parallel experiments with the injection of [3H]GABA were employed to establish the transmitter-selective nature of the retrograde labelling found with D-[3H]aspartate. Following infusion of D-[3H]aspartate, perikaryal labelling was heaviest in myencephalon, where at least 16 discrete nuclei were labelled. Heaviest labelling was localized bilaterally in the trigeminal nucleus with cells extending through its subdivisions and the entire rostrocaudal axis. Intense labelling was also obtained in the inferior olive, predominantly contralaterally, and non-perikaryal labelling noted. Vestibular, reticular and raphe nuclei contained heavily labelled perikarya. In cervical spinal cord, a moderate density of labelled cells was found in dorsal horn, adjoining the central canal (lamina X) and in the central cervical nucleus, along with appreciable labelling of processes and non-perikaryal labelling. The relative density of labelled perikarya in mesencephalic nuclei was much lower than found in myencephalon, although D-[3H]aspartate produced topographic and precise labelling of a small number of cells in the periaqueductal gray, medial parabrachial nucleus and Koelliker-Fuse nucleus. Only weak labelling was found in cortex and hypothalamus. Labelled cells were not consistently observed in other regions (stria terminalis, amygdala, fastigial nucleus, locus coeruleus and rostral ventrolateral medulla) known to innervate the nucleus tractus solitarius. Lower densities of labelled perikarya were found after the microinjection of [3H]GABA, and the only regions in which a small number of cells were labelled by both D-[3H]aspartate and [3H]GABA were trigeminal nucleus, reticular nuclei and raphe obscurus. An exception was the ventrolateral medulla, where [3H]GABA produced precise labelling in the nucleus ambiguus and facial nucleus consistent with previous evidence for a GABAergic pathway from this area to the nucleus tractus solitarius. Our findings confirm the selectivity of the retrograde transport of D-[3H]aspartate and [3H]GABA. Overall, the transport of D-[3H]aspartate revealed a complex topographic and convergent network of afferent pathways to the nucleus tractus solitarius likely to utilize an excitatory amino acid transmitter.

Contribution of the vagus nerve in mediating the memory-facilitating effects of substance P

The present study determined whether the effects of peripherally administered substance P on memory are mediated via activation of the vagus nerve. Rats were submitted to subdiaphragmatic vagotomy, sham vagotomy or non-operated, and trained in a step-down inhibitory avoidance task and tested 24 h later. Posttraining administration of 50 micrograms/kg of SP facilitated retention performance in non-operated and sham-operated groups. The facilitating effects of 50 micrograms/kg of SP was blocked by vagotomy, although vagotomy did not attenuate the memory-enhancing effects of larger doses (250 and 500 micrograms/kg). These results suggest that the mnemotropic effects of peripherally administered SP are sensitive to the functional integrity of the vagus nerve. Alternatively, the vagus nerve may be one pathway but not the only pathway by which systemic SP influences the memory storage processes in the brain.

Direct and indirect retinohypothalamic projections to the supraoptic nucleus in the female albino rat

Earlier studies have shown that retinohypothalamic projections terminate extensively within the hypothalamus of the rat. Recently, we identified a light retinal projection to the supraoptic nucleus as well as a larger, well-focused projection resulting in a peri-supraoptic nucleus terminal field. In this study, we employed a double labeling method with cholera toxin conjugated to horseradish peroxidase (CT-HRP) and pseudorabies virus, a transsynaptic neural tracer, to evaluate retinorecipient neurons in both the supraoptic nucleus and peri-supraoptic nucleus terminal field. In addition, we looked for evidence that cells in the peri-supraoptic nucleus terminal field project into the supraoptic nucleus. Three strains of pseudorabies virus were compared. A direct retinosupraoptic nucleus circuit was confirmed with all three strains. Retinorecipient neurons in the peri-supraoptic nucleus terminal field were also confirmed. However, there was a strain-based difference in the identification of these neurons. The wild-type Becker strain labeled cells in the peri-supraoptic nucleus terminal field in a manner paralleling the early, intermediate and late stages of infection of the suprachiasmatic nucleus. This parallel time course suggests that retinal ganglion cells terminate directly on cells in the peri-supraoptic nucleus terminal field. Conversely, the Bartha and PRV-91 strains showed appreciable labeling of peri-supraoptic neurons only at long survival times. This longer time course suggests that these mutant strains label neurons in the peri-supraoptic nucleus terminal field indirectly, after passing through additional neurons. In addition, experiments with monocular injection of CT-HRP and posterior pituitary injection of pseudorabies virus showed retrogradely labeled second-order cells in the peri-supraoptic nucleus amidst the CT-HRP labeled terminal field of the retinohypothalamic tract. These results demonstrate a direct projection from the retina to the supraoptic nucleus and provide evidence for an indirect circuit from the retina to the supraoptic nucleus via neurons located in the peri-supraoptic nucleus terminal field. The strain-based differences imply that those retinal ganglion cells that project to the peri-supraoptic nucleus terminal field differ from those that project to the suprachiasmatic nucleus. In addition, these results suggest a neuroanatomic basis for photic effects on physiological mechanisms that are not mediated by the circadian timing system.