Targeting specific neuronal populations using adeno- and lentiviral vectors: applications for imaging and studies of cell function

Central Autonomic Network Regions and Hypertension: Unveiling Sympathetic Activation and Genetic Therapeutic Perspectives

INTRODUCTION

Hypertension, a leading cause of death, was investigated in this study to understand the role of specific brain regions in regulating blood pressure. The lateral parabrachial nucleus (LPBN), Kolliker-fuse nucleus (KF), and periductal grey matter (PAG) were examined for their involvement in hypertension.

METHODS

Lentiviral vectors were used to alter the activity of these brain regions in hypertensive rats. Over a 75-day period, blood pressure, heart rate, reflex responses, and heart rate variability were measured.

RESULTS

Decreasing the activity in the LPBN resulted in a reduced sympathetic outflow, lowering the blood pressure and heart rate. In the KF, the sympathetic activity decreased and chemoreflex variation was attenuated, without affecting the blood pressure. Silencing the PAG had no significant impact on blood pressure or sympathetic tone, but decreased cardiac baroreflex gain.

DISCUSSION

These findings highlight the significant role of the LPBN in hypertension-related sympathetic activation. Additionally, LPBN and KF neurons appear to activate mechanisms that control respiration and sympathetic outflow during chemoreceptor activation.

CONCLUSIONS

The study provided insights into the contribution of the midbrain and pontine regions to neurogenic hypertension and offers potential avenues for future genetic interventions and developing novel treatment approaches.

Current and potential new treatment strategies for creatine deficiency syndromes

Creatine deficiency syndromes (CDS) are inherited metabolic disorders caused by mutations in GATM, GAMT and SLC6A8 and mainly affect central nervous system (CNS). AGAT- and GAMT-deficient patients lack the functional brain endogenous creatine (Cr) synthesis pathway but express the Cr transporter SLC6A8 at blood-brain barrier (BBB), and can thus be treated by oral supplementation of high doses of Cr. For Cr transporter deficiency (SLC6A8 deficiency or CTD), current treatment strategies benefit one-third of patients. However, as their phenotype is not completely reversed, and for the other two-thirds of CTD patients, the development of novel more effective therapies is needed. This article aims to review the current knowledge on Cr metabolism and CDS clinical aspects, highlighting their current treatment possibilities and the most recent research perspectives on CDS potential therapeutics designed, in particular, to bring new options for the treatment of CTD.

Novel therapeutic interventions for combating Parkinson's disease and prospects of Nose-to-Brain drug delivery

Parkinson disease (PD) is a progressive neurodegenerative disorder prevalent mainly in geriatric population. While, L-DOPA remains one of the major choices for the therapeutic management of PD, various motor and non-motor manifestations complicate the management of PD. In the last two decades, exhaustive research has been carried out to explore novel therapeutic approaches for mitigating motor and non-motor symptoms of PD. These approaches majorly include receptor-based, anti-inflammatory, stem-cell and nucleic acid based. The major limitations of existing therapeutic interventions (of commonly oral route) are low efficacy due to low brain bioavailability and associated side effects. Nanotechnology has been exploited and has gained wide attention in the recent years as an approach for enhancement of bioavailability of various small molecule drugs in the brain. To address the challenges associated with PD therapy, nose-to-brain delivery utilizing nanomedicine-based approaches has been found to be encouraging in published evidence. Therefore, the present work summarises the major challenges and limitations with antiparkinsonian drugs, novel therapeutic interventions, and scope of nanomedicine-based nose-to-brain delivery in addressing the current challenges of antiparkinsonian therapy. The manuscript tries to sensitize the researchers for designing brain-targeted nanomedicine loaded with natural/synthetic scaffolds, biosimilars, and nucleic acids that can bypass the first-pass effect for the effective management of PD.

Gene Therapy Approach with an Emphasis on Growth Factors: Theoretical and Clinical Outcomes in Neurodegenerative Diseases

The etiology of many neurological diseases affecting the central nervous system (CNS) is unknown and still needs more effective and specific therapeutic approaches. Gene therapy has a promising future in treating neurodegenerative disorders by correcting the genetic defects or by therapeutic protein delivery and is now an attraction for neurologists to treat brain disorders, like Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, spinal muscular atrophy, spinocerebellar ataxia, epilepsy, Huntington’s disease, stroke, and spinal cord injury. Gene therapy allows the transgene induction, with a unique expression in cells’ substrate. This article mainly focuses on the delivering modes of genetic materials in the CNS, which includes viral and non-viral vectors and their application in gene therapy. Despite the many clinical trials conducted so far, data have shown disappointing outcomes. The efforts done to improve outcomes, efficacy, and safety in the identification of targets in various neurological disorders are also discussed here. Adapting gene therapy as a new therapeutic approach for treating neurological disorders seems to be promising, with early detection and delivery of therapy before the neuron is lost, helping a lot the development of new therapeutic options to translate to the clinic.

Efficient infection of organotypic hippocampal slice cultures with adenovirus carrying the transgene REST/NRSF

Organotypic hippocampal slice cultures provide a useful platform maintaining hippocampal structure and synaptic connections of the brain over weeks in culture with ease of in vitro manipulations. Gene transfer is a particularly desirable tool for using with them but current difficulties with transformation of transgenes into these cultures is a barrier to their use in research. Previous quantifications of viral infections have shown low transformation rates and have relied upon invasive microinjections. In this paper we present an efficient way of infecting organotypic cultures with adenovirus at the acute slice stage that does not require injection. We use the adenoviral delivery system to introduce the transcription factor REST and a GFP marker, providing around 41 % cellular infection spread throughout the entire slice culture and promoting transgene expression for weeks in vitro. GFP expression was observed most intensely in the slices when they were infected just a few hours after plating and was shown to infect neurons and microglia. We decided to use the transcription factor REST/NRSF as an example transgene which was delivered into cells via the adenoviral construct, conferring overexpression of REST in addition to the GFP marker. This outlines a technique whereby adenoviral infection of organotypic cultures can infect neurons with good efficiency and confer successful manipulation of genetic factors within the cell.

Long-term stimulation of cardiac vagal preganglionic neurons reduces blood pressure in the spontaneously hypertensive rat

BACKGROUND

Arterial hypertension is associated with autonomic nervous system dysfunction. Different interventional strategies have been implemented in recent years for the reduction of sympathetic activity in patients with hypertension. However, the therapeutic benefit of increasing vagal tone in hypertensive patients remains largely unexplored.

OBJECTIVE

Here, we describe the effects of long-term activation of vagal neural pathways on arterial pressure, heart rate arterial pressure variability and spontaneous baroreflex sensitivity in spontaneously hypertensive rats (SHR) and normotensive Wistar rats.

METHODS

Brainstem vagal preganglionic neurons residing in the dorsal vagal motor nucleus (DVMN) were targeted with a lentiviral vector to induce the expression of an artificial G(s) protein-coupled receptor termed designer receptors exclusively activated by designer drugs (DREADD-Gs). The transduced neurons were activated daily by systemic administration of otherwise inert ligand clozapine-n-oxide. Arterial pressure measurements were recorded in conscious freely moving animals after 21 consecutive days of DVMN stimulation.

RESULTS

Resting arterial pressure was significantly lower in SHRs expressing DREADD-Gs in the DVMN, compared with control SHRs expressing enhanced green fluorescent protein. No changes in arterial pressure were detected in Wistar rats expressing DREADD-Gs compared with rats expressing enhanced green fluorescent protein in the DVMN. Pharmacogenetic activation of DREADD-Gs-expressing DVMN neurons in SHRs was accompanied with increased baroreflex sensitivity and a paradoxical decrease in cardio-vagal components of heart rate and systolic arterial pressure variability in SHRs.

CONCLUSION

These results suggest that long-term activation of vagal parasympathetic pathways is beneficial in restoring autonomic balance in an animal model of neurogenic hypertension and might be an effective therapeutic approach for the management of hypertension.

Gene Therapy Tools for Brain Diseases

Neurological disorders affecting the central nervous system (CNS) are still incompletely understood. Many of these disorders lack a cure and are seeking more specific and effective treatments. In fact, in spite of advancements in knowledge of the CNS function, the treatment of neurological disorders with modern medical and surgical approaches remains difficult for many reasons, such as the complexity of the CNS, the limited regenerative capacity of the tissue, and the difficulty in conveying conventional drugs to the organ due to the blood-brain barrier. Gene therapy, allowing the delivery of genetic materials that encodes potential therapeutic molecules, represents an attractive option. Gene therapy can result in a stable or inducible expression of transgene(s), and can allow a nearly specific expression in target cells. In this review, we will discuss the most commonly used tools for the delivery of genetic material in the CNS, including viral and non-viral vectors; their main applications; their advantages and disadvantages. We will discuss mechanisms of genetic regulation through cell-specific and inducible promoters, which allow to express gene products only in specific cells and to control their transcriptional activation. In addition, we will describe the applications to CNS diseases of post-transcriptional regulation systems (RNA interference); of systems allowing spatial or temporal control of expression [optogenetics and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)]; and of gene editing technologies (CRISPR/Cas9, Zinc finger proteins). Particular attention will be reserved to viral vectors derived from herpes simplex type 1, a potential tool for the delivery and expression of multiple transgene cassettes simultaneously.

Putative Receptors Underpinning l-Lactate Signalling in Locus Coeruleus

The importance of astrocytic l-lactate (LL) for normal functioning of neural circuits such as those regulating learning/memory, sleep/wake state, autonomic homeostasis, or emotional behaviour is being increasingly recognised. l-Lactate can act on neurones as a metabolic or redox substrate, but transmembrane receptor targets are also emerging. A comparative review of the hydroxy-carboxylic acid receptor (HCA1, formerly known as GPR81), Olfactory Receptor Family 51 Subfamily E Member 2 (OR51E2), and orphan receptor GPR4 highlights differences in their LL sensitivity, pharmacology, intracellular coupling, and localisation in the brain. In addition, a putative Gs-coupled receptor on noradrenergic neurones, LLRx, which we previously postulated, remains to be identified. Next-generation sequencing revealed several orphan receptors expressed in locus coeruleus neurones. Screening of a selection of these suggests additional LL-sensitive receptors: GPR180 which inhibits and GPR137 which activates intracellular cyclic AMP signalling in response to LL in a heterologous expression system. To further characterise binding of LL at LLRx, we carried out a structure–activity relationship study which demonstrates that carboxyl and 2-hydroxyl moieties of LL are essential for triggering d-lactate-sensitive noradrenaline release in locus coeruleus, and that the size of the LL binding pocket is limited towards the methyl group position. The evidence accumulating to date suggests that LL acts via multiple receptor targets to modulate distinct brain functions.

Emerging role of viral vectors for circuit-specific gene interrogation and manipulation in rodent brain

Over the past half century, novel tools have allowed the characterization of myriad molecular underpinnings of neural phenomena including synaptic function, neurogenesis and neurodegeneration, membrane excitability, and neurogenetics/epigenetics. More recently, transgenic mice have made possible cell type-specific explorations of these phenomena and have provided critical models of many neurological and psychiatric diseases. However, it has become clear that many critical areas of study require tools allowing the study and manipulation of individual neural circuits within the brain, and viral vectors have come to the forefront in driving these circuit-specific studies. Here, we present a surface-level review of the general classes of viral vectors used for study of the brain, along with their suitability for circuit-specific studies. We then cover in detail a new long-lasting, retrograde expressing form of herpes simplex virus termed LT-HSV that has become highly useful in circuit-based studies. We detail some of its current uses and propose a variety of future uses for this critical new tool, including circuit-based transgene overexpression, gene editing, and gene expression profiling.

Advances in optogenetic and chemogenetic methods to study brain circuits in non-human primates

Over the last 10 years, the use of opto- and chemogenetics to modulate neuronal activity in research applications has increased exponentially. Both techniques involve the genetic delivery of artificial proteins (opsins or engineered receptors) that are expressed on a selective population of neurons. The firing of these neurons can then be manipulated using light sources (for opsins) or by systemic administration of exogenous compounds (for chemogenetic receptors). Opto- and chemogenetic tools have enabled many important advances in basal ganglia research in rodent models, yet these techniques have faced a slow progress in non-human primate (NHP) research. In this review, we present a summary of the current state of these techniques in NHP research and outline some of the main challenges associated with the use of these genetic-based approaches in monkeys. We also explore cutting-edge developments that will facilitate the use of opto- and chemogenetics in NHPs, and help advance our understanding of basal ganglia circuits in normal and pathological conditions.

An optimized method for high-titer lentivirus preparations without ultracentrifugation

Lentiviral technology has proven to be a powerful tool to express exogenous genes in dividing and non-dividing cells. Currently, most protocols for generating high-titer lentivirus require ultracentrifugation, which can be an instrumental barrier for routine operations in a laboratory. In this study, the effect of relative centrifugal force (RCF) on the concentration efficiency of the lentivirus was systematically explored, and it was found that sucrose gradient centrifugation with a relatively low speed (≤10,000 g) robustly produces a high-titer virus (up to 2 × 10⁸ TU/ml). The optimal sucrose concentration is 10%, and the recovery rate of the functional virus is greater than 80%. The infection efficiency of both concentrated and un-concentrated lentivirus decreases rapidly when the viruses are stored at 4 °C (τ ≈ 1.3 days) or subjected to multiple freeze-thaw cycles (τ = 1.1 rounds). In summary, we describe an efficient and easy-to-handle protocol for high-titer lentivirus purification.

Fast targeted gene transfection and optogenetic modification of single neurons using femtosecond laser irradiation

A prevailing problem in neuroscience is the fast and targeted delivery of DNA into selected neurons. The development of an appropriate methodology would enable the transfection of multiple genes into the same cell or different genes into different neighboring cells as well as rapid cell selective functionalization of neurons. Here, we show that optimized femtosecond optical transfection fulfills these requirements. We also demonstrate successful optical transfection of channelrhodopsin-2 in single selected neurons. We extend the functionality of this technique for wider uptake by neuroscientists by using fast three-dimensional laser beam steering enabling an image-guided "point-and-transfect" user-friendly transfection of selected cells. A sub-second transfection timescale per cell makes this method more rapid by at least two orders of magnitude when compared to alternative single-cell transfection techniques. This novel technology provides the ability to carry out large-scale cell selective genetic studies on neuronal ensembles and perform rapid genetic programming of neural circuits.

Pharmacosynthetics: Reimagining the pharmacogenetic approach

Pharmacology, in its broadest interpretation, is defined as the study of the interaction between physiological entities and drugs. In modern neuropsychopharmacology, this interaction is viewed as the drug itself on one side and signal transducer (receptor), the signal transduction cascade (effector proteins, second messengers), the cellular response (transcriptional regulation, activity modulation), the organ response (brain circuitry modulation), and, finally, the whole organism response (behavior) on the other. In other words, pharmacology has structured itself around the idea that the exogenous molecule (the drug) encodes a "signal" leading to everything on the other side including, in extreme renditions, a physiological response. The inference is that engaging a particular signal transduction pathway in a defined cell type leads inexorably to a prototypic physiological response. Thus, for instance, serotonergic activation of 5-HT(2A) receptors in rat aortic smooth muscle cells leads to an increase in intracellular Ca(++) (via IP₃ release) and smooth muscle contraction (Roth et al., 1986). Here, we suggest that the invention of synthetic ligand--GPCR pairs (aka DREADDs, RASSLS, 'pharmacogenetics') permits the study of pharmacology using a shifted equation: more of the signal transduction elements moved to the left and, subsequently, under experimental control. For the purposes of disambiguation and to clarify this new interpretation as a creation of pharmacological manipulation, we present the term pharmacosynthetics to describe what has heretofore been called pharmacogenetics or chemicogenetics. This review discusses this new interpretation and reviews recent applications of the technology and considerations of the approach. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Efferent projections of C3 adrenergic neurons in the rat central nervous system

C3 neurons constitute one of three known adrenergic nuclei in the rat central nervous system (CNS). While the adrenergic C1 cell group has been extensively characterized both physiologically and anatomically, the C3 nucleus has remained relatively obscure. This study employed a lentiviral tracing technique that expresses green fluorescent protein behind a promoter selective to noradrenergic and adrenergic neurons. Microinjection of this virus into the C3 nucleus enabled the selective tracing of C3 efferents throughout the rat CNS, thus revealing the anatomical framework of C3 projections. C3 terminal fields were observed in over 40 different CNS nuclei, spanning all levels of the spinal cord, as well as various medullary, mesencephalic, hypothalamic, thalamic, and telencephalic nuclei. The highest densities of C3 axon varicosities were observed in Lamina X and the intermediolateral cell column of the thoracic spinal cord, as well as the dorsomedial medulla (both commissural and medial nuclei of the solitary tract, area postrema, and the dorsal motor nucleus of the vagus), ventrolateral periaqueductal gray, dorsal parabrachial nucleus, periventricular and rhomboid nuclei of the thalamus, and paraventricular and periventricular nuclei of the hypothalamus. In addition, moderate and sparse projections were observed in many catecholaminergic and serotonergic nuclei, as well as the area anterior and ventral to the third ventricle, Lamina X of the cervical, lumbar, and sacral spinal cord, and various hypothalamic and telencephalic nuclei. The anatomical map of C3 projections detailed in this survey hopes to lay the first steps toward developing a functional framework for this nucleus.

Cardioprotection evoked by remote ischaemic preconditioning is critically dependent on the activity of vagal pre-ganglionic neurones

Aims

Innate mechanisms of inter-organ protection underlie the phenomenon of remote ischaemic preconditioning (RPc) in which episode(s) of ischaemia and reperfusion in tissues remote from the heart reduce myocardial ischaemia/reperfusion injury. The uncertainty surrounding the mechanism(s) underlying RPc centres on whether humoral factor(s) produced during ischaemia/reperfusion of remote tissue and released into the systemic circulation mediate RPc, or whether a neural signal is required. While these two hypotheses may not be incompatible, one approach to clarify the potential role of a neural pathway requires targeted disruption or activation of discrete central nervous substrate(s).

Methods and results

Using a rat model of myocardial ischaemia/reperfusion injury in combination with viral gene transfer, pharmaco-, and optogenetics, we tested the hypothesis that RPc cardioprotection depends on the activity of vagal pre-ganglionic neurones and consequently an intact parasympathetic drive. For cell-specific silencing or activation, neurones of the brainstem dorsal motor nucleus of the vagus nerve (DVMN) were targeted using viral vectors to express a Drosophila allatostatin receptor (AlstR) or light-sensitive fast channelrhodopsin variant (ChIEF), respectively. RPc cardioprotection, elicited by ischaemia/reperfusion of the limbs, was abolished when DVMN neurones transduced to express AlstR were silenced by selective ligand allatostatin or in conditions of systemic muscarinic receptor blockade with atropine. In the absence of remote ischaemia/reperfusion, optogenetic activation of DVMN neurones transduced to express ChIEF reduced infarct size, mimicking the effect of RPc.

Conclusion

These data indicate a crucial dependence of RPc cardioprotection against ischaemia/reperfusion injury upon the activity of a distinct population of vagal pre-ganglionic neurones.

High-throughput single-cell manipulation in brain tissue

The complexity of neurons and neuronal circuits in brain tissue requires the genetic manipulation, labeling, and tracking of single cells. However, current methods for manipulating cells in brain tissue are limited to either bulk techniques, lacking single-cell accuracy, or manual methods that provide single-cell accuracy but at significantly lower throughputs and repeatability. Here, we demonstrate high-throughput, efficient, reliable, and combinatorial delivery of multiple genetic vectors and reagents into targeted cells within the same tissue sample with single-cell accuracy. Our system automatically loads nanoliter-scale volumes of reagents into a micropipette from multiwell plates, targets and transfects single cells in brain tissues using a robust electroporation technique, and finally preps the micropipette by automated cleaning for repeating the transfection cycle. We demonstrate multi-colored labeling of adjacent cells, both in organotypic and acute slices, and transfection of plasmids encoding different protein isoforms into neurons within the same brain tissue for analysis of their effects on linear dendritic spine density. Our platform could also be used to rapidly deliver, both ex vivo and in vivo, a variety of genetic vectors, including optogenetic and cell-type specific agents, as well as fast-acting reagents such as labeling dyes, calcium sensors, and voltage sensors to manipulate and track neuronal circuit activity at single-cell resolution.

Imaging microcircuit function in healthy and diseased brain

Neural microcircuits are the computational units of the mammalian brain. Recent evidence suggests that they are not composed exclusively of neurons but also involve other cell types such as astrocytes and microglia. In the healthy brain microglia, the resident immune cell, closely interacts with synapses and is likely to be involved in their structural plasticity. The interaction between the nervous and the immune systems is even more prominent under pathological conditions such as brain injury, inflammation and neurodegenerative diseases. This review discusses the techniques for high resolution imaging of microcircuit function in health and disease by focusing on some of the most recent advances in the field of in vivo calcium imaging of neurons, astrocytes and microglia.

A comprehensive concept of optogenetics

Fundamental questions that neuroscientists have previously approached with classical biochemical and electrophysiological techniques can now be addressed using optogenetics. The term optogenetics reflects the key program of this emerging field, namely, combining optical and genetic techniques. With the already impressively successful application of light-driven actuator proteins such as microbial opsins to interact with intact neural circuits, optogenetics rose to a key technology over the past few years. While spearheaded by tools to control membrane voltage, the more general concept of optogenetics includes the use of a variety of genetically encoded probes for physiological parameters ranging from membrane voltage and calcium concentration to metabolism. Here, we provide a comprehensive overview of the state of the art in this rapidly growing discipline and attempt to sketch some of its future prospects and challenges.

Transfection of osteoclasts and osteoclast precursors

Osteoclasts and their precursors have traditionally been considered difficult cells to transfect using standard approaches. Here, we describe several methods for transfection of mature osteoclasts and their precursors using the Amaxa™ Nucleofector system, lentiviruses, and adenoviruses.

Preferential and bidirectional labeling of the rubrospinal tract with adenovirus-GFP for monitoring normal and injured axons

The rodent rubrospinal tract (RST) has been studied extensively to investigate regeneration and remodeling of central nervous system (CNS) axons. Currently no retrograde tracers can specifically label rubrospinal axons and neurons (RSNs). The RST can be anterogradely labeled by injecting tracers into the red nucleus (RN), but accurately locating the RN is a technical challenge. Here we developed a recombinant adenovirus carrying a green fluorescent protein reporter gene (Adv-GFP) which can preferentially, intensely, and bi-directionally label the RST. When Adv-GFP was injected into the second lumbar spinal cord, the GFP was specifically transported throughout the entire RST, with peak labeling seen at 2 weeks post-injection. When Adv-GFP was injected directly into the RN, GFP was anterogradely transported throughout the RST. Following spinal cord injury (SCI), injection of Adv-GFP resulted in visualization of GFP in transected, spared, or sprouted RST axons bi-directionally. Thus Adv-GFP could be used as a novel tool for monitoring and evaluating strategies designed to maximize RST axonal regeneration and remodeling following SCI.

Epileptogenic ion channel mutations: from bedside to bench and, hopefully, back again

Mutations of genes coding for ion channels cause several genetically determined human epileptic syndromes. The identification of a gene variant linked to a particular disease gives important information, but it is usually necessary to perform functional studies in order to completely disclose the pathogenic mechanisms. The functional consequences of epileptogenic mutations have been studied both in vitro and in vivo with several experimental systems, studies that have provided significant knowledge on the pathogenic mechanisms that leads to inherited human epilepsies, and possibly also on the pathogenic mechanisms of non-genetic human epilepsies due to "acquired channelopathies". However, several open issues remain and difficulties in the interpretation of the experimental data have arisen that limit translational applications. We will highlight the value and the limits of different approaches to the study of epileptogenic channelopathies, focussing on the importance of the experimental systems in the assessment of the functional effects of the mutations and on the possible applications of the obtained results to the clinical practice.

Astroglia are a possible cellular substrate of angiotensin(1-7) effects in the rostral ventrolateral medulla

AIMS

Angiotensin(1-7) (Ang1-7) acting at the level of the rostral ventrolateral medulla (RVLM) affects arterial pressure. The cellular substrate of Ang1-7 remains unknown. We sought to determine which cell types in RVLM could mediate its actions and whether these are altered in the spontaneously hypertensive rat (SHR).

METHODS AND RESULTS

Astrocytes, catecholaminergic (CA-ergic) and non-CA-ergic neurones were targeted with adenoviral vectors in organotypic slice cultures from Wistar rats and SHR. Astrocytic Ca(2+) signalling was monitored using a genetically engineered Ca(2+) sensor Case12. CA-ergic neurones expressed enhanced green fluorescent protein (EGFP) under control of the PRS x 8 promoter, whereas non-CA-neurones expressed EGFP under control of the synapsin-1 promoter. Neurones were recorded in whole cell mode while [Ca(2+)](i) was monitored using Rhod-2. RVLM astrocytes responded to Ang1-7 (200-1000 nM) with concentration-dependent [Ca(2+)](i) elevation. In SHR, the response to 1000 nM was significantly attenuated. The competitive Ang1-7 receptor antagonist A779, but not the AT(1) receptor blocker (losartan), suppressed Ang1-7-induced [Ca(2+)](i) elevations, which were also antagonized by blocking intracellular Ca(2+) stores. Ang1-7 evoked no consistent changes in [Ca(2+)](i) or membrane excitability in CA-ergic or non-CA-ergic neurones in either rat strain.

CONCLUSION

Astroglia are a plausible cellular target of Ang1-7 in RVLM. Our data suggest that astrocytic responsiveness to Ang1-7 is reduced in SHR. We hypothesise that Ang1-7 modulates astrocytic signalling which in vivo may affect local metabolism and microcirculation, resulting in changes in activity of RVLM pre-sympathetic neurones and hence blood pressure.

The use of viral gene transfer in studies of brainstem noradrenergic and serotonergic neurons

In contrast to some other neuronal populations, for example hippocampal or cortical pyramidal neurons, mechanisms of synaptic integration and transmitter release in central neurons that contain noradrenaline (NA) and serotonin (5HT) are not well understood. These cells, crucial for a wide range of autonomic and behavioural processes, have long un-myelinated axons with hundreds of varicosities where transmitters are synthesized and released. Both seem to signal mostly in 'volume transmission' mode. Very little is known about the rules that apply to this type of transmission in the brain and the factors that regulate the release of NA and 5HT. We discuss some of our published studies and more recent experiments in which viral vectors were used to investigate the physiology of these neuronal populations. We also focus on currently unresolved issues concerning the mechanism of volume transmission by NA and 5HT in the brain. We suggest that clarifying the role of astroglia in this process could be essential for our understanding of central noradrenergic and 5HT signalling.

Viral vector approaches to modify gene expression in the brain

The use of viral vectors as gene transfer tools for the central nervous system has seen a significant growth in the last decade. Improvements in the safety, efficiency and specificity of vectors for clinical applications have proven to be beneficial also for basic neuroscience research. This review will discuss the viral systems currently available to neuroscientists and some of the recent achievements in the study of synaptic function, memory and drug addiction.

Adenoviral vectors for highly selective gene expression in central serotonergic neurons reveal quantal characteristics of serotonin release in the rat brain

Background

5-hydroxytryptamine (5 HT, serotonin) is one of the key neuromodulators in mammalian brain, but many fundamental properties of serotonergic neurones and 5 HT release remain unknown. The objective of this study was to generate an adenoviral vector system for selective targeting of serotonergic neurones and apply it to study quantal characteristics of 5 HT release in the rat brain.

Results

We have generated adenoviral vectors which incorporate a 3.6 kb fragment of the rat tryptophan hydroxylase-2 (TPH-2) gene which selectively (97% co-localisation with TPH-2) target raphe serotonergic neurones. In order to enhance the level of expression a two-step transcriptional amplification strategy was employed. This allowed direct visualization of serotonergic neurones by EGFP fluorescence. Using these vectors we have performed initial characterization of EGFP-expressing serotonergic neurones in rat organotypic brain slice cultures. Fluorescent serotonergic neurones were identified and studied using patch clamp and confocal Ca²⁺ imaging and had features consistent with those previously reported using post-hoc identification approaches. Fine processes of serotonergic neurones could also be visualized in un-fixed tissue and morphometric analysis suggested two putative types of axonal varicosities. We used micro-amperometry to analyse the quantal characteristics of 5 HT release and found that central 5 HT exocytosis occurs predominantly in quanta of ~28000 molecules from varicosities and ~34000 molecules from cell bodies. In addition, in somata, we observed a minority of large release events discharging on average ~800000 molecules.

Conclusion

For the first time quantal release of 5 HT from somato-dendritic compartments and axonal varicosities in mammalian brain has been demonstrated directly and characterised. Release from somato-dendritic and axonal compartments might have different physiological functions. Novel vectors generated in this study open a host of new experimental opportunities and will greatly facilitate further studies of the central serotonergic system.

Area-specific differences in transmitter release in central catecholaminergic neurons of spontaneously hypertensive rats

The link among blood pressure, sympathetic output, and brain neurons producing catecholamines is well documented. Nevertheless, their intrinsic properties and any alterations in signaling characteristics between normotensive and hypertensive phenotypes remain unknown. Here, we directly compared neurophysiological properties of catecholamine release of C1 and A2 neurons of the spontaneously hypertensive rat and Wistar rat in organotypic slices. C1 and A2 areas were studied because both are widely implicated in the pathophysiology of hypertension. Catecholaminergic neurons were visualized using viral vectors to express green fluorescent protein. Microamperometry revealed that C1 axonal varicosities of spontaneously hypertensive but not normotensive Wistar rats release a transmitter predominantly (approximately 86%) in very large quanta, comparable in catecholamine load to adrenal chromaffin granules. Because quantal size affects the spread of transmitter in the extracellular space, this may enhance the impact of C1 varicosities on their downstream targets and increase sympathetic drive in the hypertensive rat. Electrophysiological properties and Ca2+ handling were studied using patch clamp and confocal imaging. Although overall electrophysiological characteristics of C1 and A2 neurons were comparable between strains, the characteristic angiotensin-II-induced Ca2+ mobilization was reduced in A2 neurons of the spontaneously hypertensive rat. Because A2 neurons are a part of a homeostatic antihypertensive circuit, this could reduce their restraining influence on blood pressure. Thus, we have revealed an increased quantal size in C1 varicosities and a reduced responsiveness of A2 neurons of the spontaneously hypertensive rat to angiotensin II. Both effects could contribute to elevated sympathetic activity and blood pressure in the spontaneously hypertensive rat.

Transduction of brain by herpes simplex virus vectors

An imposing obstacle to gene therapy is the inability to transduce all of the necessary cells in a target organ. This certainly applies to gene transfer to the brain, especially when one considers the challenges involved in scaling up transduction from animal models to use in the clinic. Non-neurotropic viral gene transfer vectors (e.g., adenovirus, adeno-associated virus, and lentivirus) do not spread very far in the nervous system, and consequently these vectors transduce brain regions mostly near the injection site in adult animals. This indicates that numerous, well-spaced injections would be required to achieve widespread transduction in a large brain with these vectors. In contrast, herpes simplex virus type 1 (HSV-1) is a promising vector for widespread gene transfer to the brain owing to the innate ability of the virus to spread through the nervous system and form latent infections in neurons that last for the lifetime of the infected individual. In this review, we summarize the published literature of the transduction patterns produced by attenuated HSV-1 vectors in small animals as a function of the injection site, and discuss the implications of the distribution for widespread gene transfer to the large animal brain.

Exocytosis of norepinephrine at axon varicosities and neuronal cell bodies in the rat brain

Norepinephrine secretion from central neurons was widely assumed to occur by exocytosis, but the essential characteristics of this process remained unknown. We developed an approach to study it directly by amperometry using carbon fiber microelectrodes in organotypic rat brainstem slice cultures. Noradrenergic neurons from areas A1 and A2 were fluorescently labeled by an adenoviral vector with noradrenergic-specific promoter. Quantal events, consistent with exocytotic release of norepinephrine, were registered at noradrenergic axonal varicosities as well as at cell bodies. According to their charge integrals, events were grouped into two populations. The majority (approximately 40 fC) were compatible with full exocytotic fusion of small clear and dense core vesicles shown in previous morphometric studies. The quantal size distribution was modulated by treatment with reserpine and amitriptyline. In addition, much larger quantal events (>1 pC) occurred at predominantly axonal release sites. The time course of signals was severalfold faster than in adrenal chromaffin cells, suggesting profound differences in the release machinery between these cell types. Tetrodotoxin eliminated the majority of events, indicating that release was partially, but not entirely, action potential driven. In conclusion, central norepinephrine release has unique characteristics, distinguishing it from those of other monoaminergic cells in periphery and brain.

Limitations of cellular models in Parkinson's disease research

Cell cultures for Parkinson's disease research have the advantage of virtually unlimited access, they allow rapid screening for disease pathogenesis and drug candidates, and they restrict the necessary number of animal experiments. Limitations of cell cultures, include that the survival of neurons is dependent upon the culture conditions; that the cells do not develop their natural neuronal networks. In most cases, neurons are deprived from the physiological afferent and efferent connections. In Parkinson's disease research, mesencephalic slice cultures, primary immature dopaminergic neurons and immortalized cell lines--either in a proliferating state or in a differentiated state--are used. Neuronal cultures may be plated in the presence or absence of glial cells and serum. These different culture conditions as well as the selection of outcome parameters (morphological evaluation, viability assays, biochemical assays, metabolic assays) have a strong influence on the results of the experiments and the conclusions drawn from them. A primary example is the question of whether L-Dopa is toxic to dopaminergic neurons or whether it provides neurotrophic effects: In pure, neuronal-like cultures, L-Dopa provides toxicity, whereas in the presence of glial cells, it provides trophic effects when applied. The multitude of factors that influence the data generated from cell culture experiments indicates that in order to obtain clear-cut and unambiguous results, investigators need to choose their model carefully and are encouraged to verify their main results with different models.

Mechanism of nitric oxide action on inhibitory GABAergic signaling within the nucleus tractus solitarii

The cellular mechanisms mediating nitric oxide (NO) modulation of the inhibitory transmission in the nucleus tractus solitarii (NTS) remain unclear, even though this could be extremely important for various physiological and pathological processes. Specifically, in the NTS NO-evoked glutamate and gamma-aminobutyric acid (GABA) release might contribute to pathological hypertension. In cultured rat brainstem slices, NTS GABAergic neurons were targeted using an adenoviral vector to express enhanced green fluorescent protein and studied with a combination of patch clamp and confocal microscopy. Low nanomolar concentrations of NO increased intracellular Ca2+ concentration ([Ca2+]i) in somata, dendrites, and putative axons of GABAergic neurons, with axons being the most sensitive compartment. This effect was cGMP mediated and not related to depolarization or indirect presynaptic effects on glutamatergic transmission. Blockade of the cyclic adenosine diphosphate ribose (cADPR)/ryanodine-sensitive stores but not the inositol triphosphate-sensitive stores, inhibited NO effect. Since cADPR/ryanodine-sensitive stores are implicated in the Ca2+-induced Ca2+ release, NO can be expected to potentiate GABA release. In support of this notion, a cADPR antagonist abolished the NO-induced potentiation of GABAergic inhibitory postsynaptic potentials in the NTS. Thus, the NO-cGMP-cADPR-Ca2+ pathway, previously described in sea urchin eggs, also operates in mammalian GABAergic neurons. Potentiation of GABA release by NO may have implications for numerous brain functions.

Improved Golgi-like Visualization in Retrogradely Projecting Neurons after EGFP-Adenovirus Infection in Adult Rat and Monkey

An adenovirus vector was generated using a neuron-specific promoter synapsin I and enhanced green fluorescent protein (EGFP) reporter (AdSynEGFP). In addition, two modifications were identified that resulted in robust and reliable retrograde transport and EGFP expression after injection of the virus into three different brain regions in adult rats (medial prefrontal cortex, posterior thalamic nuclear group, and CA1). These are post-injection survival times of 14 days and addition of high concentrations of NaCl (≥600 mM) to the injection buffer. These modifications resulted in obvious improvement in the intensity of the EGFP signal and in the number of labeled cells. Use of anti-EGFP in immunofluorescence or immunoperoxidase processing further enhanced the signal so that Golgi-like filling of dendritic spines and axon collaterals was routinely achieved. Effectiveness of the AdSynEGFP for Golgi-like filling was confirmed in one rhesus monkey with injections in visual area V4. Because of the long-term viability of the infected neurons (at least up to 28 days in rats and 22 days in monkey), this AdSynEGFP is suitable for use in microcircuitry studies in combination with other fluorescently tagged elements, including anterogradely labeled extrinsic projections. The native EGFP signal (without antibody enhancement) may be sufficient for studies involving cultured cells or slices. (J Histochem Cytochem 54:539-548, 2006)