Dual fluorescence combined with a two-color immunoperoxidase technique: a new way of visualizing diverse neuronal elements

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Catecholamine inputs to expiratory laryngeal motoneurons in rats

Many respiration-related interneurons and motoneurons receive a catecholaminergic input, but the extent and distribution of this input to recurrent laryngeal motoneurons that innervate intrinsic muscles of the larynx are not clear. In the present study, we examined the catecholaminergic input to expiratory laryngeal motoneurons in the caudal nucleus ambiguus by combining intracellular labeling of single identified motoneurons, with immunohistochemistry to reveal tyrosine hydroxylase immunoreactive (catecholaminergic) terminal varicosities. Close appositions were found between the two structures, with 18 ± 5 close appositions per motoneuron (n = 7). Close appositions were more frequently observed on distal rather than proximal dendrites. Axosomatic appositions were not seen. In order to determine the source of this input, microinjections of cholera toxin B subunit (1%, 20 nl) were made into the caudal nucleus ambiguus. Retrogradely labeled neurons, located in the ipsilateral nucleus tractus solitarius and the area postrema, were tyrosine hydroxylase-positive. Our results not only demonstrate details of the extent and distribution of potential catecholamine inputs to the expiratory laryngeal motoneuron, but further indicate that the inputs, at least in part, originate from the dorsomedial medulla, providing a potential anatomical basis for previously reported catecholaminergic effects on the laryngeal adductor reflex.

Cholinergic inputs to laryngeal motoneurons functionally identified in vivo in rat: a combined electrophysiological and microscopic study

The intrinsic laryngeal muscles are differentially modulated during respiration as well as other states and behaviors such as hypocapnia and sleep. Previous anatomical and pharmacological studies indicate a role for acetylcholine at the level of the nucleus ambiguus in the modulation of laryngeal motoneuron (LMN) activity. The present study investigated the anatomical nature of cholinergic input to inspiratory- (ILM) and expiratory-modulated (ELM) laryngeal motoneurons in the loose formation of the nucleus ambiguus. Using combined in vivo intracellular recording, dye filling, and immunohistochemistry, we demonstrate that LMNs identified in Sprague-Dawley rat receive several close appositions from vesicular acetylcholine transporter-immunoreactive (VAChT-ir) boutons. ELMs receive a significantly greater number of close appositions (mean ± standard deviation [SD]: 47 ± 11; n = 5) than ILMs (32 ± 9; n = 8; t-test P < 0.05). For both LMN types, more close appositions were observed on the cell soma and proximal dendrites compared to distal dendrites (two-way analysis of variance [ANOVA], P < 0.0001). Using fluorescence confocal microscopy, almost 90% of VAChT-ir close appositions (n = 45 boutons on n = 4 ELMs) were colocalized with the synaptic marker synaptophysin. These results support a strong influence of cholinergic input on LMNs and may have implications in the differential modulation of laryngeal muscle activity.

A DIGE-based approach to study interacting proteins

A full spectrum of high-throughput protein identification and characterization approaches has been developed for protein profiling. However, the most demanding field to better understanding protein interactions known as the "interactome" is still of a perpetual need for modern proteomics. Recently developed DIGE (difference in-gel electrophoresis) system may be of potential use when studying interacting proteins. In this work we applied DIGE technique on native gel electrophoresis to study protein-protein interactions. As a proof of principle, we utilized an in vitro interaction model between p53 and HDM2 proteins. In parallel, we also showed interaction of these proteins using fluorescently labelled p53- or HDM2-immunoprecipitation pellets. Thus, we believe this study shows a good potential for investigating various interacting partners and benefits towards creation of interactome.

Detection of amino acid and peptide transmitters in physiologically identified brainstem cardiorespiratory neurons

Most of the CNS neurons that regulate circulation and respiration reside in regions of the brain characterized by extreme cellular heterogeneity (nucleus of the solitary tract, reticular formation, parabrachial nuclei, periaqueductal gray matter, hypothalamus, etc.). The chemical neuroanatomy of these regions is correspondingly complex and teasing out specific circuits in their midst remains a problem that is usually very difficult if not impossible to solve by conventional tract-tracing methods, Fos methodology or electrophysiology in slices. In addition, identifying the type of amino acid or peptide transmitter used by electrophysiologically recorded neurons has been until recently an especially difficult task either for lack of a specific marker or because such markers (many peptides for example) are exported to synaptic terminals and thus undetectable in neuronal cell bodies. In this review, we describe a general purpose method that solves many of these problems. The approach combines juxtacellular labeling in vivo with the histological identification of mRNAs that provide definitive neurochemical phenotypic identification (e.g. vesicular glutamate transporter 1 or 2, glutamic acid decarboxylase). The results obtained with this method are discussed in the general context of amino acid transmission in brainstem cardiorespiratory pathways. The presence of markers of amino acid transmission in specific aminergic pre-sympathetic neurons is especially emphasized as is the extensive co-localization of markers of GABAergic and glycinergic transmission in the brainstem reticular formation.

Estrogen-induced dendritic spine elimination on female rat ventromedial hypothalamic neurons that project to the periaqueductal gray

Neurons of the ventromedial hypothalamic nucleus (VMH) that project to the periaqueductal gray (PAG) form a crucial segment of the motor pathway that produces the lordosis posture, the hallmark of female rat sexual behavior. One suggested mechanism through which estrogen facilitates lordosis is by remodeling synaptic connectivity within the VMH. For instance, estrogen alters VMH dendritic spine density. Little is known, however, about the local VMH microcircuitry governing lordosis nor how estrogen alters synaptic connectivity within this local circuit to facilitate sexual behavior. The goal of this study was to define better the neuron types within the VMH microcircuitry and to examine whether estrogen alters synaptic connectivity, as measured by dendritic spine density, on VMH projection neurons. A retrograde tracer was injected into the PAG of ovariectomized rats treated with vehicle or estradiol. Retrogradely labeled VMH neurons were filled with Lucifer yellow, then immunostained for estrogen receptor-alpha (ER alpha). VMH neurons that project to the PAG had more dendrites than functionally unidentified neurons. Additionally, VMH projection neurons could be subdivided into those located within the cluster of ER alpha-containing neurons and those medial to the cluster. Estrogen decreased spine density by 57% on the long primary dendrites of VMH projection neurons located within the ER alpha cluster but not on projection neurons medial to the cluster. Only 4% of the VMH projection neurons expressed ER alpha. These results suggest that estrogen may facilitate sexual behavior by decreasing spines selectively, via an indirect mechanism, on a subset of VMH neurons that project to the PAG.

Estrogen selectively regulates spine density within the dendritic arbor of rat ventromedial hypothalamic neurons

Estrogen acts in the hypothalamic ventromedial nucleus (VMH) to promote female sexual behavior. One potential mechanism through which estrogen may facilitate this behavior is by reconfiguring synaptic connections within the VMH. Estrogen treatment increases the number of synapses and dendritic spines in the VMH, but how this remodeling occurs within the context of the local, behaviorally relevant microcircuitry is unknown. The goal of this study was to localize estrogen-induced changes in spine density within the VMH and relate these to dendritic morphology and the presence of nuclear estrogen receptor. The hypothalami from ovariectomized rats, treated with either vehicle or estradiol, were lightly fixed, and VMH neurons were iontophoretically filled with Lucifer yellow. Confocal microscopy was used to examine neuronal morphology. Estrogen treatment increased dendritic spine density by 48% in the ventrolateral VMH but had no effect on spine density in the dorsal VMH. The primary dendrites of VMH neurons were differentially affected by estrogen. Estrogen treatment increased spine density twofold on the short primary dendrites but did not affect spine density on long primary dendrites. Immunocytochemical staining showed that none of the filled neurons expressed estrogen receptor-alpha. Thus, although the effect of estrogen on spine density is localized to a VMH subdivision where estrogen receptor is expressed, estrogen treatment induces spines on neurons that lack estrogen receptor. Taken together, our results suggest that the effect of estrogen on ventrolateral VMH spines is selective within the dendritic arbor of a neuron and may be mediated by an indirect, possibly transynaptic, mechanism.

Immunocytochemical detection of two nuclear proteins within the same neuron using light microscopy

We developed a method of double immunocytochemistry (ICC) that can be used with conventional light microscopy for localizing two different nuclear proteins. The procedure involves two sequential rounds of ICC that both employ the avidin and biotin conjugated enzyme (ABC) amplification method, separated by an Avidin D and biotin blocking step to reduce non-specific avidin-biotin reactions. Round one of ICC employs the use of avidin and biotin conjugated alkaline phosphatase (ABC-AP) and the Vector Red (VR) substrate, which produces a red colorimetric reaction product. The second round of ICC makes use of avidin and biotin conjugated peroxidase (ABC-HRP) and the Vector(R) SG substrate, which produces a gray colorimetric reaction product. Neuronal nuclei that are double-labeled for both proteins appear red with a gray core. This protocol allows the simultaneous detection of two proteins within the same subcellular compartment of a single neuron, without the need for epifluorescence or scanning confocal laser microscopy.

Injection of dye into neurones in rat and human post-mortem brain, in combination with acetylcholinesterase histochemistry: permanent preparations

We describe a protocol for the intracellular injection of dye into neurones in thick sections of fixed, post-mortem rat and human brain tissue. To render the sections with the intracellularly injected neurones permanent, they are sectioned again, and the resulting subsections are either immunocytochemically treated or stained histochemically for acetylcholinesterase (AChE) activity. The resultant preparations can be stored at room temperature for prolonged periods. Background staining produced by accumulation of erythrocytes in blood vessels is greatly reduced or virtually eliminated by exposure of the sections to ultraviolet radiation prior to the intracellular injection. The pattern of AChE staining is not affected by this procedure. This ability to stain sections according to a histochemical AChE procedure after the intracellular injection of dyes into striatal neurons opens the possibility to study the relationship of neuronal dendritic trees with the striosome/matrix compartmental boundaries in post-mortem (human) brain tissue of Huntington's disease patients.

Triple immunofluorescence labelling of parvalbumin, calbindin-D28k and calretinin in rat and monkey brain

This study presents novel techniques for the concomitant cytochemical detection of the calcium-binding proteins parvalbumin, calbindin-D28k and calretinin which are frequently used neuronal markers. For the triple immunofluorescence labelling of such antigens in rat and monkey brain--with emphasis on the cortex--we developed four different protocols which revealed obviously identical distribution patterns in consecutive sections. These methods included the simultaneous use of purified monoclonal antibodies directed against parvalbumin and calbindin--D28k--haptenized with biotin or digoxigenin--and subsequent visualization with fluorochromated hapten-recognizing immunoreagents. For the combined visualization of the calcium-binding proteins we applied the bright red fluorescent carbocyanine Cy3, blue fluorescent 7-amino-4-methylcoumarin-3-acetic acid (AMCA) and as green fluorophore either fluorescein or the newly introduced carbocyanine Cy2. The latter showed a higher fluorescence intensity and more resistance against photobleaching than fluorescein. In addition to clearly distinguished distribution patterns of the calcium-binding proteins, neurons co-expressing parvalbumin and calbindin-D28k in the parietal and piriform cortex of rat were demonstrated. The elaborated methods might stimulate the further detailed investigation of spatial and functional relationships between structures immunopositive for selected neuroanatomical markers.

Barosensitive neurons in the rostral ventrolateral medulla of the rat in vivo: morphological properties and relationship to C1 adrenergic neurons

The aim of this study, conducted in anaesthetized rats, was to examine the morphology of barosensitive neurons in the rostral ventrolateral medulla and their immunoreactivity for a catecholamine synthesizing enzyme, tyrosine hydroxylase. Thirty neurons displaying inhibitory postsynaptic potentials following stimulation of the aortic depressor nerve were intracellularly labelled with Lucifer Yellow or Neurobiotin. Some of these neurons could be excited antidromically from the second thoracic segment of the spinal cord, with conduction velocities of spinal axons ranging from 1.9 to 7.2 m/s. The filled somas were found immediately caudal to the facial nucleus and ventral or ventromedial to compact formation of the nucleus ambiguus. Some dendrites reached the ventral medullary surface. Axons usually projected dorsomedially and then made a sharp rostral and/or caudal turn. The caudally projecting axon could, in some cases, be followed to the first cervical segment of the spinal cord. Seven cells issued fine axon collaterals on the ipsilateral side. These were identified mainly in two areas: in the rostral ventrolateral medulla (or immediately dorsomedial to that region), and within the dorsal vagal complex. Seven of 27 examined cells (26%) were tyrosine hydroxylase-immunoreactive and were classified as C1 adrenergic neurons. No clear relationship was found between the presence or absence of adrenergic phenotype and the morphology of filled cells. However, the amplitude of aortic nerve-evoked inhibitory postsynaptic potentials was significantly larger in tyrosine hydroxylase-positive neurons. Possible reasons for the low percentage of barosensitive cells with tyrosine hydroxylase immunoreactivity found in this study, in comparison with previously published estimates, are discussed. This is the first study describing the morphology of neurons in this part of the medulla identified as barosensitive in vivo, and directly demonstrating adrenergic phenotype in a subset of these neurons.

Morphology and dendritic domains of neurons in the lateral parabrachial nucleus of the rat

The present study provides a description of the dendritic morphology and the dendritic domains of neurons in the lateral parabrachial nucleus (PB) of the rat. The cells were intracellularly stained in vitro with Lucifer yellow. A subpopulation of these cells was characterized beforehand as neurons projecting to the amygdaloid complex by retrograde transport with rhodamine beads. With respect to their dendritic arborization, different types of "spatially" organized PB neurons were discriminated. One major cell type in the external lateral PB (PBel) is characterized by long, elongated dendritic trees that are preferentially oriented parallel to the superior cerebellar peduncle. The majority of their dendrites appears to respect subnuclear boundaries, yet their distal dendrites often exceed the limits of the PBel to encroach upon adjacent subnuclei located dorsally and ventrolaterally to the PBel. Another prominent cell type in the PBel has fairly small and locally restricted dendritic trees that are also elongated, running with their main axis from ventrolateral to dorsomedial. The dendrites of the majority of these neurons apparently stay within the confines of the PBel. A distinct group of neurons is found in the ventral portion of the PBel. The majority of their dendrites is mediolaterally oriented and not confined to the PBel subnucleus. In addition, we found a smaller number of neurons scattered within the lateral PB whose dendrites do not show a preferential orientation but travel across subnuclear boundaries into several different PB subnuclei. Our data show that the dendrites of a large proportion of neurons in the lateral PB either stay within the confines of a particular subnucleus or slightly extend across subnuclear limits. In any case, they appear to match with terminal territories of afferent axons and, thus, maintain the functional specificity of inputs by their relay through the PB. In contrast, PB neurons that extend their dendrites across subnuclear boundaries or known terminal territories are likely to receive inputs of different qualities from a variety of sources and therefore transmit a more general, integrated signal to the forebrain.

Substance P and serotonergic inputs to sympathetic preganglionic neurons

Sympathetic preganglionic neurons are the final central links in the sympathetic pathways that control the heart and blood vessels. The neurotransmitters present in the supraspinal pathways that control the activity of sympathetic preganglionic neurons include amino acids, amines and peptides. In this paper we discuss evidence that suggests a role for serotonin and substance P in these pathways. Both of these neurotransmitters are present in bulbospinal neurons. Our results suggest that they have an important physiological role in the central regulation of blood pressure.

Projections from inspiratory neurons of the ventral respiratory group to the subretrofacial nucleus of the cat

Arterial blood pressure and the activity of many sympathetic nerves are known to be affected by changes in central respiratory activity. The central neurons responsible for this respiratory modulation are unknown. In the present study we have labelled inspiratory neurons (n = 24) in the rostral ventral respiratory group and Bötzinger complex in the medulla oblongata of the cat using intracellular injection of biocytin. The filled neurons were examined to see if they had axonal projections to the subretrofacial nucleus, an important brainstem nucleus in the tonic and reflex control of blood pressure. The subretrofacial nucleus was identified histologically as a cluster of neurons in the rostral ventrolateral medulla, some of which are tyrosine hydroxylase immunoreactive. Varicose axons arising from labelled inspiratory neurons were mostly found dorsal to this cluster, within the area corresponding to the Bötzinger complex. A small number of axon varicosities were seen in the subretrofacial nucleus. The results suggest that a part of the respiratory modulation of sympathetic nerve activity may be due to a direct synaptic input from inspiratory neurons of the ventral respiratory group to neurons of the subretrofacial nucleus.

Substance P immunoreactive boutons form synapses with feline sympathetic preganglionic neurons

In this study, the relationship between substance P-immunoreactive boutons and antidromically activated sympathetic preganglionic neurons was examined by light and electron microscopy. Sympathetic preganglionic neurons in the T2-T4 spinal segments of the cat were identified by intracellular recording and antidromic activation from the corresponding white ramus. Neurons were filled with lucifer yellow and then stained to reveal, simultaneously, substance P and lucifer yellow immunoreactivity. All of the neurons examined with the light microscope (n = 13) received appositions from substance P-immunoreactive boutons. Appositions were found on all parts of the neuron, including the somata, dendrites, and axon initial segment. In most cases (11/13) few close appositions were seen; however, two neurons received large numbers of appositions from substance P-immunoreactive boutons. On one neuron, 16 substance P-immunoreactive varicosities that were identified as being closely apposed at the light microscope level were serially sectioned and examined with the electron microscope. Of these 16 varicosities, eight either directly contacted the neuron or formed morphologically identifiable synapses. The remaining eight varicosities were separated from the neuron by thin glial processes. Two other sympathetic preganglionic neurons that were examined ultrastructurally also received substance P-immunoreactive synapses and close contacts. These findings suggest that substance P-containing nerve fibres could affect all sympathetic preganglionic neurons but are likely to be important in regulating the activity of only a small proportion of these neurons.