Immunohistochemical identification of Lucifer Yellow-labeled neurons in the rat supraoptic nucleus

Approaches to Study Gap Junctional Coupling

Astrocytes and oligodendrocytes are main players in the brain to ensure ion and neurotransmitter homeostasis, metabolic supply, and fast action potential propagation in axons. These functions are fostered by the formation of large syncytia in which mainly astrocytes and oligodendrocytes are directly coupled. Panglial networks constitute on connexin-based gap junctions in the membranes of neighboring cells that allow the passage of ions, metabolites, and currents. However, these networks are not uniform but exhibit a brain region-dependent heterogeneous connectivity influencing electrical communication and intercellular ion spread. Here, we describe different approaches to analyze gap junctional communication in acute tissue slices that can be implemented easily in most electrophysiology and imaging laboratories. These approaches include paired recordings, determination of syncytial isopotentiality, tracer coupling followed by analysis of network topography, and wide field imaging of ion sensitive dyes. These approaches are capable to reveal cellular heterogeneity causing electrical isolation of functional circuits, reduced ion-transfer between different cell types, and anisotropy of tracer coupling. With a selective or combinatory use of these methods, the results will shed light on cellular properties of glial cells and their contribution to neuronal function.

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.

Dual channel confocal laser scanning microscopy of lucifer yellow-microinjected human brain cells combined with Texas red immunofluorescence

A method for visualization of individual human brain cells and their dendritic extensions in combination with immunofluorescence is described. Microinjection of Lucifer Yellow was used to reveal the dendritic morphology of cortical brain cells. Indirect immunofluorescence with Texas Red as label was used to investigate the distribution of 3 different groups of immunogens: enzymes (monoamine oxidase A and B), receptors (beta-adrenoceptor protein), and synaptic vesicle proteins (synapsin I and synaptophysin) in each cortical slice. A dual-channel confocal laser scanning microscope with an argon/krypton laser was used for imaging these double-stained fluorescent specimens. Lucifer Yellow and Texas Red were recorded simultaneously or separately, taking advantage of the different activating lines (488 lambda and 568 lambda) of the laser and using the two filter blocks (K1 and K2) supplied with the instrument (BioRad MRC-600) for recording the emission of either fluorophore. Using this technique we have demonstrated the localization of immunoreactive material in relation to the dendritic morphology of cortical cells.

Comparison of three intracellular markers for combined electrophysiological, morphological and immunohistochemical analyses

Hypothalamic paraventricular and supraoptic neurons were recorded intracellularly in coronal slices and injected with Lucifer yellow, ethidium bromide or biocytin. Electrical properties, morphological staining and neurophysin immunohistochemistry were compared among the 3 markers. Lucifer yellow electrodes had a high resistance and frequently blocked during experiments. Neurons recorded with Lucifer yellow electrodes had low input resistances and low-amplitude, broad spikes. Lucifer yellow labeling in whole mount was highly fluorescent, revealing distal dendrites and axons. Of cells injected with Lucifer yellow, 64% were recovered but were faint after immunohistochemical processing. Recordings with ethidium bromide electrodes were similar to controls, although electrode blockage sometimes occurred. Only somata and proximal dendrites of ethidium bromide-filled neurons were visible in whole-mount. Forty percent of cells injected with ethidium bromide were recovered after immunohistochemical processing; these were invariably faint. Recordings with biocytin-filled electrodes were similar to control recordings. Biocytin-filled, HRP-labeled cells showed distal dendrites and often dendritic spines and axons in 50-75-microns sections. Seventy percent of biocytin-injected cells labeled with fluorescent markers were recovered and remained strongly labeled after immunohistochemical processing. Biocytin had the best electrical and staining properties for combined electrophysiological and anatomical studies.

Excitation of supraoptic vasopressin cells by stimulation of the A1 noradrenaline cell group: failure to demonstrate role for established adrenergic or amino acid receptors

The effects of adrenergic and excitatory amino acid antagonists on supraoptic nucleus (SON) neurosecretory cell responses to stimulation of the A1 noradrenaline (NA) cell group were examined in anaesthetized male rats. As in previous studies, delivery of cathodal pulses (100 microA, 1 ms pulses, 1 Hz) to the A1 region of the caudal ventrolateral medulla excited spontaneously active, antidromically identified neurosecretory cells, the majority of which were identified as arginine vasopressin (AVP) secreting on the basis of basal discharge patterns and responses to abrupt increases in arterial blood pressure. Administration of alpha- and beta-adrenoreceptor antagonists, by systemic or intracerebroventricular delivery of a bolus, or by direct pressure injection into the SON, did not alter neurosecretory cell responses to A1 stimulation, even when doses applied exceeded that required for blockade of excitations elicited by local application of NA. Application of the broad spectrum excitatory amino acid antagonist kynurenic acid (5-40 mM) blocked the excitatory effects of locally applied glutamate (100 microM) and transiently inhibited spontaneous activity, but failed to alter the excitatory effects of A1 region stimulation on SON cells. Identical effects were obtained with a selective kainate/quisqualate receptor antagonist. These data indicate that neurosecretory cell responses to activation of the A1 cell group are unaltered by antagonists of alpha- and beta-adrenoreceptors, or excitatory amino acid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y potentiates excitation of supraoptic neurosecretory cells by noradrenaline

The effects of neuropeptide Y (NPY) and noradrenaline (NA) on the activity of rat supraoptic nucleus (SON) neurosecretory cells were examined using perfused hypothalamic slices. Bath application of either NPY (10(-9)-10(-6) M) or NA (10(-6)-10(-3) M) excited SON cells, although only NA elicited consistent, dose-dependent effects. Application of NPY at a dose having virtually no direct effects (10(-8) M) produced a 5-fold increase in SON cell responsiveness to NA at the sub-maximal response dose of 10(-5) M, but did not alter the minimum concentration of NA required to excite SON cells or increase the maximal response elicited by higher NA concentrations. The effects of NA, alone or in combination with NPY, were abolished by alpha-adrenoreceptor blockade. These data suggest that NPY has only weak direct effects on neurosecretory cells, but may have important neuromodulatory actions, significantly enhancing the excitatory effects of NA.

Thermosensitivity of neurons in the paraventricular nucleus of the rat slice preparation

The thermosensitivity of 65 spontaneously active neurons in the paraventricular nucleus (PVN) was investigated by extracellular recording in the rat hypothalamic slice preparation. The firing rate of these cells was comparatively low, ranging from 0.03 to 10.0 (mean 2.46) impulses/s at 37 degrees C, and only a minority showed a phasic firing pattern. Of 65 neurons tested, 23 (35%) increased their firing rate when the slice was warmed (warm-sensitive neurons) and 9 (14%) showed the opposite response (cold-sensitive neurons). Thermosensitivity was also tested in solutions with reduced [Ca2+] and high [Mg2+]. Eight out of 10 warm-sensitive neurons and 5 of 7 cold-sensitive neurons retained thermosensitivity after synaptic blockade. Out of 6 phasic firing neurons tested, one showed warm-sensitivity and another one showed cold-sensitivity. The thermosensitive neurons were diffusely distributed throughout the PVN and were not located in particular areas of the nucleus. Thus a group of cells in the PVN, including probably both magno- and parvocellular neurons, showed an inherent thermosensitivity, which suggests an important role for the PVN in thermoregulation.

Characterization of hypothalamic noradrenaline receptors in the supraoptic nucleus and periventricular region of the paraventricular nucleus of mice in vitro

In an attempt to determine the basis for apparently conflicting reports of the effects of noradrenaline (NA) on the neurohypophyseal system and its effects on the parvocellular periventricular region of the paraventricular nucleus (PVN), recordings were made from the neurons in the supraoptic nucleus (SON) and the periventricular region in the mouse hypothalamic slice preparation. Of 47 SON neurons, 43 (91%) were excited and two (4%) were inhibited by NA. Seven SON neurons increased the firing rate with increase of NA concentration (10(-7)-10(-4) M). Both the alpha 1-agonists phenylephrine and methoxamine also increased the activity of all SON neurons tested whereas application of the alpha 2-agonist clonidine and the beta-agonist isoproterenol had weak and inconsistent effects. While the alpha 2-antagonist yohimbine had no consistent influence, the alpha 1-antagonist prazosin blocked or reversed the effects of NA. Another group of 37 neurons in the periventricular region of the PVN was also tested; 13 (35%) were excited and 22 (59%) inhibited by application of NA (10(-5) M). When tested with phenylephrine or methoxamine, 6 of the 7 neurons were excited and one inhibited but all the 4 neurons tested were excited by isoproterenol. Clonidine strongly depressed the activity of all 12 neurons tested. The NA-induced excitatory effects were suppressed or reversed by pre-application of prazosin and the beta-antagonist propranolol while the inhibitory ones were suppressed or reversed by yohimbine. Synaptic blockade did not affect the excitatory responses of SON cells to NA nor the inhibitory responses of periventricular neurons to NA or clonidine. We conclude that SON neurons receive adrenergic excitatory effects mainly through alpha 1-receptors. The periventricular neurons receive the excitatory effects through alpha 1- or beta-receptors and receive the inhibitory effects through alpha 2-receptors.

Immunoreactivity to vasopressin- but not oxytocin-associated neurophysin antiserum in phasic neurons of rat hypothalamic paraventricular nucleus

Bursts of action potentials were recorded intracellularly from 11 phasically firing magnocellular neurons in the paraventricular nucleus in slices of rat hypothalamus. The bursts of overshooting, often broadening action potentials (63-87 mV peak-to-peak) were superimposed on depolarizing plateau potentials. Phasic activity was recorded before and/or after the neurons were injected with the fluorescent dye Lucifer Yellow CH. Injected neurons were first examined in whole slices, and subsequently, in sectioned material, characterized immunocytochemically using antisera to vasopressin- and oxytocin-associated neurophysins (VP-NP and OT-NP respectively). The 11 injections produced 8 single dye filled neurons and 3 pairs of dye-coupled neurons, 14 dye-filled cells in all. Six of the single cells and all the dye coupled pairs were immunoreactive with VP-NP antiserum and not reactive with OT-NP antiserum. Most of these neurons were in areas of the nucleus in which VP-NP reactive cells predominated, but two were surrounded by OT-NP reactive cells. Two single, dye-filled, phasically active, magnocellular neurons failed to show immunoreactivity to either antiserum.

Possible target neurons of 5-hydroxytryptamine fibers in the lamprey spinal cord: immunohistochemistry combined with intracellular staining with Lucifer yellow

Intracellular recordings were made from 76 neurons belonging to various cell types in the lamprey spinal cord, and these neurons were subsequently stained with Lucifer yellow. Sections were made of spinal cords containing Lucifer-yellow-filled neurons, and in the same sections 5-hydroxytryptamine (5-HT)-containing neurons and fibers were made visible with immunohistochemical methods. Motoneurons and lateral cells appeared to send part of their dendrites into a dense ventromedial 5-HT plexus, and these dendrites were adjacent to 5-HT varicosities. No or few 5-HT varicosities have been found adjacent to cell bodies or dendrites of sensory dorsal cells, giant interneurons, and edge cells. The combined application of intracellular staining and immunohistochemistry appeared to be suited to screen for possible transmitter-identified contacts on morphologically identified neurons.

A reliable method for immunocytochemical identification of Lucifer Yellow injected, peptide-containing mammalian central neurons

An immunocytochemical procedure is described for reliably determining the hormone content of magnocellular neuroendocrine neurons that have been injected with Lucifer Yellow in slices of rat hypothalamus. The chief advantages of this procedure over others currently available are: (a) it permits whole mount observation of the tissue, and thus, of the morphology of filled cell(s) as well as of such phenomena as dye-coupling; (b) the reliability of tissue preparation and peptide determination has been optimized so that about 85% of injected cells are identified immunocytochemically; and (c) the final immunostained product is permanent, permitting bright-field examination of the injected cell. Relative advantages and limitations of this and other recently published methods are discussed.