Transneuronal transport of intracellularly injected biotinamide in primary afferent axons

Mesencephalic trigeminal neuron dendritic structures in the macaque

It is presumed that the unusual central location of mesencephalic trigeminal neurons is a specialization that allows them to receive synaptic input. However, relatively few synaptic terminals were observed on the somata of mesencephalic trigeminal neurons in macaque monkeys via electron microscopy. This leaves the question of dendritic synaptic terminals open. Unlike the pseudounipolar neurons found in the trigeminal ganglion, some mesencephalic trigeminal neurons have been reported to be multipolar cells exhibiting a number of dendritic processes in non-primate species. To examine whether this morphological feature was also present in macaque monkeys, we retrogradely filled these cells with biotinylated dextran amine by injecting it into the trigeminal nerve entry zone. A portion of the mesencephalic trigeminal neurons exhibited short, poorly branched, dendritic processes. They also exhibited very fine, short processes believed to be somatic spines. Thus, primate trigeminal mesencephalic neurons appear to have specializations aimed at increasing the membrane surface area available for synaptic input.

Mechanosensory pathways of scorpion pecten hair sensillae-Adjustment of body height and pecten position

Scorpions' sensory abilities are intriguing, especially the rather enigmatic ventral comb-like chemo- and mechanosensory organs, the so-called pectines. Attached ventrally to the second mesosomal segment just posterior to the coxae of the fourth walking leg pair, the pectines consist of the lamellae, the fulcra, and a variable number of pecten teeth. The latter contain the bimodal peg sensillae, used for probing the substrate with regard to chemo- and mechanosensory cues simultaneously. In addition, the lamellae, the fulcra and the pecten teeth are equipped with pecten hair sensillae (PHS) to gather mechanosensory information. Previously, we have analyzed the neuronal pathway associated with the peg sensillae unraveling their somatotopic projection pattern in dedicated pecten neuropils. Little is known, however, regarding the projections of PHS within the scorpion nervous system. Behavioral and electrophysiological assays showed involvement of PHS in reflexive responses but how the information is integrated remains unresolved. Here, we unravel the innervation pattern of the mechanosensory pecten hair afferents in Mesobuthus eupeus and Euscorpius italicus. By using immunofluorescent labeling and injection of Neurobiotin tracer, we identify extensive arborizations of afferents, including (i) ventral neuropils, (ii) somatotopically organized multisegmental sensory tracts, (iii) contralateral branches via commissures, and (iv) direct ipsilateral innervation of walking leg neuromeres 3 and 4. Our results suggest that PHS function as sensors to elicit reflexive adjustment of body height and obstacle avoidance, mediating accurate pecten teeth alignment to guarantee functionality of pectines, which are involved in fundamental capacities like mating or navigation.

Neurotropic virus tracing suggests a membranous-coating-mediated mechanism for transsynaptic communication

Swine hemagglutinating encephalomyelitis virus (HEV) has been shown to have a capability to propagate via neural circuits to the central nervous system after peripheral inoculation, resulting in acute deadly encephalomyelitis in natural host piglets as well as in experimental younger rodents. This study has systematically examined the assembly and dissemination of HEV 67N in the primary motor cortex of infected rats and provides additional evidence indicating that membranous‐coating‐mediated endo‐/exocytosis can be used by HEV for its transsynaptic transfer. In addition, our results suggested that this transsynaptic pathway could adapted for larger granular materials, such as viruses. These findings should help in understanding the mechanisms underlying coronavirus infections as well as the intercellular exchanges occurring at the synaptic junctions. J. Comp. Neurol. 521:203–212, 2013. © 2012 Wiley Periodicals, Inc.

Participation of Kv1 channels in control of membrane excitability and burst generation in mesencephalic V neurons

The function and biophysical properties of low threshold Kv1 current in control of membrane resonance, subthreshold oscillations, and bursting in mesencephalic V neurons (Mes V) were examined in rat brain stem slices (P8-P12) using whole cell current and voltage patch-clamp methods. alpha-dendrotoxin application, a toxin with high specificity for Kv1.1, 1.2, and 1.6 channels, showed the presence of a low-threshold K(+) current that activated rapidly around -50 mV and was relatively noninactivating over a 1-s period and had a V(1/2)max of -36.2 mV. Other toxins, specific for individual channels containing either Kv 1.1, 1.2, or 1.3 alpha-subunits, were applied individually, or in combination, and showed that Kv1 channels are heteromeric, composed of combinations of subunits. In current-clamp mode, toxin application transformed the high-frequency resonant properties of the membrane into a low-pass filter and concomitantly reduced the frequency of the subthreshold membrane oscillations. During this period, rhythmical bursting was transformed into low-frequency tonic discharge. Interestingly, in a subset of neurons that did not show bursting, low doses of alpha-dendrotoxin (alpha-DTX) sufficient to block 50% of the low threshold Kv1 channels induced bursting and increased the resonant peak impedance and subthreshold oscillations, which was replicated with computer simulation. This suggests that a critical balance between inward and outward currents is necessary for bursting. This was replicated with computer simulation. Single cell RT-PCR and immunohistochemical methods confirmed the presence of Kv1.1, 1.2, and 1.6 alpha-subunits in Mes V neurons. These data indicate that low threshold Kv1 channels are responsible for membrane resonance, contribute to subthreshold oscillations, and are critical for burst generation.

Thalamo-telencephalic pathways in the fire-bellied toad Bombina orientalis

It was suggested that among extant vertebrates, anuran amphibians display a brain organization closest to the ancestral tetrapod condition, and recent research suggests that anuran brains share important similarities with the brains of amniotes. The thalamus is the major source of sensory input to the telencephalon in both amphibians and amniote vertebrates, and this sensory input is critical for higher brain functions. The present study investigated the thalamo-telencephalic pathways in the fire-bellied toad Bombina orientalis, a basal anuran, by using a combination of retrograde tract tracing and intracellular injections with the tracer biocytin. Intracellular labeling revealed that the majority of neurons in the anterior and central thalamic nuclei project to multiple brain targets involved in behavioral modulation either through axon collaterals or en passant varicosities. Single anterior thalamic neurons target multiple regions in the forebrain and midbrain. Of note, these neurons display abundant projections to the medial amygdala and a variety of pallial areas, predominantly the anterior medial pallium. In Bombina, telencephalic projections of central thalamic neurons are restricted to the dorsal striato-pallidum. The bed nucleus of the pallial commissure/thalamic eminence similarly targets multiple brain regions including the ventral medial pallium, but this is accomplished through a higher variety of distinct neuron types. We propose that the amphibian diencephalon exerts widespread influence in brain regions involved in behavioral modulation and that a single dorsal thalamic neuron is in a position to integrate different sensory channels and distribute the resulting information to multiple brain regions.

Cytoarchitecture of the accessory olfactory bulb in the salamander Plethodon shermani

Plethodontid terrestrial salamanders are emerging models in the study of the evolution of chemical communication in vertebrates. Their vomeronasal system is well defined. It comprises sensory neurons in the epithelium of the vomeronasal organ, whose axons form the vomeronasal nerve projecting to the accessory olfactory bulb (AOB), which in turn projects to the vomeronasal amygdala through the accessory olfactory tract. A detailed description of the cellular elements of the urodele AOB is lacking. Neuronal morphology in the AOB was studied by means of biocytin intracellular injections and retrograde tract tracing in the salamander Plethodon shermani. The AOB exhibits the characteristic lamination of olfactory bulbs, except that it displays a mixed periglomerular and mitral somata layer superficially. Mitral cells are the only AOB neurons projecting to the vomeronasal amygdala. Each mitral cell sends multiple axonal branches, generally through both dorsal and ventral portions of the accessory olfactory tract. Some mitral cells additionally send axon collaterals in the white matter immediately ventral to the AOB. AOB interneurons are divided into superficial periglomerular and deep granule cells, each category exhibiting morphological variety. Some neurons in the granule cell layer of the AOB or the region ventral to the AOB have dendritic trees that cover both regions. The present study is the first to highlight the full anatomical extent of single AOB neurons and surprisingly suggests that the ventrolateral telencephalon found below the AOB is part of the salamander vomeronasal system.

Organization of the pallium in the fire-bellied toad Bombina orientalis. I: Morphology and axonal projection pattern of neurons revealed by intracellular biocytin labeling

The cytoarchitecture and axonal projection pattern of pallial areas was studied in the fire-bellied toad Bombina orientalis by intracellular injection of biocytin into a total of 326 neurons forming 204 clusters. Five pallial regions were identified, differing in morphology and projection pattern of neurons. The rostral pallium receiving the bulk of dorsal thalamic afferents has reciprocal connections with all other pallial areas and projects to the septum, nucleus accumbens, and anterior dorsal striatum. The medial pallium projects bilaterally to the medial pallium, septum, nucleus accumbens, mediocentral amygdala, and hypothalamus and ipsilaterally to the rostral, dorsal, and lateral pallium. The ventral part of the medial pallium is distinguished by efferents to the eminentia thalami and the absence of contralateral projections. The dorsal pallium has only ipsilateral projections running to the rostral, medial, and lateral pallium; septum; nucleus accumbens; and eminentia thalami. The lateral pallium has ipsilateral projections to the olfactory bulbs and to the rostral, medial, dorsal, and ventral pallium. The ventral pallium including the striatopallial transition area (SPTA) has ipsilateral projections to the olfactory bulbs, rostral and lateral pallium, dorsal striatopallidum, vomeronasal amygdala, and hypothalamus. The medial pallium can be tentatively homologized with the mammalian hippocampal formation, the dorsal pallium with allocortical areas, the lateral pallium rostrally with the piriform and caudally with the entorhinal cortex, the ventral pallium with the accessory olfactory amygdala. The rostral pallium, with its projections to the dorsal and ventral striatopallidum, resembles the mammalian frontal cortex.

Hodological characterization of the septum in anuran amphibians: I. Afferent connections

On the basis of Nissl-stained sections, we subdivided the septum of the gray treefrog Hyla versicolor in the lateral, central, and medial septal complex. The afferent projections of the different septal nuclei were studied by combined retrograde and anterograde tracing with biotin ethylendiamine (Neurobiotin). The central and medial septal complex receives direct input from regions of the olfactory bulb and from all other limbic structures of the telencephalon (e.g., amygdalar regions, nucleus accumbens), whereas projections to the lateral septal complex are absent or less extensive. The medial pallium projects to all septal nuclei. In the diencephalon, the anterior thalamic nucleus provides the main ascending input to all subnuclei of the anuran septum, which can be interpreted as a limbic/associative pathway. The ventromedial thalamic nucleus projects to the medial and lateral septal complex and may thereby transmit multisensory information to the limbic system. Anterior preoptic nucleus, suprachiasmatic nucleus, and hypothalamic nuclei innervate the central and lateral septal complex. Only the nuclei of the central septal complex receive input from the brainstem. Noteworthy is the relatively strong projection from the nucleus raphe to the central septal complex, but not to the other septal nuclei.

Morphology and axonal projection pattern of neurons in the telencephalon of the fire-bellied toadBombina orientalis: An anterograde, retrograde, and intracellular biocytin labeling study

The connectivity and cytoarchitecture of telencephalic centers except dorsal and medial pallium were studied in the fire-bellied toad Bombina orientalis by anterograde and retrograde biocytin labeling and intracellular biocytin injection (total of 148 intracellularly labeled neurons or neuron clusters). Our findings suggest the following telencephalic divisions: (1) a central amygdala-bed nucleus of the stria terminalis in the caudal midventral telencephalon, connected to visceral-autonomic centers; (2) a vomeronasal amygdala in the caudolateral ventral telencephalon receiving input from the accessory olfactory bulb and projecting mainly to the preoptic region/hypothalamus; (3) an olfactory amygdala in the caudal pole of the telencephalon lateral to the vomeronasal amygdala receiving input from the main olfactory bulb and projecting to the hypothalamus; (4) a medial amygdala receiving input from the anterior dorsal thalamus and projecting to the medial pallium, septum, and hypothalamus; (5) a ventromedial column formed by a nucleus accumbens and a ventral pallidum projecting to the central amygdala, hypothalamus, and posterior tubercle; (6) a lateral column constituting the dorsal striatum proper rostrally and the dorsal pallidum caudally, and a ventrolateral column constituting the ventral striatum. We conclude that the caudal mediolateral complex consisting of the extended central, vomeronasal, and olfactory amygdala of anurans represents the ancestral condition of the amygdaloid complex. During the evolution of the mammalian telencephalon this complex was shifted medially and involuted. The mammalian basolateral amygdala apparently is an evolutionary new structure, but the medial portion of the amygdalar complex of anurans reveals similarities in input and output with this structure and may serve similar functions.

Connectivity and cytoarchitecture of the ventral telencephalon in the salamanderPlethodon shermani

The cytoarchitecture and axonal connection pattern of centers in the ventral telencephalon of the salamander Plethodon shermani were studied using biocytin for anterograde and retrograde labeling of cell groups, as well as by intracellular injections. Application of biocytin to the main and accessory olfactory bulbs identified the olfactory pallial regions and the vomeronasal portion of the amygdala, respectively. According to our results, the amygdala of Plethodon is divided into (1) a rostral part projecting to visceral and limbic centers and receiving afferents from the dorsal thalamus, and (2) a caudal part receiving accessory olfactory input. The striatopallial transition area (SPTA) lies rostrodorsally to the caudal (vomeronasal) amygdala and is similar in connections and possibly in function. The rostral striatum has few descending projections to the medulla, whereas the intermediate striatum sends strong projections to the tegmentum and medulla. The caudal striatum has strong ascending projections to the striatum and descending projections to the ventral hypothalamus. The dendritic trees of neurons labeled below the striatum and in the SPTA spread laterally from the soma, whereas dendrites of striatal neurons converge into the laterally situated striatal neuropil. In the caudal amygdala, three distinct types of neurons are found differing in dendritic arborization. It is concluded that, hodologically, the rostral part of the urodele amygdala corresponds to the central and basolateral amygdala and the caudal part to the cortical/medial amygdala of mammals. The urodele striatum is divided into a rostral striatum proper, an intermediate dorsal pallidum, and a caudal part, with distinct connections described here for the first time in a vertebrate.

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the fire-bellied toad Bombina orientalis

Intracellular recording and biocytin labeling were carried out in the fire-bellied toad Bombina orientalis to study the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation. Labeled neurons (n = 142) were divided into the following groups: TH1 neurons projecting to the dorsal striatum; TH2 neurons projecting to the amygdala, nucleus accumbens, and septal nuclei; TH3 neurons projecting to the medial or dorsal pallium; TH4 neurons with projections ascending to the dorsal striatum or ventral striatum/amygdala and descending to the optic tectum, tegmentum, and rostral medulla oblongata; TH5 neurons with projections to the tegmentum, rostral medulla oblongata, prectectum, or tectum; and TH6 neurons projecting to the hypothalamus. TH1 neurons are found in the central, TH2 neurons in the anterior and central, TH3 neurons in the anterior dorsal nucleus, and TH4 and TH5 neurons in the posterior dorsal or ventral nucleus. Neurons with descending projections arborize in restricted parts of retinal afferents; neurons with ascending projections do not substantially arborize within retinal afferents. At electrical optic nerve stimulation, neurons in the ventral thalamus respond with excitation at latencies of 10.8 msec; one-third of them follow repetitive stimulation and possibly are monosynaptically driven. Neurons in the dorsal thalamus respond mostly with inhibition at latencies of 42.3 msec and are polysynaptically driven. This corroborates the view that neurons in the dorsal thalamus projecting to the telencephalon receive no substantial direct retinal input and that the thalamopallial pathway of amphibians is not homologous to the mammalian retinogeniculocortical pathway.

Jaw-muscle spindle afferent pathways to the trigeminal motor nucleus in the rat

Neural pathways conveying proprioceptive feedback from the jaw muscles were studied in rats by combining retrograde and intracellular neuronal labeling. Initially, horseradish peroxidase was iontophoresed unilaterally into the trigeminal motor nucleus (Vmo). Two days later, 1-5 jaw-muscle spindle afferent axons located in the mesencephalic trigeminal nucleus were physiologically identified and intracellularly stained with biotinamide. Stained mesencephalic trigeminal jaw-muscle spindle afferent axon collaterals and boutons were predominantly distributed in the supratrigeminal region (Vsup), Vmo, dorsomedial trigeminal principal sensory nucleus (Vpdm), parvicellular reticular formation (PCRt), alpha division of the parvicellular reticular formation (PCRtA), and dorsomedial portions of the spinal trigeminal subnuclei oralis (Vodm), and interpolaris (Vidm). Numerous neurons retrogradely labeled with horseradish peroxidase from the trigeminal motor nucleus were found bilaterally in the PCRt, PCRtA, Vodm, and Vidm. Retrogradely labeled neurons were also present contralaterally in the Vsup, Vpdm, Vmo, peritrigeminal zone, and bilaterally in the dorsal medullary reticular field. Putative contacts between intracellularly stained mesencephalic trigeminal jaw-muscle spindle afferent boutons and trigeminal premotor neurons retrogradely labeled with horseradish peroxidase were found in the ipsilateral Vodm, PCRtA, and PCRt, as well as the contralateral Vsup, Vmo, Vodm, PCRt, and PCRtA. Thus, multiple disynaptic jaw-muscle spindle afferent-motoneuron circuits exist. These pathways are likely to convey long-latency jaw-muscle stretch reflexes and may contribute to stiffness regulation of the masticatory muscles.

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.

Modulation of jaw muscle spindle afferent activity following intramuscular injections with hypertonic saline

Transient noxious chemical stimulation of small diameter muscle afferents modulates jaw movement-related responses of caudal brainstem neurons. While it is likely that the effect is mediated from the spindle afferents in the mesencephalic nucleus (Vmes) via the caudally projecting Probst's tract, the mechanisms of pain induced modulations of jaw muscle spindle afferents is not known. In the present study, we tested the hypothesis that jaw muscle nociceptors gain access to muscle spindle afferents in the same muscle via central mechanisms and alter their sensitivity. Thirty-five neurons recorded from the Vmes were characterized as muscle spindle afferents based on their responses to passive jaw movements, muscle palpation, and electrical stimulation of the masseter nerve. Each cell was tested by injecting a small volume (250 microl) of either 5% hypertonic and/or isotonic saline into the receptor-bearing muscle. Twenty-nine units were tested with 5% hypertonic saline, of which 79% (23/29) showed significant modulation of mean firing rates (MFRs) during one or more phases of ramp-and-hold movements. Among the muscle spindle primary-like units (n = 12), MFRs of 4 units were facilitated, five reduced, two showed mixed responses and one unchanged. In secondary-like units (n = 17), MFRs of 9 were facilitated, three reduced and five unchanged. Thirteen units were tested with isotonic saline, of which 77% showed no significant changes of MFRs. Further analysis revealed that the hypertonic saline not only affected the overall output of muscle spindle afferents, but also increased the variability of firing and altered the relationship between afferent signal and muscle length. These results demonstrated that activation of muscle nociceptors significantly affects proprioceptive properties of jaw muscle spindles via central neural mechanisms. The changes can have deleterious effects on oral motor function as well as kinesthetic sensibility.

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the salamander Plethodon jordani

In the salamander Plethodon jordani, the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling under in vitro, whole-brain conditions. Based on their axonal projections, labeled neurons (n = 76) were divided into the following groups: TH1 neurons, with mostly ipsilateral projections to the striatum; TH2 neurons, with ipsilateral or bilateral projections to the medial amygdala and nucleus accumbens; TH3 neurons, with bilateral projections to the medial and dorsal pallium; TH4 neurons, with mostly ipsilateral projections to the striatum and ipsilateral projections to the tectum opticum, tegmentum, and rostral medulla oblongata; and TH5 neurons, with ipsilateral projections to the tegmentum, medulla oblongata, and rostral spinal cord without (TH5.1) or with (TH5.2) additional projections to the optic tectum. TH1-TH4 neurons are found in the dorsal thalamus and around the sulcus medialis, and TH5 neurons are found in the ventral thalamus. Labeled neurons with ascending projections, i.e., the more dorsally situated TH1-TH4 neurons, are mostly inhibited by electrical stimulation of the optic nerve and have significantly longer latencies (mean +/- S.D., 42.1 +/- 11.6 msec) than neurons with exclusively descending projections, i.e., the ventrally located TH5 neurons (8.5 +/- 6.1 msec), which receive the bulk of retinal afferents and show excitation at electrical optic nerve stimulation. Neurons recorded without labeling in the dorsal thalamus likewise exhibit mostly inhibition and have significantly longer latencies (35.7 +/- 18.9 msec) than those recorded in the ventral thalamus (10.9 +/- 7.7 msec), which mostly show excitation. None of the neurons recorded in the dorsal thalamus followed repetitive stimulation of the optic nerve. Thus, neurons situated in the dorsal thalamus and projecting to pallial or subpallial telencephalic targets are unlikely to receive monosynaptic or oligosynaptic, excitatory retinal input. Accordingly, no retino-thalamo-telencephalic pathway homologous to that found in amniotes appears to exist in salamanders.

Experimental muscle pain produces central modulation of proprioceptive signals arising from jaw muscle spindles

The aim of the present study was to investigate the effects of intramuscular injection with hypertonic saline, a well-established experimental model for muscle pain, on central processing of proprioceptive input from jaw muscle spindle afferents. Fifty-seven cells were recorded from the medial edge of the subnucleus interpolaris (Vi) and the adjacent parvicellular reticular formation from 11 adult cats. These cells were characterized as central units receiving jaw muscle spindle input based on their responses to electrical stimulation of the masseter nerve, muscle palpation and jaw stretch. Forty-five cells, which were successfully tested with 5% hypertonic saline, were categorized as either dynamic-static (DS) (n=25) or static (S) (n=20) neurons based on their responses to different speeds and amplitudes of jaw movement. Seventy-six percent of the cells tested with an ipsilateral injection of hypertonic saline showed a significant modulation of mean firing rates (MFRs) during opening and/or holding phases. The most remarkable saline-induced change was a significant reduction of MFR during the hold phase in S units (100%, 18/18 modulated). Sixty-nine percent of the DS units (11/16 modulated) also showed significant changes in MFRs limited to the hold phase. However, in the DS neurons, the MFRs increased in seven units and decreased in four units. Finally, five DS neurons showed significant changes of MFRs during both opening and holding phases. Injections of isotonic saline into the ipsilateral masseter muscle had little effect, but hypertonic saline injections made into the contralateral masseter muscle produced similar results to ipsilateral injections with hypertonic saline. These results unequivocally demonstrate that intramuscular injection with an algesic substance, sufficient to produce muscle pain, produces significant changes in the proprioceptive properties of the jaw movement-related neurons. Potential mechanisms involved in saline-induced changes in the proprioceptive signals and functional implications of the changes are discussed.

Recent techniques for tracing pathways in the central nervous system of developing and adult mammals

Over the last 20 years, the choice of neural tracers has increased manyfold, and includes newly introduced anterograde tracers that allow quantitation of single-axon morphologies, and retrograde tracers that can be combined with intracellular fills for the study of dendritic arbors of neurons which have a specific projection pattern. The combination of several different tracers now permits the comparison of multiple connections in the same animal, both quantitatively and qualitatively. Moreover, the finding of new virus strains, which infect neural cells without killing them, provides a tool for studying multisynaptic connections that participate in a circuit. In this paper, the labeling characteristics, mechanism of transport and advantages/disadvantages of use are discussed for the following recently introduced neural tracers: carbocyanine dyes, fluorescent latex microspheres, fluorescent dextrans, biocytin, dextran amines, Phaseolus vulgaris leucoagglutinin, cholera toxin and viruses. We also suggest the choice of specific tracers, depending on the experimental animal, age and type of connection to be studied, and discuss quantitative methodologies.

Dual intracellular recording of neocortical neurons in a neuron-glia co-culture system

Measurement of synchronized activities in neocortical networks is critical for understanding integrated brain function. We describe here a procedure for intracellular recording of activity in two functionally connected neurons using patch electrodes in neuron-glia co-cultures. Rat neocortical neurons survive on monolayers of astrocytes for far longer periods (3-4 weeks) than in their absence. This prolonged survival facilitates the detection of functional synapses which are readily identified in vitro by means of dual intracellular recording using two patch electrodes. The neuron-glia co-culture system appears to be useful not only for studying the formation of neural networks in vitro but also for determining the effects of physiologically active substances and chemicals on synapse formation.

Ultrastructural anatomy of physiologically identified jaw-muscle spindle afferent terminations onto retrogradely labeled jaw-elevator motoneurons in the rat

Neuronal microcircuits involving jaw-muscle spindle afferents and jaw-elevator motoneurons were studied via retrograde and intracellular labeling in rats. Initially, trigeminal motoneurons were retrogradely labeled from horseradish peroxidase (HRP) injections into the temporalis and masseter muscles. The intracellular response of jaw-muscle spindle afferent neurons was then characterized during palpation, ramp and hold, and sinusoidal stretching of the jaw-closing muscles. Biotinamide was injected into these neurons, and the tissue was processed for the visualization of HRP and biotinamide. The ultrastructure of 243 intracellularly stained jaw-muscle spindle afferent boutons located within the trigeminal motor nucleus (Vmo) was examined. Eighty-five of these boutons synapsed with motoneurons retrogradely labeled with HRP, and 158 boutons synapsed with unlabeled structures within the Vmo. All spindle afferent boutons contained clear, spherical synaptic vesicles. Although the majority of boutons were S type, a few labeled jaw-muscle spindle afferent boutons possessed a long, narrow cleft, with a subsynaptic cistern comparable to previous descriptions of C-type boutons. Sixty-eight percent of spindle afferent boutons synapsed with large or medium-sized, retrogradely labeled motoneuron dendrites, and 32% synapsed with retrogradely labeled somata. In numerous instances, spindle afferent boutons synapsed with trigeminal motoneuron dendritic or somatic spines. Most of the synapses between spindle afferent boutons and trigeminal motoneuron dendrites were asymmetric, and the greatest percentage of axosomatic synapses between spindle afferents and trigeminal motoneurons were symmetric. Approximately 24% of spindle afferent boutons constituted the intermediate element of a axoaxodendritic or axoaxosomatic assemblage, implying that some jaw-muscle spindle afferent synapses with trigeminal motoneurons are presynaptically modulated.

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. II: intracellular recording and labeling experiments

In the plethodontid salamanders Plethodon jordani and P. glutinosus, the morphology and axonal projections of 140 tectal neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling. Six types of neurons are distinguished morphologically. TO1 neurons have wide dendritic trees that arborize mainly in tectal layers 1 and 3; they project bilaterally to the tegmentum and contralaterally to the medulla oblongata. TO2 neurons have very wide dendritic trees that arborize mainly in layers 2 and 3; axons project bilaterally or unilaterally to the pretectum and thalamus and ipsilaterally to the medulla oblongata. TO3 neurons have very wide and flat dendritic trees confined to layers 3-5; some have the same axonal projection as TO2 neurons, whereas others have descending axons that reach only the level of the cerebellum. TO4 neurons have narrower dendritic trees that arborize in layers 2 and 3; they project to the ipsilateral pretectum, thalamus, and medulla oblongata. TO5 neurons have dendritic trees that arborize in layers 1 and 2 or 1-3 and project bilaterally or unilaterally to the pretectum and thalamus. TO-IN are interneurons, with a number of subtypes with respect to variations in dendritic arborization pattern. TO1-TO5 neurons generally have short latencies of 2-16 ms (average = 8.4 ms) at electrical optic nerve stimulation; first responses are always excitatory, often followed by inhibition. They are likely to be mono- or oligosynaptically driven by retinal afferents. TO-IN interneurons have long latencies of 20-80 ms (average = 38.6 ms) and appear to receive no direct retinal input. With their specific dendritic arborization, consequent dominant retinal input, specific axonal projections, the different types of tectal projection neurons constitute separate ascending and descending visual pathways. Hypotheses are presented regarding the nature of the information processed by these pathways.

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.