Electron microscopical demonstration of horseradish peroxidase by use of tetramethylbenzidine as chromogen and sodium tungstate as stabilizer (TMB-ST method): a tracing method with high sensitivity and well preserved ultrastructural tissue

Morphological investigations of endomorphin-2 and spinoparabrachial projection neurons in the spinal dorsal horn of the rat

It has been proved that endomorphin-2 (EM2) produced obvious analgesic effects in the spinal dorsal horn (SDH), which existed in our human bodies with remarkable affinity and selectivity for the μ-opioid receptor (MOR). Our previous study has demonstrated that EM2 made synapses with the spinoparabrachial projection neurons (PNs) in the SDH and inhibited their activities by reducing presynaptic glutamate release. However, the morphological features of EM2 and the spinoparabrachial PNs in the SDH have not been completely investigated. Here, we examined the morphological features of EM2 and the spinoparabrachial PNs by using triple fluorescence and electron microscopic immunohistochemistry. EM2-immunoreactive (-ir) afferents directly contacted with the spinoparabrachial PNs in lamina I of the SDH. Immunoelectron microscopy (IEM) were used to confirm that these contacts were synaptic connections. It was also observed that EM2-ir axon terminals contacting with spinoparabrachial PNs in lamina I contained MOR, substance P (SP) and vesicular glutamate transporter 2 (VGLUT2). In lamina II, MOR-ir neurons were observed to receive direct contacts from EM2-ir varicosities. The synaptic connections among EM2, MOR, SP, VGLUT2, and the spinoparabrachial PNs were also confirmed by IEM. In sum, our results supply morphological evidences for the analgesic effects of EM2 on the spinoparabrachial PNs in the SDH.

A Special Cranial Nucleus (CSF-Contacting Nucleus) in Primates

BACKGROUND

There is a unique nucleus (CSF-contacting nucleus) in the brain of rat. It has been demonstrated in our previous research. The extraordinary feature of this nucleus is that it is not connected to any parenchymal organ but to the CSF. In primates, however, the presence or absence of this nucleus has not been proven. Confirmation of the presence of this nucleus in primates will provide the structural basis for brain-CSF communication and help to understand the neurohumoral regulatory mechanisms in humans.

METHODS

The tracer cholera toxin B subunit conjugated to horseradish peroxidase (CB-HRP) was injected into the CSF in the lateral ventricle (LV) of primate rhesus monkeys. After 48 h, the monkeys were perfused and the brain was dissected out, and sectioned for CB-HRP staining. The CB-HRP positive structures were observed under confocal and electron microscopy. The three-dimensional (3D) structure of the CB-HRP positive neurons cluster was reconstructed by computer software.

RESULTS

(1) CB-HRP labeling is confined within the ventricle, but not leakage into the brain parenchyma. (2) From the midbrain inferior colliculus superior border plane ventral to the aqueduct to the upper part of the fourth ventricle (4V) floor, a large number of CB-HRP positive neurons are consistently located, form a cluster, and are symmetrically located on both sides of the midline. (3) 3D reconstruction shows that the CB-HRP positive neurons cluster in the monkey brain occupies certain space. The rostral part is large and caudal part is thin appearing a "rivet"-like shape. (4) Under electron microscopy, the CB-HRP positive neurons show different types of synaptic connections with the non-CSF-contacting structures in the brain. Some of the processes stretch directly into the ventricle cavity.

CONCLUSION

Same as we did in rats, the CSF-contacting nucleus is also existed in the primate brain parenchyma. We also recommend listing it as the XIII pair of cranial nucleus, which is specialized in the communications between the brain and the CSF. It is significant to the completing of innervation in the organism.

Top-down descending facilitation of spinal sensory excitatory transmission from the anterior cingulate cortex

Spinal sensory transmission is under descending biphasic modulation, and descending facilitation is believed to contribute to chronic pain. Descending modulation from the brainstem rostral ventromedial medulla (RVM) has been the most studied, whereas little is known about direct corticospinal modulation. Here, we found that stimulation in the anterior cingulate cortex (ACC) potentiated spinal excitatory synaptic transmission and this modulation is independent of the RVM. Peripheral nerve injury enhanced the spinal synaptic transmission and occluded the ACC-spinal cord facilitation. Inhibition of ACC reduced the enhanced spinal synaptic transmission caused by nerve injury. Finally, using optogenetics, we showed that selective activation of ACC-spinal cord projecting neurons caused behavioral pain sensitization, while inhibiting the projection induced analgesic effects. Our results provide strong evidence that ACC stimulation facilitates spinal sensory excitatory transmission by a RVM-independent manner, and that such top-down facilitation may contribute to the process of chronic neuropathic pain.

It is known that descending facilitation of spinal responses may contribute to chronic pain, however many studies have focussed on brainstem mechanisms. Here the authors show that stimulation of the anterior cingulate cortex increases excitatory transmission in the dorsal horn, and that this may be via a direct pathway independent of the brainstem.

VGLUT1 or VGLUT2 mRNA-positive neurons in spinal trigeminal nucleus provide collateral projections to both the thalamus and the parabrachial nucleus in rats

The trigemino-thalamic (T-T) and trigemino-parabrachial (T-P) pathways are strongly implicated in the sensory-discriminative and affective/emotional aspects of orofacial pain, respectively. These T-T and T-P projection fibers originate from the spinal trigeminal nucleus (Vsp). We previously determined that many vesicular glutamate transporter (VGLUT1 and/or VGLUT2) mRNA-positive neurons were distributed in the Vsp of the adult rat, and most of these neurons sent their axons to the thalamus or cerebellum. However, whether VGLUT1 or VGLUT2 mRNA-positive projection neurons exist that send their axons to both the thalamus and the parabrachial nucleus (PBN) has not been reported. Thus, in the present study, dual retrograde tract tracing was used in combination with fluorescence in situ hybridization (FISH) for VGLUT1 or VGLUT2 mRNA to identify the existence of VGLUT1 or VGLUT2 mRNA neurons that send collateral projections to both the thalamus and the PBN. Neurons in the Vsp that send collateral projections to both the thalamus and the PBN were mainly VGLUT2 mRNA-positive, with a proportion of 90.3%, 93.0% and 85.4% in the oral (Vo), interpolar (Vi) and caudal (Vc) subnucleus of the Vsp, respectively. Moreover, approximately 34.0% of the collateral projection neurons in the Vc showed Fos immunopositivity after injection of formalin into the lip, and parts of calcitonin gene-related peptide (CGRP)-immunopositive axonal varicosities were in direct contact with the Vc collateral projection neurons. These results indicate that most collateral projection neurons in the Vsp, particularly in the Vc, which express mainly VGLUT2, may relay orofacial nociceptive information directly to the thalamus and PBN via axon collaterals.

Perineural capsaicin induces the uptake and transganglionic transport of choleratoxin B subunit by nociceptive C-fiber primary afferent neurons

The distribution of spinal primary afferent terminals labeled transganglionically with the choleratoxin B subunit (CTB) or its conjugates changes profoundly after perineural treatment with capsaicin. Injection of CTB conjugated with horseradish peroxidase (HRP) into an intact nerve labels somatotopically related areas in the ipsilateral dorsal horn with the exceptions of the marginal zone and the substantia gelatinosa, whereas injection of this tracer into a capsaicin-pretreated nerve also results in massive labeling of these most superficial layers of the dorsal horn. The present study was initiated to clarify the role of C-fiber primary afferent neurons in this phenomenon. In L5 dorsal root ganglia, analysis of the size frequency distribution of neurons labeled after injection of CTB-HRP into the ipsilateral sciatic nerve treated previously with capsaicin or resiniferatoxin revealed a significant increase in the proportion of small neurons. In the spinal dorsal horn, capsaicin or resiniferatoxin pretreatment resulted in intense CTB-HRP labeling of the marginal zone and the substantia gelatinosa. Electron microscopic histochemistry disclosed a dramatic, ∼10-fold increase in the proportion of CTB-HRP-labeled unmyelinated dorsal root axons following perineural capsaicin or resiniferatoxin. The present results indicate that CTB-HRP labeling of C-fiber dorsal root ganglion neurons and their central terminals after perineural treatment with vanilloid compounds may be explained by their phenotypic switch rather than a sprouting response of thick myelinated spinal afferents which, in an intact nerve, can be labeled selectively with CTB-HRP. The findings also suggest a role for GM1 ganglioside in the modulation of nociceptor function and pain.

Morphological evidence for a neurotensinergic periaqueductal gray-rostral ventromedial medulla-spinal dorsal horn descending pathway in rat

Neurotensin (NT) is an endogenous neuropeptide that exerts potent opioid-independent analgesic effects, most likely via the type 2 NT receptor (NTR2). Previous morphological and electrophysiological studies suggested that the NT-NTR2 system is primarily localized in structures that constitute the descending pain control pathway, such as the periaqueductal gray (PAG), the rostral ventromedial medulla (RVM), and the spinal dorsal horn (SDH). However, relevant morphological evidence for this neurotensinergic (NTergic) circuit is lacking. Thus, the aim of the present study was to morphologically elucidate the potential sites and connections in the NT-NTR2 system that are involved in the descending pain control pathway. Based on light and electron microscopy combined with anterograde and retrograde tracing, we found evidence that NTR2-immunoreactive (IR) neurons in the RVM receive NT-IR projections originating from the PAG; express NT, serotonin (5-HT), or both; and send projections that terminate in laminae I and II of the SDH. These results suggest that NTR2 may contribute to pain control by binding to NT in the PAG-RVM-SDH pathway. In conclusion, our data provide morphological evidence for an NTergic PAG-RVM-SDH pathway, implicating novel mechanisms of NT-induced analgesia.

Whisker-related circuitry in the trigeminal nucleus principalis: ultrastructure

Trigeminal (V) nucleus principalis (PrV) is the requisite brainstem nucleus in the whisker-to-barrel cortex model system that is widely used to reveal mechanisms of map formation and information processing. Yet, little is known of the actual PrV circuitry. In the ventral "barrelette" portion of the adult mouse PrV, relationships between V primary afferent terminals, thalamic-projecting PrV neurons, and gamma-aminobutyric acid (GABA)-ergic terminals were analyzed in the electron microscope. Primary afferents, thalamic-projecting cells, and GABAergic terminals were labeled, respectively, by Neurobiotin injections in the V ganglion, horseradish peroxidase injections in the thalamus, and postembedding immunogold histochemistry. Primary afferent terminals (Neurobiotin- and glutamate-immunoreactive) display asymmetric and multiple synapses predominantly upon the distal dendrites and spines of PrV cells that project to the thalamus. Primary afferents also synapse upon GABAergic terminals. GABAergic terminals display symmetric synapses onto primary afferent terminals, the somata and dendrites (distal, mostly) of thalamic-projecting neurons, and GABAergic dendrites. Thus, primary afferent inputs through the PrV are subject to pre- and postsynaptic GABAergic influences. As such, circuitry exists in PrV "barrelettes" for primary afferents to directly activate thalamic-projecting and inhibitory local circuit cells. The latter are synaptically associated with themselves, the primary afferents, and with the thalamic-projecting neurons. Thus, whisker-related primary afferent inputs through PrV projection neurons are pre- and postsynaptically modulated by local circuits.

Regeneration of facial nerve defects with xenogeneic acellular nerve grafts in a rat model

BACKGROUND

Because of ease of harvest and low immunogenicity, xenogeneic acellular nerve graft (XANG) may be an alternative to autologous nerve to repair facial nerve defects.

METHODS

Facial nerve defects of Wistar rats were repaired by XANG, and nerve gap regeneration was investigated by electrophysiological test, horseradish peroxidase (HRP) retrograde tracing and histomorphometric analysis, as compared to autograft.

RESULTS

Twenty weeks after the grafting, electrophysiology showed that whisker pad muscles responded to the electrical stimuli given at the site proximal to the transplantation in 2 groups. Some HRP-labeled facial motorneurons were located on the facial nucleus of the operated side, and an abundance of myelinated axons were found at the middle of the grafts and obvious motor endplates in the target muscles in 2 groups, although they were inferior to the contralateral side in numbers.

CONCLUSION

XANG represents an alternative approach for the reconstruction of peripheral facial nerve defects.

Involvement of ryanodine receptors in tetanic sciatic stimulation-induced long-term potentiation of spinal dorsal horn and persistent pain in rats

Tetanic stimulation of the sciatic nerve induces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal dorsal horn and persistent pain, suggesting that spinal LTP may be a substrate for central sensitization of the pain pathway. However, its cellular mechanism remains unclear. The present study provides electrophysiological and behavioral evidence for the involvement of ryanodine receptor (RyR) in the induction of spinal LTP and persistent pain in rats. The specific inhibitor of ryanodine receptor, ryanodine and dantrolene, dose dependently blocked the induction, but not maintenance, of spinal LTP and reduced persistent pain behaviors induced by tetanic sciatic stimulation. Both cyclic ADP ribose (cADPR), an endogenous agonist of RyR, and (±)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644), an agonist of L-type calcium channel, attenuated ryanodine-induced inhibition. Immunohistochemistry and electron microscopic observation showed that RyR subtypes RyR1 and RyR3 were located in the spinal dorsal horn. The results suggest that RyRs are involved in synaptic plasticity of the spinal pain pathway and may be a novel target for treating pain. © 2012 Wiley Periodicals, Inc.

Nociceptive afferents to the premotor neurons that send axons simultaneously to the facial and hypoglossal motoneurons by means of axon collaterals

It is well known that the brainstem premotor neurons of the facial nucleus and hypoglossal nucleus coordinate orofacial nociceptive reflex (ONR) responses. However, whether the brainstem PNs receive the nociceptive projection directly from the caudal spinal trigeminal nucleus is still kept unclear. Our present study focuses on the distribution of premotor neurons in the ONR pathways of rats and the collateral projection of the premotor neurons which are involved in the brainstem local pathways of the orofacial nociceptive reflexes of rat. Retrograde tracer Fluoro-gold (FG) or FG/tetramethylrhodamine-dextran amine (TMR-DA) were injected into the VII or/and XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the caudal spinal trigeminal nucleus (Vc). The tracing studies indicated that FG-labeled neurons receiving BDA-labeled fibers from the Vc were mainly distributed bilaterally in the parvicellular reticular formation (PCRt), dorsal and ventral medullary reticular formation (MdD, MdV), supratrigeminal nucleus (Vsup) and parabrachial nucleus (PBN) with an ipsilateral dominance. Some FG/TMR-DA double-labeled premotor neurons, which were observed bilaterally in the PCRt, MdD, dorsal part of the MdV, peri-motor nucleus regions, contacted with BDA-labeled axonal terminals and expressed c-fos protein-like immunoreactivity which induced by subcutaneous injection of formalin into the lip. After retrograde tracer wheat germ agglutinated horseradish peroxidase (WGA-HRP) was injected into VII or XII and BDA into Vc, electron microscopic study revealed that some BDA-labeled axonal terminals made mainly asymmetric synapses on the dendritic and somatic profiles of WGA-HRP-labeled premotor neurons. These data indicate that some premotor neurons could integrate the orofacial nociceptive input from the Vc and transfer these signals simultaneously to different brainstem motonuclei by axonal collaterals.

Substance P expression in the distal cerebrospinal fluid-contacting neurons and spinal trigeminal nucleus in formalin-induced the orofacial inflammatory pain in rats

The distal cerebrospinal fluid-contacting neuron (dCSF-CN) is a peculiar type of neuron whose body is in the parenchyma of the brain and processes extend into cerebrospinal fluid (CSF) in the cavity of the ventricle in the central nervous system. Because most of the dCSF-CNs are found in the dorsal raphe nucleus (DR) of the brainstem, we presume these neurons relate to pain modulation. Many experiments have demonstrated that substance P (SP) plays an important role in the development of pain and hyperalgesia. To explore whether the dCSF-CNs modulated the orofacial inflammatory pain in rats, we examined behavioral changes with the orofacial formalin test and SP-expression in the dCSF-CNs and spinal trigeminal nucleus (STN) with dual immunohistochemical labeling for light microscopy or immunoelectron microscopy. It is revealed that 2h after 2.5% formalin injection SP-expression in the dCSF-CNs and STN is upregulated significantly, which may be considered an important role for nociceptive processing.

Nigrostriatal neurons in rat express the glial cell line-derived neurotrophic factor receptor subunit c-RET

The substantia nigra neurons expressing c-RET, a glial cell line-derived neurotrophic factor (GDNF) receptor intracellular tyrosine kinase subunit, were investigated in rats by using a double labeling method which combined retrograde horseradish peroxidase (HRP) labeling after injection into the striatum with immunohistochemistry to c-RET. It was revealed that the distribution of c-RET-immunoreactive neurons and HRP-labeled nigrostriatal neurons overlapped. Numerous double-labeled HRP/c-RET neurons were found in the substantia nigra pars compacta with predominate distribution ipsilateral to the injected striatum. Semiquantitative cell count indicated that a large percentage (97%) of HRP-labeled neurons showed c-RET immunoreactivity. Furthermore, double-labeled HRP/c-RET ones constituted only 61% of total c-RET-immunoreactive neurons in the substantia nigra ipsolateral to the injected striatum. Taken together with previous observations on glial cell line-derived neurotrophic factor in the basal ganglia, this study provides evidence that the c-RET protein may mediate biological activity of GDNF family ligands in most of projecting neurons in the substantia nigra pars compacta where the dopaminergic neurons are numerously distributed. Specially, it suggests that c-RET-mediating signaling cascades may play important roles in neuron-glial interaction that support and sustain nigrostriatal neuronal circuits in the basal ganglia.

The methodology for labeling the distal cerebrospinal fluid-contacting neurons in rats

The distal cerebrospinal fluid-contacting neurons (dCSF-CNs) are one special type of neurons, whose bodies are in the parenchyma of the brain and processes extend into the ventricle, which suggested that dCSF-CNs might play the important roles in neuromodulation and neuroendocrinal regulation. Without special tracing method, the dCSF-CNs is hard to identify in parenchyma of the brain. Thus, up to present, these structure and function are seldom investigated. To explore dCSF-CNs, the cholera toxin subunit B conjugated with horseradish peroxidase (CB-HRP) as a tracer was injected into one of the rats' lateral ventricles, 48 h later following tracer injection rats were deeply anesthetized and fixed by intracardiac perfusion of 450 ml fixative at 25-30 ml/min. The brainstem were dissected out and stored in the fixative (postfix) for 4-6 h at 4 degrees C, then cryoprotected by immersion for 24-48 h in gradient sucrose (5%, 10%, 15%, 20%, and 30%) with 0.01 mol/l PBS at 4 degrees C. The brainstem segment was embedded, transverse or horizontal sections were cut with a cryostat and collected in phosphate-buffered saline (PBS). Three methodology were described to label the dCSF-CNs including tetramethylbenzidine-sodium tungstate (TMB-ST) method, CB-HRP-immunofluorescence procedures and CB-HRP-immunoelectron microscopic procedures.

The distributions and signaling directions of the cerebrospinal fluid contacting neurons in the parenchyma of a rat brain

Many studies have been made on the distributions of CSF contacting neurons (CSF-CNs) in the parenchyma of the brain with horseradish peroxidase (HRP) or autoradiographics. A significant amount of data has shown that both HRP and autoradiographical substances could pass freely through the spaces of ependyma into the parenchyma of the brain. It is therefore possible that the results were not exact. We found that CB-HRP was a dependable tracer to CSF-CNs and studied the distributions and the signaling directions of cerebrospinal fluid contacting neurons (CSF-CNs) in the parenchyma of the brain with the cholera toxin subunit B with horseradish peroxidase (CB-HRP) tracing combined with transmission electron microscopy. The results were as follows: (1) CSF contacting tanycytes existed not only in the wall of the third ventricle (3V), but also in the walls of the lateral ventricle (LV), the fourth ventricle (4V) and the central canal (CC) of the spinal cord. (2) Some CSF contacting glia cells were observed in the lateral septal nucleus (LS). (3)The distal CSF-CNs in the parenchyma were found in LS, the anterodorsal thalamic nucleus (AD), the supramammillary nucleus (SuM), the dorsal raphe nucleus (DR), the floor of 4V and the lateral superior olive (LSO), but they were mainly found in DR and divided into groups A and B. (4) Axon terminals labeled by CB-HRP were found in the cavity of the brain ventricle. (5) The synaptic relationships between the neurons were labeled by CB-HRP in DR and no-labeled by CB-HRP in the parenchyma. Both synapses Gray I and II were found. It was significant that the presynaptic elements were formed by the neurons no-labeled CB-HRP and the postsynaptic elements labeled CB-HRP. Our results suggested firstly that the signaling directions of CSF-CNs in DR were only from the parenchyma to CSF.

Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

Synaptological characteristics of synapses between axonal boutons of the trigeminal mesencephalic nucleus (Vme) neurons and the hypoglossal nucleus (XII) motoneurons (MNs) were studied using biotinylated dextran amine (BDA) anterograde labeling combined with horseradish peroxidase (HRP) retrograde transport in the rat. BDA was initially iontophoresed into Vme unilaterally and 7 days later HRP was injected into the anterior two-thirds of the ipsilateral tongue. After histochemical reactions, BDA anterogradely labeled boutons were seen to appose closely to somata and dendrites of HRP retrogradely labeled MNs in XII by light microscopy. A total of 212 BDA-labeled Vme boutons were examined ultrastructurally, which had an average diameter of 1.3 +/- 0.4 microm and contain small clear spherical vesicles. Eighty-eight percent of Vme boutons (187/212) synapsed on dendrites of HRP-labeled XII MNs. Twenty-five Vme boutons (25/212, 12%) made synapses with somata of HRP-labeled XII MNs. Thirty-five percent (74/212) of BDA-labeled Vme boutons were also contacted by unlabeled P-type terminals. Presynaptic P-type terminals contained spherical (47%, 35/74), pleomorphic (43%, 32/74), and flattened (10%, 7/74) synaptic vesicles. Thus, P-type terminals (as a presynaptic element), BDA-labeled Vme boutons, and XII MNs constitute axoaxodendritic and axoaxosomatic synaptic triads. There are four types of synaptic microcircuits in XII neuropil: synaptic convergence, synaptic divergence, presynaptic inhibition synaptic circuits, and feedforward regulation circuits. This detailed ultrastructure examination of the synaptic organization between Vme neurons and XII MNs provides insights into the synaptic mechanisms of the trigeminal proprioceptive afferents involved in the jaw-tongue reflex and coordination during oral motor behaviors.

Transganglionic transport of choleragenoid by capsaicin-sensitive C-fibre afferents to the substantia gelatinosa of the spinal dorsal horn after peripheral nerve section

Choleratoxin B subunit-binding thick myelinated, A-fibre and unmyelinated, capsaicin-sensitive nociceptive C-fibre primary afferent fibres terminate in a strict topographic and somatotopic manner in the spinal cord dorsal horn. Injection of choleratoxin B subunit-horseradish peroxidase conjugate into injured but not intact peripheral nerves produced transganglionic labelling of primary afferents not only in the deeper layers (Rexed's laminae III-IV), but also in the substantia gelatinosa (Rexed's laminae II) of the spinal dorsal horn. This was interpreted in terms of a sprouting response of the Abeta-myelinated afferents and suggested a contribution to the pathogenesis of neuropathic pain [Nature 355 (1992) 75; J Comp Neurol 360 (1995) 121]. By utilising the selective neurotoxic effect of capsaicin, we examined the role of C-fibre sensory ganglion neurons in the mechanism of this phenomenon. Elimination of these particular, capsaicin-sensitive C-fibre afferents by prior intrathecal or systemic capsaicin treatment inhibited transganglionic labelling by the choleratoxin B subunit-horseradish peroxidase conjugate of the substantia gelatinosa evoked by chronic sciatic nerve section. More importantly, prior perineural capsaicin treatment of the transected nerve proximal to the anticipated site of injection of choleragenoid 12 hours later prevented the labelling of the substantia gelatinosa, but not that of the deeper layers. Electron microscopic examination of the dorsal roots revealed no significant difference in the proportion of labelled myelinated fibres relating to the intact (54.4+/-5.5%) and the transected (62.4+/-5.4%) sciatic nerves. In contrast, the proportion of labelled unmyelinated dorsal root axons relating to the transected, but not the intact nerves showed a significant, six-fold increase after sciatic nerve transection (intact: 4.9+/-1.3%; transected: 35+/-6.7%). These observations indicate that peripheral nerve lesion-induced transganglionic labelling of the substantia gelatinosa by choleratoxin B subunit-horseradish peroxidase may be primarily accounted for by the uptake and transganglionic transport of choleragenoid by injured capsaicin-sensitive C-fibre afferents rather than a sprouting response of A-fibre afferents. The present findings suggest an essential role of capsaicin-sensitive primary sensory neurons in lesion-induced spinal neuroplastic changes and provide further support for C-fibre nociceptor neurons being promising targets for the development of new strategies in pain management.

The supratrigeminal region of the rat sends GABA/glycine-cocontaining axon terminals to the motor trigeminal nucleus on the contralateral side

The supratrigeminal region (STR), a reticular zone capping the motor trigeminal nucleus (Tm), contains gamma-aminobutyric acid (GABA)ergic and glycinergic neurons which send axons to the contralateral Tm (J. Comp. Neurol. 373 (1996) 498). In the present study we observed that some single synaptic terminals upon Tm motoneurons showed immunoreactivities (IRs) for both glutamic acid decarboxylase (GAD) and glycine transporter 2 (GlyT2). After injecting biotinylated dextran amine (BDA) into the STR, we further observed in the Tm contralateral to the BDA injection that some BDA-labeled axon terminals in close contact with Tm motoneurons showed both GAD- and GlyT2-IRs. Thus, the STR was indicated to send GABAergic/glycinergic axon terminals contralaterally to Tm motoneurons.

Distribution of nestin immunoreactivity in the normal adult human forebrain

The distribution of nestin immunoreactivity was studied in the whole normal adult human forebrains using new anti-human nestin mouse monoclonal and rabbit polyclonal antiserum. The nestin immunoreactive cells could be divided into three types according to their morphological characteristics. The first type contained neuron-like nestin immunoreactive cells, distributed in CA1-3 of hippocampus, septum, the nucleus of diagonal band, amygdala and basal nucleus of Meynert. The second type contained astrocyte-like cells, distributed in the subependymal zone and subgranular layer of dentate gyrus. The third type of cells had smaller cell bodies and fewer processes, also distributed in the subependymal zone and subgranular layer of dentate gyrus. Double immunohistochemical staining showed that the nestin positive, neuron-like cells in the nucleus of diagonal band and hippocampus also expressed NSE. However, the astrocyte-like nestin immunoreactive cells of the subependymal zone and subgranular layer of dentate gyrus were not double labeled with GFAP. Although some nestin immunoreactive fibers were distributed in the infundibulum, no nestin-immunoreactive cells were detected in the cortex. These data indicate that nestin exist in the adult human brain outside of the subependymal zone and dentate gyrus and also implies that nestin-immunoreactive cells may play a role in the modulation of basal forebrain function.

Synaptic connections between trigemino-parabrachial projection neurons and gamma-aminobutyric acid- and glycine-immunoreactive terminals in the rat

The synaptic connections between gamma-aminobutyric acid (GABA)- and glycine-immunoreactive terminals and neurons projecting to the lateral parabrachial region were examined by a combination of retrograde tracing and immunohistochemical staining in the rat medullary dorsal horn. After injection of horseradish peroxidase (HRP) into the right lateral parabrachial region, HRP retrogradely labeled neurons were observed bilaterally in laminae I, II and III of the medullary dorsal horn with an ipsilateral predominance. GABA- and glycine-like immunoreactive terminals were found in laminae I, II and III. Some of these GABA- and glycine-like immunoreactive terminals were observed chiefly to make symmetric synapses with HRP-labeled neuronal cell bodies and dendritic processes. The present results indicate that neurons in the medullary dorsal horn projecting to the lateral parabrachial region might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons, which might be significantly involved in the regulation of the noxious information transmission.

Light and electron microscopic observations of a direct projection from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

A direct projection from rat mesencephalic trigeminal nucleus (Vme) neurons to the hypoglossal nucleus (XII) motoneurons was studied using a double labeling method of anterogradely biotinylated dextran amine (BDA) tracing combined with retrogradely horseradish peroxidase (HRP) transport at both light and electron microscopic levels. BDA was iontophoresed unilaterally into the caudal Vme, and 7 days later HRP was injected into the ipsilateral tongue to label hypoglossal motoneurons. The BDA-labeled fibers were seen descended along Probst' tract and were traced to the caudal medulla. In this course, the fibers gave off axon collaterals bearing varicosities in the trigeminal motor nucleus (Vmo), the parvicellular reticular formation (PCRt), the dorsomedial portions of the subnuclei of oralis (Vodm) and interpolaris (Vidm) and in the XII ipsilaterally. The labeling of terminals was most dense in the PCRt at the levels of caudal pons and rostral medulla, which displayed a "dumbbell-shaped" form in the transverse planes. In the XII, labeled terminals were distributed mainly in the dorsal compartment of the nucleus. One hundred sixty-eight appositions made by BDA-labeled terminals on HRP-labeled motoneurons were seen in the dorsal compartment (71%) and in the lateral subcompartment (24%) of the ventral XII. Under electron microscopy BDA-labeled boutons containing clear, spherical synaptic vesicles were found to form synaptic contacts with the somata and dendrites of hypoglossal motoneurons with asymmetric specializations. The present study provides new evidence that the trigeminal proprioceptive afferent neurons terminate in the XII and make synaptic contacts with their motoneurons.

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.

Cross-modal plasticity of the corticothalamic circuits in rats enucleated on the first postnatal day

Reorganization of the reciprocal corticothalamic connections was studied as a possible anatomical substrate of the cross-modal compensation of the missing visual input of the visual cortex by somatosensory-evoked activities in neonatally enucleated rats. The use of quantitative retrograde tract-tracing techniques revealed that the contribution of the lateral posterior thalamic nucleus (LP) is significantly increased following enucleation, while that of the dorsolateral geniculate and the lateral dorsal nuclei is decreased in the thalamocortical afferentation of a region in visual cortical area 17. In contrast with the control rats, a dense terminal arborization of afferents was labelled in the LP after the injection of anterograde tracer into the barrel cortex of the enucleated rats. The injection of anterograde tracer into the visual cortex also demonstrated a massive afferentation into the LP of the enucleated rats. Visual and somatosensory corticothalamic afferents exhibited similar ultrastructural features in the LP after enucleation, but their synaptic organizations differed as regards the diameter of the postsynaptic dendrites. Taken together with the previous observations, these results suggest a central role for the LP in the transmission of the somatosensory-evoked activities to the visual cortex after early blindness.

Possible routes of visceral information in the rat brain in formation of conditioned taste aversion

When ingestion of a taste stimulus is paired with internal malaise, the animal remembers the taste and rejects its ingestion thereafter. This learning is referred to as conditioned taste aversion (CTA). To establish CTA in adult male Wistar rats, 0.1% saccharin and an i.p. injection of 0.15 M LiCl were used as the conditioned and unconditioned stimuli, respectively. Neuroanatomical study using the tracer method was performed to examine the ascending routes from the lateral part of the parabrachial nucleus (PBlat) which receives general visceral information and suggested the three possible routes to the amygdala: (1) direct route to the central nucleus of the amygdala (CeA); (2) diencephalic route to the basolateral nucleus of the amygdala (BLA) involving the zona incerta (ZI) and the midline and intralaminar thalamic complex (MITC); and (3) cortical route to the BLA involving insular cortex (IC). Rats with excitotoxic lesions of each of the CeA, ZI, MITC or IC had only a small or negligible effect on the acquisition of CTA. However, single lesions of the BLA and combined lesions of the ZI and IC, but not CeA and IC, almost completely abolished the acquisition of CTA. These results together with previous findings suggest that visceral (or unconditioned stimulus) information in the PBlat is sent to the BLA which is essential for the acquisition of CTA via the functionally important two parallel routes, the diencephalic and cortical routes, with either being able to create the aversion.

Direct projections from the medial preoptic area to spinally-projecting neurons in Barrington's nucleus: an electron microscope study in the rat

Direct projections from the medial preoptic area (MPO) to the pontine micturition center neurons directly projecting to the lumbosacral spinal cord were revealed electron microscopically in the rat by a double labeling method. Biotinylated dextran amine (BDA) was injected into the MPO and horseradish peroxidase (HRP) was injected into the lumbosacral cord segments. At light microscopic level, BDA-labeled presumptive axon terminals completely overlapped with HRP-labeled neurons in Barrington's nucleus. Electron microscopic observation showed that some BDA-labeled axon terminals made synaptic contacts with dendrites of HRP-labeled neurons in Barrington's nucleus. The present results indicated that the MPO may be involved in the modulation of the pontine micturition reflex in the rat.

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.

Ultrastructure of glutamate and GABA immunoreactive axon terminals of the rat nucleus tractus solitarius, with a note on infralimbic cortex afferents

The principal fast neurotransmitters in the CNS are glutamate and GABA. Our aim was to provide a baseline account on the ultrastructure of the axon terminals immunoreactive to glutamate or GABA present in the nucleus tractus solitarius (NTS) of the rat. In addition, we wanted to complete our study of cortico-solitary afferents at the electron microscopic level, by analyzing the inputs from the infralimbic cortex. Using post-embedding immunogold, we found that nearly 61% of the axon terminals were glutamatergic, and 36% were GABAergic in the rat visceral NTS. In general, axons making asymmetric synaptic contacts were enriched in glutamate, compared to axons involved in symmetric synapses. In contrast, the vast majority of the GABAergic axon terminals made symmetric synaptic contacts. We could discern five types of glutamatergic and two types of GABAergic axon terminals that differed in their fine structure. Afferents from the infralimbic cortex were small, with clear synaptic vesicles and no dense core vesicles; they made asymmetric contacts with fine dendrites, and were glutamatergic. We conclude that most axon terminals in the NTS use glutamate or GABA as fast transmitters, in addition to being a heterogeneous population of morphological types.

Projections from the caudal spinal trigeminal nucleus to commissural interneurons in the supratrigeminal region: an electron microscope study in the rat

Electron microscopic double-labeling study in the rat indicated that projection fibers from the caudal spinal trigeminal nucleus (Vc) were distributed ipsilaterally within the supratrigeminal region (STR) capping the trigeminal motor nucleus (Tm) and made synaptic contact with neurons projecting to the contralateral Tm. Nociceptive inputs to the Vc may reflexly control, via interneurons in the STR, the activities of Tm neurons innervating the masticatory, tensor tympani, and/or tensor veli palatine muscles.

Contralateral cortical projection to the mediodorsal thalamic nucleus: origin and synaptic organization in the rat

The origin of the corticothalamic projections to the contralateral mediodorsal nucleus, the collateralization of cortical fibers and their synaptic organization in the ipsi- and contralateral mediodorsal nuclei were investigated in adult rats with double retrograde fluorescent and anterograde tracing. After tracer injections in the mediodorsal nuclei on either side, neurons were retrogradely labeled in all the areas of the contralateral prefrontal cortex in which ipsilateral labeling was also observed. Contralateral corticothalamic cells accounted for 15% of the labeled neurons in the orbital and agranular insular areas, while their proportion was lower (3%) in the anterior cingulate cortex. Up to 70% of the contralateral cortical neurons were double labeled by bilateral injections in the mediodorsal nuclei. At the electron microscopic level, unilateral injections of biotinylated dextran-amine in the orbitofrontal cortex resulted in anterograde labeling of small terminals and a few large boutons in the ipsilateral mediodorsal nucleus, while only small boutons were identified contralaterally. The diameter of postsynaptic dendritic profiles contacted by labeled small cortical endings was significantly larger in the ipsilateral mediodorsal nucleus than contralaterally. These findings demonstrate that dense contralateral cortical projections to the mediodorsal nucleus derive from the orbital and agranular insular areas, and that crossed corticothalamic afferents are mostly formed by collaterals of the ipsilateral connections. Our observations also point out the heterogeneity of corticothalamic boutons in the rat mediodorsal nucleus and morphological differences in the synaptic organization of prefrontal fibers innervating the two sides, indicating that ipsilateral cortical afferents may be more proximally distributed than crossed cortical fibers on dendrites of mediodorsal neurons.

Direct projections from the periaqueductal gray to pontine micturition center neurons projecting to the lumbosacral cord segments: an electron microscopic study in the rat

Direct projections from the periaqueductal gray (PAG) to the pontine micturition center neurons directly projecting to the lumbosacral cord segments were observed electron microscopically in the rat by a double labeling method. Biotinylated dextran amine (BDA) was injected into the PAG and horseradish peroxidase (HRP) was injected into the lumbosacral cord segments. After injection of BDA into the ventrolateral part of the PAG, many BDA-labeled axons were seen light microscopically in Barrington's nucleus; a moderate number of them were found in the pontine tegmental region just ventral to Barrington's nucleus (D-region [Ding, Y-Q., Takada, M., Tokuno, H. and Mizuno, N., J. Comp. Neurol., 357 (1996) 318-330]). On the other hand, after injection of BDA into the dorsolateral part of the PAG, only a few BDA-labeled axons were seen in Barrington's nucleus or the D-region. BDA-labeled axon terminals were electron microscopically confirmed to be in synaptic contact with HRP-labeled dendrites and somata in Barrington's nucleus and the D-region. The results indicate that the ventrolateral part of the PAG is implicated in regulation of the micturition reflex.

Tract-tracing in the nervous system of vertebrates using horseradish peroxidase and its conjugates: tracers, chromogens and stabilization for light and electron microscopy

Surprisingly the first generation of tract-tracing techniques based on intra-axonal transport, that is the methods utilizing the uptake and transport of horseradish peroxidase (HRP), still rank among the most widely used neuroanatomical tracing techniques. The success of these methods can be ascribed to several characteristics. They are fast and easy to implement. Complicated injection apparatus is unnecessary. The reaction products are visualized through simple histochemical reactions, and they are permanent or can be stabilized into permanency. Most usefully, the reaction products are visible in the light and electron microscopes. HRP (mol. wt. 44 kDa) is extracted from the roots of the horseradish plant (Cochlearia armoracia L.). Uptake of HRP into cells occurs via a passive process of endocytosis. Since lectins like wheat germ agglutinin (WGA) and bacterial toxin fragments (subunit B of cholera toxin (CTB)) strongly induce active, receptor-mediated uptake mechanisms, conjugates of these substances with HRP have been successfully applied in sensitive tract-tracing HRP and its conjugates are transported both in anterograde and retrograde direction. Retrograde transport occurs in small vesicles that are incorporated in lysosome-like vacuoles and in the Golgi apparatus. These vesicles differ in membrane properties from the anterogradely transported HRP vesicles. The retrogradely transported vesicles tend to fuse and thus accumulate HRP at high densities, facilitating the visibility of the final reaction product. The anterogradely transported HRP does not accumulate directly in lysosome-like bodies and is distributed diffusely and therefore often requires specific visualization methods. HRP and WGA-HRP may therefore be used in anterograde and retrograde transneuronal (multineuron) transport studies. Even in fixed material, labeling through diffusion of HRP can provide details on neural connections. Visualization of transported HRP is achieved by means of using the oxidative enzymatic activity of HRP to precipitate a chromogen according to the following reaction: [symbol: see text] The final reaction product may be soluble in buffer or ethanol and may require stabilization to prevent fading. In this protocol we discuss the widely used chromogens 3,3'-diaminobenzidine tetrahydrochloride (DAB), 3,3',5,5'-tetramethylbenzidine (TMB), benzidine dihydrochloride (BDHC) and p-phenylenediamine dihydrochloride with pyrocatechol (PPD-PC). Other possible chromogens, not discussed here, are 4-chloro-1-naphthol (4C1N), 3-amino-9-ethylcarbazole (AEC) and o-phenylenediamine (OPD). The visualization of the reaction product can be further improved by intensification with metal salts. At the light microscopic level (LM) this intensification enables color differentiation between distinct markers. In the present protocol we provide an up-to-date guideline for the application of HRP and its conjugates in tracing with special emphasis on electron microscopic (EM) visualization. Some modifications for stabilization and of metal intensification to enhance visibility are incorporated in conjunction with specific methods for multiple labeling in combined tract-tracing experiments.

Glutamate immunoreactivity of insular cortex afferents to the nucleus tractus solitarius in the rat: a quantitative electron microscopic study

Corticosolitary axons and their terminals were labeled by the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, after injections into the rat insular cortex. The ultrastructure of these cortical afferents was analysed in the medial and commissural subnuclei of the nucleus tractus solitarius. Cortical terminals had a mean area of 0.36 microns 2, and were among the smallest terminals in the nucleus. They made single, asymmetric synaptic contacts with thin dendritic stems or with spines. The average diameter of the dendrites postsynaptic to cortical axons was 0.59 microns, and significantly smaller (P < 0.01, Kolmogorov-Smirnov test) than the mean (0.87 microns) of the population of dendrites in the same region of the nucleus tractus solitarius. Cortical boutons contained closely packed round and clear synaptic vesicles of diameter ca. 28 nm, a few mitochondria, and no dense core vesicles. Postembedding immunogold analysis showed that the anterogradely labeled cortical axon terminals were immunoreactive to glutamate, but not to GABA. Cortical afferents had on average four times the glutamate immunoreactivity (assessed by gold particle density) than local dendrites or terminals making symmetric synaptic contacts. Similarly, most of the unlabeled axon terminals participating in asymmetric synaptic contacts were highly enriched in glutamate immunoreactivity, suggesting that glutamate may be a most prevalent transmitter in the nucleus tractus solitarius. Terminals immunoreactive to GABA always made symmetric synapses, mostly with dendritic shafts and perikarya. We concluded that insular cortex axons made single, asymmetric synaptic contacts with thin, probably distal dendrites in the nucleus tractus solitarius. Cortical terminals are immunoreactive to glutamate, and morphologically different from primary afferents and from terminals immunoreactive to GABA.

Morphological evidence for recurrent jaw-muscle spindle afferent feedback within the mesencephalic trigeminal nucleus

Horseradish peroxidase was injected into the jaw-elevator muscles of rats. Jaw-muscle spindle afferent axons were then intracellularly stained with biotinamide. Eleven intracellularly stained somata were closely apposed by intracellularly labeled spindle afferent boutons. Spindle afferent boutons closely apposed another 10 mesencephalic trigeminal nucleus (Vme) perikarya labeled from the jaw muscles. These results indicate that an anatomical substrate exists for recurrent feedback between jaw-muscle spindle afferents within Vme.

Transneuronal transport of intracellularly injected biotinamide in primary afferent axons

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

Ultrastructural basis for synaptic transmission between jaw-muscle spindle afferents and trigeminothalamic neurons in the rostral trigeminal sensory nuclei of the rat

Trigeminothalamic neurons were retrogradely labeled by injection of horseradish peroxidase into the ventroposteromedial nucleus of the thalamus in rats. Jaw-muscle spindle afferent axons were then physiologically identified and intracellularly stained with biotinamide. The ultrastructure of labeled spindle afferent boutons was then studied in the caudolateral supratrigeminal region (Vsup) and dorsomedial trigeminal principal sensory nucleus (Vpdm). A total of 418 stained spindle afferent boutons were identified in Vsup and Vpdm; approximately 75% of these synapsed with dendrites, 10% synapsed with somata, and 15% synapsed with axons. Most jaw-muscle spindle afferent boutons were postsynaptic to unlabeled P-type boutons. Reciprocal synapses between spindle afferent boutons and unlabeled boutons were occasionally observed. A few dendrites in Vsup and Vpdm received synapses from multiple spindle afferent boutons. Conversely, some large (from 3 x 6 to 4 x 8 microns) and giant (from > 4 x 8 to 5 x 10 microns) spindle afferent boutons simultaneously contacted two to five dendrites and/or somata. Jaw-muscle spindle afferent boutons also formed synapses with retrogradely labeled trigeminothalamic neurons in Vsup and Vpdm. Numerous unlabeled S-and F-type boutons converged onto the same trigeminothalamic dendrite or soma contacted by a spindle afferent bouton. A small number of synaptic triads consisting of an unlabeled P-type bouton, a spindle afferent bouton, and either a dendrite or soma were also encountered. These data indicate that sensory feedback from the masticatory muscles is subject to presynaptic inhibition and integration prior to reaching the thalamus. This pathway is likely to be important in the relay of proprioceptive and kinesthetic information from the muscles of mastication to the thalamus.

Catecholaminergic neurons in the nucleus tractus solitarii which send their axons to the midbrain periaqueductal gray express Fos protein after noxious stimulation of the stomach: a triple labeling study in the rat

In rats which were injected with horseradish peroxidase (HRP) into the midbrain periaqueductal gray (PAG) and then administered with formaldehyde into the stomach, Fos-like immunoreactivity was found in tyrosine hydroxylase-like immunoreactive neurons in the nucleus tractus solitarii (NTS) which were retrogradely labeled with HRP. The results indicate that catecholaminergic NTS neurons may mediate nociceptive visceral information to the PAG.

A modification of biotinylated dextran amine histochemistry for labeling the developing mammalian brain

Biotinylated dextran amine (BDA) has proven to be an excellent anterograde tracer in adult mammalian brains, having some advantages over other anterograde tracers such as Phaseolus vulgaris-leucoagglutinin (PHA-L) and biocytin. However, results are inferior when BDA is used in neonatal mammals. To improve the sensitivity and quality of BDA labeling in neonatal mammalian brains, the tetramethylbenzidine-sodium tungstate (TMB-ST) method for horseradish peroxidase (HRP) histochemistry was modified and used in BDA histochemistry. After BDA application to the visual cortex of neonatal rat and cat, contralateral and ipsilateral cortical and subcortical regions were examined for BDA-labeled exons and terminals. The modified BDA histochemistry produced corpus callosum (CC) axons in neonatal rat and cat that were heavily and continuously labeled. The distribution, trajectories, branching and termination of individual CC axons, and even possible axon-axon contracts, were clearly identified in exquisite detail, even at low magnification. The quality of BDA labeling in the ipsilateral lateral geniculate nucleus and superior colliculus was similar to that of the CC axonal labeling. These results indicate that the modified BDA histochemistry provides a very sensitive and reliable approach to revealing the detailed distribution and morphology of projecting axons and terminals in the developing mammalian nervous system.

Inputs from identified jaw-muscle spindle afferents to trigeminothalamic neurons in the rat: a double-labeling study using retrograde HRP and intracellular biotinamide

Projections from physiologically identified jaw-muscle spindle afferents onto trigeminothalamic neurons were studied in the rat. Trigeminothalamic neurons were identified by means of retrograde transport of horseradish peroxidase from the ventroposteromedial nucleus of the thalamus. Labeled neurons were found contralaterally in the supratrigeminal region (Vsup), the trigeminal principal sensory nucleus, the ventrolateral part of the trigeminal subnucleus oralis, the spinal trigeminal subnuclei interpolaris and caudalis, the reticular formation, and an area ventral to the trigeminal motor nucleus (Vmo) and medial to the trigeminal principal sensory nucleus (AVM). Jaw-muscle spindle afferents were physiologically identified by their increased firing during stretching of the jaw muscles and intracellularly injected with biotinamide. Axon collaterals and boutons from jaw-muscle spindle afferents were found in Vmo; Vsup; the dorsomedial part of the trigeminal principal sensory nucleus (Vpdm); the dorsomedial part of the spinal trigeminal subnuclei oralis, interpolaris (Vidm) and caudalis; the parvicellular reticular formation (PCRt); and the mesencephalic trigeminal nucleus. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, PCRt, and AVM were associated with axon collaterals and boutons from intracellularly stained jaw-muscle spindle afferents. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, and PCRt were closely apposed by one to 14 intracellularly labeled boutons from jaw-muscle spindle afferents, suggesting a powerful input to some trigeminothalamic neurons. These data demonstrate that muscle length and velocity feedback from jaw-muscle spindle afferents is projected to the contralateral thalamus via multiple regions of the trigeminal system and implicates these pathways in the projection of trigeminal proprioceptive information to the cerebral cortex.

Ultrastructure of calcitonin gene-related peptide-immunoreactive, unmyelinated afferents to the cat carotid body: a case of volume transmission

To relate the ultrastructure of unmyelinated afferents to the cat carotid body with the known electrophysiological properties of cat chemosensory C-fibers, we took advantage of the fact that the calcitonin gene-related peptide is exclusively present in a population of sparsely branched afferents to the carotid body. They have a morphology identical to the afferents originating from carotid sinus nerve unmyelinated axons. Immunoreactive axons were stained using pre-embedding protocols and horseradish peroxidase-labeled secondary antibody. Labeling was present only in unmyelinated axons and boutons distributed in the interstitial and parenchymal tissue. The varicosities had an average diameter of 0.7 micron, and contained both small, clear vesicles and larger dense-core vesicles. No labeled axons were ever seen to contact glomus cells, but could be observed as close as 0.2 micron to a glomus cell, always with an interposed glial process. With a very sensitive protocol, that used tungstate-stabilized tetramethylbenzidine as the chromogen, amorphous deposits of reaction product were often detected in the extracellular space around a labeled bouton. We interpret these findings as indicating that the reciprocal chemical transmission between the oxygen-sensitive glomus cells and the unmyelinated afferents takes place through non-synaptic transmission, via the rather large extracellular space of the carotid body. In addition, the larger distances between glomus cells and unmyelinated afferents could explain the lowered sensitivity and sluggishness of chemosensory C-fibers, compared to the A-fibers.

Confirmation of the existence of transitory corpus callosum axons in area 17 of neonatal cat: an anterograde tracing study using biotinylated dextran amine

Corpus callosum (CC) axons in visual cortex were labeled anterogradely by in vivo biotinylated dextran amine (BDA) in neonatal cat at postnatal day (PND) 6, 10 and 15. Labeled CC axons were distributed throughout the visual cortex including medial area 17. The number of CC axons in medial area 17 increased from PND 6 to PND 10, and then decreased from PND 10 to PND 15. At PND 15, few CC axons could be followed into the grey matter in medial area 17. Thus, BDA labels transitory CC axons that extend through all cortical layers in medial area 17, confirming the results revealed by in vitro DiI labeling.

Multicolour preparations for in situ hybridization using precipitating enzyme cytochemistry in combination with reflection contrast microscopy

We have further developed a method for the detection of different enzyme cytochemical reaction products by means of reflection contrast microscopy (RCM). By embedding these enzyme precipitates in a protein matrix, we were able to prevent the reaction products from dissolving in immersion oil, which is required for RCM analysis. The applicability of the RCM procedure is, therefore, extended to a range of cytochemical enzyme precipitation methods, which normally result in oil soluble reaction products. To test their usefulness, these enzyme precipitates have been used in single- and well as double-label in situ hybridization (ISH) procedures to visualize a number of DNA target sequences by several different reflection colours, i.e. white, yellow and red. Three repetitive DNA probes for the (sub)centromeric regions of chromosomes 1, 7 and 17, as well as a repetitive DNA probe for the telomeric region of chromosome 1, and two cosmid DNA probes (40 kb each) for both arms of chromosome 11 could be detected with high efficiency in both interphase and metaphase preparations. Moreover the enzyme precipitates were shown to be stable upon exposure to excitation light or upon storage. It may be concluded that these findings render RCM a sensitive method for the visualization of multiple targets in biological specimens.

Demonstration of habenular neurons which receive afferent fibers from the nucleus accumbens and send their axons to the midbrain periaqueductal gray

After injecting Phaseolus vulgaris leucoagglutinin (PHA-L) and wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP), respectively, into the medial part of the nucleus accumbens (NA) and the midbrain periaqueductal gray including the dorsal raphe nucleus (PAG/DR) on one side of single rat brains, we observed that axon terminals labeled anterogradely with PHA-L made possible synaptic contact with neurons in the habenula (Hb) which were retrogradely labeled with WGA-HRP. These Hb neurons were distributed in the medial part of the lateral Hb. The results indicate that the medial part of the lateral Hb relays neuronal information from the medial part of the NA to the PAG/DR.

Demonstration of axon terminals of projection fibers from the periaqueductal gray onto neurons in the nucleus raphe magnus which send their axons to the trigeminal sensory nuclei

Axon terminals of projection fibers from the periaqueductal gray (PAG) onto neurons in the nucleus raphe magnus (NRM) sending their axons to the principal sensory trigeminal nucleus (Vp) or caudal spinal trigeminal nucleus (Vc) were demonstrated light microscopically: In the rats which were injected with Phaseolus vulgaris leucoagglutinin (PHA-L) and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) respectively into the PAG and trigeminal sensory complex (Vp or Vc), presumed axon terminals labeled anterogradely with PHA-L appeared to make synaptic contacts with NRM neurons labeled retrogradely with WGA-HRP.