Changes in neurons of medulla oblongata nuclei under conditions of chronic NO-synthase inhibition.

Nucleus tractus solitarius and giant-cell and lateral reticular nuclei were studied using the reaction to NADPH-diaphorase in 7-, 10-, 14-, 30-, 45-, 60-day-and 3- and 6-month-old rats receiving L-NAME (50 µg/kg, 2 times a day) on days 1-6 of life. In 7-14-day-old rats, the compound reduced NO-synthase activity in the majority of NO-neurons and the total number and to a lesser degree the relative number of these neurons, while cell cross-section areas remained practically unchanged. The differences in the corresponding quantitative parameters between the control (D-NAME administration) and experimental groups decreased with time after the last L-NAME injection and became undetectable starting from the age of 30-45 days. In the nucleus tractus solitarius, the changes in metric parameters after exposure to NO-synthase inhibitor were more pronounced than in the reticular formation nuclei.

The macaque midbrain reticular formation sends side-specific feedback to the superior colliculus.

The central mesencephalic reticular formation (cMRF) likely plays a role in gaze control, as cMRF neurons receive tectal input and provide a bilateral projection back to the superior colliculus (SC). We examined the important question of whether this feedback is excitatory or inhibitory. Biotinylated dextran amine (BDA) was injected into the cMRF of M. fascicularis monkeys to anterogradely label reticulotectal terminals and retrogradely label tectoreticular neurons. BDA labeled profiles in the ipsi- and contralateral intermediate gray layer (SGI) were examined electron microscopically. Postembedding GABA immunochemistry was used to identify putative inhibitory profiles. Nearly all (94.7%) of the ipsilateral BDA labeled terminals were GABA positive, but profiles postsynaptic to these labeled terminals were exclusively GABA negative. In addition, BDA labeled terminals were observed to contact BDA labeled dendrites, indicating the presence of a monosynaptic feedback loop connecting the cMRF and ipsilateral SC. In contrast, within the contralateral SGI, half of the BDA labeled terminals were GABA positive, while more than a third were GABA negative. All the postsynaptic profiles were GABA negative. These results indicate the cMRF provides inhibitory feedback to the ipsilateral side of the SC, but it has more complex effects on the contralateral side. The ipsilateral projection may help tune the "winner-take-all" mechanism that produces a unified saccade signal, while the contralateral projections may contribute to the coordination of activity between the two colliculi.

Perturbation of syndapin/PACSIN impairs synaptic vesicle recycling evoked by intense stimulation.

Synaptic vesicle recycling has been proposed to depend on proteins which coordinate membrane and cytoskeletal dynamics. Here, we examine the role of the dynamin- and N-WASP (neural Wiskott-Aldrich syndrome protein)-binding protein syndapin/PACSIN at the lamprey reticulospinal synapse. We find that presynaptic microinjection of syndapin antibodies inhibits vesicle recycling evoked by intense (5 Hz or more), but not by light (0.2 Hz) stimulation. This contrasts with the inhibition at light stimulation induced by perturbation of amphiphysin (Shupliakov et al., 1997). Inhibition by syndapin antibodies was associated with massive accumulation of membranous cisternae and invaginations around release sites, but not of coated pits at the plasma membrane. Cisternae contained vesicle membrane, as shown by vesicle-associated membrane protein 2 (VAMP2)/synaptobrevin 2 immunolabeling. Similar effects were observed when syndapin was perturbed before onset of massive endocytosis induced by preceding intense stimulation. Selective perturbation of the Src homology 3 domain interactions of syndapin was sufficient to induce vesicle depletion and accumulation of cisternae. Our data show an involvement of syndapin in synaptic vesicle recycling evoked by intense stimulation. We propose that syndapin is required to stabilize the plasma membrane and/or facilitate bulk endocytosis at high release rates.

Cranial visceral afferent pathways through the nucleus of the solitary tract to caudal ventrolateral medulla or paraventricular hypothalamus: target-specific synaptic reliability and convergence patterns.

Cranial visceral afferents activate central pathways that mediate systemic homeostatic processes. Afferent information arrives in the brainstem nucleus of the solitary tract (NTS) and is relayed to other CNS sites for integration into autonomic responses and complex behaviors. Little is known about the organization or nature of processing within NTS. We injected fluorescent retrograde tracers into two nuclei to identify neurons that project to sites involved in autonomic regulation: the caudal ventrolateral medulla (CVLM) or paraventricular nucleus of the hypothalamus (PVN). We found distinct differences in synaptic connections and performance in the afferent path through NTS to these neurons. Anatomical studies using confocal and electron microscopy found prominent, primary afferent synapses directly on somata and dendrites of CVLM-projecting NTS neurons identifying them as second-order neurons. In brainstem slices, afferent activation evoked large, constant latency EPSCs in CVLM-projecting NTS neurons that were consistent with the precise timing and rare failures of monosynaptic contacts on second-order neurons. In contrast, most PVN-projecting NTS neurons lacked direct afferent input and responded to afferent stimuli with highly variable, intermittently failing synaptic responses, indicating polysynaptic pathways to higher-order neurons. The afferent-evoked EPSCs in most PVN-projecting NTS neurons were smaller and unreliable but also often included multiple, convergent polysynaptic responses not observed in CVLM-projecting neurons. A few PVN-projecting NTS neurons had monosynaptic EPSC characteristics. Together, we found that cranial visceral afferent pathways are structured distinctly within NTS depending on the projection target. Such, intra-NTS pathway architecture will substantially impact performance of autonomic or neuroendocrine reflex arcs.

Kappa opioid receptors in the rostral ventromedial medulla of male and female rats.

Kappa opioid receptor (KOR) ligands alter nociceptive responses when applied to the rostral ventromedial medulla (RVM). However, the effects of kappa opioid receptor ligands are distinct in males and females. The present study examined the distribution of kappa opioid receptor immunoreactivity in the RVM of male and female rats. KOR immunoreactivity was found at pre- and postsynaptic sites within the RVM of both sexes. The most common KOR-immunoreactive (KOR-ir) neuronal structures were unmyelinated axons, followed by axon terminals, dendrites, and somata. Different proportions of KOR-ir axon terminals and dendrites were found in females at different estrous stages. Specifically, dendrites containing KOR immunoreactivity were less abundant in proestrus females compared with estrus females and showed a trend toward being less abundant in males, suggesting that KOR ligands applied to the RVM may be less potent in proestrus females. These findings suggest that the distribution of KORs in the RVM may be influenced by reproductive hormone levels. We also found KOR immunoreactivity in many spinally projecting neurons within the RVM of female rats. These findings are consistent with the hypothesis that KOR ligands influence nociceptive behaviors by altering the activity of specific populations of neurons within the RVM. The abundance of KOR in axons and axon terminals in RVM indicates a substantial role for presynaptic effects of KOR ligands through pathways that have not been clearly delineated. Altering the balance between pre- and postsynaptic receptive sites may underlie differences in the effects of KOR agonists on nociceptive responses in males and females.

An unusual pathologic feature associated with dermatomyositis.

We present a case of juvenile dermatomyositis with unusual histopathologic findings. The child presented with a course consistent with dermatomyositis, a diagnosis confirmed by finding reticulotubular aggregates in endothelial cells on electron microscopy. However, histopathology of his muscle biopsy revealed a striking pattern of glycogen accumulation, to an extent similar to that seen in glycogen storage diseases; this degree of accumulation could potentially confound histopathologic diagnosis.

GABAergic structures in the ventral part of the oral pontine reticular nucleus: An ultrastructural immunogold analysis.

GABA mediates inhibitory effects in neurons of the ventral part of the oral pontine reticular nucleus (vRPO). Evidence increasingly suggests that GABA plays an important role in the modulation of rapid eye movement (REM) sleep generation in the cat vRPO. Here, we investigate the anatomical substrate of this modulation using GABA immunocytochemistry. Immunoperoxidase labeling revealed a few small GABA-immunoreactive cell bodies scattered throughout the vRPO. The numerical densities of all vRPO synapses and the GABA-immunoreactive synapses were estimated, at the electron microscopical level, by using a combination of the physical disector and the post-embedding immunogold techniques. We estimated that 30% of all vRPO synaptic terminals were immunoreactive to GABA. Our findings support the hypothesis that vRPO neuron activity is significantly controlled by inhibitory GABAergic terminals that directly target somata and the different parts of the dendritic tree, including distal regions. GABAergic input could inhibit vRPO REM sleep-inducing neurons during other states of the sleep-wakefulness cycle such as wakefulness or non-REM sleep.

Cellular and subcellular distributions of delta opioid receptor activation sites in the ventral oral pontine tegmentum of the cat.

The ventral division of the reticular oral pontine nucleus (vRPO) is a pontine tegmentum region critically involved in REM sleep generation. Previous reports of morphine microinjections in the cat pontine tegmentum have shown that opioid receptor activation in this region modulates REM sleep. Even though opiate administration has marked effects on sleep-wake cycle architecture, the distribution of opioid receptors in vRPO has only been partially described. Using an antiserum directed against delta opioid receptor (DOR), to which morphine binds, in the present study, we use (1) light microscopy to determine DOR cellular distribution in the rostral pontine tegmentum and (2) electron microscopy to determine DOR subcellular distribution in the cat vRPO. In the dorsal pons, DOR immunoreactivity was evenly distributed throughout the neuropil of the reticular formation and was particularly intense in the parabrachial nuclei and locus coeruleus; the ventral and central areas of the RPO and locus coeruleus complex were especially rich in DOR-labeled somata. Within the vRPO, DOR was localized mainly in the cytoplasm and on plasma membranes of medium to large dendrites (47.8% of DOR-labeled profiles), which received both symmetric and asymmetric synaptic contacts mainly from non-labeled (82% of total inputs) axon terminals. Less frequently, DOR was distributed presynaptically in axon terminals (19% of DOR-labeled profiles). Our results suggest that DOR activation in vRPO regulates REM sleep occurrence by modulating postsynaptic responses to both excitatory and inhibitory afferents. DOR activation in vRPO could have, however, an additional role in direct modulation of neurotransmitter release from axon terminals.

[Ultrastructural characteristics of the reticular thalamic nucleus neurons in WAG/Rij rats].

The aim of this study was to investigate the ultrastructure of the reticular thalamic nucleus (RTN) in rats of WAG/Rij strain, an established model for human absence epilepsy. Most RTN neurons are medium-to large-sized and have either dark or light appearance, depending on their functional state. Moreover, small-sized neurons with short axons are present, their characteristics being described for the first time.

Effects of an ionophore, X-537A, on mouse CNS cultures: a light and electron microscope study.

The effects of the ionophore X-537A on organotypic CNS tissue culture were investigated with bright field and electron microscopy. In addition to condensation of mitochondria this agent was found to induce a relatively selective vacuolation of the Golgi apparatus of neurons and glial cells. At the dosages used (50 mug/ml for 2-3 hours and 100 mug/ml for 1 hour) these changes were irreversible. Additional experiments suggested that the cellular effects of X-537A were primarily due to its ability to translocate Na+ across cellular membranes.

Cytology and synaptology of the lateral reticular nucleus of the cat.

The organization of lateral reticular nucleus (LRN) of the cat was investigated using electron microscopy and Golgi techniques. Golgi-Cox preparations revealed that the LRN consists of allodendritic and, especially, isodendritic neurons. The latter have been associated with neural centres that have important roles integrating signals from distant sources. Several forms of spines were identified with the Golgi method, and their ultrastructural correlates were determined. Somatic spines resembled stubby protrusions, while dendritic spines, where were usually observed on distal dendrites, appeared as pedunculated spines, racemose appendages and spine-crowned appendages. Ultrastructural examination of this nuclease revealed various synaptic relationships. The majority of the synaptic terminals were small (1.5--2.5 micrometer in diameter), contained round vesicles and usually contacted dendrites and spines. Other small terminals contained pleomorphic vesicles and contacted distal dendrites and spines. Large terminals (greater than 2.5 micrometer in diameter) with round or pleomorphic vesicles contacted the somata or proximal dendrites. Three types of "synaptic configurations," which consisted of discrete aggregations of neuronal processes invested by astrocytic lamellae, were also identified. These structural arrangements likely provide a basis for the integration of inputs to the LRN from spinal and supraspinal centres.

Membrane specializations in the developmentally transient perireticular nucleus of the rat.

The perireticular nucleus lies among the fibres of the internal capsule and in the rat is relatively large at birth. Like the subplate of the neocortex, it is dramatically reduced in the adult (Mitrofanis, 1992). On embryonic day 17 (E17), cells of the perireticular nucleus project to the cortex (Adams and Baker, 1995); at birth there is also a projection to the thalamus (Mitrofanis and Baker, 1993). When corticofugal axons reach the perireticular nucleus, they separate into the descending tracts and the corticothalamic pathway. The axons then enter the internal capsule on their way to the thalamus, defasciculate and contribute to a complex zone of interweaving fibres seen in this region (Adams and Guillery, 1994). We have found that the cells of the perireticular nucleus are immunopositive for an antibody (3A10) to a phosphorylated neurofilament protein (Yamada et al., 1991), and we have shown that these 3A10-positive perireticular cells extend their dendrites across the path of cortical fibres at the earliest age studied (E17). We have confirmed this dendritic orientation by Golgi staining at a later stage (P3). Electron micrographs show that axo-somatic and axo-dendritic synapses first appear in the perireticular nucleus at about the day of birth. Prenatally, between E14 and E19, the major specialized membrane interrelationships seen are omega formations between adjacent profiles that often contain vesicles. Omega formations are rarely seen at later stages. Puncta adhaerentia typified by opposing membrane densities with no associated vesicles were seen at all ages studied. We suggest that early membrane contacts (omega formations) may represent transient organisational or guidance influences since they occur during a period when axonal pathways are being defined.

An electron microscopic study of the afferent connections of the lateral reticular nucleus of the cat.

The mode and pattern of termination of the afferents to the lateral reticular nucleus (LRN) of the cat were examined at the cellular level through the ultrastructural localization of induced degeneration. Examination of the LRN following hemicordotomy at the fifth and sixth cervical levels revealed that most of the degenerating terminals were in contact with intermediate and distal dendrites, and that most of these degenerating terminals were small and contained round vesicles. Fewer degenerating terminals were observed on the somata and proximal dendrites after spinal hemisection, and most of these terminals were large and contained round vesicles. Following lesions of the pericruciate cortex, small degenerating terminals were occasionally observed making contact onto intermediate and distal dendrites. Degenerating rubral terminals were observed synapsing on somata, somatic and dendritic spines, proximal dendrites and most commonly on intermediate and distal dendrites following lesioning of the red nucleus. The degenerating axosomatic rubro-LRN terminals belonged to the large, round-vesicle terminal population, while those degenerating terminals contacting intermediate and distal dendrites belonged to the small, round-vesicle population. Small, degenerating terminals were occasionally seen following lesions of the fastigial nucleus, and they made synaptic contact mainly onto intermediate and distal dendrites and dendritic spines. The present ultrastructural observations taken together with the convergence pattern of LRN afferents and the available electrophysiological data on inputs to the LRN suggest an extensive integration of converging impulses from two or more afferent sources to the rostral LRN neurons. The results of this study therefore support the veiw that the rostral LRN functions as a comparator of command signals from the motor cortex and red nucleus and feedback signals from the spinal cord and cerebellum during ongoing movement.

Influence of the tectal zone on the distribution of synaptic boutons in the brainstem of goldfish.

This study investigated whether the topographic differences in the functional properties of the tectal motor map of goldfish are related to particular patterns of connections with downstream structures. With this aim, the distribution of synaptic boutons in the mesencephalic and rhombencephalic structures was studied after discrete injections of the tracer biotinylated dextran amine were placed at separate sites along the tectal anteroposterior axis. Irrespective of the location of the injection site, the boutons were more abundant in the mesencephalon than in the rhombencephalon, and they were located chiefly ipsilaterally all throughout the brainstem. In the mesencephalon, the boutons were found in its ventrolateral reticular formation and, to a lesser extent, in the nucleus of the medial longitudinal fasciculus, the oculomotor and isthmi nuclei, and the torus semicircularis. In the mesencephalic reticular formation, the bouton location was distributed topographically with respect to the injection site. Terminals were also observed in the nucleus of the medial longitudinal fasciculus after injections into anteromedial or middle tectal zones. In the oculomotor nucleus, boutons were present exclusively in the case of the anteromedial injection. In the rhombencephalon, most boutons were found in the superior reticular formation, and their number decreased in the medial and inferior reticular formations. A topographic distribution could be observed within the superior reticular formation, although its density was attenuated compared with that observed in the mesencephalic reticular formation. The domains of synaptic endings on the ipsilateral side were different from those on the contralateral side: The ipsilateral synaptic endings were located more medially. Finally, a few boutons were also found in the vestibulocerebellar area on either the ipsilateral or the contralateral side, depending on the injection site. From these data, the authors conclude that, in goldfish, irrespective of the tectal injection site, the endings are in similar nuclei in the brainstem; however, the distribution of synaptic boutons within such nuclei can be related to the functional properties of each tectal zone.

Evidence for growth of supraspinal axons through the lesion after transection of the thoracic spinal cord in the developing opossum Didelphis virginiana.

In the present study, we asked whether supraspinal axons grow through a complete transection of the spinal cord in the developing opossum Didelphis virginiana. When the thoracic cord was transected at postnatal day (PD) 5 and bilateral injections of Fast Blue (FB) were made four segments caudal to the lesion 30-40 days later, FB-containing neurons were found in each of the supraspinal nuclei labeled by comparable injections in age-matched unlesioned controls. Continuity between the cut ends of the cord was obviously gross when the animals were killed, and histologically recognizable spinal cord was present at the lesion site. When the same procedure was followed on pups subjected to transection at PD12, FB-containing neurons were still present at supraspinal levels, but they appeared to be fewer in number than in the PD5 cases or the age-matched controls, and none were found within the medial pontine reticular and lateral vestibular nuclei. When the lesion was made at PD20, labeled neurons were even fewer in number, and when it was made at PD26, they were restricted to the medullary raphe and the red nuclei. There was no evidence for growth of supraspinal axons across lesions made at PD33. We conclude that supraspinal axons grow through the lesion after transection of the spinal cord in neonatal opossums and that the critical period for growth of reticulospinal and vestibulospinal axons through the lesion ends earlier than that for comparable growth of raphespinal and rubrospinal axons.

Ultrastructural organization of lamprey reticulospinal synapses in three dimensions.

The giant reticulospinal synapse in lamprey provides a unique model to study synaptic vesicle traffic. The axon permits microinjections, and the active zones are often separated from each other, which makes it possible to track vesicle cycling at individual release sites. However, the proportion of reticulospinal synapses with individual active zones ("simple synapses") is unknown and a quantitative description of their organization is lacking. Here, we report such data obtained by serial section analysis, intermediate-voltage electron microscopy, and electron tomography. The simple synapse was the most common type (78%). It consisted of one active zone contacting one dendritic process. The remaining synapses were "complex," mostly containing one vesicle cluster and two to three active zones synapsing with distinct dendritic shafts. Occasional axosomatic synapses with multiple active zones forming synapses with the same cell were also observed. The vast majority of active zones in all synapse types contained both chemical and electrotonic synaptic specializations. Quantitative analysis of simple synapses showed that the majority had active zones with a diameter of 0.8-1.8 microm. The number of synaptic vesicles and the height of the vesicle cluster in middle sections of serially cut synapses correlated with the active zone length within, but not above, this size range. Electron tomography of simple synapses revealed small filaments between the clustered synaptic vesicles. A single vesicle could be in contact with up to 12 filaments. Another type of filament, also associated with synaptic vesicles, emerged from dense projections. Up to six filaments could be traced from one dense projection.

Projection of jaw-muscle spindle afferents to the caudal brainstem in rats demonstrated using intracellular biotinamide.

Intracellular staining with biotinamide was used to study the axonal projection and synaptic morphology of rat jaw-muscle spindle afferents. Intracellular recordings in the mesencephalic trigeminal nucleus (Vme) were identified as spindle afferent responses by their increased firing during stretching of the jaw-elevator muscles. Biotinamide-stained axon collaterals with boutons were found in the trigeminal motor nucleus (Vmo), Vme, the region dorsal to Vmo including the supratrigeminal region, the dorsomedial portion of the trigeminal principal sensory nucleus, and the dorsomedial part of the rostral spinal trigeminal subnucleus oralis. Additional, previously undescribed projections of jaw-muscle spindle afferents were found to the dorsomedial portion of the caudal spinal trigeminal subnucleus oralis (Vodm), the dorsomedial part of the spinal trigeminal subnucleus interpolaris (Vidm), the caudal parvicellular reticular formation, laminae IV and V of the spinal trigeminal subnucleus caudalis (Vc), and the dorsal division of the medullary reticular field. Labeled spindle boutons in Vodm formed predominately axodendritic synapses. Some of these boutons received presynaptic inputs from unlabeled P-type boutons containing clear, spherical, or flattened vesicles. In Vidm, labeled collaterals and boutons were densely clustered into glomerular-like structures. Labeled boutons in Vidm made axodendritic, axosomatic, and axoaxonic synapses and received synaptic contacts from unlabeled boutons containing clear, spherical, or flat and pleomorphic vesicles. Unlabeled presynaptic boutons in Vidm occasionally contained dense core vesicles. Labeled boutons in Vc mainly formed synaptic contacts with large diameter dendrites. This projection of jaw-muscle spindle afferents to caudal brainstem regions may play a significant role in masticatory-muscle stretch reflexes and in the integration of trigeminal proprioceptive information and its transmission to higher centers.

Synaptic distribution of afferents from reticular nucleus in ventroposterior nucleus of cat thalamus.

This study was aimed at determining the synaptic circuitry that contributes to the alterations in thalamic function that accompany changes in behavioral states. The somatosensory sector of the thalamic reticular nucleus (RTN) was identified by microelectrode recording in cats and injected with Phaseolus vulgaris-leucoagglutinin (PHA-L). The axons of labeled RTN cells gave rise to collaterals within the RTN and continued into the dorsal thalamus where they terminated predominately in the ventral posterior lateral nucleus (VPL). After small injections in the upper limb representation of RTN, most labeled terminations in VPL were confined to its medial part, suggesting the presence of a topographic organization in the projection. Terminations were concentrated in localized, focal aggregations of boutons. Combined electron microscopic immunocytochemistry, using immunogold labeling for gamma-aminobutyric acid (GABA), showed that the PHA-L labeled boutons were GABA-positive terminals that ended in symmetrical synapses. Eighty-two percent of these synapses were on dendrites of relay neurons, 8.5% on dendrites of interneurons, and 9.3% on somata. The terminals of RTN axons form the majority of axon terminals ending in symmetrical synapses in VPL. Their concentration on relay neurons probably underlies the capacity of the RTN projection to reduce background activity of VPL relay neurons in the awake state and to maintain oscillatory behavior of these neurons in drowsiness and early phases of sleep.

Somatodendritic morphology of on- and off-cells in the rostral ventromedial medulla.

The rostral ventromedial medulla (RVM) contains two classes of physiologically defined neurons, on-cells and off-cells, that are implicated in nociceptive modulation. In a continuing effort to detail the neural circuitry that underlies the activity of these two distinct neuronal types, the somatodendritic morphology of on- and off-cells was studied in the cat, rat, and ferret. In lightly anesthetized animals, on-cells increased and off-cells decreased their discharge rate during a withdrawal reflex evoked by noxious stimuli. Following their physiological characterization by using intracellular recording, on- and off-cells were injected with either horseradish peroxidase or biocytin and their somatodendritic arborizations were examined. Labeled on- and off-cells included fusiform and stellate cells of all sizes as well as large multipolar neurons. Although the somatic shape of both on- and off-cells in RVM was heterogeneous, off-cells tended to be fusiform neurons whose long axis was oriented mediolaterally. The dendritic domains of both on- and off-cells extended bilaterally past the lateral edge of the trapezoid body or pyramid and ventrally to, and sometimes including, the trapezoid body or pyramid. In contrast to their extensive mediolateral spread, the dendritic domains of both cell types were limited to the ventral half of the reticular formation and were compressed along the rostrocaudal axis. The dendritic arbor of individual on- and off-cells extended well beyond the cytoarchitectonic boundaries of any single nuclear region, within the domain delineated as the RVM. The spatial domains of the dendritic arbors of on- and off-cells are further evidence that the on- and off-cells throughout the RVM constitute an integrated unit in the modulation of nociceptive transmission.

Monosynaptic projections from the medullary gigantocellular reticular formation to sympathetic preganglionic neurons in the thoracic spinal cord.

Microinjection of L-glutamate into a restricted area of the medullary gigantocellular reticular formation, the gigantocellular depressor area (GiDA), lowers arterial pressure. Unlike the nuclei tractus solitarii and the caudal ventrolateral medulla, the two principle medullary vasodepressor areas, the GiDA projects directly to the spinal cord and not to the rostral ventrolateral medulla (Aicher et al. [1994] Neuroscience 60:761-779). We investigated whether neurons within GiDA directly innervate autonomic areas of the thoracic spinal cord. Fluoro-Gold injected into the thoracic spinal cord labeled neurons within functionally defined vasodepressor sites in the GiDA in the same animal. To examine the morphology of GiDA efferents to the spinal cord, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoresed into the GiDA, and efferent processes in the intermediolateral cell column and nucleus intercalatus spinalis were examined by electron microscopy. Labeling was confined to axons and axon terminals (n = 144) that usually contained primarily small clear vesicles, contacted large and small dendrites, and formed symmetric (inhibitory) synapses. To determine whether some of the postsynaptic targets of GiDA efferent terminals in the thoracic spinal cord were sympathoadrenal preganglionic neurons, these neurons were retrogradely labeled from the adrenal gland with Fluoro-Gold in rats that had deposits of the anterograde tracer, biotinylated dextran amine (BDA), in the GiDA. Some BDA-containing terminals formed symmetric synapses with dendrites containing Fluoro-Gold. We conclude that a population of neurons in the GiDA monosynaptically innervates some sympathetic preganglionic neurons. The findings suggest the presence of a novel reticulospinal sympathoinhibitory projection originating in the GiDA.