Descending projections from the nucleus retroambiguus to the iliopsoas motoneuronal cell groups in the female golden hamster: Possible role in reproductive behavior

In the cat, the nucleus retroambiguus (NRA) projects to expiratory motoneurons in the brainstem and spinal cord. Recently, it has been demonstrated that the NRA sends fibers to a specific set of motoneurons in the lumbosacral cord, which pathway is thought to play a crucial role in mating behavior. The question is whether such projections exist in the hamster, because the female of this species displays a very distinctive receptive behavior. In the hamster, lumbosacral cord injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) combined with hemisection 1 or 2 segments rostral to injection sites in three of the five cases demonstrated retrogradely labeled neurons in the NRA at levels 1.0–2.25 mm caudal to the obex, contralateral to the injection sites. Injections of WGA-HRP into the NRA and adjoining reticular formation revealed that NRA fibers crossed the midline in the caudal medulla and descended in the contralateral lateral and ventrolateral funiculi to terminate bilaterally, but mainly contralaterally, in the motoneuronal cell groups of the abdominal wall and iliopsoas muscles. NRA projections to levels caudal to lumbar segment 5 were virtually absent. Electron microscopic examination revealed that, of the 162 labeled NRA terminal profiles found in the ultrathin sections, 144 (89%) made monosynaptic contacts with retrogradely labeled dendrites of iliopsoas motoneurons. These NRA terminals formed asymmetrical synapses and contained spherical vesicles indicative of an excitatory function. The results indicate that, in the hamster, direct contralateral NRA projections exist to iliopsoas motoneurons. A concept is discussed in which this pathway plays a crucial role in mating behavior. J. Comp. Neurol. 403:219–228, 1999. © 1999 Wiley-Liss, Inc.     

Subregions of the periaqueductal gray topographically innervate the rostral ventral medulla in the rat.

Previous anatomical and physiological studies have revealed a substantial projection from the periaqueductal gray (PAG) to the nucleus paragigantocellularis (PGi). In addition, physiological studies have indicated that the PAG is composed of functionally distinct subregions. However, projections from PAG subregions to PGi have not been comprehensively examined. In the present study, we sought to examine possible topographic specificity for projections from subregions of the PAG to PGi. Pressure or iontophoretic injections of wheat germ agglutinin-conjugated horseradish peroxidase, or of Fluoro-Gold, placed into the PGi of the rat retrogradely labeled a substantial number of neurons in the PAG from the level of the Edinger-Westphal nucleus to the caudal midbrain. Retrogradely labeled neurons were preferentially aggregated in distinct subregions of the PAG. Rostrally, at the level of the oculomotor nucleus, labeled neurons were i) compactly aggregated in the ventromedial portion of the PAG corresponding closely to the supraoculomotor nucleus of the central gray, ii) in the lateral and ventrolateral PAG, and iii) in medial dorsal PAG. More caudally, retrogradely labeled neurons became less numerous in the dorsomedial PAG but were more widely scattered throughout the lateral and ventrolateral parts of the PAG. Only few retrogradely labeled neurons were found in the ventromedial part of the PAG at caudal levels. Injections of retrograde tracers restricted to subregions of the PGi suggested topography for afferents from the PAG. Injections into the lateral portion of the PGi yielded the greatest number of labeled neurons within the rostral ventromedial PAG. Medially placed injections yielded numerous retrogradely labeled neurons in the lateral and ventrolateral PAG. Injections placed in the rostral pole of the PGi (medial to the facial nucleus) produced the greatest number of retrogradely labeled neurons in the dorsal PAG. To examine the pathways taken by fibers projecting from PAG neurons to the medulla, and to further specify the topography for the terminations of these afferents in the PGi, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoretically deposited into subregions of the PAG that contained retrogradely labeled neurons in the above experiments. These results revealed distinct fiber pathways to the rostral medulla that arise from the dorsal, lateral/ventrolateral, and ventromedial parts of the PAG. These injections also showed that there are differential but overlapping innervation patterns within the PGi. Consistent with the retrograde tracing results, injections into the rostral ventromedial PAG near the supraoculomotor nucleus yielded anterograde labeling immediately ventral to the nucleus ambiguus in the ventrolateral medulla, within the retrofacial portion of the PGi.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ultrastructural evidence for direct excitatory retroambiguus projections to cutaneous trunci and abdominal external oblique muscle motoneurons in the cat

The nucleus retroambiguus (NRA) is a group of neurons, located laterally in the caudal medulla oblongata. The NRA is thought to modulate abdominal pressure in the framework of respiration, vomiting, vocalization, probably parturition, and, in all likelihood mating behavior. The NRA exerts this control through its projections to motoneurons to the nucleus ambiguus in the lateral medulla (innervating pharynx, larynx), and spinal cord (innervating cutaneous trunci, intercostal, abdominal, pelvic floor, and lower limb muscles). The nature of these NRA-motoneuronal projections is unknown. In this study we have determined the ultrastructure of the NRA-motoneuronal projections, and especially those to the abdominal external oblique and cutaneous trunci muscles. In four cats 0.1% cholera toxin subunit b was injected in the external oblique and cutaneous trunci muscles to retrogradely label their motoneurons in the spinal cord. Wheat germ agglutinin-conjugated horseradish peroxidase was injected into the NRA to anterogradely label its contralaterally descending fibers to the motoneurons of both muscles. In order to prevent anterograde labeling of ipsilaterally descending systems not originating from the NRA, a hemisection was made at the level of C2 prior to the NRA injection. The ultrastructural results indicate that the majority (60-74%) of the anterogradely labeled NRA-terminals made monosynaptic contacts with retrogradely labeled dendrites of the external oblique and the cutaneous trunci muscle motoneurons. The majority (86-95%) of the NRA terminals made asymmetric synaptic contacts and 79-84% contained round vesicles. These results demonstrate the existence of direct, presumably excitatory, projections from NRA to external oblique and cutaneous trunci muscle motoneurons.     

Two parts of the nucleus prepositus hypoglossi project to two different subdivisions of the dorsolateral periaqueductal gray in cat

The dorsolateral column of the mesencephalic periaqueductal gray (PAG) is a separate part of the PAG. Its afferent sources, efferent targets, and neurochemical properties differ from the adjacent PAG columns. The dorsolateral PAG is thought to be associated with aversive behaviors, but it is not yet understood how these behaviors are brought about. To elucidate the function of the PAG further, in the present study we investigated which brainstem regions project to the dorsolateral PAG. Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections involving the dorsolateral PAG, but extending into the lateral part, resulted in many retrogradely labeled cells in the pontine and medullary tegmentum bilaterally. However, it was concluded that these neurons were labeled from the lateral PAG, because no anterograde labeling was found in the dorsolateral PAG after a large injection into the tegmentum. Retrogradely labeled cells were also found in the nucleus prepositus hypoglossi (PPH), mainly contralaterally. Injections of [3H]leucine or WGA-HRP in the PPH resulted in anterogradely labeled fibers in the dorsolateral PAG. Two separate distribution patterns were found. The caudal and intermediate PPH projected to a small region on the dorsolateral edge of the dorsolateral column, whereas the supragenual PPH distributed labeled fibers to all other parts of the dorsolateral PAG, except the area on the dorsolateral edge. These separate PPH projections suggest that two subdivisions exist within the dorsolateral PAG. The present findings suggest a role for the dorsolateral PAG in the oculomotor system.     

Anatomical study of the final common pathway for vocalization in the cat

Vocalization, the nonverbal production of sound, can be elicited in many vertebrates by stimulation in several regions of the limbic system but most easily in the caudal periaqueductal gray (PAG). This study shows that a specific cell group in the lateral part of the caudal PAG and in the tegmentum just lateral to it projects bilaterally to the nucleus retroambiguus (NRA) in the caudal medulla oblongata. Similar but much weaker projections are derived from the dorsal PAG. Neurons in the NRA in turn project via a contralateral pathway through the ventral funiculus of the spinal cord to motoneuronal cell groups, innervating intercostal and abdominal muscles. These projections are stronger on the contralateral side, although at lower thoracic and upper lumbar levels, many fibers recross to terminate in the ipsilateral motoneuronal cell groups. In the brainstem NRA neurons project to the motoneuronal cell groups innervating mouth-opening and perioral muscles as well as to motoneurons innervating the pharynx, soft palate, and tongue, and probably to the larynx. All these muscles are active in vocalization. The present anatomical results, combined with the physiological results of others, indicate that the projections from PAG via NRA to vocalization motoneurons form the final common pathway in vocalization. The role of this pathway in the total framework of emotional behavior is discussed.     

Ultrastructural evidence for a direct excitatory pathway from the nucleus retroambiguus to lateral longissimus and quadratus lumborum motoneurons in the female golden hamster

During mating, the female golden hamster displays a stereotyped specific receptive posture, characterized by lordosis of the back, elevation of the tail, and extension of the legs. Muscles involved in this posture are thought to be iliopsoas, cutaneus trunci, lateral longissimus (LL), and quadratus lumborum (QL). Lesion studies in rats suggest that mating behavior is controlled by the mesencephalic periaqueductal gray (PAG). The PAG does not project directly to the motoneurons innervating the muscles involved in mating, but is thought to make use of the nucleus retroambiguus (NRA) as relay. The NRA is located ventrolaterally in the most caudal medulla, and projects directly to iliopsoas and cutaneus trunci motoneuronal cell groups. The question is whether this is also true for LL and QL muscles. Retrograde HRP tracing experiments revealed that LL and QL motoneurons are located medially in the ventral horn of the T12-L6 and T13-L4 segments, respectively. A subsequent ultrastructural study combined wheatgerm agglutinin-conjugated horseradish peroxidase injections in the NRA with cholera-toxin B-subunit injections in LL and QL muscles. The results revealed monosynaptic contacts between anterogradely labeled NRA-fiber terminals with retrogradely labeled dendrites of both LL and QL motoneurons. Almost all these terminals had asymmetrical synapses and contained spherical vesicles, suggesting an excitatory function of this NRA-motoneuronal pathway. These results correspond with the hypothesis that in hamster the PAG-NRA-motoneuronal projection not only involves motoneurons of iliopsoas and cutaneus trunci but also of LL and QL.     

The efferent projections of the periaqueductal gray in the rat: A Phaseolus vulgaris-leucoagglutinin study. II. Descending projections

The descending projections of the periaqueductal gray (PAG) have been studied in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The tracer was injected into the dorsolateral or ventrolateral subdivisions of the PAG at rostral or caudal sites. It was found that the patterns of the descending projections of the rostral and caudal parts of the dorsolateral PAG were the same and that the patterns of the descending projections of the rostral and caudal parts of the ventrolateral PAG were the same. However, the patterns of projections of the dorsolateral and ventrolateral PAG subregions were substantially different. These results suggest that the dorsolateral and ventrolateral parts of the PAG are organized into longitudinal columns that extend throughout the length of the PAG. The axons of PAG neurons descended through the pons and medulla via two routes. A small fiber bundle was present in the periaqueductal gray and in the periventricular area. This bundle distributed fibers and terminals locally within the periaqueductal gray and in the locus coeruleus and Barrington's nucleus. A larger bundle had a diffuse arrangement in the pontine reticular formation, however, and it had a more restricted distribution in the medulla, where it occupied a position dorsolateral to the pyramid. This bundle supplied structures in the pontine and medullary tegmentum. The dorsolateral column preferentially supplied the locus coeruleus, subcoeruleus, the gigantocellular nucleus pars alpha, the rostral part of the paragigantocellular nucleus, and the region of the A5 noradrenergic cell group. The ventrolateral column preferentially supplied the nucleus raphe magnus, the caudal part of the lateral paragigantocellular nucleus, and the rostroventrolateral reticular nucleus. © 1995 Willy-Liss, Inc.     

大鼠中脑导水管周围灰质向孤束核的直接投射

用ＰＨＡ－Ｌ和ＷＧＡ－ＨＲＰ顺行追踪方法,对大鼠中脑导水管周围灰质（ＰＡＧ）向孤束核的投射进行了研究。结果如下：（１）ＰＨＡ－Ｌ和ＷＧＡ－ＨＲＰ顺标纤维和终末在孤束核内的分布状态基本一致,内侧亚核最多,连合亚核、腹外侧亚核和腹侧亚核次之,中间亚核内最少。无论注射区在ＰＡＧ的尾段还是吻段,孤束核的尾中段内的标记纤维和终末均多于吻段。（２）ＰＡＧ尾段向孤束核的投射多于ＰＡＧ吻段。（３）ＰＡＧ腹外侧区向孤束核的投射较多,而背外侧区较少（尾段）或缺如（吻段）。根据作者过去的结果和本实验证据,可见ＰＡＧ向孤束核的投射通路存在定位投射关系。     

Monosynaptic projections from the nucleus retroambiguus to motoneurons supplying the abdominal wall, axial, hindlimb, and pelvic floor muscles in the female rhesus monkey

The nucleus retroambiguus (NRA) consists of premotor neurons in the caudal medulla. It is involved in expiration, vomiting, vocalization, and probably reproductive behavior by means of projections to distinct motoneuronal cell groups. Because no information is available about the NRA and its efferent pathways in primates, the present study examines NRA projections to the lumbosacral spinal cord in female rhesus monkeys. To identify the NRA, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the lumbosacral cord in three monkeys. To study the distribution of NRA axons in the lumbosacral cord, WGA-HRP injections were made into the NRA in seven monkeys. To identify motoneuronal cell groups receiving input from the NRA, the same seven monkeys also received cholera toxin subunit b (CTb) injections into different hindlimb, axial, and pelvic floor muscles. The results show that NRA neurons projecting to the lumbosacral cord are mainly located between 1 to 4 mm caudal to the obex. They send numerous axons to external oblique and pelvic floor motoneurons, whereas projections to iliopsoas and axial motoneurons are less numerous. The projections are bilateral, but show a clear contralateral predominance in the iliopsoas, axial, and pelvic floor motoneuronal cell groups. At the ultrastructural level, NRA-terminal profiles make asymmetrical contacts with labeled and unlabeled dendrites in these motoneuronal cell groups and contain large amounts of spherical and a few dense core vesicles. It is concluded that the NRA is well developed in the monkey and that there exists a direct pathway from the NRA to lumbosacral motoneurons in this species. The finding that the NRA projects to a somewhat different set of motoneuronal cell groups compared with other species fits the concept that it is not only involved in expiration-related activities but also in species specific receptive and submissive behavior.     

Monosynaptic projections from the lateral periaqueductal gray to the nucleus retroambiguus in the rhesus monkey: implications for vocalization and reproductive behavior

The periaqueductal gray (PAG) is known to be essential for vocalization and reproductive behavior. The PAG controls components of these behaviors by means of projections to the nucleus retroambiguus (NRA), a group of premotor neurons in the caudal medulla oblongata. In the accompanying study (VanderHorst et al., 2000 [accompanying study]), the NRA and its lumbosacral projections have been identified in the rhesus monkey. The present light and electron microscopical tracing study describes the PAG-NRA pathway in primates. To locate midbrain neurons projecting to the NRA, wheat germ agglutinin horseradish peroxidase (WGA-HRP) was injected into the NRA in six monkeys. To determine the distribution pattern of PAG axons in the medulla oblongata, WGA-HRP was injected into the PAG and adjacent tegmentum in three additional monkeys. In one of these three monkeys, biotinylated dextran amine and cholera toxin subunit b were injected into the lumbosacral cord to retrogradely identify NRA neurons. The results show that a compact group of neurons in the medial part of the lateral PAG at the intercollicular level sends a dense projection to the NRA. The projection is bilateral with a clear ipsilateral predominance. At the ultrastructural level, there are monosynaptic contacts between PAG fibers and NRA neurons, including NRA neurons that project to the lumbosacral cord. The synaptic contacts were primarily asymmetrical and the labeled terminal profiles contained spherical and dense core vesicles. It is concluded that there exists a strong and direct PAG-NRA pathway in the rhesus monkey. Because NRA neurons projecting to the lower lumbar cord are included, the PAG-NRA projection is likely to be involved not only in vocalization but also in other behaviors, such as receptive posture.     

Changes in the topographically organized connections between the nucleus isthmi and the optic tectum after partial tectal ablation in adult goldfish

The projection of the nucleus isthmi to the ipsilateral optic tectum was examined in normal goldfish. This was compared to the projection in animals in which the entire visual field had been induced to compress onto a rostral half tectum by caudal tectal ablation. The isthmo-tectal projection was examined by making localized injections of horseradish peroxidase into the optic tecta and observing the patterns of labeled cells within the nucleus isthmi. The teleost nucleus isthmi consists of a cell sparse medulla covered by a cellular cortex, which is thick on the rostral, medial, and dorsal surfaces of the nucleus. Almost all isthmic cells projecting to the tectum were located in the area of thick cortex. In normal fish, rostral tectal injections labeled cells in the rostroventral portion of the thick cortex; injections midway in the rostrocaudal tectal axis labeled more caudodorsally located cells, and caudal tectal injections labeled cells a little further caudally in extreme dorsal cortex. The rostroventral to caudodorsal isthmic axis was therefore seen to project rostrocaudally along the tectum. This topography contrasts somewhat with the situation seen in amphibia where the rostrocaudal tectal axis receives projections from the rostrocaudal isthmic axis. In fish with half-tectal ablations, injections near the caudal edge of the half tectum (at a site that had originally been midtectal) labeled cells that had previously projected to caudal tectum. Rostral tectal injections in fish with compression of the visual field gave a normal pattern of labeled isthmic cells. The results indicate that a topographically ordered isthmo-tectal projection exists in goldfish that may be induced to compress onto a half tectum.     

Nucleus retroambiguus-spinal pathway in the mouse: Localization, gender differences, and effects of estrogen treatment

Nucleus retroambiguus (NRA)-motoneuronal projections are species-specific and serve expiration, Valsalva maneuvers, vocalization, and sexual behavior. In cat and monkey, estrogen induces sprouting of NRA-spinal axons. This pathway may thus serve as a model to study mechanisms through which estrogen induces neuronal plasticity. In this study, NRA-spinal projections are described in adult mice by using anterograde and retrograde tracing techniques, with attention to gender, strain (CD-1 and C57BL/6), and estrogen-induced changes (in ovariectomized females). Labeled NRA-spinal neurons at the level of the decussation of the corticospinal tract were most numerous after tracer injections into the thoracic and upper lumbar cord. They were medium-sized and had axons that descended through the contralateral cord. A group of small neurons was labeled in the NRA immediately rostral to the decussation of the corticospinal tract after cervical and thoracic, but not after lumbar injections. This group projected mainly via an ipsilateral pathway. The main projections from the caudal NRA involved motoneurons in the thoracic and upper-lumbar cord that supply abdominal wall and cremaster muscles. Pelvic floor motoneurons did not receive substantial input. NRA-spinal projections, especially those involving the upper lumbar cord, were sexually dimorphic, being more extensive in males than in females. Moreover, they were more distinct in estrogen-treated females than in control females. Strain differences were not observed. The unique features of the caudal NRA-spinal pathway in the mouse are discussed in the framework of possible functions of this system, such as mating behavior and related social behaviors, parturition, thermoregulation, and control of balance.     

Afferent projections to pharynx and soft palate motoneurons: a light and electron microscopical tracing study in the cat

Pharynx and soft palate are muscles for respiration, vocalization, swallowing, and vomiting. In cat, motoneurons innervating pharynx/soft palate are located in the dorsal group of the nucleus ambiguus (dgNA) in the medulla oblongata. In cat, dgNA is the only part of nucleus ambiguus that can be distinguished as a separate cell group, which makes it possible to study its afferent input. In two cats, WGA-HRP injections in dgNA and surrounding tegmentum resulted in retrogradely labeled cells at several levels of the neuraxis. In 170 cases anterograde tracers were injected in areas in which the cells of origin were identified. Results demonstrate that dgNA afferents originate from the tegmentum dorsolateral to the superior olivary complex, medullary ventromedial tegmentum, caudal raphe nuclei, medullary lateral tegmental field, nucleus retroambiguus (NRA), and adjoining tegmentum, extending into the first cervical segment of the spinal cord. In order to determine whether periaqueductal gray (PAG) and parabrachial nuclei (PB) make synaptic contacts with dgNA, ultrastructural studies combined anterograde tracing from PAG, PB, and NRA with retrograde tracing of pharyngeal and soft palate motoneurons. The results showed that PB, but not PAG, projects to the dgNA and that NRA afferent synapses are three times as numerous as those from PB. The morphology of PB and NRA synapses is consistent with excitatory input. In conclusion, pharyngeal and soft palate motoneurons receive their afferents almost exclusively from the pontine and medullary tegmentum and first cervical spinal segment.     

Reciprocal connections between the medial preoptic area and the midbrain periaqueductal gray in rat: a WGA-HRP and PHA-L study.

The midbrain periaqueductal gray (PAG) participates in diverse functions such as analgesia, autonomic regulation, sexual behavior, and defense/escape responses. Anatomical studies of the circuits involved in such functions have largely focused on the connections of PAG with the medulla. Projections to PAG from forebrain structures are extensive, but their organization has received little attention. Previous anatomic studies indicate that the medial preoptic area (MPO), involved in a variety of physiological and behavioral functions, is a major source of afferent input to the periaqueductal gray. Here, we have examined the topography of reciprocal connections between these two structures in the rat by using wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and Phaseolus vulgaris leucoagglutinin (PHA-L). Multiple WGA-HRP injections at several rostrocaudal levels of PAG retrogradely labeled large numbers of neurons in the medial preoptic area; labeled cells were primarily located in the medial preoptic nucleus, the median preoptic nucleus, and the region lateral to the medial preoptic nucleus. The distribution of labeled cells shifted medially to laterally along the rostral to caudal axis of the medial preoptic area. Rostrally, there was selective retrograde labeling in the central and lateral divisions of medial preoptic nucleus, whereas caudally, labeled cells were primarily located only in the lateral subdivision of medial preoptic nucleus. Tracer injections in PAG also produced strong anterograde labeling in MPO. WGA-HRP and PHA-L injections in the medial preoptic area resulted in dense anterograde labeling along the entire rostrocaudal axis of PAG. The terminal labeling in PAG from the medial preoptic area was not uniformly distributed throughout PAG, however. Instead, this projection formed one or two rostrocaudally oriented longitudinal columns that terminated in different subregions of PAG along the entire rostrocaudal axis of this structure. Rostrally, inputs from the medial preoptic area project heavily to dorsomedial PAG, and at mid-PAG levels, the projection becomes distinctly bipartite with two discrete longitudinal terminal columns in dorsomedial and lateral PAG; caudally, the heaviest labeling is in ventrolateral PAG. The projection also exhibited a central to peripheral (radial) gradient; labelled fibers and terminals were heaviest near the aqueduct and much lower in the peripheral parts of PAG. WGA-HRP injections in MPO also produced retrograde labeling of neurons at all rostrocaudal levels of PAG; more neurons were labeled in the rostral than the caudal half of PAG. The majority of labeled cells were located in dorsomedial and ventral/ventrolateral parts of PAG; only a few neurons in the dorsal raphe region appear to project to MPO.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projections from the superior colliculus to the trigeminal system and facial nucleus in the rat

To determine the influence of the superior colliculus (SC) in orienting behaviors, we examined SC projections to the sensory trigeminal complex, the juxtatrigeminal region, and the facial motor nucleus in rats. Anterograde tracer experiments in the SC demonstrated predominantly contralateral colliculotrigeminal projections. Microinjections in the deep layers of the lateral portion showed labeled nerve fibers and terminals in the ventromedial parts of the caudal principal nucleus and of the rostral oral subnucleus and in the medial part of the interpolar subnucleus. Some terminals were also observed in the juxtatrigeminal region and in the dorsolateral part of the facial motor nucleus contralaterally, overlying the orbicularis oculi motoneuronal region. Verification by retrograde tracer injections into the trigeminal target regions showed labeled SC neurons mostly in lateral portions of layers 4-7. When the juxtatrigeminal region was involved, a remarkable increase of labeled neurons was observed, having a patch-like arrangement with a decreasing gradient from lateral to medial SC portions. Retrograde tracer injections in the dorsolateral VII nucleus showed bilateral labeled neurons mainly in the deep lateral SC portion. Retrograde BDA microinjections into the same trigeminal or juxtatrigeminal regions, followed by gold-HRP into the dorsolateral VII nucleus, demonstrated a significant number of SC neurons in deep layers 6-7 projecting to both structures by axon collaterals. These neurons are mediolaterally grouped in patches along the rostrocaudal SC extent; a subset of them are immunoreactive for glutamic acid decarboxylase (GAD). They could be involved in the coordination of facial movements. Simultaneous anterograde and retrograde tracer injections into the lateral SC portion and the VII nucleus respectively localized trigeminofacial neurons receiving collicular input in the trigeminal principal nucleus and pars oralis. Therefore the SC should play a crucial role in regulating motor programs of both eye and eyelid movements.     

Thalamic projections to retrosplenial cortex in the rat

The topographic relationships between anterior thalamic neurons and their terminal projection fields in the retrosplenial cortex of the rat were characterized by experiments with the fluorescent dye retrograde labeling technique. The results demonstrate that the anterodorsal (DAD) and anteroventral (AV) nuclei project heavily to retrosplenial granular cortex (Rg) and to a lesser extent to retrosplenial agranular cortex (Rag). In contrast, the anteromedial (AM) and lateral dorsal (LD) nuclei project heavily to Rag and more lightly to Rg. Irrespective of terminal field in Rg or Rag, the neuronal cell bodies in AD and AV are organized topographically so that the neurons in the caudal part of each nucleus project to rostral retrosplenial cortex and the neurons in the rostral portion of each nucleus project to the caudal retrosplenial cortex. Further, the ventromedial AD and AV neurons project to rostral retrosplenial cortex, whereas dorsolateral neurons in both nuclei project to caudal retrosplenial cortex. LD neurons display a different topographic organization. The neurons in the medioventral part of LD project primarily to the rostral retrosplenial cortex, and the neurons in lateral LD project to the caudal retrosplenial cortex. This latter projection to the caudal retrosplenial cortex is also contributed to by neurons residing in the mediodorsal part of caudal LD. The neurons in AM that project to the retrosplenial cortex display less segregation than the AV, AD, or LD neurons. In all experiments, a number of neurons in the dorsal ventro-anterolateral nucleus were labeled by retrosplenial injections. The largest number of cells in this nucleus were labeled after Rag injections, and these were topographically organized such that the neurons projecting to the rostral Rag were located immediately deep to the internal medullary lamina, and the neurons projecting to the caudal Rag were more ventrally located. Very few thalamic neurons have axon collaterals to different areas of the retrosplenial cortex as shown by double labeling experiments. Together, these results demonstrate a highly organized thalamic projection to the retrosplenial cortex.     

Distribution and connections of inspiratory premotor neurons in the brainstem of the pigeon (Columba livia)

We have recorded extracellular, inspiratory-related (IR) unit activity in the medulla at locations corresponding to those of neurons retrogradely labeled by injections of retrograde tracers in the lower brachial and upper thoracic spinal cord, injections that covered cell bodies and dendrites of motoneurons innervating inspiratory muscles. Bulbospinal neurons were distributed throughout the dorsomedial and ventrolateral medulla, from the spinomedullary junction through about 0.8 mm rostral to the obex. Almost all of the 104 IR units recorded were located in corresponding parts of the ventrolateral medulla, rostral to nucleus retroambigualis, where expiratory related units are found. Injections of biotinylated dextran amine at the recording sites labeled projections both to the spinal cord and to the brainstem. In the lower brachial and upper thoracic spinal cord, bulbospinal axons traveled predominantly in the contralateral dorsolateral funiculus and terminated in close relation to the dendrites of inspiratory motoneurons retrogradely labeled with cholera toxin B-chain. In the brainstem, there were predominantly ipsilateral projections to the nucleus retroambigualis, tracheosyringeal motor nucleus (XIIts), ventrolateral nucleus of the rostral medulla, infraolivary superior nucleus, ventrolateral parabrachial nucleus, and dorsomedial nucleus of the intercollicular complex. In all these nuclei, except XIIts, retrogradely labeled neurons were also found, indicating reciprocity of the connections. These results suggest the possibility of monosynaptic connections between inspiratory premotor neurons and inspiratory motoneurons, which, together with connections of IR neurons with other brainstem respiratory-vocal nuclei, seem likely to mediate the close coordination that exists in birds between the vocal and respiratory systems. The distribution of IR neurons in birds is similar to that of the rostral ventral respiratory group (rVRG) in mammals. J. Comp. Neurol. 379:347–362, 1997. © 1997 Wiley-Liss, Inc.     

Amygdaloid and pontine projections to the ventromedial nucleus of the hypothalamus

Amygdaloid and pontine projections to the feline ventromedial nucleus of the hypothalamus (HVM) were studied with retrograde transport of horseradish peroxidase (HRP) and anterograde transport of tritiated amino acids. Following injections of HRP into HVM, amygdaloid neurons were labeled in the ipsilateral cortical and medial nuclei and the ventral portion of the parvocellular part of the basal nucleus. In experiments in which HRP was injected into the tuberal hypothalamus following stria terminalis lesions, it was determined that amygdaloid neurons projecting to HVM by way of the stria terminalis were located in the cortical and medial nuclei while those projecting through another route, presumably the ventral amygdalofugal pathway, were found in the rostral part of the medial nucleus and the parvocellular basal nucleus. Following HRP injection into lateral hypothalamus at the level of HVM, labeled neurons were seen in the magnocellular basal nucleus. After preoptic injections, neurons containing the HRP reaction product were in cortical and medial nuclei and magnocellular and parvocellular parts of the basal nucleus. In addition to cells in the amygdala, rostral pontine neurons were labeled after HRP injections into HVM. The cells were located ipsilateral to the injection, mostly in the dorsal nucleus of the lateral lemniscus, lateral and dorsolateral to the brachium conjunctivum. The pontine cells labeled following HVM injections of HRP were different from those labeled following lateral hypothalamic and preoptic region injections. The pontine projection to HVM was confirmed using axoplasmic transport autoradiography. A mixture of tritiated leucine and tritiated proline was injected into the lateral pontine region labeled after HRP injections into HVM. Labeled axons ascending in the medial forebrain bundle terminated throughout the rostro-caudal extent of HVM.     

Direct projections from the centre median-parafascicular complex to the subthalamic nucleus in the cat and rat

Single injections of a mixture of [³H] leucine and L-[³H] made into the centre median-parafascicular complex (CM-Pf) of the cat. The ipsilateral subthalamic nucleus (STN) was most heavily labeled at its ros-tral pole and moderately in the ventral and ventromedial portions of its rostral third, At middle subthalamic levels, label was more sparse and disappeared over the caudal third of the nucleus. Labeled fibers appeared to outline the borders of the STN throughout its rostral half. Control in-jections of isotope into several other thalamic andmesodiencephalic regions produced no terminal labeling in the cat STN. After horseradish peroxidase (HRP) injections into the cat STN, a small number of the CM-Pf neurons were labeled retrogradely. The labeled neurons were scattered diffusely in the CM-Pf and were not obviously distinguishable from other unlabeled neurons in the nucleus. Single injections of L-[³H] leucine were made into the CM-Pf of the rat. Anterograde labeling was seen in the rostral half of the ipsilateral STN. Heavy labeling was present throughout the most rostral STN, became less prominent more caudally, and was absent from the caudal half of the STN. HRP injections in the rat CM-Pf produced a distribution of anterograde labeling in the STN similar to the isotope injections. It was concluded that a small number of scattered CM-Pf neurons project primarily to rostral region of the STN. Thus, the CM-Pf is in a unique position to regulate the basal ganglia by way of a newly established thalamosubthal-amic pathway as well as a widespread, conspicuous thalamostriate pathway in those two mammalian species.     

The physiological motor patterns produced by neurons in the nucleus retroambiguus in the rat and their modulation by vagal,peripheral chemosensory,and nociceptive stimulation

The nucleus retroambiguus (NRA) is a neuronal cell group in the medullary ventrolateral tegmentum, rostrocaudally between the obex and the first cervical spinal segment. NRA neurons are premotor interneurons with direct projections to the motoneurons of soft palate, pharynx, and larynx in the nucleus ambiguus in the lateral medulla as well as to the motoneurons in the spinal cord innervating diaphragm, abdominal, and pelvic floor muscles and the lumbosacral motoneurons generating sexual posture. These NRA premotor interneurons receive very strong projections from the periaqueductal gray (PAG) in the context of basic survival mechanisms as fight, flight, freezing, sound production, and sexual behavior. In the present study in rat we investigated the physiological motor patterns generated by NRA neurons, as the result of vagal, peripheral chemosensory, and nociceptive stimulation. The results show that the NRA contains phasic respiratory modulated neurons, as well as nonphasic tonically modulated neurons. Stimulation in the various rostrocaudal levels of the NRA generates site‐specific laryngeal, respiratory, abdominal, and pelvic floor motor activities. Vagal and peripheral chemosensory stimulation induces both excitatory and inhibitory modulation of phasic NRA‐neurons, while peripheral chemosensory and nociceptive stimulation causes excitation and inhibition of nonphasic NRA‐neurons. These results are in agreement with the concept that the NRA represents a multifunctional group of neurons involved in the output of the emotional motor system, such as vomiting, vocalization, mating, and changes in respiration.