Direct projections from the nucleus retroambiguus to cricothyroid motoneurons in the cat

Vocalization can be elicited by stimulation in the periaqueductal gray (PAG). Light-microscopical tracing and physiological studies have revealed that the PAG uses the nucleus retroambiguus (NRA) as a relay to excite the vocalization muscle motoneurons. Direct NRA projections have been demonstrated to pharyngeal and abdominal wall muscle motoneurons, but not to laryngeal motoneurons. In two cats 0.1% cholera toxin subunit b was injected in the cricothyroid muscle of the larynx to retrogradely label its motoneurons, and 2.5% wheat germ agglutinin-horseradish peroxidase was injected into the NRA to anterogradely label its fibers. The electronmicroscopical results indicate that the NRA fibers make monosynaptic contacts with cricothyroid motoneuronal dendrites. Almost all NRA terminal profiles had asymmetrical synapses and contained mostly round or pleiomorphic vesicles, which strongly suggests that the NRA-cricothyroid motoneuronal projection is an excitatory pathway.

Estrogen receptor-alpha immunoreactivity in parasympathetic preganglionic neurons innervating the bladder in the adult ovariectomized cat

Estrogen affects autonomic functions such as micturition. The sacral cord is important in the control of micturition and contains numerous estrogen receptor-alpha immnoreactive (ER-alpha IR) neurons. Therefore, the present double labeling study examines whether sacral parasympathetic preganglionic neurons innervating the bladder are immunoreactive for ER-alpha. In the sacral cord of seven female ovariectomized cats, the distribution of ER-alpha IR neurons was studied using the H222 and 1D5 antibodies. Choleratoxin subunit b (CTb) was injected into the bladder wall to visualize its preganglionic neurons. ER-alpha IR was present in the nuclei of cells in laminae I, II, V, VII, and X, and in nuclei and cytoplasm of neurons in the sacral parasympathetic nucleus. The vast majority of CTb labeled neurons contained ER-alpha IR nuclei, indicating that preganglionic neurons innervating the bladder express ER-alpha. The results suggest that estrogen modulates micturition in the cat via ER-alpha in bladder preganglionic neurons.

Ultrastructural evidence for direct projections from the pontine micturition center to glycine-immunoreactive neurons in the sacral dorsal gray commissure in the cat

During micturition, according to the concept of Blok, Holstege, and colleagues ([1997] Neurosci. Lett. 233:109-112), the pontine micturition center (PMC) elicits bladder contraction by way of direct excitation of the parasympathetic bladder motoneurons. At the same time, the PMC elicits relaxation of the external urethral sphincter (EUS) by excitation of gamma-aminobutyric acid (GABA)-ergic interneurons in the sacral dorsal gray commissure (DGC), which, in turn, inhibit EUS motoneurons. The question is whether the inhibitory neurotransmitter glycine is also involved in this system. The present study investigated, first, whether there are glycine immunoreactive interneurons in the sacral DGC and, second, whether they receive direct PMC afferents. Finally, it was determined whether glycine and GABA are colocalized in DGC interneurons. In two adult male cats, the PMC was identified by electrical stimulation. Subsequently, the identified region was injected with the anterograde tracer WGA-HRP. Sections of sacral cord segments were processed for light and electron microscopic detection of anterograde labeling, as well as for glycine and GABA, using postembedding immunogold labeling with antibodies. In total 128 labeled PMC terminals were found in the DGC, which contained many round vesicles and asymmetric synapses. About 31.3% (40 of 128) made contact with glycine-immunoreactive dendrites. Eleven of them were selected for serial sectioning, which showed that 54.6% (6 of 11) of the glycine-immunoreactive dendrites were also immunoreactive for GABA. The results demonstrate that the PMC projects directly to dendrites of interneurons in the sacral DGC, which are immunoreactive for both glycine and GABA. These interneurons are thought to inhibit the EUS motoneurons during micturition.

Lamina I-periaqueductal gray (PAG) projections represent only a limited part of the total spinal and caudal medullary input to the PAG in the cat

The periaqueductal gray is well known for its involvement in nociception control, but it also plays an important role in the emotional motor system. To accomplish these functions the periaqueductal gray receives input from the limbic system and from the caudal brainstem and spinal cord. Earlier studies gave the impression that the majority of the periaqueductal gray projecting cells in caudal brainstem and spinal cord are located in the contralateral lamina I, which is involved in nociception. The present study in the cat, however, demonstrates that of all periaqueductal gray projecting neurons in the contralateral caudal medulla less than 7% was located in lamina I. Of the spinal periaqueductal gray projecting neurons less than 29% was located in lamina I. However, within the spinal cord large segmental differences exist: in few segments of the enlargements the lamina I-periaqueductal gray projecting neurons represent a majority. In conclusion, although the lamina I-periaqueductal gray projection is a very important nociceptive pathway, it constitutes only a limited part of the total projection from the caudal medulla and spinal cord to the periaqueductal gray. These results suggest that a large portion of the medullo- and spino-periaqueductal gray pathways conveys information other than nociception.

Segmental and laminar organization of the spinal neurons projecting to the periaqueductal gray (PAG) in the cat suggests the existence of at least five separate clusters of spino-PAG neurons

The present retrograde tracing study in the cat describes the spinal cord projections to the periaqueductal gray (PAG), taking into account different regions of the PAG and all spinal segments. Results show that injecting different parts of the PAG leads to different laminar and segmental distributions of labeled spinal neurons. The impression was gained that at least five separate clusters of spinal neurons exist. Cluster I neurons are found in laminae I and V throughout the length of the cord and are probably involved in relaying nociceptive information to the PAG. Cluster II neurons lie in the ventrolateral part of laminae VI-VII of the C1-C4 spinal cord and were labeled by injecting the ventrolateral or lateral part of the rostrocaudal PAG or the deep tectum. Cluster III neurons are located in lamina X of the thoracic and upper lumbar cord and seem to target the PAG and the deep tectum. Cluster IV neurons are located in the medial part of laminae VI-VII of the lumbosacral cord and seem to project predominantly to the lateral and ventrolateral caudal PAG. These neurons may play a role in conveying tactile stimuli to the PAG during mating behavior. Neurons of cluster V are located in the lateral part of lamina I of L6-S2 and in laminae V-VII and X of S1-S3. They are labeled only after injections into the central portion of the lateral and ventrolateral caudal PAG and probably relay information concerning micturition and mating behavior.

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.

Retroambiguus projections to the cutaneus trunci motoneurons may form a pathway in the central control of mating

Our laboratory has proposed that the nucleus retroambiguus (NRA) generates the specific motor performance displayed by female cats during mating and that it uses direct pathways to the motoneurons of the lower limb muscles involved in this activity. In the hamster a similar NRA-projection system could generate the typical female mating posture, which is characterized by lordosis of the back as well as elevation of the tail. The present study attempted to determine whether this elevation of the tail is also part of the NRA-mating control system. The basic assumption was that elevation of the tail is a function of the cutaneous trunci muscle (CTM), which was verified by bilateral tetanic stimulation of the lateral thoracic nerves innervating the CTM. It resulted in upward movement of the tail to a position similar to the tail-up position during the lordosis posture. Retrograde tracing results showed that CTM motoneurons are located in the ventral and ventrolateral part of the C(7)-C(8) ventral horn, those innervating the tail region ventrolateral to those innervating the axillary region. Anterograde tracing studies showed that NRA fibers terminate bilaterally in both parts of the CTM motoneuronal cell groups. Electron microscopical studies revealed that labeled NRA terminals make monosynaptic contacts with retrogradely labeled dendrites of CTM motoneurons. Almost all of these terminal profiles had asymmetric synapses and contained spherical vesicles, which suggests an excitatory function. The observation that 15% of the labeled NRA terminals make more than one synaptic contact with a retrogradely labeled CTM motoneuronal dendrite within the same section indicates how powerful the NRA-CTM projection is. The results indicate that during mating the NRA not only could generate the lordosis posture but also the elevation of the tail.

The pontine micturition center in rat receives direct lumbosacral input. An ultrastructural study

The act of micturition differs strongly among species. For example, adult cats and humans urinate primarily in a safe environment ('guarded urination'), while rats urinate more reflexively ('reflex urination'). This study in adult rats investigates the existence of direct lumbosacral cord projections to spinally projecting neurons in the pontine micturition center (PMC). Bilateral injections of wheat germ agglutinin horseradish peroxidase in the caudal lumbar and rostral sacral cord resulted in labeled profiles, including retrogradely labeled neurons in the PMC. At the ultrastructural level, anterogradely labeled terminals in the PMC were found, which were filled with many round and some pleiomorphic and flat vesicles. About eleven percent of the terminals contacted retrogradely labeled dendrites. Of the labeled terminals 80% contained asymmetric synaptic clefts, and 20% symmetric synaptic clefts. The results provide evidence that in the rat, unlike the cat, a direct lumbosacral pathway to the PMC exists, which might explain the differences in micturition behavior between rats and cats.

Estrogen receptor-alpha-immunoreactive neurons in the mesencephalon, pons and medulla oblongata of the female golden hamster

Recent studies have revealed brainstem-spinal pathways involved in the generation of receptive behavior in hamster and cat, and the enormous influence of estrogen on these pathways. The present study gives an overview of the location of estrogen receptor-alpha-immunoreactive neurons (ER-alpha-IR) in the brainstem of the female hamster. In the mesencephalon, ER-alpha-IR cells were found in the arcuate and peripeduncular nuclei as well as throughout the rostrocaudal extent of the periaqueductal gray (PAG), and the laterally adjoining tegmentum. In the caudal brainstem, groups of ER-alpha-IR cells were present in the ventrolateral parabrachial nucleus, the solitary nucleus, and in contrast to the cat, in the nucleus retroambiguus. No ER-alpha-IR cells were found in any other part of the brainstem. The functional implications of these findings are discussed.

Two pontine micturition centers in the cat are not interconnected directly: implications for the central organization of micturition

The urinary bladder muscle and its external urethral sphincter are innervated, respectively, by the parasympathetic preganglionic motoneurons in the sacral intermediolateral cell column and somatic motoneurons in Onuf's nucleus. Neurons coordinating the activity of these muscles during micturition and urinary continence are not located in the sacral cord but in two pontine regions, the medial (M)-region (or pontine micturition center) and the lateral (L)-region (or pontine storage center). The M-region excites the bladder muscle through projections to its motoneurons and inhibits the urethral sphincter through excitatory projections to sacral cord gamma-amino butyric acid (GABA)-immunoreactive interneurons, which, in turn, inhibit urethral sphincter motoneurons. The L-region, through direct projections, excites urethral sphincter motoneurons. The present study investigated whether there are interconnections between the M- and L-regions. Anterograde tracing injections in the M-region resulted in labeled fibers to the intermediolateral cell column containing bladder motoneurons but not to Onuf's nucleus. No specific projections were found to the L-regions or to the contralateral M-region. L-region injections resulted in distinct projections to the Onuf's nucleus but not to the sacral intermediolateral cell column. No specific projections were observed either to the M-region or to the contralateral L-region. In conclusion, the M- and L-regions have direct long fiber projections, respectively, to the motoneurons of the bladder muscle and the external urethral sphincter, but they do not influence one another through direct pathways. The results strongly suggest that the M- and L-regions represent separate functional systems that act independently.

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.

Brain activation during micturition in women

Experiments in the cat have led to a concept of how the CNS controls micturition. In a previous study this concept was tested in a PET study in male volunteers. It was demonstrated that specific brainstem and forebrain areas are activated during micturition. It was unfortunate that this study did not involve women, because such results are important for understanding urge incontinence, which occurs more frequently in women than in men. Therefore, a similar study was done in 18 right-handed women, who were scanned during the following four conditions: (i) 15 min prior to micturition (urine withholding); (ii) during micturition; (iii) 15 min after micturition; and (iv) 30 min after micturition. Of the 18 volunteers, 10 were able to micturate during scanning and eight were not, despite trying vigorously. Micturition appeared to be associated with significantly increased blood flow in the right dorsal pontine tegmentum and the right inferior frontal gyrus. Decreased blood flow was found in the right anterior cingulate gyrus during urine withholding. The eight volunteers who were not able to micturate during scanning did not show significantly increased regional cerebral blood flow in the right dorsal, but did so in the right ventral pontine tegmentum. In the cat this region controls the motor neurons of the pelvic floor. In the same unsuccessful micturition group, increased blood flow was also found in the right inferior frontal gyrus. In all 18 volunteers, decreased blood flow in the right anterior cingulate gyrus was found during the period when they had to withhold their urine prior to the micturition condition. The results suggest that in women and in men the same specific nuclei exist in the pontine tegmentum responsible for the control of micturition. The results also indicate that the cortical and pontine micturition sites are more active on the right than on the left side.

Three times as many lamina I neurons project to the periaqueductal gray than to the thalamus: a retrograde tracing study in the cat

The number and distribution of lamina I neurons projecting to the periaqueductal gray (PAG) were examined by a retrograde tracing study in the cat. WGA-HRP injections in the intermediate and caudal PAG resulted in as much as 1600 labeled lamina I neurons throughout the length of the spinal cord, counted in a 1:4 series of sections. The lamina I-PAG projection was predominantly contralateral and most labeled lamina I neurons were found in the enlargements. Comparing these results with the number of lamina I-thalamic neurons leads to the conclusion that in the cat about three times as many lamina I neurons project to the PAG than to the thalamus. Considering this, one can conclude that the spino-PAG system is a virtually neglected area in pain research.

The anatomy of the central control of posture: consistency and plasticity

Posture is usually thought to be steered by brainstem and cortical structures that have access to the motoneurons and their premotor interneurons of the axial and neck muscles. The present paper describes these pathways and their relation with gaze control structures. All these systems belong to the medial component of the so-called voluntary motor system. On the other hand, in the cat there also exist several postures that are not steered by the somatic, but by the emotional motor system. Examples are arching of the back and mating postures. The pathways thought to be involved in these behaviours are described. They belong to the lateral component of the emotional motor system and, especially in case of mating postures, are extremely plastic. They have been demonstrated to be almost 10 times as strong in oestrus than in non-oestrus cats.

Electrical stimulation of the sacral dorsal gray commissure evokes relaxation of the external urethral sphincter in the cat

Stimulation of the pontine micturition center (PMC) results in micturition, i.e. an immediate relaxation of the urethral sphincter and a contraction of the detrusor muscle of the bladder. The PMC generates the bladder contraction by way of a direct excitatory pathway to the parasympathetic bladder motoneurons in the sacral cord. The idea is that the PMC produces the relaxation of the urethral sphincter via direct projections to GABAergic neurons in the dorsal gray commissure (DGC), which, in turn, inhibit the urethral sphincter motoneurons. According to this hypothesis, electrical stimulation in the DGC in three cats should result in relaxation of the urethral sphincter. The results were in total agreement with this concept. During DGC stimulation a sharp decrease of the urethral pressure was found, the strength of which depended completely on the amplitude of the electrical stimulation.

The central nervous system control of micturition in cats and humans

Recent findings concerning the central control of micturition in cats are compared to findings obtained from dynamic imaging studies in humans. In the cat, three areas in the brainstem and diencephalon are specifically implicated in the control of micturition: (1) Barrington's nucleus or the pontine micturition center in the dorsomedial pontine tegmentum directly excites bladder motoneurons and indirectly inhibits, via inhibitory interneurons in the medial sacral cord, urethral sphincter motoneurons; (2) the periaqueductal grey receiving bladder filling information; and (3) the pre-optic area of the hypothalamus possibly involved in determining the beginning of micturition. According to PET-scan studies, in humans the same supraspinal regions are active during micturition. In the cat another area, located in the ventrolateral pontine tegmentum and is called the L-region, which controls the motoneurons of the pelvic floor, including the external urethral sphincter. This region might be considered as the pontine storage center. In humans the L-region is especially active in volunteers who tried but did not succeed to micturate. The results suggest that in cats and humans at the brainstem and diencephalic levels micturition is organized in the same way.

Sensory and motor components of reproductive behavior: pathways and plasticity

Reproductive behavior in most mammalian species consists of a highly stereotyped pattern of movements, is elicited by specific sensory stimuli and is sex steroid dependent. The present paper describes a concept of the pathways in the midbrain, brainstem and spinal cord which control the receptive posture of the female cat. The midbrain periaqueductal gray (PAG), which is an important structure in the Emotional Motor System (EMS), receives direct input from a distinct group of neurons in the dorsal horn of the lumbosacral cord. This cell group overlaps with the location of pelvic and to lesser extent, pudendal nerve primary afferents, which convey information from the pelvic viscera and sex organs to the central nervous system. The PAG, in turn, controls various motor components of female receptive behavior using different pathways. For example, immobility, which is one of the characteristics of receptive behavior, might be mediated by a diffuse pathway from the PAG, via the ventral part of the medial medullary tegmentum, to all parts of the spinal ventral horn. More specific components, such as hindlimb treading, lateral deviation of the tail and elevation of the lower back, are thought to be controlled by a circumscribed projection from the PAG to the nucleus retroambiguus (NRA). The NRA is a group of interneurons at the transition between brainstem and spinal cord and projects directly to distinct lumbosacral motoneuronal cell groups, which innervate muscles that are likely to be involved in the female receptive posture. Estrogen induces axonal sprouting of the NRA-lumbosacral pathway in adult female cats, which explains why female cats only display receptive behavior when estrogen levels are high.

Premature ejaculation and serotonergic antidepressants-induced delayed ejaculation: the involvement of the serotonergic system

Premature ejaculation has generally been considered a psychosexual disorder with psychogenic aetiology. Although still mainly treated by behavioural therapy, in recent years double-blind studies have indicated the beneficial effects of some of the serotonergic antidepressants (SSRIs) in delaying ejaculation. We describe here the neurophysiology and the peripheral neuroanatomy of ejaculation and provide a review of the involvement of serotonin in the central nervous system in relation to serotonergic nuclei and their projections. A hypothesis of the role of 5-HT1A and 5-HT2C receptors in premature ejaculation is postulated.

Estrogen receptor-alpha-immunoreactive neurons in the periaqueductal gray of the adult ovariectomized female cat

Anatomical and physiological studies in rodent and cat have shown that distinct parts of the midbrain periaqueductal gray (PAG) are important for the estrogen dependent, female reproductive behavior. The present study gives a detailed overview of the estrogen receptor-alpha-immunoreactive (ER-IR) neurons in the PAG in the cat. ER-IR neurons were found throughout the rostrocaudal extent of the PAG and laterally adjacent tegmentum, but were most numerous at caudal levels. The lateral and dorsal PAG contained most ER-IR neurons, whereas moderate numbers were found dorsolaterally. In these areas, only very few ER-IR neurons were found near the border of the ependymal layer. Except for the rostral dorsal raphe nucleus, the ventral PAG contained only few ER-IR neurons.

A PET study on cortical and subcortical control of pelvic floor musculature in women

The pelvic floor musculature plays an important role in behaviors such as defecation, micturition, mating behavior, and vomiting. A recent positron emission tomography (PET) study revealed that structures belonging to the emotional motor system are involved in the control of the pelvic floor during micturition. However, there also exist brain structures involved in the voluntary motor control of the pelvic floor, and the present PET study was designed to identify these structures. Six adult female volunteers were scanned with the bolus injection of H2(15)O during the following four conditions: (1) rest, (2) repetitive pelvic floor straining, (3) sustained pelvic floor straining, and (4) sustained abdominal straining. The results revealed that the superomedial precentral gyrus, the most medial portion of the motor cortex, is activated during pelvic floor contraction and the superolateral precentral gyrus during contraction of the abdominal musculature. In these conditions, significant activations were also found in the cerebellum, supplementary motor cortex, and thalamus. The right anterior cingulate gyrus was activated during sustained pelvic floor straining. No activations were found in subcortical structures belonging to the emotional motor system. The results are discussed in light of the existing literature on human control of the pelvic floor and micturition.

Large segmental differences in the spinal projections to the periaqueductal gray in the cat

The periaqueductal gray (PAG) is involved in motor activities, such as movements of the neck, back and hind limbs, cardiovascular regulation, micturition, vocalization, and mating behavior, as well as in nociception control. To accomplish these functions the PAG uses information from other parts of the limbic system, from the lower brainstem, and from the spinal cord. To study the ascending projections from the spinal cord to the PAG, tracer was injected in different parts of the PAG, and the number of retrogradely labeled neurons were counted for each spinal segment. Results show that large segmental differences exist in the number of PAG projecting neurons throughout the length of the spinal cord and that different parts of the spinal cord project to specific areas in the PAG.

The lumbar cord location of the motoneurons innervating psoas and iliacus muscles: a single and double labeling study in the female Syrian golden hamster

The spinal cord location of the motoneurons innervating the psoas and iliacus muscles was determined in the golden hamster. The results of single and double labeling studies, using the retrograde tracers horseradish peroxidase (HRP) and cholera toxin B-subunit (CTB), showed that both psoas and iliacus motoneurons were present ventrolaterally in the ventral horn in the caudal L1 to rostral L5 lumbar spinal segments with their motoneurons intermingled in one cell group. Further retrograde tracing studies demonstrated abdominal muscle motoneurons ventrolaterally in the ventral horn of the L1 and upper L2 segments. Double labeling experiments revealed that at these levels (caudal L1 and rostral L2), the abdominal muscle motoneurons were located dorsomedial to the psoas and iliacus motoneurons.

Estrogen receptor-immunoreactive neurons in the lumbosacral cord projecting to the periaqueductal gray in the ovariectomized female cat

The periaqueductal gray (PAG) plays a crucial role in reproductive behavior. The present study investigates whether lumbosacral PAG-projecting neurons contain estrogen receptors. In four ovariectomized adult female cats, injections with cholera toxin subunit (CTb) were made into the PAG to retrogradely label PAG projecting neurons in the lumbosacral cord. Estrogen receptor immunoreactive ER-IR neurons in the lumbosacral cord were identified immunohistochemically using the antibody H222. PAG-projecting neurons that were immunoreactive for the estrogen receptor were very scarce, and predominantly present in the medial part of the ventral horn. The results indicate that only very few of the neurons relaying information from the urogenital organs to the PAG contain estrogen receptors.

The pontine micturition center projects to sacral cord GABA immunoreactive neurons in the cat

Stimulation of the pontine micturition center (PMC) results in micturition, i.e. an immediate relaxation of the bladder sphincter and a contraction of the detrusor muscle of the bladder. Earlier studies have shown that the bladder contraction is brought about by a direct excitatory pathway from the PMC to the parasympathetic bladder motoneurons in the sacral cord. How the PMC produces the inhibition of the bladder sphincter is not known. The present study in two adult male cats demonstrates at the ultrastructural level a direct pathway from the PMC to the dorsal gray commissure of the sacral cord. More than half (55%) of these terminals made contact with gamma amino butyric acid (GABA) immunoreactive neurons or somata, the others with non-GABA immunoreactive profiles. The PMC terminals contained many round vesicles, some dense cored vesicles and exclusively asymmetric synaptic clefts, which correspond with an excitatory pathway. A concept is put forward in which this pathway produces the relaxation of the bladder sphincter during micturition.

The late blink reflex response abnormality due to lesion of the lateral tegmental field

We report on a blink reflex abnormality observed in two patients, which provides additional information on the central pathways mediating this reflex. Autopsy was performed in one patient and MRI in the other. In the first patient there was a small lesion at the dorsal middle third of the lateral tegmental field and in the second patient at the level of the dorsal lower third of the medulla oblongata. In both patients the common finding was the absence of the late response (R2) ipsilateral to the side of the lesion, while the R2 response on the unaffected side was normal regardless of the side of the supraorbital nerve stimulation. The R1 responses were normal. This type of blink reflex abnormality has not been reported before and is referred to by us as 'tegmental type' of R2 abnormality. The results led to the conclusions that: (i) the crossed and uncrossed ascending trigeminofacial connections are mediated through the lateral tegmental field; (ii) the uncrossed trigeminofacial connection originates at the level of at least the lower medulla oblongata; (iii) the contralateral R2 response is established by way of an ascending pathway, which crosses the midline at the level of at least the lower third of the medulla oblongata.

Evidence for a periaqueductal gray-nucleus retroambiguus-spinal cord pathway in the rat

The nucleus retroambiguus in the cat has been shown to receive strong projections from the periaqueductal gray and to send fibres to distinct motoneuronal cell groups in brainstem and spinal cord. The nucleus retroambiguus plays a role in the production of vocalization and possibly copulatory (lordosis and mounting) behaviour. The question arises of whether a periaqueductal gray nucleus retroambiguus-spinal cord projection also exists in the rat. In the present study, using the retrograde wheatgerm agglutinin-horseradish peroxidase tracing technique, the nucleus retroambiguus was defined as the area in the caudal medulla oblongata (1.0-2.0 mm caudal to the obex) which sends its fibres mainly through the contralateral spinal cord. Further retrograde tracing experiments demonstrated that a relatively large number of neurons in the lateral and ventral periaqueductal gray and immediately adjacent tegmentum projects to the caudal medullary lateral tegmentum. Anterograde wheatgerm agglutinin-horseradish peroxidase tracing studies finally showed that neurons in the lateral periaqueductal gray and immediately adjoining tegmentum project specifically to the nucleus retroambiguus and not to the lateral tegmentum in general, which seems to be the case for the neurons in the ventral periaqueductal gray. The results indicate that in the rat a periaqueductal gray nucleus retroambiguus spinal cord projection also exists, which may be of crucial importance for the study of the anatomical and physiological framework of respiration, vocalization, and female and male reproductive behaviour in this animal.

Organization of lumbosacral motoneuronal cell groups innervating hindlimb, pelvic floor, and axial muscles in the cat

In a study on descending pathways from the nucleus retroambiguus (NRA) to hindlimb motoneurons (see accompanying paper), it appeared impossible, using data from the literature, to precisely determine which muscles were innervated by the motoneurons receiving the NRA fibers. This lack of data made it necessary to produce a detailed map of the lumbosacral motoneuronal cell groups in the cat. Therefore, 50 different muscles or muscle compartments of hindlimb, pelvic floor and lower back were injected with horseradish peroxidase (HRP) in 135 cases. The respective muscles were divided into ten groups: I, sartorius and iliopsoas; II, quadriceps; III, adductors; IV, hamstrings; V, gluteal and other proximal muscles of the hip; VI, posterior compartment of the distal hindlimb; VII, anterior compartment of the distal hindlimb; VIII, long flexors and intrinsic muscles of the foot; IX, pelvic floor muscles; and X, extensors of the lower back and tail. The L4-S2 segments were cut and incubated, and labeled motoneurons were counted and plotted. A new method was developed that made it possible, despite variations in size and segmental organization between the different cases, to compare the results of different cases. The results show that the spatial interrelationship between the hindlimb and pelvic floor lumbosacral motoneuronal cell groups remains constant. This finding enabled the authors to compose an accurate overall map of the location of lumbosacral motoneuronal cell groups. The general distribution of the motoneuronal cell groups is also discussed in respect to their dorsoventral, mediolateral, and rostrocaudal position within the lumbosacral ventral horn.

Nucleus retroambiguus projections to lumbosacral motoneuronal cell groups in the male cat

Recently, in the female cat, nucleus retroambiguus (NRA) projections have been described as distinct motoneuronal cell groups in the lumbar enlargement, possibly involved in lordosis behavior. The present study deals with the NRA-lumbosacral pathway in the male cat. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify retrogradely labeled neurons in the caudal medulla. These injections were preceded by spinal hemisections to distinguish between neurons with ipsi- and contralaterally descending axons. The NRA-lumbosacral fibers descended almost exclusively contralaterally, but neurons in areas surrounding the NRA projected mainly ipsilaterally. Injections of WGA-HRP were made in the region of the NRA to determine its targets in the lumbosacral cord. To distinguish between the contralateral NRA pathways and the ipsilateral projections from neurons in the adjoining lateral tegmentum, the injections were preceded by ipsilateral hemisections in C2. A new scaling method was used to compare the results of the different cases, despite variations in size and segmental organization. The results show that the distribution pattern of anterogradely labeled fibers in the lumbosacral cord matched precisely the location of certain motoneuronal cell groups. The NRA projected densely to the abdominal wall and pelvic floor motoneurons in Onuf's nucleus, moderately to adductor longus, semimembranosus, and biceps femoris anterior motoneuronal cell groups, and only sparsely to iliopsoas and semitendinosus motoneuronal cell groups. Compared with the findings in the female, the NRA in the male cat projects more heavily to the biceps anterior and adductor longus and only sparsely to the iliopsoas and semitendinosus motoneuronal cell groups. These male-female differences are discussed.

Motoneuronal location of the external urethral and anal sphincters: a single and double labeling study in the male and female golden hamster

The location of external urethral (EUS) and anal sphincter (EAS) motoneurons was investigated in the golden hamster using the retrograde tracers horseradish peroxidase and cholera toxin B-subunit. Single and double labeling studies revealed that the motoneurons of the EUS and EAS were present in the same nucleus (nucleus of Onuf) ventrolaterally in the ventral horn of the caudal first sacral segment and throughout the second sacral segment. Within Onuf's nucleus the EAS motoneurons were located dorsomedial to the EUS motoneurons, which were located at the border of the gray and white matter. This location is similar to that in cat, dog, monkey and man, but differs from that in rat, Mongolian gerbil and domestic pig.

Evidence for monosynaptic projections from the nucleus retroambiguus to hindlimb motoneurons in the cat

The nucleus retroambiguus (NRA) is a group of premotor neurons at the transition between brainstem and spinal cord. It projects to certain motoneuronal cell groups, among which is a distinct set of motoneurons in the lumbar enlargement innervating muscles including iliopsoas, adductor longus, and hamstrings. To find out whether these NRA-motoneuronal projections are monosynaptic, injections of wheat germ-agglutinin horseradish peroxidase (WGA-HRP) into the NRA were combined with injections of cholera toxin subunit b (CTb) into the hamstring muscles. Electron microscopical examination revealed that the NRA terminal profiles make monosynaptic contacts with dendrites of motoneurons innervating these muscles. The NRA terminal profiles formed asymmetrical synapses, and contained spherical and a few dense core vesicles. These findings provide evidence of monosynaptic NRA-hindlimb motoneuronal projections which are likely to be excitatory.

Ultrastructural evidence for a direct pathway from the pontine micturition center to the parasympathetic preganglionic motoneurons of the bladder of the cat

Light microscopy tracing studies have provided evidence that the pontine micturition center (PMC) projects to the area of the intermediolateral cell column of the sacral spinal cord. Although this region contains parasympathetic preganglionic motoneurons of the bladder and colon, it also contains many local interneurons and neurons projecting to supraspinal levels. The present study demonstrates that neurons in the PMC indeed project to preganglionic bladder motoneurons. Wheat germ agglutinin horseradish peroxidase was injected in the PMC and cholera toxin B subunit was injected in the bladder wall. Many anterogradely labeled fibers from the PMC were found to terminate on somata and dendrites of the retrogradely labeled preganglionic bladder motoneurons. The terminals were filled with many round vesicles and possessed an asymmetric synaptic cleft, suggesting an excitatory function.

Estrogen induces axonal outgrowth in the nucleus retroambiguus-lumbosacral motoneuronal pathway in the adult female cat

In 1995, we discovered a new pathway in the cat, which originates from the nucleus retroambiguus (NRA) and terminates in a distinct set of lumbosacral hindlimb, axial, and pelvic floor motoneuronal cell groups [VanderHorst VGJM, Holstege G (1995) Caudal medullary pathways to lumbosacral motoneuronal cell groups in the cat: evidence for direct projections possibly representing the final common pathway for lordosis. J Comp Neurol 359:457-475]. The NRA is a compact group of interneurons located laterally in the caudal medulla oblongata. Its projection to lumbosacral motoneurons is thought to represent the final common pathway for male mounting and for female receptive or lordosis behavior. However, females only display lordosis behavior. However, females only display lordosis behavior when they are in estrus, which suggests that the NRA-lumbosacral pathway is only active during estrus. This raised the question of whether estrogen affects this pathway. The effect of estrogen on the NRA-lumbosacral projection was studied light microscopically, using wheat-germ agglutinin horseradish peroxidase (WGA-HRP) as a tracer. The rubrospinal pathway served as control. The density of labeled NRA fibers in their target hindlimb motoneuronal cell groups appeared abundant in estrous and very weak in nonestrous cats. Such differences were not found in the rubrospinal pathway. For electron microscopical study, the NRA projection to the semi-membranosus motoneuronal cell group was selected. In this cell group, an almost ninefold increase of labeled profiles was found in estrous versus nonestrous cats. Moreover, the semimembranous motoneuronal cell group contained labeled growth cones in estrous, but not in nonestrous, cats. The present study is the first to show that estrogen induces axonal outgrowth of a precisely identified pathway in the adult mammalian central nervous system. The possible mechanisms underlying this outgrowth are discussed.

A PET study on brain control of micturition in humans

Although the brain plays a crucial role in the control of micturition, little is known about the structures involved. Identification of these areas is important, because their dysfunction is though to cause urge incontinence, a major problem in the elderly. In the cat, three areas in the brainstem and diencephalon are specifically implicated in the control of micturition: the dorsomedial pontine tegmentum, the periaqueductal grey, and the preoptic area of the hypothalamus. PET scans were used to test whether these areas are also involved in human micturition. Seventeen right-handed male volunteers were scanned during the following four conditions: (i) 15 min prior to micturition during urine withholding: (ii) during micturition; (iii) 15 min after micturition; (iv) 30 min after micturition. Ten of the 17 volunteers were able to micturate during scanning. micuturition was associated with increased blood flow in the right dorsomedial pontine tegmentum, the periaqueductal grey, the hypothalamus and the right inferior frontal gyrus. Decreased blood flow was found in the right anterior cingulate gyrus when urine was withheld. The other seven volunteers were not able to micturate during scanning, although they had a full bladder and tried vigorously to do so. In this group, during these unsuccessful attempts to micturate, increased blood flow was found in the right ventral pontine tegmentum, which corresponds with the hypothesis, formulated from results in cats, that this area controls the motor neurons of the pelvic floor. Increased blood flow was also found in the right inferior frontal gyrus during unsuccessful attempts at micturition, and decreased blood flow in the right anterior cingulate gyrus was found during the withholding of urine. The results suggest that, as that of the cat, the human brainstem contains specific nuclei responsible for the control of micturition, and that the cortical and pontine micturition sites are predominantly on the right side.

Distinct cell groups in the lumbosacral cord of the cat project to different areas in the periaqueductal gray

The periaqueductal gray (PAG) is involved in aggressive and defensive behavior, micturition, and lordosis. Especially for the latter two functions, PAG afferents from the lumbosacral cord are of vital importance because, in addition to information regarding homeostasis and thermoregulation, they convey information from the pelvic viscera and sex organs. In the present retro- and antero-grade tracing study, the projection patterns of different lumbosacral cell groups in the PAG were determined. In the retrograde study, wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injections were made in the PAG and/or adjacent tegmentum, and in the anterograde study, WGA-HRP was injected in different lumbosacral segments. The results revealed that lumbosacral-PAG neurons could be divided into three groups. The first and largest group was present in lumbar 7-sacral 3 segments (L7-S3) and consisted of small, oval, and fusiform neurons. It extended from the dorsolateral part of lamina I in L7, along the lateral part of the dorsal horn in S1, and into lamina V of S2. In the lateral part of S2, some of its neurons formed clusters with intervals of +/- 230 microns. The location of the first group overlapped extensively with the termination area of pelvic and pudendal afferents. The main midbrain target of the first group was the medial part of the lateral PAG. The second group consisted of small to large multipolar neurons in laminae VIII and medial VII of caudal L6, L7, and rostral S1. This group projected strongly to a distinct region in the lateral part of the lateral PAG and the laterally adjacent tegmentum. About 10% of the labeled neurons did not fit in the two groups. They were evenly distributed throughout lumbar 4-coccygeal 3 segments (L4-Co3) and consisted of large multipolar lamina V neurons and small lamina I neurons that projected diffusely to the lateral and dorsal PAG. The large lamina V neurons also targeted the laterally adjacent tegmentum. The possible involvement of the lumbosacral-PAG projections in micturition, lordosis, and defensive and aggressive behavior is discussed.

Dorsal border periaqueductal gray neurons project to the area directly adjacent to the central canal ependyma of the C4-T8 spinal cord in the cat

In a previous study horseradish peroxidase (HRP) injections in the upper thoracic and cervical spinal cord revealed some faintly labeled small neurons at the dorsal border of the periaqueductal gray (PAG). The present light microscopic and electronmicroscopic tracing study describes the precise location of these dorsal border PAG-spinal neurons and their terminal organization. Wheat germ agglutinin-conjugated HRP (WGA-HRP) injections into cervical and upper thoracic spinal segments resulted in several hundreds of small retrogradely labeled neurons at the dorsal border of the ipsilateral caudal PAG. These neurons were not found after injections in more caudal segments. WGA-HRP injections in the dorsal border PAG region surprisingly resulted in anterogradely labeled fibers terminating in the area dorsally and laterally adjoining the central canal ependyma of the C4-T8 spinal cord. No anterogradely labeled fibers were found more caudal in the spinal cord. The labeled fibers found in the upper cervical cord were not located in the area immediately adjoining the ependymal layer of the central canal, but in the lateral part of laminae VI, VII and VIII and in area X bilaterally. Electronmicroscopic results of one case show that the dorsal border PAG-spinal neurons terminate in the neuropil of the subependymal area and in the vicinity of the basal membranes of capillaries located laterally to the central canal. The terminal profiles contain electron-lucent and densecored vesicles, suggesting a heterogeneity of possible transmitters. A striking observation was the lack of synaptic contacts, suggesting nonsynaptic release from the profiles. The function of the dorsal border PAG-spinal projection is unknown, but considering the termination pattern of the dorsal border PAG neurons on the capillaries the intriguing similarity between this projection system and the hypothalamohypophysial system is discussed.

Location of external anal sphincter motoneurons in the sacral cord of the female domestic pig

The location of the striated external anal sphincter motoneurons in the spinal cord was investigated in 12, between 3 and 4 months old, female domestic pigs using the retrograde tracer horseradish peroxidase (HRP). Their motoneuronal cell bodies were found in the spinal segments S1-S3, and were not located in the ventral horn, but dorsolateral to the central canal. This location within the spinal gray matter strongly differs from the location of the external and sphincter motoneurons in rat, cat, dog, monkey and humans, but is similar to that in the Mongolian gerbil. The possible relevance of this 'aberrant' location is discussed.

Pontine and medullary projections to the nucleus retroambiguus: a wheat germ agglutinin-horseradish peroxidase and autoradiographic tracing study in the cat

The nucleus retroambiguus (NRA) in the caudal medulla oblongata plays a role in expiration, vocalization, vomiting, and possibly lordosis. The present study tried to determine which structures, in turn, control the NRA. One cell group is the periaqueductal gray (PAG), which is considered to be the final integrator of defensive and aggressive behaviors, micturition, vocalization, and lordosis. Structures rostral to the PAG seem to bypass the NRA. With respect to the existence of cell groups caudal to the PAG projecting to the NRA, the situation is less clear. Therefore, in five adult female cats, injections of wheat germ agglutinin-horseradish peroxidase were centered on the NRA, and the resulting retrogradely labeled neurons were plotted. In the areas containing retrogradely labeled cells, the anterograde autoradiographic tracer [3H]-leucine was injected in 66 cats. The combined results demonstrated that NRA afferents not only originate from the PAG but also from specific cell groups in the pontine and medullary lateral tegmental field, i.e., the ventrolateral parabrachial nucleus, the nucleus Kölliker-Fuse, the retrotrapezoid nucleus, and the ventrolateral part of the medulla caudal to the facial nucleus including the Bötzinger and pre-Bötzinger complex and the periambigual region. Afferents also originate from the solitary nucleus and two cell groups in the ventral part of the medullary medial tegmental field, one at the level of facial nucleus and one just rostral to the hypoglossal nucleus. It can be concluded that many respiratory-related cell groups have direct access to the NRA. The cell groups in the medial tegmental field, which have not yet been found to play an important role in respiration, might serve as relay for certain limbic system cell groups to reach the NRA in the context of specific emotional behavior.

Blepharospasm in association with a lower pontine lesion

A patient with neurofibromatosis type I and breast carcinoma developed a bilateral but asymmetric blepharospasm and paresis of the left abducens muscle over a 2-week course. MRI disclosed a small lesion in the left dorsomedial lower pontine region. Electrodiagnostic investigations revealed bilateral R1 responses after stimulation of the left supraorbital nerve and enhancement of R1 and R2 recovery curves. We concluded that lesions in the lower pontine tegmentum may cause blepharospasm.