Possible origins of substance P-like immunoreactive axons within Onuf's nucleus of the cat

The Dual Nature of Onuf's Nucleus: Neuroanatomical Features and Peculiarities, in Health and Disease

Onuf's nucleus is a small group of neurons located in the ventral horns of the sacral spinal cord. The motor neurons (MNs) of Onuf's nucleus innervate striated voluntary muscles of the pelvic floor and are histologically and biochemically comparable to the other somatic spinal MNs. However, curiously, these neurons also show some autonomic-like features as, for instance, they receive a strong peptidergic innervation. The review provides an overview of the histological, biochemical, metabolic, and gene expression peculiarities of Onuf's nucleus. Moreover, it describes the aging-related pathologies as well as several traumatic and neurodegenerative disorders in which its neurons are involved: indeed, Onuf's nucleus is affected in Parkinson's disease (PD) and Shy-Drager Syndrome (SDS), whereas it is spared in Amyotrophic Lateral Sclerosis (ALS), Spinal Muscular Atrophy (SMA), Duchenne Muscular Dystrophy (DMD). We summarize here the milestone studies that have contributed to clarifying the nature of Onuf's neurons and in understanding what makes them either vulnerable or resistant to damage. Altogether, these works can offer the possibility to develop new therapeutic strategies for counteracting neurodegeneration.

Identified motoneurons involved in sexual and eliminative functions in the rat are powerfully excited by vasopressin and tachykinins

The pudendal motor system is constituted by striated muscles of the pelvic floor and the spinal motoneurons that innervate them. It plays a role in eliminative functions of the bladder and intestine and in sexual function. Pudendal motoneurons are located in the ventral horn of the caudal lumbar spinal cord and send their axon into the pudendal nerve. In the rat, binding sites for vasopressin and tachykinin are present in the dorsomedial and dorsolateral pudendal nuclei, suggesting that these neuropeptides may affect pudendal motoneurons. The aim of the present study was to investigate possible effects of vasopressin and tachykinins on these motoneurons. Recordings were performed in spinal cord slices of young male rats using the whole-cell patch-clamp technique. Before recording, motoneurons were identified by 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine, 4-chlorobenzenesulfonate retrograde labeling. The identification was confirmed, a posteriori, by choline acetyltransferase immunocytochemistry. Vasopressin and tachykinins caused a powerful excitation of pudendal motoneurons. The peptide-evoked depolarization, or the peptide-evoked inward current, persisted in the presence of tetrodotoxin, indicating that these effects were mainly postsynaptic. By using selective receptor agonists and antagonist, we determined that vasopressin acted via vasopressin 1a (V1a), but not V1b, V2, or oxytocin receptors, whereas tachykinins acted via neurokinin 1 (NK1), but not NK2 or NK3, receptors. Vasopressin acted by enhancing a nonselective cationic conductance; in some motoneurons, it also probably suppressed a resting K+ conductance. Our data show that vasopressin and tachykinins can excite pudendal motoneurons and thus influence the force of striated perineal muscles involved in eliminative and sexual functions.

Testosterone prevents synaptic loss in the perineal motoneuron pool in the spinal cord in male rats exposed to chronic stress

Chronic stress is known to induce disorders of reproductive neuroendocrine functions. Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in male rats play an important role in copulatory behavior. In the present study, it was examined whether chronic stress would alter synaptic organization of the SNB motoneurons and whether androgen would modify the changes under chronic stress. Five male rats were under restraint stress for 5 days per week for 3 weeks, and five males implanted subcutaneously with Silastic capsules containing testosterone were also exposed to stress. Five males served as unstressed controls. After 3 weeks of restraint stress, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the SNB were dissected, processed with a modified tetramethylbenzidine (TMB) method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 SNB motoneurons (total for three groups) were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts. The mean percentage of somatic membranes covered by synapses in males exposed to chronic stress was significantly less than that in controls or stressed males treated with testosterone. Size and number of synaptic contacts per unit length of somatic membranes in males exposed to stress were also significantly less than those in controls or stressed males treated with testosterone. There was no significant difference in any of the parameters between controls and stressed males treated with testosterone. Changes in plasma levels of testosterone showed the same profile as changes in the synaptic contacts. These results suggest that the SNB motoneurons of male rats exposed to chronic stress retain a considerable synaptic plasticity in response to androgen, and that androgen treatment can rescue the SNB system in male rats when under chronic restraint stress.

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex. The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.

Androgen stimulates neuronal plasticity in the perineal motoneurons of aged male rats

Ten aged male rats (24 months of age) were castrated and implanted subcutaneously with Silastic capsules containing testosterone (T)(5 males) or nothing (5 males). Five sham-castrated males (25 months of age) served as controls. Four weeks after castration, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 CT-HRP-labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the control and experimental SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in castrated, aged males treated with T was significantly greater than that in control or castrated animals. Size and number of synaptic contacts per unit length of somatic membranes in castrated, aged males treated with T were also significantly greater than those in control or castrated animals. Plasma levels of T in castrated, aged males treated with T were significantly greater than that in controls. These results suggest that the SNB motoneurons of aged male rats retain a considerable synaptic plasticity in response to androgen, and that androgen may be, at least in part, involved in the process of aging of the SNB system in male rats.

Synaptic changes in the perineal motoneurons of aged male rats

Cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles of young (2 months of age) and old (19-20 months of age) male rats, and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 120 CT-HRP-labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the young and old SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in old rats was significantly smaller than that in young ones. Size and number of synaptic contacts per unit length of somatic membranes in old animals were also significantly reduced. Plasma levels of testosterone in old males were significantly smaller than those in young ones. These age-related changes in synaptic inputs to SNB motoneurons and plasma levels of androgen seem to correlate with aging of the SNB system.

Evidence for supraspinal nervous control of external anal sphincter motility in the cat

The aim of this study was to investigate the role of noradrenergic descending nervous pathways in external anal sphincter motility. For this purpose, the effects of intravenously injected adrenoceptor antagonist and agonist on the tonic electrical activity of this sphincter were studied in anesthetized cats. The effects of stimulating the region of the locus coeruleus and the effects of intravenous, intracerebroventricular and intrathecal injection of the above drugs on the electromyographic responses of this muscle to pudendal nerve stimulation were also investigated. The tonic sphincteric activity and the reflex response triggered by electrically stimulating pudendal afferent nerve fibers were inhibited by alpha1-adrenoceptor antagonist nicergoline and enhanced by alpha1-adrenoceptor agonist phenylephrine. Stimulation of the locus coeruleus area either inhibited or enhanced the reflex responses. Intracerebroventricular and intrathecal injection of the alpha2-adrenoceptor agonists, morphine and leu-enkephalin decreased the amplitude of these reflex responses. All the effects of opioids were blocked by naloxone and by spinalization performed at the cervical and lumbar levels. The direct response elicited by stimulating the sphincteric motor axons was not affected either by these drugs or by the brainstem stimulation. These results suggests the existence of a pontine neuronal network controlling the motility of the external anal sphincter via noradrenergic and opioid neurons.

Morphological characterization of substance P receptor-immunoreactive neurons in the rat spinal cord and trigeminal nucleus caudalis

Although there is considerable evidence that primary afferent-derived substance P contributes to the transmission of nociceptive messages at the spinal cord level, the population of neurons that expresses the substance P receptor, and thus are likely to respond to substance P, has not been completely characterized. To address this question, we used an antibody directed against the C-terminal portion of the rat substance P receptor to examine the cellular distribution of the receptor in spinal cord neurons. In a previous study, we reported that the substance P receptor decorates almost the entire dendritic and somatic surface of a subpopulation of spinal cord neurons. In the present study we have taken advantage of this labeling pattern to identify morphologically distinct subpopulations of substance P receptor-immunoreactive neurons throughout the rostral-caudal extent of the spinal cord. We observed a dense population of fusiform substance P receptor-immunoreactive neurons in lamina I at all segmental levels. Despite having the highest concentration of substance P terminals, the substantia gelatinosa (lamina II) contained almost no substance P receptor-immunoreactive neurons. Several distinct populations of substance P receptor-immunoreactive neurons were located in laminae III-V; many of these had a large, dorsally directed dendritic arbor that traversed the substantia gelatinosa to reach the marginal layer. Extensive labeling was also found in neurons of the intermediolateral cell column. In the ventral horn, we found that labeling was associated with clusters of motoneurons, notably those in Onuf's nucleus in the sacral spinal cord. Finally, we found no evidence that primary afferent fibers express the substance P receptor. These results indicate that relatively few, but morphologically distinct, subclasses of spinal cord neurons express the substance P receptor. The majority, but not all, of these neurons are located in regions that contain neurons that respond to noxious stimulation.

Enkephalin-, thyrotropin-releasing hormone- and substance P-immunoreactive axonal innervation of the ventrolateral dendritic bundle in the cat sacral spinal cord: an ultrastructural study

The distribution and synaptic arrangement of thyrotropin-releasing hormone-, substance P- and enkephalin-immunoreactive axonal boutons have been studied in the ventrolateral nucleus (Onuf's nucleus) of the upper sacral spinal cord segments in the cat. For this purpose, the peroxidase-antiperoxidase immunohistochemical technique was used. Immunoreactive axonal boutons were traced in complete series of sections in order to reveal synaptic contacts with the bundled dendrites of the ventrolateral nucleus. As judged from the cross-sectional diameter of the postsynaptic dendrites, the distribution of immunoreactive boutons was non-random. Enkephalin-immunoreactive axonal boutons, presumed to be mostly of segmental origin, displayed a rather restricted distribution to mainly (> 80%) medium-to-large dendrites. Thyrotropin-releasing hormone-immunoreactive boutons, that derive from supraspinal levels, were also found to impinge on medium-to-large dendrites (> 80%), indicating a proximal location within the dendritic trees. The skewness toward large postsynaptic dendrites was even more marked for thyrotropin-releasing hormone- than for enkephalin-immunoreactive boutons. Substance P-immunoreactive boutons, that are of either supraspinal or spinal origin, showed a more even distribution throughout the dendritic trees, including both thin distal branches and thick proximal dendrites. In view of the well-known fact that virtually all thyrotropin-releasing hormone-immunoreactive boutons in the ventral horn co-contain substance P (and serotonin) it was assumed that substance P-immunoreactive boutons in synaptic contact with the finest-calibre dendrites as well as most of those with a very proximal juxtasomatic location on the dendritic trees were of segmental origin, while those impinging on medium-to-large dendrites could be of either spinal or supraspinal origin. Fine-calibre dendrites (< 1 micron) represent about 25% of the dendritic branches in the ventrolateral nucleus, but receive, with the exception of substance P (8%), very little (< 3%) peptidergic or GABAergic (Ramírez-León and Ulfhake, 1993) input, although the degree of dendritic membrane covering by bouton profiles in the ventrolateral nucleus does not seem to vary much with the calibre of the postsynaptic dendrite (Ramírez-León and Ulfhake, 1993). Both substance P- and enkephalin-immunoreactive axonal boutons established synaptic contact with more than one dendrite. Furthermore, one and the same bouton could be found in contact with two dendrites that were coupled to each other by a dendro-dendritic contact of desmosomal or puncta adherentia type.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotoninergic, peptidergic and GABAergic innervation of the ventrolateral and dorsolateral motor nuclei in the cat S1/S2 segments: an immunofluorescence study

Indirect single- and double-staining immunofluorescence techniques were used to study the serotoninergic, peptidergic and GABAergic innervation of the ventrolateral (Onuf's nucleus) and dorsolateral (innervating intrinsic foot sole muscles) nuclei, located in the S1/S2 segments of the cat spinal cord. The relative density of 5-hydroxytryptamine-, thyrotropin-releasing hormone-, substance P- and gamma-aminobutyric acid-immunoreactive axonal varicosities was similar in both nuclei. The highest relative density was recorded for varicosities immunoreactive to gamma-aminobutyric acid, while those immunoreactive to 5-hydroxytryptamine or thyrotropin-releasing hormone yielded the lowest values. The density of enkephalin-immunoreactive varicosities was higher in the ventrolateral than in the dorsolateral nucleus. Calcitonin gene-related peptide-like immunoreactivity could be seen in neurons of the ventrolateral and dorsolateral nuclei. Occasionally, calcitonin gene-related peptide-immunoreactive axonal fibers were also encountered in these nuclei. Virtually all thyrotropin-releasing hormone-immunoreactive varicosities in the ventrolateral and dorsolateral nuclei also contained 5-hydroxytryptamine-like immunoreactivity, while a somewhat smaller number of them were co-localized with substance P. About 5-10% of the 5-hydroxytryptamine-immunoreactive varicosities were devoid of peptide-like immunoreactivity, and the number of 5-hydroxytryptamine-immunoreactive varicosities lacking thyrotropin-releasing hormone-like immunoreactivity was higher in the dorsolateral than in the ventrolateral nucleus. Finally, the free fraction of substance P-immunoreactive varicosities, i.e., those lacking both 5-hydroxytryptamine and thyrotropin-releasing hormone, was about 39% in the ventrolateral and 26% in the dorsolateral nucleus. Spinal cord transection at the lower thoracic level induced a depletion of 5-hydroxytryptamine and thyrotropin-releasing hormone-immunoreactive fibers from the ventrolateral and dorsolateral nuclei, indicating an exclusive supraspinal origin for these fibers. A reduction in substance P-like immunoreactivity following spinal cord transection alone or spinal cord transection combined with unilateral dorsal rhizotomy was also detected in both nuclei, suggesting a dual origin for substance P-immunoreactive fibers, i.e., both supra- and intraspinal. The decrease in number of substance P-immunoreactive fibers was however smaller than expected from the analysis of the fraction of substance P-immunoreactive fibers co-localized with 5-hydroxytryptamine, indicating thus that the experimental lesions may have triggered a sprouting of substance P-immunoreactive axons originating from spinal cord sources. The distribution of gamma-aminobutyric acid in the ventrolateral and dorsolateral nuclei was not affected by the different lesion paradigms. It is therefore assumed that these inputs are intrinsic to the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of enkephalin and its relation to serotonin in cat and monkey spinal cord and brain stem

The distribution of enkephalin (ENK)-like immunoreactivity (LI) in spinal cord and medulla oblongata of cat and gray monkey (Macaca fascicularis) was studied by use of immunofluorescence and peroxidase antiperoxidase (PAP) techniques. Possible coexistence between ENK- and 5-hydroxytryptamine (5-HT)-LI was also analyzed with double labeling immunofluorescence. Furthermore, in situ hybridization was used to demonstrate cell bodies in the brain stem expressing mRNA encoding for ENK. ENK-immunoreactive (IR) axonal varicosities and fibers were demonstrated throughout the spinal cord gray matter, with the highest density in the superficial dorsal horn, the area around the central canal, the intermediolateral cell column, the sacral parasympathetic nucleus, and in Onuf's nucleus. In the monkey ventral horn, ENK-IR varicose fibers could in some cases be demonstrated in very close apposition to cell bodies. A low degree of co-localization between ENK- and 5-HT-LI was seen in the spinal cord of both species. Still, fibers containing both compounds could as a rule be demonstrated in every section studied. The highest degree of coexistence was encountered in the motor nucleus of the ventral horn. Six weeks after a low thoracic spinal cord transection a decreased staining for ENK-LI was demonstrated in the ventral horn motor nucleus, whereas other parts of the spinal cord appeared unaffected. In the brain stem of cats after colchicine treatment, ENK-LI was found in a majority of the 5-HT-IR cell bodies in the raphe nuclei (nucleus raphe magnus, pallidus and obscurus) and in the lateral reticular nucleus (rostroventrolateral reticular nucleus). In cat not pretreated with colchicine, a few weakly stained ENK-IR cell bodies could be found in the midline raphe nuclei and in the lateral reticular nucleus with the PAP technique. In the monkey brain stem without colchicine treatment, using the PAP technique, heavily stained ENK-IR cell bodies could be seen in the lateral reticular nucleus whereas, as in the cat, only a few, weakly stained ENK-IR cell bodies could be seen in the midline raphe nuclei. Using in situ hybridization technique, ENK mRNA expressing cells were demonstrated in the lateral reticular nucleus while no convincing mRNA signal could be found over cell bodies in the raphe nuclei. It is concluded that part of the ENKergic innervation of the cord in both species derives from supraspinal or suprasegmental levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Axon collaterals indicate broad intraspinal role for sacral preganglionic neurons

The classic view of preganglionic neurons in spinal autonomic nuclei is that they convey information exclusively from the central nervous system to autonomic neurons in peripheral ganglia. The present morphological study in the cat sacral spinal cord demonstrates that these neurons may also make abundant synaptic connections within the spinal cord. Neurons labeled intracellularly with neurobiotin or horseradish peroxidase exhibited an expansive distribution of axon collaterals in spinal cord laminae I, V, VII, VIII, IX, X, and the ventrolateral funiculi. This broad-ranging axon-collateral system, which has the potential for multiple neuronal contacts, indicates widespread integrative functions for sacral preganglionic neurons within the spinal cord, in addition to functions currently known in the periphery.