The relationship between serotonin, dopamine beta hydroxylase and GABA immunoreactive inputs and spinal preganglionic neurones projecting to the major pelvic ganglion of wistar rats

Exogenous serotonin regulates colorectal motility via the 5-HT2 and 5-HT3 receptors in the spinal cord of rats

BACKGROUND

We previously reported that intrathecal injection of noradrenaline or dopamine causes enhancement of colorectal motility. As these monoamines are neurotransmitters of descending pain inhibitory pathways in the spinal cord, we hypothesized that serotonin, which is one of the neurotransmitters involved in descending pain inhibition, also influences the lumbosacral defecation center. Therefore, we examined whether serotonin acting on the spinal defecation center enhances colorectal motility.

METHODS

Colorectal intraluminal pressure and propelled liquid volume were recorded in vivo in anesthetized rats.

KEY RESULTS

Intrathecal injection of serotonin into the L6-S1 spinal cord elicited periodic increases in colorectal intraluminal pressure, being associated with increases in liquid output. Pharmacological experiments revealed that the effect of serotonin is mediated by both 5-HT₂ and 5-HT₃ receptors. The serotonin-induced enhancement of colorectal motility was unaffected even after disconnection of the defecation center from supraspinal regions by cutting the T8 spinal cord, while transection of the parasympathetic pelvic nerves prevented the colokinetic effect of serotonin. Finally, we investigated interactions among serotonin, noradrenaline and dopamine. Simultaneous administration of sub-effective doses of these monoamine neurotransmitters into the spinal cord caused propulsive colorectal motility slightly but substantially.

CONCLUSIONS AND INFERENCES

These results demonstrate that exogenous serotonin acts on 5-HT₂ and 5-HT₃ receptors in the lumbosacral defecation center and activates the parasympathetic nervous system to enhance colorectal motility in cooperation with noradrenaline and dopamine.

Human autoantibodies to amphiphysin induce defective presynaptic vesicle dynamics and composition

See Irani (doi:10.1093/awv364) for a scientific commentary on this article. 

Stiff-person syndrome is the prototype of a central nervous system disorder with autoantibodies targeting presynaptic antigens. Patients with paraneoplastic stiff-person syndrome may harbour autoantibodies to the BAR (Bin/Amphiphysin/Rvs) domain protein amphiphysin, which target its SH3 domain. These patients have neurophysiological signs of compromised central inhibition and respond to symptomatic treatment with medication enhancing GABAergic transmission. High frequency neurotransmission as observed in tonic GABAergic interneurons relies on fast exocytosis of neurotransmitters based on compensatory endocytosis. As amphiphysin is involved in clathrin-mediated endocytosis, patient autoantibodies are supposed to interfere with this function, leading to disinhibition by reduction of GABAergic neurotransmission. We here investigated the effects of human anti-amphiphysin autoantibodies on structural components of presynaptic boutons ex vivo and in vitro using electron microscopy and super-resolution direct stochastic optical reconstruction microscopy. Ultrastructural analysis of spinal cord presynaptic boutons was performed after in vivo intrathecal passive transfer of affinity-purified human anti-amphiphysin autoantibodies in rats and revealed signs of markedly disabled clathrin-mediated endocytosis. This was unmasked at high synaptic activity and characterized by a reduction of the presynaptic vesicle pool, clathrin coated intermediates, and endosome-like structures. Super-resolution microscopy of inhibitory GABAergic presynaptic boutons in primary neurons revealed that specific human anti-amphiphysin immunoglobulin G induced an increase of the essential vesicular protein synaptobrevin 2 and a reduction of synaptobrevin 7. This constellation suggests depletion of resting pool vesicles and trapping of releasable pool vesicular proteins at the plasma membrane. Similar effects were found in amphiphysin-deficient neurons from knockout mice. Application of specific patient antibodies did not show additional effects. Blocking alternative pathways of clathrin-independent endocytosis with brefeldin A reversed the autoantibody induced effects on molecular vesicle composition. Endophilin as an interaction partner of amphiphysin showed reduced clustering within presynaptic terminals. Collectively, these results point towards an autoantibody-induced structural disorganization in GABAergic synapses with profound changes in presynaptic vesicle pools, activation of alternative endocytic pathways, and potentially compensatory rearrangement of proteins involved in clathrin-mediated endocytosis. Our findings provide novel insights into synaptic pathomechanisms in a prototypic antibody-mediated central nervous system disease, which may serve as a proof-of-principle example in this evolving group of autoimmune disorders associated with autoantibodies to synaptic antigens.

Neurogenic mechanisms in bladder and bowel ageing

The prevalence of both urinary and faecal incontinence, and also chronic constipation, increases with ageing and these conditions have a major impact on the quality of life of the elderly. Management of bladder and bowel dysfunction in the elderly is currently far from ideal and also carries a significant financial burden. Understanding how these changes occur is thus a major priority in biogerontology. The functions of the bladder and terminal bowel are regulated by complex neuronal networks. In particular neurons of the spinal cord and peripheral ganglia play a key role in regulating micturition and defaecation reflexes as well as promoting continence. In this review we discuss the evidence for ageing-induced neuronal dysfunction that might predispose to neurogenic forms of incontinence in the elderly.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

Nuclear expression of PG-21, SRC-1, and pCREB in regions of the lumbosacral spinal cord involved in pelvic innervation in young adult and aged rats

In rats, ageing results in dysfunctional patterns of micturition and diminished sexual reflexes that may reflect degenerative changes within spinal circuitry. In both sexes the dorsal lateral nucleus and the spinal nucleus of the bulbospongiosus, which lie in the L5-S1 spinal segments, contain motor neurons that innervate perineal muscles, and the external anal and urethral sphincters. Neurons in the sacral parasympathetic nucleus of these segments provide autonomic control of the bladder, cervix and penis and other lower urinary tract structures. Interneurons in the dorsal gray commissure and dorsal horn have also been implicated in lower urinary tract function. This study investigates the cellular localisation of PG-21 androgen receptors, steroid receptor co-activator one (SRC-1) and the phosphorylated form of c-AMP response element binding protein (pCREB) within these spinal nuclei. These are components of signalling pathways that mediate cellular responses to steroid hormones and neurotrophins. Nuclear expression of PG-21 androgen receptors, SRC-1 and pCREB in young and aged rats was quantified using immunohistochemistry. There was a reduction in the number of spinal neurons expressing these molecules in the aged males while in aged females, SRC-1 and pCREB expression was largely unchanged. This suggests that the observed age-related changes may be linked to declining testosterone levels. Acute testosterone therapy restored expression of PG-21 androgen receptor in aged and orchidectomised male rats, however levels of re-expression varied within different nuclei suggesting a more prolonged period of hormone replacement may be required for full restoration.

The soma and proximal dendrites of sympathetic preganglionic neurons innervating the major pelvic ganglion in female rats receive predominantly inhibitory inputs

Sympathetic preganglionic neurons (SPNs) in the intermediolateral (IML) and dorsal commissural nucleus (DCN) of the thoracolumbar segments of the spinal cord contribute to the autonomic control of the pelvic visceral organs. We examined the morphology of these neurons at the light and electron microscopic level and quantified the boutons apposing the soma and proximal dendrites of the SPNs innervating the major pelvic ganglion (MPG) in female rats. The majority of these cells resided in the DCN (61.6±6.2%) and IML (33.2±4.4%) nuclei. Measurements of cell volume and shape revealed no differences between SPNs sampled from the DCN and IML populations. Ultrastructural studies of DCN and IML SPNs revealed that coverage of SPNs by synaptic inputs is sparse, with an average of 11.60±2.41% of the soma membrane and 16.33±6.18% of proximal dendrites apposed by boutons, though some somata exhibited no synaptic coverage. Three distinct types of boutons were found to appose the SPN somata and dendrites. The putatively inhibitory F-type bouton covered a significantly greater percentage of membrane on the soma (8.48±2.12%) and dendrites (12.65±4.34%), than the S-type bouton, a putatively excitatory bouton, which only covered 2.94±0.70% of the somatic and 3.68±2.98% of the dendritic membranes. Boutons with dense-core vesicles were rare. Our results demonstrate that SPNs of the DCN and IML of female rats are similar morphologically, and that synaptic input on these cells, though sparse, is predominantly inhibitory.

Serotonergic innervation of the caudal spinal stump in rats after complete spinal transection: effect of olfactory ensheathing glia

Spinal cord injury studies use the presence of serotonin (5-HT)-immunoreactive axons caudal to the injury site as evidence of axonal regeneration. As olfactory ensheathing glia (OEG) transplantation improves hindlimb locomotion in adult rats with complete spinal cord transection, we hypothesized that more 5-HT-positive axons would be found in the caudal stump of OEG- than media-injected rats. Previously we found 5-HT-immunolabeled axons that spanned the transection site only in OEG-injected rats but detected labeled axons just caudal to the lesion in both media- and OEG-injected rats. Now we report that many 5-HT-labeled axons are present throughout the caudal stump of both media- and OEG-injected rats. We found occasional 5-HT-positive interneurons that are one likely source of 5-HT-labeled axons. These results imply that the presence of 5-HT-labeled fibers in the caudal stump is not a reliable indicator of regeneration. We then asked if 5-HT-positive axons appose cholinergic neurons associated with motor functions: central canal cluster and partition cells (active during fictive locomotion) and somatic motor neurons (SMNs). We found more 5-HT-positive varicosities in lamina X adjacent to central canal cluster cells in lumbar and sacral segments of OEG- than media-injected rats. SMNs and partition cells are less frequently apposed. As nonsynaptic release of 5-HT is common in the spinal cord, an increase in 5-HT-positive varicosities along motor-associated cholinergic neurons may contribute to the locomotor improvement observed in OEG-injected spinal rats. Furthermore, serotonin located within the caudal stump may activate lumbosacral locomotor networks.

Neurochemistry of the paraventricular nucleus of the hypothalamus: implications for cardiovascular regulation

The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic and endocrine homeostasis. The PVN integrates specific afferent stimuli to produce an appropriate differential sympathetic output. The neural circuitry and some of the neurochemical substrates within this circuitry are discussed. The PVN has at least three neural circuits to alter sympathetic activity and cardiovascular regulation. These pathways innervate the vasculature and organs such as the heart, kidney and adrenal medulla. The basal level of sympathetic tone at any given time is dependent upon excitatory and inhibitory inputs. Under normal circumstances the sympathetic nervous system is tonically inhibited. This inhibition is dependent upon GABA and nitric oxide such that nitric oxide potentiates local GABAergic synaptic inputs onto the neurones in the PVN. Excitatory neurotransmitters such as glutamate and angiotensin II modify the tonic inhibitory activity. The neurotransmitters oxytocin, vasopressin and dopamine have been shown to affect cardiovascular function. These neurotransmitters are found in neurones of the PVN and within the spinal cord. Oxytocin and vasopressin terminal fibres are closely associated with sympathetic preganglionic neurones (SPNs). Sympathetic preganglionic neurones have been shown to express receptors for oxytocin, vasopressin and dopamine. Oxytocin causes cardioacceleratory and pressor effects that are greatest in the upper thoracic cord while vasopressin cause these effects but more significant in the lower thoracic cord. Dopaminergic effects on the cardiovascular system include inhibitory or excitatory actions attributed to a direct PVN influence or via interneuronal connections to sympathetic preganglionic neurones.

Differential synaptic inputs to the cell body and proximal dendrites of preganglionic parasympathetic neurons in the rat conus medullaris

Preganglionic parasympathetic neurons (PPNs) reside in the intermediolateral (IML) nucleus of the rat lumbosacral spinal cord and contribute to the autonomic control of visceral pelvic organs. PPNs provide the final common pathway for efferent parasympathetic information originating in the spinal cord. We examined the detailed ultrastructure of the type and organization of synaptic inputs to the cell body and proximal dendrites of PPNs in the rat conus medullaris. The PPNs were retrogradely labeled by a systemic administration of the B subunit of cholera toxin conjugated to horseradish peroxidase. We demonstrate four distinct types of synaptic boutons in apposition with PPN somata and proximal dendrites: S-type boutons show clear, spheroid vesicles; F-type boutons show flattened vesicles; dense-cored vesicle-type (DCV-type) boutons show a mixture of clear and dense-cored vesicles; L-type boutons were rare, but large, exhibited clear spheroid vesicles, and were only encountered in apposition with the PPN dendrites in our sample. The membrane surface covered by apposed boutons was markedly higher for the proximal dendrites of PPNs, compared with their somata. The inhibitory synaptic influence was markedly higher over the PPN somata compared with their proximal dendrites, as suggested by the higher proportion of putative inhibitory F-type boutons in apposition with the soma and a higher frequency of S-type boutons per membrane length for the proximal dendrites. Our studies suggest that the synaptic input to PPNs originates from multiple distinct sources and is differentially distributed and integrated over the cell membrane surface.