The serotoninergic bulbospinal system and brainstem-spinal cord content of serotonin-, TRH-, and substance P-like immunoreactivity in the aged rat with special reference to the spinal cord motor nucleus

Ageing reduces persistent inward current contribution to motor neurone firing: Potential mechanisms and the role of exercise

Nervous system deterioration is a primary driver of age-related motor impairment. The motor neurones, which act as the interface between the central nervous system and the muscles, play a crucial role in amplifying excitatory synaptic input to produce the desired motor neuronal firing output. For this, they utilise their ability to generate persistent (long-lasting) depolarising currents that increase cell excitability, and both amplify and prolong the output activity of motor neurones for a given synaptic input. Modulation of these persistent inward currents (PICs) contributes to the motor neurones' capacities to attain the required firing frequencies and rapidly modulate them to competently complete most tasks. Thus, PICs are crucial for adequate movement generation. Impairments in intrinsic motor neurone properties can impact motor unit firing capacity, with convincing evidence indicating that the PIC contribution to motor neurone firing is reduced in older adults. Indeed, this could be an important mechanism underpinning the age-related reductions in strength and physical function. Furthermore, resistance training has emerged as a promising intervention to counteract age-associated PIC impairments, with changes in PICs being correlated with improvements in muscular strength and physical function after training. In this review, we present the current knowledge of the PIC magnitude decline during ageing and discuss whether reduced serotonergic and noradrenergic input onto the motor neurones, voltage-gated calcium channel dysfunction or inhibitory input impairments are candidates that: (i) explain age-related reductions in the PIC contribution to motor neurone firing and (ii) underpin the enhanced PIC contribution to motor neurone firing following resistance training in older adults.

Facilitation-inhibition control of motor neuronal persistent inward currents in young and older adults

A well-coordinated facilitation-inhibition control of motor neuronal persistent inward currents (PICs) via diffuse neuromodulation and local inhibition is essential to ensure motor units discharge at required times and frequencies. Present best estimates indicate that PICs are reduced in older adults; however, it is not yet known whether PIC facilitation-inhibition control is also altered with ageing. We investigated the responses of PICs to (i) a remote handgrip contraction, which is believed to diffusely increase serotonergic input onto motor neurones, and (ii) tendon vibration of the antagonist muscle, which elicits reciprocal inhibition, in young and older adults. High-density surface electromyograms were collected from soleus and tibialis anterior of 18 young and 26 older adults during triangular-shaped plantar and dorsiflexion contractions to 20% (handgrip experiments) and 30% (vibration experiments) of maximum torque (rise-decline rate of 2%/s). A paired-motor-unit analysis was used to calculate ∆F, which is assumed to be proportional to PIC strength. ΔF increased in both soleus (0.55 peaks per second (pps), 16.0%) and tibialis anterior (0.42 pps, 11.4%) after the handgrip contraction independent of age. Although antagonist tendon vibration reduced ΔF in soleus (0.28 pps, 12.6%) independent of age, less reduction was observed in older (0.42 pps, 10.7%) than young adults (0.72 pps, 17.8%) in tibialis anterior. Our data indicate a preserved ability of older adults to amplify PICs following a remote handgrip contraction, during which increased serotonergic input onto the motor neurones is expected, in both lower leg muscles. However, PIC deactivation in response to reciprocal inhibition was impaired with ageing in tibialis anterior despite being preserved in soleus. KEY POINTS: Motor neuronal persistent inward currents (PICs) are facilitated via diffuse neuromodulation and deactivated by local inhibition to ensure motor units discharge at required times and frequencies, allowing normal motor behaviour. PIC amplitudes appear to be reduced with ageing; however, it is not known whether PIC facilitation-inhibition control is also altered. Remote handgrip contraction, which should diffusely increase serotonergic input onto motor neurones, facilitated PICs similarly in both soleus and tibialis anterior of young and older adults. Antagonist tendon vibration, which induces reciprocal inhibition, reduced PICs in soleus in both young and older adults but had less effect in tibialis anterior in older adults. Data from lower-threshold motor units during low-force contractions suggest that PIC facilitation is preserved with ageing in soleus and tibialis anterior. However, the effect of reciprocal inhibition on the contribution of PICs to motor neurone discharge seems reduced in tibialis anterior but preserved in soleus.

Breathing during sleep

The depth, rate, and regularity of breathing change following transition from wakefulness to sleep. Interactions between sleep and breathing involve direct effects of the central mechanisms that generate sleep states exerted at multiple respiratory regulatory sites, such as the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles, as well as effects secondary to sleep-related changes in metabolism. This chapter discusses respiratory effects of sleep as they occur under physiologic conditions. Breathing and central respiratory neuronal activities during nonrapid eye movement (NREM) sleep and REM sleep are characterized in relation to activity of central wake-active and sleep-active neurons. Consideration is given to the obstructive sleep apnea syndrome because in this common disorder, state-dependent control of upper airway patency by upper airway muscles attains high significance and recurrent arousals from sleep are triggered by hypercapnic and hypoxic episodes. Selected clinical trials are discussed in which pharmacological interventions targeted transmission in noradrenergic, serotonergic, cholinergic, and other state-dependent pathways identified as mediators of ventilatory changes during sleep. Central pathways for arousals elicited by chemical stimulation of breathing are given special attention for their important role in sleep loss and fragmentation in sleep-related respiratory disorders.

Intrinsic motoneuron excitability is reduced in soleus and tibialis anterior of older adults

Age-related deterioration within both motoneuron and monoaminergic systems should theoretically reduce neuromodulation by weakening motoneuronal persistent inward current (PIC) amplitude. However, this assumption remains untested. Surface electromyographic signals were collected using two 32-channel electrode matrices placed on soleus and tibialis anterior of 25 older adults (70 ± 4 years) and 17 young adults (29 ± 5 years) to investigate motor unit discharge behaviors. Participants performed triangular-shaped plantar and dorsiflexion contractions to 20% of maximum torque at a rise-decline rate of 2%/s of each participant's maximal torque. Pairwise and composite paired-motor unit analyses were adopted to calculate delta frequency (ΔF), which has been used to differentiate between the effects of synaptic excitation and intrinsic motoneuronal properties and is assumed to be proportional to PIC amplitude. Soleus and tibialis anterior motor units in older adults had lower ΔFs calculated with either the pairwise [-0.99 and -1.46 pps; -35.4 and -33.5%, respectively] or composite (-1.18 and -2.28 pps; -32.1 and -45.2%, respectively) methods. Their motor units also had lower peak discharge rates (-2.14 and -2.03 pps; -19.7 and -13.9%, respectively) and recruitment thresholds (-1.50 and -2.06% of maximum, respectively) than young adults. These results demonstrate reduced intrinsic motoneuron excitability during low-force contractions in older adults, likely mediated by decreases in the amplitude of persistent inward currents. Our findings might be explained by deterioration in the motoneuron or monoaminergic systems and could contribute to the decline in motor function during aging; these assumptions should be explicitly tested in future investigations.

Estimates of persistent inward currents are reduced in upper limb motor units of older adults

Ageing is a natural process causing alterations in the neuromuscular system, which contributes to reduced quality of life. Motor unit (MU) contributes to weakness, but the mechanisms underlying reduced firing rates are unclear. Persistent inward currents (PICs) are crucial for initiation, gain control and maintenance of motoneuron firing, and are directly proportional to the level of monoaminergic input. Since concentrations of monoamines (i.e. serotonin and noradrenaline) are reduced with age, we sought to determine if estimates of PICs are reduced in older (>60 years old) compared to younger adults (<35 years old). We decomposed MU spike trains from high-density surface electromyography over the biceps and triceps brachii during isometric ramp contractions to 20% of maximum. Estimates of PICs (ΔFrequency; or simply ΔF) were computed using the paired MU analysis technique. Regardless of the muscle, peak firing rates of older adults were reduced by ∼1.6 pulses per second (pps) (P = 0.0292), and ΔF was reduced by ∼1.9 pps (P < 0.0001), compared to younger adults. We further found that age predicted ΔF in older adults (P = 0.0261), resulting in a reduction of ∼1 pps per decade, but there was no relationship in younger adults (P = 0.9637). These findings suggest that PICs are reduced in the upper limbs of older adults during submaximal isometric contractions. Reduced PIC magnitude represents one plausible mechanism for reduced firing rates and function in older individuals, but further work is required to understand the implications in other muscles and during a variety of motor tasks. KEY POINTS: Persistent inward currents play an important role in the neural control of human movement and are influenced by neuromodulation via monoamines originating in the brainstem. During ageing, motor unit firing rates are reduced, and there is deterioration of brainstem nuclei, which may reduce persistent inward currents in alpha motoneurons. Here we show that estimates of persistent inward currents (ΔF) of both elbow flexor and extensor motor units are reduced in older adults. Estimates of persistent inward currents have a negative relationship with age in the older adults, but not in the young. This novel mechanism may play a role in the alteration of motor firing rates that occurs with ageing, which may have consequences for motor control.

Expression Analysis of Serotonin Receptors, Serotonin Transporter and l-Amino Acid Decarboxylase in the Mouse Sphenopalatine Ganglion by RT-PCR, Northern Blot Analysis and In Situ Hybridization

The sphenopalatine ganglion (SPG) is a gathering of the cell bodies of parasympathetic fibers that dominate the nasal gland, lacrimal gland and cerebral blood vessels. The SPG controls nasal secretions, tears, and the dilation of cerebral blood vessels. However, it is unclear how serotonin regulates SPG functions. In this study, we investigated the expression of genes involved in the serotonergic system in the mouse SPG. We examined the mRNA expression levels of 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_1F, 5-HT_2A, 5-HT_2B, 5-HT_2C, 5-HT_3A, 5-HT_3B, 5-HT₄, 5-HT_5A, 5-HT_5B, 5-HT₆ and 5-HT₇ receptors, as well as serotonin transporter, tryptophan hydroxylases 1 and 2, and L-amino acid decarboxylase (AADC) by RT-PCR. It revealed that the 5-HT_3A and 5-HT_3B ionotropic receptors and AADC were likely to be highly expressed in the SPG, as measured by RT-PCR. We next performed in situ hybridization on the SPG to examine the expression of these three genes at the cellular level after validating the specificity of each cRNA probe by northern blotting. The 5-HT_3A receptor, 5-HT_3B receptor, and AADC were expressed in 96.5% ± 1.0%, 29.7% ± 10.7%, and 57.4% ± 2.9% of neuronal cell bodies in the SPG, respectively, indicating that the 5-HT_3A receptor was virtually expressed in all SPG neurons. Our results on the expression of these critical serotonin system genes in the parasympathetic SPG provide insight into the pathogenetics of rhinitis, conjunctivitis and headache. Furthermore, our findings suggest that targeting the 5-HT_3A receptor might have therapeutic potential in the treatment of these ailments.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

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.

Changes in behaviors of male C57BL/6J mice across adult life span and effects of dietary restriction

Behavioral analysis is a high-end read-out of aging impact on an organism, and here, we have analyzed behaviors in 4-, 22-, and 28-month-old male C57BL/6J with a broad range of tests. For comparison, a group of 28-month-old males maintained on dietary restriction (DR) was included. The most conspicuous alteration was the decline in exploration activity with advancing age. Aging also affected other behaviors such as motor skill acquisition and grip strength, in contrast to latency to thermal stimuli and visual placement which were unchanged. Object recognition tests revealed intact working memory at 28 months while memory recollection was impaired already at 22 months. Comparison with female C57BL/6J (Fahlström et al., Neurobiol Aging 32:1868-1880, 2011) revealed that alterations in aged males and females are similar and that several of the behavioral indices correlate with age in both sexes. Moreover, we examined if behavioral indices in 22-month-old males could predict remaining life span as suggested in the study by Ingram and Reynolds (Exp Aging Res 12(3):155-162, 1986) and found that exploratory activity and motor skills accounted for up to 65% of the variance. Consistent with that a high level of exploratory activity and preserved motor capacity indicated a long post-test survival, 28-month-old males maintained on DR were more successful in such tests than ad libitum fed age-matched males. In summary, aged C57BL/6J males are marked by a reduced exploratory activity, an alteration that DR impedes. In light of recently published data, we discuss if a diminishing drive to explore may associate with aging-related impairment of central aminergic pathways.

Nesfatin-1 exerts long-term effect on food intake and body temperature

OBJECTIVE

To determine whether the anorexigenic peptide, nesfatin-1 affects energy expenditure, and to follow the time course of its effects.

DESIGN

Food intake duration, core body temperature, locomotor activity and heart rate of rats were measured by telemetry for 48 h after a single intracerebroventricular injection of 25 or 100 pmol nesfatin-1 applied in the dark or the light phase of the day. Body weight, food and water intake changes were measured daily. Furthermore, cold-responsive nesfatin-1/NUCB2 neurons were mapped in the brain.

RESULTS

Nesfatin-1 reduced duration of nocturnal food intake for 2 days independently of circadian time injected, and raised body temperature immediately, or with little delay depending on the dose and circadian time applied. The body temperature remained higher during the next light phases of the 48 h observation period, and the circadian curve of temperature flattened. After light phase application, the heart rate was elevated transiently. Locomotion did not change. Daily food and water intake, as well as body weight measurements point to a potential decrease in all parameters on the first day and some degree of compensation on the second day. Cold-activated (Fos positive) nesfatin-1/NUCB2 neurones have been revealed in several brain nuclei involved in cold adaptation. Nesfatin-1 co-localised with prepro-thyrotropin-releasing hormone in cold responsive neurones of the hypothalamic paraventricular nucleus, and in neurones of the nucleus raphe pallidus and obscurus that are premotor neurones regulating brown adipose tissue thermogenesis and skin blood flow.

CONCLUSION

Nesfatin-1 has a remarkably prolonged effect on food intake and body temperature. Time course of nesfatin-1's effects may be varied depending on the time applied. Many of the nesfatin-1/NUCB2 neurones are cold sensitive, and are positioned in key centres of thermoregulation. Nesfatin-1 regulates energy expenditure a far more potent way than it was recognised before making it a preferable candidate anti-obesity drug.

Differential effect of aging on axon sprouting and regenerative growth in spinal cord injury

The demographics of acute spinal cord injury (SCI) are changing with an increased incidence in older age. However, the influence of aging on the regenerative growth potential of central nervous system (CNS) axons following SCI is not known. We investigated axonal sprouting along with the efficiency of the infusion of the stromal cell-derived growth factor-1 (SDF-1/CXCL12) and regenerative growth along with the anti-scarring treatment (AST) in young (2-3 months) and geriatric (22-28 months) female rats following SCI. AST included local injection of iron chelator (2,2'-dipyridine-5,5'-dicarboxylic acid) and 8-bromo-cyclic adenosine monophosphate solution into the lesion core. Axon outgrowth was investigated by immunohistological methods at 5 weeks after a partial dorsal hemisection at thoracic level T8. We found that aging significantly reduces spontaneous axon sprouting of corticospinal (CST), serotonergic (5-HT) raphespinal and catecholaminergic (TH) coerulospinal tracts in distinct regions of the spinal cord rostral to the lesion. However, impairment of axon sprouting could be markedly attenuated in geriatric animals by local infusion of SDF-1. Unexpectedly and in contrast to rostral sprouting, aging does not diminish the regenerative growth capacity of 5-HT-, TH- and calcitonin gene-related peptide (CGRP)-immunoreactive axons at 5 weeks after SCI. Moreover, 5-HT and TH axons maintain the ability to react upon AST with significantly enhanced regeneration in aged animals. These data are the first to demonstrate, that old age compromises axonal plasticity, but not regenerative growth, after SCI in a fiber tract-specific manner. Furthermore, AST and SDF-1 infusions remain efficient, which implicates that therapy in elderly patients is still feasible.

Discharge characteristics of biceps brachii motor units at recruitment when older adults sustained an isometric contraction

The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference (P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences (P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.

Neurochemistry of bulbospinal presympathetic neurons of the medulla oblongata

This review focuses on presympathetic neurons in the medulla oblongata including the adrenergic cell groups C1-C3 in the rostral ventrolateral medulla and the serotonergic, GABAergic and glycinergic neurons in the ventromedial medulla. The phenotypes of these neurons including colocalized neuropeptides (e.g., neuropeptide Y, enkephalin, thyrotropin-releasing hormone, substance P) as well as their relative anatomical location are considered in relation to predicting their function in control of sympathetic outflow, in particular the sympathetic outflows controlling blood pressure and thermoregulation. Several explanations are considered for how the neuroeffectors coexisting in these neurons might be functioning, although their exact purpose remains unknown. Although there is abundant data on potential neurotransmitters and neuropeptides contained in the presympathetic neurons, we are still unable to predict function and physiology based solely on the phenotype of these neurons.

Developmental profiles of neurotransmitter receptors in respiratory motor nuclei

We discuss the time course of postnatal development of selected neurotransmitter receptors in motoneurons that innervate respiratory pump and accessory respiratory muscles, with emphasis on other than classic respiratory signals as important regulatory factors. Functions of those brainstem motoneurons that innervate the pharynx and larynx change more dramatically during early postnatal development than those of spinal respiratory motoneurons. Possibly in relation to this difference, the time course of postnatal expression of distinct receptors for serotonin differ between the hypoglossal (XII) and phrenic motoneurons. In rats, distinct developmental patterns include a decline or increase that extends over the first 3-4 postnatal weeks, a rapid increase during the first 2 weeks, or a transient decline on postnatal days 11-14. The latter period coincides with major changes in many transmitters in brainstem respiratory regions that may be related to a brain-wide reconfiguration of sensorymotor processing resulting from eye and ear opening and beginning of a switch from suckling to mature forms of food seeking and processing. Such rapid neurochemical changes may impart increased vulnerability on the respiratory system. We also consider rapid eye movement sleep as a state during which some brain functions may revert to conditions typical of perinatal period. In addition to normal developmental processes, changes in the expression or function of neurotransmitter receptors may occur in respiratory motoneurons in response to injury, perinatal stress, or disease conditions that increase the load on respiratory muscles or alter the normal levels and patterns of oxygen delivery.

Caloric Restriction Does Not Offset Age-Associated Changes in the Biophysical Properties of Motoneurons

Age-associated changes in neuromuscular function may be due to a loss of motor neurons as well as changes in their biophysical properties. Neuronal damage imposed by reactive oxygen species may contribute to age-related deficits in CNS function. Thus we hypothesized that aging would alter the functional properties of motoneurons and that caloric-restriction would offset these changes. Intracellular recordings were made from lumbar motoneurons of old Fisher Brown Norway (FBN) fed ad libitum (oldAL, 30.8 ± 1.3 mo) or on a fortified calorie-restricted diet from 14 wk of age (oldCR, 31.0 ± 1.8 mo). Basic and rhythmic firing properties recorded from these aged motoneurons (MNs) were compared with properties recorded from young FBN controls (young, 8.4 ± 4.6 mo). Compared with young MNs, old MNs had a 104% greater ( P < 0.001) afterhyperpolarization potential (AHP), a 21.1% longer AHP half-decay time ( P < 0.05), 28.7% lower rheobase ( P < 0.001), 49.7% greater ( P < 0.001) input resistance, 21.1% ( P < 0.0001) less spike frequency adaptation, lower minimal (30.2%, P < 0.0001) and maximal (16.7%, P < 0.0001) steady-state firing frequencies, a lower (35.5%, P < 0.0001) frequency-current slope, and an increased incidence of persistent inward current. Because basic properties became more diverse in old MNs and the slope of the frequency-current relationship, which is normally similar for high- and low-threshold MNs, was lower in the old group, we conclude that aging alters the biophysical properties of MNs in a fashion that cannot be simply attributed to a loss of high-threshold MNs. Surprisingly, caloric restriction, which is known to attenuate aging-associated changes in hindlimb muscles, had no effect on the progress of aging in the innervating MNs.

Factors contributing to neuromuscular impairment and sarcopenia during aging

Motor disturbances and wasting of skeletal muscles (sarcopenia) causes significant impairment of daily life activities and is a major underlying cause for hospitalization in senescence. Herein we review data and present new findings on aging-specific changes in motoneurons, skeletal muscle and the interplay between motoneurons and target muscle fibers. Although many of the changes occurring during aging may be specific to motoneurons and myofibers, respectively, evidence indicates that myofiber regeneration in sarcopenic muscle is halted at the point where reinnervation is critical for the final differentiation into mature myofibers. Combined, evidence suggests that sarcopenia to a significant extent depend on a decreased capacity among motoneurons to innervate regenerating fibers. There are also conspicuous changes in the expression of several cytokines known to play important roles in establishing and maintaining neuromuscular connectivity during development and adulthood. We also present data showing the usefulness of rodent models in studies of successful and unsuccessful patterns of aging. Finally, we show that not only dietary restriction (DR) but also activity and social environment may modulate the pattern of aging.

The effects of ageing on the distribution of vesicular acetylcholine transporter immunoreactive inputs to pelvic motoneurons of male Wistar rats

Age-related changes in the number and size of large cholinergic terminals immunoreactive for vesicular acetylcholine transporter (VAChT), were documented for the dorsolateral nucleus (DLN), retrodorsolateral nucleus (RDLN) and spinal nucleus of the bulbospongiosus (SNB) of the lumbosacral spinal cord of male rats. The most significant changes were a large increase in the number and size of cholinergic terminals within the DLN of aged animals, together with a small decrease in terminal number within the RDLN. No significant age-associated differences in VAChT labeling were seen within the SNB. In both age groups, SNB motoneurons projecting to the levator ani muscle received about 9 to 10 contacts from large cholinergic terminals. Ultrastructural examination of the terminals revealed structures likely to be postsynaptic subsurface cisterns that are characteristic of type C terminal boutons. Since both the DLN and SNB contain motoneurons innervating pelvic muscles and sphincters, these findings provide further evidence for a central cholinergic influence on micturition and sexual reflexes and suggest that this may remain robust in the face of ageing.

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

Preganglionic neurones in the lumbosacral spinal cord give rise to nerves providing the parasympathetic and sympathetic innervation of pelvic organs. These neurones are modulated by neurotransmitters released both from descending supra-spinal pathways and spinal interneurones. Though serotonin has been identified as exerting a significant influence on these neurones, few studies have investigated the circuitry through which it achieves this particularly in relation to sympathetic preganglionic neurones. Using a combination of neuronal tracing and multiple immunolabeling procedures, the current study has shown that pelvic preganglionic neurones receive a sparse, and probably non-synaptic, axosomatic/proximal dendritic input from serotonin-immunoreactive terminals. This was in marked contrast to dopamine beta hydroxylase-immunoreactive terminals, which made multiple contacts. However, the demonstration of both serotonin, and dopamine beta hydroxylase immunoreactive terminals on both parasympathetic and sympathetic preganglionic neurones provides evidence for direct modulation of these cells by both serotonin and norepinephrine. Serotonin-containing terminals displaying conventional synaptic morphology were often seen to contact unlabeled somata and dendritic processes in regions surrounding the labeled preganglionic cells. It is possible that these unlabeled structures represent interneurones that might allow the serotonin containing axons to exert an indirect influence on pelvic preganglionic neurones. Since many spinal interneurones employ GABA as a primary fast acting neurotransmitter we examined the relationship between terminals that were immunoreactive for serotonin or GABA and labeled pelvic preganglionic neurones. These studies were unable to demonstrate any direct connections between serotonin and GABA terminals within the intermediolateral or sacral parasympathetic nuclei. Colocalization of serotonin and GABA was very rare but terminals immunoreactive for each were occasionally seen to contact the same unlabeled processes in close proximity. These results suggest that in the rat, the serotonin modulation of pelvic preganglionic neurones may primarily involve indirect connections via local interneurones.

Involvement of Dorsal Column Nucleus Neurons in Nociceptive Transmission in Aged Rats

To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P–like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.

The organization of the brainstem and spinal cord of the mouse: relationships between monoaminergic, cholinergic, and spinal projection systems

Information regarding the organization of the CNS in terms of neurotransmitter systems and spinal connections in the mouse is sparse, especially at the level of the brainstem. An overview is presented of monoaminergic and cholinergic systems in the brainstem and spinal cord that were visualized immunohistochemically in inbred C57BL/6 and outbred CD-1 mice. This information is complemented with data on spinal cord-projecting systems that were characterized using retrograde tracing, spinal hemisections, and double labeling techniques. Attention is given to differences in these systems related to spinal levels. The data are discussed with reference to studies in the rat, and to standardized information as provided in the atlas of the mouse brain. Although the overall organization of these systems in the mouse is largely similar to those in the rat, species differences are present in relative location, size and/or connectivity of cell groups. For example, catecholaminergic neurons in the (ventro)lateral pons (A5 and A7 cell groups) in the mouse project to the spinal cord mainly via contralateral, and not ipsilateral, pathways. The data further supplement information as provided in standardized brainstem sections of the C57BL/6 mouse [Paxinos, G., Franklin, K.B.J., 2001. The mouse brain in stereotaxic coordinates. Academic Press, San Diego], especially with respect to the size and/or location of the catecholaminergic retrorubral field (A8 group), A5, A1, and C1 cell groups, and the serotonergic B4 group, reticulotegmental nucleus (B9 group), lateral paragigantocellular nucleus and raphe magnus nucleus (B3 group). Altogether this study provides a comprehensive overview of the spatial relationships of neurochemically and anatomically defined neuronal systems in the mouse brainstem and spinal cord.

Changes in the substance P-containing innervation of the lumbosacral spinal cord in male Wistar rats as a consequence of ageing

Quantitative image analysis was used to determine age-related changes in the substance P-containing innervation of autonomic and somatic nuclei in the lumbosacral spinal cord, which are associated with the control of micturition and sexual reflexes. In the upper lumbar segments (L1-L2), significant declines in the distribution density of substance P-containing processes were observed in the dorsal grey commissure, the intermediolateral cell column and the ventral horn. More caudally, at levels corresponding to L5 through S1, significant reductions were seen in the dorsal grey commissure and within the sacral parasympathetic nucleus. In contrast to these observations, the substance P-immunoreactive innervation of the dorsolateral nucleus remained robust in aged animals and was not significantly different from young adults. It is possible that these distinct age-related patterns of change in substance P-containing innervation, are reflected in the urinary/sexual dysfunction's in aged animals.

MHC class I, beta2 microglobulin, and the INF-gamma receptor are upregulated in aged motoneurons

During aging, spinal cord motoneurons show characteristic changes including the loss of afferent boutons, a selective process that associates with gliosis and behavioral motor impairment. Evidence suggests that the major histocompatibility complex Class I (MHC I) system may be involved in synaptic plasticity of neurons during development and regeneration. In search of a mechanism governing senescent changes in synaptic connectivity, we report evidence for increased expression of MHC I and beta2 microglobulin (beta2M) in motoneurons and glial-like profiles of 30-month-old rats. The regulatory signal(s) for MHC I expression in normal neurons remains unresolved but among tentative molecules are cytokines such as interferon-gamma (INF-gamma) and tumor necrosis factor alpha (TNF-alpha). Interestingly, aged motoneurons, overlapping with those showing increased levels of MHC I, contained increased levels of INF-gamma receptor message. INF-gamma mRNA was detected at low levels in most (8/9) of the aged spinal cords but only infrequently (2/9) in young adult spinal cords; however, the cellular localization of INF-gamma mRNA could not be determined. Our data also indicates that TNF-alpha is upregulated in the senescent spinal cord but that TNF-alpha immunoreactive protein does not associate with motoneurons. We report evidence for an increased expression of MHC I and beta2M in senescent spinal motoneurons and discuss the possibility that this regulation associates with INF-gamma or changes in neurotrophin signaling and neuron activity in senescence.

Locomotor recovery and mechanical hyperalgesia following spinal cord injury depend on age at time of injury in rat

We tested the effect of age at the time of spinal cord injury (SCI) on locomotor recovery, in open field tests, and mechanical hyperalgesia, using paw withdrawal frequency (PWF) in response to noxious mechanical stimuli, in male Sprague-Dawley rats after spinal hemisection at T13 in young (40 days), adult (60 days) and middle-age (1 year) groups. Behavioral outcomes were measured weekly for 4 weeks in both SCI and sham groups. Following SCI, the young and adult groups recovered significantly more locomotor function, at a more rapid rate, than did the middle-age group. The PWF of the young group was significantly increased, the adult group was significantly decreased, and the middle-age group showed no significant change in fore- and hindlimbs when compared to other age groups, pre-injury and sham controls. These results support age-dependent behavioral outcomes after SCI.

Age-associated changes in the monoaminergic innervation of rat lumbosacral spinal cord

The effects of ageing on the innervation patterns of lumbosacral spinal nuclei involved in controlling lower urinary tract functions, including micturition, were studied using immunohistochemistry for serotonin (5-HT) and tyrosine hydroxylase (TH) in male Wistar rats of 3 and 24 months. Quantitative image analysis revealed significant age-associated declines in the innervation of most regions including the intermediolateral cell nucleus, sacral parasympathetic nucleus, dorsal grey commissure and in the ventral horn including the dorsolateral nucleus which in the rat is one of the component nuclei homologous to Onuf's nucleus in man. Notable exceptions to this generalised decline were observed in the 5-HT innervation of the sacral parasympathetic nucleus, which was maintained, and in the region of the dorsolateral motor nucleus where TH-like immunoreactivity did not significantly decline. These results suggest that the changes in micturition characteristics observed in aged rats may in part be a consequence of the alterations in, and decline of, aminergic inputs to both autonomic and somatic spinal nuclei associated with bladder function.

Differential regulation of Shc adaptor proteins in skeletal muscle, spinal cord and forebrain of aged rats with sensorimotor impairment

The Shc family of proteins participates in mitogenic and survival signalling through binding to receptor tyrosine kinases. We report here on the expression of Shc in forebrain, spinal cord and hind limb muscles from 30-month-old rats with different degrees of sensorimotor impairment. ShcA (mRNA and protein) is up-regulated in skeletal muscles and spinal cord of aged rats, and this change relates to biological age, i.e. degree of behavioural incapacitation, rather than to chronological age. Western blot and RT-PCR revealed that the increase in ShcA selectively affected the p46 isoform in the spinal cord, whereas in muscle tissue a robust increase of p66(ShcA) was also evident. Furthermore, in parallel with the up-regulation of ShcA, an increase of p75(NTR) mRNA in the aged animals was observed. ShcB mRNA showed a tendency for down-regulation in both spinal cord and skeletal muscles, whereas the expression of ShcC was unaltered. Our data show that the regulation of Shc mRNAs in senescence is region as well as isoform specific. The regulatory changes may reflect changes in mitogenic/survival signalling induced by age-related cell and tissue damage. The coup-regulation of p66(ShcA) and p75(NTR) is interesting since both molecules have been associated with apoptosis.

Evidence for loss of myelinated input to the spinal cord in senescent rats

Impaired sensory perception is a well-established stigma of aging and whereas loss of dorsal root ganglion (DRG) neurons is marginal there is a specific pattern of reduced peripheral sensory innervation. To resolve if similar regressive processes occur in the central innervation, peripheral nerves were injected with markers for unmyelinated (isolectin B4) or myelinated (cholera toxin B subunit; CTB) DRG neurons. The results were a dramatic decrease of primary sensory endings in the spinal cord of aged rats following transganglionic labeling with CTB, and also to a lesser degree with B4. Profile counting and frequency estimates showed that the reduction of CTB labeled profiles not was caused by impaired axonal uptake, slowed axonal transport of CTB, or by a loss of myelinated fibers in the peripheral nerve. At the ultrastructural level, peripheral nerves showed the classical hallmarks of aging, with more pronounced alterations in myelinated than unmyelinated axons. Taken together, sensory deprivation in senescence appears to be a distal process in DRG neurons involving both peripheral and central target disconnection. Finally, preliminary data indicates that the substantial reduction in mechanoreceptive input to the central nervous system co-varies with the degree of sensorimotor impairment of the aged individuals.

Differential susceptibility to ageing of rat preganglionic neurones projecting to the major pelvic ganglion and of their afferent inputs

We have analysed age-related changes in the morphology of preganglionic neurones in the lumbosacral spinal cord, labelled following injection of retrograde tracers into the major pelvic ganglion of young adult and aged male rats. We have also examined changes in neurotransmitter-characterised spinal afferent inputs to these neurones, or to the nuclei in which they lie, using light and electron microscope immunohistochemistry. In previous investigations of the major pelvic ganglion, the sympathetic, but not parasympathetic, postganglionic neurones were seen to exhibit age-related changes and the same pattern is seen in the preganglionic neurones. This included an apparent reduction in the numbers of sympathetic preganglionic neurones, and a reduction in the length of their dendrites and the complexity of their branches. Ultrastructural immunohistochemical studies described here reveal significant reductions in the area of synaptic contact made by glutamate-immunoreactive boutons onto the dendrites of sympathetic (but not parasympathetic) preganglionic neurones, while contacts from boutons immunoreactive for glycine or gamma-aminobutyric acid (GABA) were unchanged. There is also a reduction in synaptic contacts received by sympathetic somata from boutons immunoreactive for none of these amino acids. Serotonin-immunoreactive terminals are closely associated with preganglionic autonomic neurones, and these are reduced in number in sympathetic, but not parasympathetic, spinal nuclei of aged rats. However, serial section electron microscopy has so far failed to demonstrate conventional synaptic contacts between serotonergic terminals and the dendrites or somata of the preganglionic autonomic neurones. In young animals, axon terminals immunoreactive for thyrotropin-releasing hormone (TRH) are abundant in all spinal laminae including area X, but in aged animals, such terminals are significantly reduced in number in regions containing preganglionic sympathetic, but not parasympathetic, neurones. These results indicate that the sympathetic preganglionic neuron populations that project to the major pelvic ganglion, and the spinal inputs they receive, show a number of degenerative changes in aged rats which are not seen parasympathetic preganglionic neuronal populations.

Age-associated changes in the serotonergic system in rat superior colliculus and pretectum

The purpose of this study was to investigate whether aging alters serotonergic innervation of the superior colliculus and pretectum in rats. The superior colliculus has one of the highest concentrations of serotonin in the rat central nervous system. Young and old male F344 rats (<6 months, and >18 months, albino and pigmented) were used in all experiments. Coronal sections through the superior colliculus and pretectum were incubated with antibodies to serotonin, the serotonin 2A receptor, and the serotonin transporter. Immunocytochemical staining was analyzed semi-quantitatively. The results indicate that with age there is an increase in serotonin immunoreactivity throughout the entire superior colliculus and pretectum, a decrease in levels of serotonin 2A receptor staining in select layers of superior colliculus, and no change in serotonin transporter immunoreactivity. Albino rats differ from pigmented rats in that they have enhanced serotonergic immunoreactivity in the superficial layers of superior colliculus, a region that receives direct retinal input. These data suggest that the age-related changes in the serotonergic system in the superior colliculus and pretectum may account for some of the alterations in light-mediated behaviors with aging.

Temporal facilitation of the flexor reflex induced by C-fiber activity: comparison between adult and aged rats

We investigated the wind-up phenomenon of the flexor reflex in adult and aged rats. The sural nerve was stimulated at C-fiber strength and reflex activity was recorded from the semitendinosus muscle. The wind-up rate, the increment rate of the C-fiber response (i.e. activity from 100 to 600 ms after stimulation) by successive stimuli (five train pulses), was decreased exponentially with increasing stimulus intervals from 3 to 20 s. The time constant of the decay for the aged rats was 9.2+/-3.2 s (mean+/-SD), which was significantly longer than for the adult rats (6.4+/-2.9 s). The findings indicate that the effects of C-fiber activation on the spinal nociceptive pathways attenuate more slowly in aged rats as compared with adult rats.

Microglial activation, emergence of ED1-expressing cells and clusterin upregulation in the aging rat CNS, with special reference to the spinal cord

With advancing age, the incidence of neuronal atrophy and dystrophy increases and, in parallel, behavioural sensorimotor impairment becomes overt. Activated microglia has been implicated in cytotoxic and inflammatory processes in neurodegenerative diseases as well as during aging. Here we have used immunohistochemistry and in situ hybridization to examine the expression of OX42, ED1, ED2, GFAP and clusterin in CNS of young adult and behaviourally tested aged rats (30-month-old), to study the occurrence of activated microglia/ED1 positive macrophages in senescence and to what extent this correlates with astrogliosis and signs of sensorimotor impairment among the individuals. The results show a massive region-specific increase in activated microglia and ED1 expressing cell profiles in aged rats. The infiltration was most prominent in the spinal cord dorsal columns, including their sensory relay nuclei, and the outer portions of the lateral and ventral columns. At such sites the occurrence of macrophages coincided with increased levels of GFAP and positive correlations were evident between the labeling for, on the one hand, OX42 and, on the other, GFAP and ED1. Also, the ventral and dorsal roots were heavily infiltrated by ED1 positive cells. The signs of gliosis were most pronounced among aged rats with advanced sensorimotor impairment. In contrast, the grey matter of aged rats showed very few activated microglia/ED1 labeled cells despite signs of focal astrogliosis. ED2 expression was confined to perivascular cells and leptominges with a similar labeling pattern in young and aged rats. In aged rats increased expression of clusterin was observed in GFAP-immunoreactive profiles of the white matter only. It is suggested that this increase may reflect a response to degenerative/inflammatory processes.

Neuropeptides in hippocampus and cortex in transgenic mice overexpressing V717F beta-amyloid precursor protein--initial observations

Immunohistochemistry was used to analyse 18- and 26-month-old transgenic mice overexpressing the human beta-amyloid precursor protein under the platelet-derived growth factor-beta promoter with regard to presence and distribution of neuropeptides. In addition, antisera/antibodies to tyrosine hydroxylase, acetylcholinesterase, amyloid peptide, glial fibrillary acidic protein and microglial marker OX42 were used. These mice have been reported to exhibit extensive amyloid plaques in the hippocampus and cortex [Masliah et al. (1996) J. Neurosci. 16, 5795-5811]. The most pronounced changes were related to neuropeptides, whereas differences between wild-type and transgenic mice were less prominent with regard to tyrosine hydroxylase and acetylcholinesterase. The main findings were of two types; (i) involvement of peptide-containing neurites in amyloid beta-peptide positive plaques, and (ii) more generalized changes in peptide levels in specific layers, neuron populations and/or subregions in the hippocampal formation and ventral cortices. In contrast, the parietal and auditory cortices were comparatively less affected. The peptide immunoreactivities most strongly involved, both in plaques and in the generalized changes, were galanin, neuropeptide Y, cholecystokinin and enkephalin. This study shows that there is considerable variation both with regard to plaque load and peptide expression even among homozygotes of the same age. The most pronounced changes, predominantly increased peptide levels, were observed in two 26-month-old homozygous mice, for example, galanin-, enkephalin- and cholecystokinin-like immunoreactivities in stratum lacunosum moleculare, and galanin, neuropeptide Y, enkephalin and dynorphin in mossy fibers. Many peptides also showed elevated levels in the ventral cortices. However, decreases were also observed. Thus, galanin-like immunoreactivity could not any longer be detected in the diffusely distributed (presumably noradrenergic) fiber network in all hippocampal and cortical layers, and dynorphin-like immunoreactivity was decreased in stratum moleculare, cholecystokinin-like immunoreactivity in mossy fibers and substance P-like immunoreactivity in fibers around granule cells. The significance of generalized peptide changes is at present unclear. For example, the increase in the mainly inhibitory peptides galanin, neuropeptide Y, enkephalin and dynorphin and the decrease in the mainly excitatory peptide cholecystokinin in mossy fibers (and of substance P fibers around granule cells) indicate a shift in balance towards inhibition of the input to the CA3 pyramidal cell layer. Moreover, it may be speculated that the increase in levels of some of the peptides represents a reaction to nerve injury with the aim to counteract, in different ways, the consequences of injury, for example by exerting trophic actions. Further studies will be needed to establish to what extent these changes are typical for Alzheimer mouse models in general or are associated with the V717F mutation and/or the platelet-derived growth factor-beta promoter.

GM1 ganglioside restores abnormal responses to acute thermal and mechanical stimuli in aged rats

We investigated the effect of aging on the responses to thermal and mechanical stimuli in rats. Young (3-5 months old) and aged (22-24 months old) male Sprague-Dawley rats were tested in the hot plate, high- and low-intensity radiant heat tail flick, and von Frey hair assays. Compared to young rats, aged rats displayed longer latencies in the hot plate and the high-intensity tail flick assays (hypoalgesia), but there was no difference in the low-intensity tail flick assay. In addition, aged rats had decreased thresholds to mechanical stimuli produced by von Frey hairs compared with young rats (mechanical allodynia). Administration of GM1 ganglioside, 30 mg/kg, i.p., once daily for 30 days, to aged rats partially restored the responses in the hot plate and von Frey hair assays. GM1 had no effect on the altered responses in the tail flick test in aged rats, and in general, had no effect on any sensory modality tested in young rats.

Upregulation of GFRalpha-1 and c-ret in primary sensory neurons and spinal motoneurons of aged rats

Aging is associated with a decline in neuromuscular and somatosensory functions. Senile muscle atrophy, considered to be of neurogenic origin, is prevalent, and sensory thresholds increase with age. However, the loss of motoneurons and primary sensory neurons is small, while sensory and motor innervation appears disturbed due to aging-related axon lesions. One mechanism which may play a role in this process is altered trophin signaling. We here report that the glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha-1 mRNA and GFRalpha-1 protein-like immunoreactivity are upregulated in spinal motoneurons, and in dorsal root ganglion neurons of 30-month-old rats. The established signaling mechanism for the GDNF/GFRalpha-1 complex is through binding to the tyrosine kinase receptor encoded by the c-ret proto-oncogene, and we also show here that c-ret mRNA is upregulated in both motoneurons and primary sensory neurons of aged rats. The findings reported here, combined with evidence presented in other studies of changes in p75(NTR) and trk receptor expressions in aging primary sensory neurons and motoneurons, point at marked alterations in trophin signaling in senescence.

Early appearance of acetylcholinergic, serotoninergic, and peptidergic neurons and fibers in the developing human central nervous system

Animal experiments have already shown that neurotransmitters and neuropeptides are not only important for normal functioning of the adult central nervous system (CNS) but are also crucial to its development. However, information on the spatio-temporal distribution of these endogenous substances in the developing human CNS is still scarce. With the use of immunocytochemical staining and a constant supply of properly fixed human abortuses from southern China, an early appearance of acetylcholinesterase, enkephalin, and substance P immunoreactivities was detected first in the spinal cord (weeks 5 to 7 of gestation), then in the brainstem nuclei (weeks 11 to 12). Their overlapping localizations in many regions of the CNS suggest possible interactions among neurons containing these substances, which are in turn important for the proper establishment of the neuronal circuitry. Immunoreactivity for neuropeptide Y appeared initially in the lateral region of upper segments of the spinal cord at week 12 of gestation, then spread latero-medially and cranio-caudally to the sacral region. In the hippocampus, neuropeptide Y neurons appeared from week 15 onwards. Serotoninergic neurons were found in the dorsal raphe nucleus at week 10 and then decreased in number as the fetus grew older. Somatostatin releasing inhibitory factor, vasopressin, and oxytocin were detected in the hypothalamus from weeks 12 to 14 onwards, and monoamine oxidase, succinic dehydrogenase, parvalbumin, calbindin D28K, and vasoactive intestinal peptide were found in the visual cortex at midgestation. The early appearance and the abundance of the neurotransmitters and neuropeptides in the developing CNS indicate that they may play a key role in neuronal differentiation.

Age-related changes in nociception, behavior, and monoamine levels in rats

1. Pain threshold, behavioral parameters, and monoamine levels were compared in two groups of rats: adult (12 months old) and old rats (25 months old). 2. No differences in nociception were found between the two groups using the tail-shock test. 3. Behavioral experiments with the holeboard test showed that locomotor activity and exploration activity were lower in aged animals, whereas no significant differences were found in emotivity. 4. Using high-performance liquid chromatography (HPLC) techniques, we found that serotonin and dopamine showed lower levels in the old group.

Neuropeptides and neurotrophin receptor mRNAs in primary sensory neurons of aged rats

Neuropeptides and neurotrophin receptors are regulated in primary sensory neurons in response to axonal injury, and axonal lesions are characteristic stigmata of aging primary sensory neurons. We have therefore examined the expression of neuropeptides and neurotrophin receptor mRNAs in 30-month-old (median survival age) Sprague-Dawley rats to see if similar adaptive mechanisms operate in senescence. The content of neuropeptides was examined with immunohistochemistry (IHC) and in situ hybridization (ISH), and the cellular mRNA expression of neurotrophin receptors was studied with ISH. All of the aged rats had symptoms of hind limb incapacity (posterior paralysis), but fore limbs did not seem affected. The size-distribution of neuronal profiles in cervical and lumbar dorsal root ganglia (DRGs) was similar in aged and young adult (2-3 months old) rats. In aged rats, the DRG neurons showed an increase in both immunolabelling and mRNA content of neuropeptide tyrosine (NPY), as well as an increased cellular expression of galanin mRNA. In the same animals, there were decreased cellular levels of calcitonin gene-related peptide (CGRP; IHC and ISH) and substance P (SP; IHC and ISH), while the difference in neuronal somatostatin (IHC and ISH) was small. The distribution of neuropeptide immunoreactivities in the dorsal horn of the corresponding spinal cord segments revealed a decreased labelling for CGRP-, SP-, and somatostatin-like immunoreactivities (LI) in the aged rats at both cervical and lumbar levels. NPY- and galanin-LI had a similar distribution in aged and young adult rats. NPY-immunoreactive fibers were also encountered in the dorsal column of aged but not young adult rats. ISH revealed that most of the primary sensory neurons express mRNA for the p75 low-affinity neurotrophin receptor (p75-LANR) and that there was no discernible difference between young adult and aged rats. The labelling intensity for mRNA encoding high-affinity tyrosine kinase receptors (TrkA, TrkB, and TrkC) was decreased in aged rat DRG neurons, while the percentage of neuronal profiles expressing mRNA for TrkA/B/C was similar in young adult and aged rats. The changed pattern of neuropeptide expression in primary sensory neurons of aged rats resembled that seen in young adult rats subjected to axonal injury of peripheral sensory nerves and may, thus, indicate aging-related lesions of sensory fibers. Since NPY is primarily present in large and galanin in small DRG neurons, the stronger effect on NPY as compared to galanin expression may indicate that aging preferentially affects neurons associated with mechanoreception (A alpha and A beta fibers) as compared to nociceptive units (A delta and C fibers). Furthermore, the observed changes in neuropeptide expression were most pronounced in lumbar DRGs, that harbors the sensory neurons supplying the affected hindlimbs of the rats.

Intracerebroventricular CRF inhibits cold restraint-induced c-fos expression in the dorsal motor nucleus of the vagus and gastric erosions in rats

Acute exposure to cold-restraint induces vagal-dependent gastric erosions associated with activation of neurons in the dorsal motor nucleus of the vagus (DMN) in rats. The influence of intracerebroventricular (i.c.v.) injection of corticotropin-releasing factor (CRF) (10 micrograms) on c-fos expression in the brain and gastric erosions induced by 3 h cold-restraint was investigated in conscious rats. In cold-restraint exposed rats, CRF injected i.c.v. inhibited gastric erosions and the number of Fos positive neurons in the DMN by 93 and 72%, respectively, while Fos labelling in the nucleus tractus solitarius (NTS) was increased by 5-fold compared with vehicle group. c-fos expression was also induced in the central amygdala by i.c.v. CRF, unlike the vehicle-injected group exposed to cold-restraint. c-fos expression induced by cold-restraint in the raphe pallidus (Rpa) and paraventricular nucleus of the hypothalamus was not altered by i.c.v. CRF. These data indicate that central CRF-induced gastric protection results from the inhibition of DMN neuronal activity enhanced by cold-restraint. CRF action on DMN neurons may be related to the increase in the NTS and central amygdala inputs leading to inhibition of DMN neurons rather than to the decrease in the excitatory input from the caudal raphe projections to the DMN.

Decreased expression of TrkB and TrkC mRNAs in spinal motoneurons of aged rats

Several studies have indicated that a decrease in availability and/or responsiveness to neurotrophin(s) may be of importance in ageing and disease-related neurodegeneration. Using in situ hybridization we have studied the mRNA expression of the full-length neurotrophin receptors TrkB and TrkC in spinal cord motoneurons of aged rats (30 months old) with symptoms of hindlimb incapacity and in young adult rats (2-3 months old). The labelling intensity for TrkB of the individual cell profile was decreased by 25% (P < 0.001) in both the cervical and lumbar motor nuclei of aged rats. In thoracic motoneurons of aged and young adult rats the difference in expression of TrkB mRNA was smaller (down by 15%; P < 0.05). The labelling for TrkC mRNA was much weaker than that for TrkB in both aged and young adult rats, but TrkC mRNA expression also seemed to decrease. Comparison of the different motor nuclei along the spinal cord axis revealed that the motoneurons of the L6/S1 nuclei showed the strongest hybridization signal for the two Trk receptors in both aged and young adult rats. The possibility that a decrease in TrkB mRNA may contribute to age-related motor disturbances is discussed.

Increased expression of serotonin transporter messenger RNA in raphe neurons of the aged rat

The action of serotonin (5-hydroxytryptamine; 5HT) in the nervous system is terminated by reuptake of the neurotransmitter into the presynaptic nerve terminal via a plasma membrane-bound transporter protein. Recently, cDNAs encoding serotonin transporter protein were cloned. Using in situ hybridization we have studied the expression of serotonin transporter mRNA in the bulbospinal 5HT system of adult (2-3 months) and aged (30 months) rats. In adult rats, serotonin transporter mRNA labelled neurons were detected in the nucleus raphe obscurus, pallidus and magnus. In all aged rats, the optical density of the serotonin transporter mRNA labelling of individual cell profiles was increased by 20-30% (P < 0.001) when compared with adult rats. Five of the six aged rats studied had various degrees of hindlimb motor dysfunctions. In rats with the most pronounced signs of hindlimb motor dysfunction the number of labelled neurons was increased by 25-75% (P < 0.01) when compared with young adult rats. It is concluded that an increased serotonin transporter gene expression is present in the bulbospinal 5HT system of aged animals.

Increase in alpha-CGRP and GAP-43 in aged motoneurons: a study of peptides, growth factors, and ChAT mRNA in the lumbar spinal cord of senescent rats with symptoms of hindlimb incapacities

Sprague-Dawley rats develop progressive motor dysfunctions during the third year of life. We use this as a model to examine possible neuronal mechanism(s) that may cause motor impairments occuring during aging. In this study we have used indirect immunofluorescence histochemistry (IF) and in situ hybridization histochemistry (ISH) to study quantitatively and qualitatively the staining pattern and mRNA expression of calcitonin gene-related peptide (alpha-CGRP), growth-associated protein 43 (GAP-43), and acidic fibroblast growth factor (aFGF) in spinal lumbar motoneurons of young adult (2-3 months) and aged (30 months) Sprague-Dawley rats. In addition, mRNAs encoding choline acetyltransferase (ChAT), beta-CGRP, and cholecystokinin (CCK) were analyzed. All aged rats used in this study disclosed symptoms of hindlimb incapacity, ranging from mild weight-bearing insufficiency to paralysis of the hind limbs. The symptoms were confined to the musculature of the hindlimb and hip regions. Only a small number (approximately 15%) of the large motoneurons that innervate the hindlimb muscles were lost in those aged rats that had clinical symptoms of hindlimb motor incapacities. The remaining motoneurons expressed ChAT mRNA at levels similar to those of young adult rats. The vast majority of these motoneurons showed increased mRNA levels for alpha-CGRP and GAP-43. Aged motoneurons contained more CGRP like immunoreactivity (LI), but the number of immunoreactive neurons was smaller than in adult rats. GAP-43-LI could be detected in motoneurons in aged, but not in adult, rats. GAP-43-LI was always colocalized with CGRP-LI in aged motoneurons. Studies of individual aged rats revealed that the increase of GAP-43 mRNA-positive cell bodies occurred in cases with the most severe clinical symptoms, whereas the increase in alpha-CGRP was even evident in rats with mild symptoms. No alterations in content of aFGF-LI or aFGF mRNA could be detected in the aged rat, and the content of CCK and beta-CGRP mRNAs was also normal. The usefulness of this rat model for studies of neuromuscular aging and possible functional roles for GAP-43 and CGRP in plastic and regenerative processes during aging are discussed.

Caudal raphe-dorsal vagal complex peptidergic projections: role in gastric vagal control

Subpopulations of raphe pallidus (Rpa) and raphe obscurus (Rob) neurons containing TRH, serotonin (5-HT), and substance P contribute projections to the dorsal vagal complex (DVC). Activation of Rpa and Rob neurons induces a vagal cholinergic-dependent stimulation of gastric secretory and motor function and modulates resistance of the gastric mucosa to gastric injury in rats and cats. The caudal raphe nuclei-DVC pathways containing TRH/5-HT are involved in mediating cold-induced vagal stimulation of gastric function and erosion formation. These results suggest that Rpa/Rob-DVC projections containing TRH/5-HT may be an important pathways in the medullary regulation of vagal activity to the viscera.