Integrative responses of neurons in nucleus tractus solitarius to visceral afferent stimulation and vestibular stimulation in vertical planes

Anatomical studies have demonstrated that the vestibular nuclei project to nucleus tractus solitarius (NTS), but little is known about the effects of vestibular inputs on NTS neuronal activity. Furthermore, lesions of NTS abolish vomiting elicited by a variety of different triggering mechanisms, including vestibular stimulation, suggesting that emetic inputs may converge on the same NTS neurons. As such, an emetic stimulus that activates gastrointestinal (GI) receptors could alter the responses of NTS neurons to vestibular inputs. In the present study, we examined in decerebrate cats the responses of NTS neurons to rotations of the body in vertical planes before and after the intragastric administration of the emetic compound copper sulfate. The activity of more than one-third of NTS neurons was modulated by vertical vestibular stimulation, with most of the responsive cells having their firing rate altered by rotations in the head-up or head-down directions. These responses were aligned with head position in space, as opposed to the velocity of head movements. The activity of NTS neurons with baroreceptor, pulmonary, and GI inputs could be modulated by vertical plane rotations. However, injection of copper sulfate into the stomach did not alter the responses to vestibular stimulation of NTS neurons that received GI inputs, suggesting that the stimuli did not have additive effects. These findings show that the detection and processing of visceral inputs by NTS neurons can be altered in accordance with the direction of ongoing movements.     

Neurogenic adaptation contributes to the afferent response to mechanical stimulation

This study aimed to characterize the effect of mechanical stimuli on mesenteric afferent nerve signaling in the isolated rat jejunum in vitro. This was done to determine the effect of mechanical stresses and strains relative to nonmechanical parameters (neurogenic adaptation). Mechanical stimulations were applied to a segment of jejunum from 15 rats using ramp distension with water at three rates of distension, a relaxation test (volume maintained constant from initial pressure of 20 or 40 mmHg), and a creep test (pressure maintained constant). Circumferential stress and strain and the spike rate increase ratio were calculated for evaluation of afferent nerve activity during the mechanical stimulations. Ramp distension evoked two distinct phases of afferent nerve signaling as a function of circumferential stress or strain. Changing the volume distension rate did not change the stress-strain relationship, but faster distension rate increased the afferent firing rate (P < 0.05). In the stress relaxation test, the spike rate declined faster and to a greater extent than the stress. In the creep test, the spike rate declined, despite a small increase in the strain. Three classes of mechanosensitive single-afferent units (low, wide dynamic range, and high threshold units) showed different response profiles against stress and strain. Low-threshold units exhibited a near linear relationship against the strain (R(2) = 0.8095), whereas high-threshold units exhibited a linear profile against the stress (R(2) = 0.9642). The afferent response is sensitive to the distension speed and to the stress and strain level during distension. However, the afferent nerve response is not a simple function of either stress or strain. Nonmechanical time-dependent adaptive responses other than those related to viscoelasticity also play a role.     

The responses of primary and secondary sensory hair cells in the squid statocyst to mechanical stimulation

Summary Intracellular recordings were obtained from primary and secondary sensory hair cells in the anterior transverse crista segment of the squid (Alloteuthis subulata) statocyst during imposed displacements of the overlying cupula. The secondary sensory hair cells were depolarized by ventral movements of the cupula and hyperpolarized by dorsal cupula movements. The displacement/response curve was asymmetric around the zero position and sigmoidal in shape, similar to that already described for vertebrate hair cells. The cells are estimated to have a sensitivity of at least 0.5 mV per degree angle of cilia displacement. The responses showed pronounced adaptation and could be blocked by bath applied alcohols, such as heptanol or octanol, or by high concentrations of aminoglycosides.The primary sensory hair cells were depolarized by dorsal movements of the cupula, usually responding with a burst of action potentials. The displacement/response curve was also sigmoidal in shape and the firing pattern showed strong adaptation to maintained displacements of the cupula.The cupula itself appeared to be irregular in shape, extending much further into the statocyst cavity in its central part than at its edges. This is likely to result in differences in the responses of the underlying hair cells along the length of the crista ridge.     

Age-related changes in sensory neurons containing calcitonin gene-related peptide under conditions of afferentation deficit in rats

Morphological features of calcitonin gene-related peptide (CGRP)-immunoreactive neurons were studied in the sensory ganglia of the vagus and thoracic nerves in 3-, 10-, 20-, 30-, 60-, 90-, and 180-day-old rats under conditions of chemically-induced deafferentation. We found that, in rats, CGRP-containing neurons appeared in both ganglia immediately after they were born and their number decreased with aging. Most of CGRP-immunoreactive neurons were small in size, i.e., up to 600 ??m². Administration of capsaicin modified age-related changes in the number of CGRP-immunopositive neurons. In the thoracic nerve ganglion, the mean square of these cells and their number substantially decreased, whereas, in the vagus nerve ganglion, positive cells were not observed.     

Age-related changes in postural responses to backward platform translation

The aim of the study was to investigate age-related changes in postural responses to platform translation with 3 various velocities. We focused on the influence of linear velocity using the smoothed profile of platform acceleration (till 100 cm.s(-2)). Eleven healthy young (20-31 years) and eleven healthy elderly (65-76 years) subjects were examined. The subjects stood on the force platform with their eyes closed. Each trial (lasting for 8 sec) with different velocity (10, 15, 20 cm.s(-1)) of 20 cm backward platform translation was repeated 4 times. We have recorded displacements of the centre of pressure (CoP) and the EMG activity of gastrocnemius muscle (GS) and tibialis anterior muscle (TA). The results showed increased maximal values of CoP responses to the platform translation. There was also observed a scaling delay of CoP responses to platform translation with different velocities in elderly. The EMG activity of GS muscle during backward platform translation was of about similar shape in both groups during the slowest platform velocity, but it increased depending on rising velocity. EMG activity of TA was not related to the platform velocity. Early parts of postural responses showed significant co-activation of TA and GS muscles of elderly. It is likely that elderly increased body stiffening in order to help their further balance control.     

Beta-catenin levels influence rapid mechanical responses in osteoblasts

Mechanical loading of bone initiates an anabolic, anticatabolic pattern of response, yet the molecular events involved in mechanical signal transduction are not well understood. Wnt/beta-catenin signaling has been recognized in promoting bone anabolism, and application of strain has been shown to induce beta-catenin activation. In this work, we have used a preosteoblastic cell line to study the effects of dynamic mechanical strain on beta-catenin signaling. We found that mechanical strain caused a rapid, transient accumulation of active beta-catenin in the cytoplasm and its translocation to the nucleus. This was followed by up-regulation of the Wnt/beta-catenin target genes Wisp1 and Cox2, with peak responses at 4 and 1 h of strain, respectively. The increase of beta-catenin was temporally related to the activation of Akt and subsequent inactivation of GSK3beta, and caveolin-1 was not required for these molecular events. Application of Dkk-1, which disrupts canonical Wnt/LRP5 signaling, did not block strain-induced nuclear translocation of beta-catenin or up-regulation of Wisp1 and Cox2 expression. Conditions that increased basal beta-catenin levels, such as lithium chloride treatment or repression of caveolin-1 expression, were shown to enhance the effects of strain. In summary, mechanical strain activates Akt and inactivates GSK3beta to allow beta-catenin translocation, and Wnt signaling through LRP5 is not required for these strain-mediated responses. Thus, beta-catenin serves as both a modulator and effector of mechanical signals in bone cells.     

Age-related intrinsic changes in human bone-marrow-derived mesenchymal stem cells and their differentiation to osteoblasts

In vivo and in vitro studies indicate that a subpopulation of human marrow-derived stromal cells (MSCs, also known as mesenchymal stem cells) has potential to differentiate into multiple cell types, including osteoblasts. In this study, we tested the hypothesis that there are intrinsic effects of age in human MSCs (17-90 years). We tested the effect of age on senescence-associated beta-galactosidase, proliferation, apoptosis, p53 pathway genes, and osteoblast differentiation in confluent monolayers by alkaline phosphatase activity and osteoblast gene expression analysis. There were fourfold more human bone MSCs (hMSCs) positive for senescence-associated beta-galactosidase in samples from older than younger subjects (P < 0.001; n = 17). Doubling time of hMSCs was 1.7-fold longer in cells from the older than the younger subjects, and was positively correlated with age (P = 0.002; n = 19). Novel age-related changes were identified. With age, more cells were apoptotic (P = 0.016; n = 10). Further, there were age-related increases in expression of p53 and its pathway genes, p21 and BAX. Consistent with other experiments, there was a significant age-related decrease in generation of osteoblasts both in the STRO-1+ cells (P = 0.047; n = 8) and in adherent MSCs (P < 0.001; n = 10). In sum, there is an age-dependent decrease in proliferation and osteoblast differentiation, and an increase in senescence-associated beta-galactosidase-positive cells and apoptosis in hMSCs. Up-regulation of the p53 pathway with age may have a critical role in mediating the reduction in both proliferation and osteoblastogenesis of hMSCs. These findings support the view that there are intrinsic alterations in human MSCs with aging that may contribute to the process of skeletal aging in humans.     

Contribution of neurons to habituation to mechanical stimulation in Caenorhabditis elegans

In Caenorhabditis elegans, a light touch induces a locomotor response. Repeated touches, however, result in an attenuation of response, that is, habituation. Withdrawal responses elicited by anterior touch are controlled by anterior mechanosensory neurons (AVM and ALMs), and by four pairs of interneurons (AVA, AVB, AVD, and PVC) (Chalfie et al., 1985; White et al., 1986). To identify the neurons that participate in habituation, we ablated these neurons with a laser microbeam and investigated the resulting habituation of the operated animals. The animals lacking both left and right homologues AVDLR were habituated more rapidly than intact animals. We propose that chemical synapses at AVD play a critical role in the habituation of intact animals.     

Synaptic responses produced in lobster abdominal postural motor neurons by mechanical stimulation of the swimmeret

1. Intracellular recordings were obtained from the somata of identified abdominal postural motor neurons in lobster to examine their subthreshold and suprathreshold responses to tactile stimulation of the swimmeret. 2. Pressure stimulation of the swimmeret surface evoked abdominal extension by producing tonic spiking in the extensor excitors and the synergistic flexor inhibitor (f5) and hyperpolarizing responses in the extensor inhibitor and antagonistic flexor excitors. These responses often continued for several seconds following the termination of the stimulus. The receptive fields of these motor responses extended over most of the swimmeret surface. 3. More localized tactile stimulation of the swimmeret surface elicited EPSPs in f5 and the extensor excitors, and IPSPs in the flexor excitors. The amplitude of these synaptic potentials decreased as the stimulus intensity was reduced. 4. Stimulation of feathered hair (both sexes) and smooth hair (female only) sensilla produced responses characteristic of extension whereas bristly spines on the male accessory lobe excited only two flexor excitors without affecting any of the other postural motor neurons. 5. Summed synaptic responses recorded from the motor neurons differed in their amplitudes and latencies according to the type of mechanoreceptor stimulated-cuticular receptors, feathered hairs or smooth hairs. Stimulation of the swimmeret cuticle produced the strongest responses (shortest latency, largest amplitude), while feathered hair stimulation initiated the weakest responses (longest latency, smallest amplitude). 6. The relatively long latencies (greater than 35 ms) and the complex form of the EPSPs and IPSPs indicate the involvement of multisynaptic interneuronal pathways in the reflex arcs.     

Discharge pattern of reticular neurons subjected to direct mechanical stimulation

Changes in neuronal discharge pattern due to direct mechanical stimulation through the recording extracellular micropipette were studied in 30 neurons of the rat's mesencephalic reticular formation. Results parallel closely the polarization experiments, i.e. changing the firing rate one group of neurons shows stable pattern whereas the other change their pattern significantly. It is concluded that the first group presumably reflects the decrease in membrane potential only, whereas in the second group overall synaptic inflow is influenced.     

Effect of psychostimulants on responses of neocortical neurons to afferent and subcortical stimuli

The effect of psychostimulants on unit responses of the pericruciate region of the cortex during sensory and subcortical stimulation was studied in experiments on 52 immobilized cats. Amphetamine (2 mg/kg) caused a definite increase in the number of spontaneously active cortical cells. Caffeine (40 mg/kg) had a weaker action. After administration of psychostimulants afferent stimuli more often induced tonic changes in the spontaneous unit activity. More polymodal neurons were found. Phasic responses were often facilitated but were recorded at the same frequency as in the control. Amphetamine and caffeine weakened inhibition of the unit discharges during stimulation of the caudate nucleus and potentiated activation caused by stimulation of the reticular formation. The character of interaction between sensory and reticular (caudate) stimuli was not modulated by the psychostimulants although its scale and intensity were varied. The possible role of the effects of amphetamine and caffeine in therapeutic psychostimulation is discussed.     

Extra spike formation in sensory neurons and the disruption of afferent spike patterning

The peculiar pseudounipolar geometry of primary sensory neurons can lead to ectopic generation of "extra spikes" in the region of the dorsal root ganglion potentially disrupting the fidelity of afferent signaling. We have used an explicit model of myelinated vertebrate sensory neurons to investigate the location and mechanism of extra spike formation, and its consequences for distortion of afferent impulse patterning. Extra spikes originate in the initial segment axon under conditions in which the soma spike becomes delayed and broadened. The broadened soma spike then re-excites membrane it has just passed over, initiating an extra spike which propagates outwards into the main conducting axon. Extra spike formation depends on cell geometry, electrical excitability, and the recent history of impulse activity. Extra spikes add to the impulse barrage traveling toward the spinal cord, but they also travel antidromically in the peripheral nerve colliding with and occluding normal orthodromic spikes. As a result there is no net increase in afferent spike number. However, extra spikes render firing more staccato by increasing the number of short and long interspike intervals in the train at the expense of intermediate intervals. There may also be more complex changes in the pattern of afferent spike trains, and hence in afferent signaling.     

Morphologic characteristics and responses of the neurons of the right parietal ganglion in Lymnaea stagnalis to stimulation of the sensory structures

Intracellular recordings have been made of the responses of 22 neurons of the central part of the dorsal surface of the right parietal ganglion of the snail Lymnaea stagnalis to adequate stimulation of chemo-, photo- and mechanoreceptor cells of the mantle and head skin including tentacles and lips. It was shown that the main bulk of the neurons investigated has broad receptive fields in the body wall and mantle, being able to respond to all types of the applied stimuli. Alongside, single neurons were revealed which receive single-mode input, either a mechanosensory or chemosensory one. Morphological studies indicate that the neurons are unipolar and have usually one, sometimes several projections. They differ in the pattern of branching as well as in the projections to peripheral nerves. However, almost all of them have vast dendritic regions in the central nervous system including central sensory nucleus of the right parietal ganglion.     

Differential responses of regional sympathetic activity and blood flow to visceral afferent stimulation

The sympathetic nervous system is essential for the cardiovascular responses to stimulation of visceral afferents. It remains unclear how the reflex-evoked sympathetic output is distributed to different vascular beds to initiate the hemodynamic changes. In the present study, we examined changes in regional sympathetic nerve activity and blood flows in anesthetized cats. Cardiovascular reflexes were induced by either electrical stimulation of the right splanchnic nerve or application of 10 microg/ml of bradykinin to the gallbladder. Blood flows were measured using colored microspheres or the Transonic flow meter system. Sympathetic efferent activity was recorded from the left splanchnic, inferior cardiac, and tibial nerves. Stimulation of visceral afferents decreased significantly blood flows in the celiac (from 49 +/- 4 to 25 +/- 3 ml/min) and superior mesenteric (from 35 +/- 4 to 23 +/- 2 ml/min) arteries, and the vascular resistance in the splanchnic bed was profoundly increased. Consistently, stimulation of visceral afferents decreased tissue blood flows in the splanchnic organs. By contrast, activation of visceral afferents increased significantly blood flows in the coronary artery and portal vein but did not alter the vascular resistance of the femoral artery. Furthermore, stimulation of visceral afferents increased significantly sympathetic efferent activity in the splanchnic (182 +/- 44%) but not in the inferior cardiac and tibial nerves. Therefore, this study provides substantial new evidence that stimulation of abdominal visceral afferents differentially induces sympathetic outflow to the splanchnic vascular bed.     

Excitatory responses of trigeminal neurons to substance P suggest involvement in sensory transmission

Responses to substance P application were studied with intracellular recording techniques in in vitro preparations of trigeminal root ganglion neurons of guinea pigs. Perfusion of substance P in micromolar concentrations markedly depolarized neurons and reduced their input conductances. Also, the threshold for spikes evoked by injections of depolarizing current pulses was decreased. Single electrode voltage-clamp recordings showed that substance P increased inward, and decreased outward currents evoked by hyperpolarizing voltage steps from holding potentials near rest. Depolarizing responses to substance P were attenuated in Na+-deficient solutions. The excitatory actions of this endogenous peptide on the perikarya of primary sensory neurons give rise to the possibility of physiological actions of substance P at multiple sites in the trigeminal system.     

Primary afferent neurons intrinsic to the guinea-pig intestine,like primary afferent neurons of spinal and cranial sensory ganglia,bind the lectin,IB4

The plant lectin, IB4, binds to the surfaces of primary afferent neurons of the dorsal root and trigeminal ganglia and is documented to be selective for nociceptive neurons. Physiological data suggest that the intrinsic primary afferent neurons within the intestine are also nociceptors. In this study, we have compared IB4 binding to each of these neuron types in the guinea-pig. The only neurons in the intestine to be readily revealed by IB4 binding have Dogiel-type-II morphology; these neurons have been previously identified as intrinsic primary afferent neurons. Most of the neurons that are IB4-positive in the myenteric plexus are calbindin-immunoreactive, whereas those in the submucosal ganglia are immunoreactive for NeuN. The neurons that bind IB4 strongly have a similar appearance in enteric, dorsal root and trigeminal ganglia. Binding is to the cell surface, to the first part of axons and to cytoplasmic organelles. A low level of binding was found in the extracellular matrix. A few other neurons in all ganglia exhibit faint staining with IB4. Strongly reactive neurons are absent from the gastric corpus. Thus, IB4 binding reveals primary afferent neurons with similar morphologies, patterns of binding and physiological roles in enteric, dorsal root and trigeminal ganglia.This work was supported by a grant from the National Health and Medical Council of Australia.