Afferent Stimuli Research Articles

Central neuronal pathways of the cardioinhibitory reflex in the isopod crustacean Bathynomusdoederleini

We have already identified central neurons for cardioinhibition and cardioacceleration in Bathynomus, an isopod crustacean. The 1st thoracic ganglion (TG1) has cardioinhibitory neurons, which we call CIs, while the 2nd and 3rd thoracic ganglia (TG2 and TG3) have cardioacceleratory neurons, which we call CA1s and CA2s. We examined neuronal pathways for cardioinhibitory reflexes in whole animal preparations, using intracellular and extracellular recording methods. Cardiac inhibition in response to a variety of external stimuli was mediated by activation of CIs and inhibition of both CAs. When preparations had the ventral nerve cord intact, CIs were activated by excitatory postsynaptic potentials and CAs were inhibited by inhibitory postsynaptic potentials in response to tactile stimuli applied to sensilla setae on appendages and afferent stimuli applied to ganglionic roots of the thoracic ganglia. However, stimulation of ganglionic nerve roots of TG2 and TG3, or tactile stimulation of the body surface, failed to evoke inhibition of CAs in preparations in which both the cerebral ganglion and TG1 had been excised. These results suggest that TG1 is an indispensable central region for the excitation of CI and for inhibition of CA neurons, induced by tactile stimuli and by stimuli applied to nerve roots of TG2 and TG3.

Metabolic activity changes in the rat spinal cord during adjuvant monoarthritis

The development of chronic pain is associated with activity-dependent plastic changes in neuronal structures in the peripheral and central nervous system. In order to investigate the time-dependent processing of afferent noxious stimuli in the spinal cord we employed the quantitative autoradiographic 2-deoxyglucose technique in a model of chronic monoarthritic pain in the rat. Spinal metabolic activity was determined at various time-points (two, four and 14 days) after the injection of complete Freund's adjuvant into the left tibiotarsal joint. In addition, the effect of acute noxious mechanical stimulation of the arthritic joint was investigated at 14 days of monoarthritis. Local glucose utilization was determined in lumbar segments L 2–L 5, ipsi- and contralateral to the inflamed hindpaw, and compared with saline-injected controls. In general, monoarthritic animals had bilaterally increased metabolic activity in all laminae of the spinal cord. Detailing the time-course showed that in rats with two days of monoarthritis metabolic activity was significantly increased to a similar extent on both sides of all spinal laminae. In contrast, at four days, glucose utilization in deep laminae of the dorsal horn (laminae V–VI), the central gray area (laminae X) and the ventral horn (laminae VII–IX) tended to return to control levels. At 14 days of monoarthritis, however, metabolic activity showed a further increase in all laminae of the spinal cord. This increase was more pronounced on the side ipsilateral to inflammation, reaching 65% above corresponding control levels in laminae V, VI. Animals with 14 days of monoarthritis which were subjected to mechanical noxious stimulation of the arthritic joint displayed clear behavioral signs of acute pain. Although in this group metabolic activity was above control levels, it was lower than in animals with 14 days of monoarthritis that were not additionally stimulated. The data show not only a general increase of spinal cord metabolic activity during the time-course of the development of a chronic pain state, but also show a region-specific non-linear time profile. This may reflect the complexity of transducing and suppressive transmitter systems involved in the central processing of ongoing pain.

Single Synaptic Events Evoke NMDA Receptor–Mediated Release of Calcium from Internal Stores in Hippocampal Dendritic Spines

We have used confocal microscopy to monitor synaptically evoked Ca 2+ transients in the dendritic spines of hippocampal pyramidal cells. Individual spines respond to single afferent stimuli (<0.1 Hz) with Ca 2+ transients or failures, reflecting the probability of transmitter release at the activated synapse. Both AMPA and NMDA glutamate receptor antagonists block the synaptically evoked Ca 2+ transients; the block by AMPA antagonists is relieved by low Mg 2+. The Ca 2+ transients are mainly due to the release of calcium from internal stores, since they are abolished by antagonists of calcium-induced calcium release (CICR); CICR antagonists, however, do not depress spine Ca 2+ transients generated by backpropagating action potentials. These results have implications for synaptic plasticity, since they show that synaptic stimulation can activate NMDA receptors, evoking substantial Ca 2+ release from the internal stores in spines without inducing long-term potentiation (LTP) or depression (LTD).

Fundamentals of pain perception in animals

Pain is mediated by functionally distinctive components. It may involve acute high threshold afferent stimuli (thermal, mechanical or chemically damaged tissue), protracted afferent input (long-lasting hyperalgesia) or low-threshold input (allodynia as related to pain from light touch). Behavioral patterns will be associated with the effects of noxious stimulus on excitatory transmitters. Humane studies using mechanical, thermal (cold pressor test), audio-evoked potentials, or other noxious stimuli during anesthesia and analgesia provide clues to perception of pain in animals and help us determine guidelines for the clinical relief of animal pain. There is a better understanding of cutaneous somatosensory responses than for deep sensation (e.g., subcutaneous tissue, muscle, bone, viscera). The prevention or treatment of pain can best be accomplished when there is a diagnosis of the neuroanatomical distribution of the pain based on evidence of sensory dysfunction involving a peripheral nerve, plexus, nerve root or central pathway using different modalities for quantitative sensory testing. Anatomical studies have demonstrated that unmyelinated primary afferent fibers contain a variety of neuroactive substances which may be released by high-intensity peripheral stimulation. Fast depolarization of the dorsal horn nociresponsive neurons is mediated by tissue damage. These neuroreceptors may be activated by glutamate, substance P, and neurokinin A. The major ascending pathway relaying nociceptive information relating to pain is the spinothalamic tract. Most of the neurons in this tract are nociceptive or thermoreceptive. The thalamus is the final relay nucleus for all the sensory systems, except olfactory tissue. Injury or disease processes in deeper tissue or visceral areas with extensive innervation may result in pain thresholds not adequately controlled by current available analgesics or at their recommended dosage levels. These issues are complex and diagnosis is even more difficult due to species and breed differences in outward expression to painful insult. Medications can now be targeted for specific receptors.

Do different levels and patterns of sympathetic activation all provoke renal vasoconstriction?

Renal sympathetic nerve activity (RSNA) is postulated to influence renal function in selective ways with changes in renal hemodynamics only occurring during high stimulus intensities. The aim of this study was to determine the renal blood flow (RBF) response to a number of stimuli designed to increase RSNA by a modest amount and assess the possibility that different afferent stimuli produce differential levels of vasoconstriction by differentially altering the pattern of RSNA. Experiments were performed in eight conscious rabbits subjected to 20 min periods of three stimuli noise stress, air jet stress or hypoxia (10% O 2). RSNA was significantly increased 12±4, 31±8 and 14±5% (means of 20 min periods±SEM) and these effects were mirrored in the significant changes in RBF over the period of each stimuli with mean reductions of 8±1, 10±3 and 8±4% during noise, air jet stress and hypoxia respectively. Changes in plasma renin activity did not occur without changes in RBF. With regard to the pattern of RSNA discharges, hypoxia selectively increased the amplitude (number of recruited nerves) while noise and air jet stress increased both the amplitude and frequency of discharges. The role of the renal nerves in these responses and in providing a tonic level of vasoconstriction within the kidney, was demonstrated in experiments on a group of eight renal denervated animals. The renal denervated rabbits had greater resting RBF than the intact rabbits (54±1 denervated vs. 38±1 ml min −1 intact), and RBF was not altered by any of the afferent stimuli. We conclude that small changes in RSNA, irrespective of the stimulus, modulate renal blood flow.

Restoration of Micturition in Patients With Acontractile and Hypocontractile Detrusor by Transurethral Electrical Bladder Stimulation

The aim of transurethral electrical bladder stimulation (TEBS) treatment in patients with diminished detrusor contractility is to attain complete voiding, i.e., micturition without residual urine, which is under volitional control. In contrast to other types of electrical bladder stimulation, the physiological basis for this kind of treatment is sensitizing of the bladder mechanoreceptor afferents. The latency of afferent stimuli in normal conditions indicates that myelinated A-δ-fibers are used for neurotransmission from these mechanoreceptors. Forty-eight patients with hypocontractile or acontractile detrusor underwent intravesical electrical stimulation. All patients had a history pointing to neurogenic pathology. Forty patients had an acontractile and 8 a hypocontractile bladder. Prior to therapy, the mean residual urine volume was 461 ml. After stimulation, 32 of 48 patients were able to void with a mean residual of 57 ml. Detrusor contraction was achieved in 39% and bladder sensation was perceived by 75% of patients who failed these conditions before treatment. Following therapy, 19 of 35 patients were able to empty their bladder without the previously needed catheterization. Side effects were epididymitis in 1 and cystitis in 7 patients. © 1996 Wiley-Liss, Inc.

Intercostal nerve loop as an original treatment for long-term post-thoracotomy pain

Up to now, the treatment for long-term post-thoracotomy pain has been problematic. Conservative methods, surgical, or percutaneous rhizotomies or implantation of a permanent nerve stimulator have failed in many patients, and were not without risk. Because of this, we looked for a fundamental physiologically based approach to manage this pain problem with minimal risks but long lasting effect. As most patients obtained complete temporary pain relief after an intercostal nerve block of serial dermatomes bordering the thoracotomy, nerve ischemia, nerve entrapment in scar tissue, nerve compression due to abnormal bone formation and neuroma function may all have been mechanisms resulting in pain. Subsequently, three intercostal nerve loops between the six intercostal nerves, belonging to the three ribs above and below the thoracotomy, were performed. This technique creates permanent anesthetic dermatomes, thus avoiding nociceptive afferent stimuli and preventing recurrence of aberrant nerve regeneration responsible for chronic pain. This technique was applied successfully in five cases with a mean follow-up of 13 months. As nerve regeneration has stopped at that time, neuroma formation is prevented and pain recurrence is not to be expected. We, therefore, propose that if an anatomical pain distribution pattern along the thoracotomy scar is found in patients with long-term post-thoracotomy pain, serial intercostal nerve loops might be indicated as primary treatment.

Kinetics of muscarinic reduction of IsAHP in hippocampal neurons: effects of acetylcholinesterase inhibitors.

The present experiments were designed to elucidate the time frame in which an evoked cholinergic impulse decreases the Ca2+-dependent K+ current (IsAHP) in hippocampal CA1 neurons, and to determine to what extent acetylcholinesterase (AChE) inhibitors enhance the efficacy of the cholinergic impulse. Whole cell voltage-clamp recordings were performed on hippocampal CA1 neurons of rat brain slices and IsAHPs were evoked by constant depolarizing pulses. Cholinergic afferent fibers in stratum oriens were stimulated electrically and the time interval between the afferent stimulus and the depolarizing pulse was varied from 1 to 30 s. In slices perfused with the standard external medium, the afferent stimulus caused a profound decrease in the following IsAHP only when the stimulus preceded the depolarizing pulse by 1-2 s. The stimulus was without effects on the IsAHP when applied >/=5s before the depolarizing pulse. The effects of the afferent stimulus were greatly enhanced in CA1 neurons exposed to the catalytic AChE inhibitors neostigmine, physostigmine, or 9-amino-1,2,3, 4-tetrahydro-acridine. A substantial decrease in the IsAHP was observed even when the stimulus preceded the depolarizing pulse by >/=30 s. However applications of peripheral site AChE inhibitors decamethonium and propidium caused only minor or no enhancement of the IsAHP reduction after the afferent stimulus. We suggest in physiological conditions that muscarinic modulation of ionic conductances of CNS neurons has a limited time course after a cholinergic impulse and that the modulation is greatly enhanced and prolonged when catalytic activities of AChEs are suppressed pharmacologically.

Antisense oligonucleotides in neuroendocrinology: Enthusiasm and frustration

Antisense oligodeoxynucleotides (ODN) offer the potential advantage to manipulate neuropeptide or neuropeptide receptor expression within the brain transiently and site-specifically, thus providing a tool for neuroendocrinological research into the physiological function of a particular neuropeptide system. In this study, various approaches are introduced which reveal that antisense ODN may exert acute, short-term effects on neuronal responsiveness to afferent stimuli, as well as long-term effects on neuropeptide/receptor protein availability in a given system depending on the duration of treatment. Short-term effects were seen in that oxytocin (OXT) and vasopressin (AVP) antisense ODN affected electrophysiological and secretory parameters of oxytocinergic and vasopressinergic neurons, respectively, as well as their ability to express the Fos protein in response to afferent stimulation a few hours after a single infusion into the hypothalamic supraoptic nucleus. In this study, two methodological approaches to study long-term effects of the antisense ODN are exemplified, in which antisense ODN directed against the mRNA coding for the neuropeptide itself or its receptor were used. The repeated infusion of corticotropin releasing hormone (CRH) antisense ODN into the paraventricular nucleus resulted in reduced immunoreactive CRH, but not AVP, in the external zone of the median eminence. Furthermore, in order to evaluate the receptor-mediated effects of CRH and AVP released locally within the paraventricular nucleus on adrenocorticotropin (ACTH) release from the pituitary, CRH receptor (and also AVP receptor) antisense ODN were repeatedly infused into the hypothalamic nuclei; this treatment resulted in an elevation of stimulated, but not basal, ACTH release into the blood. However, in addition to these obvious antisense effects, results are discussed which demonstrate sequence-unspecific effects of phosphorothioated ODN, suggesting that some of their mechanisms of action are not yet understood.

Atropine inhibits gastric distension and pharyngeal receptor mediated lower oesophageal sphincter relaxation

Background—Atropine decreases the frequency of transient lower oesophageal sphincter relaxation (TLOSR) through an unknown mechanism. Gastric distension and pharyngeal receptor excitation are two possible sources for the afferent stimulus responsible...

Afferent vs Efferent Motion Evoked Potentials: Relationship to Dynamic Visual Performance

Visual evoked potentials (VEPs) were elicited by a computer-generated random-dot kinematogram in which a Landolt ring target moved from monitor centre to the right in front of a stationary background. Motion contrast was varied in terms of percentage of moving pixels within the ring (100%, 50%, 30%, 20%, 10%). Subjects indicated one of four gap locations (left, right, upper, lower) while fixating a small dot which was either screen centred (afferent task) or centred within the moving Landolt ring (efferent task). Correctly reported gap locations and reaction times were recorded as well as N2-amplitudes and N2-latencies. Decreasing contrast lead to decreased correct responses, decreased N2-amplitudes, and increased reaction times which were linearly correlated with increased N2-latencies. While these effects were obtained for both the afferent and efferent tasks, they were significantly stronger for the latter. At 10% motion contrast, efferent reaction times and peak N2-latencies were as much as 370 and 100 ms greater, respectively. To determine whether the stronger efferent effects were due to differences in retinal afference between the two conditions, a third stimulus was employed in which the retinal image motion of the efferent task was simulated in the absence of eye movements (stationary fixation point and Landolt ring on a leftward moving background). The effects on performance and N2-latencies and amplitudes obtained fell between those of the two initial conditions. Implications of these findings for the processing of afferent vs efferent dynamic visual stimuli and their relationship to VEPs are discussed.

Hyperexcitability in combined entorhinal/hippocampal slices of adult rat after exposure to brain-derived neurotrophic factor.

Effects of brain-derived neurotrophic factor (BDNF) in area CA3, the dentate gyrus, and medial entorhinal cortex were examined electrophysiologically by bath application of BDNF in slices containing the hippocampus and entorhinal cortex. Bath application of 25-100 ng/ml BDNF for 30-90 min increased responses to single afferent stimuli in selective pathways in the majority of slices. In area CA3, responses to mossy fiber stimulation increased in 73% of slices and entorhinal cortex responses to white matter stimulation increased in 64% of slices. After exposure to BDNF, these areas also demonstrated evidence of hyperexcitability, because responses to repetitive stimulation (1-Hz paired pulses for several s) produced multiple population spikes in response to mossy fiber stimulation in CA3 or multiple field potentials in response to white matter stimulation in the entorhinal cortex. Repetitive field potentials persisted after repetitive stimulation ended and usually were followed by spreading depression. Enhancement of responses to single stimuli and hyperexcitability were never evoked in untreated slices or after bath application of boiled BDNF or cytochrome C. The tyrosine kinase antagonist K252a (2 microM) blocked the effects of BDNF. In area CA3, both the potentiation of responses to single stimuli and hyperexcitability showed afferent specificity, because responses to mossy fiber stimulation were affected but responses to fimbria or Schaffer collateral stimulation were not. In addition, regional specificity was demonstrated in that the dentate gyrus was much less affected. The effects of BDNF in area CA3 were similar to those produced by bath application of low doses of kainic acid, which is thought to modulate glutamate release from mossy fiber terminals by a presynaptic action. These results suggest that BDNF has acute effects on excitability in different areas of the hippocampal-entorhinal circuit. These effects appear to be greatest in areas that are highly immunoreactive for BDNF, such as the mossy fibers and the entorhinal cortex. Although the present experiments do not elucidate mechanism(s) definitively, the afferent specificity, similarity to the effects of kainic acid, and block by K252a are consistent with previous hypotheses that BDNF affects acute excitability by a presynaptic action on trkB receptors that modulate excitatory amino acid transmission. However, we cannot rule out actions on inhibitory synapses or postsynaptic processes.

8-Cyclopentyltheophylline, an adenosine A 1 receptor antagonist, inhibits the reversal of long-term potentiation in hippocampal CA1 neurons

The effects of an adenosine A 1 receptor antagonist, 8-cyclopentyltheophylline (8-CPT, 1 μM), on the reduction of long-term potentiation were studied in CA1 neurons of guinea pig hippocampal slices. Reduction of long-term potentiation (depotentiation) was achieved by delivering a train of low-frequency afferent stimuli (low-frequency stimulation, 1000 pulses, 1 Hz) 20 min after the tetanus (100 Hz, 100 pulses). In control experiments, low-frequency stimulation reduced the potentiated component of the slope of the field EPSP and the amplitude of the population spike by 68.5±14.4% and 80.1±8.8%, respectively ( n=6); these values were significantly reduced to 13.4±9.7% and 9.0±10.9% ( n=7) when the low-frequency stimulation was applied during the perfusion with 8-CPT (1 μM). These results indicate that activation of adenosine A 1 receptors enhances the depotentiation of long-term potentiation.

Superior laryngeal nerve stimulation in the cat: effect on oropharyngeal swallowing, oesophageal motility and lower oesophageal sphincter activity.

Superior laryngeal nerve (SLN) stimulation can activate the brainstem swallowing mechanism to produce a complete swallowing sequence consisting of oropharyngeal, oesophageal and lower oesophageal sphincter (LOS) components. However, little is known of the effect of SLN stimulation (peripheral-sensory input from the pharynx) on the characteristics of oesophageal motor activity, especially in the smooth muscle portion. The present study examined the effect of varying stimulus train length and frequency on each of the three components of the reflex. Acute studies were performed in urethane anaesthetized cats. Oesophageal motility was monitored using conventional manometric techniques, and oropharyngeal swallowing by the mylohyoid electromyogram. SLN stimulus train length (1-10 sec) and frequency (5-30 Hz) were varied independently. Increased train length or frequency resulted in (1) an increase in oropharyngeal swallowing and incidence of the complete swallowing response, (2) an increase in latency to onset of the oesophageal peristaltic wave, (3) reduction of the amplitude of the evoked peristaltic contraction in the smooth muscle portion, without altering its velocity, (4) increased LOS relaxation, and increased LOS after-contraction. The LOS contraction was abolished by atropine (100 micrograms kg-1). Therefore, increased SLN stimulation not only results in excitation of the central swallowing program and the oropharyngeal stage of swallowing, but has major effects on the oesophageal and LOS stages of swallowing. Afferent SLN stimuli can impact on the control mechanisms for each stage, to inhibit or excite the stages in different ways.

BASIC PHARMACOLOGY AND PHYSIOLOGY OF ACUTE PAIN PROCESSING

Chemical, mechanical, or thermal stimuli of sufficient quality or intensity to threaten or destroy tissue or to disrupt vasculature integrity typically lead to autonomic (changes in heart rate or blood pressure) or hormonal (adrenal and pituitary secretion) responses as well as to the subjective sensation of pain. This article discusses several of the components of the physiology and pharmacology involved in the generation of the acute pain state. Emphasis is on the afferent and spinal components. Space does not permit a consideration of the supraspinal linkages that are believed to play a crucial role in the decoding of the sensory message. Moreover, it is clear that this understanding of the properties of this afferent linkage has provided important insights into the properties of the encoding of the afferent stimulus in a manner that usefully predicts the pain state generated by tissue-damaging stimuli.

Noradrenergic regulation of synaptic plasticity in the hippocampal CA1 region.

The effects of norepinephrine (NE) and related agents on long-lasting changes in synaptic efficacy induced by several patterns of afferent stimuli were investigated in the CA1 region of rat hippocampal slices. NE (10 microM) showed little effect on the induction of long-term potentiation (LTP) triggered by theta-burst-patterned stimulation, whereas it inhibited the induction of long-term depression (LTD) triggered by 900 pulses of 1-Hz stimulation. In nontreated slices, 900 pulses of stimuli induced LTD when applied at lower frequencies (1-3 Hz), and induced LTP when applied at a higher frequency (30 Hz). NE (10 microM) caused a shift of the frequency-response relationship in the direction preferring potentiation. The effect of NE was most prominent at a stimulus frequency of 10 Hz, which induced no changes in control slices but clearly induced LTP in the presence of NE. The facilitating effect of NE on the induction of LTP by 10-Hz stimulation was blocked by the beta-adrenergic receptor antagonist timolol (50 microM), but not by the alpha receptor antagonist phentolamine (50 microM), and was mimicked by the beta-agonist isoproterenol (0.3 microM), but not by the alpha1 agonist phenylephrine (10 microM). The induction of LTD by 1-Hz stimulation was prevented by isoproterenol but not by phenylephrine, indicating that the activation of beta-receptors is responsible for these effects of NE. NE (10 microM) also prevented the reversal of LTP (depotentiation) by 900 pulses of 1-Hz stimulation delivered 30 min after LTP induction. In contrast to effects on naive (nonpotentiated) synapses, the effect of NE on previously potentiated synapses was only partially mimicked by isoproterenol, but fully mimicked by coapplication of phenylephrine and isoproterenol. In addition, the effect of NE was attenuated either by phentolamine or by timolol, indicating that activation of both alpha1 and beta-receptors is required. These results show that NE plays a modulatory role in the induction of hippocampal synaptic plasticity. Although beta-receptor activation is essential, alpha1 receptor activation is also necessary in determining effects on previously potentiated synapses.

Frequency and amplitude of sympathetic discharges by baroreflexes during hypoxia in conscious rabbits.

Sympathetic nerve activity (SNA) from multifiber preparations exhibits two distinct components: the frequency at which discharges occur and their relative amplitude (reflecting the number of activated nerve fibers within each burst). These two components may respond independently to various afferent inputs, indicating separate central controlling processes. We examined the response in the frequency and amplitude of renal SNA to changes in baroreceptor activity and the effect of two forms of hypoxia on this relationship in nine conscious rabbits. Rabbits breathed either room air or one of two hypoxic gas mixtures (10% O2 or 10% O2 + 3% CO2) for 20 min, during which baroreflexes were stimulated by ramp increases and then decreases in arterial pressure with phenylephrine (0.5 mg/ml iv) and nitroprusside (1 mg/ml) (total arterial pressure range induced was 80 mmHg). Hypoxia with 10% O2 significantly increased the resting frequency of SNA before baroreflex modifications from 2.15 +/- 0.18 to 2.82 +/- 0.25 discharges/s and with 10% O2 + 3% CO2 to 3.20 +/- 21 discharges/s. The amplitude of sympathetic discharges was increased 44 +/- 5% over control levels during 10% O2 but was not further increased by the addition of 3% CO2. The baroreflex curve for total SNA (1-s averages of the integrated neurogram) showed a graded response to the two hypoxic stimuli, with significant increases in the upper plateau, gain, and resting point on the curves. However, the baroreflex curve for the frequency or amplitude of sympathetic discharges did not show graded responses to each hypoxic treatment. The frequency baroreflex curve was sigmoidal and not changed from air during 10% O2. During 10% O2 + 3% CO2, the gain (responsiveness) of the curve was increased although the range of frequencies occurring was unaltered. The baroreflex curve for the amplitude showed similar responses to the two hypoxic stimuli, namely, increases in the upper plateau, gain, and resting point of the curve. We conclude that the frequency and amplitude of sympathetic discharges are able to respond differentially to changes in afferent stimuli. Given that alterations in the frequency and recruitment of sympathetic fibers (amplitude) to the kidney may have differing effects, this phenomenon may provide a previously unknown level of renal hemodynamic control through the interaction of specific afferent inputs to the central nervous system.

Plateau‐generating neurones in the dorsal horn in an in vitro preparation of the turtle spinal cord.

1. In transverse slices of the spinal cord of the turtle, intracellular recordings were used to characterize and analyse the responses to injected current and activation of primary afferents in dorsal horn neurones. 2. A subpopulation of neurones, with cell bodies located laterally in the deep dorsal horn and dendrites radiating towards the pial surface, was distinguished by the ability to generate plateau potentials. Activation of the plateau potential by a suprathreshold depolarizing current pulse produced an increasing firing frequency during the first few seconds and a sustained after-discharge. 3. The plateau potential was assumed to be mediated by L-type Ca2+ channels since it was blocked by Co2+ (3 mM) and nifedipine (10 microM) and enhanced by Bay K 8644 (0.5-2 microM). 4. The threshold for activating the plateau potential declined during the first few seconds of depolarization. The decline in threshold gradually subsided over 3-10 s after repolarization. 5. Frequency potentiation of the plateau potential contributed to wind-up of the response to depolarizing current pulses and primary afferent stimuli repeated at frequencies higher than 0.1-0.3 Hz. 6. The sustained after-discharge mediated by the plateau potential was curtailed by a slow after-hyperpolarization (sAHP) evoked by strong depolarizations. The relative strength of the plateau potential and sAHP varied among cells. In some cells the plateau potential and sAHP interacted to produce damped oscillations upon depolarization. The sAHP was mediated by both apamin and tetraethylammonium (TEA)-sensitive K+ channels. 7. Our findings suggest that basic properties of sensory integration may reside with the specialized intrinsic response properties of particular subtypes of neurones in the dorsal horn.

Burst‐generating neurones in the dorsal horn in an in vitro preparation of the turtle spinal cord.

1. In transverse slices of the spinal cord of the turtle, intracellular recordings were used to characterize and analyse the responses to injected current and activation of primary afferents in dorsal horn neurones. 2. A subpopulation of neurones, with cell bodies located centrally in the dorsal horn, was distinguished by the ability to generate a burst response following a hyperpolarization from rest or during a depolarization from a hyperpolarized holding potential. The burst response was inactivated at the resting membrane potential. 3. The burst response was mediated by a low threshold Ca2+ spike assumed to be mediated by T-type Ca2+ channels since it resisted tetrodotoxin and was blocked by 3 mM Co2+ or 100-300 microM Ni2+ and resembled the low threshold spike (LTS) described elsewhere. 4. Some burst-generating cells also displayed plateau potentials mediated by L-type Ca2+ channels. In these cells the burst following a hyperpolarizing current pulse, applied from the resting membrane potential, facilitated the activation of the plateau potential. Wind-up of the plateau potential was produced when the hyperpolarizing pulse generating the burst was repeated at 0.1-0.3 Hz or faster. 5. The burst response and the underlying low threshold Ca2+ spike were activated synaptically by primary afferent stimuli in a voltage range hyperpolarized from the resting membrane potential. 6. Cells with bursts were morphologically distinguishable from cells with bursts and plateau properties. 7. Our findings in this and the preceding paper show that the intrinsic response properties of particular subtypes of neurones in the dorsal horn have a profound influence on the amplitude and time course of the responses mediated by primary afferent fibres. We predict that these postsynaptic properties are probable targets for synaptic modulation.

Hyperventilation during bronchial challenges in asthmatics: reproducibility and assessment of contributing factors

Among asthmatics, the ventilatory response is heterogeneous during bronchial challenge. This study aimed to investigate the reproducibility of the response and to assess possible causes for hyperventilation. Repeated bronchial histamine and methacholine challenges ( HiCh MeCh ) were performed in 10 asthmatic adolescents. Ventilation was monitored by respiratory inductive plethysmography (RIP), in order to minimally affect the spontaneous breathing pattern. FEV 1 and the volume of trapped gas (measured as the volume of air mobilized by five maximal breaths after a multiple breath nitrogen washout to 2% N 2), were used to assess mainly central and peripheral airways obstruction, respectively. When FEV 1 had decreased by at least 20%, mean inspiratory flow ( V TI T I ) increased by 21% and minute ventilation ( V′ I) by 21% and 23% during HiCh and MeCh, respectively (both P<0·05). No correlation was found between the magnitude of the ventilatory response and either: the degree of FEV 1 decline, the increase in gas trapping, SaO 2 decline or the increase in dyspnoea score. Histamine challenge after β 2-agonist pre-treatment was associated with increased ventilatory drive in one patient despite the absence of bronchial obstruction, indicating that histamine might directly stimulate afferent airway nerves which cause hyperventilation. The intra-individual variability of the ventilatory response (increase in V′ I and V TI T I ) was more than 100% of the mean ventilatory response, while the variability of the bronchomotor response was about 25% of the mean bronchomotor response. Thus, during induced bronchial obstruction in asthmatics, the occurrence of hyperventilation and its intensity are not related to either the degree of central or peripheral airways obstruction, or to the degree of dyspnoea. The reproducibility of the ventilatory response is poor. The ventilatory response appears to be the result of a complex interaction between several afferent stimuli and central ventilatory control.