Biomedical subjects

K M Spyer

Publications and source records attributed to K M Spyer.

At least 19 recordsLinked to original sources

PubMed2001

Respiratory effects of stimulation of cell bodies of the A5 region in the anaesthetised rat.

To assess the importance of the pontine A5 region in modulating respiratory activity, electric current or microinjections of glutamate (10-30 nl, 1-3 nmol) were used to stimulate discrete zones within this region in the spontaneously breathing, anaesthetised rat. These stimuli evoked an expiratory facilitatory response, consisting of a decrease in respiratory rate (P < 0.01 electrical, P < 0.001 chemical) due to an increase of expiratory time (P < 0.01 in both cases) as measured from recordings of phrenic nerve activity. No changes were observed in inspiratory time. To avoid changes in PCO2, which could modulate the respiratory response, stimulation was also made during artificial ventilation. Under these conditions the expiratory facilitatory response elicited by glutamate was still present (P < 0.05), although its duration was reduced (P < 0.05), as was the magnitude of the phrenic burst (P < 0.05). At all the sites at which electrical stimulation and glutamate injection had evoked a respiratory response, electrical stimulation evoked a concomitant increase in both blood pressure and heart rate. Glutamate injection evoked a pressor response in 21 out of 30 animals. In eight animals the rise in blood pressure was followed by a fall in blood pressure and in one animal, a depressor response was observed. In all cases glutamate evoked an increase in heart rate. The expiratory facilitatory response was not evoked as a consequence of the increase of blood pressure since it was still present after the administration of guanethidine, which abolished the blood pressure changes. As glutamate is believed to excite perikarya rather than axons of passage these data indicate that expiratory facilitatory responses and the accompanying cardiovascular changes are the consequence of activating neurones located within the A5 region. The possible interactions between the A5 region and the medullary respiratory complex in eliciting these changes are discussed.

Anesthesia↗

PubMed2000

Somatosympathetic reflex in a working heart-brainstem preparation of the rat.

The purpose of the present study was to examine the cardiorespiratory responses (CR) evoked by a somatosympathetic reflex (SSR) in the working heart-brainstem preparation (WHBP). Sprague-Dawley rats (75-100 g) were anesthetized with halothane, bisected sub-diaphramatically and decerebrated pre-collicularly (n = 15). The preparation was transferred to a recording chamber and perfused via the thoracic aorta with Ringer's solution containing an oncotic agent (Ficoll, 1.25%). SSR was activated by electrical stimulation (5 s) of the brachial nerve (0.5-40 Hz, 1-20 V, 0.1 ms) or the forelimb (0.5-40 Hz, 5-60 V, 2 ms). Stimulation at 40 Hz significantly increased heart rate (HR, 366 +/- 10 to 374 +/- 9 beats/min), systemic perfusion pressure (PP, 83 +/- 5 to 89 +/- 6 mmHg) and phrenic nerve discharge (PND, 0.4 +/- 0.1 to 1.4 +/- 0.3 Hz). Ganglionic blockade with hexamethonium (300 microM) eliminated the tachycardia and pressor response but did not alter the tachypnea to forelimb stimulation (n = 3). Transection of the brachial nerve plexus abolished the increase in PP and PND (n = 4). This indicates that a neural reflex mediated these responses. Spinal transection (C1-C2) completely abolished all responses indicating that they were mediated via a supraspinal pathway (n = 2). Based upon these findings, we conclude that activation of somatosensory afferent fibers in the WHBP evokes a programmed pattern of autonomic responses altering the activity-state of both the cardiovascular and respiratory systems. The WHBP provides a unique opportunity to investigate the medullary circuits and neuronal mechanisms that may be involved in coupling cardiorespiratory and somatomotor activity during locomotion/exercise.

Afferent Pathways↗

PubMed2000

A role for adenosine in modulating cardio-respiratory responses: a mini-review.

Evidence is provided that purine nucleotides play a major role in modulating cardio-respiratory control through actions at specific sites in the central nervous system (CNS). The accumulation of adenosine in the extracellular space of the nucleus tractus solitarius (NTS) on hypothalamic defence area stimulation is described, and is shown to result from the extracellular breakdown of neurally released adenosine triphosphate (ATP). The actions of adenosine in modulating synaptic actions in the NTS are discussed in relation to the potential presynaptic action of adenosine in modulating transmitter release. Actions of adenosine and ATP at other sites in the medulla concerned with cardio-respiratory control are discussed. The inter-relationships between ATP and adenosine are mentioned.

Adenosine↗

PubMed2000

In vivo modulation of nucleus tractus solitarius (NTS) neurones by activation of 5-hydroxytryptamine(2) receptors in rats.

In in vivo experiments, 5-hydroxytryptamine (5-HT) and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl (DOI), a 5-HT(2) receptor agonist, were applied by ionophoresis to rat nucleus tractus solitarius (NTS) neurones identified by their vagal and cardiopulmonary afferent inputs to test whether the response of NTS cells to 5-HT(2) receptor activation was related to whether they received mono- or polysynaptic vagal inputs and their presumed function as defined by their afferent input. Cells were classified on the basis of the variability of the latency of the vagal-evoked spikes: this varied by less than 3 ms for Group 1, from 3 to 5 ms for Group 2, and more than 5 ms for Group 3. Both 5-HT and DOI inhibited most Group 1 cells (16/18) and inactive (without ongoing activity) cells (8/13) in Group 2. Cells inhibited by DOI were also inhibited by cardiopulmonary afferent stimulation, evoked by atrial phenylbiguanide administration. By contrast, application of 5-HT and DOI excited the majority of Group 3 cells (14/19) and Group 2 with ongoing activity (7/9). Cells excited by DOI were also activated by cardiopulmonary stimulation. Both actions of DOI were reversed by application of ketanserin (n=15). In conclusion, these data demonstrate that activation of 5-HT(2) receptors in the NTS produces different effects dependent on whether the neurones received mono- or polysynaptic vagal input and their response to cardiopulmonary afferent stimulation.

Afferent Pathways↗

PubMed2000

Sensing arterial CO(2) levels: a role for medullary P2X receptors.

ATP has been shown to act as an excitatory neurotransmitter in the central nervous system. In this review, evidence is presented to indicate that when ATP is micro-injected into the ventrolateral medulla (VLM) of the rat, changes in respiratory activity are elicited. These effects, and accompanying changes in heart rate and blood pressure are mediated by P2X purinoreceptors. Immunocytochemistry indicates a prevalence of P2X(2) and P2X(6) purinoreceptors in this region of the medulla. The P2 purinoceptor antagonists, suramin and PPADS blunt the respiratory responses to changes in arterial CO(2) levels when micro-injected into the VLM. This effect is shown electrophysiologically to be mediated by purinoreceptors located primarily on respiratory neurones of the VLM including the Bötzinger complex. As the effects of agonist activation of P2X(2) purinoceptors expressed in HEK293 cells and Xenopus oocytes are potentiated by lowering pH, these data imply that the central respiratory response to CO(2) depends in part on the pH sensitivity of purinoreceptors located on inspiratory neurones. The implications for respiratory activity and control are discussed.

Adenosine Triphosphate↗

PubMed2000

Coexpression of rat P2X2 and P2X6 subunits in Xenopus oocytes.

Transcripts for P2X(2) and P2X(6) subunits are present in rat CNS and frequently colocalize in the same brainstem nuclei. When rat P2X(2) (rP2X(2)) and rat P2X(6) (rP2X(6)) receptors were expressed individually in Xenopus oocytes and studied under voltage-clamp conditions, only homomeric rP2X(2) receptors were fully functional and gave rise to large inward currents (2-3 microA) to extracellular ATP. Coexpression of rP2X(2) and rP2X(6) subunits in Xenopus oocytes resulted in a heteromeric rP2X(2/6) receptor, which showed a significantly different phenotype from the wild-type rP2X(2) receptor. Differences included reduction in agonist potencies and, in some cases (e.g., Ap(4)A), significant loss of agonist activity. ATP-evoked inward currents were biphasic at the heteromeric rP2X(2/6) receptor, particularly when Zn(2+) ions were present or extracellular pH was lowered. The pH range was narrower for H(+) enhancement of ATP responses at the heteromeric rP2X(2/6) receptor. Also, H(+) ions inhibited ATP responses at low pH levels (<pH 6.3). The pH-dependent blocking activity of suramin was changed at this heteromeric receptor, although the potentiating effect of Zn(2+) on ATP responses was unchanged. Thus, the rP2X(2/6) receptor is a functionally modified P2X(2)-like receptor with a distinct pattern of pH modulation of ATP activation and suramin blockade. Although homomeric P2X(6) receptors function poorly, the P2X(6) subunit can contribute to functional heteromeric P2X channels and may influence the phenotype of native P2X receptors in those cells in which it is expressed.

Adenosine Triphosphate↗

PubMed2000

Inhibition of rat nucleus tractus solitarius neurones by activation of 5-HT2C receptors.

In vivo, nucleus tractus solitarius (NTS) neurones receiving monosynaptic vagal input and inactive intermediate neurones were inhibited by both DOI and a selective 5-HT2C receptor agonist, MK-212. Cells receiving a more polysynaptic input were excited by DOI and although MK-212 also excited a few of these cells, the majority of cells in these groups were unaffected by MK-212. The inhibitory, but not the excitatory actions of both MK-212 and DOI were prevented by a selective 5-HT2C receptor antagonist, RS-102221. In contrast, most dorsal vagal preganglionic neurones were unaffected by application of either DOI or MK-212, the few remaining cells being excited by both agonists. These data demonstrate that DOI-evoked inhibition of NTS cells activated by vagal afferent input and DOI-evoked excitation of vagal preganglionic neurones is mediated by 5-HT2C receptors.

Amphetamines↗

PubMed2000

P2X purinoceptor-mediated excitation of trigeminal lingual nerve terminals in an in vitro intra-arterially perfused rat tongue preparation.

A novel in vitro intra-arterially perfused adult rat tongue-nerve preparation was used to explore the possible actions of P2X purinoceptor agonists (ATP and alpha,beta-methylene ATP (alpha, beta-meATP)) on sensory nerve terminals innervating the rat tongue. We made whole-nerve recordings of the trigeminal branch of the lingual nerve (LN), which conducts general sensory information (pain, temperature, touch, etc.), and the chorda tympani (CT), which conducts taste information. Changes in LN and CT activity following intra-arterial application of P2X agonists were compared. In seven preparations, bolus close-arterial injection of ATP (30-3000 microM, 0.1 ml) or alpha,beta-meATP (10-300 microM, 0.1 ml) induced a rapid (< 1 s after injection), dose-related increase in LN activity that decayed within a few seconds. The minimal concentration of ATP (100 microM) required to elicit a response was about 10-fold higher than that of alpha,beta-meATP (10 microM). Bolus injection of ATP or alpha,beta-meATP induced a moderate decrease in firing frequency in three of seven CT preparations. LN responses to P2X agonists showed signs of rapid desensitisation with the peak frequency of discharge being smaller when the agonists were applied at short intervals. Suramin (200 microM) or PPADS (200 microM) applied by intra-arterial perfusion each antagonised the rapid increase in LN activity following application of alpha,beta-meATP (100 microM). Capsaicin (10 microM, 0.1 ml, n = 5 preparations) was injected intra-arterially to desensitise nociceptive fibres. This was found to block (n = 2) or greatly reduce (n = 3) the excitatory effects of alpha,beta-meATP (100 microM, 0.1 ml) on LN activity, implying that only capsaicin-sensitive nociceptive fibres in LN were responsive to P2X agonists. In contrast to the consistent excitatory responses in LN activity following fast application of P2X agonists as bolus, a variable and moderate change in discharge rate of LN and no change in CT activity (n = 5) was observed after applying ATP (100-300 microM, n = 21) or alpha,beta-meATP (100-300 microM, n = 14) by intra-arterial perfusion. The variable responses in LN activity to slow perfusion in contrast to close-arterial bolus injection are consistent with activation of the rapidly desensitising P2X3 receptors. In summary, ATP and alpha,beta-meATP preferentially activate general sensory afferent fibres (LN) but not taste fibres (CT). We suggest that the increase in whole-nerve activity of LN following application of P2X agonists represents activation of nociceptive fibres which possess P2X3 receptors. Our data indicate that ATP and P2X3 receptors may play a role in nociception, rather than taste sensation in the tongue.

Adenosine Triphosphate↗

PubMed2000

ATP as a mediator of mammalian central CO2 chemoreception.

1. A role for P2 purinoceptors in the chemosensory response of respiratory neurones localised in the ventrolateral medulla to changes in arterial CO2 levels was investigated in the anaesthetised rat. Extracellular recordings were made from different classes of respiratory neurone and the effects of P2 receptor blockade on CO2-evoked changes in activity investigated. 2. Increasing inspired CO2 excited 85 % of inspiratory neurones in the pre-Botzinger complex. In all cases, CO2-evoked excitation was blocked by ionophoretic application of the P2 receptor antagonists suramin (0.02 M) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 100 microM), but not the adenosine receptor antagonist 8-phenyltheophylline (8-PT; 100 microM). Suramin and PPADS often reduced ongoing activity, and blocked the excitatory effects of ATP. Inspiratory neurones were also excited by the P2X receptor agonist alphabeta-methyleneATP, suggesting a specific role for P2X receptors. 3. Sixty-six per cent of pre-inspiratory neurones were also excited by CO2. This effect was reduced or abolished by prior application of P2 receptor antagonists. Although post-inspiratory and expiratory neurones were excited by increasing levels of CO2, and also by ionophoretically applied ATP, the CO2-evoked effects were unaffected by P2 receptor blockade. 4. We suggest that ATP, possibly acting via P2X purinoceptors localised within the ventral respiratory group, is involved in central chemoreception. Specifically, these distinctive CO2-P2X-mediated actions were observed only in inspiratory neurones (incrementing inspiratory neurones and pre-inspiratory neurones), which appear to have purinoceptors with pH sensitivity that can account for the actions of CO2 in modifying ventilatory activity.

Adenosine Triphosphate↗

PubMed2000

Localization and action of adenosine A2a receptors in regions of the brainstem important in cardiovascular control.

In vitro autoradiography and central microinjections of a P1 adenosine A2a receptor antagonist have been employed to investigate a possible role for centrally located adenosine A2a receptors in modulation of the baroreceptor reflex. In vitro autoradiography using [125I]4-(2-[7-amino-2-[2-furyl][3,2,4]triazolol[2,3-a][1,3,5]tr iazin-5-yl-amino]ethyl)phenol ([125I]ZM241385), the high-affinity adenosine A2a receptor antagonist, revealed a heterogeneous distribution of adenosine A2a binding sites within the lower brainstem of the rat. Image analysis showed high levels of binding in rostral regions of both the nucleus tractus solitarius and the ventrolateral medulla. Intermediate levels of binding were observed in the commissural nucleus tractus solitarius and the dorsal vagal motor nucleus, with low levels of binding in caudal regions of the nucleus tractus solitarius and the ventrolateral medulla, and the hypoglossal nucleus. Unilateral microinjections of unlabelled ZM241385 into the nucleus tractus solitarius had no effect on baseline levels of arterial pressure, heart rate and phrenic nerve activity recorded in anaesthetized, artificially ventilated rats. However, microinjections of ZM241385 reduced the bradycardia evoked by stimulation of the ipsilateral aortic nerve. In contrast, ZM241385 had no effect on the depressor response or the reduction in phrenic nerve activity evoked by aortic nerve stimulation. Our results indicate that adenosine A2a binding sites are located in a number of brainstem regions involved in autonomic function, consistent with the idea that adenosine acts as a neuromodulator of a variety of cardiorespiratory reflexes. Specifically, the data support the hypothesis that adenosine A2a receptors located within the nucleus tractus solitarius are activated during baroreceptor stimulation and have an important modulatory role in the pattern of cardiovascular changes associated with this reflex.

Adenosine↗

PubMed2000

In vivo modulation of vagal-identified dorsal medullary neurones by activation of different 5-Hydroxytryptamine(2) receptors in rats.

1. In in vivo experiments, DOI (a 5-HT(2) receptor agonist), MK-212 (a 5-HT(2C) receptor agonist), and BW-723C86 (a 5-HT(2B) receptor agonist) were applied by ionophoresis to neurones in the rat nucleus tractus solitarius (NTS) receiving vagal afferent input. 2. The majority of the putative 'monosynaptically' vagal activated cells were inhibited by both MK-212 (4/6) and DOI (2/4), but unaffected by BW-723C86 (12/14). In contrast, 'polysynaptically' activated NTS cells were excited by both BW-723C86 (13/19) and DOI (9/10). Inactive 'intermediate' cells were inhibited by BW-723C86 (9/12), MK-212 (5/6) and DOI (3/4), whilst active cells of this group were excited by BW-723C86 (7/13) and DOI (5/5). 3. The selective 5-HT(2B) receptor antagonist LY-202715 significantly reduced the excitatory actions of BW-723C86 on 'intermediate' and 'polysynaptic' cells (13/13), but not the inhibitory effects observed on inactive Group 2 cells (n=5) whereas the selective 5-HT(2C) receptor antagonist RS-102221 reversed the inhibitory effects of MK-212 and DOI on 'monosynaptic and 'intermediate' neurones. 4. Cardio-pulmonary afferent stimulation inhibited two of four putative 'monosynaptically' activated calls and all four inactive intermediate cells. These were also inhibited by DOI and MK-212. In contrast, cardio-pulmonary afferents excited all five active intermediate cells and all six putative 'polysynaptically' activated NTS cells, while all were also previously excited by BW-723C86 and/or DOI. 5. In conclusion, these data demonstrate that neurones in the NTS are affected differently by 5-HT(2) receptor ligands, in regard of their vagal postsynaptic location, the type of cardio-pulmonary afferent they receive and the different 5-HT(2) receptors activated.

Afferent Pathways↗

PubMed2000

Dorsal column nuclei neurons recorded in a brain stem-spinal cord preparation: characteristics and their responses to dorsal root stimulation.

Recordings were obtained from dorsal column nucleus (DCN) neurons in a neonatal rat brain stem-spinal cord preparation to study their basic electrophysiological properties and responses to stimulation of a dorsal root. Whole-cell patch-clamp recordings were made from 21 neurons that responded to dorsal root stimulation with a fast excitatory postsynaptic potential (EPSP). These neurons were located lateral to, but at the level of, the area postrema at depths of 100-268 microm below the dorsal surface of the brain. The neurons could be divided into groups according to the shape of their action potentials or voltage responses to hyperpolarizing current steps; however, the response profiles of the groups of neurons to dorsal root stimulation were not significantly different and all neurons were considered together. Dorsal root stimulation elicited excitatory postsynaptic potentials (EPSPs) in all neurons with a very low variability in onset latency and an ability to follow 100-Hz stimulation, indicating that they were mediated by activation of a monosynaptic pathway. The peak amplitude of the EPSP increased with membrane hyperpolarization, and applications of the non-NMDA receptor antagonists 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2, 3-dione (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the amplitude of the EPSP to 14.2% of the control response (n = 6). The descending phase of the EPSP decreased with membrane hyperpolarization and was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 (n = 2). The EPSPs were also reduced in amplitude by applications of the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, which had no effect on membrane potential or input resistance. These results show that fast EPSPs in DCN neurons elicited by dorsal root stimulation are mediated by an excitatory amino acid acting at both non-NMDA and, to a lesser extent, NMDA receptors. In addition, GABA acting at presynaptic GABA(B) receptors can inhibit these responses.

Adrenergic Fibers↗

PubMed2000

Inhibition of the carotid baroreflex by urinary bladder distension.

OBJECTIVE: The cardiovascular responses to urinary bladder distension include a rise in arterial blood pressure, an increase in vascular resistance and heart rate, which are abolished by sectioning the pelvic and hypogastric nerves. There are indications in the literature that inputs from the arterial baroreceptors and urinary bladder converge at the same autonomic efferent pathways. Recent data provided evidence of a partial inhibitory action of the carotid baroreflex on the reflex cardiovascular responses evoked by bladder distension. In this study, we investigate the influence of urinary bladder distention upon cardiovascular responses elicited by carotid baroreceptor stimulation. METHODS: Wistar Rats were anaesthetised with alpha-choloralose (80 mg/kg, i.p.) supplemented as necessary. The carotid sinus was stimulated randomly with different volumes of saline (0.5-2.0 ml). The ureters were ligated and the bladder was cannulated. Arterial blood pressure (BP), ECG, heart rate (HR), and bladder pressure (BLP) were monitored. Baroreceptor stimulation was undertaken on two levels of BLP (2 and 20 cmH2O). The cardiovascular response to baroreceptor activation was assessed as the maximal variation of BP, HR and R-R interval for different volumes of saline injected. RESULTS: Our results showed that in all tests baroreceptor stimulation evoked a decrease in BP and HR but the magnitude of these responses was significantly different for each volume of saline and for two levels of BLP. The effectiveness and reproducibility of responses to these stimuli were confirmed by injecting a fixed volume of saline into the carotid sinus of two animals at two levels of BLP. In five animals, the variation in BP increased as the magnitude of the stimulus to the baroreceptor increased. However, the response was reduced with increased BLP. The fitted regression line was calculated for all the tests (n = 82; s = 8.2; Yint = 0.62 for BLP = 2 cmH2O; s = 9.5; Yint = 2.9; r = 0.69 for BLP = 20 cmH2O). Similar effects were observed and similar patterns of regression lines were obtained with regard to HR changes (n = 48; s = 13.3; Yint = 2.7; r = 0.82 for BLP = 2 cmH2O; s = 15.8; Yint = 0.89; r = 0.76 for BLP = 20 cmH2O) and interval changes (n = 48; s = 7.1; Yint = 4.3; r = 0.78 for BLP = 2 cmH2O; s = 8.5; Yint = 0.2; r = 0.76 for BLP = 20 cm H2O). CONCLUSIONS: These data suggest that the cardiovascular responses evoked by carotid baroreceptor stimulation are significantly different when the urinary bladder is empty or full with the response diminishing as the urinary bladder is distended.

Animals↗

PubMed2000

Interaction of the Bezold-Jarisch reflex with the urinary bladder function.

OBJECTIVES: The Bezold-Jarisch reflex involves a marked increase in vagal (parasympathetic) efferent discharge to the heart. During micturition, there is a marked increase in parasympathetic activity to the bladder (pelvic nerve activity) and this coincides with periods of phasic bladder contractions. In recent studies, we have shown that regions of the CNS that modulate pelvic nerve activity also affect cardiovascular variables. Recent anatomical studies have shown neuronal connections between the areas of the CNS that modulate both reflexes. Accordingly, in the present study we investigate if during the Bezold-Jarisch reflex there are simultaneous changes in pelvic nerve activity. METHODS: Experiments were performed in Wistar female rats, anaesthetised with alpha-chloradose i.p. (80 micrograms/Kg) supplemented i.v. as required. Arterial blood pressure, ECG heart rate, urinary bladder pressure and pelvic nerve activity were recorded. RESULTS: Stimulation of cardiac chemosensitive nerve endings with veratridine evoked a decrease in arterial blood pressure from 88.6 +/- 1.72 to 66.1 +/- 2.50 mmHg (n = 30; p < 0.0001) and of heart rate from 420.3 +/- 5.56 to 248.8 +/- 17.58 bpm (n = 13; p < 0.0001) There were simultaneous changes of pelvic nerve activity that preceded an increase in urinary bladder pressure from 10.2 +/- 0.28 to 26.3 +/- 1.52 cmH2O (n = 30; p < 0.0001) and bladder contractions. We believe that the reflex changes resulted from activation of cardiac chemosensory nerve endings, as equivalent injections of warm saline into the left ventricle produced no changes in either cardiovascular variables or pelvic nerve activity. CONCLUSIONS: These observations suggest a significant role for cardiac afferents in modulating bladder contractions. The integration and functional significance of these inputs remain to be elucidated. The clinical importance of this physiological result may contribute to a better understanding of the mechanism of some semiologic findings, notably the association of urgency to micturate with myocardial ischemia and myocardial infarction.

Animals↗

PubMed1999

Central CO2 chemoreception: a mechanism involving P2 purinoceptors localized in the ventrolateral medulla of the anaesthetized rat.

1. The involvement of P2 purinoceptors in chemosensory function in the ventrolateral regions of the medulla oblongata was investigated in the anaesthetized rat. We have investigated the effect of antagonizing, or desensitizing, P2 receptors in the retrofacial area of the ventrolateral medulla on factors modifying respiratory activity. 2. Bilateral microinjection of suramin (50 nl, 0.02 M), a P2 purinoceptor antagonist, into the retrofacial area in the artificially ventilated rat reduced resting phrenic nerve discharge. It also markedly affected the response of the phrenic nerve to increases in arterial CO2. Under conditions of hyperoxic, hypocapnic apnoea, the mean threshold for inducing phrenic nerve activity was raised significantly (from an end-tidal CO2 of 2.5 % to 4.5 %, n = 9). 3. In addition, the slope of the respiratory response curve to increases in CO2 was reduced after suramin. A similar effect was observed after desensitization of certain P2X receptors with alphabeta-methyleneATP. As arterial levels of O2 were greater than 100 mmHg, and an equivalent pattern of response was observed in sino-aortically denervated and vagotomized animals, we believe any contribution of the peripheral chemoreceptors to be minimal. 4. Our data suggest that respiratory neurones within the retrofacial area (Botzinger complex) represent part of the central site of action of CO2 on respiration. Moreover, our observations lead us to suggest that CO2-evoked changes in respiration are mediated at least in part by P2X purinoceptors.

Adenosine Triphosphate↗

PubMed1999

A novel influence of adenosine on ongoing activity in rat rostral ventrolateral medulla.

We have investigated whether exogenously applied adenosine modulates neuronal activity in a region of the central nervous system crucial for cardiovascular regulation. Extracellular recordings were made from neurons in the rostral ventrolateral medulla of the anaesthetized rat. Ionophoretic application of adenosine altered ongoing activity in 91% of neurons, evoking either a long-lasting depression or a short-lasting increase in firing rate. Both responses were blocked by application of the broad spectrum adenosine receptor antagonist 8-sulphophenyltheophylline, indicating that the responses were mediated by specific cell surface receptors. The adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine blocked the increase, and partially blocked the decrease in firing rate in response to adenosine. The GABA(A) receptor antagonist bicuculline also blocked the increase in firing rate in response to adenosine, suggesting that adenosine may inhibit release of GABA from axon terminals in this region. The adenosine A2a receptor agonist CGS 21680 produced a long-lasting depression of ongoing activity. These results suggest that A1 receptors mediate an increase in firing rate, whilst A1 and A2a receptors mediate decreases in firing rate in some rostral ventrolateral medulla neurons. Thus, adenosine has been shown to modulate the ongoing activity of neurons in the rostral ventrolateral medulla by acting at both A1 and A2a receptors. Accordingly, we suggest, and provide some evidence to support the idea, that adenosine acts as an important neuromodulator in this region of the central nervous system, possibly by modulating the presynaptic release of neurotransmitters such as GABA.

Adenosine↗

PubMed1999

Late vagal inhibition in neurons of the ventrolateral medulla oblongata in the rat.

Stimulation of cervical vagal afferents elicits long-lasting inhibitory effects in a variety of neuronal populations, although little is known concerning the cellular mechanisms that are involved in these effects. In the present study, the electrophysiological characteristics of responses elicited by cumulative activation of vagal afferents were examined in neurons of the rostral ventrolateral medulla oblongata, which play an important role in the coordination of cardiovascular and other visceral activities. The study has focused on the late-onset, slow inhibitory component of vagal responses, which is likely to affect the temporal modulation of postsynaptic effects. Vagal stimulation elicited four distinct response patterns in intracellularly penetrated neurons (n = 78): excitation, inhibition, excitation-inhibition and inhibition-inhibition. The late inhibitory component was encountered in 43 (55%) of the cells, including five putative medullospinal neurons. It was due to a postsynaptic hyperpolarization which reversed at potentials more negative than -83 mV. The voltage dependency, as well as the average onset latency (93+/-3.0 ms), duration (270+/-16.5 ms) and amplitude (1.3+/-0.2 mV as measured at resting membrane potentials), of late inhibition were clearly different from those of the short-latency inhibitory response. The differences in the voltage dependency and time-course of the short-latency responses and the late inhibition indicate that they are mediated by different central relays. In the majority of neurons, late inhibition could be elicited by stimulating only myelinated vagal afferents. The magnitude of the response was, however, significantly enhanced in 63% of the examined cells when the intensity of stimulation was raised to recruit further myelinated and non-myelinated fibres. This indicates that late vagal inhibition is often elicited by a cumulative activation of convergent afferent inputs. The intracellularly labelled vagally responsive neurons were present at all rostrocaudal levels of the rostral ventrolateral medulla, with an accumulation in the region of the lateral paragigantocellular nucleus. Neurons that exhibited late vagal inhibition were dominant in the juxtafacial region of this nucleus. Due to its slow time-course, late vagal inhibition may contribute to a tonic modulation of the activity of neurons in the rostral ventrolateral medulla oblongata. It is proposed that late vagal inhibition plays an important role in the temporal integration of sensory inputs in neurons of the rostral ventrolateral medulla oblongata. The time-course and strength of this modulatory effect are related to the level of activity in those visceral sensory inputs that converge onto the inhibitory interneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons.

Animals↗

PubMed1999

Vagal modulation of responses elicited by stimulation of the aortic depressor nerve in neurons of the rostral ventrolateral medulla oblongata in the rat.

Stimulation of cervical vagal afferents inhibits central sympathetic outflows in part by inhibiting the ongoing activity of putative baroreceptive neurons in the rostral ventrolateral medulla oblongata. The aim of the present study was to examine the electrophysiological characteristics of vagal responses and their interactions with responses elicited by stimulation of the aortic nerve in neurons there. The study focused on the role of the long-lasting, late-onset vagal inhibition, which is likely to play an important role in the tonic inhibitory effects of vagal afferent stimulation. In vivo intracellular recordings were obtained from 33 neurons that received convergent inputs from aortic and vagal afferents. Sixty-four percent of these neurons exhibited a late inhibition following electrical stimulation of myelinated vagal afferents (mean onset latency of 100+/-5 ms). The average duration of late inhibition (294+/-19 ms) exceeded the duration of the cardiac cycle. As a consequence of this, sustained vagal stimulation diminished the effect of rhythmic baroreceptor inputs in neurons that exhibited late vagal inhibition. Simultaneous activation of aortic and vagal afferents significantly increased the magnitude of late inhibition, even in those neurons where stimulation of the aortic nerve alone did not elicit a response (n = 15). This suggested that the convergence between vagal and aortic afferent inputs occurred in inhibitory inteneurons antecedent to the recorded rostral ventrolateral medulla oblongata neurons. Focal stimulation of the caudal part of the nucleus of the solitary tract also elicited a late-onset inhibition in 73% of the neurons that responded to stimulation of the aortic nerve. This inhibition appeared to be similar to the late vagal inhibition, except for its shorter average onset latency (64+/-7 ms). Based on this observation, it is proposed that inhibitory inteneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons may lie within the caudal part of the nucleus of the solitary tract. The present study established that activation of myelinated vagal afferents exerts a complex modulation over the ongoing and evoked activity of neurons that respond to stimulation of the aortic nerve. The complex interaction that occurs between aortic and vagal inputs in neurons of the rostral ventrolateral medulla may be implicated in long-term modulation of sympathetic outflows in response to changes in the activation of visceral receptors supplied by vagus afferents. The modulation elicited by late vagal inhibition may help to adjust cardiovascular outflows according to requirements set by the thoraco-abdominal visceral environment.

Animals↗