Francis A. Hopp

Effects of Isoflurane on the Baroreceptor Reflex

The baroreceptor reflex has been found to be attenuated during anesthesisa, but the effects of the relatively new anesthetic, isoflurance, on baroreflex function have not been examined throughly. This study was performed to determine the effects of isoflurane on each component of the baroreceptor reflex arc, including the receptors, afferent and efferent nerve pathways, central integratory centers, peripheral ganglia, and the heart. Baroreflex effects on heart rate initiated by systemic pressure changes were examined in conscious and anesthetized dogs (1.3% and 2.6% isoflurane). The effects on individual components of the reflex are were determined by examining carotid sinus baroreceptor afferent activity, sympathetic efferent nerve activity, and heart rate response to direct sympathetic and parasympathetic efferent nerve stimulation in anesthetized dogs. Preganglionic and postganglionic nerve activities were recorded simultaneously during baroreflex activation to determine ganglionic effects of isoflurane. Baroreflex-induced changes in heart rate were not depressed significantly until 2.6% isoflurane if blood pressure changes due to anesthetic administration were prevented. Significant decreases in baseline sympathetic efferent nerve activity were found at 1.3% and 2.6% isoflurane, with depression of postganglionic activity significantly greater than preganglionic activity at 2.6% isoflurane, indicating a ganglionic effect of isoflurance. Cardiac chronotropic responses to direct stimulation of sympathetic and vagal fibers were attenuated significantly by isoflurane, with sympathetic stimulation showing the greater sensitivity to the anesthetic. Carotid baroreceptor afferent activity was increased by isoflurane, and this sensitization of the baroreceptors appeared to contribute to the decreased levels of sympathetic tone. Therefore, although isoflurane was found to alter the baroreceptor reflex through its effects at multiple sites of the baroreflex arc, significant depression of the cardiac chronotropic component of the reflex was seen only at 2.6% isoflurane.

Selective contribution of two types of carotid sinus baroreceptors to the control of blood pressure.

This study was performed to determine if selective elimination of afferent input from two different types of previously described baroreceptors altered the ability of the dog to regulate blood pressure (BP), examining specifically if there was differential loss of baroreceptor control of tonic levels of baseline pressure versus dynamic changes in pressure. In the first series of experiments in this study, anodal block of the carotid sinus nerve was used to selectively block afferent input in a sequence from large-diameter A-fiber carotid baroreceptors (mostly type I) to smaller A-fiber and nonmyelinated C-fiber baroreceptors (mostly type II). In the second series of experiments, anesthetic block of the carotid sinus nerve with bupivacaine was used to selectively eliminate afferent input in reverse order from anodal block, first blocking input from baroreceptors with small afferent fibers and then additionally eliminating input from the larger-diameter A-fiber baroreceptors. The effects of selective elimination of each baroreceptor type were determined by monitoring baseline BP during constant carotid sinus pressure (CSP) perfusion of a vascularly isolated carotid sinus (tonic control) and obtaining baroreflex sensitivity (slope) during ramp pressure stimulations of the carotid sinus (dynamic control) under various blocking conditions. Low levels of anodal block significantly attenuated baroreflex sensitivity (-0.84 +/- 0.11 versus -0.63 +/- 0.10 mm Hg BP/mm Hg CSP) at levels of block that had no effect on tonic baseline BP (158.41 +/- 9.5 versus 160.7 +/- 9.5 mm Hg BP). In contrast, low levels of bupivacaine block produced significant increases in tonic BP (158.8 +/- 6.4 versus 169.0 +/- 6.5 mm Hg BP), whereas there was no effect on dynamic baroreflex sensitivity (-0.85 +/- 0.08 versus -0.73 +/- 0.08 mm Hg BP/mm Hg CSP). Thus, blocking large A-fiber baroreceptors resulted in significant decreases in baroreflex sensitivity without changes in baseline levels of BP, indicating primarily an attenuation in dynamic baroreflex regulation. Blocking of smaller A-fiber and unmyelinated C-fiber baroreceptors resulted in smaller decreases in baroreflex sensitivity and significant elevations in baseline BP, indicating a loss of tonic control of pressure. These results suggest that the two types of baroreceptors contribute differently to the regulation of blood pressure.

Modulation of the synaptic drive to respiratory premotor and motor neurons.

The characteristics of GABAergic inhibitory modulation of respiratory bulbospinal neuronal activity and short-term potentiation (STP) of phrenic motoneuronal activity were studied. Extracellular unit recording and picoejection techniques in anesthetized dogs showed that both the spontaneous rhythmic and reflexly induced discharge patterns of inspiratory (I) and expiratory (E) premotor neurons were proportionately amplified by the localized application of picomole amounts of bicuculline (Bic), a competitive GABAA antagonist. Intracellular recording and paired-pulse stimulation techniques in anesthetized rats demonstrated an STP of phrenic motor output that appears to be mediated by NMDA receptors and is associated with facilitation of EPSPs and prolonged depolarization of individual phrenic motoneurons. We speculate that both GABAergic gain modulation of premotor neuronal activity and NMDA-mediated STP of phrenic activity may be neural substrates which are involved with the optimization of respiratory and non-respiratory behaviors, via adaptive and/or differential control of breathing.

Halothane and the carotid sinus reflex: evidence for multiple sites of action.

&NA; Baroreceptor reflexes have been found to be attenuated during halothane anesthesia in humans and experimental animals. The baroreceptor reflex are is comprised of a number of components, including receptors, afferent and efferent nerve pathways, central integratory centers, peripheral ganglia, and effector organs, at which halothane might exert an inhibitory effect. This study was performed to determine the effect of halothane at each component in order to identify the site or sites of baroreflex attenuation due to halothane. The baroreflex effects on heart rate initiated by carotid sinus pressure changes were examined in conscious and anesthetized (0.0%, 0.75%, and 1.5% halothane in 50% N2O and O2, plus 25 mg/kg thiopental) dogs. In addition, carotid sinus afferent activity, cardiac sympathetic efferent activity, and heart rate responses to direct sympathetic and parasympathetic efferent stimulation were examined in anesthetized dogs. Preganglionic and postganglionic sympathetic nerve activities were recorded simultaneously during baroreceptor activation to determine ganglionic effects of halothane. All levels of anesthesia significantly (P < 0.05) attenuated reflex changes in heart rate produced by the pressure changes compared to conscious dogs. Significant decreases in cardiac sympathetic efferent activity were produced at 1.5% halothane (P < 0.05). The depression in postganglionic activity was significantly greater than that for preganglionic activity, indicating a ganglionic‐blocking effect by halothane. Cardiac chronotropic changes produced by direct efferent stimulation of sympathetic and vagal fibers were attenuated significantly by halothane (P < 0.05). On the other hand, baroreceptor afferent activity was increased at 1.5% halothane. This sensitization of baroreceptors appeared to contribute to decreased levels of sympathetic tone, leading to blunted reflex changes in nerve activity. Therefore, halothane was found to have multiple sites of action, leading to depression of the baroreflex.

Respiratory responses to selective blockade of carotid sinus baroreceptors in the dog

Activation of carotid sinus (CS) baroreceptors has been shown to increase inspiratory time (Ti) and expiratory time (Te) and to have a varied effect on tidal volume. The contribution of two functionally different types of baroreceptors to changes in respiratory function were examined in the current study. The techniques of DC anodal block and bupivacaine anesthetic block were used to selectively block fibers, from largest (type I) to smallest (type II) and smallest to largest, respectively, in the CS nerve (CSN) from an isolated CS in an anesthetized, paralyzed, vagotomized, artificially ventilated dog. Anodal blocking currents from 25 to 60 μA, which blocked primarily large A fibers, produced significant decreases in Ti and Te and increased the slope of the average phrenic neurogram [PNG( t)], with no change in peak PNG( t). Further increases in blocking current to levels that also blocked small C fibers did not result in additional changes. Bupivacaine blockade using concentrations that blocked primarily C fibers did not block changes in Ti and Te to step CS pressure changes. Increasing bupivacaine concentration to 20 mg/100 ml blocked all CSN conduction, and respiratory responses were eliminated. Therefore respiratory responses arising from CS baroreceptors appear to originate from the larger type I baroreceptors.Activation of carotid sinus (CS) baroreceptors has been shown to increase inspiratory time (TI) and expiratory time (TE) and to have a varied effect on tidal volume. The contribution of two functionally different types of baroreceptors to changes in respiratory function were examined in the current study. The techniques of DC anodal block and bupivacaine anesthetic block were used to selectively block fibers, from largest (type I) to smallest (type II) and smallest to largest, respectively, in the CS nerve (CSN) from an isolated CS in an anesthetized, paralyzed, vagotomized, artificially ventilated dog. Anodal blocking currents from 25 to 60 microA, which blocked primarily large A fibers, produced significant decreases in TI and TE and increased the slope of the average phrenic neurogram [PNG(t)], with no change in peak PNG(t). Further increases in blocking current to levels that also blocked small C fibers did not result in additional changes. Bupivacaine blockade using concentrations that blocked primarily C fibers did not block changes in TI and TE to step CS pressure changes. Increasing bupivacaine concentration to 20 mg/100 ml blocked all CSN conduction, and respiratory responses were eliminated. Therefore respiratory responses arising from CS baroreceptors appear to originate from the larger type I baroreceptors.

Differential control of sympathetic activity to kidney and skeletal muscle by ventral medullary neurons

A rise in arterial blood pressure can be evoked by microinjections of D,L-homocysteic acid into localized regions of the ventrolateral medulla of the cat. Three patterns of sympathetic discharge can be identified during the pressor response. A differential pattern consisting of an increase in renal nerve activity and no change in sympathetic activity to skeletal muscle vasculature can be elicited from sites ventromedial to the caudal pole of the facial nucleus. From more lateral and caudal sites, a generalized sympatho-excitation is evoked in the outflow to both the kidney and hindlimb muscle vasculature. A third response consisting of a differential increase in muscle sympathetic activity simultaneous with a small decrease in renal nerve activity could be evoked from caudal sites, lateral to the inferior olives and superficial to the ventral surface. The results show that ventral medullary neurons can selectively activate sympathetic outflow to control specific vascular beds. These data may support the hypothesis that the ventrolateral medulla contains discrete groups of topographically arranged neurons that can differentially control sympathetic tone to various end-organs.

Effects of Sevoflurane on excitatory neurotransmission to medullary expiratory neurons and on phrenic nerve activity in a decerebrate dog model

Background Sevoflurane is a new volatile anesthetic with a pronounced respiratory depressant effect. Synaptic neurotransmission in canine expiratory bulbospinal neurons is mainly mediated by excitatory N-methyl-d-aspartatic acid (NMDA) receptor input and modulated by inhibitory &ggr;-aminobutyric acid type A (GABAA) receptors. The authors investigated the effect of sevoflurane on these mechanisms in decerebrate dogs. Methods Studies were performed in decerebrate, vagotomized, paralyzed and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration (MAC; 2.4%) sevoflurane on extracellularly recorded neuronal activity was measured during localized picoejection of the glutamate agonist NMDA and the GABAA receptor blocker bicuculline in a two-part protocol. First, complete blockade of the GABAAergic mechanism by bicuculline allowed differentiation between the effects of sevoflurane on overall GABAAergic inhibition and on overall glutamatergic excitation. In a second step, the neuronal response to exogenous NMDA was used to estimate sevoflurane’s effect on postsynaptic glutamatergic neurotransmission. Results One minimum alveolar concentration sevoflurane depressed the spontaneous activity of 16 expiratory neurons by 36.7 ± 22.4% (mean ± SD). Overall glutamatergic excitation was depressed 19.5 ± 16.2%, and GABAAergic inhibition was enhanced 18.7 ± 20.6%. However, the postsynaptic response to exogenous NMDA was not significantly altered. In addition, 1 MAC sevoflurane depressed peak phrenic nerve activity by 61.8 ± 17.7%. Conclusions In the authors’in vivo expiratory neuronal model, the depressive effect of sevoflurane on synaptic neurotransmission was caused by a reduction of presynaptic glutamatergic excitation and an enhancement of GABAAergic inhibition. The effects on expiratory neuronal activity were similar to halothane, but sevoflurane caused a stronger depression of phrenic nerve activity than halothane.

Sympathetic Efferent Nerve Activity in Conscious and Isoflurane-anesthetized Dogs

The hypotension accompanying isoflurane suggests that the anesthetic produces an attenuation of sympathetic tone. Previous studies examining the effects of isoflurane on sympathetic efferent nerve activity have required concomitant use of a basal anesthetic or decerebration, both of which independently alter sympathetic activity. This study was performed to examine the effects of isoflurane on sympathetic efferent nerve activity in the absence of basal anesthetic or decerebration. Five mongrel dogs were anesthetized with 4% isoflurane by mask. Platinum electrodes chronically were implanted around a renal nerve adjacent to the renal artery in order to measure renal sympathetic efferent nerve activity in the conscious and anesthetized animal. After 5–24 h for recovery, renal nerve activity and arterial pressure (via an implanted femoral artery cannula) were measured in the conscious, resting animal (control); during induction (4% isoflurane) and intubation; in the anesthetized animal (1.5% and 2.5% isoflurane); and during recovery and extubation.Isoflurane produced a significant dose-dependent depression of arterial blood pressure but did not significantly change heart rate from control. Renal sympathetic efferent nerve activity at 1.5% isoflurane was not significantly different from that in conscious animals, but nerve activity at 2.5% isoflurane was depressed significantly from both control and 1.5% isoflurane. Both intubation and extubation were accompanied by an increase in sympathetic nerve activity. Isoflurane appeared to directly depress sympathetic activity at both levels of anesthesia, but the direct depression of activity at 1.5% isoflurane seemed to be countered by reflex increases in sympathetic tone due to the hypotension accompanying the anesthesia. At 2.5% isoflurane, the central depression of reflex activity by isoflurane combined with direct depression of sympathetic efferent activity resulted in the attenuation of renal nerve activity.

NMDA receptor-mediated transmission of carotid body chemoreceptor input to expiratory bulbospinal neurones in dogs.

1. This study tested the hypothesis that excitatory amino acid receptors mediate the excitatory response of expiratory bulbospinal neurones to carotid body chemoreceptor inputs. 2. Studies were carried out in thiopental sodium anaesthetized, paralysed, ventilated, vagotomized dogs. 3. Brisk, short‐duration chemoreceptor activation was produced by bilateral bolus injections of CO2‐saturated saline (PCO2 > 700 mmHg) into the autoperfused carotid arteries. A pressurized‐reservoir‐solenoid valve system was used to deliver the CO2 bolus injections just prior to the onset of the neural expiratory phase, as determined from the phrenic neurogram, about once per minute. 4. Multibarrelled micropipettes were used to record neuronal unit activity and deliver neurotransmitter agents. Net responses of expiratory bulbospinal neurones to peripheral chemoreceptor activation were determined by subtracting the mean discharge frequencies (Fn) during three control expiratory cycles from the Fn during administration of a CO2 test bolus. The role of excitatory amino acid receptors in mediating this response was determined by comparing the baseline and bolus expiratory neuronal Fn before, during and after the pressure microejection of the NMDA receptor antagonist 2‐amino‐5‐phosphonovalerate (AP5) or the non‐NMDA receptor antagonist 2,3‐dihydroxy‐6‐nitro‐7‐sulphamoyl‐ benzo(f)quinoxaline (NBQX). Ejection rates of AP5 and NBQX were measured by monitoring the movement of the pipette meniscus. 5. AP5 reduced Fn during both the control and bolus cycles, as well as reducing the change in Fn between control and bolus cycles. NBQX had no effect on either baseline or bolus responses. 6. AP5 did not prevent excitation of expiratory bulbospinal neurones by AMPA. Coadministration of AMPA with AP5 prevented the AP5‐mediated decrease in Fn but not the dose‐dependent reduction in the CO2 bolus response. 7. Taken together, these data strongly suggest that the carotid chemoreceptor‐mediated excitation of expiratory bulbospinal neurones is dependent on NMDA but not non‐NMDA glutamate receptors.

Role of glutamate receptors in transmission of vagal cardiac input to neurones in the nucleus tractus solitarii in dogs

1 Vagal afferent input from cardiac mechanoreceptors excites neurones in the nucleus tractus solitarii (NTS), but discharge patterns evoked by physiological activation of pressure‐sensitive cardiac mechanoreceptors have not been studied in vivo. The role of glutamate receptor subtypes in transmission of afferent activity to the NTS neurones has not been determined. The present study therefore has two aims: first, to characterise the discharge patterns of neurones in the NTS that receive pressure‐sensitive vagal cardiac receptor input and second, to determine the roles of ionotropic glutamate receptor subtypes in the transmission of this putative cardiac mechanoreceptor‐related activity to NTS neurones. 2 Pulse‐synchronous activity of neurones in the NTS evoked by vagal afferent input was recorded extracellularly in an anaesthetised dog model using multibarrel glass electrodes, which allowed picoejection of the glutamate receptor antagonists NBQX or AP5 to block either non‐NMDA or NMDA receptors, respectively, during the neuronal recording. Pressure sensitivity of the recorded neurones was examined by monitoring their response to a small increase in arterial blood pressure. Selective pressure activation of carotid sinus baroreceptors in an isolated sinus or selective denervation of aortic baroreceptors were used to test for convergent excitation of the neurones by arterial baroreceptors. 3 Pulse‐synchronous cardiac‐related neuronal activity recorded from neurones in both the right and left NTS was eliminated following section of the left (n= 17) or right (n= 1) vagus nerves. No spontaneous, non‐pulsatile activity was observed in these neurones before or after vagotomy. Activity transmitted via left vagal afferents was found to be sensitive to changes in arterial blood pressure. In these neurones, activity was blocked in 13 of 17 neurones by picoejection of NBQX, with the remainder requiring both NBQX and AP5. None of the cardiac‐related neurones responded to activation of carotid baroreceptors or denervation of aortic baroreceptors, indicating no convergence of activity from carotid baroreceptors or aortic baroreceptors with pressure thresholds of approximately 130 mmHg or less. 4 The results suggest that vagal pressure‐sensitive afferent input from cardiac mechanoreceptors is transmitted primarily by left vagal afferent fibres via non‐NMDA receptors to neurones in both the ipsilateral and contralateral NTS. NMDA receptors were also found to have a role in the activation of a small subpopulation of neurones.