Role of trigeminal, olfactory, carotid sinus and aortic nerves in the respiratory and circulatory response to nasal inhalation of cigarette smoke and other irritants in the rabbit

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

Direct reticular projections of trigeminal sensory fibers immunoreactive to CGRP: potential monosynaptic somatoautonomic projections

Few trigeminal sensory fibers project centrally beyond the trigeminal sensory complex, with only projections of fibers carried in its sensory anterior ethmoidal (AEN) and intraoral nerves described. Fibers of the AEN project into the brainstem reticular formation where immunoreactivity against substance P and CGRP are found. We investigated whether the source of these peptides could be from trigeminal ganglion neurons by performing unilateral rhizotomies of the trigeminal root and looking for absence of label. After an 8–14 days survival, substance P immunoreactivity in the trigeminal sensory complex was diminished, but we could not conclude that the sole source of this peptide in the lateral parabrachial area and lateral reticular formation arises from primary afferent fibers. Immunoreactivity to CGRP after rhizotomy however was greatly diminished in the trigeminal sensory complex, confirming the observations of others. Moreover, CGRP immunoreactivity was nearly eliminated in fibers in the lateral parabrachial area, the caudal ventrolateral medulla, both the peri-ambiguus and ventral parts of the rostral ventrolateral medulla, in the external formation of the nucleus ambiguus, and diminished in the caudal pressor area. The nearly complete elimination of CGRP in the lateral reticular formation after rhizotomy suggests this peptide is carried in primary afferent fibers. Moreover, the arborization of CGRP immunoreactive fibers in these areas mimics that of direct projections from the AEN. Since electrical stimulation of the AEN induces cardiorespiratory adjustments including an apnea, peripheral vasoconstriction, and bradycardia similar to those seen in the mammalian diving response, we suggest these perturbations of autonomic behavior are enhanced by direct somatic primary afferent projections to these reticular neurons. We believe this to be first description of potential direct somatoautonomic projections to brainstem neurons regulating autonomic activity.

Parasympathetic preganglionic cardiac motoneurons labeled after voluntary diving

A dramatic bradycardia is induced by underwater submersion in vertebrates. The location of parasympathetic preganglionic cardiac motor neurons driving this aspect of the diving response was investigated using cFos immunohistochemistry combined with retrograde transport of cholera toxin subunit B (CTB) to double-label neurons. After pericardial injections of CTB, trained rats voluntarily dove underwater, and their heart rates (HR) dropped immediately to 95 ± 2 bpm, an 80% reduction. After immunohistochemical processing, the vast majority of CTB labeled neurons were located in the reticular formation from the rostral cervical spinal cord to the facial motor nucleus, confirming previous studies. Labeled neurons caudal to the rostral ventrolateral medulla were usually spindle-shaped aligned along an oblique line running from the dorsal vagal nucleus to the ventrolateral reticular formation, while those more rostrally were multipolar with extended dendrites. Nine percent of retrogradely-labeled neurons were positive for both cFos and CTB after diving and 74% of these were found rostral to the obex. CTB also was transported transganglionically in primary afferent fibers, resulting in large granular deposits in dorsolateral, ventrolateral, and commissural subnuclei of the nucleus tractus solitarii (NTS) and finer deposits in lamina I and IV-V of the trigeminocervical complex. The overlap of parasympathetic preganglionic cardiac motor neurons activated by diving with those activated by baro- and chemoreceptors in the rostral ventrolateral medulla is discussed. Thus, the profound bradycardia seen with underwater submersion reinforces the notion that the mammalian diving response is the most powerful autonomic reflex known.

Persistence of the nasotrigeminal reflex after pontomedullary transection

Most behaviors have numerous components based on reflexes, but the neural circuits driving most reflexes rarely are documented. The nasotrigeminal reflex induced by stimulating the nasal mucosa causes an apnea, a bradycardia, and variable changes in mean arterial blood pressure (MABP). In this study we tested the nasotrigeminal reflex after transecting the brainstem at the pontomedullary junction. The nasal mucosae of anesthetized rats were stimulated with ammonia vapors and their brainstems then were transected. Complete transections alone induced an increase in resting heart rate (HR; p<0.001) and MABP (p<0.001), but no significant change in ventilation. However, the responses to nasal stimulation after transection were similar to those seen prior to transection. HR still dropped significantly (p<0.001), duration of apnea remained the same, as did changes in MABP. Results from rats whose transection were incomplete are discussed. These data implicate that the neuronal circuitry driving the nasotrigeminal reflex, and indirectly the diving response, is intrinsic to the medulla and spinal cord.

The rat: a laboratory model for studies of the diving response

Underwater submersion in mammals induces apnea, parasympathetically mediated bradycardia, and sympathetically mediated peripheral vasoconstriction. These effects are collectively termed the diving response, potentially the most powerful autonomic reflex known. Although these physiological responses are directed by neurons in the brain, study of neural control of the diving response has been hampered since 1) it is difficult to study the brains of animals while they are underwater, 2) feral marine mammals are usually large and have brains of variable size, and 3) there are but few references on the brains of naturally diving species. Similar responses are elicited in anesthetized rodents after stimulation of their nasal mucosa, but this nasopharyngeal reflex has not been compared directly with natural diving behavior in the rat. In the present study, we compared hemodynamic responses elicited in awake rats during volitional underwater submersion with those of rats swimming on the water's surface, rats involuntarily submerged, and rats either anesthetized or decerebrate and stimulated nasally with ammonia vapors. We show that the hemodynamic changes to voluntary diving in the rat are similar to those of naturally diving marine mammals. We also show that the responses of voluntary diving rats are 1) significantly different from those seen during swimming, 2) generally similar to those elicited in trained rats involuntarily "dunked" underwater, and 3) generally different from those seen from dunking naive rats underwater. Nasal stimulation of anesthetized rats differed most from the hemodynamic variables of rats trained to dive voluntarily. We propose that the rat trained to dive underwater is an excellent laboratory model to study neural control of the mammalian diving response, and also suggest that some investigations may be done with nasal stimulation of decerebrate preparations to decipher such control.

Brainstem projections from recipient zones of the anterior ethmoidal nerve in the medullary dorsal horn

Stimulation of the anterior ethmoidal nerve or the nasal mucosa induces cardiorespiratory responses similar to those seen in diving mammals. We have utilized the transganglionic transport of a cocktail of horseradish peroxidase conjugates and anterograde and retrograde tract tracing techniques to elucidate pathways which may be important for these responses in the rat. Label was seen throughout the trigeminal sensory complex after the horseradish peroxidase conjugates were applied to the anterior ethmoidal nerve peripherally. Reaction product was most dense in the medullary dorsal horn, especially in laminae I and II. Injections were made of biotinylated dextran amine into the recipient zones of the medullary dorsal horn from the anterior ethmoidal nerve, and the anterogradely transported label documented. Label was found in many brainstem areas, but fibers with varicosities were noted in specific subdivisions of the nucleus tractus solitarii and parabrachial nucleus, as well as parts of the caudal and rostral ventrolateral medulla and A5 (noradrenergic cell group in ventrolateral pons) area. The retrograde transport of FluoroGold into the medullary dorsal horn after injections into these areas showed most neurons in laminae I, II, and V. Label was especially dense in areas which received primary afferent fibers from the anterior ethmoidal nerve. These data identify potential neural circuits for the diving response of the rat.

Target site of inhibition of baroreflex vagal bradycardia by nasal stimulation

We have previously reported that stimulation of nasal mucosa inhibits baroreflex vagal bradycardia (BVB) and this inhibition was mediated exclusively by the trigeminal nerve, and occurred principally at pontomedullary level. In this study, to identify the target site of the inhibition, several types of experiments were conducted in chloralose-urethane-anesthetized, beta-adrenergic receptor-blocked rats. Afferent discharges in the ethmoidal nerve (EN5) were increased in response to nasal stimulation by smoke, and electrical stimulation of the EN5 suppressed BVB induced by electrical stimulation of the aortic depressor nerve (ADN). Electrical stimulation of the EN5 inhibited vagal bradycardia evoked by either electrical or chemical stimulation of the nucleus tractus solitarius (NTS), while it rather facilitated bradycardia by stimulation of the nucleus ambiguus (NA) region. Microstimulation of the NTS induced antidromic compound spike potential along the ADN but this was not affected by stimulation of the EN5. ADN-evoked field potentials and unitary responses of neurons in the NTS were suppressed by stimulation of the EN5. These results suggested that barosensitive neurons in the NTS are the major target sites of inhibition of BVB by nasal stimulation in rats.

Method for in vivo calibration of renal sympathetic nerve activity in rabbits

A major difficulty of recording from peripheral sympathetic nerves is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. In this study we assessed methods of calibrating renal sympathetic nerve activity (RSNA) in conscious rabbits. Calibration values were obtained from maximum RSNA responses to nasopharyngeal stimulation, airjet stress or unloading baroreceptors. Curves relating RSNA to blood pressure were produced by raising and lowering blood pressure with vasoactive drugs. To assess whether normalization would eliminate differences between RSNA curves which were most likely due to recording conditions, rabbits were first divided into two groups with high or low basal microvolt levels of RSNA, then again into two groups with high or low heart rate. In both cases, curves were similar if values were normalized by nasopharyngeal stimulation or by the upper plateau value. In hypertensive rabbits, where the baroreflex is suppressed, only the nasopharyngeal method showed this attenuated pattern. This method also eliminated the 50% decay in basal RSNA measured over 5 weeks. We conclude that expressing RSNA in terms of the maximum response to nasopharyngeal stimulation provides a calibration method suitable for comparing nerve activity over the long term as well as showing valid differences in baroreflex curves between different experimental groups.

Acute effects of L- and T-type calcium channel antagonists on cardiovascular reflexes in conscious rabbits

1. The effects of the relatively selective T-type voltage- operated calcium channel (VOCC) antagonist mibefradil were compared with verapamil, an L-type VOCC antagonist, on a range of autonomic reflexes in conscious rabbits. 2. Mean arterial pressure (MAP), heart rate (HR), the baroreceptor-HR reflex, postural adaptation reflex (90 degrees head-up tilt), Bezold-Jarisch-like reflex and the vasoconstrictor component of the nasopharyngeal reflex were assessed before and during i.v. infusion of vehicle (saline), mibefradil or verapamil. Doses of mibefradil that gave low (M1; 0.45 +/- 0.02 microg/mL) and high (M2; 0.93 +/- 0.05 microg/mL) plasma concentrations, or verapamil (0.059 +/- 0.004 microg/mL; n = 6 each) were chosen to mimic clinically observed therapeutic levels. 3. At steady state infusion over 30-90 min, MAP was significantly lower in M2 (- 7 mmHg) and verapamil (- 6 mm Hg) treatments, but only verapamil caused a significant tachycardia (+ 31 b.p.m.) compared with vehicle. Mibefradil (M2) and verapamil decreased the HR range of the baroreflex by 27 and 29%, respectively, but neither treatment affected the vagal or sympathetic constrictor components of the Bezold-Jarisch-like and nasopharyngeal reflexes, respectively. 4. In response to 90 degrees tilt, vehicle- and verapamil-treated rabbits responded with small rises in MAP of 4 +/- 2 and 8 +/- 2 mm Hg, respectively, 5 s into the upright posture, while M1 and M2 caused falls in MAP of 6 +/- 4 and 9 +/- 3 mm Hg, respectively, at 5 s. 5. Thus, both L- and T-type VOCC antagonists, at plasma concentrations in the clinical range, lowered MAP in the conscious rabbit, but only mibefradil caused postural hypotension. We conclude that T-type VOCC may play an important role in the venoconstrictor reflex in response to tilt in the rabbit.

Neurons in amygdala mediate ear pinna vasoconstriction elicited by unconditioned salient stimuli in conscious rabbits

We determined whether functional integrity of neurons in the amygdala is necessary for sudden episodes of cutaneous vasoconstriction that occur when the conscious animal detects a salient alerting stimulus. To inhibit neuronal function, muscimol (5 nmol in 300 nl), a long acting and potent GABA-A receptor agonist that hyperpolarizes neurons, was injected bilaterally into the amygdala or into a more dorsal control site in conscious rabbits. Cutaneous blood flow was measured in the ear pinna flow using an ultrasonic Doppler probe chronically implanted around the central ear artery. Ear flow responses to salient unconditioned alerting stimuli (fur touch, slight cage movement. removal of drape covering cage) were examined before and after injection of the muscimol, and the effects compared with effects of muscimol on the ear flow response to more nociceptive stimuli, including ear pinch. Muscimol injections into the dorsal control site did not significantly alter alerting-related episodes of ear pinna vasoconstriction. Muscimol injections into the amygdala almost completely abolished ear vasoconstriction elicited by fur touch (0/5 positive responses), drape removal (0/7 positive responses) and cage movement (0/7 positive responses). Muscimol injections into the amygdala reduced the mean ear flow coefficient of variation for a 15 min observation period from 47+/-5 before injection to 15+/-33% after injection (P<0.01, n=7 rabbits). Muscimol injections into the amygdala did not alter the vigorous ear pinna vasoconstriction elicited by ear pinch (7/7 positive responses). Our results indicate that neuronal function in the amygdala, probably the central nucleus of the amygdala, is necessary for the occurrence of ear pinna vasoconstriction episodes elicited by unconditioned salient stimuli but not for the occurrence of corresponding vasoconstriction elicited by nociceptive stimuli.

Protective and defensive airway reflexes evoked by nasal exposure to wood smoke in anesthetized rats

We investigated the airway responses evoked by nasal wood smoke in anesthetized Sprague-Dawley rats. Wood smoke (5 ml, 1.4 ml/s) was delivered into an isolated nasal cavity while animals breathed spontaneously. In study 1, nasal wood smoke triggered either an apneic response (n = 26) or a sniff-like response (n = 16) within 1 s after smoke exposure in 42 normal rats. Both airway responses were abolished by trigeminal nerve denervation and by nasal application of a local anesthetic or a hydroxyl radical scavenger, but they were not significantly affected by removal of smoke particulates or nasal application of a saline vehicle. In study 2, nasal wood smoke only triggered a mild apneic response in two rats neonatally treated with capsaicin and had no effect on breathing in the other six; the treatment is known to chronically ablate C fibers and some Adelta fibers. In contrast, nasal wood smoke evoked an apneic response in six rats neonatally treated with the vehicle of capsaicin and elicited a sniff-like response in the other two. These results suggest that the apneic and sniff-like responses evoked by nasal wood smoke result from the stimulation of trigeminal nasal C-fiber and Adelta-fiber afferents by the gas-phase smoke and that hydroxyl radical is the triggering chemical factor.

Cardiovascular effects of odors

Several occupational and residential settings can expose both normal and sensitive human subjects to odors and irritants. These settings include intensive agricultural operations housing swine and poultry, cigarette-smoke-filled bars, landfills and manufacturing processes. The literature suggests that adverse sensory reactions to strong odors and irritants may lead to the release of catecholamines and stress hormones. Physiological and biochemical measurements related to cardiovascular risk, e.g., blood pressure, heart rate, high-density lipoprotein (HDL) cholesterol level and serum triglyceride level, may be altered as a result of exposure to odor and irritant-induced release of catecholamines. Further work in the form of field studies and chamber exposure protocols is required to determine whether the physiological and biochemical changes observed to date represent an increase in cardiovascular risk, or are reversible changes within the normal homeostatic range.

The rostral ventrolateral medulla mediates the sympathoactivation produced by chemical stimulation of the rat nasal mucosa

1. We sought to outline the brainstem circuit responsible for the increase in sympathetic tone caused by chemical stimulation of the nasal passages with ammonia vapour. Experiments were performed in alpha-chloralose-anaesthetized, paralysed and artificially ventilated rats. 2. Stimulation of the nasal mucosa increased splanchnic sympathetic nerve discharge (SND), elevated arterial blood pressure (ABP), raised heart rate slightly and inhibited phrenic nerve discharge. 3. Bilateral injections of the broad-spectrum excitatory amino acid receptor antagonist kynurenate (Kyn) into the rostral part of the ventrolateral medulla (RVLM; rostral C1 area) greatly reduced the effects of nasal mucosa stimulation on SND (-80 %). These injections had no effect on resting ABP, resting SND or the sympathetic baroreflex. 4. Bilateral injections of Kyn into the ventrolateral medulla at the level of the obex (caudal C1 area) or into the nucleus tractus solitarii (NTS) greatly attenuated the baroreflex and significantly increased the baseline levels of both SND and ABP. However they did not reduce the effect of nasal mucosa stimulation on SND. 5. Single-unit recordings were made from 39 putative sympathoexcitatory neurons within the rostral C1 area. Most neurons (24 of 39) were activated by nasal mucosa stimulation (+65.8 % rise in discharge rate). Responding neurons had a wide range of conduction velocities and included slow-conducting neurons identified previously as C1 cells. The remaining putative sympathoexcitatory neurons were either unaffected (n = 8 neurons) or inhibited (n = 7) during nasal stimulation. We also recorded from ten respiratory-related neurons, all of which were silenced by nasal stimulation. 6. In conclusion, the sympathoexcitatory response to nasal stimulation is largely due to activation of bulbospinal presympathetic neurons within the RVLM. We suggest that these neurons receive convergent and directionally opposite polysynaptic inputs from arterial baroreceptors and trigeminal afferents. These inputs are integrated within the rostral C1 area as opposed to the NTS or the caudal C1 area.

Inhibition of baroreflex vagal bradycardia by nasal stimulation in rats

Nasal stimulation provokes hypertension and bradycardia. We report here that such stimulation inhibits baroreflex vagal bradycardia (BVB). In chloralose- and urethan-anesthetized, beta-adrenergic receptor-blocked rats, the aortic depressor nerves were cut and electrically stimulated to induce BVB. Nasal application of smoke, warm distilled water, or cold or hot Ringer solution suppressed BVB, but application of warm Ringer solution did not. Smoke-induced inhibition was abolished by trigeminal but not olfactory denervation. Neither suprapontine decerebration nor C3 spinal cord transection affected the inhibition. Bradycardia induced by electrical stimulation of the peripheral cut end of the cervical vagus nerve (VIB) was suppressed by long-lasting smoke application. Intravenous prazosin, a proposed blocker of prejunctional inhibition of acetylcholine release from the vagus terminals, abolished VIB inhibition but attenuated BVB inhibition only slightly. Thus nasal stimulation inhibits BVB, and this inhibition is mediated exclusively by the trigeminal nerve and occurs principally at the pontomedullary level, although the potential exists for contribution of the prejunctional mechanism. The inhibition of BVB might contribute to cardiovascular regulation associated with protection from atmospheric hazards.

Renal hemodynamic responses to dynamic exercise in rabbits

Cardiovascular hemodynamics, including renal blood flow, were measured in rabbits with one intact and one denervated kidney during various intensities of treadmill exercise. Within the first 10 s of exercise, there was rapid vasoconstriction in the innervated kidney associated with decreases in renal blood flow (range -10 to -17%). The vasoconstriction in the innervated kidney was evident at all workloads and was intensity dependent. There was no significant vasoconstriction or change in renal blood flow (range 0.5 to -3.1%) in the denervated kidney at the onset of exercise. However, a slowly developing vasoconstriction occurred in the denervated kidney as exercise progressed to 2 min at all workloads. Examination of responses to exercise performed under alpha-adrenergic blockade with phentolamine (5 mg/kg iv) revealed that the vasoconstriction in the innervated kidney at the onset of exercise and the delayed vasoconstriction in the denervated kidney were due primarily to activation of alpha-adrenergic receptors. In addition, a residual vasoconstriction was also present in the innervated kidney after alpha-adrenergic blockade, suggesting that, during exercise, activation of other renal vasoconstrictor mechanisms occurs which is dependent on the presence of renal nerves.

Exogenous NPY modulation of cardiac autonomic reflexes and its pressor effect in the conscious rabbit

1. Neuropeptide Y (NPY) may inhibit sympathetic and vagal transmission via presynaptic Y2 receptors and cause vasoconstriction via postsynaptic Y1 receptors. We examined the effects of NPY and related peptides on cardiovascular parameters and autonomic reflexes in the conscious rabbit. Further, the postjunctional effects of NPY and related peptides were assessed on acetylcholine (ACh) and isoprenaline agonist dose-chronotropic response curves. 2. In conscious rabbits the cardiac baroreceptor-heart rate reflex (baroreflex), Bezold-Jarisch like and nasopharyngeal reflexes were assessed in control, propranolol-treated or methscopolamine-treated (baroreflex only) groups, before and 30 min after i.v. administration of NPY (10 microg kg[-1] + 5 microg kg[-1] min[-1]) or vehicle (saline, 10 ml h[-1]). The effects of equivalent pressor doses of [Leu31, Pro34]NPY or methoxamine on the baroreflex were also examined. In separate animals, dose-heart rate (HR) response curves to isoprenaline or ACh were constructed before and 15 min after administration of NPY, [Leu31,Pro34]NPY (ACh only) or [Leu31,Pro34]NpY + sodium nitroprusside (ACh only). 3. Administration of NPY-receptor agonists caused sustained bradycardia (in the absence of methscopolamine) and rightward shifts of the barocurves in all 3 groups. The range of sympathetically-mediated tachycardia was significantly decreased by NPY or [Leu31,Pro34]NPY in the methscopolamine-treated group. However, these changes in the baroreflex were no different from those elicted by equipressor doses of methoxamine. There was no vagal inhibition by any NPY-receptor agonist in all three autonomic reflexes examined. ACh or isoprenaline dose-HR response curves were not affected by NPY peptide administration. 4. We conclude that in the conscious rabbit, at a single dose that elicits a significant pressor response, exogenous NPY has no direct effect on modulation of cardiac and autonomic reflexes. Non-specific effects of exogenous NPY on the baroreflex may be fully explained by its pressor action. There was no effect of NPY on postjunctional ACh or isoprenaline agonist dose-response curves. Therefore, it is unlikely that endogenous NPY has a functional role in directly modulating cardiac autonomic neurotransmission in the rabbit.

Cerebral blood flow in rabbits during the nasopharyngeal reflex elicited by inhalation of noxious vapor

We used chronically implanted Doppler ultrasonic flow probes to measure internal carotid and vertebral blood flow during the nasopharyngeal reflex elicited by inhalation of formaldehyde vapor in conscious rabbits. Internal carotid flow gradually increased to 157 +/- 5% of baseline and vertebral artery increased to 123 +/- 9% of baseline, with maximum values reached approximately 20-40 s after administration of vapor, at a time when arterial pO2 had decreased from 80 +/- 3 to 53 +/- 4 mmHg. Increases in flow were associated with increases in vascular conductance. The delayed increases in cerebral blood flow contrasted with rapid decreases in ear and distal aortic flows, measured at the same time. Our results indicate that forebrain vascular conductance increases in response to inhalation of noxious vapor, possibly reflecting cerebrovascular events associated with hypoxemia.

Sympathoexcitatory response to cyclosporin A and baroreflex resetting

We postulate that the sympathoexcitatory response associated with the immunosuppressive agent cyclosporin A is due to an upward resetting of the arterial baroreflex. We performed studies in conscious intact and sinoaortic-denervated rabbits instrumented with catheters and renal nerve electrodes. In intact rabbits, cyclosporin A (20 mg/kg i.v., 30 minutes) produced significant increases in renal sympathetic nerve activity (100% to 269 +/- 74%, P < .05) but did not increase mean arterial pressure. In intact rabbits, we determined arterial baroreflex curves relating renal sympathetic nerve activity and heart rate to mean arterial pressure by producing ramp increases (intravenous phenylephrine) and decreases (intravenous nitroprusside) in mean arterial pressure. Cyclosporin A treatment produced a shift of the midrange of the baroreflex control of heart rate (78.0 +/- 4.1 to 84.6 +/- 4.7 mm Hg, P < .05) and renal sympathetic nerve activity (74.6 +/- 3.9 to 87.0 +/- 4.8 mm Hg, P < .05). Vehicle administration produced no effects on arterial baroreflex curves relating renal sympathetic nerve activity and heart rate to mean arterial pressure. Compared with vehicle treatment, cyclosporin A reduced the maximum gain of heart rate (-5.6 +/- 0.6 versus -3.1 +/- 0.8 beats per minute per millimeter of mercury, P < .05) but had no effect on the maximum gain of renal sympathetic nerve activity. In conscious sinoaortic-denervated rabbits, cyclosporin A had no effect on mean arterial pressure (95.7 +/- 7.3 to 91.8 +/- 10.8 mm Hg), renal sympathetic nerve activity (100% to 110 +/- 6%). and heart rate (287 +/- 10 to 279 +/- 8 beats per minute). However, when the same sinoaortic-denervated rabbits were anesthetized with sodium pentobarbital, cyclosporin A (20 mg/kg i.v.) produced increases in renal sympathetic nerve activity (100% to 189 +/- 27%). These data indicate (1) that the sympathoexcitatory response to cyclosporin A depends on baroreceptor afferent input in the conscious state and (2) that this response involves an upward resetting of the arterial baroreflex.

Trigeminally-mediated alteration of cardiorespiratory rhythms during nasal application of carbon dioxide in the rat

Stimulation of the upper respiratory tract with air-borne irritants can result in dramatic alterations of cardiorespiratory rhythms that include apnea, bradycardia and selective peripheral vasoconstriction. Since carbon dioxide can stimulate receptors in the nasal passages, we wanted to determine if this odorless gas can induce the same autonomic changes as air-borne irritants. Passing 100% carbon dioxide through the nasal passages of rats anesthetized with chloralose-urethane produced apnea, a vagally-mediated bradycardia and a sympathetically-mediated increase in mean arterial blood pressure. Application of atropine blocked the bradycardia without affecting respiratory or blood pressure changes, while injection of prazosin eliminated blood pressure responses but did not affect heart rate or apnea. There were no significant autonomic responses to nasal application of 10, 25 or 50% carbon dioxide. The responses were mediated through the trigeminal innervation of the nasal mucosa since they could be blocked when the anesthetic procaine was applied to the nasal cavity. We conclude that these cardiorespiratory responses are due to stimulation of trigeminal nociceptors located within the nasal mucosa.

Renal sympathetic and heart rate baroreflex function in conscious and isoflurane anaesthetized normotensive and chronically hypertensive rabbits

1. Baroreflex control of heart rate (HR) has been studied in normotensive (NT) and hypertensive (HT) awake and anaesthetized animals and man, but baroreflex control of sympathetic nerve activity has not been well studied. We investigated baroreflex control of HR and renal sympathetic nerve activity (RSNA) over a wide range of arterial pressure (AP) in conscious and isoflurane (ISO) anaesthetized NT and HT rabbits. 2. Animals were instrumented to record AP, HR and RSNA. Hypertension was accomplished by renal encapsulation. AP-HR and AP-RSNA baroreflex function curves were obtained while awake and after 1.0, 1.5, 2.0 and 2.5% ISO. All baroreflex curves were fit to sigmoid or exponential functions. 3. In conscious rabbits, HT for 3-5 weeks, AP was significantly higher (75.6 +/- 0.8 vs 102.3 +/- 8.9 mmHg); HR significantly lower (218.0 +/- 5.5 vs 189.5 +/- 5.5 beats/min); and RSNA not different than NT rabbits (14.9 +/- 2.2 vs 9.9 +/- 3.2% max RSNA). 4. ISO shifted AP-HR and AP-RSNA baroreflex curves to the left in NT and HT animals, and significantly attenuated baroreflex range and slope. At low ISO concentrations, baroreflex compensation for decreases in AP is limited to small increases in HR and sympathetic nerve activity. At higher ISO concentrations, baroreflex responses to decreases in AP are lost. RSNA responses to increases in AP are preserved with increasing ISO concentrations while HR responses are progressively attenuated. The sole effect of chronic hypertension was to shift the AP-HR and AP-RSNA barocurves to the right along the pressure axis in both conscious and ISO anaesthetized animals with no additional change in range or slope. 5. At this stage of hypertension development, ISO anaesthesia affects baroreflex function equally in normotensive and hypertensive rabbits.

A medullary dorsal horn relay for the cardiorespiratory responses evoked by stimulation of the nasal mucosa in the muskrat Ondatra zibethicus: evidence for excitatory amino acid transmission

Stimulation of the upper respiratory tract, including the nasal mucosa, with water, vaporous irritants, or gases, induces a collation of several cardiorespiratory responses including an apnea and bradycardia and often some change in arterial blood pressure. Since the nasal mucosa is innervated by branches of the trigeminal nerve, it implies that some part of the trigeminal system within the central nervous system mediates the autonomic responses induced by nasal stimulation. In the present study, respirations, heart rate and arterial pressure were monitored in muskrats anesthetized with a mixture of chloralose-urethane. We induced a bradycardia and apnea by stimulating the nasal mucosa of muskrats with brief (5 s) transnasal application of vapors of ammonia hydroxide. In an effort to determine the central site where the trigeminal mediation of the cardiorespiratory responses occurs, small nanoliter injections of 2% lidocaine were made bilaterally into the subnucleus caudalis of the spinal trigeminal nucleus (referred to as the medullary dorsal horn) to determine if the responses could be blocked. The responses could be blocked when the lidocaine injections on both sides were placed in the rostral, ventral parts of the medullary dorsal horn, but persisted when the injections were placed elsewhere. Since lidocaine blocks both neurons and fibers of passage, nanoliter injections of kynurenate, a general excitatory amino acid antagonist, were used in a similar paradigm to circumvent the problem of blocking only fibers of passage.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoconstriction induced by inhalation of irritant vapour is associated with appearance of Fos protein in C1 catecholamine neurons in rabbit medulla oblongata

The medulla oblongata was examined with Fos and tyrosine hydroxylase (TH) immunohistochemistry after 2 h of intermittent nasopharyngeal stimulation with formaldehyde vapour in the conscious rabbit. The stimulation caused apnoea, bradycardia and a rise in blood pressure known to be associated with vigorous vasoconstriction. Fos-positive neurons occurred in the spinal trigeminal nucleus, the nucleus tractus solitarius, the raphe nuclei and the ventrolateral medulla. In the rostral ventrolateral medulla, 68% of the Fos-positive neurons were TH-positive C1 cells. Our data indicate that nasopharyngeally-evoked peripheral vasoconstriction is associated with activation of C1 neurons.

Trigeminal mediation of the diving response in the muskrat

Stimulation of the nasal cavity elicits powerful cardiorespiratory responses similar to the diving response. In the present study, bradycardia and apnea were elicited in muskrats by stimulation of the nasal cavity with ammonia vapors. These responses could be blocked by injections of 2% lidocaine made bilaterally into the medullary dorsal horns of the trigeminal sensory complex. However, the bradycardia due to activation of the baroreceptor reflex with intravenous phenylephrine was retained. These data implicate trigeminal neurons in the medullary dorsal horn as modulators of autonomic activity, especially in the cardiorespiratory adjustments after nasal stimulation.

Influence of cardiac afferents on time-dependent changes in the renal sympathetic baroreflex of conscious rabbits

1. The stability of the renal sympathetic baroreflex and nasopharyngeal reflex, and the role of cardiac sensory receptors, was studied in conscious rabbits over a 5 h experimental period. 2. Renal sympathetic nerve activity (SNA) was recorded during (i) slow ramp changes in mean arterial pressure (MAP) of 1-2 mmHg/s induced by inflating perivascular balloon cuffs, and (ii) the inhalation of cigarette smoke. Experiments were repeated in other rabbits after blocking cardiac afferents with 5% intrapericardial procaine. 3. Baroreflex responses to the first two caval cuff inflations of the day were significantly greater than subsequent responses. After this, triplicate sets of reflex curves were relatively stable during a 2 h period in the morning. When the experiment was repeated in the afternoon, there was a significant attenuation of baroreflex range and a small fall in resting renal SNA which were abolished by pericardial procaine. 4. Changes in baroreflex properties were minimal when the reflex was assessed only twice, at the beginning and end of a 5 h period. No change was seen in the nasopharyngeal reflex whether the rabbits had been subjected to few or to many cuff inflations. 5. We conclude that time dependent changes can occur in the renal sympathetic baroreflex of conscious rabbits which must be allowed for by appropriate protocol design. These include increasing inhibitory influences from cardiac sensory receptors in experimental situations requiring multiple reflex estimations.

Controlled bradycardia induced by nasal stimulation in the muskrat, Ondatra zibethicus

Respiration was disrupted and bradycardia induced in anesthetized muskrats by stimulating the nasal cavity with a stream of either water or various concentrations of ammonia vapors. When responses induced by either ammonia or water were compared, ammonia vapors were considered preferable because the responses could be maintained reliably through relatively rapid periods of stimulation, and the post-stimulus recovery of heart rate and respiration was more predictable. Moreover, the bradycardia induced in the first 5 s of stimulation by dilutions of ammonia vapors was graded. After injections of lidocaine were made into the nucleus tractus solitarius a profound bradycardia to ammonia stimulation persisted despite disruption of normal respiratory rhythms and an inhibition of the baroreceptor reflex induced by phenylephrine administration. These results show that ammonia vapors stimulating the nasal chambers effectively elicit cardio-respiratory adjustments in anesthetized muskrats and that the bradycardia may be controlled by varying the intensity of the peripheral stimulus. The trigeminal contribution for this is emphasized since the bradycardia persists after reversible blockade of the solitary complex. These data suggest that the trigeminal input to cardiac motorneurons is via relatively few synapses and is over circuits which run parallel to those modulating cardiac activity in response to chemoreceptors, baroreceptors and pulmonary afferent fibers.

Inhibitory effects of formaldehyde inhalation on the cardiovascular and respiratory systems in unanesthetized rabbits

The mechanisms of the marked inhibitory effects of 10 ppm formaldehyde (HCHO) inhalation on heart rate and respiratory movement were investigated in unanesthetized rabbits. Inhibition of the heart rate and respiratory movement induced by HCHO inhalation was caused by a reflex reaction during sensory irritation of the upper respiratory tract, mainly the nasal mucosa, but not of the lower respiratory tract, mainly the lung. These reflex reactions, particularly decreases in the heart rate, were not blocked by vagotomy, atropine or prazosin, but were blocked by propranolol, phenoxybenzamine, phentolamine, yohimbine and guanethidine. These results suggest that these reflex reactions are derived from sympathetic nervous activity rather than parasympathetic nervous activity, and the reflex bradycardia is caused by inhibiting the transmitter release at the adrenergic nerve endings.

The renal sympathetic baroreflex in the rabbit. Arterial and cardiac baroreceptor influences, resetting, and effect of anesthesia

Curves relating renal sympathetic nerve activity and mean arterial pressure were derived in conscious rabbits during ramp changes in mean arterial pressure, elicited by perivascular balloon inflation. The renal sympathetic nerve activity-mean arterial pressure relationship consisted of a high-gain sigmoidal region about resting, where renal sympathetic nerve activity rose or fell in response to moderate falls and rises of mean arterial pressure. With larger pressure rises, renal sympathetic nerve activity first fell to a lower plateau and then reversed at even higher mean arterial pressure. When mean arterial pressure was lowered below resting, renal sympathetic nerve activity rose to an upper plateau and then reversed abruptly toward resting at low mean arterial pressure. Both arterial and cardiac baroreceptors exerted substantial inhibitory influences on renal sympathetic nerve activity at all pressure levels. These effects appeared additive over the central high gain region of the curve, but beyond this region there were non-additive interactions. The latter were affected considerably by alfathesin anesthesia. In other experiments, we studied the effects of sustained alterations in resting mean arterial pressure induced by infusing nitroprusside and phenylephrine, which produced rapid resetting of the renal baroreflex. The latter could be accounted for, in part, by resetting of the threshold of the arterial baroreceptors and in part by contributions from other afferents, probably the cardiac receptors. During resetting associated with nitroprusside-induced falls in resting blood pressure, high-gain reflex adjustments in renal sympathetic nerve activity to moderate changes in mean arterial pressure were preserved, but during resetting associated with phenylephrine-induced rises in resting mean mean arterial pressure, the resting renal sympathetic nerve activity lay on the lower curve plateau, resulting in reduction in the apparent gain of the reflex renal sympathetic nerve activity response to moderate changes in mean arterial pressure.

Role of peripheral chemoreceptors in response to smoke-induced apnea vs tracheal occlusion

Reflex autonomic changes which occur after cigarette smoke enters the upper airways are partially due to peripheral chemoreceptor stimulation. Chemoreceptor denervation attenuates but does not abolish smoke induced bradycardia. Denervation nearly abolishes bradycardia induced by tracheal occlusion. Hypertension accompanies smoke induced apnea but does not occur during tracheal occlusion.

Species differences in circulatory responses to nasopharyngeal perfusion with smoke

Cardiovascular changes were observed in five species of laboratory mammals during upper airway perfusion with cigarette smoke. Apnoea occurred, both before and after vagotomy, in all species. Blood pressure always went up in rabbits, rats and hamsters but was not much affected in cats and guinea-pigs. Before vagotomy heart rate fell in rabbits and rats, rose in hamsters but did not change in cats and guinea-pigs. Vagotomy affected heart rate changes only in rabbits and rats. Results demonstrate markedly different autonomic responses in different species.

Chemosensitivity of rat nasal trigeminal receptors

Electrophysiological responses to odorants delivered via an air dilution olfactometer were recorded from the ethmoid branch of the trigeminal nerve innervating the nasal cavity. Thresholds were obtained for nine compounds with those for heptanol (21-137 ppm) and propionic acid (39-49) ppm consistently being the lowest. Not all odorants e.g., phenethyl alcohol, elicited responses in all rats even at vapor saturation. A striking degree of correlation was present between the rat whole-nerve electrophysiological response magnitudes of this study and the human anosmic intensity ratings established in the work of Doty et al. [9] to vapor saturated stimuli. These results suggest that the rat is an excellent model for assessing the stimulatory effectiveness of odorants on human trigeminal receptors. The possible role of the trigeminal system in the perception of odors as well as the physiologic effects of odorants due to trigeminal stimulation are discussed.

Identification of the primary afferent fiber group and adequate stimulus initiating the trigeminal depressor response

In anesthetized and immobilized rabbits we sought to identify the adequate stimulus and the primary afferent fiber group initiating hypotension and inhibition of sympathetic vasoconstrictor discharges in the trigeminal depressor response (TDR). In the first series of experiments we investigated the effects of electrical stimulation of different fiber groups of the infraorbital nerve on arterial pressure (AP) and renal sympathetic nerve activity (RNA). Stimulation of A-beta-fibers at frequencies between 1 and 200 Hz caused little or no reduction in AP or RNA. Recruitment of A-delta-fibers at stimulus frequencies between 1 and 30 Hz always resulted in falls in AP and RNA (by up to 30 mm Hg, and 75% of control, respectively). Decreases in AP and RNA were augmented to some extent by additional excitation of C-fibers over the same frequency range. In the second series of experiments we applied noxious or innocuous mechanical or thermal stimuli to the hairy skin of the upper lip. Hypotension and inhibition of RNA were elicited by any of the following noxious stimuli of the facial skin: (a) pricking by pins; (b) pinching by serrated forceps, (c) heating above 43% C by a contact thermoprobe; and (d) actual burning of the skin. In contrast, innocuous mechanical or thermal (below 40 degrees C) stimuli failed to diminish AP and RNA. These results indicate that noxious stimuli to the face initiate hypotension and inhibition of the sympathetic nerve activity in the TDR through activation of the A-delta primary afferent fiber group, alone or in combination with the C-fiber group.

Nasal-cardiopulmonary reflexes: a role of the larynx

A brief review of the literature from otorhinolaryngology and the basic sciences shows the existence and role of nasal-cardiopulmonary reflexes in animals and man. There is ample evidence that odors, fluids and mechanical stiumlation of the nasal mucosa will induce changes in the lungs and cardiovascular system. The proposition that nasal obstruction also produces cardiopulmonary changes is briefly reviewed. The suggestion is made that one of the functions of the nose is to act as an expiratory brake. the removal of this brake could result in changes in laryngeal resistance that lead to poor ventilation.