Hreday N. Sapru

Aortic nerve stimulation in the rat: Cardiovascular and respiratory responses

Cardiovascular response and changes in the phrenic and recurrent laryngeal nerve activities to the electrical stimulation of the aortic nerves were studied in the decerebrate male Wistar rats. Stimulus parameters were varied over a wide range (intensities 0.1-10 volts, frequencies 2-150 pulses per sec, durations 0.01-10 msec) and stimulation was continued for 1 min to ensure that steady state was reached. Stimulation of the aortic nerves consistently produced decreases in blood pressure and heart rate (baroceptor responses) while increase in blood pressure and heart rate (chemoreceptor responses) were never obtained. Inhibition of phrenic and recurrent laryngeal nerve activities (baroceptor responses) always followed stimulation of aortic nerves while facilitation of these nerve activities (chemoreceptor responses) was never observed. These results indicate that the aortic nerves of the rat contain mainly baroceptor afferent fibers and there are few, if any, functional chemoreceptor fibers in these nerves.

Evidence for a sympathoexcitatory pathway from the nucleus tractus solitarii to the ventrolateral medullary pressor area

The role of the ventrolateral medullary pressor (VLPA) and depressor (VLDA) areas in mediating cardiovascular responses evoked from the nucleus tractus solitarius (NTS) was investigated. Male Wistar rats, anesthetized with pentobarbital or urethane, were artificially ventilated and blood pressure (BP) and heart rate (HR) were monitored. The VLPA, VLDA, and the NTS were identified bilaterally with microinjections of L-glutamate. Unilateral microinjections of muscimol or lidocaine into the VLPA or the VLDA blocked the decrease in BP produced by microinjections of L-glutamate (1.77 nmol) into the NTS. These findings indicate that both areas are essential for mediating depressor responses elicited from the NTS. When neuronal activity in the VLDA was depressed unilaterally (leaving the ipsilateral VLPA intact), with the microinjection of muscimol or lidocaine, microinjection of a larger dose (5.0 nmol) of L-glutamate into the ipsilateral NTS elicited a pressor response. This response was blocked by depressing neuronal activity in the ipsilateral VLPA by microinjection of muscimol into this site. This pressor response evoked from the NTS was not due to non-specific effects of L-glutamate since repeated microinjections of L-glutamate (5.0 nmol/site) into the NTS consistently produced decreases in BP and HR. The stimulation of the contralateral NTS by glutamate continued to elicit the usual decreases in BP and HR. Microinjections of either dose (1.77 or 5 nmol) of L-glutamate into the areas adjacent to the NTS (e.g. 1.0 mm rostral or lateral to the NTS, the gracile or cuneate nuclei and area postrema) failed to evoke any cardiovascular responses indicating that the responses were mediated by neurons localized within the intermediate one-third of the NTS. These results indicate that: (1) the depressor responses elicited from the NTS involve the pathways from the NTS to the VLDA and VLDA to VLPA and (2) there may be a pathway from the NTS to the VLPA which is sympathoexcitatory and is unmasked when neuronal activity in the VLDA is depressed.

Putative neurotransmitters involved in medullary cardiovascular regulation.

Experiments were carried out in artificially ventilated pentobarbital-anesthetized male Wistar rats. Following microinjection of muscimol (GABA-mimetic) or kynurenic acid (KYN; glutamate antagonist) into the ventrolateral medullary depressor area (VLDA), microinjection of L-glutamate (GLU; 4.5 nmol) into the NTS elicited a pressor response. This pressor response was attenuated in a dose-dependent manner by microinjection of KYN (0.5-5 nmol) into the ventrolateral medullary pressor area (VLPA). A GLU-induced pressor response could also be elicited from the NTS when GABA receptors in the VLPA were blocked with the microinjection of bicuculline (GABA antagonist, 200 pmol) into this site. The same dose of bicuculline in the VLPA also blocked the depressor responses elicited from the VLDA. With the VLDA or VLPA functionally unimpaired, microinjection of GLU (4.5 nmol) into the NTS elicited a fall in blood pressure and heart rate. This depressor response was attenuated in a dose-dependent manner by the microinjections of KYN (2-20 nmol) into the VLDA. These results indicate that: (1) The NTS sends glutamatergic inputs to both the VLDA and the VLPA; the projection from the NTS to the VLPA mediates pressor responses while that from the NTS to the VLDA represents one component of the pathway mediating the depressor responses elicited from the NTS. (2) The pathway from the VLDA to the VLPA is GABA-ergic and represents another component of the pathway mediating depressor responses evoked from the NTS. (3) The bradycardia evoked from the NTS may involve a pathway from the NTS to the VLDA.

Excitatory amino acid receptors in the nucleus tractus solitarius mediate the responses to the stimulation of cardio-pulmonary vagal afferent C fiber endings

Stimulation of cardio-pulmonary vagal afferent C fiber endings by right atrial injections of phenylbiguanide (PBG, 40 micrograms/kg) elicited apnea, bradycardia and hypotension. These responses were abolished by bilateral vagotomy low in the neck. Stimulation of neurons in a specific nucleus tractus solitarius site (0.5 mm rostral, 0.5 mm lateral and 0.5 mm deep with reference to the calamus scriptorius) by injections of L-glutamate produced responses similar to those following the stimulation of cardio-pulmonary vagal afferent C fiber endings by PBG. Inhibition of neurons in this NTS site by injections of muscimol abolished the responses to PBG. Specific blockade of NMDA receptors by microinjections of AP-7 (100 pmol) or non-NMDA receptors (KA and AMPA) by injections of DNQX (10-25 pmol) into this NTS site did not block the responses to PBG. Microinjections of kynurenic acid (1-4 nmol) into this NTS site blocked the responses to right atrial injections of PBG. These results indicate that: (1) blockade of either NMDA receptors or non-NMDA receptors (KA and AMPA) alone in the above-mentioned NTS site does not abolish the responses to the stimulation of cardio-pulmonary vagal afferent C fiber endings; (2) it is necessary to block NMDA as well as non-NMDA receptors in this NTS site for abolishing the responses to the stimulation of these nerve endings.

Central resetting of baroreflex in the spontaneously hypertensive rat.

The role of central nervous system in the resetting of baroreflex was investigated in 5-month-old spontaneously hypertensive rats (SHR) of Okamoto strain. Age-matched Wistar-Kyoto (WKY) rats were used as normotensive controls. The aortic nerves, which in the rat, contain few or no chemoreceptor fibers, were stimulated electrically using a wide range of stimulus frequencies. The depressor responses (expressed as percent decrease in blood pressure as compared to its blood pressure value prior to aortic nerve stimulation) produced by these stimulations were significantly smaller in SHR than those in WKY. In another series of experiments, changes in the efferent limb of the baroreflex arc (i.e., greater splanchnic nerve activity) in response to stimulation of the baroreceptor afferents in the aortic nerve were recorded. Inhibition of the greater splanchnic nerve activity due to aortic nerve stimulation was found to be significantly smaller in SHR than in the WKY. Control sympathetic nerve activity was greater in SHR than in WKY. These results suggest that the central bulbospinal nervous system may be another site for resetting of baroreflex in hypertension.

M2 muscarinic receptors mediate pressor responses to cholinergic agonists in the ventrolateral medullary pressor area

Microinjections of cholinergic agonists into the ventrolateral medullary pressor area (VLPA) evoke increase in blood pressure (BP) and heart rate (HR). Recently two major subtypes of muscarinic receptors (M1 and M2) have been identified. This investigation was designed to study the role of these muscarinic receptor subtypes in pressor responses of cholinergic agonists in the VLPA. Male Wistar rats were anesthetized with pentobarbital or decerebrated at mid-collicular level. The rats were artificially ventilated and BP and HR were recorded. Ventral medulla was exposed and the VLPA identified bilaterally by microinjections of L-glutamate. Microinjections of cis-methyldioxolane (CD, a specific agonist of M2 receptors) in the doses of 0.004-4 nanomol (nmol)/site into the VLPA evoked an increase in BP (13-56 mm Hg) and HR (7-24 bpm) which lasted for 10-50 min. Intravenous injections of the same doses of this agent failed to evoke a response. AFDX-116 (a specific M2 muscarinic receptor antagonist) microinjected into the VLPA (0.2-1.6 nmol-/site) evoked depressor responses (6-20 mm Hg). Microinjections of this agent into the VLPA prevented the pressor responses to subsequent microinjections of CD at the same sites, indicating that AFDX-116 blocked M2 receptors. AFDX-116 rendered neurons in the VLPA unresponsive to L-glutamate but this effect lasted for 30-40 min while the hypotensive and M2 receptor blocking effect lasted for 60-150 min. McN-A343 (a specific agonist for M1 receptors) or pirenzepine (PZ, a specific antagonist of M1 receptors) injected into the VLPA (0.4-4 nmol/site) failed to evoke any response.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular and respiratory effects of some anesthetics in the decerebrate rat.

The cardiovascular and respiratory effects of pentobarbital, ketamine, chloralose and urethane were studied in decerebrate rats. All the anesthetics reduced blood pressure and heart rate. Pentobarbital, chloralose and ketamine, but not urethane, reduced respiratory rate. None of these agents had a significant effect on tidal volume. All these anesthetics significantly attenuated the carotid occlusion, tilt and sodium cyanide chemo-receptor response. These results suggest that the decerebrate rat may be a more suitable preparation, compared to the anesthetized one, for studying cardiovascular and respiratory responses in this species.

Differential regulation of regional vascular resistance by the rostral and caudal ventrolateral medulla in the rat

Regional vascular resistance changes were determined following chemical excitation and inhibition of the rostral vasopressor (RVLM) and caudal vasodepressor (CVLM) areas in the ventrolateral medulla. Mesenteric, renal and hindquarter vascular resistances were assessed in paralyzed and artificially ventilated urethane-anesthetized rats instrumented with pulsed-Doppler flow probes. Microinjection of the excitatory amino acid L-glutamate in the RVLM elicited a significant dose-related transient increase in blood pressure, heart rate and resistance of mesenteric, renal and hindquarter vascular beds. A similar dose-related hemodynamic profile was obtained following microinjection of muscimol, a GABAmimetic, in the CVLM. In contrast, significant dose-related decrease in blood pressure, heart rate and resistance in mesenteric and hindquarter vascular beds was observed following glutamate-induced excitation of the CVLM and muscimol-induced inhibition of the RVLM. Changes in renal vascular resistance were inconsistent in this second hemodynamic profile. Intravenous administration of the alpha 1 adrenergic antagonist, prazosin, abolished all of the hemodynamic effects elicited by excitation of the RVLM except the tachycardia. Intravenous atropine methylnitrate blocked the bradycardia associated with excitation of the CVLM but did not alter the vascular resistance changes. These results indicate that the changes in heart rate did not contribute significantly to the resistance profiles described. The changes in vascular resistance elicited by excitation and inhibition of the RVLM were correlated with increase and decrease in the greater splanchnic nerve activity, respectively. In conclusion, neuron pools in the RVLM and CVLM exert differential effects upon resistance in different vascular beds via changes in sympathetic outflow.(ABSTRACT TRUNCATED AT 250 WORDS)

Procedure for the decerebration of the rat.

A procedure for the decerebration of the rat was devised. The internal and external carotid arteries were ligated under halothane anesthesia. A transection was made at the mid-collicular level of the brain and the transected forebrain was removed. The posterior communicating arteries were occluded by suction and oxidized cellulose was packed around these vessels. The cut surface of the brain was covered with oxidized cellulose and the cranial cavity was filled with cotton balls. Head-up tilt, carotid occlusion and sodium cyanide responses indicated that the orthostatic, baro-, and chemo- reflexes were intact in these preparations. The stability of the preparation (for at least 8-10 hr) and the integrity of these reflexes indicate that the decerebrate rat may be a suitable alternative for studies involving brain stem regulatory mechanisms in this species when it is desirable to avoid general anesthetics.

Pulmonary opiate receptor activation evokes a cardiorespiratory reflex

The administration of [D-Ala2,Met5]enkephalinamide (D-AME) and [D-Ala2,Leu5]enkephalinamide (D-ALE) into the right atrium of decerebrate rats caused bradycardia, a slight transient biphasic blood pressure response and apnea within 1-2 sec. Apnea was followed by rapid shallow breathing. These effects were dose related (1-1000 micrograms/kg) and blocked by pretreatment with naloxone. Atropine blocked the bradycardia. Sectioning the vagi at the level of the diaphragm did not affect the responses, whereas bivagotomy below the cardiac branches abolished all responses. The triad of responses was attributed to a reflex action arising from vagal afferents within the lung. These results were confirmed in paralyzed , artificially ventilated animals. In these animals, the enkephalin analogues produced a cessation of phrenic nerve (PN) activity followed by a decrease in the duration of bursts. The recurrent laryngeal nerve (RLN) was concomitantly excited in a continuous decremental fashion. This excitation was independent of PN inhibition. Recordings of single and near single pulmonary vagal afferents demonstrated no effect of D-AME or D-ALE on stretch and irritant receptors. However, type J-receptors were stimulated.