Mamoru Kumada

A1 noradrenergic neurons tonically inhibit sympathoexcitatory neurons of C1 area in rat brainstem

In rats anesthetized with urethane and paralyzed, bilateral microinjections of kainic acid (KA) into the region of caudal ventrolateral medulla (CVL) containing noradrenergic neurons of the A1 group (A1 area) elicited a decrease followed by an increase in arterial pressure (AP), heart rate (HR) and sympathetic renal nerve activity (RNA). The sympathoinhibitory and sympathoexcitatory effects of KA were prevented by bilateral microinjection of tetrodotoxin into an area of the rostral ventrolateral medulla (RVL) containing C1 adrenergic neurons (the C1 area). In contrast, the autonomic responses were not altered by interruption of the two other principal projections of A1 area neurons, namely to the hypothalamus or to the nucleus tractus solitarii. Bilateral microinjections of tyramine, clonidine, alpha-methylnoradrenaline or histamine into the C1 area elicited a dose-dependent, anatomically specific and reversible decrease in AP, HR and RNA. The effect of tyramine was blocked by previous microinjection of reserpine, 6-hydroxydopamine (6-OHDA), or phentolamine into the C1 area. Pretreatment with phentolamine unveiled a hypertensive effect of alpha-methylnoradrenaline. All effects of alpha-methylnoradrenaline were blocked by pretreatment of the C1 area with phentolamine plus DL-propranolol, whereas those elicited by histamine prevailed. Pretreatment of the C1 area with 6-OHDA abolished all changes in AP and HR elicited by microinjections of KA into the A1 area. We conclude that (1) neurons of the CVL tonically inhibit sympathetic activity, (2) this effect is mediated by an action upon vasomotor neurons of the C1 area of RVL, (3) the inhibition is mediated by noradrenergic projections from A1 neurons into the C1 area, and (4) this tonic sympathoinhibitory effect is independent of the baroreceptor reflex.

Sympathoinhibition by A1-noradrenergic neurons is mediated by neurons in the C1 area of the rostral medulla☆

In anesthetized, paralyzed rats, bilateral microinjections of kainic acid (KA) into an area of caudal ventrolateral medulla containing A1 noradrenergic neurons of the A1 group (A1 area) first reduced and then elevated arterial pressure (AP), heart rate (HR) and sympathetic renal nerve activity. These effects are attributable to initial excitation and then paralysis of local neurons by KA. The microinjection of tetrodotoxin into an area of rostral ventrolateral medulla containing adrenaline neurons of the C1 group (C1 area) and which is innervated by neurons in A1 area abolished all the effects of KA. The pretreatment of the C1 area with 6-hydroxy-dopamine (6-OHDA) also abolished all the effects of KA. In contrast interruption of projections of the A1 area to the hypothalamus or nucleus tractus solitarii had no effects. The latency for response evoked in renal nerve by stimulation of A1 area was about 10 ms longer than that elicited from C1 area. Tyramine microinjected into the C1 area elicited a dose-dependent decrease in AP prevented by local application of desmethylimipramine. We conclude that neurons of the A1 area tonically inhibit sympathetic activity by inhibiting neurons of the C1 area probably by release of NA.

Physiological and pharmalogical properties of the three subsystems constituting the aortic nerve-renal sympathetic reflex in rabbits

Electrical stimulation of the aortic nerve of anesthetized rabbits reflexly evoked both excitation and inhibition of renal nerve activity. The excitatory component of the reflex, observed in about 75% of the animals, was elicited by activation of aortic C-fibers. It was selectively suppressed by chronic treatment of the animal with capsaicin. Intracisternal injection of either [D-ala2]-met-enkephalinamide or beta-endorphin markedly attenuated this excitatory component, although neither affected the excitatory component mediated by chemoreceptor fibers in response to stimulation of the carotid sinus nerve. It seems most likely that nociceptive C-fibers of the rabbits aortic nerve were responsible for the excitatory component. On the other hand, the inhibitory component was reflexly elicited by stimulation of the aortic A- or C-fiber group activated separately or in combination. In agreement with previous reports, the sympatho-inhibitory action of C-fibers was more powerful and longer-lasting than that of A-fibers. We found that the inhibitory component induced by C-fibers was markedly attenuated by the two opioid peptides mentioned above, but was resistant to pentobarbital. On the contrary, the component mediated by A-fibers was suppressed by pentobarbital but was relatively resistant to the opioid peptides. Thus, the rabbits aortic nerve-renal sympathetic reflex consists of the following 3 subsystems characterized by different physiological and pharmacological properties: sympatho-inhibitory systems activated by barosensory A- or C-fibers and a sympatho-excitatory system attributable to C-fibers probably of nociceptive modality.

Cardiorespiratory effects of topical application of endothelin-1 to the ventral surface of the rat medulla

In urethane-anesthetized and vagotomized rats, we examined the cardiorespiratory effects of a topical application of endothelin-1 (ET-1) to the ventral surface of the medulla (VSM) and surveyed subregions of the VSM influenced by these effects. ET-1 (0.1 fmol) delivered to the S area of the VSM via a needle (i.d. of approximately 100 microns) gently pressed on the VSM had no effect on mean arterial pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), phrenic nerve activity (PNA), or the burst rate of PNA. However, a dose of 1 fmol of ET-1 induced transient but significant increases in MAP, HR, RSNA, and burst rate while at a dose of 10 fmol or more, PNA also increased and simultaneously longer-lasting decreases in MAP, RSNA, and PNA followed the initial increase. The subregion of the VSM in which ET-1 most prominently elicited these effects was the S and caudal part of the M area, where topical application of 50 nmol of L-glutamate caused cardiorespiratory changes. Additionally, there was a restricted region within the caudal VSM in which ET-1 caused a decrease in PNA with an increase in burst rate. These results support our hypothesis that the VSM is crucially involved in the cardiorespiratory changes induced by centrally administered ET-1.

Barosensory neurons in the rostral ventrolateral medulla mediate the renal-sympathetic reflex in rabbits.

In urethane-anesthetized and vagotomized rabbits, activation of C-fibers of renal afferents resulted in a reflex change in multifiber renal nerve activity (RNA) which consisted of inhibitory (I) and excitatory (E) components, either alone or in combination. The I and E components were individually and reversibly blocked by bilateral application of bicuculline and kynurenic acid, respectively, to the ventral surface of medulla. Bicuculline further eliminated the sympathoinhibiton produced by stimulation of the aortic nerve. Within the subjacent rostral ventrolateral medulla (RVLM), we recorded 23 spontaneously active single units that responded to electrical stimulation of renal afferents and were antidromically activated by stimulation of the dorsolateral funiculus at the C2 level. Usually, the neuronal response preceded that of the RNA by about 100 ms. These bulbospinal RVLM neurons were barosensory, since they responded to stimulation of the aortic nerve. We conclude that barosensory neurons in the RVLM mediate the renal-sympathetic reflex in rabbits.

Two Types of Hypotensive Effect of Beta-Blocking Agents

In anesthetized and immobilized rats, an hour-long continuous intravenous injection of dl-propranolol (PR; 3 mg/kg), pindolol (PI; 1 mg/kg), oxprenolol (OX; 3 mg/kg) or atenolol (AT; 3 mg/kg) invariably resulted in moderate hypotension. When the drug-induced hypotension was plotted against the control arterial pressure (AP), two types of correlation were found. The hypotension induced by PR or PI, both known to accumulate in the brain at a high concentration was positively correlated to the control AP, whereas the hypotension produced by OX or AT, both known to penetrate the blood-brain-barrier poorly, was not. To test the hypothesis that the observed difference was attributable to the presence or lack of sympathoinhibitory action of the drug, the effect of these agents on the renal nerve activity (RNA) was examined. PR or PI diminished the tonic and reflexly evoked RNA, when the evoked RNA was elicited by sciatic nerve stimulation. No such changes were induced by OX or AT. These results demonstrate a modulatory role of sympathoinhibitory effect of beta-blocking agents in their hypotensive action.

The aortic nerve-sympathetic reflex in the rat

The effects of stimulation of aortic nerve A- and C-fibers on the renal and cardiac sympathetic nerve activities in anesthetized and immobilized Sprague-Dawley rats were investigated. A separate aortic nerve was found in 46 rats (90%) out of 51. Activation of A- and C-fiber groups, alone or in combination, resulted in an inhibition of renal and cardiac nerve activities. However, an excitatory component preceding the inhibitory component, representing the reflex response to stimulation of non-barosensory afferent fibers contained in the carotid sinus or aortic nerve, was never observed. This result provides electrophysiological evidence supporting the view that the rats aortic nerve does not contain a significant amount of functionally active non-barosensory afferents. As with the aortic nerve reflex in the rabbit and cat, the sympatho-inhibitory action of C-fibers was more powerful and longer-lasting than that of A-fibers. Furthermore, the C-fiber reflex was elicited at stimulus frequencies as low as 2 Hz. No significant difference was found between the reflex response of cardiac and renal nerves. On the other hand, stimulation of the superior laryngeal nerve, which constitutes an important pathway carrying arterial baroreceptor fibers, caused a reflex sympathetic response typically consisting of excitatory and inhibitory components. Thus, the rats aortic nerve provides a useful experimental means to activate selectively central neural structures associated with barosensory afferents and to elicit the reflex response homologous to that in the arterial baroreceptor reflex in rabbits and cats.