Area postrema-induced inhibition of the exercise pressor reflex

Area postrema, a brain circumventricular organ, is the site of antidipsogenic action of circulating atrial natriuretic peptide in eels

Accumulating evidence indicates that circulating atrial natriuretic peptide(ANP) potently reduces excess drinking to ameliorate hypernatremia in seawater(SW) eels. However, the cerebral mechanism underlying the antidipsogenic effect is largely unknown. To localize the ANP target site in the brain, we examined the distribution of ANP receptors (NPR-A) in eel brain immunohistochemically using an antiserum specific for eel NPR-A. The immunoreactive NPR-A was localized in the capillaries of various brain regions. In addition, immunoreactive neurons were observed mostly in the medulla oblongata, including the reticular formation, glossopharyngeal-vagal motor complex, commissural nucleus of Cajal, and area postrema (AP). Trypan Blue, which binds serum albumin and does not cross the blood–brain barrier, was injected peripherally and stained the neurons in the AP but not other NPR-A immunopositive neurons. These histological data indicate that circulating ANP acts on the AP, which was further confirmed by physiological experiments. To this end, the AP in SW eels was topically destroyed by electric cauterization or were by chemical lesion of its neurons by kainic acid, and ANP (100 pmol kg–1) was then injected into the circulation. Both heat-coagulative and chemical lesions to the AP greatly reduced an antidipsogenic effect of ANP, but the ANP effect was retained in sham-operated eels and in those with lesions outside the AP. These results strongly suggest that the AP, a circumventricular organ without a blood–brain barrier, serves as a functional window of access for the circulating ANP to inhibit drinking in eels.

Suppression of vagal motor activities evokes laryngeal afferent-mediated inhibition of gastric motility

We previously reported that the activation of water-responsive afferents in the superior laryngeal nerve was responsible for the inhibition of gastric motility. The present study was undertaken to clarify the roles of the vagal preganglionic neurons responsible for laryngeal afferent-mediated inhibition of gastric motility. Intravenous injection of atropine abolished the inhibition of motility in both the distal and the proximal stomach induced by water administration into the larynx. The neurons in the dorsal motor nucleus of the vagus (DMV), which project to the abdominal viscera, were exclusively inhibited by water administration. Taken together, inhibition of neurons in the DMV induces inhibition of gastric motility evoked by laryngeal water-responsive afferents via a cholinergic pathway. Because chemical lesions of the intermediate DMV, but not the caudal DMV, abolished the inhibition of the distal stomach motility induced by water administration, the intermediate DMV is responsible for the inhibition shown in the distal stomach.

Central blockade of vasopressin V(1) receptors attenuates postexercise hypotension

We tested the hypothesis that central arginine vasopressin (AVP) mediates postexercise reductions in arterial pressure (AP) and heart rate (HR). To test this hypothesis, nine spontaneously hypertensive rats (SHR) were instrumented with a 22-gauge stainless steel guide cannula in the right lateral cerebral ventricle and with a carotid arterial catheter. After the rats recovered, AP and HR were assessed before and after a single bout of dynamic exercise with the central administration of vehicle or the selective AVP V(1)-receptor antagonist d(CH(3))(5) Tyr(Me)-AVP (AVP-X). AP and HR were significantly decreased below preexercise values with central administration of vehicle [P < 0.05, change (Delta)-21 +/- 4 mmHg and Delta-20 +/- 6 beats/min, respectively]. In sharp contrast, after exercise with central administration of AVP-X, both AP (Delta+8 +/- 5 mmHg) and HR (Delta+24 +/- 9 beats/min) were not significantly different from preexercise values (P > 0.05). Furthermore, AVP-X at rest did not significantly alter AP (181 +/- 11 vs. 178 +/- 11 mmHg, P > 0.05) or HR (328 +/- 24 vs. 331 +/- 22 beats/min, P > 0.05). Thus central blockade of AVP V(1) receptors prevented postexercise reductions in AP and HR. These data suggest that AVP, acting within the central nervous system, mediates postexercise reductions in AP and HR in the SHR.