Depressor and bradycardic actions of L-proline injected into the nucleus tractus solitarii of anesthetized rats.
L-Glutamate has been considered to be a neurotransmitter in the nucleus tractus solitarius (NTS) of the afferent baroreflex pathway, though this has not yet been decisively shown. A bolus injection of a neurotransmitter candidate amino acid L-proline into the cisterna magna and that of L-glutamate shows the same pressor action in the freely moving rat, but the actual nuclei responding L-proline remain undetermined. Besides L-glutamate, L-proline might be another candidate amino acid in the NTS. The present study was therefore performed to characterize the circulatory action of L-proline injected into the NTS where responses to glutamate in the anesthetized rat had already been shown. The NTS was first determined as a site on the dorsal surface of the medulla where a microinjection of L-glutamate decreased arterial pressure and heart rate. Microinjected L-proline (1.65 to 13.2 nmol, 33 nl) into the NTS decreased arterial pressure and heart rate in a dose-dependent manner. The injection of a mixed solution (66 nl) of kynurenate, an ionotropic excitatory amino acid receptors antagonist (1.32 nmol), and L-proline (6.6 nmol) into the NTS abolished the depressor and bradycardic actions with L-proline alone (6.6 nmol, 66 nl). However, a mixture of an increased concentration of kynurenate (6.6 nmol) with glutamate augmented the actions seen with glutamate alone (0.66 nmol, 66 nl). D-Proline (13.2 nmol, 66 nl), the optic isomer of L-proline, produced no change in arterial pressure or heart rate, suggesting that the actions of L-proline in the NTS were optically specific. The results indicate that L-proline but not D-proline induces its depressor and bradycardic actions through ionotropic excitatory amino acid receptors in the NTS of the anesthetized rat. L-Proline may become a candidate transmitter of baroreceptor information in the NTS.
Takemoto
Y
Y
Department of Physiology II, Hiroshima University Faculty of Medicine, Hiroshima, 734-8551 Japan. yumitake@hiroshima-u.ac.jp
D000758:Anesthesia; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D001919:Bradycardia / Q000139:chemically induced / Q000503:physiopathology*; D004305:Dose-Response Relationship, Drug; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology; D007736:Kynurenic Acid / Q000494:pharmacology; D008297:Male; D008845:Microinjections; D011392:Proline / Q000737:chemistry / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D017552:Solitary Nucleus / Q000187:drug effects / Q000503:physiopathology*; D013237:Stereoisomerism
D018691:Excitatory Amino Acid Antagonists; D018698:Glutamic Acid; D011392:Proline; D007736:Kynurenic Acid