Yumi Takemoto

Chemical structures of angiotensins formed by incubating plasma with the kidney and the corpuscles of Stannius in the chum salmon, Oncorhynchus keta

The chemical structures of salmon angiotensins produced by incubating tissue extract of the kidney or the corpuscles of Stannius (CS) with homologous plasma are proposed. Two angiotensins, [Asp1, Val5, Asn9] and [Asn1, Val5, Asn9] angiotensin I, were proposed from both kidney and CS incubations by amino acid analysis and the fluorescent peptide-mapping techniques. CS angiotensins were not organ specific, because these two angiotensins were produced by both kidney and CS incubations in a ratio of 1:2 under the same conditions. Whether [Asp1, Val5, Asn9] angiotensin I is a naturally occurring form remains to be clarified; however, [Asn1, Val5, Asn9] angiotensin I may be the major form of angiotensin formed from plasma by salmon kidney and CS.

Immunohistochemical evidence for the localization of neurons containing the putative transmitter L-proline in rat brain.

We examined whether there are the neurotransmitter candidate amino acid L-proline containing neurons localized in the rat brain. Antibodies against L-proline conjugated with rabbit serum albumin were raised in a rabbit and purified with affinity chromatography. Strong L-proline-like immunoreactivity was confined to several groups of neurons in the arcuate nucleus (n) and supraoptic n in the hypothalamus and area postrema. The brainstem had markedly stained fibers in the medial longitudinal fasciculus and localized neuronal cell body labeling in the red n, mesencephalic trigeminal n, lateral reticular n, raphe obscurus n, solitary n, compact ambiguus n, motor trigeminal n and n of trapezoid body. Our findings are consistent with the hypothesis that L-proline may function as a neurotransmitter or neuromodulator in the brain.

Depressor responses to L-proline microinjected into the rat ventrolateral medulla are mediated by ionotropic excitatory amino acid receptors.

The essential amino acid L-proline produces a depressor response when microinjected into the caudal ventrolateral medulla (CVLM) of anesthetized rats. L-proline may activate some excitatory amino acid (EAA) receptors. The present study tested this hypothesis by investigating the effects of two ionotropic excitatory amino acid receptor antagonists on the depressor response to L-proline in the CVLM: the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the N-methyl-D-aspartate (NMDA) receptor-selective antagonist MK801. Urethane-anesthetized rats received arterial catheters and their ventrolateral medulla surface was exposed. Injections of the antagonists CNQX and MK801 (2 mM, 68 nl in each case) into the CVLM completely blocked depressor responses to subsequent administration of AMPA (2 pmol/34 nl) and NMDA (2 pmol/34 nl), respectively. The depressor response to L-proline (3.4 nmol/34 nl) was strongly inhibited by prior injection of CNQX (2 mM, 68 nl) and significantly attenuated by prior injection of a high dose (20 mM, 68 nl), but not a low dose (2 mM, 68 nl), of MK801. The results indicate that the depressor response to L-proline in the CVLM includes mechanisms of ionotropic excitatory amino acid receptors.

Kynurenic acid inhibits circulatory responses to intracisternally injected L-proline in conscious rats.

To investigate the modes of action of potential neurotransmitters for cardiovascular control, amino acids and an antagonist were injected intracisternally into conscious rats. Blood pressure and superior mesenteric flow were measured with cannulae and electromagnetic flow probes that had been implanted in a previous operation under pentobarbitone anaesthesia. L-Proline, L-glutamate and L-arginine (10, 2 and 10 micromol, respectively) caused similar increases in blood pressure and mesenteric vascular resistance. Prior injection of kynurenic acid (0.1 micromol), a broad spectrum antagonist of ionotropic excitatory amino acid receptors, completely blocked the circulatory effects of L-proline, significantly reduced those of L-glutamate but had little effect on responses to L-arginine. These results suggest that the central pressor pathways activated by L-proline, a potential endogenous neurotransmitter, are mediated by ionotropic excitatory amino acid receptors.

Amino Acids That Centrally Influence Blood Pressure and Regional Blood Flow in Conscious Rats

Functional roles of amino acids have increasingly become the focus of research. This paper summarizes amino acids that influence cardiovascular system via the brain of conscious rats. This paper firstly describes why amino acids are selected and outlines how the brain regulates blood pressure and regional blood flow. This section includes a concise history of amino acid neurotransmitters in cardiovascular research and summarizes brain areas where chemical stimulations produce blood pressure changes mainly in anesthetized animals. This is followed by comments about findings regarding several newly examined amino acids with intracisternal stimulation in conscious rats that produce changes in blood pressure. The same pressor or depressor response to central amino acid stimulations can be produced by distinct mechanisms at central and peripheral levels, which will be briefly explained. Thereafter, cardiovascular actions of some of amino acids at the mechanism level will be discussed based upon findings of pharmacological and regional blood flow measurements. Several examined amino acids in addition to the established neurotransmitter amino acids appear to differentially activate brain structures to produce changes in blood pressure and regional blood flows. They may have physiological roles in the healthy brain, but pathological roles in the brain with cerebral vascular diseases such as stroke where the blood-brain barrier is broken.

Pressor response to l -cysteine injected into the cisterna magna of conscious rats involves recruitment of hypothalamic vasopressinergic neurons

The sulfur-containing non-essential amino acid l-cysteine injected into the cisterna magna of adult conscious rats produces an increase in blood pressure. The present study examined if the pressor response to l-cysteine is stereospecific and involves recruitment of hypothalamic vasopressinergic neurons and medullary noradrenergic A1 neurons. Intracisternally injected d-cysteine produced no cardiovascular changes, while l-cysteine produced hypertension and tachycardia in freely moving rats, indicating the stereospecific hemodynamic actions of l-cysteine via the brain. The double labeling immunohistochemistry combined with c-Fos detection as a marker of neuronal activation revealed significantly higher numbers of c-Fos-positive vasopressinergic neurons both in the supraoptic and paraventricular nuclei and tyrosine hydroxylase containing medullary A1 neurons, of l-cysteine-injected rats than those injected with d-cysteine as iso-osmotic control. The results indicate that the cardiovascular responses to intracisternal injection of l-cysteine in the conscious rat are stereospecific and include recruitment of hypothalamic vasopressinergic neurons both in the supraoptic and paraventricular nuclei, as well as of medullary A1 neurons. The findings may suggest a potential function of l-cysteine as an extracellular signal such as neuromodulators in central regulation of blood pressure.

The mapped pattern of kainate on blood pressure responses is similar to that of l-proline in the ventrolateral medulla of the rat

Kainate is an excitatory amino acid receptor agonist with a structure similar to the amino acid L-proline. Our previous studies demonstrated that microinjections of L-proline into the ventrolateral medulla (VLM) of the rat induce a mapped pattern of blood pressure responses distinct from L-glutamate, and the depressor response to L-proline in the caudal VLM (CVLM) is abolished by the kainate/AMPA receptor antagonist CNQX. The present study investigated whether kainate produces the L-proline-mapped pattern of responses in the VLM, compared with the pattern by AMPA. Kainate is known to activate AMPA receptors at higher concentrations. Therefore, responses to kainate were investigated at a low concentration. Microinjections of AMPA or NMDA showed the pattern of the L-glutamate-type; a pressor response in the rostral VLM and caudal pressor area (CPA) and a depressor response in the CVLM. Microinjections of kainate showed depressor responses in the CVLM but minor pressor responses in the rostral VLM, suggesting the same responses to L-proline. However, the response sites in the CPA did not enable us to clearly determine the L-proline-type. Further trials at sites defined by a pressor response to L-glutamate in the CPA, successive injections of L-proline and kainate produced no response, indicating that L-glutamate responding neurons in the CPA are not sensitive to L-proline and kainate. These results suggest that kainate stimulation in the VLM produces a mapped pattern of ABP responses similar to the mapped pattern with L-proline. Kainate receptors could therefore be involved in the depressor response to L-proline in the medulla.

Hindquarters Vasoconstriction Through Central GABAB Receptors in Conscious Rats

The exogenous application of GABA into the cisterna magna of the freely moving rat decreases hindquarters vascular tone as well as arterial pressure. GABA could influence GABA receptor subtypes A, B or C. However, the hindquarters vascular response to the stimulation of each receptor subtype has not yet been investigated. The present study therefore characterized the response to the GABAB receptor agonist baclofen injected into the cisterna magna of the conscious rat. Intracisternally injected baclofen induced long‐lasting increases in hindquarters vascular resistance and arterial pressure in a dose‐dependent manner. Both actions induced by baclofen were completely blocked by a prior intracisternal injection with the GABAB receptor antagonist CGP 35348 (p‐[3‐aminopropyl]‐p‐diethoxymethylphosphinic acid), and systemically by ganglionic blockade. These actions of baclofen were also abolished centrally by sodium pentobarbital anaesthesia. The results suggest that GABAB receptor stimulation via the cisterna magna induced hindquarters vasoconstriction, probably due to central disinhibition of sympathetic activity.

Cardiovascular actions of l-cysteine and l-cysteine sulfinic acid in the nucleus tractus solitarius of the rat

Abstract The sulfur-containing excitatory amino acid (EAA) l-cysteine sulfinic acid (CSA), a neurotransmitter candidate, is endogenously synthesized from l-cysteine (Cys). Exogenous Cys administration into the brain produces cardiovascular effects; these effects likely occur via synaptic stimulation of central nervous system (CNS) neurons that regulate peripheral cardiovascular function. However, the cardiovascular responses produced by CNS Cys administration could result from CSA biosynthesized in synapse. The present study examined the role of CSA in Cys-induced cardiovascular responses within the nucleus tractus solitarius (NTS) of anesthetized rats. The NTS receives input from various visceral afferents that gate autonomic reflexes, including cardiovascular reflexes. Within the NTS, both Cys and CSA microinjections produced decrease responses in arterial blood pressure and heart rate that were similar to those produced by l-glutamate. Co-injection of the ionotropic EAA receptor antagonist kynurenic acid abolished Cys-, but not CSA-, induced cardiovascular responses. This finding suggests that only Cys-induced cardiovascular responses are mediated by kynurenate-sensitive receptors. This study provides the first demonstration that Cys- and CSA-induced cardiovascular responses occur via different mechanisms in the NTS of rats. Further, this study also indicates that Cys-induced cardiovascular responses do not occur via CSA. Thus, within the NTS, endogenous Cys and/or CSA might be involved in cardiovascular regulation.

Intracisternally injected L-proline activates hypothalamic supraoptic, but not paraventricular, vasopressin-expressing neurons in conscious rats.

When injected into specific rat brain regions, the neurotransmitter candidate L-proline produces various cardiovascular changes through ionotropic excitatory amino acid receptors. The present study used an immunohistochemical double-labeling approach to determine whether intracisternally injected L-proline in freely moving rats, which increases blood pressure, activates hypothalamic vasopressin-expressing neurons and ventral medullary tyrosine-hydroxylase- (TH-) containing neurons. Following injection of L-proline, the number of activated hypothalamic neurons that coexpressed vasopressin and c-Fos was much greater in the supraoptic nucleus (SON) than in the paraventricular nucleus (PVN) of rats with increased blood pressure. The number of activated TH-containing neurons was significantly greater following L-proline treatment than following control injections of artificial cerebrospinal fluid (ACSF). These results clearly demonstrate that intracisternally injected L-proline activates hypothalamic supraoptic, but not paraventricular, vasopressin-expressing neurons and medullary TH-containing (A1/C1) neurons in freely moving rats.