Central and peripheral actions of a nonpeptidic angiotensin II receptor antagonist.

Nonpeptidic imidazole derivatives were recently reported to be angiotensin II receptor antagonists with acute blood pressure-lowering activity. In the present study, we characterized the angiotensin II receptor antagonist properties of one such derivative, 4'-([2-butyl-4-chloro-5-(hydroxymethyl)-1H-imidazol-1-yl]methyl)- [1,1'-biphenyl]-2-carboxylic acid (IMI). In receptor binding studies, IMI displaced bound [125I]angiotensin II from rat uterine membranes with an IC50 of 0.17 microM. In isolated rabbit aortic rings, IMI shifted the angiotensin II concentration-response curve to the right in a parallel and concentration-dependent manner. A Schild plot of these data indicated a pA2 of 7.13 +/- 0.16 and a slope of 0.94 +/- 0.06. In rat kidney slices, IMI shifted the concentration-response curve for angiotensin II-induced inhibition of renin release to the right. Antagonism of the angiotensin II pressor response by IMI was dose dependent and reversible in ganglion-blocked, anesthetized rats. The water intake and pressor responses to intracerebroventricular angiotensin II (100 pmol) were inhibited by intracerebroventricular IMI (25 or 50 nmol) in conscious Sprague-Dawley rats. Similarly, the drinking and pressor responses to intravenous angiotensin II were blocked by intravenous IMI in conscious rats. IMI alone had no effects on mean arterial pressure or drinking when administered either intravenously or intracerebroventricularly. IMI decreased mean arterial pressure throughout 5 days of infusion in spontaneously hypertensive rats. In summary, IMI was a full competitive antagonist without partial agonist activity in peripheral tissues and the central nervous system. Moreover, the chronic administration of this angiotensin II receptor antagonist was antihypertensive in spontaneously hypertensive rats.

Structure-activity relationships for the carboxy-terminus truncated analogues of angiotension II, a new class of angiotensin II antagonists.

A series of analogues of the recently reported angiotensin II (AII) antagonist [Sar1]AII-(1-7)-amide or des-Phe8[Sar1]AII (3) have been prepared by solid-phase synthesis and purified by reverse-phase liquid chromatography. The agonist and antagonist properties of these carboxy-truncated analogues of AII were determined in the isolated rabbit aorta assay. In the analogues tested, replacement of aspartic acid in position 1 by sarcosine was found necessary to produce significant antagonist activity. At position 7 of the des-Phe8 analogues, prolinamide could be replaced by proline without significant change in the biological activity. However, substitution of 7-prolinamide by either glycinamide or sarcosinamide provided inactive peptides. Methylation of the 4-tyrosine in [Sar1]AII-(1-7)-NH2 preserved the antagonist potency in isolated rabbit aorta. Deletion of the proline at position 7 resulted in inactive hexapeptides, both in the Asp1 and Sar1 series. However synthesis of the N,N-dimethyl amide at the N-terminus afforded hexapeptide [Sar1]AII-(1-6)-N(CH3)2 (10) with a pA2 value of 7.05. All the antagonistic peptides synthesized were fully reversible, competitive antagonists in vitro. These findings indicate that the structural requirements for receptor blockade by these C-truncated analogues are quite stringent with respect to the nature of the amino acid at positions 1 and 6/7. The analogues 2, 3, 7, 10, 11 (saralasin), and 12 (sarmesin) were tested in vivo in the anesthetized rat and were found to inhibit the AII pressor response. In addition, 3 inhibited angiotensin II stimulated aldosterone release from isolated rat adrenal zona glomerulosa cells and had no agonist activity by itself at the doses tested. Interestingly, analogue 3, when injected intracerebroventricularly in conscious rats, failed to antagonize the dipsogenic response to an angiotensin II icv injection and this reflects some heterogeneity in the AII receptor population. Peptide 3 is the first example of an antagonist that discriminates between peripheral and brain receptor subtypes.

Sensitivity of the developing rat brain to hypobaric/ischemic damage parallels sensitivity to N-methyl-aspartate neurotoxicity.

The endogenous excitotoxin, glutamate (Glu), acting at the N-methyl-aspartate (NMA) subtype of Glu receptor, is thought to play a major role in hypoxic/ischemic neuronal degeneration. In the present study, the sensitivities of the developing rat CNS to hypoxic/ischemic neuronal degeneration and to the neurotoxic action of NMA were compared at various postnatal ages. In the hypoxic/ischemic experiments, ischemia was produced by unilateral common carotid artery ligation and hypoxia by subjecting the pups to a partial vacuum. Keeping the duration of the hypobaric episode constant at 75 min for all age groups, we observed that the vulnerability of the immature brain to hypobaric/ischemic damage increased during the early neonatal period (days 2-4), reached a peak at day 6 and then diminished progressively with increasing age. In the second part of the study, NMA was microinjected unilaterally into the head of the caudate nucleus at various postnatal ages (2-80 d). In the early neonatal period (days 2-6), injections of relatively small doses of NMA (6-15 nmol) produced a dose-dependent widespread excitotoxic reaction throughout the forebrain with peak sensitivity being observed on day 6. The cytotoxic reaction to NMA was identical in appearance and time course to that induced by hypobaric/ischemic methods. With increasing age, the excitotoxic response to a given dose of NMA decreased progressively and the lesions became more strictly confined to the injection site. Cell populations most sensitive to NMA toxicity in the 2-10 d period closely correlated with those most vulnerable to hypoxia/ischemia, and sensitivity to both types of injury reached a peak at 6 d. These findings reinforce other evidence linking an excitotoxic mechanism and the NMA subtype of Glu receptor to hypoxic/ischemic brain damage and suggest that there may be a period during development when NMA receptors are hypersensitive to excitotoxic stimulation, thus rendering the neurons possessing such receptors hypervulnerable to hypoxic/ischemic damage.

Hypobaric-ischemic conditions produce glutamate-like cytopathology in infant rat brain.

We present a new animal model of perinatal hypoxic/ischemic brain damage and compare this type of brain damage with the excitotoxic type of damage previously described in the brains of infant rats and monkeys treated systemically with glutamate (Glu). Ten-d-old rats with unilateral occlusion of the common carotid artery were subjected to hypobaric conditions for 75 min and sacrificed 0-4 hr later for light and electron microscopic brain examination. The mortality rate was relatively low (12%), and brain damage was evident ipsilateral to the ligated carotid in 94% of surviving animals 4 hr after termination of the hypobaric event. Regions most frequently affected were the medial habenulum, dentate gyrus, caudate nucleus, frontoparietal neocortices, olfactory tubercle, and several thalamic nuclei. The acute cytopathological changes, primarily edematous degeneration of neuronal dendrites and cell bodies, evolved very rapidly, with some neurons manifesting end-stage necrosis at 0 hr (immediately after hypobaric exposure) and others developing such changes over a 1-4-hr period. We conclude that the neurodegenerative reaction induced in infant rat brain by hypoxia/ischemia is indistinguishable from the excitotoxic type of damage exogenous Glu is known to cause. Moreover, in a companion study (Olney et al., 1989) we show that MK-801, a powerful antagonist of the N-methyl-D-aspartate receptor complex (subtype of Glu receptor), protects against neuronal degeneration in this hypobaric/ischemic model. Our results reinforce other recent evidence suggesting that hypoxic/ischemic brain damage is mediated by endogenous Glu or related excitotoxins.

L-homocysteic acid: an endogenous excitotoxic ligand of the NMDA receptor.

L-Homocysteic acid (L-HCA) has been proposed as a natural transmitter at the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor based on recent evidence that L-HCA occurs L-HCA occurs naturally in the mammalian CNS, is released from K+ stimulated brain slices in a calcium-dependent manner and may be contained in nerve terminals located in certain brain regions that have a high density of NMDA receptors. Here we report that L-HCA potently induces a pattern of cytopathology in the ex vivo chick retina which mimics the pattern of NMDA but not kainic acid (KA) neurotoxicity. We also show that known NMDA antagonists, including Mg++, D-aminophosphonopentanoate and certain anesthetics, analgesics, and sedative hypnotics block the neurotoxic actions of L-HCA in direct proportion to their efficacy in blocking NMDA neurotoxicity. While there is a perfect correspondence between agents that block NMDA and L-HCA neurotoxicity, only a few such agents are active against KA neurotoxicity. We find that 3H-Glu binding is inhibited more potently by L-HCA (Ki = 67 microM). Moreover the patterns with which L-HCA and NMDA displace 3H-Glu binding in autoradiograms appear essentially identical. These findings are consistent with the proposal that L-HCA is an endogenous ligand at NMDA receptors.

MK-801 powerfully protects against N-methyl aspartate neurotoxicity.

Using the ex vivo chick embryo retina to study the efficacy of antagonists in blocking the excitotoxic effects of excitatory amino acid agonists, we previously identified phencyclidine as the most powerful known anti-excitotoxin. Here we show that MK-801 is 5 times more powerful than phencyclidine as an anti-excitotoxin, that its antagonism is specific for N-methyl-asparate toxicity, is non-competitive and does not entail inhibition of excitatory amino acid receptor binding.

Behavioral effect of selective and non-selective lesions of median raphe nucleus in the rat.

The role of the serotonergic (5-HT) system in the regulation of locomotor activity and consummatory behaviors was studied. Electrolytic and chemical (5,7-dihydroxytryptamine: 5,7-DHT) lesions were made in the median raphe nucleus (MR). The effects of these lesions on spontaneous locomotor activity, food and water intake and body weight were then studied. In these experiments, rats on an ad libitum feeding-drinking regimen were housed individually in cages with activity wheels located in a room kept at a temperature of 22 + 1 degrees C. For each rat, records were kept of its spontaneous locomotor activity, food and water intake, and body weight for 8 weeks before and after the placement of lesion in MR. It was observed that: (1) electrolytic destruction of the MR produced a permanent decrease in locomotor activity, and (2) there were a reduced food intake and a decreased spontaneous activity after intra-MR injection of 5,7-DHT.

Cyclic AMP-induced changes in the electroencephalogram and in behavior during morphine dependence and abstinence in the rat.

Rats were prepared with permanent electrodes for recording activities of the electroencephalogram (EEG) and electromyogram (EMG) and made morphine dependent by the subcutaneous implantation of two morphine pellets. Abstinence was then precipitated by removal of the pellets 72 hr later. During the first 2 days after pellet implantation, high voltage slow activity appeared in the EEG during behavioral wakefulness and stupor, and sleep and REM sleep were greatly reduced. By the third day, tolerance to these effects developed. After pellet removal, REM sleep time during the first day was reduced. Wet dog shake activity reached a maximum during the second day of abstinence. Treatment of rats with cyclic adenosine 3',5'-monophosphate (cAMP), 50 mg/kg intraperitoneally once daily prior to and during induction of morphine dependence and during withdrawal, markedly reduced the appearance of morphine-induced high voltage EEG slow activity after implantation of the morphine pellets. Behavioral arousal during the first 2 days after pellet implantation, on the other hand, was increased. After removal of the morphine pellets, sleep episodes occurred less frequently during the first day than in the control rats. The incidence of wet shakes at this time was also elevated. These results provide direct evidence for an involvement of brain cAMP in the mediation of the central effects of morphine.

Involvement of brain cyclic AMP in the acute and chronic effects of morphine in the rat.

Incubation of cerebral cortical slices of rat brain with 3H-adenosine in the presence of varying concentrations of morphine in vitro resulted in a dose-related increase in 3H-cAMP formation. In in vivo experiments, the rate of 3H-cAMP formation in cortical slices both 45 min and 4 h after the acute s.c. administration of a 10 mg/kg dose of morphine was significantly greater than that for saline-treated controls. A significant enhancement of cortical 3H-cAMP formation likewise became apparent 72 hr after s.c. implantation of two morphine pellets. This was not evident after only 24 h. In similar experiments undertaken with hypothalamic tissue, however, the rate of conversion of 3H-adenosine to 3H-cAMP remained similar to that of the controls. Administration of exogenous cAMP antagonized the analgesic response to morphine in both nontolerant and tolerant rats and accelerated the development of tolerance to morphine. These results may provide further evidence for a role of cAMP in the mediation of the central actions of morphine.