nitroarginine and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic-acid

The objective of this study was to investigate the effects of the HIV-1 envelope protein gp120 and its peptide fragments on the function of N-methyl-D-aspartate (NMDA) receptors mediating release of cholecystokinin (CCK) and somatostatin (SRIF). These are nonconventional NMDA receptors recently found to be activated by glycine or D-serine 'only'. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused rat neocortex synaptosomes was potently increased by gp120, its cyclic V3 loop and the linear V3 sequence BRU-C-34-A, but not by RP-135 (a central portion of BRU-C-34-A). The EC50 values of gp120 were 0.02 nM (CCK-LI release) and 0.01 nM (SRIF-LI release). The releasing effect of gp120 was prevented by blocking the glycine site or the ion channel of NMDA receptors, but not the glutamate recognition site; in addition, the gp120 effect was strongly inhibited by nanomolar concentrations of Zn2+ ions and by low micromolar concentrations of ifenprodil. It is concluded that gp120 acts as a very potent agonist at the glycine site of NMDA receptors sited on CCK- and SRIF-releasing nerve endings; the protein is able to activate the receptor channel in the absence of glutamate. Gp120 activates the receptors through its V3 loop as peptide fragments related to V3 retain near-maximal activity. The sensitivity of the gp120 effect to both Zn2+ and ifenprodil would not be incompatible with the idea that these NMDA receptors contain the triple subunit combination NR1/NR2A/NR2B.

Topics: 2-Amino-5-phosphonovalerate; Animals; Cerebral Cortex; Cholecystokinin; Dizocilpine Maleate; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; HIV Envelope Protein gp120; HIV-1; Indoles; Kynurenic Acid; Male; Maleimides; Neurons; Nitroarginine; Peptide Fragments; Pipecolic Acids; Piperazines; Piperidines; Potassium; Protein Structure, Tertiary; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Somatostatin; Synaptic Transmission; Synaptosomes; Zinc

Neuroprotection against cerebral ischemia can be realized if the brain is preconditioned by previous exposure to a brief period of sublethal ischemia. The present study was undertaken to test the hypothesis that nitric oxide (NO) produced from the neuronal isoform of NO synthase (NOS) serves as a necessary signal for establishing an ischemia-tolerant state in brain. A newborn rat model of hypoxic preconditioning was used, wherein exposure to sublethal hypoxia (8% oxygen) for 3 hours renders postnatal day (PND) 6 animals completely resistant to a cerebral hypoxic-ischemic insult imposed 24 hours later. Postnatal day 6 animals were treated 0.5 hour before preconditioning hypoxia with the nonselective NOS inhibitor L-nitroarginine (2 mg/kg intraperitoneally). This treatment, which resulted in a 67 to 81% inhibition of calcium-dependent constitutive NOS activity 0.5 to 3.5 hours after its administration, completely blocked preconditioning-induced protection. However, administration of the neuronal NOS inhibitor 7-nitroindazole (40 mg/kg intraperitoneally) before preconditioning hypoxia, which decreased constitutive brain NOS activity by 58 to 81%, was without effect on preconditioning-induced cerebroprotection, as was pretreatment with the inducible NOS inhibitor aminoguanidine (400 mg/kg intraperitoneally). The protective effects of preconditioning were also not blocked by treating animals with competitive [3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate; 5 mg/kg intraperitoneally] or noncompetitive (MK-801; 1 mg/kg intraperitoneally) N-methyl-D-aspartate receptor antagonists prior to preconditioning hypoxia. These findings indicate that NO production and activity are critical to the induction of ischemic tolerance in this model. However, the results argue against the involvement of the neuronal NOS isoform, activated secondary to a hypoxia-induced stimulation of N-methyl-D-aspartate receptors, and against the involvement of the inducible NOS isoform, but rather suggest that NO produced by the endothelial NOS isoform is required to mediate this profound protective effect.

Topics: Animals; Animals, Newborn; Brain Ischemia; Calcium; Dizocilpine Maleate; Enzyme Inhibitors; Guanidines; Hypoxia; Indazoles; Nitric Oxide; Nitric Oxide Synthase; Nitric Oxide Synthase Type II; Nitric Oxide Synthase Type III; Nitroarginine; Oxygen; Piperazines; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate

The role of nitric oxide in learning and memory processes has been tested in the albino rat by a histochemical and a behavioral study, following behavioral habituation to spatial novelty. Histochemically, the neural consequences of behavioral testing were mapped in the brain by staining for NADPH-d, known to be a NOS, whereas behaviorally the formation of LTH has been interfered with by posttrial NOS-inhibition. In the histochemical study, adult male Sprague-Dawley rats were tested in a Làt-maze and sacrificed at different time intervals thereafter. Handled unexposed rats served as controls. The brains were perfused with aldheide and processed for NADPH-d staining. In unexposed control rats the basal expression of NADPH-d was low and scattered. It pertained to few cells in the neostriatum, cerebral cortex, and CA1 hippocampal regions. In contrast, rats that had been exposed for the first time to the maze (spatial novelty) showed NADPH-d activity in the dorsal hippocampus (granule cells, few hilar neurons, and some CA1 pyramidal cells), the caudate-putamen complex, the cerebellum, and in all layers of somatosensory cortex. The positivity was not due to activity per se, since immediately after exposure it was not different from baseline. In contrast, it was present by 2 h and decreased significantly 24 h later. In addition, a strong neuronal discharge induced by the convulsant pentylentetrazol did not induce NADPH-d 2 h afterwards. The staining was prevented by pretreatment with the NMDA receptor antagonist CPP (5 mg/kg) or with the NOS inhibitor L-NOARG (10 mg/kg). In the behavioral study, rats were given an intraperitoneal injection of 1-10 mg/kg (L-NOARG) or vehicle immediately following exposure to a Làt-maze. The highest dose used (10 mg/kg) disrupted habituation of the vertical component only, known to be mainly of emotional meaning. Conversely, both doses disrupted emotional habituation based on defecation scores. The data indicate that the formation of LTH to novelty triggers a cascade of neurochemical events also involving NOS neurons. Further, the widespread induction of NADPH-d by exposure to novelty suggests that spatial and emotional information processing activate neural networks across different organizational levels of the CNS.

Topics: Amino Acid Oxidoreductases; Animals; Arginine; Brain; Habituation, Psychophysiologic; Hippocampus; Histocytochemistry; Learning; Male; Memory; NADPH Dehydrogenase; Nerve Fibers; Nitric Oxide; Nitric Oxide Synthase; Nitroarginine; Piperazines; Pyramidal Cells; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Time Factors