This study sought to determine whether the activity of nitric oxide synthase (NOS) is an important physiological link required to mediate increases in cortical cerebral blood flow (CBF) elicited by electrical microstimulation of the basal forebrain (BF). Changes in cortical CBF were assessed in urethane anesthetized rats using laser-Doppler flowmetry. Microstimulation of the BF elicited stimulus-locked increases in CBF that were dependent on frequency and current intensity (up to 280% of control at 50 Hz). Infusion of the potent NOS inhibitor NG-nitro-L-arginine (L-NNA) resulted in significant dose-related reductions in the BF-elicited response at 50 Hz (3.75-60 mg/kg, i.v.), significant elevation in resting mean arterial pressure (MAP) from 106 to 160 mmHg, and modest 21% reductions in resting CBF. The stereoisomer NG-nitro-D-arginine (D-NNA) was without any effect on CBF, although at higher concentrations MAP was elevated to levels comparable to those obtained with L-NNA. Infusion of arginase was also without effect on resting or BF-elicited CBF responses. In contrast, L-arginine (100-400 mg/kg, i.v.) significantly potentiated the BF-elicited response up to an additional 38%, without affecting resting CBF or MAP. This study suggests that NO, or a related nitroso precursor formed by NOS, has a critical role in mediating regulation of cortical CBF by BF neurons.
NPS 1506 is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. NPS 1506 is neuroprotective in rodent models of ischemic stroke, hemorrhagic stroke, and head trauma, with a 2-hr window of opportunity. Neuroprotectant doses of NPS 1506 ranged from approximately 0.1-1.0 mg/kg, with peak plasma concentrations ranging from 8-80 ng/mL. Even at doses producing behavioral toxicity, NPS 1506 did not elicit MK-801-like behaviors, did not generalize to phencyclidine (PCP), and did not elicit neuronal vacuolization. In a Phase I study, intravenous (i.v.) doses of NPS 1506 from 5-100 mg were well tolerated and provided plasma concentrations in excess of those required for neuroprotection in rodents. Adverse events at the 100-mg dose included mild dizziness and lightheadedness, and mild to moderate ataxia. Neither PCP-like psychotomimetic effects nor cardiovascular effects were noted. The long plasma half-life of NPS 1506 (approximately 60 hr) suggests that a single i.v. dose will provide prolonged neuroprotection in humans.
In this set of analogs, a minimum of three basic nitrogens in the side chain was required for maximum potency as NMDAR antagonists. Isosteric nitrogen substitutions in the polyamine chain reduced the in vitro potency of these analogs. An analog binding-conformation model was proposed to rationalize the inactivity of these isosterically substituted analogs.
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