of regionsand a litmusis nottest forand better dissect the multiple cognitive constructs that contribute to WCST performance and assess their relations to the integrity of different integrated functionalcerebral systems.-R.B.lobe damage;it is hopedthat futurewill exploreSchuman EM, Madison DV: A requirement the intercellular messenger nitric oxide in long-term potentiation. Science 1991;254:1503-1506for evidenceUsing thesethe workers oxidestandard haveon nearby slice theaction potentiationneurons. preparation, of inhibitorssynaptic trans-hippocampal investigatedof nitricecent studies have focused attention on the intriguing hypothesis that nitric oxide, originally identified as an endogenous vasodilator, acts as a novel type of messenger molecular in brain. In vessels, nitric oxide is produced by endothelial cells lining the vessel wall. This gas then diffuses out of these cells and causes relaxation by acting on adjacent smooth-muscle cells. Theobservation oxide that neurons are capable of producing nitricmission. In particular, they these inhibitors on long-termthe (LIP),effect of a para-in response to stimulation by neurotransmitters has raised the possibility that this novel messenger also mediate neuronal signaling. As a gas, it wouldrequire the conventional because on its synaptic it can diffuse role in regulating machinery ular release branes. Based across cell vascularmay notmemtone,of vesic-nitric oxide was act on cerebralviewed initially as an agent that might microvessels to raise blood flow inarticle byto increasedneuronal activity. However, this Madison provides compellingas a model of synaptic plasticity underlying learning and memory. In this model, brief tetanic stimulation of neuronal afferents causes a longterm increase in subsequent responses to synaptic stimulation. Although it is clear that LIP is triggered by postsynaptic excitation, it...