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Solomon Halbert Snyder

(b. 1938)

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(1938–) American psychopharmacologist

Snyder was born in Washington DC and educated at Georgetown University. After receiving his MD in 1962 he moved to the National Institute of Health, Bethesda, Maryland, as a research associate. He later (1965) joined the staff of Johns Hopkins where he has served since 1970 as professor of psychiatry and pharmacology.

Many drugs, hormones, and neurotransmitters are effective at very low concentrations. The synthetic opiate etorphine produces euphoria and relieves pain in doses as low as one ten-thousandth of a gram. It was inferred from this that for such small doses to be effective they must bind to highly selective receptor sites. Snyder, in collaboration with C. Pert, began the search for such receptor sites using radioactively labeled opiates. By 1973, despite many complications, they were able to report the presence of receptors in the mammalian limbic system, a primitive region in the center of the brain associated with the perception and integration of pain and emotional experiences. To identify the receptors Snyder, working with Candice Pert, developed the widely used technique of ‘reversible ligand binding’. Brain tissue was exposed to opiates labeled with radioactive isotopes. These were quickly washed away. The assumption was that, in low concentrations, opiates would first bind tightly to receptors. The opiates which would normally bind loosely with other tissue would be washed away. The binding sites could then be identified by locating the radioactive isotopes.

The implications of such a discovery were far-reaching for it is clear that opiate receptors had not evolved to await the isolation of morphine. The alternative is that there must be natural morphinelike substances in the brain that bind at these sites. Within a few years the first such enkephalins or endorphins, as they were named, were discovered by J. Hughes and Hans Kosterlitz.

In more recent work Snyder has claimed to have identified a new kind of neurotransmitter, namely, the unlikely and highly toxic nitric oxide (NO). In 1987 it had been discovered that NO diffused from blood-vessel walls causing adjacent muscles to relax and the vessels to dilate. Snyder set out to find if NO was made in the brain. He found NO present bound with iron in an enzyme, cyclic guanosine monophosphate (cGMP). NO acts in an unusual way by initiating a three-dimensional change in the shape of the enzyme. Snyder has also suggested that nitric oxide in the brain could be involved in changes connected with learning and memory processes. He has proposed that carbon monoxide may also belong to this novel class of neurotransmitters.

In another breakthrough in 1987 Snyder succeeded in cultivating in vitro human cerebral cortex tissue. For some unknown reason neurons, which do not normally divide, taken from a child undergoing an operation have continued to divide. “They have all the properties of neurons,” Snyder emphasized, “they do everything that neurons do.” Snyder has speculated that it might in the future be possible to implant such neurons in badly damaged brains.

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