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settings (see discussion by Harris R. Lieberman and Barbara Shukitt-Hale, Chapter 23 in this volume).

For example, one important causal relationship may be the changes in brain catecholamine neurons that accompany stress. During stressful situations, catecholamine neurons increase their firing rate. Over time, the rise in firing rate may deplete the neuron's stores of transmitter, thereby compromising its ability to perform its required functions. Replenishment of transmitter might help to restore normal neuronal function, and overall cognitive function as well. One method for replenishing catecholamine involves administering tyrosine, the catecholamine precursor, to stimulate transmitter production (see Lieberman and Shukitt-Hale, Chapter 23 in this volume; for a more detailed discussion, see Ahlers et al., 1994; Lieberman, 1994; Wurtman and Lieberman, 1989). Indeed, in laboratory studies, increases in catecholamine production and release by neurons occur when tyrosine is administered to animals in a stressful setting (Luo et al., 1993; see Lieberman and Shukitt-Hale, Chapter 23 in this volume). Such tyrosine treatment improved performance in rats in these experimental situations (Ahlers et al., 1994; Luo et al., 1992; Rauch and Lieberman, 1990; Shurtleff et al., 1993). In limited field tests to date, consistent with the results of animal studies, tyrosine treatment maintained some aspects of cognitive performance in soldiers exposed to stressful situations (Ahlers et al., 1994; Banderet and Lieberman, 1989; Lieberman et al., 1990; Shurtleff et al., 1994).

A second causal relationship may be the changes in brain acetylcholine neurons that accompany stress. Acetylcholine neurons, particularly in the hippocampus, are important for normal learning and memory. Stress diminishes the functioning of these cholinergic neurons, with attendant decrements in performance. Preventing acetylcholine depletion during stress might therefore help prevent the loss of cognitive function under stressful conditions.

A variety of plant and animal extracts have been used by Asian practitioners since ancient times to manage stress and increase endurance, as presented by K. K. Srivastava at the workshop (see summary of his unpublished manuscript in Appendix A in this volume). These extracts have been termed adaptogens, and their chemical composition is relatively poorly defined. Careful studies in human subjects to evaluate the true efficacy of adaptogens are lacking. Because of the lack of information on composition and limited data on human or animal trials using these compounds, the CMNR concluded that they could not evaluate these materials or this concept.

Military Considerations

As described in this report, the many adverse physiological reactions, physical, or cognitive performance difficulties, and mood changes at high altitudes, when combined with increased risks for mountain illnesses, trauma,

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