GABA and analogs: Potential performance-enhancing metabolites of herbage

Diets of grazing livestock include substances that are benign, others that are feeding deterrents, others are toxicants and some stimulate feeding and functions of herbivores. GABA (1 amino butyric acid), is the primary non protein amino acid in herbage and the primary neuro-inhibitor of mammals, other vertebrates and some invertebrates. Researchers at KAES showed that cattle dosed with Librium, an antidepressant active at GABA receptors, calmed cattle grazing tall fescue pastures and increased live weight gain and later showed that GABA supplements increased rates of biting and short term rates of herbage intake of cattle grazing alfalfa and caused extreme agitation. Neuroactivity was unexpected because the GABA is not supposed to cross the blood brain barrier. The blood brain barrier should have protected the brain from fluxes of dietary GABA. GABA may be deaminated and hydroxylated to GHB (1 hydroxy butyric acid), which is also a neuroactive agent that, in contrast to GABA, passes through the blood-brain barrier. We hypothesize that GABA ingested by herbivores is either passes through the blood barrier intact or as GHB or some other neuroactive derivative. Initially GABA was considered to be functionless in plants but this view has changed because research has demonstrated: I. That GABA has a major role in plant stress metabolism and may be a fast, first responder stress mechanism. 2. The GABA shunt of the TCA cycle. 3. That GABA is involved in transamination. 4. That GABA acts as a feeding deterrent of phytophagous insects. 5. That GABA protective functions include that as part of an antioxidant mechanism. Other functions of GABA are also being found in plants and in animals, for example, dietary GABA was shown to promote the release of growth hormone in rats. We will determine the concentration of GABA and GBH in herbage of important species of forage grasses and legumes. As a preliminary stage in GABA metabolism/catabolism we will also determine the amount of GABA ingested by beef steers grazing alfalfa, and follow concentrations of GABA and GBH in rumen fluid, serum, dung, and urine. This area is of significance because: I. Plant GABA levels respond to selection and may be manipulated easily by plant J:>iotechnology.2. Presence of neuroactive natural plant constituents in herbage may help explain some of the observed variation in grazing behavior of grazing livestock. 3. Neuroactive substances in plants may impact human behavior when ingested. 4. GABA is part of the response of plants to stress. 5. GABA is a feeding deterrent of some phytophagous invertebrates.