||The present work was divided into three main areas of study, (1) the development of a reliable "in vivo" model for double choice (DOCH) testing in piglets avoiding the isolation time and fasted state of animals to evaluate weaned pig preferences, consumption and appetence under a fed-state for Glycine and several L-amino acids at different concentrations (-in vivo trials- Chapter 1), (2) identifying and characterizing the porcine Tas1r1 and Tas1r3 gene sequences in pigs, to construct a stable cell reporter system expressing the porcine umami taste receptor (pT1r3/pT1r1) to measure the cell responses to amino acids at physiological concentrations (0. 5, 5 and 50mM) and to compare these cell response with the pig in vivo results (-in vitro trials- Chapter 2), and (3) to determine the pattern of expression of the porcine umami taste receptor genes, pTas1r3 and pTas1r1, in several tissues of the digestive system including tongue (circumvallate and fungiform papillae), stomach (fundus), pancreas, liver, duodenum, jejunum and ileum and how that pattern responds to three factors: (a) the age of the pig -from birth to 20 days after weaning-; (b) sex -male vs. female-; and (c) the dietary crude protein content and essential amino acid supplementation (Chapter 3). Chapter 1 describes a new methodology of DOCH testing to evaluate preferences, consumption and appetence in fed pigs for amino acid solutions (D,L-Met, Gly, L-Ala, L-Gln, L-Glu, L-Lys, L-Thre, L-Trp and MSG) at different concentrations (0. 5, 5, 50 and 500mM). This methodology includes a preliminary training period (10¬minute at 9am and 12pm) based on an operant conditioning procedure using sucrose at 500mM as a reward before test sessions (2-minute at 9am and 12pm). Two models were developed where animals were maintained individually (individual model) or in pairs (pair model) during training (4 and 2 days, respectively) and test sessions. Social isolation and novelty are two important factors of stress that could influence pig learning capacity for DOCH testing, therefore, behavioural and total test consumption parameters were used as criteria for the exclusion of animals in testing sessions. The results showed that the pair model developed in this work may be more appropriate than previous models to study pig preferences and appetence for amino acid solutions or other nutrients. Weaned pigs under a fed-status were able to discriminate solutions of amino acids except when those were offered at 0. 5mM. In general, piglets showed significant preferences for non-essential amino acids with a higher appetence for potential umami tastants at high concentrations (MSG, L-Glu and L-Gln). However, this taste response changed with essential amino acids, resulting in significant aversions at high concentrations (L-Trp, L-Thr). Chapter 2 presents, after the identification and characterization of the porcine umami taste receptor (pT1r1/pT1r3), an in vitro tool based on a cell culture that expresses the heterodimer to identify Gly and L-amino acids as umami tastants at different concentrations (0. 5, 5 and 50mM) in pigs. This cell system showed significant responses to MSG, L-Glu, L-Gln, L-Ala, L-Asn and Gly at all tested concentrations. Moreover, our in vivo data (Chapter 1) was significantly correlated with our in vitro results meaning that umami agonists are highly preferred by pigs. Chapter 3 studies the expression of the porcine umami taste receptor genes, pTas1r1 and pTas1r3, in different taste (tongue's fungiform and circumvallate papillae) and non-taste tissues from the gastrointestinal tract (stomach, duodenum, jejunum, ileum and liver) of pigs of different sex (male and female) and ages (birth, preweaning- 26d old-, 48h after weaning -28d old- and 20d postweaning -46d old-) and with different levels of dietary crude protein in their postweaning diet (high-crude protein -26%, HCP-, low crude protein-17%- with -SAA- and without -LCP- essential amino acid supplementation). Both genes were more expressed in tongue and stomach, followed by small intestine and liver. Significant changes in gene expression were observed with age, sex and dietary crude protein content and the main changes occurred after weaning, with more expression in males than in females and in the LCP group that in the other two groups. It is concluded that pigs sense some amino acids as umami tastants that generate a pleasant stimulus. The umami tastants are sensed through the heterodimer receptor pT1r1/pT1r3, which is expressed in taste buds in the tongue and in non-taste tissues along the gastrointestinal tract. Changes in pTas1r1/pTas1r3 gene expression may reflect the nutritional status of the animal and a better understanding of the mechanism will help to develop new strategies (such as the use of umami ligands to stimulate their voluntary feed intake) for improving the adaptation of piglets to the postweaning period.