ROLE OF AMINO-ACIDS IN SALIVATION AND THE LOCALIZATION OF THEIR RECEPTORS IN THE RAT SALIVARY-GLAND

The distribution of gamma-aminobutyric acid (GABA) receptor subunits s uch as GABA(A)R-gamma 1 and GABA(A)R-gamma 2, and lpha-amino-3-hydroxy -5-methyl-4-isoxazolepropionic acid (AMPA) type receptor subunits such as GluR-1, GluR-2/3 and GluR-4, and N-methyl-D-aspartic acid (NMDA) t ype subunits such as NR1 were investigated by immunocytochemistry. Fur thermore, the roles of these amino acids, GABA and glutamate, on saliv ation were analyzed in the rat submandibular and sublingual glands. So me similarities were observed in the distribution patterns of GABA(A) type receptors and AMPA receptors. in the submandibular ganglion cells , collecting ducts and striated ducts, these subunits were expressed s trongly; however, there were some differences in their expression patt erns between the submandibular and sublingual gland acinar cells. Sinc e these receptor subunits were expressed in the acinar cell bodies of the submandibular gland, they were not expressed in the acinar cells b ut were expressed in the myoepithelial cells in the sublingual gland. On the other hand, no NR1 expression was observed. To examine the role s of GABA and glutamate in salivation, the submandibular and sublingua l glands were perfused partially with Ringer's solution via a facial a rtery to avoid systemic influence, and substrates were infused into th e perfusion solution. No salivary secretion was evoked by GABA or glut amate infusion in the absence of electrical stimulation (2-3 V, 5 ms, 20 Hz). Salivary flow evoked by electrical stimulation of the chorda-l ingual nerve caused significant inhibition by GABA (10(-6), 10(-5), 10 (-4) and 10(-3) M) and the GABA(A)R agonist muscimol (10(-3) and 10(-6 ) M) (n = 6, P < 0.05). Such GABA-induced inhibition was antagonized b y the GABA(A)R antagonists bicuculline (BCC; 10(-6) and 10(-3) M) and picrotoxin (PTX; 10(-6) and 10(-3) M). On the other hand, salivary flo w evoked by electrical stimulation (8-10 V, 5 ms, 20 Hz) of the superi or cervical ganglion (SCG) was not affected by GABA. While high doses of glutamate (10(-1) M) and NMDA (10-(1) M) showed no effects on saliv ary now despite application of electrical stimulation, AMPA at a high concentration (10(-1) M) significantly inhibited salivary secretion (n = 6, P < 0.05). These studies revealed that inhibitory and excitatory amino acid receptors such as GABA(A) and AMPA type receptors are coex pressed in the rat salivary glands, and that GABA inhibits salivary se cretion via GABA(A) receptors which may act with acetylcholine. Howeve r, the role of glutamate in salivation remains unclear despite the pre sence of AMPA type receptors. The present findings suggest that glutam ate does not act alone but with other substances such as peptides and/ or other amino acids.