The biogenic amine, serotonin (5-hydroxytrptamine, [5-HT]) is an important extracellular regulator in the central nervous system and many peripheral tissues including the gastrointestinal tract, urinary tract, adrenal cortex, and vasculature. Published data from the Principal Investigator's laboratory have demonstrated that, in the intact perfused rat submandibular gland, 5-HT decreases acetylcholine-induced saliva flow and increases saliva protein content. 5-HT also increases cAMP production in dispersed SMG (as well as sublingual and parotid) cell aggregates, but has no effect on SMG cell intracellular free Ca2+ concentrations. The data presented in this proposal suggest that rat SMG co-expresses two 5-HT receptor subtypes, 5-HT4b and 5-HT7, both of which are positively coupled to cAMP production. The main hypothesis of this proposal is that cAMP-coupled 5-HT receptors in the rat SMG play physiological roles in the regulation of gland function. Such receptors might be potential therapeutic targets in diseases affecting salivary gland function. The goals of the research are: 1. To define conclusively the 5-HT receptor complement in rat SMG in terms of subtypes and receptor density of each subtype using molecular, pharmacological, and radioligand binding techniques. 2. To determine the cell-type specific distribution of 5-HT receptor subtypes using functional assays in dispersed duct and acinar -enriched cell aggregates from SMG in conjunction with in situ hybridization studies designed to co-localize 5-HT receptor subtype mRNAs with cell-type specific marker proteins using dual fluorescence techniques, 3. To verify that the adenyl cyclase-cAMP-cAMP dependent protein kinase signaling pathway mediates the 5-HT effects in the SMG, through use of selective pharmacological tools which activate or inhibit this pathway in conjunction with subtype-specific 5-HTR ligands; and 4. To begin to assess the physiological relevance of SMG 5-HT receptors by determining the 5-HT levels in the gland, detecting the key 5-HT synthetic enzyme in SMG and determining the presence and level of high affinity 5-HT transporters (SERT) in SMG. Successful completion of these goals will help to resolve the question of whether 5-HT is one of the growing list of agents involved in the regulation of mammalian salivary gland function and might stimulate interest in 5-HT receptors as therapeutic targets in patients with salivary gland dysfunction.
