Identification of the adenylyl cyclase-activating 5-hydroxytryptamine receptor subtypes expressed in the rat submandibular gland

1 Serotonin (5-hydroxytryptamine, 5-HT) has been shown to increase cyclic A MP production in dispersed cell aggregates from the major salivary glands o f the rat. The goal of the present study was to identify the 5-HT receptor subtypes that mediate these effects in rat submandibular glands (SMG). 2 Among the 5-HT receptor subtypes identified in the rat, 5-HT4(a,b), 5-HT6 and 5-HT7(a,b,c) activate adenylyl cyclase (AC). We used subtype specific primers to screen rat SMG by reverse transcription-PCR. Results indicate th e presence of mRNA for 5-HT4(b) and 5-HT7(a) but not for 5-HT4(a), 5-HT6 an d 5-HT7(b,c). 3 In dispersed SMG cells, 5-carboxyamidotryptamine (5-CT), a 5-HT7 receptor agonist, stimulated cyclic AMP synthesis with higher potency (EC50=27+/-5 nM) but lower efficacy than 5-HT, suggesting a 5-HT7 component and an addit ional component in the response to 5-HT. The 5-HT7 contribution was further supported by antagonism of the 5-CT effect by metergoline, a 5-HT7 antagon ist, which exhibited an affinity (K-i=50 nM) similar to that obtained at th e cloned 5-HT7 receptor. 4 In the presence of a maximally effective concentration of 5-CT, 5-HT prod uced an additional increase in cyclic AMP production that was inhibited by the 5-HT4 receptor antagonist, GR113808, suggesting that the second compone nt of cyclic AMP production is mediated by 5-HT4 receptors. 5 These findings indicate the presence in rat SMG of both 5-HT4(b) and 5-HT 7(a) receptors positively coupled to AC.