Previous work from this laboratory has shown that the serotonin (5-HT) induced response is significantly augmented in differentiated NG108-15 (NG) cells treated with dibutyryl cAMP (Bt(2)cAMP) due to qualitative and quantitative changes in the expression of the 5-HT3 receptor as demonstrated by specific [H-3] LY-278584 (a selective 5HT(3) receptor antagonist) binding. In this study, we investigated whether there is any change in the relative expression of the 5-HT3A and 5-HT3B subunits in NG cells differentiated following Bt(2)cAMP treatment cells. The major findings of this study were that the relative amount of 5-HT3B subunit mRNA in Bt(2)cAMP-treated NG cells 5 days following Bt(2)cAMP-treatment was greater than that in the untreated cells. In contrast, the relative expression of the 5-HT3B subunit protein in the Bt(2)cAMP-treated NG cells was much less than in the untreated cells, but the relative expression of the 5-HT3A subunit in the Bt(2)cAMP-treated NG cells was similar to the untreated cells. Therefore, no relationship between mRNA and protein expression for 5-HT3A and 5-HT3B subunits in Bt(2)cAMP treated and untreated NG cells were observed. It was also found that fluorescent intensity for the 5-HT3B subunit in the cell body of the Bt(2)cAMP treated and untreated NG cells gradually decreased from the day 1-5 after Bt(2)cAMP treatment. However, in specific areas such as the varicosity and nerve endings of the Bt(2)cAMP treated cells, staining intensity for the 5-HT3B subunits was stronger than in the untreated cells at the all time points, peaking at day 5 post-treatment. These results suggest that the augmented response induced by 5-HT acting via 5-HT3 receptors in differentiated NG cells may be due to changes in the relative amount of the 5-HT3B subunit, particularly the ratio and distribution of the 5-HT3A to (3B) subunits.