Investigation of serotonin receptors in the isolated penile bulb of rats

The aim of this study was to investigate serotonin (5-HT) receptors in the penile bulb, which have been suggested to play a role in penile erection. Serotonin (10 -7 -3 x 10 -4 M) contracted penile bulbs in a concentration-dependent manner. Ketanserin (5-HT 2A antagonist, 10 -9 -10 -7 M) and prazosin (α 1 -adrenergic receptor blocker, 10 -9 -10 -7 M) suppressed the lower and upper parts of concentration-response curves to 5-HT, respectively. Guanethidine (adrenergic neuron blocker, 5 x 10 -5 M) reduced the responses to 5-HT at only 10 -4 and 3 × 10 -4 M concentrations. NAN-190 (5-HT 1A antagonist, 10 -8 , 10 -7 M) shifted the concentration-response curve to the right with a reduction in the maximum response to 5-HT. While ondansetron (5-HT 3 antagonist, 10 -6 -10 -5 M) and GR55562 (5-HT 1B/1D antagonist, 10 -6 -10 -5 M) had no effect on the concentration-response curve to 5-HT. The 5-HT 1A agonist 8-OH-DPAT (10 -7 -3 x 10 -4 M) contracted penile bulbs in a concentration-dependent manner with a lower pD 2 value than that of 5-HT. Sumatriptan (5-HT 1B/1D agonist, 10 -8 -10 -4 M) did not produce any contractile response in the penile bulbs. Prucalopride, a selective 5-HT 4 agonist (R093877, 10 -7 -3 × 10 -4 M) produced concentration-dependent relaxation in penile bulbs contracted by phenylephrine (10 -5 M). 5-HT 4 agonists cisapride (10 -7 -10 -4 M) and metoclopramide (10 -7 -3 × 10 -4 M) also relaxed the tissue, concentration-dependently. Selective 5-HT 4 antagonists GR125487 (10 -6 -10 -5 M) and GR113808 (10 -6 -10 -5 M) slightly, but not significantly, decreased prucalopride- and cisapride-induced relaxation. Propranolol (β-adrenergic receptor blocker, 10 -6 -10 -5 M) and L-NOARG (nitric oxide synthase inhibitor, 10 -4 M) had no effect on prucalopride-induced relaxation. These results suggest the existence of α 1 -adrenergic, 5-HT 1A and 5-HT 2A serotonergic receptors in the penile bulb of rats, which are responsible for 5-HT-induced contraction. Additionally, a serotonergic receptor resembling a 5-HT 4 -type plays a role in the relaxation. The latter receptor is activated by 5-HT 4 agonists, but is not antagonized by 5-HT 4 antagonists.