Tramadol-induced block of hyperpolarization-activated cation current in rat pituitary lactotrophs

The hyperpolarization-activated cation current (I (h)) in rat pituitary lactotrophs (GH(3) cells) was characterized. Tramadol-induced block of this current was investigated. Effects of various related compounds on I (h) in GH(3) cells were also compared. Tramadol caused a time- and concentration-dependent reduction in the amplitude of I (h) with an IC50 value of 13.6 mu M. ZD7288 (30 mu M), CsCl (2 mM), and propofol (30 mu M) were effective in suppressing the amplitude of I (h). 2',5'-dideoxyadenosine (100 mu M) suppressed I (h), while sp-cAMPS (100 mu M) had no effect on it. Tramadol (10 mu M) shifted the activation curve of I (h) to a more negative potential by approximately -20 mV, although no change in the slope factor was observed. Under current-clamp configuration, tramadol (10 mu M) could reduce the firing frequency of action potentials. Intracellular Ca2+ measurements revealed its ability to reduce spontaneous Ca2+ oscillations in GH(3) cells. The results suggests that during cell exposure to tramadol used at clinically relevant concentration, the tramadol-mediated inhibition of I (h) could be direct and mediated via a non-opioid mechanism and would be one of the ionic mechanisms underlying reduced cell excitability.