Effects of acute tryptophan depletion on prepulse inhibition of the acoustic startle (eyeblink) response and the N1/P2 auditory evoked response in man

Contraction of the orbicularis oculi muscle in response to a sudden loud so und (acoustic startle response) and the N1/P2 component of the auditory evo ked potential are both attenuated when a brief low-intensity stimulus is pr esented 30-500 ms before the 'startle-eliciting' stimulus (prepulse inhibit ion). Here, we report the effect of acute tryptophan depletion on prepulse inhibition of these responses. Thirteen males (21-52 years) participated in two sessions separated by 7 days, in which they ingested a drink containin g a mixture of amino-acids, which either included (+ TP) or did not include (- TP) tryptophan, according to a balanced double-blind design. Electromyo graphic (EMG) responses of the orbicularis oculi muscle and N1/P2 auditory evoked potentials were recorded in a 20-min session, 6 h after ingestion of the mixture. Subjects received 40 trials in which 1-kHz sounds were presen ted: (i) 40 ms, 115 dB ('pulse alone' trials) and (ii) 40 ms, 85 dB, follow ed after 120 ms by 40 ms, 115 dB ('prepulse/pulse' trials). Mean amplitudes of the EMG response and the N1/P2 potential were derived from the pulse-al one trials and, in each case, percentage prepulse inhibition was calculated . Plasma tryptophan levels were measured from blood samples taken before an d 7 h after each treatment. Under the + TP condition, both the EMG response and the N1/P2 complex showed > 60% prepulse inhibition. The - TP condition was associated with (i) significant suppression of prepulse inhibition of the EMG response, with no significant change in response amplitude and (ii) reduction of the amplitude of the N1/P2 potential, with no significant cha nge in prepulse inhibition of this response. Tryptophan levels rose by 90 /- 15% under the + TP condition and fell by 81 +/- 3% under the - TP condit ion. The suppression of prepulse inhibition of the acoustic startle respons e under the - TP condition suggests that central 5-hydroxytryptaminergic me chanisms may be involved in regulating prepulse inhibition of this response . The lack of effect of tryptophan depletion on prepulse inhibition of the N1/P2 potential suggests that different mechanisms are involved in prepulse inhibition of the startle response and the N1/P2 complex.