5-HT modulation of pain in SI and SII revealed by fMRI

To observe the different cerebral activations between pre-administration and post-administration of tryptophan-depleted amino acid mixture (TRP-)/balanced amino acid mixture (BAL) in healthy volunteers through functional magnetic resonance imaging (fMRI) when they received electrical stimulation at the same time. Furthermore, the effects of 5-hydroxytryptamine (5-HT) were explored in pain modulation network.

Two-phase crossover design was adopted. Six right-handed healthy volunteers participated in the experiment. TRP- and BAL were administered to each participant according to the Young's method. Volunteers were scanned under fMRI with or without administration of TRP- or BAL when receiving electrical stimulation simultaneously. Analysis of functional neuroimages (AFNI) software was used to show the different activations in the human brain between pre- and post-administration of TRP-/BAL. Then all the results were analyzed by SPSS 14.0.

(1) Bilateral primary somatosensory cortex (SI) and secondary somatosensory cortex (SII) were activated during pre-administration and post-administration of TRP-, but the activated volumes of bilateral SI and SII during post-administration of TRP- were found significantly larger than that during pre-administration of TRP (P<0.05). (2) All the activated regions showed no difference between pre- and post-administration of BAL. (3) There were significant differences of pain threshold between pre- and post-administration of TRP- (P<0.01), which meant that pain threshold reduced remarkably after TRP- administration, while it showed no difference between pre- and post-administration of BAL (P>0.05).

5-HT plays an important role in pain modulation in the central nervous system, TRP- causes acute tryptophan depletion (ATD) through reducing the level of 5-HT in the human brain, which can induce the decrease of volunteers' pain threshold. The activated volumes of bilateral SI and SII display significant difference between pre- and post-administration of TRP-, suggesting these brain areas may not only involve in the 5-HT related activities but in pain modulation network as well.