5-Hydroxytryptamine and pain modulation in man: a clinical pharmacological approach with tryptophan and parachlorophenylalanine.

Monoamines are involved in the central nervous assimilation and modulation of the pain flow. According to a personal hypothesis, a disorder of this biochemical control (in particular a precariousness of brain 5-hydroxytryptamine turnover), is the basic mechanism of some painful conditions, such as migraine and other essential headaches. Acute (infusion) and chronic (ingestion) administration of tryptophan to migraine-headache sufferers improved the clinical course significantly in respect to placebo. Few patients with untractable pain from disseminated cancer received daily infusion of tryptophan and ingested a few gr of this amnioacid: improvement of pain and reduction of morphine necessity was observed. Parachlorophenylalanine chronic administration in migraine-headache sufferers lowered the pain threshold so far as to provoke (in 20% of cases) spontaneous pains in the trunk, legs and arms. This systemic pain syndrome was promptly reversible by discontinuing the treatment. Spontaneous pain syndrome was not reported by others in the healthy subject; this suggests an apparent vulnerability of 5HT turnover in essential headaches.