Effects of orthodontic force on methionine enkephalin and substance P concentrations in human pulpal tissue

Orthodontic treatment typically involves intermittent periods of patient discomfort caused by forces on the teeth and adjacent tissues. This sensation of discomfort presumably is caused by the action of neuropeptides in the peripheral and central nervous systems. The effects of orthodontic force on the concentrations of two endogenous neuropeptides, methionine enkephalin (ME) and substance P (SP), measured as immunoreactive-methionine enkephalin (ir-ME) and immunoreactive-substance P (ir-SP), in human tooth pulp were evaluated in 20 patients from whom premolars were extracted before orthodontic treatment. The teeth from nine controls were not subjected to a force, whereas the 11 experimental patients had force applied to their maxillary premolars either by a transpalatal spring ligature or, in one case, by a headgear. The ligature applied a force within the range of 120 to 245 gm; the headgear applied 600 gm. Reversed-phase high-performance liquid chromatography (RP-HPLC) was used to purify the neuropeptides in the pulp homogenate, and radioimmunoassay (RIA) was used to quantify ir-ME and ir-SP in their appropriate HPLC fractions. (1) Females subjected to orthodontic force had significantly greater ir-ME concentrations than males. (2) The ir-SP concentration decreased significantly from the first to the third tooth extracted, then increased from the third to the fourth tooth. (3) Ir-SP and ir-ME concentrations are positively intercorrelated. The association was highest in the first tooth extracted from controls; surgical extraction decreased the correlation, although it continued to be positive. (4) The concentrations of ir-ME and ir-SP each correlated negatively with the magnitude of the orthodontic force and that correlation was enhanced when the value of the force was log-transformed.