In vitro percutaneous penetration of topically applied capsaicin in relation to in vivo sensation responses.

Capsaicin, the primary pungent element in several spices, elicits a variety of physiological effects which are due to neurogenic responses. The aim of the study was to explore the in vivo sensation responses of capsaicin and to compare the results with the in vitro percutaneous absorption of the substance. The overall objectives were to determining an in vitro-in vivo correlation for capsaicin. Capsaicin was applied in a chamber on the volar forearm of twelve volunteers and in a flow-through diffusion chamber on excised human epidermal membranes. Topical administration of capsaicin produced a complex cutaneous sensation that changed in intensity and quality as a function of time and was characterized by sting, prick, burn and pain. Percutaneous steady-state penetrations of capsaicin with a receptor fluid consisting either of 4% bovine serum albumin in phosphate buffered saline or 50% ethanol in water were 28.2+/-2.7 and 29.6+/-2.9 microg/cm(2) per h, respectively. The corresponding cumulative penetrated amounts of capsaicin after 30 min were 14. 7+/-1.7 and 19.2+/-2.1 microg/cm(2), respectively. The present investigation indicates that there is a good correlation between in vivo physiological responses and in vitro percutaneous penetration of topically applied capsaicin.

Regional glucose metabolism in the rabbit brain in control and TRH-treated animals.

The local cerebral metabolism in urethane anaesthetized control and TRH-treated rabbits was studied with the [14C]2-deoxyglucose method. In the controls, the glucose use was found to be highest in regions known to have a high blood flow and low in regions with low flow. The glucose consumption was, calculated using the constants found by Kennedy et al. in monkeys, 23.5 +/- 6.0 mumol 100 g-1 min-1 in parietal cortex. The TRH was infused at a dose of 0.06 mg kg-1 min-1 which is known to cause vasodilation in the brain. No marked influence of the peptide on the glucose use was detected. It was concluded that the previously reported cerebral vasodilation caused by TRH is not due to an increase in cerebral metabolism.