Changes in substance P concentrations after protein synthesis inhibition provide an index of substance P utilization.

In all but one brain region sampled, substance P-like immunoreactivity (SPLI) was unchanged 4 h after a dose of cycloheximide that produced a near-total inhibition of rat brain protein synthesis. This suggests that brain substance P (SP) utilization is slow under basal conditions. The administration of the dopamine antagonist haloperidol to cycloheximide-pretreated rats apparently accelerated striatonigral SP utilization, as evidenced by large depletions in SPLI in the substantia nigra and striatum. Measuring the decline of SPLI a short time after protein synthesis inhibition may provide an index of brain SP utilization.