Evidence for a small functional pool of serotonin in neurohumoral transmission.

"Scottie Cramp" is a genetic neurologic disease which occurs in the Scottish terrier breed of dogs. Decreasing central nervous system (CNS) concentrations of serotonin (5-HT) via para-chlorophenylalanine (pCPA) will profoundly increase the severity of the disease although the basic defect does not involve serotonergic neurons. The purpose of this study was to attempt to correlate the effect of p-CPA on the clinical signs of the disease with the alteration in serotonin synthesis and concentration in the CNS. Synthesis was estimated by following the rise in cisternal cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA) concentrations with time following probenecid injection. Concentration of serotonin in the CNS was estimated by measuring cisternal CSF 5-HIAA concentrations. The results suggest that serotonin may be synthesized in excess of neuronal transmitter needs and that the estimation of whole brain turnover rates and concentrations of 5-HT may not yield a true measure of serotonergic neuronal activity.