IMMUNOLOCALIZATION OF TRIPEPTIDYL PEPTIDASE II, A CHOLECYSTOKININ-INACTIVATING ENZYME, IN RAT BRAIN P. FACCHINETTI,* C. ROSE,† PH. ROSTAING,‡ A. TRILLER‡ and J.-C. SCHWARTZ†§ *Laboratoire de Physiologie, Faculte ´ de Pharmacie, 4 Avenue de l’Observatoire, 75006 Paris, France †Unite ´ de Neurobiologie et Pharmacologie (U.109) de l’INSERM, Centre Paul Broca, 2ter rue d’Ale ´sia, 75014 Paris, France ‡Laboratoire de Biologie Cellulaire de la Synapse, INSERM CJF 9410, Ecole Normale Supe ´rieure, 46 rue d’Ulm, 75005 Paris, France Abstract––Tripeptidyl peptidase II (EC 3.4.14.10) is a serine peptidase apparently involved in the inactivation of cholecystokinin octapeptide [Rose C. et al. (1996) Nature 380, 403–409]. We have compared its distribution with that of cholecystokinin in rat brain, using a polyclonal antibody raised against a highly purified preparation for immunohistochemistry at the photon and electron microscope levels. Tripeptidyl peptidase II-like immunoreactivity was mostly detected in neurons, and also in ependymal cells and choroid plexuses, localizations consistent with a possible participation of the peptidase in the inactivation of cholecystokinin circulating in the cerebrospinal fluid. Immunoreactivity was mostly detected in cell bodies, large processes and, to a lesser extent, axons of various neuronal populations. Their localization, relative to that of cholecystokinin terminals, appears to define three distinct situations. The first corresponds to neurons with high immunoreactivity in areas containing cholecystokinin terminals, as in the cerebral cortex or hippocampal formation, where pyramidal cell bodies and processes surrounded by cholecystokinin axons were immunoreactive. A similar situation was encountered in many other areas, namely along the pathways through which cholecystokinin controls satiety, i.e. in sensory vagal neurons, the nucleus tractus solitarius and hypothalamic nuclei. The second situation corresponds to cholecystokinin neuronal populations containing tripeptidyl peptidase II-like immunoreactivity, as in neurons of the supraoptic or paraventricular nuclei, axons in the median eminence or nigral neurons. In both situations, localization of tripeptidyl peptidase II-like immunoreactivity is consistent with a role in cholecystokinin inactivation. The third situation corresponds to areas with mismatches, such as the cerebellum, a region devoid of cholecystokinin, but in which Purkinje cells displayed high tripeptidyl peptidase II-like immunoreactivity, possibly related to a role in the inactivation of neuropeptides other than cholecystokinin. Also, some areas with cholecystokinin terminals, e.g., the molecular layer of the cerebral cortex, were devoid of tripeptidyl peptidase II-like immunoreactivity, suggesting that processes other than cleavage by tripeptidyl peptidase II may be involved in cholecysto- kinin inactivation. Tripeptidyl peptidase II-like immunoreactivity was also detected at the ultrastructural level in the cerebral cortex and hypothalamus using either immunoperoxidase or silver-enhanced immunogold detection. It was mainly associated with the cytoplasm of neuronal somata and dendrites, often in the vicinity of reticulum cisternae, Golgi apparatus or vesicles, and with the inner side of the dendritic plasma membrane. Hence, whereas a fraction of tripeptidyl peptidase II-like immunoreactivity localization at the cellular level is consistent with its alleged function in cholecystokinin octapeptide inactivation, its association with the outside plasma membrane of neurons remains to be confirmed.  1998 IBRO. Published by Elsevier Science Ltd. Key words: EC 3.4.14.10, cholecystokinin octapeptide, electron microscope immunolocalization, light microscope immunolocalization, satiety, serine proteases. In contrast with the mechanisms responsible for turning off the signals generated by amino acids and monoamines, of which the inactivating enzymes or transporter systems are well identified, those respon- sible for turning off neuropeptide signals are, in most cases, ill defined. One possibility is that the generally long-lived peptidergic signals are turned off by diffusion and/or hydrolysis of the neurotransmit- ter by a series of non-specific peptidases. 28 In some cases, however, well-defined peptidases were found to play a critical role in the inactivation of the neuropeptide due to a combination of adequate localization and catalytic properties. The first and still best example is that of enkephalins, 47 whose physiological inactivation is attributable to the action of enkephalinase (neprilysin, EC 3.4.24.11) 23 and §To whom correspondence should be addressed. Abbreviations: BSA, bovine serum albumin; CCK, chole- cystokinin; CCK-8, cholecystokinin octapeptide; LI, -like immunoreactivity; PBS, phosphate-buffered saline; TBS, Tris-buffered saline; TPPII, tripeptidyl peptidase II. Pergamon Neuroscience Vol. 88, No. 4, pp. 1225–1240, 1998 Copyright  1998 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0306–4522/99 $19.00+0.00 PII: S0306-4522(98)00257-7 1225