ULTRASTRUCTURAL-STUDY OF PHENYLETHANOLAMINE-N-METHYLTRANSFERASE, CORTICOTROPIN-RELEASING FACTOR AND NEUROTENSIN-IMMUNOREACTIVE NEURONS IN THE EXTERNAL CUNEATE NUCLEUS OF THE GERBIL

The present study examined the existence of catecholamine-, corticotro pin-releasing factor (CRF)- and neurotensin (NT)-containing neurons in the external cuneate nucleus (ECN) of the gerbil using single label p re-embedding immunocytochemistry in an attempt to shed light on the in creasing evidence for autonomic involvement of the ECN. Peroxidase imm unoreactivity of phenylethanolamine-N-methyltransferase (PNMT), CRF or NT was identified in the heterogeneous population of the ECN neurons characterized by a deeply infolded nucleus. The label was localized in their somata, dendrites, myelinated axons and axon terminals. The imm unolabelled dendrites were contacted by spherical (S) and flattened (F ) types of presynaptic boutons containing spherical and flattened syna ptic vesicles, respectively. The PNMT-labelled dendrites, however, wer e postsynaptic to an additional type of axon terminals containing pleo morphic (P) synaptic vesicles. Among the immunoreactive axon terminals , the PNMT-labelled boutons consisted of two types: S and F; in the CR F- and NT-labelled axon terminals, only the S type was observed. The c atecholamine-containing ECN neurons differed from the CRF- and NT-immu noreactive neurons in their synaptic organization. The latter two were considered to be of the same cell population because of their similar ities in ultrastructural features and synaptic relations. In view of a high frequency (48% for PNMT, 50% for CRF and 46% for NT) of the F-ty ped boutons associated with the three categories of immunolabelled neu rons in the ECN, it is possible that they are under considerable inhib itory control. The presence of catecholamine, CRF and NT in the ECN su ggests that the nucleus may be involved in the integration of proprioc eption-, exercise- or stress-evoked autonomic responses.