CORTICOTROPIN-RELEASING FACTOR-CONTAINING AXON TERMINALS SYNAPSE ONTOCATECHOLAMINE DENDRITES AND MAY PRESYNAPTICALLY MODULATE OTHER AFFERENTS IN THE ROSTRAL POLE OF THE NUCLEUS LOCUS-COERULEUS IN THE RAT-BRAIN

Physiological and immunohistochemical studies have suggested that cort icotropin-releasing factor (CRF), the hypophysiotropic peptide that in itiates endocrine responses to stress, may serve as a neurotransmitter to activate noradrenergic neurons in the nucleus locus coeruleus (LC) . We combined immunoperoxidase labeling for CRF and immunogold-silver localization of the catecholamine-synthesizing enzyme tyrosine hydroxy lase (TH) in single sections through the rat LC to determine potential substrates for interactions between these two transmitters. Light mic roscopic analysis indicated that CRF processes are dense and highly va ricose in the rostral LC region in the vicinity of noradrenergic dendr ites. Electron microscopy of this rostral region revealed that immunop eroxidase labeling for CRF was mainly restricted to axons and axon ter minals and was rarely seen in somata or dendrites. Axon terminals cont aining CRF immunoreactivity varied in size, content of synaptic vesicl es, and formation of synaptic specializations. The postsynaptic target s of the CRF-labeled axon terminals consisted of both TH-labeled dendr ites and dendrites lacking detectable TH-immunoreactivity. Of 113 CRF- immunoreactive axon terminals, approximately 70% were in direct contac t with TH-labeled and unlabeled dendrites. Of the CRF-labeled axon ter minals forming synapses with TH-labeled and unlabeled dendrites, they were either of the asymmetric (excitatory type; 19%) or symmetric (inh ibitory type; 11%) variety or did not form identifiable contacts in th e plane of section analyzed. Unlabeled axon terminals and glial proces ses were also commonly located adjacent to the plasma membranes of CRF -labeled axon terminals. These results provide the first direct ultras tructural evidence that axon terminals containing CRF-immunoreactivity 1) directly contact catecholamine-containing dendrites within the ros tral pole of the LC, 2) may presynaptically modulate other afferents, and 3) are often enveloped by astrocytic processes. (C) 1996 Wiley-Lis s, Inc.