C-FOS AND RELATED IMMEDIATE-EARLY GENE-PRODUCTS AS MARKERS OF ACTIVITY IN NEUROENDOCRINE SYSTEMS

Expression of c-Fos, or other immediate early gene products, by indivi dual neurons can be used as a marker of cell activation, making staini ng of these proteins an extremely useful technique for functional anat omical mapping of neuroendocrine systems. Because these proteins are l ocated in the nucleus, identification of the phenotype of the activate d neuron using substances located within the cytoplasm can be accompli shed with standard double-labeling immunocytochemical techniques. Alth ough it is clear that neurons have the capacity to express a number of immediate early gene products, what remains to be established is whet her there is a different pattern of expression following various stimu li. In our studies, we focus primarily on expression of one immediate early gene product, the c-Fos protein. We also include some experiment s using expression of other members of the Fos family and Jun proteins as markers for neuronal activation. Our studies describe uses of c-Fo s expression in both parvocellular and magnocellular hypothalamic syst ems to address the following issues: (a) identification of neuroendocr ine cells activated by specific treatments and conditions, (b) ascerta inment of functional differences in subpopulations activated by specif ic stimuli, (c) evaluation of neuronal activity in complex areas conta ining multiple neuroendocrine systems, (d) identification of other bra in areas activated in conjunction with neuroendocrine systems followin g specific stimuli, (e) analysis of connectivity of activated neuroend ocrine systems with other parts of the brain, and (f) identification o f stimuli that decrease neuronal activity. The neuroendocrine systems studied include those that secrete arginine vasopressin (AVP), oxytoci n (OT), corticotropin-releasing hormone (CRH), luteinizing hormone-rel easing hormone (LHRH), and dopamine (DA). The use of c-Fos expression has permitted functional neuroanatomical mapping of these systems in r esponse to specific stimuli such as cholecystokinin (CCK), hyperosmola lity, and volume depletion, or during various physiological states suc h as the proestrous ovulatory luteinizing hormone (LH) surge and lacta tion. Although the use of c-Fos as a marker of neuronal activation wil l continue to be an extremely powerful technique, future studies will also be directed at relating immediate early gene expression to change s in neuroendocrine gene expression. To this end, we have shown that b oth c-Fos and c-Jun are expressed in neuroendocrine neurons in respons e to a number of stimuli, setting the stage for potential regulatory d rive to genes containing AP-1 binding sites. In addition, by using dou ble-label in situ hybridization techniques, it should be possible to c ompare gene expression in activated neurons (as determined by the pres ence of c-Fos mRNA) and unactivated neurons, thus permitting immediate early gene expression to move beyond its present use as a marker of n euronal activation to an assessment of its role in the promotion of sp ecific neuropeptide gene expression.