Intraspecific fighting induced by 6-8 weeks of social isolation results in massive release of nerve growth factor (NGF) into the bloodstream of adult male mice. The amount of circulating NGF is highly correlated with the number of fighting episodes. Biological, radioimmunological, immunohistochemical, and ultrastructural studies show that NGF is discharged from the salivary gland into the blood within minutes after fighting and reaches the highest level 3-4 hr later. Adrenergic innervation of the salivary gland or adrenalectomy does not abolish the NGF release. Corticotropic hormones do not induce NGF increase in the blood. Daily administrations of highly purified NGF (3 ,ug per g of body weight) result in a considerable increase in the volume of adrenal glands. These findings are unequivocable evidence for a physiological role of the mouse salivary glands as a major source of blood NGF.Almost 3 decades ago, it was reported that mouse submaxillary salivary glands (SSGs) synthesize and release an extraordinarily large amount of a nerve growth factor (NGF) molecule (1,2) endowed with a potent nerve growth-promoting activity on sympathetic and sensory nerve cells (3)(4)(5)(6). This finding raised the question whether this molecule serves any specific function and, if so, whether it is discharged into the saliva, into the blood, or into both. All the results obtained so far seem to favor the hypothesis that this salivary product, the NGF, is routed from the gland secretory ducts into the saliva, where it plays some still uncertain function (7,8). This hypothesis springs from the almost generally agreed upon observation that the injection ofpharmacological agents shows NGF not to be present in mouse blood, whereas it is present in large amounts in the saliva (9-11). Structural, ultrastructural, and immunohistochemical studies have also favored this hypothesis and shown that the granular products are localized in the apical part of the cell lining in the gland tubular portion where NGF has been identified (11, 12). However, results from this laboratory provided convincing evidence that, upon pharmacological stimulation (13, 14), NGF from SSG can also be found in the bloodstream.A recent report that aggressive behavior results in the release into the blood of a large amount of another biologically active protein synthesized in the gland tubular portion, renin (15), prompted us to investigate the effects of aggressiveness induced by social isolation on NGF content in the blood. The present study was aimed at the investigation of this problem and, more importantly, at the possible functional significance of such a large amount of NGF synthesis and release by mouse salivary glands. The results of these studies showed that aggressive behavior induces massive NGF discharge and that the adrenal gland might be a primary target for this endogenous NGF release. MATERIALS AND METHODSBehavioral Studies. Male mice of a Swiss-derived (CD-1) strain (25-27 g), purchased from Charles River Italy, were used. They were either in...