Figure 8. Model of corazonin neuronal function in Drosophila. Corazonin neurons in both males and females express the receptors for DH44 and DH31 (Johnson et al., 2005) but may also have sex-specific inputs that are currently unknown. It may be that stress status is related either through these inputs or yet unidentified inputs. Corazonin neurons participate either directly or indirectly to regulate locomotor activity, dopamine levels, and triglycerides but exhibit sex-specific differences in the extent and nature of these phenotypes. Arrow size reflects the magnitude of interactions. Additionally, it may be that there are sex-specific targets of corazonin neurons. doi:10.1371/journal.pone.0009141.g008 stress, males continue to court, whereas in contrast, female receptivity is significantly lowered [31]. It could be that corazonin signaling is altering male-specific responses and has novel roles in females, or alternatively corazonin has similar roles in both sexes but the effects of which are compensated by other dimorphic aspects of the circuitry. Notably, our reports here on the functional differences in between the sexes of corazonin neuronal function are supported by a recent anatomical study that identified four corazonin-expressing neurons present only in the abdominal ganglia of adult males [40]. Furthermore, given that the corazonin receptor is ancestrally related to vertebrate GnRH receptors [42] and that GnRH signaling throughout the vertebrates is involved in dimorphic behaviors [e.g., 43], the sexual dimorphisms we observed further support the hypothesis of a functional conserva- tion between corazonin and GnRH signaling. The differences between the actions of corazonin in the different sexes suggest fundamental differences in regulatory elements of the neural circuitry in males and females. In males, the loss of corazonin signaling produced phenotypes similar to those with the silencing of corazonin neurons, and as expected, the activation of corazonin neurons consistently produced opposite phenotypes. This suggests a simple model in males, in which the absolute amount of corazonin regulates stress behaviors and sensitivity. In contrast, the lack of corazonin signaling results in a stress-resistant phenotype, whereas the activation or silencing of these neurons results in varied phenotypes. We speculate that in females the absolute presence of corazonin may help determine stress sensitivity, since ablation of female corazonin neurons leads to changes in multiple stress physiologies and behaviors. We speculate that females are resistant to changes in the levels of corazonin by specific mechanisms regulating the timing of corazonin release in females. In support of this notion, it has been noted that the efficacy of the elements employed here (ork and NaChBac) to manipulate excitability are in part dependent Dimorphic Stress Hormones upon fundamental aspects of the circuitry targeted. Specifically, these elements have been determined to be more effective in circuits in which temporal coding is not the critical component [22–24,44–45]. If this is the case with corazonin neurons, this would suggest that there are fundamental differences in corazonin circuits between males and females, and may explain the differences we have observed utilizing this experimental strategy. Alternatively, it could be that there are some compensatory mechanisms of downstream components that may explain some of the different effects of altered corazonin neuronal function in females. However, we note that a significant contribution of the dimorphic aspects is localized to corazonin neurons, suggesting that a major component is likely to be some intrinsic differences in regulation of corazonin secretion. The connections between corazonin and altered dopamine implicate a complex neuroendocrine circuitry that mediates behavioral and physiological responses to stress. Dopamine signaling in Drosophila contributes to many different behavioral and physiological processes that are targets of stress modulation. For instance, dopamine elevates heart rate [46], mediates female receptivity [47], affects general arousal state [48], alters egg laying behaviors [49], and contributes to circadian rhythmicity [50]. These various phenotypes are largely consistent with behavioral and physiological modifications in the general adaptation syndrome proposed by Selye. [1,51]. Dopamine levels increase with the presentation of heat stress, as measured via high performance liquid chromatography (HPLC) on Drosophila heads [33]. In addition, the activity of the rate limiting enzyme in dopamine synthesis, tyrosine dehydroxylase (TH), is altered under several types of heterotypic stresses in Drosophila [31–34]. Previous studies have shown that dopaminergic neuron survival is diminished during oxidative stress [35]. Notably, the behavioral deficits accompanying the loss of dopaminergic neurons can be tempered through application of the dopamine precursor, DOPA [52–53]. Our results showing a strong resistance of animals lacking corazonin neurons to oxidative stress is consistent with the observations of increased dopamine levels in these same animals. Furthermore, dopamine actions are known to be dimorphic in Drosophila, and our results are consistent with previous observations documenting the contributions of dopamine to dimorphic be- haviors [54]. Notably, the connection between dopamine and GnRH signaling in the vertebrates is well-documented [e.g., 55] and lends additional support for functional parallels between mammalian GnRH and insect corazonin signaling. Corazonin may be altering dopamine levels either at the level of synthesis, degradation, or release, the latter may, in part explain the failure to discern differences in dopamine levels in the brain. Alternatively or additionally, altered dopamine levels may be selectively altered only in the periphery via corazonin neuronal manipulations. Future studies that identify the distribution of the corazonin receptor in specific regards to dopaminergic cells may assess the specific mechanisms of corazonin actions on dopamine. Our observation of inhibition of corazonin transcript expression during starvation and osmotic stress suggests that stress reduces corazonin signaling, consistent with previous results employing microarray analysis, which also found a reduction of corazonin transcript under stress [56]. Furthermore, analysis of the ensemble of stress phenotypes also supports a model of stress-induced inhibition of corazonin signaling. We speculate that this inhibition may be caused by signaling through either the DH31 or DH44 peptide transmitters, as corazonin neurons express both of these receptors [9] and in mammals, the homologs of these peptide hormones are both involved in the stress-induced suppression of GnRH release [4]; this observation suggests that this particular PLoS ONE | www.plosone.org 12 February 2010 | Volume 5 | Issue 2 | e9141