Neuropeptide kisspeptin has been suggested to be an essential central regulator 
of reproduction in response to changes in serum gonadal steroid concentrations. 
However, in spite of wide kisspeptin receptor distribution in the brain, 
especially in the preoptic area and hypothalamus, the research focus has mostly 
been confined to the kisspeptin regulation on GnRH neurons. Here, by using 
medaka whose kisspeptin (kiss1) neurons have been clearly demonstrated to be 
regulated by sex steroids, we analyzed the anatomical distribution of kisspeptin 
receptors Gpr54-1 and Gpr54-2. Because the both receptors were shown to be 
activated by kisspeptins (Kiss1 and Kiss2), we analyzed the anatomical 
distribution of the both receptors by in situ hybridization. They were mainly 
expressed in the ventral telencephalon, preoptic area, and hypothalamus, which 
have been suggested to be involved in homeostatic functions including 
reproduction. First, we found gpr54-2 mRNA expression in nucleus preopticus pars 
magnocellularis and demonstrated that vasotocin and isotocin (Vasopressin and 
Oxytocin ortholog, respectively) neurons express gpr54-2 by dual in situ 
hybridization. Given that kisspeptin administration increases serum oxytocin and 
vasopressin concentration in mammals, the present finding are likely to be 
vertebrate-wide phenomenon, although direct regulation has not yet been 
demonstrated in mammals. We then analyzed co-expression of kisspeptin receptors 
in three types of GnRH neurons. It was clearly demonstrated that 
gpr54-expressing cells were located adjacent to GnRH1 neurons, although they 
were not GnRH1 neurons themselves. In contrast, there was no gpr54-expressing 
cell in the vicinities of neuromodulatory GnRH2 or GnRH3 neurons. From these 
results, we suggest that medaka kisspeptin neurons directly regulate some 
behavioral and neuroendocrine functions via vasotocin/isotocin neurons, whereas 
they do not regulate hypophysiotropic GnRH1 neurons at least in a direct manner. 
Thus, direct kisspeptin regulation of GnRH1 neurons proposed in mammals may not 
be the universal feature of vertebrate kisspeptin system in general.