Using quantitative in‐situ hybridization, this study monitored diurnal changes in the abundance of the gene transcripts of two corticotropin‐releasing peptides, arginine vasotocin (AVT) and isotocin in hypothalamic neurones, and of pro‐opiomelanocortin (POMC) mRNA in the pituitary of the rainbow trout (Oncorhynchus mykiss). A significant diurnal pattern of gene expression was only displayed in the hypothalamus by the parvocellular AVT neurones of the preoptic nucleus. Abundance of AVT mRNA in these neurones was low at lights on (06.00 h), increased during the morning to reach a plateau of peak values between 14.00 h and 22.00 h, and then declined during the dark phase. This pattern was the inverse of that shown by plasma cortisol values. Changes in AVT transcript abundance are also considered in terms of the reported diurnal change in circulating AVT concentration. Pituitary and hypothalamic AVT peptide content did not change. Transcripts of both POMC genes (POMC‐A and POMC‐B) were monitored in pituitary corticotropes and melanotropes. Only POMC‐A mRNA was detected in corticotropes where it showed no diurnal change in abundance. Transcripts of both POMC genes were found in the melanotropes, although, judging from autoradiographic intensity, POMC‐A mRNA predominated. Both genes showed diurnal differences in their transcription with POMC‐A mRNA showing peak values at 10.00 h and a nadir at 02.00 h, while POMC‐B mRNA showed an inverse pattern. The results indicate that the two POMC genes can be independently regulated.
