Abstract
In a previous study we have shown that in ovo administration of growth hormone-releasing hormone (GHRH) influences catecholaminergic neuronal expression as assessed by tyrosine hydroxylase activity ( Dev. Brain Res., 49 (1989) 275–280). In this study we examined the effects of GHRH and somatostatin (SRIF) on cholinergic neuronal neurotransmitter expression both in ovo and in vitro. Chick embryos were administered GHRH or SRIF in ovo via the air sac on embryonic days 1, 3, 5 and 7, sacrificed at day 8 and the activity of choline acetyltransferase (ChAT) was assayed in whole brain homogenates. ChAT activity was significantly higher in peptide-treated embryos as compared to controls. Similar results were obtained when GHRH or SRIF (50 ng/50 μl) was administered in a single dose at day 2 or 3; ChAT activity was significantly increased in peptide-treated versus control embryos. In contrast, embryos treated with GHRH or SRIF on day 1 or 5 or with growth hormone (100 ng/50 μl) on day 3 of incubation showed no difference in ChAT activity as compared to controls. More direct effects of GHRH and SRIF were tested in neuron-enriched cultures derived from 3- (E3) or 6-day-old (E6) chick embryos. Cultures were grown in either serum-supplemented or serum-free medium for 6 days in the presence of GHRH or SRIF concentrations ranging from 0.01 to 100 nM. As observed in ovo, ChAT activity was increased in E3 cultures treated with peptides. In addition, the composition of the culture medium influenced the response to peptide treatment. In the presence of serum, GHRH produced a more pronounced effect on ChAT activity, whereas, the response to SRIF treatment was more pronounced in the absence of serum. In contrast to E3 cultures, enzyme activity in E6 cultures was unaffected by GHRH or SRIF. These data suggest that GHRH and SRIF possess important neurotrophic properties with respect to cholinergic development. These cholinotrophic actions are exerted during early embryonic development and only during a well-defined critical period.
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