The impact of physiological stimuli on the expression of corticotropin-releasing hormone (CRH) and other neuropeptide genes.

The article reviews some of the recent work showing how physiological stimuli act to alter neuropeptide gene expression. It describes how neural and humoral factors activated by physiological stimuli interact with the mechanisms regulating neuropeptide gene expression in neurons with either vascular (neurosecretory) or cellular (centrally directed) synapses. Although the focus will be on corticotropin-releasing hormone (CRH) in the hypothalamic paraventricular nucleus, comparisons will be made between this neurosecretory cell group and others that express this gene. The regulation of neuropeptide genes colocalized in neurons that synthesize CRH is also considered. The review begins with a brief historical introduction, placing peptides in the overall functional perspective of neurosecretory and centrally directed neurons. It then describes studies using in vitro preparations that reveal details of the signal transduction mechanisms responsible for altering the expression of neuropeptide genes. For the CRH gene they are providing the foundations for future work on how physiological stimuli alter mRNA levels in the whole animal. Physiological stimuli provide a very broad range of signals to neuropeptide neurons commensurate with the wide variety of motor responses they initiate. One important humoral signal impacting neuropeptide neurons is plasma corticosterone, and many workers have addressed this aspect of its function. Corticosterone appears capable of interacting with at least two different neuronal mechanisms to regulate CRH mRNA levels: one is clearly seen in paraventricular neurosecretory neurons, where increasing plasma corticosteroid reduces CRH mRNA levels; the other, seen in neurons in the central nucleus of the amygdala, acts to increase them. Since physiological stimuli present a complex mixture of humoral and neural signals to the CNS, integration of these two signal types is a critical aspect of peptide metabolism that requires detailed attention. Studies that are beginning to address this important question are described. Circadian influences play an important role in organizing homeostatic processes, and their influence on CRH gene expression is considered. The viscerosensory-motor integration associated with dehydration offers a useful model for investigating the role of peptides in neuronal function and motor architecture. Much of our work has concentrated on how peptide genes are regulated by alterations to fluid homeostasis, and these studies, along with those of other investigators, are described in this integrative context. Finally, consideration is given to the many studies that have addressed the impact of nonviscerosensory stimulation on neuropeptide gene expression.