Plasticity in expression of neuropeptides

Peptides of different sizes ranging from a few to more than 40 amino acids are widely and abundantly distributed in the nervous system. In many, if not all cases, these peptides have been demonstrated to occur in neurons which also contain a classic transmitter such as a catecholamine, 5-hydroxytryptamine (5HT), acetylcholine or 7-aminobutyric acid (GABA). As judged from immunohistochemical, and more recently in situ hybridization studies, the levels of peptides and their mRNAs differ markedly between various peptide systems. Moreover, various experimental manipulations have been shown to have a strong and rapid effect on peptide expression. This can be particularly well demonstrated using in situ hybridization. For example, brief electrical, preganglionic stimulation in man dramatically increases levels of mRNA for neuropeptide tyrosine (NPY), as well as for tyrosine hydroxylase and dopamine/,¢-hydroxylase in the postsynaptic sympathetic cell bodies (Schalling et al., 1989). In the present brief article we would like to focus on some of the neuronal systems that produce peptides normally at very low levels or at levels which can not be detected with our histochemical techniques. However, after experimental manipulations such as mechanical damage or drug treatment, there is a marked upregulation to detectable levels, both of peptides and of their mRNAs. Here we will describe four systems, striatal neurons in basal ganglia, hypothalamic neurons in the paraventricular nucleus, chromaffin cells in the adrenal gland and primary sensory neurons.