Neuropeptides II: function

As noted in Chapter 11, the human genome contains about 90 genes that encode neuropeptide precursors (pre-propeptides). The biologically active neuropeptide products of the prepropeptides number at least 100, and there are likely to be many more waiting to be discovered. Neuropeptides are synthesized in a wide variety of neurons in many brain regions and more often than not are co-localized and co-released with classical neurotransmitters. Should neuropeptides therefore be categorized as neurotransmitters? An alternative description is that of neuromodulator, since in many instances they modify the neural effects of classical neurotransmitters. A good example of this is shown in Figure 11.2, where co-release of a neuropeptide totally modifies the influence of the co-localized neurotransmitter on a postsynaptic neuron. This chapter will illustrate the neurotransmitter and neuromodulator actions of neuropeptides on the neuroendocrine system, the autonomic nervous system (ANS) and the central nervous system. First, however, we will explore whether neuropeptides are best described as neurotransmitters or neuromodulators, or both.