Neurons with Multiple Messengers with Special Reference to Neuroendocrine Systems

Abstract

This chapter presents the neurons with multiple messengers with special reference to neuroendocrine systems. Neurons of different types in the periphery and in the central nervous system, including neurosecretory cells, produce, store, and perhaps release more than one messenger molecule. The coexisting messengers in primitive neurons are stored in the same vesicles, as they are in mammalian endocrine cells at present. With the demand for faster communication, new types of vesicles, small synaptic vesicles, developed storing and releasing exclusively classical transmitters, being present in addition to the larger vesicles storing both classical transmitter and peptide(s). Interestingly, the neurosecretory cells, representing an intermediate between endocrine cells and neurons, contain a higher proportion of large dense core vesicles than neurons releasing their messengers at more or less well defined synapses. Studies in the peripheral nervous system suggest that the classical transmitters and peptides are, indeed, co-released and may interact in a cooperative way on effector cells. Therefore, interaction between different messengers released from the same nerve endings may be of several types and may, in a general sense, provide mechanisms for differential responses and for increasing the amount of information transferred at synapses. Multi-messenger transmission may represent a principle for increasing capacity for information transfer in the nervous system, a capacity which already appears enormous when considering just the number of neurons and their nerve endings in the mammalian nervous system. At present, the importance of peptides is in many cases difficult to evaluate, and it cannot be excluded that their role is considerably less significant than that of the classical transmitters.