Principles for the Hormone Regulation of Wiring Transmission and Volume Transmission in the Central Nervous System

Abstract

We have recently suggested the existence in the central nervous system of two types of electrochemical transmission, namely wiring transmission (WT) and volume transmission (VT) (see Agnati et al. 1986a, b). The concepts are summarized in Tables 1 and 2. VT is a humoral type of chemical transmission. However, it consists not only of humoral and paracrine signals, diffusing in the extracellular fluid to reach the appropriate receptors, but also of electrotonic signals, which also operate in the extracellular fluid. In fact, the extracellular space of the brain constitutes a restricted microenvironment. Thus, ion fluxes across cellular membranes can induce substantial changes in the ion composition. These ionic fluctuations in the extracellular fluid and the ionic fluxes from sources to sinks may represent signals for communication between neural groups (Nicholson 1980). In Table 1 the possible role of glia and neurons in WT and VT is summarized. In VT the glial cells control the extracellular fluid ion composition and the shaping of the extracellular fluid pathways (i.e., the communication channels between neural groups) for signal diffusion as well as the release, uptake and metabolism of humoral and paracrine signals. With regard to the function of neurons in VT they represent the location of sources and sinks for electrotonic signals and the sites of release and recognition of humoral and paracrine signals. From a biochemical standpoint the neurons control the sources and sinks of electrotonic signals and are involved in the uptake, release and metabolism of humoral and paracrine signals. When we focus our attention on chemical signals in WT and VT it is possible to recognize some main differential features. Thus, as seen in Table 2, the VT is characterized by low speed and long-term action, a high degree of divergence and plasticity and low safety of the transmission process. WT is the classical type of transmission which is neuron-linked and operates with high speed and safety and short-term actions, the divergency and plasticity being low. It seems clear that the integrative capability of the central and peripheral nervous system is increased by the presence of VT, which is not submitted to neuroanatomical constraints and may affect the computing charateristics of the neuronal networks.