The peptide substances P (SP) and somatostatin (SOM) are present in small-diameter neurones of dorsal root ganglia (DRG) and in small-diameter fibres that project to the spinal cord dorsal horn. It is not known whether SP or SOM coexist with other transmitter molecules but, since both peptides can be released from sensory neurones and both can alter neuronal firing rates in the dorsal horn, it seems likely that they are involved in some way in synaptic transmission in the spinal cord. SP has generally excitatory effects in the spinal cord whereas SOM exerts inhibitory effects, and it is not at all clear which subclass of nociceptive afferents contain SP or SOM. Mudge, Leeman & Fischbach (1979) have studied sensory neurones derived from chick DRG grown in dissociated cell culture. When the neurones are grown in the absence of non-neuronal cells, they contain SP but relatively little SOM. The amount of SOM produced by these neurones is greatly increased (c. 50-fold) when the neurones are grown together with ganglionic non-neuronal cells or with medium ‘conditioned’ by incubation with such cells. The increase in SOM content is not accompanied by an increase in SP content or a detectable change in neuronal survival. The DRG non-neuronal cells consist of two major cell types--glial cells and fibroblasts. Indirect evidence suggests that glial cells rather then fibroblasts are responsible for the increased production of SOM by the neurones. Raff and his colleagues (Brockes, Fields & Raff, 1979) have used immunological techniques to obtain purified cultures of these two cell types. Work currently in progress is aimed at extending the above observations to the rat and firmly establishing whether glial cells can indeed influence SOM production in sensory neurones.

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