Subcellular distribution of radioimmunoassayable somatostatin in rat brain.

A specific, sensitive and reproducible radioimmunoassay is described for measurement of somatostatin in brain tissue. The sensitivity of the assay is 10 pg/tube and recovery of synthetic somatostatin added to brain homogenates was 95.8 +/- 6.2%. Dilution of tissue extracts from various brain regions showed parallelism in standard curves with labelled somatostatin. Somatostatin is shown to be widespread in the central nervous system with highest concentrations in hypothalamus, particularly the median eminence. Subcellular preparations of medial basal hypothalamus, preoptic area and amygdala indicate that over 70% of somatostatin immunoreactivity is localized to the synaptosome fraction. Recovery of activity in the P2 pellet prior to separation on sucrose gradient is approximately 100%. It is hypothesized that somatostatin, in addition to being released into blood vessels of the median eminence, may also be liberated from nerve terminals in other brain regions.