Co-expression of somatostatin SSTR-3 and SSTR-4 receptor messenger RNAs in the rat brain

In situ hybridization histochemistry was used to study the distribution and possible co-expression of the messenger RNA of the somatostatin receptor subtypes SSTR-4 and SSTR-3 in rat brain. Our results demonstrate that SSTR-3 messenger RNA is widely expressed within the rat brain, while expression of SSTR-4 messenger RNA is restricted to the telencephalon, diencephalon and granular layer of the cerebellum. It is also shown that single neurons can co-express both SSTR-4 and SSTR-3 receptor messenger RNAs. The highest density of SSTR-4 messenger RNA was found in the pyramidal cell layer of the hippocampus, especially in the CA1 and CA2 areas, anterior olfactory nuclei, amygdala, and in layers IV and VI of the cerebral cortex. SSTR-3 messenger RNA displayed a homogeneous distribution in the cerebral cortex and was expressed in the olfactory bulb, pyramidal cells of the hippocampus, granular cell layer of the dentate gyrus, motor and sensory metencephalic nuclei, and the granular and Purkinje cell layers of the cerebellum. Whether SSTR-3 and SSTR-4 messenger RNA can be expressed by the same cell was assessed by the simultaneous use of digoxigenin (SSTR-3)- and isotopic (SSTR-4)-labelled oligoprobes. Co-expression of SSTR-3 and SSTR-4 messenger RNAs was found in neurons of the CA1 and CA2 regions of the hippocampus, in the subiculum and in layer IV of the cerebral cortex. The expression of two receptor subtypes of the same neurotransmitter in a single cell, if confirmed functionally, raises questions about the consequences of the simultaneous activation of these different receptors.