Colocalization of cannabinoid receptor 1 with somatostatin and neuronal nitric oxide synthase in rat brain hippocampus

- The majority of CB1 receptor positive neurons coexpress SST in hippocampus.
- Colocalization between CB1 receptor and nNOS is frequently observed.
- nNOS is selectively expressed in SST positive interneurons.
- Majority of nNOS positive neurons express SST.

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in different parts of the brain and functions as a neurotransmitter and neuromodulator. In the central nervous system (CNS), SST inhibits Ca2+ influx and regulates neuronal excitability in the hippocampus, the brain region which plays a major role in seizure, as well as cognitive and memory function. Much like SST, cannabinoid receptor 1 (CB1 receptor) is also widely distributed in the CNS, associated with memory function ad exerts inhibitory effects on seizure. It is unknown whether overlapping functional activities of SST and CB1 receptor are also associated with coexpression in the hippocampus. In the present study, we determined the colocalization between SST and CB1 receptor in adult rat brain hippocampus. In the CNS, the majority of SST positive interneurons coexpress neuronal nitric oxide synthase (nNOS). Accordingly, colocalization studies were also performed to determine whether nNOS positive neurons display comparable colocalization with CB1 receptor. The findings suggested that SST and nNOS are expressed in most interneurons whereas CB1 receptor is present in both interneurons and projection neurons in hippocampal regions. The distinct neuronal populations either expressing CB1 receptor, SST and nNOS alone or colocalization were observed in a region specific manner. Taken together, the observations described here anticipate the possibility of crosstalk between somatostatin subtypes and CB1 receptor in regulation of physiological activities in the hippocampus.