The Role of Somatostatin‐Expressing Cells in Opioid‐Induced Respiratory Depression

Abstract

Introduction Opioids have the ability to cause respiratory depression, a severe decline in respiratory rate that can be lethal with overdose. While key brainstem respiratory regions have been shown to play a role in respiratory depression by opioids, the cells involved remain unknown. One region of interest is the preBötzinger Complex, a site that mediates respiratory depression and contains a subgroup of somatostatin (SST)-expressing cells that are required to maintain normal breathing. We aimed to determine the role of SST-expressing cells in respiratory depression by opioids. We hypothesized that deletion of mu-opioid receptors (MORs) in SST-expressing cells will prevent opioid-induced respiratory depression. Methods We developed transgenic knockout mice that lack MORs in SST-expressing cells (SST-MOR-/-) using Cre-lox recombination. SST-Cre+/+ mice (Ssttm2.1(cre)Zjh/J) were bred with Oprm1fl/fl mice (Oprm1tm1.1Cgrf/KffJ) to produce SST-MOR-/- mice. We used in situ hybridization to determine whether MOR mRNAs are co-expressed with SST mRNA in the brainstem of control (wild-type, SST-MOR+/+) and SST-MOR-/- mice. To determine the effect of removing MORs from SST-expressing cells, respiratory depression by intraperitoneal injection of fentanyl (0.3mg/kg) was quantified in control (wild-type, SST-MOR+/+) and SST-MOR-/- mice. Respiratory rate was recorded using whole-body plethysmography. Results MORs were expressed in several brainstem regions regulating breathing such as the preBötzinger Complex, the Bötzinger Complex, the nucleus ambiguus, the Kölliker-Fuse nucleus, the locus coeruleus and the nucleus tractus solitarius, while SST and MORs were co-expressed primarily in the preBötzinger Complex. Knockout of MORs in SST-expressing cells was confirmed by microscopy. Intraperitoneal injection of fentanyl (0.3mg/kg) induced a similar, significant respiratory rate depression in both wild-type (p<0.001, n=14) and SST-MOR-/- (p<0.001, n=12) mice. Conclusions SST and MORs are co-expressed in the preBötzinger Complex while MORs are expressed in other key brainstem respiratory sites. Preliminary data suggests that MORs in SST-expressing cells are not required for fentanyl-induced respiratory depression. Our previous work showed that arousal state can affect the severity of respiratory depression by opioids. For this reason, we are currently investigating the impact of arousal state (active versus inactive) on opioid-induced respiratory depression in control and SST-MOR-/- mice. We are also looking at the role of SST-expressing cells in respiratory depression by other opioid drugs such as morphine.