The effect of nitric oxide on endocannibinoid signalling at glutamate synapses in the rat dorsomedial hypothalamus

Abstract

Endocannabinoids (eCBs) and nitric oxide (NO) are both retrograde neurotransmitters that are made in the postsynaptic cell in response to increases in intracellular calcium. eCBs target type I cannabinoid receptors (CB1Rs), located on the presynaptic membrane, while NO binds soluble guanylate cyclase (sGC) inside the presynaptic cell. Both neurotransmitters can act presynaptically to regulate GABA and glutamate transmission. Previous studies have shown an interaction between NO and eCB signaling. NO prevented eCB-mediated decrease in glutamate transmission in the dorsomedial nucleus of the hypothalamus (DMH). Furthermore, another study within the DMH found that NO was required for eCB-mediated decrease in GABA transmission. There is little research, however, on the mechanism behind the interactions between eCBs and NO. The DMH has been a major focus for research due to its importance in appetite regulation. We aimed to determine how NO affects eCB signaling at glutamatergic synapses in the rat DMH. We hypothesized that NO affects eCB-mediated decrease in glutamate transmission through an NMDA, and cGMP, -dependent pathway. To test our hypothesis, male Sprague Dawley rats were used as subjects, and whole-cell electrophysiological recordings were taken at glutamate synapses in live DMH neurons. Our results show that NO disrupts eCB-mediated depression through an NMDA receptor-dependent pathway. Additionally, we further targeted the NO pathway and concluded that NO is likely affecting eCB signaling from the postsynaptic cell. We also performed an experiment targeting CB1R activation by an agonist. Previous data showed that activation of CB1Rs by an agonist significantly decreased glutamate release. We repeated this experiment in the presence of L-arginine, a NO precursor, and observed that the agonist was still able to decrease glutamate signaling. Finally, we wanted to determine if NO is affecting short-term eCB signaling. Previous research has suggested that NO does not affect short-term synaptic plasticity at glutamate synapses in the hippocampus. Our data was consistent with this as we did not see a change in short-term glutamate transmission. Overall, figuring out how these two retrograde neurotransmitters interact to affect synaptic transmission can extend well beyond appetite regulation as these neurotransmitters are ubiquitously produced at synapses in various parts of the brain.