The impact of L-arginine on cannabinoid 1 receptor expression, neuronal excitability, and synaptic transmission in the hypothalamus of young rats

Abstract

Interactions between nitric oxide (NO) and endogenous cannabinoids (eCBs) have been reported in various brain regions including dorsomedial hypothalamus (DMH). Such interactions include that eCBs diminish NO signaling and NO is required for eCB signaling. I aimed to determine if the requirement of NO for eCB signaling is mediated by CB1R expression. To determine the effects of NO on CB1R expression, young male Sprague Dawley rats were divided into two groups: (1) acute robust exposure to L-arginine (100mg/kg) by intraperitoneal injection or vehicle (saline) injection and (2) prolonged modest exposure to L-arginine by supplemented water (2.25%) and control distilled water for control. It was determined by RT-PCR that the hypothalamus of animals receiving an acute exposure to L-arginine showed a significant increase in CB1R mRNA, but not the cortex or hippocampus. Prolonged treatment with L-arginine water did not cause a significant change in CB1R expression. Results from RT-PCR experiments lead to obtaining electrophysiological recordings from the DMH, a region known to exhibit interactions between NO and eCBs. Electrophysiological recordings were used to determine the effects of L-arginine on neuronal excitability and synaptic transmission at glutamate synapses. To do this, animals received the same acute and robust exposure to L-arginine that was found to increase CB1R mRNA in the hypothalamus. Whole cell recordings from DMH neurons were performed to determine the effect of L-arginine on neuronal excitability and the ability of glutamate synapses to undergo long-term changes in synaptic strength in response to high frequency stimulation (HFS). It was found that glutamate synapses from animals receiving Larginine injections were less excitable than those from vehicle-treated animals. In animals receiving a vehicle treatment, there was a short-term potentiation of glutamate signaling observed in the first five-minutes after HFS. This short-term potentiation was abolished in animals which received L-arginine. No long-term changes in synaptic strength were observed with either treatment. These findings suggest that L-arginine may increase CB1R expression in the DMH, causing a decrease in neuronal excitability as well as masking short-term potentiation post-HFS at glutamate synapses. Future research is required to confirm if the L-arginine effects on neuronal excitability and synaptic transmission are CB1R mediated.