Transient gain of function of cannabinoid CB1 receptors in the control of frontocortical glucose consumption in a rat model of Type-1 diabetes
Introduction
Marihuana has long been known to modulate carbohydrate metabolism in animal models (de Pasquale et al., 1978) and man (Benowitz et al., 1976). One of the major targets of marihuana’s Δ9-THC in the body is the cannabinoid CB1 receptor (CB1R), the foremost metabotropic receptor of the endocannabinoid system, and is widely expressed in most mammalian organs and tissues, including the CNS, the pancreas, the liver and the skeletal muscle (Katona and Freund, 2012; Solymosi and Köfalvi, 2017). The CB1R is a major regulator of the body’s energy homeostasis, via three major targets: neural circuits, neurohumoral communication and cellular metabolism (Piazza et al., 2017; Ruiz de Azua and Lutz (2019)).
Overactivation of the endocannabinoid system may lead to systemic insulin resistance (Bowles et al., 2015; Jourdan et al., 2017; Sidibeh et al., 2017), which is a primary pathomechanism of diabetes. One theory suggests that the CB1R inhibits insulin receptor signaling via physical complexing, that is, heteromerization (Dalton and Howlett, 2012; Kim et al., 2012). Indeed, we found in the rat nucleus accumbens that the CB1R co-immunoprecipitates with the insulin receptor, and its activation prevents insulin from stimulation of [3H]deoxyglucose uptake (Pinheiro et al., 2016). Interestingly, the interaction between insulin and endocannabinoids is even more intricate. For instance, insulin stimulates endocannabinoid release in the ventral tegmental area (Labouèbe et al. (2013)), while the CB1R stimulates insulin secretion in the endocrine pancreas (Bermúdez-Silva et al., 2016). This heralds the possibility that diabetes could be associated with alterations in CB1R signaling.
Our particular area of interest is the cerebral cortex because of the widely accepted but little understood link between systemic metabolic diseases and neurodegeneration (Duarte, 2015; Rehni and Dave, 2018). It has been speculated that cerebral dysmetabolism can contribute to the onset of brain disorders (de Ceballos and Köfalvi, 2017; Zilberter and Zilberter, 2017), prompting the interest to identify new pharmacological targets to mitigate brain dysmetabolism. Recently we reported that untreated type-1 diabetes (T1D) triggered a reduction both in CB1R density and in glucose uptake in hippocampal and frontocortical slices of mice. Furthermore, the genetic deletion of the CB1R also caused similar glucose dysmetabolism, but strikingly, no further impairment was observed in the diabetic CB1R knockout animals (Moura et al., 2019). This strongly suggests that CB1R dysfunction may mediate some deleterious effects of T1D on brain glucose metabolism.
We now used ex vivo frontocortical tissue from control and diabetic rats to compare glucose metabolism, CB1R density, and the effect of CB1R ligands at 2, 4 and 8 weeks after a single i.p. injection with streptozotocin (STZ) or vehicle. We found that all the investigated parameters significantly change according to the time-course of the disease and that acute ex vivo CB1R stimulation may prevent early dysregulation of glucose metabolism in the frontocortical slices.
Section snippets
Animals and diabetes models
All studies were conducted in accordance with the principles and procedures outlined as "3Rs" in EU guidelines (86/609/EEC), FELASA, and the National Centre for the 3Rs (the ARRIVE; Kilkenny et al., 2010), and were approved by the Animal Care Committee of the Center for Neuroscience and Cell Biology of the University of Coimbra, Portugal. We also applied the ARRIVE guidelines for the design and execution of in vitro experiments (see below), as well as for data management and interpretation (
Glucose uptake in frontocortical slices of diabetic rats
The subjects of the following assay are rats in which diabetes was elicited with 1 single i.p. injection of STZ at 12 weeks of age, together with their sham-injected controls. SFig. 1 documents that the STZ-injected rats became in fact diabetic. The sham rats showed steadily increasing body weight, while the STZ-injected rats showed a reverse tendency. Blood glucose levels were highly elevated in the diabetic animals.
The following assay was carried out in pairwise manner, that is, randomly
Discussion
We here report a transient reduction of basal glucose uptake and its recovery in a CB1R-dependent fashion, in the frontal cortex of rats in the first weeks after T1D induction. We also found biphasic changes in CB1R receptor protein levels and GSK3 signaling. These changes appear counter-regulatory and sequential. Namely, after 2 weeks with T1D, the CB1R showed an increased density and a gain-of-function, as inferred from the stimulatory effect of the cannabinoid agonist, WIN55212−2. After 4
Concluding remarks
We previously reported in mice (Moura et al., 2019) and now we found in the rat that the CB1R is involved in cerebral hypometabolism in T1D. The present data suggest that targeting cerebral CB1Rs in T1D could be an interesting strategy to correct neuroglycopenia. We previously proposed a set of testable hypotheses about the mechanisms underlying the influence of CB1Rs on hippocampal and cortical glucose uptake (Moura et al., 2019). It would require a sizable effort to explore these hypotheses
Funding
This work was financed by Portuguese national funds via FCT – Fundação para a Ciência e a Tecnologia, under projects PTDC/DTP-FTO/3346/2014 (A.K.), PTDC/SAU-NSC/110,954/2009 (IT), EXCL/DTP-PIC/0069/2012 (E.C.), Centro 2020 Regional Operational Programme (CENTRO-01−0145-FEDER-000008: BrainHealth 2020) (F.I.B and F.A.A.), FEDER (QREN), through Programa Operacional Factores de Competitividade −COMPETE 2020 (POCI-01−0145-FEDER-007,440), HealthyAging2020 CENTRO-01−0145-FEDER-000,012-N2323P30 and
Declaration of Competing Interest
The authors declare no conflict of interest. The funding agencies had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
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- 1
Present addresses: VIB Discovery Sciences, Bio-incubator Leuven, 3001 Leuven (Heverlee), Belgium.
- 2
Present addresses: Research Institute for Medicines (iMed.ULisboa), Faculty of Pharmacy, Universidade de Lisboa, Av. Professor Gama Pinto, Lisbon 1649-003, Portugal.
- 3
Present addresses: Post-Graduation Program in Biochemistry, Federal University of Santa Catarina, Florianópolis, Brazil.
- 4
Present addresses: Experimental Psychiatry Unit, Center for Psychiatry and Psychotherapy, Medical University Innsbruck, Austria.
- 5
Present addresses: CERVO Brain Research Center, Université Laval, Quebec City, Canada.
- 6
Department of Psychiatry and Neuroscience, Faculty of Medicine..
- 7
These authors contributed to this paper equally.