Individual differences in hedonic response as measured by sucrose intake: Relationship with sensitivity to anxiogenic drugs and RNA m6A methylation

Animals individually differ to a large extent in their level of intake of sucrose [1;2]. Rats with higher sucrose intake are more sensitive to chronic stress in terms of behaviour, transcriptomic landscape, monoamine neurochemistry and cerebral regional oxidative metabolism [2;3;4]. Recently, epitranscriptomic regulation, especially RNA methylation of adenosine at N⁶ (m⁶A), has been found associated with anxiety [5].

The present study aimed at comparison of the behavioural and neurochemical effects of acute treatment with two anxiogenic drugs with different mechanisms of action, FG 7142 (N-methyl-ß-carboline-3-carboxamide; a benzodiazepine receptor partial inverse agonist) and yohimbine (an a₂-adrenoceptor antagonist), in rats different in hedonic response.

Wistar rats were kept in standard conditions and three weekly sucrose tests were carried out during the dark phase as described in previous studies [2-4]. Food was available throughout the study. One bottle was filled with 1% sucrose solution, the other with water, the placement of bottles was counterbalanced. Based on the median split of the average sucrose consumption in 12 h, rats were categorized into high sucrose-consuming (HSuc; mean±SEM 125±3 g) and low sucrose-consuming (LSuc; 58±2 g) phenotypes. FG 7142 (10 and 15 mg/kg) and yohimbine (2.5 and 5 mg/kg) or vehicle were administered intraperitoneally 30 min before the elevated plus-maze test, and one hour after the test the animals were sacrificed. Monoamine levels were measured by HPLC and gene expression by qPCR as in previous studies [4;5].

Control HSuc- and LSuc rats did not differ in the elevated plus-maze test. FG 7142 reduced exploration in the plus-maze with the maximal effect already at 10 mg/kg (e.g., open/total entries ratio, F(2,30)=5.58; p<0.01), with a tendency of a larger effect in the LSuc-rats. Yohimbine had a dose-dependent anxiogenic effect (e.g., open arm entries, F(2,30)=4.32; p<0.05) that, in contrast to FG 7142, was more prominent in the HSuc-rats. Yohimbine dose-dependently increased dopamine turnover and reduced serotonin turnover in the amygdala (F(2,29)=7.58 and 3.79; p<0.005 and p<0.05, respectively). Yohimbine also increased noradrenaline turnover in the striatum; the levels of homovanillic acid were increased. In the hypothalamus, the HSuc-rats had lower levels of normetanephrine, and yohimbine increased noradrenaline turnover and reduced serotonin turnover, whereas the former effect was significantly more prominent in the HSuc-rats. FG 7142 had no effect on monoamines. The HSuc-rats had a higher expression of Mettl3 (encoding RNA m⁶A methyltransferase) and lower expression of Fto (encoding RNA m⁶A demethylase) in the frontal cortex, whereas both Mettl3 and Ftowere more highly expressed in the nucleus accumbens. In the nucleus accumbens, both drugs reduced Mettl3expression in the HSuc-rats. In striatum, control groups had a similar level of Mettl3 expression; FG 7142 increased Mettl3 expression significantly only in the LSuc-rats and yohimbine only in the HSuc rats.

Conclusively, in animals different in hedonic response, anxiety-provoking drugs elicit distinct neurochemical alterations that vary by their mechanism of action. Interestingly, hedonic behaviour was also related to the expression of genes encoding the enzymes responsible for epitranscriptomic regulation by RNA m⁶A methylation, and it was sensitive to pharmacological provocation of anxiety.

References

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