Thyrotropin-releasing hormone (TRH) has anorexigenic and anxiolytic functions when injected intraventricularly. Nucleus accumbens (NAcc) is a possible brain region involved, since it expresses proTRH. TRH from hypothalamic paraventricular nucleus (PVN) has a food intake-regulating role. TRHergic pathways of NAcc and PVN are implicated in anxiety and feeding. Both behaviors depend on cAMP and phosphorylated-cAMP response element binding protein (pCREB) intracellular levels.
Intracellular levels of cAMP are controlled by the degrading activity of phosphodiesterases (PDEs). Since TRH transcription is activated by pCREB, a specific inhibitor of PDE7B may regulate TRH-induced effects on anxiety and feeding.
We evaluated the effectiveness of an intra-accumbal and intraperitoneal (i.p.) administration of a PDE7 inhibitor (BRL-50481) on rats' anxiety-like behavior and food intake; also on TRH mRNA and protein expression in NAcc and PVN to define its mediating role on the PDE7 inhibitor-induced behavioral changes.
Accumbal injection of 4 μg/0.3 μL of PDE7 inhibitor decreased rats' anxiety. The i.p. injection of 0.2 mg/kg of the inhibitor was able to increase the PVN TRH mRNA expression and to decrease feeding but did not change animals' anxiety levels; in contrast, 2 mg/kg b.w inhibitor enhanced accumbal TRH mRNA, induced anxiolysis with no change in food intake.
PDE7 inhibitor induced anxiolytic and anorexigenic like behavior depending on the dose used. Results supported hypothalamic TRH mediated feeding-reduction effects, and accumbal TRH mediation of inhibitor-induced anxiolysis. Thus, an i.p dose of this inhibitor might be reducing anxiety with no change in feeding, which could be useful for obese patients.