Differential responses to acute administration of a new 5-HT7-R agonist as a function of adolescent pre-treatment: phMRI and immuno-histochemical study

Abstract

LP-211 is a new, selective agonist of serotonin (5-hydroxytryptamine, 5-HT) receptor 7 (5-HT7-R), which is part of a neuro-transmission system with a proposed role in neural plasticity and in mood, cognitive and sleep regulation. Adolescent subchronic LP-211 treatment produces some persisting changes in rats' forebrain structural and functional parameters. Here, using pharmacological MRI (phMRI), we investigated the effect of acute administration with LP-211 (10 mg/kg i.p.), or vehicle, to adult rats previously exposed to the same drug (0.25 mg/kg/day for 5 days), or vehicle, during adolescence (44-48 post-natal days); histology and immuno-histochemistry were performed ex vivo to evaluate neuro-anatomical and physiological long-term adaptation to pharmacological pre-treatment. The phMRI signal reveals forebrain areas (i.e., hippocampus, orbital prefrontal cortex), activated in response to LP-211 challenge independently of adolescent pre-treatment. In septum and nucleus accumbens, sensitized activation was found in adolescent pre-treated rats but not in vehicle-exposed controls. Immuno-histochemical analyses showed marked differences in septum as long-term consequence of the adolescent pre-treatment: increased level of 5-HT7-R, increased number of 5-HT7-R positive cells, and enhanced astrocyte activation. For nucleus accumbens, immuno-histochemical analyses did not reveal any difference between adolescent pre-treated rats and vehicle-exposed controls. In conclusion, subchronic LP-211 administration during adolescence is able to induce persistent physiological changes in the septal 5-HT7-R expression and astrocyte response that can still be observed in adulthood. Data shed new insights into roles of 5-HT7-R for normal and pathologic behavioral regulations.

Keywords: LP-211; Ph-MRI; hippocampus; limbic/cortical loop; nucleus accumbens; septum.

Figure 1

Positioning of the six slices for phMRI, overlaid on the anatomical sagittal fast spin-echo images taken from an individual representative rat. We acquired six slices in total: three consecutive slices (thickness 1 mm), centered on the hippocampus (Hip, slice 1, 2, 3), as well, after a gap of 3 mm, other three slices. These were centered, respectively, on: dorsal striatum (dStr, slice 4), on the fibers between the prefrontal cortex and striata (slice 5), on prefrontal cortex (PFC, slice 6).

Figure 2

Right panels: localization of ROIs. Left panels: BOLD activation maps (the Student-t-values, in color scale), overlaid on anatomical images (in gray scale). Both are taken from the three brain templates, originating from rats receiving: vehicle challenge (VEH), LP-211 acutely only (in drug-naive subjects) i.e., following adolescent vehicle exposure (VEH+LP), LP-211 also in acute but following adolescent subchronic pre-treatment (LP+LP). Only pixels with significant values (p < 0.065 for slice on Hip and p < 0.05 for the others) are illustrated. ROIs were taken with the help of the Atlas (Paxinos and Watson, 1998) from: the CA1-CA2 fields of Hip (red regions at −3.30 from bregma, slice 2); from the dSTR, septum and frontal cortex (see green, blue, red ROIs, respectively, at +0.7 from bregma, slice 4); from the nucleus accumbens, medial prefrontal cortex, dorsal prefrontal cortex (see green, blue, red, ROIs, respectively, at +2.7 from bregma, slice 6).

Figure 3

Mean ± s.e.m. change (%) in BOLD signal timecourses, in the CA1-CA2 fields of Hip, septum and nucleus accumbens (ROIs shown in Figure 2), for rats receiving vehicle challenge (VEH), LP-211 acutely only (in drug-naive subjects) i.e., following adolescent vehicle exposure (VEH+LP), LP-211 also in acute but following adolescent subchronic pre-treatment (LP+LP). Time after the challenge is expressed over ten 3-min intervals, obtained by averaging the originally acquired data in ten 5-point-wide time-windows. Significant post-hoc Tukey comparison (LP+LP vs VEH+LP groups) are indicated as follows: (^*)0.05 < p < 0.1; ^*p < 0.05; ^*p < 0.01.

Figure 4

Analysis of 5-HT7-R and GFAP immunoreactivity in the nucleus accumbens (NAcc) and in the septum: statistically significant differences were observed only in the septum. Here, animals subchronically pre-treated during adolescence with LP-211 drug showed an increased 5HT7-R optical density and an increased number of GFAP-positive cells with respect to VEH-exposed control group (C). The values are given as mean ± SE; asterisks denote a statistically significant difference (p < 0.05). No differences for 5HT7-R were observed by cell count in both areas. (A,B) are representative photomicrographs at the level of the NAcc and septum in Luxol fast blue-stained slides.

Figure 5

Analysis of GFAP immunoreactivity in the nucleus accumbens (NAcc) and in the septum: significant differences were observed only in the septum. Here, animals subcronically pre-treated during adolescence with LP-211 drug (A) revealed an increased GFAP immunoreactivity, with respect to the VEH-exposed control group (B). No differences were conversely observed in the nucleus accumbens (C,D). Original magnifications 40×.