Guanfacine treatment improves ADHD phenotypes of impulsivity and hyperactivity in a neurofibromatosis type 1 mouse model

Abstract

Background: Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder with a mutation in one copy of the neurofibromin gene (NF1^+/-). Even though approximately 40-60% of children with NF1 meet the criteria for attention deficit hyperactivity disorder (ADHD), very few preclinical studies, if any, have investigated alterations in impulsivity and risk-taking behavior. Mice with deletion of a single NF1 gene (Nf1^+/-) recapitulate many of the phenotypes of NF1 patients.

Methods: We compared wild-type (WT) and Nf1^+/- mouse strains to investigate differences in impulsivity and hyperactivity using the delay discounting task (DDT), cliff avoidance reaction (CAR) test, and open field. We also investigated whether treatment with the clinically effective alpha-2A adrenergic receptor agonist, guanfacine (0.3 mg/kg, i.p.), would reverse deficits observed in behavioral inhibition.

Results: Nf1^+/- mice chose a higher percentage of smaller rewards when both 10- and 20-s delays were administered compared to WT mice, suggesting Nf1^+/- mice are more impulsive. When treated with guanfacine (0.3 mg/kg, i.p.), Nf1^+/- mice exhibited decreased impulsive choice by waiting for the larger, delayed reward. Nf1^+/- mice also exhibited deficits in behavioral inhibition compared to WT mice in the CAR test by repetitively entering the outer edge of the platform where they risk falling. Treatment with guanfacine ameliorated these deficits. In addition, Nf1^+/- mice exhibited hyperactivity as increased distance was traveled compared to WT controls in the open field. This hyperactivity in Nf1^+/- mice was reduced with guanfacine pre-treatment.

Conclusions: Overall, our study confirms that Nf1^+/- mice exhibit deficits in behavioral inhibition in multiple contexts, a key feature of ADHD, and can be used as a model system to identify alterations in neural circuitry associated with symptoms of ADHD in children with NF1.

Keywords: ADHD; Alpha 2A-adrenergic receptor agonist; Behavioral inhibition; Cliff avoidance reaction test; Delay discounting; Guanfacine; Hyperactivity; Impulsivity; Mouse; Neurofibromatosis type 1.

Fig. 1

Nf1^+/− mice exhibit hyperactivity in response to a novel open field. Graphs depict the differences in behaviors between WT and Nf1^+/− mice on the a distance (m) traveled over time, and the b total distance moved (m) in 60 min. *p < 0.05 compared to wild-type (WT). All data reported as mean + S.E.M

Fig. 2

Nf1^+/− mice exhibit deficits in behavioral inhibition in the cliff avoidance reaction (CAR) test. Graphs illustrate the effects of Nf1^+/− on the a total distance traveled (m), b number of entries made into the edge zone and outer edge zone of the CAR platform, c % mice with intact CAR, and d stereotypy rating scores. *p < 0.05 compared to wild-type (WT). All data reported as mean + S.E.M

Fig. 3

Nf1^+/− mice exhibit increased impulsive choice in the delay discounting test (DDT). Graphs illustrate a the general methods used in the DDT on a t-maze and the effects of Nf1^+/− on b the number of small reinforcements chosen during a DDT task in a t-maze and c the number of days the mice took to reach criterion prior to the initiation of the delay. *p < 0.05 compared to wild-type (WT). #p < 0.05 compared to 0 delay within the same genotype. All data reported as mean + S.E.M

Fig. 4

Guanfacine (GUAN) decreases hyperactivity in Nf1^+/− mice. Graphs depict the effects of GUAN (0.1 and 0.3 mg/kg) on the a amount of distance (m) traveled over time and the b total distance moved (m) in 60 min. *p < 0.05 compared to wild-type (WT) within the same treatment. #p < 0.05 compared to vehicle (VEH) within the same genotype. All data reported as mean + S.E.M

Fig. 5

Guanfacine (GUAN, 0.3 mg/kg) attenuates deficits in behavioral inhibition in Nf1^+/− mice in the cliff avoidance reaction (CAR) test. Graphs illustrate the effects of Nf1^+/− on the a total distance traveled (m), b % mice with impaired CAR, and c stereotypy rating scores. *p < 0.05 compared to wild-type (WT) within the same treatment. #p < 0.05 compared to vehicle (VEH) within the same genotype. All data reported as mean + S.E.M

Fig. 6

Guanfacine (GUAN, 0.3 mg/kg) decreases impulsive choice in Nf1^+/− mice in the delay discounting test (DDT). Graphs depict the effects of GUAN on a the number of small reinforcements chosen during a DDT task in a t-maze and b the number of days the mice took to reach criterion prior to the initiation of the delay. *p < 0.05 compared to wild-type (WT) within the same treatment. #p < 0.05 compared to vehicle (VEH) within the same genotype. All data reported as mean + S.E.M. Black-filled bars represent WT mice treated with VEH. Green-filled bars represent Nf1^+/− mice treated with VEH. Gray-filled bars with pattern represent WT mice treated with GUAN. Green-filled bars with pattern represent Nf1^+/− mice treated with GUAN