Methylphenidate disrupts social play behavior in adolescent rats

Abstract

Methylphenidate is the first-choice treatment for attention-deficit/hyperactivity disorder (ADHD), but its mechanism of action is incompletely understood. The cognitive effects of methylphenidate have been extensively studied, but little is known about its effects on spontaneous social behavior. During adolescence, rats display a characteristic, highly vigorous form of social behavior, termed social play behavior, which is of critical importance for social and cognitive development. We investigated the neurobehavioral mechanisms by which methylphenidate affects social play behavior in rats. Methylphenidate (0.3-3.0 mg/kg, s.c. or p.o.) abolished social play behavior, without altering general social interest. This effect of methylphenidate did not depend upon the baseline level of social play and was not secondary to changes in locomotion. Furthermore, the play-suppressant effect of methylphenidate was not subject to tolerance or sensitization. Methylphenidate blocked both the initiation to play and the responsivity to play initiation. The effect of methylphenidate was mimicked by the noradrenaline reuptake inhibitor atomoxetine, which is also used for the treatment of ADHD, and was blocked by an alpha-2 adrenoceptor antagonist. In addition, combined administration of subeffective doses of methylphenidate and atomoxetine suppressed social play. However, blockade of alpha-1 adrenoceptors, beta-adrenoceptors, or dopamine receptors did not alter the effect of methylphenidate. These data show that methylphenidate selectively blocks the most vigorous part of the behavioral repertoire of adolescent rats through a noradrenergic mechanism. We suggest that the effect of methylphenidate on social play is a reflection of its therapeutic effect in ADHD, that is, improved behavioral inhibition. However, given the importance of social play for development, these findings may also indicate an adverse side effect of methylphenidate.

Figure 1

Methylphenidate (0.3-3.0 mg/kg, s.c.) dose-dependently suppressed pinning ((a) F_3,29=14.96, p<0.0001) and pouncing ((b) F_3,29=28.90, p<0.0001) but not social exploration ((c) F_3,29=1.72, NS). Lower doses of methylphenidate (0.01 and 0.1 mg/kg, s.c.) were ineffective in reducing pinning but 1.0 mg/kg methylphenidate, s.c., suppressed pinning ((d) F_3,30=5.42, p<0.01). Methylphenidate (0.3-3.0 mg/kg, s.c.) also dose-dependently suppressed pinning in animals showing a minimal induction of social play by 0 h of social isolation ((e) F_3,31=4.18, p<0.05), or maximal induction of social play by 24 h of social isolation prior to the experiment ((f) F_3,31=53.51, p<0.0001). The potency and efficacy of methylphenidate to suppress pinning were comparable in animals isolated for 0, 3.5, or 24 h (g). Oral administration of methylphenidate also suppressed pinning ((h) F_3,29=17.50, p<0.0001) and pouncing F_3,29=6.96, p=0.001) at doses comparable to those effective after s.c. administration of methylphenidate. (a-i) 0 mg/kg methylphenidate saline vehicle. *Significantly different from saline, p<0.05 (Student-Newman-Keuls test).

Figure 2

Assessment of social play behavior when animals in a test pair received either methylphenidate (MP; 3.0 mg/kg, s.c.) or saline (SAL). ‘Subject’ represents the treatment of the animal whose behavior was scored; ‘partner’ represents the treatment of its test partner. When either one or both animals in a pair were treated with methylphenidate, pinning was completely suppressed ((a) F(treatment subject)_1,47=32.06, p<0.0001; F(treatment partner)_1,47=22.87, p<0.0001; F(treatment subject×treatment partner)_1,47=21.83, p<0.0001), whereas pouncing was attenuated in saline-treated animals interacting with methylphenidate-treated animals, and completely suppressed in methylphenidate-treated animals ((b) F(treatment subject)_1,47=58.40, p<0.0001; F(treatment partner)_1,47=5.72, p<0.0001; F(treatment subject × treatment partner)_1,47=5.06, p<0.0001)). Social exploration was not affected by methylphenidate ((c) F(treatment subject)_1,47=0.07, NS; F(treatment partner)_1,47=0.83, NS; F(treatment subject × treatment partner)_1,47=0.47, NS). *Significantly different from saline-treated rats interacting with saline-treated rats (SAL/SAL), p<0.05 (Student-Newman-Keuls test); ^#significantly different from saline-treated rats interacting with methylphenidate-treated rats (SAL/MP), p<0.05 (Student-Newman-Keuls test).

Figure 3

Pretreatment (PRE) with methylphenidate (MP 1; 1.0 mg/kg, s.c., once daily for 5 consecutive days) did not alter the effectiveness of methylphenidate (TEST) to suppress social play. (a) Effect of 1.0 mg/kg methylphenidate, s.c. (MP 1), or saline (SAL) on pinning after 5 days of pretreatment with methylphenidate (1.0 mg/kg, s.c.; MP 1) or saline (SAL) (F(pretreatment)_1,31=1.57, NS; F(treatment)_1,31=33.73, p<0.0001; F(interaction)_1,31=2.13, NS). (b) Effect of 1.0 mg/kg methylphenidate, s.c. (MP 1), or saline (SAL) on pouncing after 5 days of pretreatment with methylphenidate (1.0 mg/kg, s.c.; MP 1) or saline (SAL) (F(pretreatment)_1,31=0.34, NS; F(treatment)_1,31=48.10, p<0.0001; F(interaction)_1,31=2.61, NS). (c) Effect of 0.1 mg/kg methylphenidate, s.c. (MP 0.1), or saline (SAL) on pinning after 5 days of pretreatment with methylphenidate (1.0 mg/kg, s.c.; MP 1) or saline (SAL) (F(pretreatment)_1,31=0.002, NS; F(treatment)_1,31=0.41, NS; F(interaction)_1,31=1.43, NS). (d) Effect of 0.1 mg/kg methylphenidate, s.c. (MP 0.1), or saline (SAL) on pouncing after 5 days of pretreatment with methylphenidate (1.0 mg/kg, s.c.; MP 1) or saline (SAL) (F(pretreatment)_1,31=1.36, NS; F(treatment)_1,31=0.008, NS; F(interaction)_1,31=0.09, NS). *Significantly different from SAL/SAL (pretreated with saline, tested after saline) rats, p<0.05 (Student-Newman-Keuls test); ^#significantly different from MP 1/SAL (pretreated with methylphenidate, tested after saline) rats, p<0.05 (Student-Newman-Keuls test).

Figure 4

The selective noradrenaline reuptake inhibitor atomoxetine ((a) 0.3-3.0 mg/kg, i.p.; F_3,31=11.91, p<0.0001), but not the selective dopamine reuptake inhibitor GBR-12909 ((b) 1.0-10 mg/kg, s.c.; F_3,32=1.71, NS), dose-dependently suppressed pinning. The dopamine receptor agonist apomorphine slightly increased pinning ((c) 0.03-0.3 mg/kg, s.c.; F_3,31=5.44, p<0.01). (a-c) 0 mg/kg saline vehicle. *Significantly different from saline, p<0.05 (Student-Newman-Keuls test).

Figure 5

The effect of methylphenidate (MP; 1.0 mg/kg, s.c.) on pinning was blocked by the α-2 adrenoceptor antagonist RX821002 (RX; 0.2 mg/kg, i.p.) ((a) F(RX821002)_1,31=1.13, NS; F(methylphenidate)_1,31=7.83, p<0.001; F(interaction)_1,31=8.97, p<0.001), but not by the α-1 adrenoceptor antagonist prazosin (PRAZ; 0.3 mg/kg, i.p.) ((b) F(prazosin)_1,31=2.13, NS; F(methylphenidate)_1,31=36.12, p<0.0001; F(interaction)_1,31=0.86, NS), the β-adrenoceptor antagonist propranolol (PROP; 3.0 mg/kg, i.p.) ((c) F(propranolol)_3,27=0.23, NS; F(methylphenidate)_3,27=44.97, p<0.0001; F(interaction)_3,27=0.57, NS), or the dopamine receptor antagonist cis-(Z)-flupenthixol (FLU; 0.125 mg/kg, i.p.) ((d) F(cis-(Z)-flupenthixol)_1,30=0.04, NS; F(methylphenidate)_1,30=47.58, p<0.0001; F(interaction)_1,30=0.36, NS). RX821002 (RX; 0.2 mg/kg, i.p.) also blocked the effect of atomoxetine (ATM; 1.0 mg/kg, i.p.) on pinning ((e) F(RX821002)_1,30=35.16, p<0.0001; F(atomoxetine)_1,30=18.43, p<0.0001; F(interaction)_1,30=17.96, p<0.0001). Combined treatment with subeffective doses of methylphenidate (MP; 0.1 mg/kg, s.c.) and atomoxetine (ATM; 0.3 mg/kg, i.p.) suppressed pinning ((f) F(methylphenidate)_1,63=5.94, p<0.05; F(atomoxetine)_1,63=4.58, p<0.05; F(interaction)_1,63=0.58, NS). SAL saline; VEH=vehicle (distilled water). *Significantly different from SAL/SAL (a, c-f) or VEH/SAL (b), p<0.05 (Student-Newman-Keuls test); ^#significantly different from SAL/MP (a) or SAL/ATM (e), p<0.05 (Student-Newman-Keuls test); ⁺ significantly different from PRAZ/SAL (b), PROP/SAL (c), or FLU/SAL (d), p<0.05 (Student-Newman-Keuls test).