Spatial discrimination reversal and incremental repeated acquisition in adolescent and adult BALB/c mice

Adult and adolescent antipsychotic exposure increases delay discounting and diminishes behavioral flexibility in male C57BL/6 mice

Second-generation antipsychotics are frequently prescribed to adolescents, but the long-term consequences of their use remain understudied. These medications work via monoamine neurotransmitter systems, especially dopamine and serotonin, which undergo considerable development and pruning during adolescence. Dopamine and serotonin are linked to a wide host of behaviors, including impulsive choice and behavioral plasticity. In a murine model of adolescent antipsychotic use, male C57BL/6 mice were exposed to either 2.5 mg/kg/day risperidone or 5 mg/kg/day olanzapine via drinking water from postnatal days 22-60. To determine whether the adolescent period was uniquely sensitive to antipsychotic exposure, long-term effects on behavior were compared to an equivalently exposed group of adults where mice were exposed to 2.5 mg/kg risperidone from postnatal days 101-138. Motor activity and body weight in adolescent animals were assessed. Thirty days after exposure terminated animal's behavioral flexibility and impulsive choice were assessed using spatial discrimination reversal and delay discounting. Antipsychotic exposure produced a modest change in behavior flexibility during the second reversal. There was a robust and reproducible difference in impulsive choice: exposed animals devalued the delayed alternative reward substantially more than controls. This effect was observed both following adolescent and adult exposure, indicating that an irreversible change in impulsive choice occurs regardless of the age of exposure.

Aging, motor function, and sensitivity to calcium channel blockers: An investigation using chronic methylmercury exposure

Methylmercury (MeHg) neurotoxicity is thought to be mediated, in part, by dysregulation of calcium (Ca(2+)) homeostasis, a mechanism that may also slowly and progressively degrade neuronal function during normal aging. Longitudinal studies of MeHg exposure provide a powerful approach to studying neural and behavioral mechanisms by which both MeHg toxicity and aging affect motor function. Wheel-running and rotarod performance were assessed in two age groups of BALB/c mice chronically exposed to 0 or 1.2mg/kg/day MeHg and 0 or 20mg/kg/day nimodipine, a 1,4-dihyrdopyridine L-type calcium channel blocker (CCB), for approximately 8.5 months. Adults began exposure on postnatal day (PND) 72 and retired breeders on PND 296. A log-survivor bout analysis partitioned wheel-running into bouts that identified motor (within-bout rates) and motivational (bout-initiation rates) influences. Retired breeders ran farther, because of a higher bout-initiation rates, but performed more poorly on the rotarod than younger adults, a difference unaffected by nimodipine. MeHg produced relatively age-independent deficits in wheel-running and rotarod performance, whereas nimodipine afforded greater protection to adult mice than to retired breeders. Rotarod performance and within-bout response rate were more sensitive to and more reliable predictors of MeHg toxicity than bout-initiation rate, which was least affected by MeHg exposure. Thus the motivation to run was unimpaired as the ability to do so declined. While chronic MeHg exposure produced functionally similar behavior deficits between age groups, the age-dependent neuroprotection by nimodipine supports the notion that underlying neurobiological systems mediated by Ca(2+) signaling, are differentially affected in older adults.

Examination of clozapine and haloperidol in improving ketamine-induced deficits in an incremental repeated acquisition procedure in BALB/c mice

RATIONALE

Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, causes locomotor hyperactivity, aberrant prepulse inhibition and impaired reversal learning among other deficits. There are numerous clinical and pre-clinically uses of NMDAR antagonists and a growing need to characterize their neurobehavioral effects.

OBJECTIVES

The present study was designed to characterize 1) ketamine's effect on incremental repeated acquisition (IRA), a procedure that taps multiple neurobehavioral functions and has performance measures correlated with IQ in humans, and 2) the extent to which clozapine (CLZ) and haloperidol (HAL) block ketamine's detrimental effects.

METHODS AND RESULTS

In experiment 1 (Exp. 1), BALB/c mice nose-poked under an IRA procedure for sucrose pellets. Systemic ketamine (1-30 mg/kg) dose-dependently decreased measures of cognitive and motor function. CLZ pretreatment (CLZ 0.1-4.0 mg/kg) dose-dependently attenuated ketamine-induced (30 mg/kg) deficits; the effective dose range of CLZ was 0.3-1.0 mg/kg. HAL pretreatment (0.01-0.1 mg/kg) did not attenuate any ketamine-induced deficits. In experiment 2 (Exp. 2), BALB/c mice lever-pressed under an IRA procedure for sweetened condensed milk. Ketamine (30 mg/kg) produced a global impairment in the IRA procedure and CLZ pretreatment (0.3-1.0 mg/kg) dose-dependently attenuated that impairment; motor-based performance recovered to a greater extent than cognitive performance. When tested alone, these doses of CLZ had little effect on IRA performance.

CONCLUSIONS

These findings support the notion that CLZ is more effective than HAL at blocking ketamine-induced deficits. The IRA procedure may be beneficial for distinguishing the efficacy of drugs that seek to alleviate deficits in complex behavior that result from acute NMDAR antagonism.