Peripubertal diazepam administration prevents the emergence of dopamine system hyperresponsivity in the MAM developmental disruption model of schizophrenia

Abstract

Schizophrenia is believed to arise from an interaction of genetic predisposition and adverse environmental factors, with stress being a primary variable. We propose that alleviating anxiety produced in response to stress during a sensitive developmental period may circumvent the dopamine (DA) system alterations that may correspond to psychosis in adults. This was tested in a developmental rat model of schizophrenia based on prenatal administration of the mitotoxin methyl azoxymethanol acetate (MAM). MAM administration leads to a hyperdopaminergic state consisting of an increase in the number of DA neurons firing spontaneously, which correlates with an increased behavioral response to amphetamine. MAM-treated rats exhibited a heightened level of anxiety during adolescence. Peripubertal administration of the antianxiety agent diazepam was found to prevent the increase in DA neuron activity and blunt the behavioral hyperresponsivity to amphetamine in these rats. These data suggest that the pathophysiological factors leading to the onset of psychosis in early adulthood may be circumvented by controlling the response to stress during the peripubertal period.

Figure 1

Experimental design. Pregnant rats were administered with MAM or saline on GD 17. The elevated plus maze test was performed on litters on either PD31 or PD38-41. Litters were further divided into four groups by 10 daily oral administration of diazepam or vehicle during PD31–PD40. These rats were used for electrophysiology or behavioral tests as adults.

Figure 2

MAM rats exhibited a significantly higher level of anxiety compared with saline rats during adolescence, which was reversed by diazepam administration. (a) Adolescent (PD31) MAM rats (n=14) spent significantly less time on open arms relative to closed arms in the elevated plus maze compared with saline rats (n=12) (Mann–Whitney rank sum test, p<0.05). (b) Adolescent (PD38-41) MAM rats that were given diazepam acutely 90 min before testing (MAM:DZ, n=4) spent significantly more time on open arms relative to closed arms, compared with those treated with vehicle (MAM:Veh, n=4).

Figure 3

Peripubertal diazepam treatment (5 mg/kg, oral; daily, PD31–PD40) prevented the pathological increase in the number of spontaneously active dopamine (DA) neurons (presented as cells/track) in MAM-treated animals. (a) MAM:Veh (n=7) rats had a significantly higher number of DA neurons firing per electrode track compared with MAM:DZ (n=7), Sal:Veh (n=7), and Sal:DZ rats (n=7; Bonferroni post hot test). In contrast, diazepam treatment did not significantly alter the number of DA neurons firing in saline rats (Sal:Veh vs Sal:DZ rats, p>0.05). (b) Average firing rate and (c) percentage of spikes fired in bursts were not significantly different. ***p<0.001.

Figure 4

Rats treated peripubertally with diazepam (5 mg/kg, oral; daily PD31–PD40) showed an attenuation of the aberrant enhancement of the locomotor response to D-amphetamine (0.5 mg/kg, i.p.) that was observed in MAM-treated rats. (a) MAM:Veh (n=10) rats showed significantly higher amphetamine-induced locomotion compared with MAM:DZ (n=10), Sal:Veh (n=11), and Sal:DZ (n=9) rats. In contrast, MAM:DZ, Sal:Veh, and Sal:DZ rats were not significantly different. Repeated measures of three-way ANOVA revealed a significant effect of time (F_8,288=67.8, p<0.001), MAM treatment (F_1,36=22.0, p<0.001), and the interaction of MAM and diazepam treatment (F_1,36=10.6, p<0.01). Locomotor activity was calculated within each bin (bin width=10 min). D-amphetamine injection is indicated by the dashed line. (b) MAM:Veh rats showed significantly higher locomotor activity than MAM:DZ, Sal:Veh, and Sal:DZ rats. *p<0.05; **p<0.01; ***p<0.001. Spontaneous activity before amphetamine injection did not show significant difference among all four groups.