Reversal of scopolamine-induced disruption of prepulse inhibition by clozapine in mice

Abstract

Prepulse inhibition (PPI) of the acoustic startle reflex refers to the reduction of the startle response to an intense acoustic pulse stimulus when it is shortly preceded by a weak non-startling prepulse stimulus and provides a cross-species measure of sensory-motor gating. PPI is typically impaired in schizophrenia patients, and a similar impairment can be induced in rats by systemic scopolamine, a muscarinic cholinergic receptor antagonist that can evoke a range of cognitive and psychotic symptoms in healthy humans that are commonly referred to as the "anti-muscarinic syndrome" resembling some clinical features of schizophrenia. Scopolamine-induced PPI disruption has therefore been proposed as an anti-muscarinic animal model of schizophrenia, but parallel investigations in the mouse remain scant and the outcomes are mixed and often confounded by an elevation of startle reactivity. Here, we distinguished the PPI-disruptive and the confounding startle-enhancing effects of scopolamine (1 and 10mg/kg, i.p.) in C57BL/6 wild-type mice by showing that the latter partly stemmed from a shift in spontaneous baseline reactivity. With appropriate correction for between-group differences in startle reactivity, we went on to confirm that the PPI-disruptive effect of scopolamine could be nullified by clozapine pre-treatment (1.5mg/kg, i.p.) in a dose-dependent manner. This is the first demonstration that scopolamine-induced PPI disruption is sensitive to atypical antipsychotic drugs. In concert with previous data showing its sensitivity to haloperidol the present finding supports the predictive validity of the anti-muscarinic PPI disruption model for both typical and atypical antipsychotic drug action.

Figure 1

Spontaneous activity on no-stimulus and the reaction on single-stimulus trials. Scopolamine (SCP), but not methylscopolamine (mSCP) led to an upward-shift in the reactivity curve, mainly due to an increase in baseline spontaneous activity on no-stimulus trials (A and C). This effect of SCP could be eliminated by normalizing the ln-transformed reactivity against baseline reactivity using a difference score (B and D). All values refer to mean values ± standard error (SE).

Figure 2

Prepulse inhibition (PPI) is indexed by the reduction in startle reaction as a function of increasing prepulse intensity, and is depicted separately for each of the three pulse intensities (as indicated at the top) in A and C. The different prepulse intensities are denoted along the abscissa in dB units. Pulse-alone trials are denoted as ‘no-prepulse’. The respective pulse conditions that are explicitly compared in the cross-pulse analysis are highlighted in red and are re-plotted in B and C. Scopolamine (SCP) led to a non-significant decrease in PPI at 1 mg/kg (A and B) whilst the drug significantly reduced PPI at 10 mg/kg (C and D) as indicated by a significant drug × prepulse intensity interaction revealed by both the overall (three pulse × three prepulse) and cross-pulse analysis (p’s < 0.05). By contrast, methylscopolamine (mSCP) did not alter PPI at either dose. All values refer to mean values ± SE.

Figure 3

Summary of the results obtained in the clozapine experiment. The following data sets were derived and separately analyzed: (A) ln-transformed reactivity on single stimulus trials including spontaneous activity obtained in no-stimulus trials, (B) Normalized reactivity scores on single stimulus trials, (C) PPI as indexed by the reduction in startle reaction as a function of increasing prepulse intensity, which is depicted separately for each of the three pulse intensities (as indicated at the top). The different prepulse intensities are denoted along the abscissa in dB units. Pulse-alone trials are denoted as ‘no-prepulse’. The respective pulse conditions that are explicitly compared in the cross-pulse analysis are highlighted in red and are re-plotted in (D). (E) Represents the linear component (slope) of the reactivity curve plotted in (D). * Denotes a significant group difference (p<0.05) based on Fisher’s LSD post hoc comparison following the emergence of significant drug effect in the one-way ANOVA. All values refer to mean values ± SE.