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Apomorphine-Induced Prepulse Inhibition Disruption Is Associated with a Paradoxical Enhancement of Prepulse Stimulus Reactivity

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Apomorphine-Induced Prepulse Inhibition Disruption Is Associated with a Paradoxical Enhancement of Prepulse Stimulus Reactivity Neuropsychopharmacology (2004) 29, 240–248 & 2004 Nature Publishing Group All rights reserved 0893-133X/04 $25.00 www.neuropsychopharmacology.org Apomorphine-Induced Prepulse Inhibition Disruption is Associated with a Paradoxical Enhancement of Prepulse Stimulus Reactivity ,1 1 1 Benjamin K Yee* , Holger Russig and Joram Feldon 1 Laboratory of Behavioural Neurobiology, Swiss Federal Institute of Technology, Zurich, Schwerzenbach, Switzerland Prepulse inhibition (PPI) refers to the reduction in startle reaction to a startle-eliciting stimulus when it is shortly preceded by a subthreshold prepulse stimulus. PPI has been extensively employed as an assay for sensorimotor gating, and its disruption has been characterized in specific disease conditions, including schizophrenia. In animals, dopamine agonists disrupt PPI, and this disruption can be antagonized by antipsychotic drug treatment. The present study extended these fundamental findings to C57BL6 mice, and further evaluated the subjects’ reaction to the prepulse stimulus alone in relation to the expression of PPI. Not only did apomorphine (2.0 mg/kg, intraperitoneal (i.p.)) attenuate PPI but it also enhanced reactivity to the prepulse stimulus. The dual effects of apomorphine appear paradoxical in view of the positive correlation, detectable in both the control and apomorphine groups, between prepulse reactivity and PPI magnitude. The present findings contradict the hypothesis that apomorphine disrupts PPI via reduced detectability or perception of the prepulse, and we further propose that enhanced distractibility may provide a parsimonious account for the dual effects of apomorphine. Moreover, haloperidol pretreatment (0.4 mg/kg, i.p.) fully antagonized the effects of apomorphine upon prepulse reactivity as well as on PPI. The present results add to our understanding of the relevance and applicability of the PPI paradigm in modeling schizophrenia-like symptoms in animals. Neuropsychopharmacology (2004) 29, 240–248, advance online publication, 3 December 2003; doi:10.1038/sj.npp.1300323 Keywords: amphetamine; apomorphine; haloperidol; mice; prepulse inhibition; schizophrenia; startle INTRODUCTION ably psychosis-prone subjects (Braff et al, 1992, 2001). Although PPI impairment is not unique to schizophrenia Prepulse inhibition (PPI) refers to the reduction of startle (Braff et al, 2001; Castellanos et al, 1996; Swerdlow et al, reaction to a startle-eliciting stimulus when it is shortly 1995), the sensitivity of PPI to dopaminergic manipulations preceded by a weak stimulus (Hoffman and Searle, 1965). fits readily to the dopamine hypothesis of schizophrenia PPI of the acoustic startle reflex has been extensively (Snyder, 1976). Systemic administration of the direct studied in animals and in humans. Theoretical expositions dopamine agonist, apomorphine, reliably disrupts PPI, of PPI invariably attribute it to a competition between the and such disruption is antagonized by antipsychotic drugs prepulse and pulse stimuli for limited processing resources. (Swerdlow et al, 1994). Apomorphine-induced PPI disrup- When the prepulse and pulse stimuli are presented in close tion has therefore been commonly employed as a screening proximity, the preceding prepulse triggers a protective or test for potential antipsychotic compounds (Swerdlow and gating mechanism that limits the processing of, and Geyer, 1998). However, the psychological mechanism therefore the reaction to, the succeeding pulse stimulus whereby apomorphine leads to PPI disruption remains a (Braff et al, 1992; Graham, 1975, 1980, 1992). subject of speculation. As reviewed elsewhere, significant deficits in PPI have One suggestion contends that apomorphine attenuates been reported in schizophrenic, schizotypal, and presum- the PPI effect by reducing the detectability of the weak prepulse stimulus (Davis et al, 1990). Alternatively, *Correspondence: BK Yee, Laboratory of Behavioural Neurobiology, apomorphine may directly interfere with the gating Swiss Federal Institute of Technology, Zurich, Schorenstrasse 16, mechanism underlying PPI. Although the two possibilities Schwerzenbach 8603, Switzerland, Tel: +41 1 6557203, Fax: +41 1 6557203, E-mail: [email protected] are not mutually exclusive, they could be distinguished Received 22 May 2003; revised 01 September 2003; accepted 09 experimentally. One way is to evaluate the drug’s effect on September 2003 the processing of the prepulse stimulus. If apomorphine Online publication: 10 September 2003 at http://www.acnp.org/ impairs the perception and/or processing of the prepulse, citations/Npp09100303233/default.pdf then the reactivity towards the prepulse should be Apomorphine, prepulse and PPI BK Yee et al 241 diminished. On the other hand, if apomorphine solely 5 mg of haloperidol in 1 ml of solvent containing 6 mg of impairs the gating mechanism, no such change in prepulse lactic acid. This was diluted with sterile 0.9% NaCl solution perception would be expected (cf Braff et al, 1992). to achieve the required concentration (final pH ¼ 5.5). The The present study examined the possible relationships appropriate vehicle solution was used for the control group: that might exist between prepulse reactivity and the 0.1% ascorbic acid for apomorphine, 0.9% NaCl for expression of PPI in C57BL/6J mice under different amphetamine, and 0.9% saline/lactic acid (pH 5.5) for pharmacological interventions. To this end, our experi- haloperidol. D-amphetamine (2.5 mg/kg) and haloperidol mental design included trials in which only the prepulse (0.4 mg/kg) were administered via the intraperitoneal (i.p.) stimulus was presented. Such trials are not always present route, while apomorphine (2.0 mg/kg) was administered via in existing PPI studies, and even if they are included they the subcutaneous (s.c.) route. Haloperidol was administered are seldom subjected to vigorous analysis. The major focus 60 min prior to testing, and amphetamine and apomorphine here is to examine whether animals that reacted more were administered 15 min before testing. The volume of strongly to the prepulse also tended to express stronger PPI. injection was 5 ml/kg. The doses and pretreatment time In most studies, prepulses of various intensities are usually were determined by previous pilot studies and conformed employed, and it is a well-established finding that more to those commonly employed in studies of a similar nature intense prepulses lead to stronger PPI. The present study, in mice (for a review, see Geyer et al, 2002). on the other hand, examined this relationship in terms of individual differences among animals. A positive relation- Apparatus ship between reactivity to prepulse and PPI magnitude would be expected based on the theoretical accounts put The apparatus consisted of two acoustic startle chambers forward by Graham (1975, 1980, 1992) and Braff et al for mice (SR-LAB, San Diego Instruments, San Diego, CA, (1992). Experiments 1 and 2 examined the effects of USA). Each startle chamber comprised a nonrestrictive apomorphine and amphetamine, respectively, upon pre- cylindrical enclosure made of clear Plexiglas attached pulse reactivity, PPI magnitude, as well as the relationship horizontally on a mobile platform, which was in turn between them. Experiment 1 would constitute a direct test resting on a solid base inside a sound-attenuated isolation of the hypothesis of Davis et al (1990) that apomorphine cubicle. A high-frequency loudspeaker mounted directly attenuates PPI via reduced detectability of the prepulse. above the animal enclosure inside each cubicle produced a Experiment 3 was designed to further examine whether the continuous background noise of 65 dBA and the various ability of haloperidol to counter the disruptive effect of acoustic stimuli in the form of white noise. Vibrations of the apomorphine upon PPI was associated with any effects on Plexiglas enclosure caused by the whole-body startle the prepulse reactivity or detection. One extension of the response of the animal were converted into analog signals suggestion by Davis et al (1990) is that antipsychotic drugs by a piezoelectric unit attached to the platform. These (such as haloperidol) might lead to enhanced reaction or signals were digitized and stored by a computer. A total of detection of the prepulse stimulus, which would also predict 130 readings were taken at 0.5-ms intervals (ie spanning the finding that antipsychotic drugs are associated with across 65 ms), starting at the onset of the startle stimulus in enhanced PPI when administered on their own (Geyer et al, pulse-alone and prepulse-plus-pulse trials, and at the onset 2001). of the prepulse stimulus in prepulse-alone trials. The average amplitude over the 65 ms was used to determine the stimulus reactivity. The sensitivity of the stabilimeter METHODS was routinely calibrated to ensure consistency between Subjects chambers and across sessions. Three cohorts of naı¨ve male C57BL6/J mice, weighing 28– Procedures 35 g, were used. Littermates of three to four mice were kept together in a cage, and maintained under ad libitum food In the demonstration of PPI of the acoustic startle reflex, and water throughout the experiment. They were housed in subjects were presented with a series of discrete trials a temperature- and humidity-controlled (221C, B55%) comprising a mixture of four types of trials. These included vivarium under a reversed light–dark cycle (lights off: 0800– pulse-alone trials, prepulse-plus-pulse trials, prepulse-alone 2000). All experiments were
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