Neonatal domoic acid treatment produces alterations to prepulse inhibition and latent inhibition in adult rats

Pharmacology, Biochemistry and Behavior 103 (2012) 338–344

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Neonatal domoic acid treatment produces alterations to prepulse inhibition and latent inhibition in adult rats Amber L. Marriott a, Catherine L. Ryan b, Tracy A. Doucette a,⁎ a b

Department of Biology, University of Prince Edward Island, 550 University Avenue, PE, Canada, C1A 4P3 Department of Psychology, University of Prince Edward Island, 550 University Avenue, PE, Canada, C1A 4P3

a r t i c l e

i n f o

Article history: Received 6 July 2011 Received in revised form 20 August 2012 Accepted 25 August 2012 Available online 7 September 2012 Keywords: Domoic acid Glutamate Schizophrenia Prepulse inhibition Latent inhibition Animal models

a b s t r a c t Schizophrenia is a complex and severe mental disorder characterized by positive, negative and cognitive symptoms. Characteristic behavioral alterations reflecting these categories of symptoms have been observed in many animal models of this disorder, and are consistent with those manifested in the clinical population. The purpose of this study was to determine whether early alterations in glutamate signaling would result in alterations to prepulse inhibition (PPI) and latent inhibition (LI); two assessments used for evaluating putative novel animal models with relevance to schizophrenia. In the present experiment, daily subcutaneous (s.c.) injections of 20 μg/kg of domoic acid (DOM) were administered to rat pups from postnatal days (PND) 8–14. When tested as adults, DOM treated rats displayed deficits in PPI that were dependant on both sex and time of day. No differences in startle amplitude, habituation, or movement were found during any test, indicating that the PPI deficits seen could not be attributed to baseline startle differences. Deficits in LI were also apparent when adult rats were tested using a conditioned taste aversion task, with DOM-treated animals displaying a significantly suppressed LI. These results suggest that early treatment with DOM may serve as a useful tool to model schizophrenia which in turn may lead to a better understanding of the contribution of glutamate, and in particular, kainate receptors, to the development and/or manifestation of schizophrenia or schizophrenia-like symptoms in the clinical population. © 2012 Elsevier Inc. All rights reserved.

1. Introduction Animal models provide one of the best ways to study the underlying neurobiological mechanisms that contribute to human disorders such as schizophrenia. Currently, there are a number of well established animal models with relevance to schizophrenia including neurodevelopmental lesion-based models (e.g. Lipska et al., 1993, 1995), genetically based models (e.g. Egan et al., 2001; Millar et al., 2000; Stefansson et al., 2002; Straub et al., 2002), prenatal-immune challenge models (e.g. Zuckerman and Weiner, 2003), and drug induced models (e.g. Gambill and Kornetsky, 1976; Sams-Dodd, 1997). All of these models illustrate an important aspect of schizophrenia and all display validity, but the type and degree vary widely with each different model. Because schizophrenia is a heterogeneous disorder, composed of some combination of positive, negative and cognitive symptoms (DSM-IV-TR, 2000), no single “ideal” animal model can represent the entire population of schizophrenic patients. Rather, as pointed out by Powell and Miyakawa (2006), it is critical that novel models be

⁎ Corresponding author at: Department of Biology, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE, Canada, C1A 4P3. Tel.: +1 902 566 6055; fax: +1 902 566 0740. E-mail address: [email protected] (T.A. Doucette). 0091-3057/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.pbb.2012.08.022

discovered so that each new model might represent a subpopulation, or particular aspect or endophenotype. Issues of validity are always important to consider with respect to the development of any animal model. Understanding the validity of an animal model provides critical information about the strengths, uses and limits of the model in question (Geyer and Markou, 2000). Many animal models of schizophrenia seek, at least initially, to simulate the disease in terms of core behavioral symptoms, attempting to achieve face validity. This can be difficult to accomplish in an animal model of a complex human disorder such as schizophrenia, with the difficulty being further compounded by the fact that symptoms of the disorder vary widely between those affected. However, success in modeling certain symptoms of schizophrenia has been achieved using tests of behaviors that can be measured both in humans and rodent models. Prepulse inhibition (PPI) of the acoustic startle response and latent inhibition (LI) are two such behavioral measures which test for well known cognitive symptoms of schizophrenia (for reviews see Lubow, 2005; Swerdlow and Geyer, 1998; Swerdlow et al., 2000; Van den Buuse et al., 2005). Prepulse inhibition is the normal suppression of the startle reflex that occurs when the startling stimulus is preceded by a less intense, non-startling stimulus (Graham, 1975). This measure of sensory-motor gating is believed to be controlled by structures located in the lower brainstem and mediated by input from the forebrain (Weiss and Feldon, 2001). Latent inhibition is the process by which

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pre-exposure to a stimulus without consequence inhibits the learning of later conditioned associations with that stimulus. Latent inhibition is considered to be a measure of ability to ignore irrelevant stimuli and allocate appropriate mental resources (Lubow, 1989). Observed across many different species, including rats and humans, PPI and LI are reliably disrupted in humans with schizophrenia (Baruch et al., 1988; Braff et al., 1978) and have become widely used in studies of the neural alterations of schizophrenia as well as in the search for useful animal models of the disorder (Ellenbroek et al., 1996; Grecksch et al., 1999; Zuckerman and Weiner, 2003). Recent hypotheses propose that the neuropathology of schizophrenia is the result of an altered interaction between glutamate (Glu) and dopamine (DA) systems, with one of the most common theories being that hypofunction of the Glu system (particularly within the prefrontal cortex) results in hyperactivity of the mesolimbic DA neurons (Coyle, 2006). Contributions of early Glu dysfunction to schizophrenia have historically focused primarily on NMDA receptors (Bickel and Javitt, 2009; du Bois and Huang, 2007; Harris et al., 2003). More recently, however, the contribution of other Glu receptors to the pathophysiological manifestation of this disease has been explored. In particular, it would appear that kainate receptors, a subtype of ionotropic Glu receptor, may play a role in the etiology of schizophrenia and/or the manifestation of schizophrenia-relevant behaviors (see Meador-Woodruff and Healy, 2000 for review). It has also been shown that KA receptors play a modulatory role in the release of DA in the prefrontal cortex (Jedema and Moghddam, 1996; Wu et al., 2002). Behaviorally, Howland et al. (2004) demonstrated that an acute neonatal i.p. injection of kainic acid (1.5 mg/kg) administered on postnatal day (PND) 7 produced a significant deficit in prepulse inhibition (PPI) during early adulthood, but not during adolescence. Additionally, rats who received the kainic acid treatment displayed significantly higher spontaneous locomotor activity in response to amphetamine when compared to controls. To date, research in our lab has focused on how the administration of low, sub-convulsant doses of domoic acid (DOM) (i.e. a kainate receptor agonist) (Verdoorn et al., 1991, 1994) to neonatal rats during a critical period of CNS development (Dobbing and Smart, 1974), affects behavior in adulthood. We have shown that DOM (20 μg/kg), administered daily during postnatal days 8–14, produces deficits in PPI (Adams et al., 2008a) and increases in responses to novelty (Burt et al., 2008a), a behavior believed to reflect signs of psychomotor agitation (Powell and Miyakawa, 2006). Other previously published results from our laboratory indicate that this early DOM exposure paradigm produces changes in cognitive functioning and alterations in the functioning of the mesocorticolimbic pathway (Adams et al., 2009; Burt et al., 2008b; Doucette et al., 2007). Taken together, this pattern of anomalies is consistent with clinical manifestations of schizophrenia and also with changes seen in current animal models of the disorder. The purpose of the present study was to further explore the behavioral changes produced by early DOM exposure in two key behavioral paradigms; PPI and LI. This study characterizes these effects in order to address issues of face validity and to determine the potential usefulness of early DOM treatment as a neurodevelopmental animal model of schizophrenia. 2. Methods 2.1. Experimental animals and injection procedure Experimental animals were the offspring of 10 untimed pregnant Sprague–Dawley rats obtained from Charles River Laboratories (St. Constant, Quebec, Canada). The day of parturition was designated PND 0. Within 24 h of birth, litters were culled to 10 pups with an even number of males and females where possible, providing an average n of 8 for LI testing (with no group having an n below 7) and an average n of 12 for PPI testing (with no group having an n below

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11), as a greater number of experimental groups are required for LI testing (pre-exposure, non pre-exposure, control) as compared to PPI (dark phase, light phase). The same animals were used for both behavioral tests, with pups from every litter being used in each treatment group for LI and pups from 5 litters used for each PPI experiment (dark vs. light phase). From PND 8–14, pups were weighed, marked with non-toxic marker for identification purposes and given a single daily subcutaneous injection of either saline or 20 μg/kg of DOM (obtained from BioVectra DCL, Charlottetown, PE, Canada). Rats were weaned on PND 21 and group housed (2–3 animals per cage) with non-littermates of the same sex and from the same treatment group. All rats received ad libitum access to food and water (except during LI testing, as indicated below). Animals were maintained on a reversed 12:12 h light–dark cycle (lights off at 7:00, on at 19:00). behavioral testing began when the animals reached PND 90. All parts of this study were conducted experimenter blind and according to the guidelines established by the Canadian Council on Animal Care and in accordance with the Animal Care Committee at the University of Prince Edward Island. 2.2. Preweaning assessments and weight To ensure that the treatment procedure did not produce overt signs of toxicity, developmental measures were assessed beginning on PND 8. For eye-opening (defined as a break in the suture of both eyes) and auditory startle (defined as a visible startle to the noise made by a clicker held 10–15 cm above the pup's head), animals were tested until criterion was reached. Weight gain was also measured at various stages of development (PND 8–14, 20 and 89). 2.3. Prepulse inhibition Animals were tested during either the light (21:00–5:00, n = 49) or dark (between 9:00 and 17:00, n = 51) phase of the light–dark cycle, as time of day has been reported as a biological factor which may affect PPI (Adams et al., 2008b; Chabot and Taylor, 1992; Frankland and Ralph, 1995; Swerdlow and Geyer, 1998). The startle apparatus was an SR-Lab from San Diego Instruments (San Diego, CA, United States). Full details for PPI testing can be found in Adams et al. (2008b). In brief, all animals received a 5 minute acclimation period to the chamber before the experiment began, followed by 3 blocks of trials. The intertrial interval for all trials was an average of 15 s (ranging from 10 to 20 s) and a background white noise level of 70 dB was maintained. Average startle amplitude was obtained by measuring every 1 ms for 100 ms after the onset of the startle pulse, with startle amplitude defined as the average of the 100 readings. Block 1 consisted of 6 120 dB white noise startle pulses, each 40 ms in length. These trials were used to normalize startle, to measure initial startle (pulse 1) and to establish a startle baseline for the beginning of testing (the average of pulses 2–6). Block 3 consisted of 5 120 dB white noise startle pulses, each 40 ms in length. These trials were used to establish a startle baseline for the end of the session (the average of the 5 pulses). Together, data from Blocks 1 and 3 were used to determine if there was any difference between the two groups in their startle amplitudes independent of PPI, as well as to measure within-test habituation. The data from these trials was not included in the calculation of %PPI. Block 2, contained 3 types of trials: (1) startle alone pulses, like those in Blocks 1 and 3, (2) no stimulus trials, during which no stimulus other than the background white noise was administered, and (3) prepulse–pulse trials, which consisted of a 20 ms prepulse, either 4, 8, 12, or 16 dB above the background noise, which occurred 100 ms (onset to onset) before the startle pulse. Eight of each of these trial types were administered in pseudorandom order. The %PPI was calculated by the following formula: PPI = 100 − (P / S) ∗ 100 where P is the average startle amplitude for prepulse–pulse trials and S is the

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average startle amplitude on startle pulse alone trials. Data collected on the no stimulus trials was also used to obtain a measure of the activity of the animals during testing. 2.4. Latent inhibition Latent inhibition was assessed using a standard conditioned taste aversion task, as adapted from Bethus et al. (2005). In brief, five days before the start of the experiment, rats were single housed and divided into 3 groups (pre-exposure, non pre-exposure and control) balanced for condition and sex. Water bottles were removed from the cages 24 h before the first day of testing and the animals remained on a water restriction schedule throughout the length of the experiment. All testing was conducted during the dark phase of the light/dark cycle and daily access to water occurred during testing, as outlined below. Bottles were weighed before and after testing periods to determine the amount of solution consumed. Percent sucrose consumed was calculated using the following formula: %Suc = (sucrose consumed / [sucrose consumed + tap water consumed]) ∗ 100. Access to food remained unlimited throughout the experiment, with all testing being conducted in the homecage. 2.4.1. Pre-exposure The animals in the pre-exposure group received access to a bottle containing 50 ml of a 5% sucrose solution for 30 min. Rats in the non pre-exposure and control groups received access to a bottle containing 50 ml of tap water for 30 min. This procedure was repeated for 3 consecutive days. 2.4.2. Conditioning Twenty four hours following pre-exposure, all rats received a bottle with the sucrose solution and were allowed to drink for 30 min. Immediately after, the bottles were removed and all rats received an intraperitoneal (i.p.) injection of 75 mg/kg LiCl (pre-exposure and non pre-exposure groups) or saline (control group). 2.4.3. Testing During testing, each rat was provided with 2 bottles, one containing the 5% sucrose solution and the other containing tap water. The animals were allowed to drink for 30 min, after which time the bottles were removed and weighed, to determine the amount of each solution that was consumed by each animal. Testing was conducted 24 h after conditioning and again 7–8 days later. 2.5. Data analysis For preweaning assessments and weight measurements, 2-way analyses of variance (ANOVAs) were used (sex × neonatal treatment). For PPI measures, 3-way ANOVAs were used with sex and neonatal treatment as between subject factors and decibel level as a within subjects factor. For LI testing, 3-way ANOVAs were used (sex × neonatal treatment× LI group) (SPSS Version 11.5.2.1). All post-hoc comparisons were conducted using Bonferroni t-tests. A result of p b 0.05 indicated significance. If a significant main effect for sex, or an interaction between sex and another variable was found, data for male and female animals were analyzed separately. 3. Results 3.1. Preweaning assessments and weight While significant effects for sex were found for each variable assessed, no significant treatment effects were noted, nor were any significant interactions with this variable present (see Table 1).

Table 1 Neurodevelopmental and physical assessments. Males Eye-opening (day) Saline 14.708 ± 0.112 DOM 14.852 ± 0.103 Total 14.784 ± 0.076 Aud. startle (day) Saline 12.917 ± 0.158 DOM 12.963 ± 0.155 Total 12.941 ± 0.110 Weight (g) PND 8 Saline 17.667 ± 0.187 DOM 17.519 ± 0.202 Total 17.588 ± 0.138 Weight (g) PND 9 Saline 19.708 ± 0.204 DOM 19.444 ± 0.252 Total 19.569 ± 0.164 Weight (g) PND 10 Saline 21.958 ± 0.213 DOM 21.704 ± 0.219 Total 21.824 ± 0.153 Weight (g) PND 11 Saline 24.542 ± 0.248 DOM 24.296 ± 0.255 Total 24.412 ± 0.178 Weight (g) PND 12 Saline 26.583 ± 0.248 DOM 26.185 ± 0.272 Total 26.373 ± 0.186 Weight (g) PND 13 Saline 28.583 ± 0.240 DOM 28.444 ± 0.279 Total 28.510 ± 0.184 Weight (g) PND 14 Saline 30.875 ± 0.284 DOM 30.370 ± 0.303 Total 30.608 ± 0.210 Weight (g) PND 20 Saline 46.875 ± 0.456 DOM 46.296 ± 0.599 Total 46.569 ± 0.381 Weight (g) PND 89 Saline 483.083 ± 7.241 DOM 493.074 ± 6.827 Total 488.373 ± 5.797

Females

F-values

14.480 ± 0.117 14.458 ± 0.134 14.469 ± 0.088

F(1,96) = 7.109, p = 0.009*

12.480 ± 0.102 12.458 ± 0.120 12.470 ± 0.078

F(1,96) = 11.788, p = 0.001*

17.000 ± 0.163 16.917 ± 0.294 16.959 ± 0.165

F(1,96) = 8.608, p = 0.004*

19.120 ± 0.194 18.958 ± 0.272 19.041 ± 0.165

F(1,96) = 5.266, p = 0.024*

21.000 ± 0.191 20.875 ± 0.320 20.939 ± 0.183

F(1,96) = 13.932, p = b0.001*

23.360 ± 0.207 23.125 ± 0.291 23.245 ± 0.176

F(1,96) = 21.808, p = b0.001*

25.520 ± 0.232 25.250 ± 0.320 25.388 ± 0.195

F(1,96) = 13.690, p = b0.001*

27.520 ± 0.193 27.125 ± 0.291 27.327 ± 0.173

F(1,96) = 21.876, p = b0.001*

29.680 ± 0.287 29.250 ± 0.296 29.469 ± 0.206

F(1,96) = 15.513, p = b0.001*

45.840 ± 0.438 44.958 ± 0.483 45.408 ± 0.329

F(1,96) = 5.521, p = 0.021*

288.840 ± 7.095 286.708 ± 7.214 287.796 ± 3.936

F(1,96) = 795.217, p b 0.001*

An asterisk designates a p-value below 0.05, indicating a significant difference between male and female rats on that developmental measure.

3.2. Prepulse inhibition 3.2.1. Testing during the light phase Analysis of the %PPI revealed a significant main effect for dB level [F(3,135) = 25.410, p = b0.001] with %PPI increasing as dB increased, however, no significant effect for sex was displayed. Statistical analyses showed that DOM statistically decreased PPI only at 74 dB level [t(34.333) = − 1.986, p = 0.0275] (Fig. 1). 3.2.2. Testing during the dark phase Analysis of the %PPI in male and female animals together showed no significant effect for condition but revealed a significant main effect for dB level [F(3,141) = 97.783, p = b 0.001] with %PPI increasing as dB level increased. A significant main effect was also found for sex [F(1,47) = 5.073, p = 0.029] with males showing greater %PPI than females. Because of this difference, the %PPI data for males and females were subsequently analyzed separately. No significant differences were found between DOM and saline treated rats in the males at any dB, however, the DOM treated females showed a tendency for lowered %PPI, which was significant at the 86 dB prepulse level [t(24) = − 1.821, p = 0.0405] [p = 0.113, 0.088, and 0.054 for 74, 78 and 82 dB respectively] (Fig. 2).

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Fig. 1. Mean ± SEM %PPI in adult rats at various prepulse decibel levels when tested during the light phase of the light–dark cycle. Asterisk indicates a significant difference from saline-treated rats (p b 0.05).

3.3. Latent inhibition 3.3.1. Pre-exposure Neonatal treatment with DOM did not affect liquid consumption during the first 3 days of testing (pre-exposure), however a significant main effect for sex was found [F(1,88)= 122.818, p b 0.001] with males (22.555 ±0.489 ml) consuming on average, a greater amount of liquid than females (14.821 ± 0.498 ml). Data from males and females were then analyzed separately. No differences in liquid consumption were found between rats who received DOM compared to those that received saline. 3.3.2. Conditioning Likewise, in an analysis which considered the amount of sucrose consumed on the conditioning day, no differences were found between DOM treated rats and controls, although again, a main effect for sex was found [F(1,88) = 70.702, p b 0.001] with males (25.371 ±0.620 ml) consuming more sucrose than females (17.937± 0.630 ml). When data from both sexes were analyzed separately, no significant effect for treatment was found. 3.3.3. Testing When analyzing the %sucrose consumed on the two testing days, a main effect was found for sex [F(1,87) = 21.639, p b 0.001] with females consuming a higher percentage of sucrose (66.279 ± 1.852 ml) as compared to their male counterparts (54.136 ± 1.840 ml), as well as a main effect for day [F(1,87) = 20.556, p b 0.001], with all groups consuming a higher percentage of sucrose on the second day of testing (64.360 ± 1.639 ml) as compared to the first day (56.054 ± 1.549 ml). t-Tests which further investigated the above stated effects found that on the first testing day (24 h after conditioning) the male DOM treated rats who were pre-exposed to sucrose, consumed significantly less sucrose (47.094 ±9.655 ml) than did the saline treated males who were also pre-exposed to the sucrose (72.060 ± 5.214 ml) [t(16)= −2.275, p = 0.037] (Fig. 3). No significant effects were found for the male rats on the second day of testing (7–8 days after conditioning). For the females, t-tests revealed no significant differences between the DOM treated groups and control groups on the first testing day but did reveal a significant difference between the groups on the second day of testing [t(15) = −2.541, p = 0.023], with the pre-exposed DOM treated rats consuming a significantly lower percentage of sucrose (79.589 ± 1.949 ml) than did pre-exposed saline treated females (86.570 ± 1.925 ml) (Fig. 4).

Fig. 2. Mean ± SEM %PPI in adult female rats (A) and male rats (B) at various prepulse decibel levels when tested during the dark phase of the light–dark cycle. Asterisk indicates a significant difference from saline-treated rats (p b 0.05).

light phase of the light–dark cycle, DOM treated animals displayed significantly lowered %PPI at the 74 dB level, with a tendency for suppressed %PPI at the 78 and 82 dB levels (p = 0.177 and 0.2275 for 78 and 82 dB, respectively). It is also important to note that these effects cannot be attributed to differences in baseline startle level or activity during testing, as none of these measures showed any significant differences between groups, nor were any significant group differences noted with respect to their startle at the end of testing or startle habituation.

4. Discussion Prepulse inhibition testing revealed that early DOM treatment does indeed result in PPI deficits, although the magnitude of the effect is impacted upon by the prepulse dB level, sex of the animals and the time of day during which testing occurred. When tested during the

Fig. 3. Effects of early postnatal DOM treatment (20 μg/kg) on latent inhibition in a conditioned taste aversion task in adult male (A) and female (B) rats when tested 24 h post-conditioning. Values represent mean ± SEM, asterisk indicates a significant difference from saline-treated rats (p b 0.05).

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Fig. 4. Effects of early post-natal DOM treatment (20 μg/kg) on latent inhibition in a conditioned taste aversion task in adult male (A) and female (B) rats when tested 7–8 days post-conditioning. Values represent mean ± SEM, asterisk indicates a significant difference from saline-treated rats (p b 0.05).

Interestingly, when analyzing the data of animals tested during the dark phase of the light–dark cycle, male rats displayed a significantly greater %PPI than did their female counterparts. Furthermore, while no significant effects were found for treatment in male rats, female rats treated neonatally with DOM did display a significantly lower %PPI at the 86 dB level when compared to their saline-treated counterparts, with a tendency for lowered %PPI manifested across all dB levels (refer to Section 3.2.2.). As with animals tested during the light phase, no differences were found in baseline startle at the beginning or end of testing, startle habituation or movement during the testing period. These results support those of our previous study on PPI in DOM treated rats. Although the previous experiment found significance only in males, the DOM treated rats did show lowered %PPI. Such differences in sex and time of day testing on PPI are interesting but not entirely surprising. Many studies have found sex differences in PPI in both humans (Kumari et al., 2004) and rats (Lehmann et al., 1999), with males consistently displaying greater PPI, particularly at lower prepulse levels. It is possible that such sex differences have contributed to the variability in PPI found in this study. The time of day during which startle testing occurs has also been found to affect the outcome. A study in our lab which sought to determine the effect of various experimental parameters on PPI in female rats has shown that although startle and PPI were not affected by the estrous cycle, the time of day during which testing occurred could have an effect, with greater %PPI demonstrated when the rats were tested during the dark cycle (Adams et al., 2008b). Data from the LI trials indicates that while all animals demonstrated conditioned taste aversion (as evidenced by significantly lower sucrose consumption in all rats who received the LiCl injection but no pre-exposure to sucrose), rats treated neonatally with DOM exhibited significantly lowered latent inhibition. While significant 24 h following conditioning, for the males, the effect was not present one week later. This is not surprising, as it would be expected that the effect may not be as strong during the second test due to the passage of time, and also due to the first test, during which the animals all had access to sucrose without it being paired with LiCl (an extinction trial). Conversely, in females, deficits in LI became apparent only

7–8 days following conditioning. These findings could be due to a number of factors. It is possible that while the effect of lowered LI in female DOM treated rats is present, it may not be as robust when compared to the effect seen in males and therefore was only apparent on the second testing day. The fact that female rats consumed a higher percentage of sucrose regardless of group could also have affected the measure. However, it is clear that there were no differences between DOM and saline treated groups in their sucrose consumption during pre-exposure or conditioning, indicating that any differences found during testing cannot be attributed to baseline difference in sucrose preference, nor can they be attributed to variability in overall liquid consumption. Quite possibly, the effect manifested itself following test 1, an extinction trial. On the second test day the saline females increased their sucrose consumption (displaying less aversion) possibly due to the weakening of the association between the sucrose flavor and the feeling of nausea. The DOM treated animals appeared resistant to the effect of the test 1 extinction effect. Interestingly, resistance to extinction is a frequently reported learning effect in schizophrenia (Yee, 2000). While in this study we have not examined the potential neurobiological mechanisms responsible for the observed behavioral changes, we can speculate as to what systems may be involved. For example, it is known that KAR regulate both excitability and dopaminergic function. For instance, KAR appear to contribute greatly to maintaining the overall balance between excitatory and inhibitory tone within the CNS, as KAR have been shown to be localized both pre‐ and post‐ synaptically, are present on both glutamatergic and GABAergic neurons, and both increase and decrease synaptic transmission (Cherubini et al., 2011; Frerking et al., 2001; Frerking and Nicoll, 2000; Huettner, 2003; Lerma et al., 2001; Rodríguez-Moreno and Sihra, 2011; Sihra and Rodríguez-Moreno, 2011). In addition, data have been published demonstrating that KAR regulate the mesocorticolimic dopamine system at various levels. It has been shown, for instance, that KAR subtypes directly modulate mesoaccumbens neurons (Kalivas et al., 1989) and that they serve a modulatory function for DA release in the medial PFC (Jedema and Moghddam, 1996; Taepavarapruk et al., 2008; Wu et al., 2002). In adults, drugs usually produce a transient pharmacological effect. But in the immature organism, especially during the brain growth spurt (i.e. during the first three weeks of postnatal life in the rat) drug administration has the potential to cause permanent, irreversible insult. This insult may be expressed as either a permanent dysfunction in the neurotransmitter systems involved, or may result in “irreversible imprinting” of receptor densities, which in turn results in lasting functional and/or structural changes to the nervous system (Kaufmann, 2000; Rice and Barone, 2000; Vorhees, 1986). Therefore, we feel that, at present, it is premature to speculate on a mechanism of action for perinatal DOM-treatment. However, we believe that this is a very important consideration, and mechanistic studies are presently underway in our laboratory. 5. Conclusion A model is said to display face validity if it has phenomenological similarities to the human condition. In other words, the behavioral manifestations have the same form or appearance. Currently, in the most widely accepted neurodevelopmental animal models of schizophrenia, adult rats have been shown to display deficits in PPI and LI following some perinatal intervention. These models include the neonatal ventral hippocampal lesion model, first proposed by Lipska et al. (1993, 1995), and the polyriboinosinic–polyribocytidilic acid (polyI:C) model created by Zuckerman and Weiner (2003) (Ozawa et al., 2006 for PPI, and Zuckerman and Weiner, 2003 for LI). Although the cause of schizophrenia is not yet known, the prominent theory is that it is neurodevelopmental in origin and arises due to a combination of environmental factors and genetic susceptibility. Neonatal DOM exposure fits in well with this theory which states that factors

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occurring long before the formal onset of the illness (potentially during gestational or early developmental stages), disrupt normal CNS development, possibly leading to severe and long-lasting changes in the functional integrity of the CNS (Rapoport et al., 2005), indicating that this model displays etiological validity. The lowered PPI seen in the animals treated neonatally with DOM is similar to that found previously (Adams et al., 2008a) while the lack of differences in baseline startle between groups found in this study further solidifies the finding. The changes to LI seen in both male and female rats, and the observation that at least some of these changes are long-lasting, lend further support to the potential of chronic low-dose treatment of DOM during development as a novel animal model of schizophrenia. Conflict of interest All authors declare that there are no conflicts of interest relating to this manuscript. Funding sources had no role in the study design, in the collection, analysis and interpretation of data, in the writing of the report or in the decision to submit the paper for publication. Acknowledgments Funding for this study was provided by a grant from the Natural Science and Engineering Research Council of Canada, awarded to T.A. Doucette and a grant from the Atlantic Innovation Fund (193639). A.L. Marriott is supported by the Hooper/Klarenbach Research Graduate Scholarship and a Post-Graduate Scholarship from the Natural Science and Engineering Research Council of Canada. References Adams AL, Doucette TA, Ryan CL. Altered pre‐pulse inhibition in adult rats treated neonatally with domoic acid. Amino Acids 2008a;35:157–60. Adams AL, Hudson A, Ryan CL, Doucette TA. Effects of estrous stage and time of day on prepulse inhibition in female rats. J Neurosci Methods 2008b;173:295–8. Adams AL, Doucette TA, James R, Ryan CL. Persistent changes in learning and memory in rats following neonatal treatment with domoic acid. Physiol Behav 2009;96: 505–12. Baruch I, Hemsley DR, Gray JA. Differential performance of acute and chronic schizophrenics in a latent inhibition task. J Nerv Ment Dis 1988;176:598–606. Bethus I, Lemaire V, Lhomme M, Goodall G. Does prenatal stress affect latent inhibition? It depends on the gender. Behav Brain Res 2005;158:331–8. Bickel S, Javitt DC. Neurophysiological and neurochemical animal models of schizophrenia: focus on glutamate. Behav Brain Res 2009;204:352–62. Braff D, Stone C, Callaway E, Geyer M, Glick I, Bali L. Prestimulus effects on human startle reflex in normals and schizophrenics. Psychophysiology 1978;15:339–43. Burt MA, Ryan CL, Doucette TA. Altered responses to novelty and drug reinforcement in adult rats treated neonatally with domoic acid. Physiol Behav 2008a;93:327–36. Burt MA, Ryan CL, Doucette TA. Low dose domoic acid in neonatal rats abolishes nicotine induced place preference during late adolescence. Amino Acids 2008b;35: 247–9. Chabot CC, Taylor DH. Circadian modulation of the rat acoustic startle response. Behav Neurosci 1992;106:846–52. Cherubini E, Caiati MD, Sivakumaran S. In the developing hippocampus kainate receptors control the release of GABA from mossy fiber terminals via a metabotropic type of action. Adv Exp Med Biol 2011;717:11–26. Coyle JT. Glutamate and schizophrenia: beyond the dopamine hypothesis. Cell Mol Neurobiol 2006;26:365–84. Dobbing J, Smart JL. Vulnerability of developing brain and behaviour. Br Med Bull 1974;30:164–8. Doucette TA, Ryan CL, Tasker RA. Gender-based changes in cognition and emotionality in a new rat model of epilepsy. Amino Acids 2007;32:317–22. du Bois TM, Huang XF. Early brain development disruption from NMDA receptor hypofunction: relevance to schizophrenia. Brain Res Rev 2007;53:260–70. Egan MF, Goldberg TE, Kolachana BS, Callicott JH, Mazzanti CM, Straub RE, et al. Effect of COMT Val108/158 Met genotype on frontal lobe function and risk for schizophrenia. Proc Natl Acad Sci 2001;98:6917–22. Ellenbroek BA, Budde S, Cools AR. Prepulse inhibition and latent inhibition: the role of dopamine in the medial prefrontal cortex. Neuroscience 1996;75:535–42. Frankland PW, Ralph MR. Circadian modulation in the rat acoustic startle circuit. Behav Neurosci 1995;109:43–8. Frerking M, Nicoll RA. Synaptic kainate receptors. Curr Opin Neurobiol 2000;10(3): 342–51. Frerking M, Schmitz D, Zhou Q, Johansen J, Nicoll RA. Kainate receptors depress excitatory synaptic transmission at CA3 → CA1 synapses in the hippocampus via a direct presynaptic action. J Neurosci 2001;21:2958–66.

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