Alterations to prepulse inhibition magnitude and latency in adult rats following neonatal treatment with domoic acid and social isolation rearing.

Deficits in perceptual, informational, and attentional processing are consistently identified as a core feature in schizophrenia and related neuropsychiatric disorders. Neonatal injections of low doses of the AMPA/kainate agonist domoic acid (DOM) have previously been shown to alter various aspects of perceptual and attentional processing in adult rats. The current study investigated the effects of combined neonatal DOM treatment with isolation rearing on prepulse inhibition behaviour and relevant neurochemical measures, to assess the usefulness of these paradigms in modeling neurodevelopmental disorders. Daily subcutaneous injections of DOM (20 µg/kg) or saline were administered to male and female rat pups from postnatal days (PND) 8-14. After weaning, rats were either housed alone or in groups of 4. Both the magnitude and latency of prepulse inhibition were determined in adulthood (approximately 4.5 months of age) and post-mortem brain tissue was assayed using Western blot. Social isolation alone significantly lowered PPI magnitude in male (but not female) rats while DOM treatment appeared to make animals refractory to this effect. Combining social isolation and DOM treatment caused an additive decrease in PPI startle latency. No statistically significant differences were found in the expression of D1, D2, TH, GAD65 or GAD67 protein in either the prefrontal cortex or hippocampus, although some tendencies toward differences were noted. We conclude that both neonatal low-dose DOM and social isolation affect prepulse inhibition in rats but that each paradigm exerts these effects through different neuronal signalling systems.

Perinatal Domoic Acid as a Neuroteratogen.

In mammals, the period shortly before and shortly after birth is a time of massive brain growth, plasticity and maturation. It is also a time when the developing brain is exquisitely sensitive to insult, often with long-lasting consequences. Many of society's most debilitating neurological diseases arise, at least in part, from trauma around the time of birth but go undetected until later in life. For the past 15 years, we have been studying the consequences of exposure to the AMPA/kainate agonist domoic acid (DOM) on brain development in the rat. Domoic acid is a naturally occurring excitotoxin that enters the food chain and is known to produce severe neurotoxicity in humans and other adult wildlife. Our work, and that of others, however, has demonstrated that DOM is also toxic to the perinatal brain and that toxicity occurs at doses much lower than those required in adults. This raises concern about the current regulatory limit for DOM contamination that is based on data in adult animals, but has also allowed creation of a novel model of neurological disease progression. Herein, we review briefly the toxicity of DOM in adults, including humans, and describe features of the developing nervous system relevant to enhanced risk. We then review the data on DOM as a prenatal neuroteratogen and describe in detail the work of our respective laboratories to characterize the long-term behavioural and neuropathological consequences of exposure to low-dose DOM in the newborn rat.

Neonatal domoic acid abolishes latent inhibition in male but not female rats and has differential interactions with social isolation.

Deficits in attention have long been identified as a core feature in schizophrenia and related neuropsychiatric disorders. We have investigated the combined effects of neonatal treatment with domoic acid (DOM) and social isolation rearing (both putative animal models of schizophrenia) on latent inhibition (LI), a measure of attentional processing. Daily subcutaneous injections of 20 µg/kg DOM or saline were administered to rat pups from postnatal days (PND) 8-14. After weaning, rats were housed either alone or in groups of 4 until LI was assessed at PND 110 using a lick-suppression conditional emotional response paradigm. Neonatal treatment with DOM abolished LI behaviour in adult male rats regardless of housing condition when tested 48 h after conditioning, but this effect was not observed in female rats. Social isolation rearing also reduced LI in male rats, but not to the same extent as DOM. When tested again one week later, single-housed males treated with DOM displayed significant LI whereas saline treated or group-housed DOM males did not. No significant differences were found with females 1 week later. We conclude that neonatal DOM and social isolation both impair attentional processing in young adult male, but not female, rats although the mechanisms by which this occurs appear to be different.

Voluntary forced use of the impaired limb following stroke facilitates functional recovery in the rat.

Constraint induced movement therapy (CIMT), which forces use of the impaired arm following stroke, improves functional recovery. The mechanisms underlying recovery are not well understood, necessitating further investigation into how rehabilitation may affect neuroplasticity using animal models. Animal motivation and stress make modelling CIMT in animals challenging. We have shown that following focal ischemia, voluntary forced use therapy using pet activity balls could engage the impaired forelimb and result in a modest acceleration in recovery. In this study, we investigated the effects of a more intensive appetitively motivated regimen that included task specific reaching exercises. Adult male Sprague Dawley rats were subjected to focal unilateral stroke using intracerebral injections of endothelin-1 or sham surgery. Three days later, stroke animals were assigned to daily rehabilitation or control therapy. Rehabilitation consisted of 30 min of generalized movement sessions in activity balls, followed by 30 min of voluntary task-specific movement using reaching boxes. Rats were tested weekly to measure forelimb deficit and recovery. After 30 days, animals were euthanized and tissue was examined for infarct volume, brain derived neurotrophic factor expression, and the presence of new neurons using doublecortin immunohistochemistry. Rehabilitation resulted in a significant acceleration of forelimb recovery in several tests, and a significant increase in the number of doublecortin-expressing cells. Furthermore, while the proportion of cells expressing BDNF in the peri-infarct region did not change, there was a shift in the cellular origin of expressed BDNF, resulting in significantly more non-neuronal, non-astrocytic BDNF, presumed to be of microglial origin.

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

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 20µg/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.

Altered social interaction in adult rats following neonatal treatment with domoic acid.

Schizophrenia is a debilitating neurological disorder characterized by positive, negative, cognitive and/or emotional symptoms. Decreased social interaction is a common negative symptom. Social interaction can be readily observed in rats and is therefore an ideal target behaviour when evaluating an animal model of schizophrenia. The purpose of this study was to determine whether early alterations in glutamate signaling resulted in social withdrawal; a finding which would be consistent with existing animal models of schizophrenia and is observed within the clinical population. In the present study, male and female SD rat pups received daily injections (s.c.) of very low doses of the glutamate agonist domoic acid (DOM; 20 microg/kg) or saline during a critical period of CNS development (i.e., PND 8-14). As adults, rats were assessed for degree of social interaction. During testing, each test rat was placed in a social interaction arena and scored for social contact with and avoidance of, a same-sex untreated conspecific. No differences were found in overall activity, nor were differences present for time spent engaged in neutral behavior (i.e., not engaged in behaviour, either directed toward or in avoidance of, the stimulus rat). However, domoate-treated male rats demonstrated evidence of social withdrawal, as evidenced by a significantly greater amount of time spent in avoidance behaviour and a significantly less amount of time spent engaged in social contact. These findings are discussed in context of the significance of early alteration to glutamate signaling in the development of human neuropathological disorders.

Persistent changes in learning and memory in rats following neonatal treatment with domoic acid.

The present study examined the effects of neonatal exposure to serial low dose injections of the glutamate agonist, domoic acid (DOM), on learning and memory in two spatial memory tasks in the rat. Neonatal Sprague Dawley rats were given single daily injections of low dose DOM (20 microg/kg) over postnatal days 8-14 and assessed as adolescents and adults in the radial 8-arm maze and the Morris Water Maze, respectively. Our results indicate that the DOM-treated rats showed a complex pattern of lasting alterations in learning and memory performance measures that were task specific. Adolescent DOM-treated animals, regardless of sex, demonstrated superior choice accuracy over seven days of testing in the 8-arm baited version of the radial maze. As adults, these same animals manifested improvements in several performance measures in the water maze. These improvements were also observed in a reversal task. However, when the escape platform was returned to its original position, some regression in search strategies were observed in the DOM-treated animals, especially the females, compared to their saline counterparts. These findings demonstrate that low-dose administration of a selective kainate receptor agonist during critical periods of CNS maturation produces lasting changes in learning and memory in the rat; adding to the ever-expanding body of literature which underscores the importance of optimal glutamatergic signaling to normal neurodevelopmental processes.

Effectiveness of PSD95 inhibitors in permanent and transient focal ischemia in the rat.

BACKGROUND AND PURPOSE: Postsynaptic density-95 inhibitors reduce ischemic brain damage without inhibiting excitatory neurotransmission, circumventing the negative consequences of glutamatergic inhibition. However, their efficacy in permanent ischemia and in providing permanent neuroprotection and neurobehavioral improvement in a practical therapeutic window is unproven. These were tested here under conditions that included fever, which is a common occurrence in clinical stroke. METHODS: Six studies were performed in unfasted Sprague-Dawley rats. Two involved permanent pial vessel occlusion in male and female rats. Two involved permanent middle cerebral artery occlusion, which induced severe hyperthermia, and 2 involved transient middle cerebral artery occlusion. Animals were treated with a single intravenous injection of postsynaptic density-95 inhibitors (Tat-NR2B9c([SDV]) or Tat-NR2B9c([TDV])) 1 hour or 3 hours after stroke. Infarct volumes and neurobehavior were assessed in a blinded manner at 24 hours (pial vessel occlusion and permanent middle cerebral artery occlusion) or at 62 days (transient middle cerebral artery occlusion). RESULTS: Postsynaptic density-95 inhibitors dramatically reduced infarct size in male and female animals exposed to pial vessel occlusion (>50%), in hyperthermic animals with fever exceeding 39 degrees C exposed to permanent middle cerebral artery occlusion (approximately 50%), and at 62 days poststroke in animals exposed to transient middle cerebral artery occlusion (approximately 80%). Effectiveness of postsynaptic density-95 inhibitors was achieved without the drugs affecting body temperature. In transient middle cerebral artery occlusion, a single dose of postsynaptic density-95 inhibitor given 3 hours after stroke onset permanently maintained reduced infarct size and improved neurobehavior. CONCLUSIONS: Postsynaptic density-95 inhibitors administrated 3 hours after stroke onset reduced infarct volumes and improved long-term neurobehavioral functions in a wide therapeutic window. This raises the possibility that they may have future clinical usefulness.

Effects of estrous stage and time of day on prepulse inhibition in female rats.

Prepulse inhibition (PPI) of the acoustic startle response is a measure of sensory motor gating, and is affected in various neuropsychiatric disorders. Although PPI has been used extensively to study both the neural effects of such conditions, as well as in the search for animal models, a number of critical issues have been encountered. Published methods for testing PPI vary widely across many parameters, two of the most common being the phase of the light/dark cycle during which the subjects are tested and the inclusion or exclusion of females. While previous research has attempted to clarify the effect of these factors, results for both human and animal studies have often been contradictory. This study investigated the relevance of the estrous cycle and time of day as variables that may influence PPI in adult female Sprague-Dawley rats. Results indicate that PPI is not affected by estrous phase, but may be affected by the time of day of testing, particularly at higher prepulse levels. At the 86 dBs prepulse level, rats tested during the light phase of the light/dark cycle displayed significantly lowered PPI as compared to the animals tested during the dark phase. Additionally, other measures such as baseline startle, habituation and activity during testing did not vary across the estrous or light cycles. These findings indicate that while estrous phase does not have any effect on PPI in female Sprague-Dawley rats when tested under these parameters, the time of day during which testing occurs does have the potential to alter PPI.

Altered responses to novelty and drug reinforcement in adult rats treated neonatally with domoic acid.

Activity in the mesocorticolimbic dopamine system is linked to responses to novelty, reward, and drug-seeking behaviours. Glutamate signaling, through kainate receptors, has been shown to modulate dopamine release in this pathway. In the present study, a low, overtly non-convulsive dose of the kainate receptor agonist, domoic acid (DOM), was administered to rat pups over PND 8-14. As juveniles and adolescents, rats were assessed in the open field. During adulthood, rats were tested in an open field, a sucrose consumption task, the playground maze and in a nicotine-induced conditioned place preference paradigm. Domoic acid related effects were found in open field behavior at each time point assessed. Male rats treated neonatally with DOM displayed altered novelty-related behaviour in a novelty trial, as indicated by an increase in time spent exploring familiar objects during the novelty trial of the playground maze. In nicotine-induced conditioned place preference, DOM-treated females developed a conditioned place preference for the nicotine-paired compartment of the test arena, an effect that was maintained for at least a month following the final drug-compartment pairing. The results of this study underscore the importance of the glutamate system in the ontogeny of behaviors that rely on the functional integrity of the midbrain dopamine system.

An improved post-operative care protocol allows detection of long-term functional deficits following MCAo surgery in rats.

Developing new therapeutants for stroke requires animal models in which typical stroke outcomes can be detected. In rats, temporary occlusion of the middle cerebral artery (MCAo) closely resembles reversible human ischemic stroke, but most neuroprotection studies have used limited, short-term (1-2 weeks) behavioural and histological endpoints in this model. Further, the use of this model for assessing long-term recovery has been questioned. Long-term deficits may be difficult to detect because testing may not reflect the diversity of functional outcomes in clinical stroke, and/or high mortality rates mean that only the least severely affected rats remain to be tested. We believe that enhanced survival through intensive post-operative husbandry practices, and extensive behavioural testing that reflects multiple clinically relevant behavioural endpoints, will permit the detection of long-term functional deficits. In the present study, male SD rats (280-320 g) received transient (90 min) MCAo (n = 19) or sham surgery (n = 13). An intense post-operative care protocol was maintained, and assessments included various physical and sensorimotor parameters, tests of emotionality and tests of learning and memory. We report 0% mortality, and statistically significant deficits on all aspects of this battery, including learning and memory deficits up to 2 months post-MCAo. The current study demonstrates that with adequate post-operative care and extensive behavioural testing, assessing the potential of new therapeutants for promoting long-term functional neuroprotection following MCAo in the rat is feasible.

Low doses of non-NMDA glutamate receptor agonists alter neurobehavioural development in the rat.

While it is known that glutamate is critical to CNS development and function, less is known about the role of kainate receptors, a subclass of ionotropic glutamate receptors, during ontogeny. This is especially true with respect to the emergence and expression of behaviour. It is also known that the neonatal CNS differs from that of adults with respect to excitatory amino acid (EAA) toxicity. Our aim was to determine the effects of early low-dose stimulation of kainate receptors on physical and behavioural development in the rat. Saline, one of two subtoxic doses of domoic acid (DOM) (5 and 20 microg/kg), or in a parallel study, saline, or one of two pharmacologically equivalent doses of kainic acid (KA) (25 and 100 microg/kg), were systemically administered once daily from postnatal days (PNDs) 8-14. While DOM or KA had no effect on typical measures of toxicity such as weight gain, acoustic startle, ultrasonic vocalizations (UVs), or maternal retrieval, these doses were shown to be physiologically relevant, producing particular group differences in eye opening, conditioned place preference, and activity levels. We conclude that administration of very low doses of selective kainate receptor agonists during the second postnatal week produces changes in neurobehavioural development in the rat.