Sensory gating in rats depleted of dopamine as neonates: potential relevance to findings in schizophrenic patients

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Minocycline rescues decrease in neurogenesis, increase in microglia cytokines and deficits in sensorimotor gating in an animal model of schizophrenia

Adult neurogenesis in the hippocampus is impaired in schizophrenic patients and in an animal model of schizophrenia. Amongst a plethora of regulators, the immune system has been shown repeatedly to strongly modulate neurogenesis under physiological and pathological conditions. It is well accepted, that schizophrenic patients have an aberrant peripheral immune status, which is also reflected in the animal model. The microglia as the intrinsic immune competent cells of the brain have recently come into focus as possible therapeutic targets in schizophrenia. We here used a maternal immune stimulation rodent model of schizophrenia in which polyinosinic-polycytidilic acid (Poly I:C) was injected into pregnant rats to mimic an anti-viral immune response. We identified microglia IL-1β and TNF-α increase constituting the factors correlating best with decreases in net-neurogenesis and impairment in pre-pulse inhibition of a startle response in the Poly I:C model. Treatment with the antibiotic minocycline (3mg/kg/day) normalized microglial cytokine production in the hippocampus and rescued neurogenesis and behavior. We could also show that enhanced microglial TNF-α and IL-1β production in the hippocampus was accompanied by a decrease in the pro-proliferative TNFR2 receptor expression on neuronal progenitor cells, which could be attenuated by minocycline. These findings strongly support the idea to use anti-inflammatory drugs to target microglia activation as an adjunctive therapy in schizophrenic patients.

Physical exercise increases adult neurogenesis and telomerase activity, and improves behavioral deficits in a mouse model of schizophrenia

Epidemiological studies indicate that among other early life challenges, maternal infection with influenza during pregnancy increased the risk of developing schizophrenia in the child. One morphological manifestation of schizophrenia is hippocampal atrophy. In the hippocampus, playing a key role in learning and memory formation, new granule cell neurons are produced throughout life from resident precursor cells. We hypothesize that individuals exposed to a maternal anti-viral immune response would presumably enter life with a challenged neural precursor cell pool and might later be susceptible to psychiatric pathologies due to reduced adult neurogenesis. We used the injection of double-stranded RNA (polyriboinosinicpolyribocytidylic acid - PolyI:C) in pregnant C57Bl/6 and nestin-GFP reporter mice to induce a maternal viral-like infection and schizophrenia-like behavior in the offspring. In the progeny we found impairments in the open field test and in sensorimotor gating as measured by pre-pulse inhibition of the startle response. The behavioral deficits were accompanied by reduced baseline adult hippocampal neurogenesis. Telomerase activity in neural precursor cells was reduced from birth on and telomere shortening was found in the same cell type in adult life. When we subjected the progeny of viral-like infected dams to voluntary exercise, a known stimulus of adult hippocampal neurogenesis, we could rescue the phenotype in behavior, adult neurogenesis, and cellular senescence. In summary, maternal viral-like immune response reduced telomerase activity and resulted in telomere shortening in neural precursor cells. Further we demonstrate that beneficial behavioral and cellular effects induced by exercise can be studied in a rodent model of schizophrenia.

Dopamine is necessary for cue-dependent fear conditioning

Dopamine (DA) is implicated in many behaviors, including motor function, cognition, and reward processing; however, the role of DA in fear processing remains equivocal. To examine the role of DA in fear-related learning, dopamine-deficient (DD) mice were tested in a fear-potentiated startle paradigm. DA synthesis can be restored in DD mice through administration of 3, 4-dihydroxy-l-phenylalanine (l-Dopa), thereby permitting the assessment of fear processing in either a DA-depleted or -replete state. Fear-potentiated startle was absent in DD mice but could be restored by l-Dopa administration immediately after fear conditioning. Selective viral-mediated restoration of DA synthesis within the ventral tegmental area fully restored fear learning in DD mice, and restoration of DA synthesis to DA neurons projecting to the basolateral amygdala restored short-term memory but not long-term memory or shock sensitization. We also demonstrate that the DA D(1) receptor (D(1)R) and D(2)-like receptors are necessary for cue-dependent fear learning. These findings indicate that DA acting on multiple receptor subtypes within multiple target regions facilitates the stabilization of fear memory.

Early postnatal iron repletion overcomes lasting effects of gestational iron deficiency in rats

Iron deficiency anemia in early childhood causes developmental delays and, very likely, irreversible alterations in neurological functioning. One primary goal for the present study was to determine whether the effects of late gestational iron deficiency on brain monoamine metabolism, iron content, and behavioral phenotypes could be repaired with iron intervention in early lactation. Young pregnant rats were provided iron-deficient or control diets from mid-gestation (G15). At postnatal d 4 (P4), pups from iron-deficient dams were out-fostered either to other ID dams or control dams while pups of control dams were similarly fostered to other control dams. Dietary treatments continued to adulthood (P65) when brain iron and regional monoamines were evaluated. P4 iron repletion normalized body iron status, brain iron concentrations, monoamine concentrations, and monoamine transporter and receptor densities in most brain regions. Dopamine transporter densities in caudate and substantia nigra were lower in ID rats but were normalized with iron repletion. Serotonin transporter levels in most brain regions and open-field exploration were also normalized with iron repletion. The success of this approach of early postnatal iron intervention following iron deficiency in utero contrasts to a relative lack of success when the intervention is performed at weaning. These data suggest that a window of opportunity exists for reversing the detrimental effects of iron deficiency in utero in rats and provides strong support of intervention approaches in humans with iron deficiency during pregnancy.

Moderate iron deficiency in infancy: Biology and behavior in young rats

Iron deficiency anemia in early childhood is associated with developmental delays and perhaps, irreversible alterations in neurological functioning. The goals were to determine if dietary induced gestational and lactational iron deficiency alters brain monoamine metabolism and behaviors dependent on that neurotransmitter system. Young pregnant rats were provided iron deficient or control diets from early in gestation through to weaning of pups and brain iron concentration, regional monoamine variables and achievement of specific developmental milestones were determined throughout lactation. Despite anemia during lactation, most brain iron concentrations did not fall significantly until P25, and well after significant changes in monoamine levels, transporter levels, and D2R density changed in terminal fields. The changes in D2R density were far smaller than previously observed models that utilized severe dietary restriction during lactation or after weaning. Iron deficient pups had normal birth weight, but were delayed in the attainment of a number of milestones (bar holding, vibrissae-evoked forelimb placing). This approach of iron deficiency in utero and during lactation sufficient to cause moderate anemia but not stunt growth demonstrates that monaminergic metabolism changes occur prior to profound declines in brain iron concentration and is associated with developmental delays. Similar developmental delays in iron deficient human infants suggest to us that alterations in iron status during this developmental period likely affects developing brain monaminergic systems in these infants.

The neonate-6-hydroxydopamine-lesioned rat: a model for clinical neuroscience and neurobiological principles

In 1973, a technique of administering 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine) intracisternally to neonate rats was introduced to selectively reduce brain dopamine (neonate-lesioned rat). This neonate treatment proved unique when compared to rats lesioned as adults with 6-hydroxydopamine--prompting the discovery of differing functional characteristics resulting from the age at which brain dopamine is reduced. A realization was that neonate-lesioned rats modeled the loss of central dopamine and the increased susceptibility for self-injury in Lesch-Nyhan disease, which allowed identification of drugs useful in treating self-injury in mentally retarded patients. The neonate-lesioned rat has also been proposed to model the hyperactivity observed in attention-deficit hyperactivity disorder. Because the neonate-lesioned rat exhibits enhanced sensitization to repeated NMDA receptor antagonist administration and has functional changes characteristic of schizophrenia, the neonate lesioning is believed to emulate the hypothesized NMDA hypofunction in this psychiatric disorder. Besides modeling features of neurological and psychiatric disorders, important neurobiological concepts emerged from pharmacological studies in the neonate-lesioned rats. One was the discovery of coupling of D1/D2-dopamine receptor function. Another was the progressive increase in responsiveness to repeated D1-dopamine agonist administration referred to as "priming" of D1-dopamine receptor function. Additionally, a unique profile of signaling protein expression related to neonate reduction of dopamine has been identified. Thus, from modeling characteristics of disease to defining adaptive mechanisms related to neonatal loss of dopamine, the neonate-lesioned rat has had a persisting influence on neuroscience. Despite an extraordinary legacy from studies of the neurobiology of this treatment, a host of unknowns remain that will inspire future investigations.

Auditory inhibitory gating in the amygdala: Single-unit analysis in the behaving rat

Inhibitory sensory gating has been proposed to be a fundamental physiological process that filters neural input. Its temporal properties could allow for a rapid influence on vigilance and attention processes. Inhibitory mechanisms are reflected by reductions in neural responsiveness to repeated and well-predicted stimuli; for auditory gating, this translates into an inhibition of the neural activation to subsequent tone stimuli embedded within sequential and identical tone presentations. Here we expand previous neurophysiological data on inhibitory gating by examining gating in the amygdala using single-unit recording in freely moving animals. Previous data have shown the amygdala to be important in mediating rapid auditory sensory processing involved in emotional conditioning. We measured inhibitory gating with two matching auditory tones presented in a repetitive fashion (10 ms tones, ISI = 500 ms and 10 s between pairs) for 1 h (360 pairs). The majority of the tone responsive units showed inhibitory gating (78/95 units) located in both the medial and lateral subnuclei of the amygdala. Different types of tone responses were gated, including both shorter- and longer-duration excitatory tone responses as well as inhibitory tone responses. Different degrees of gating were found ranging from 100% inhibition (complete gating category) to 25% inhibition (graded gating category). The degree of gating varied over short-term and long-term time intervals. These findings demonstrate the existence of inhibitory gating in the amygdala and provide a detailed description of the basic properties of this rapid neural inhibition that could play an important role in filtering stimulus input.

An animal model of chronic placental insufficiency: relevance to neurodevelopmental disorders including schizophrenia

Evidence now suggests that compromised prenatal brain development may increase the risk for the manifestation of neurological disorders such as schizophrenia. We present a guinea-pig model which mimics a condition of human pregnancy, namely, chronic placental insufficiency. Previously we reported that at term there are changes in the brains of these offspring which are relevant to changes in patients with schizophrenia. The aim of this study was to examine whether deficits in brain structure persist to adolescence and young adulthood (8-12 weeks) and have implications for behavioral function. Reduced uteroplacental blood flow was induced via unilateral ligation of the uterine artery at mid-gestation. The brain was examined in control and prenatally compromised (PC) animals 8 weeks after birth using morphometric and immunohistochemical markers. In a separate cohort of animals, prepulse inhibition (PPI) of the acoustic startle response was assessed at 4, 8 and 12 weeks of age. Brain neurochemistry was examined by determining the concentrations of dopamine and its metabolite, dihydroxyphenylacetic acid (DOPAC), at 12 weeks using high performance liquid chromatography. In PC animals compared with controls there was a reduction in brain weight, persistent enlargement of the lateral ventricles, a reduction in the volume of the basal ganglia and septal region and no evidence of gliosis. No differences were observed in concentration of catecholamines in any brain region examined. At 12, but not 4 or 8, weeks of age, PPI was reduced in PC animals compared with controls. The findings of reduced brain weight, ventriculomegaly, reduced basal ganglia volume and absence of astrogliosis in the PC guinea-pig brain at adolescence parallel some of the changes observed in patients with schizophrenia. The impairment of PPI is comparable to sensorimotor gating deficits observed in patients with schizophrenia. These results indicate that adverse prenatal conditions lead to long-term alterations in brain structure and function which resemble alterations seen in patients with schizophrenia and therefore support the early neurodevelopmental hypothesis of schizophrenia.

Sustained extracellular signal-regulated kinase 1/2 phosphorylation in neonate 6-hydroxydopamine-lesioned rats after repeated D1-dopamine receptor agonist administration: implications for NMDA receptor involvement

Extracellular signal-regulated kinase (ERK) 1/2, a well known regulator of gene expression, is likely to contribute to signaling events underlying enduring neural adaptations. Phosphorylated (phospho)-ERK was examined immunohistochemically after both single and repeated (i.e., sensitizing) doses of the partial D1-dopamine (DA) receptor agonist SKF-38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benazepine HCl) to adult rats lesioned as neonates (neonate lesioned) with 6-hydroxydopamine. Remarkably, prolonged phospho-ERK accumulated primarily in layers II-III of medial prefrontal cortex (MPC), where it declined gradually yet remained significantly elevated for at least 36 d after repeated doses of SKF-38393. Sustained (> or =7 d) phospho-ERK was observed for shorter periods in various other cortical regions but was not detectable in striatum or nucleus accumbens. At 36 d, an additional injection of SKF-38393 to sensitized rats restored phospho-ERK to maximal levels only in MPC when examined 7 d later. Phosphorylated cAMP response element-binding protein (CREB), examined 7 d after the sensitizing regimen, was observed exclusively in MPC, where it was abundant throughout all layers. Systemic injections of SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile), an inhibitor of the upstream ERK activator mitogen ERK kinase, attenuated both ERK and CREB phosphorylation in layers II-III of MPC. Pretreatment with the D1 antagonist SCH-23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-OL maleate) inhibited the prolonged increase in MPC phospho-ERK, whereas the 5-HT2 receptor antagonist ketanserin (3-[2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl]-2,4(1H,3H)-quinazolinedione tartrate) was ineffective. Competitive and noncompetitive NMDA receptor antagonists also blocked sustained ERK phosphorylation. Collectively, the present results demonstrate coupling of D1 and NMDA receptor function reflected in sustained activation of the ERK signaling pathway in MPC of SKF-38393-sensitized neonate-lesioned rats. Ultimately, long-lasting phosphorylation of ERK and CREB in MPC may play a pivotal role in any permanent adaptive change(s) in these animals.

Effect of acute and chronic olanzapine treatment on phencyclidine-induced behavioral sensitization in rats with neonatal dopamine loss

In agreement with previous work, adult rats given selective lesions to dopamine (DA)-containing neurons as neonates exhibited a greater behavioral sensitization to repeated phencyclidine (PCP) treatment in comparison to sham-lesioned controls. Acute administration of olanzapine (1-5 mg/kg ip) or clozapine (15 mg/kg ip) decreased sensitized PCP-induced activity in both lesioned and control animals. Acute haloperidol (0.5 mg/kg ip) had no impact on PCP responsiveness in lesioned animals, but significantly antagonized PCP effects in sham-lesioned controls. Ketanserin, a selective 5-HT(2A)/5-HT(2C)-receptor antagonist, significantly reduced PCP activation in both lesioned and control rats, suggesting that the efficacy of atypical antipsychotics against PCP-induced sensitized responses may be mediated by one of the 5-HT(2)-receptor subtypes. A 6-week chronic regimen of orally administered olanzapine, clozapine, or haloperidol failed to block the sensitization induced by repeated PCP exposure. However, a 10-month oral olanzapine treatment significantly blunted the behavioral sensitization to repeated PCP exposure in lesioned animals, even after withdrawal from chronic olanzapine for more than 3 weeks. A 10-month oral haloperidol treatment had no effect on the sensitization induced by repeated PCP dosing. The persistent effect of chronic olanzapine administration on PCP sensitization may be relevant to the chronic therapeutic efficacy of atypical antipsychotics treating schizophrenia-a clinical syndrome linked to enhanced sensitivity to N-methyl-d-aspartate (NMDA)-receptor antagonists.

The effects of early maternal deprivation on auditory information processing in adult Wistar rats

BACKGROUND

There is now ample evidence that schizophrenia is due to an interaction between genetic and (early) environmental factors which disturbs normal development of the central nervous system and ultimately leads to the development of clinical symptoms. Recently, we showed that a single 24-hour period of maternal deprivation of rat pups at postnatal day 9 leads to a disturbance in prepulse inhibition, similar to what is seen in schizophrenia. The present set of experiments was designed to further characterize the information processing deficits of maternally deprived Wistar rats.

METHODS

Wistar rats were deprived from their mother for 24 hours on postnatal day 9. At adult age, rats were tested in the acoustic startle paradigm for prepulse inhibition and startle habituation. Rats were also tested in the evoked potentials paradigm for auditory sensory gating.

RESULTS

The results show that maternal deprivation led to a reduction in acoustic startle habituation and auditory sensory gating in adult rats. Moreover, maternal deprivation disrupted prepulse inhibition but only when the prepulses were given shortly (50-100 milliseconds) before the startle stimulus. At longer intervals (250-1000 milliseconds), no effect was seen.

CONCLUSIONS

The implications for the model and the development of disturbances in information processes are discussed.

Prenatal protein deprivation in rats induces changes in prepulse inhibition and NMDA receptor binding

Epidemiological studies suggest that prenatal malnutrition increases the risk of developing schizophrenia. Animal models indicate that prenatal protein deprivation (PPD) affects many aspects of adult brain function. We tested the hypothesis that PPD in rats would alter prepulse inhibition (PPI), which is an operational measure of sensorimotor gating that is deficient in schizophrenia patients. Additionally, we examined dopaminergic and glutaminergic receptor binding in the striatum and hippocampus, which have been suggested to play a role in the etiology of schizophrenia. Rat dams were fed normal (25%) or low (6%) protein diets beginning 5 weeks prior to, and throughout pregnancy. The pups were tested at postnatal days (PND) 35 and 56 for PPI. Striatal and hippocampal NMDA receptor, and striatal dopamine receptor binding were quantified post-mortem in a subset of these rats. Female rats exposed to PPD had reduced levels of PPI at PND 56, but not PND 35, suggesting the emergence of a sensorimotor gating deficit in early adulthood. Striatal NMDA receptor binding was increased in PPD females. A decrease in initial startle response (SR) was also observed in all PPD rats relative to control rats. These results suggest that PPD causes age- and sex-dependent decreases in PPI and increases in NMDA receptor binding. This animal model may be useful for the investigation of neurodevelopmental changes that are associated with schizophrenia in humans.

Prepulse inhibition deficits of the startle reflex in neonatal ventral hippocampal-lesioned rats: reversal by glycine and a glycine transporter inhibitor

BACKGROUND

Neonatal ventral hippocampal (NVH) lesions in rats induce behavioral abnormalities at adulthood thought to simulate some aspects of the positive, negative, and cognitive deficits classically observed in schizophrenic patients. Such lesions induce a postpubertal emergence of prepulse inhibition (PPI) deficits of the startle reflex reminiscent of the sensorimotor gating deficits observed in a majority of schizophrenic patients. To study the potential involvement of the glycinergic neurotransmission in such deficits, we investigated the capacity of glycine (an obligatory N-methyl-D-aspartate [NMDA] receptor co-agonist) and ORG 24598 (a selective glycine transporter 1 inhibitor) to reverse NVH lesion-induced PPI deficits in rats.

METHODS

Ibotenic acid was injected bilaterally into the ventral hippocampus of 7-day-old pups. Prepulse inhibition of the startle reflex was measured at adulthood.

RESULTS

Glycine (.8 and 1.6 g/kg IP) and ORG 24598 (10 mg/kg IP) fully and partially reversed lesion-induced PPI deficits, respectively.

CONCLUSIONS

These findings confirm that an impaired glutamatergic neurotransmission may be responsible for PPI deficits exhibited by NVH-lesioned rats and support the hypoglutamatergic hypothesis of schizophrenia. They also suggest that drugs acting either directly at the NMDA receptor glycine site or indirectly on the glycine transporter 1 could offer promising targets for the development of novel therapies for schizophrenia.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Prepulse inhibition of acoustic startle in aromatase knock-out mice: effects of age and gender

Estrogen has been suggested to play a neuromodulatory and neuroprotective role on the brain dopamine system. We used aromatase knockout (ArKO) mice that lack a functional aromatase enzyme and are unable to convert testosterone into estrogen, and assessed prepulse inhibition of acoustic startle, locomotor hyperactivity to amphetamine treatment and rotarod performance. Mice were tested at either 1 month, 4-5 months or 12-18 months of age. In male, but not female ArKO mice, there was an age-related reduction of prepulse inhibition. The 12-18 months old male ArKO mice also showed significantly greater amphetamine-induced hyperactivity. Mice heterozygous for the mutation showed no deficits or were in-between wildtype mice and ArKO mice. We postulate that these data indicate a neuroprotective role of estrogen, particularly in male mice, on ageing of brain mechanisms involved in pre-pulse inhibition and locomotor activity regulation. It is likely that these brain mechanisms are or include dopaminergic activity.

Schizophrenia and viral infection during neurodevelopment: a focus on mechanisms

The task of defining schizophrenia pathogenesis has fascinated and frustrated researchers for nearly a century. In recent years, unprecedented advances from diverse fields of study have given credence to both viral and developmental theories. This review considers possible mechanisms by which viral and developmental processes may interact to engender schizophrenia. Many of the current controversies in schizophrenia pathogenesis are reviewed in light of the viral hypothesis, including: epidemiological findings and the role of a genetic diathesis, phenotype heterogeneity, abnormalities in excitatory and inhibitory neurotransmitter systems, anomalous cerebral latereralization, and static vs progressive disease. The importance of animal models in elucidating the impact of viral infections on developing neurons is illustrated by recent studies in which neonatal rats are infected with lymphocytic choriomeningitis virus in order to examine alterations in hippocampal circuitry. Finally, consideration is given to a new hypothesis that some cases of schizophrenia could be instigated by a viral infection that disrupts developing inhibitory circuits, consequently unleashing glutamatergic neurotransmission leading to selective excitotoxicity, and a degenerative disease course.

Animal models for the negative symptoms of schizophrenia

Negative or defect symptoms refer to a reduction in normal functioning. In schizophrenia, negative symptoms encompass, among others, anhedonia, flat affect, avolition and social withdrawal. These symptoms have been found to be particularly prominent in the more chronic phase of the illness and seem to be virtually insensitive to current antipsychotic treatment. This review focuses on the possibilities and limitations of animal models for the negative symptoms of schizophrenia. Following a review of the negative symptoms in schizophrenia, attention is focused on the two symptoms most often modelled in animals - anhedonia and social withdrawal. We then look at the important question of how to model schizophrenic pathology in animals. Since the exact pathology is still far from clear, most efforts have in the past concentrated on using psychotomimetic drugs such as amphetamine or phencyclidine. The recently accumulated knowledge that schizophrenia probably results from disturbances in the normal development of the brain has led to a surge of new animal models in which the long-term consequences of early manipulations are investigated. However, so far these models have predominantly concentrated on the positive rather than the negative symptoms of schizophrenia. The last part of this review is dedicated to the question of validation of animal models for anhedonia and social withdrawal. The general conclusion is that very few models have so far been adequately tested. The lack of currently effective treatment further hampers the study of such validation.

Prenatal heroin exposure. Effects on development, acoustic startle response, and locomotion in weanling rats

The purpose of this study was to investigate the effects of prenatal heroin exposure on the offspring in postnatal behavioral development. Pregnant Sprague-Dawley rats were injected daily (s.c.) with 10mg/kg of heroin from gestational day 8 to 20. The control dam received saline injections and the pair-fed dam received saline and was yoked to a weight-matched heroin-treated dam. Litters were culled to eight to ten pups and weighed at postnatal day (PND) 1, 8, 15, and 22. Acoustic prepulse inhibition and habituation were parameters used for evaluating the sensorimotor gating and simple form of learning respectively. Locomotor activity and rearing were assessed using the photobeam activity system. All behavioral tests were performed on the offspring at PND 21 to 23. Results showed that heroin treatment significantly reduced maternal food intake, water consumption, and weight gain. Both heroin-exposed and pair-fed groups showed a marked reduction in birth weight in both male and female pups when compared with controls; however the postnatal weight gain in heroin-exposed pups was significantly lower than the pair-fed group by 3 weeks postnatally, particularly in the female pups. These female pups also showed a significant increase in ambulation and rearing when compared to the pair-fed pups. The habituation rate in both types of behavioral tests was also decreased in these female pups as compared to control and pair-fed groups. The present study indicated that prenatal heroin exposure could result in a marked retardation of postnatal development and learning. These effects are sex related.

The effects of an early stressful life event on sensorimotor gating in adult rats

There is increasing evidence that patients suffering from schizophrenia have disturbances in the brain and other parts of the body indicative of a disturbed development. These findings have led to the so-called neurodevelopmental hypotheses of schizophrenia, which state that schizophrenia (or a predisposition for this disease) results from perinatal disturbances which affect the normal development of the central nervous system. In order to study such a possible relationship we have used early short-lasting (24 h) maternal deprivation, and studied the influence of this life event on prepulse inhibition of the acoustic startle at adult age in rats, since it has been shown that schizophrenic patients show a disruption of prepulse inhibition. The results show that early maternal deprivation significantly reduced prepulse inhibition when the animals were tested at postnatal day (pnd) 69 (birth being pnd 0). The effects were qualitatively similar when deprivation took place on pnd 3, 6 or 9, although at the later days the effects were stronger. There was little influence on baseline startle response (except for a small reduction seen after deprivation on pnd 6). In separate experiments it was shown that the effect of maternal deprivation on prepulse inhibition was not seen before puberty and was similar for male and female offspring. Moreover, the effects could be reversed by treatment with the classical antipsychotic, haloperidol, or the putative atypical antipsychotic, quetiapine (both given 15 min before the prepulse inhibition experiment). In summary, the results show that an early stressful life event can have a delayed influence on prepulse inhibition in rats, qualitatively similar to the disturbances seen in schizophrenic patients. These data suggest that maternal deprivation (i.e., a 24 h separation of rat pups from their mother early in life) may represent an interesting animal model for investigating the influence of early life events on the information processing and general functioning of an individual at adult age.

Dissociation between startle and prepulse inhibition in rats exposed to gamma radiation at day 15 of embryogeny

The role of prenatal trauma in disordered sensory gating was explored in albino rats of the Sprague-Dawley strain. Pregnant rats were exposed to 1.5 Gy (0.15 Gy/min) of the whole-body gamma radiation on days 15, 17, or 19 of gestation. Controls were sham-exposed during 10 min in the same conditions. Exposed and control offsprings were evaluated for the auditory startle response (ASR) and its gating by either the habituation process or by the preceding weak sensory stimulus in the prepulse inhibition of startle (PPI) procedure. The tests were conducted when the animals reached 27 and 57 days of age. A noticeable hyperresponding and delayed habituation of startle were found in rats exposed at E15, with meager effects in rats exposed at E17 and E19. Maximal deficit was obtained on tests conducted on P57 but not on P27. However, in rats pretreated with amphetamine, dysfunctional startle was unmasked already on the P27 test. By contrast, PPI was insensitive to the damaging effect of prenatal irradiation at either period. This dissociation is reminiscent of one observed in patients with posttraumatic stress disorder (PTSD).

Effects of haloperidol and SCH 23390 on acoustic startle in animals depleted of dopamine as neonates: implications for neuropsychiatric syndromes

Animals depleted of dopamine (DA) in the neonatal period and tested in adulthood exhibit some similarities to patients with schizophrenia, including increased sensitivity to DA agonists, altered sensitivity to DA receptor antagonists, and abnormalities of the acoustic startle response (ASR). In this study, we examined the contributions of D1-like and D2-like DA receptors to ASR measures in animals depleted of DA as neonates. Male rat pups received intracerebroventricular injections of 6-hydroxydopamine (DA depleted) or its vehicle (controls) at 3 days of age. Animals underwent startle testing as adults (60-75 days of age) after administration of DA antagonists (haloperidol: 0.1 or 0.3 mg/kg, SCH 23390:0.01 or 0.05 mg/kg) with and without DA agonist administration (apomorphine 0.5 mg/kg). ASR amplitude and prepulse inhibition (PPI: percentage decrease in startle amplitude due to a low intensity prepulse) were measured. DA depleted animals showed increased ASR amplitude and reduced PPI compared to controls. Administration of D1-like or D2-like DA antagonists significantly reduced overall ASR and increased PPI in both control and DA depleted animals, with DA depleted animals showing a relatively greater sensitivity to the D1-like antagonist SCH 23390. Findings are discussed in terms of the role of residual DA in mediating ASR phenomena in depleted animals, differences between D1/D2 DA receptor mediation of ASR compared to other behaviors in DA depleted animals, and potential implications for neuropsychiatric syndromes such as schizophrenia.

Increased sensitivity to the sensorimotor gating-disruptive effects of apomorphine after lesions of medial prefrontal cortex or ventral hippocampus in adult rats

Sensorimotor gating of the startle reflex is impaired in humans with schizophrenia and in rats after mesolimbic D2 dopamine receptor activation. The loss of startle gating after D2 activation in rats has been used as an animal model of impaired sensorimotor gating in schizophrenia, because the ability of antipsychotics to restore startle gating in D2-activated rats correlates significantly with antipsychotic clinical potency. Substantial evidence indicates that the pathophysiology of schizophrenia includes structural and functional deficits in prefrontal and temporal regions, particularly the dorsolateral prefrontal cortex and the hippocampus and parahippocampal gyrus. The present study assessed startle gating in adult rats after ibotenic acid lesions of the medial prefrontal cortex or ventral hippocampus. Medial prefrontal cortex lesioned rats exhibited normal startle amplitude and normal sensorimotor gating, as reflected by prepulse inhibition (PPI) of the startle reflex. Hippocampus lesioned rats exhibited elevated startle amplitude, and similar to rats with medial prefrontal cortex lesions, did not show significant changes in basal PPI. Low doses of the mixed dopamine agonist apomorphine did not significantly reduce PPI in sham lesioned rats, but significantly disrupted PPI in both medial prefrontal cortex- and ventral hippo-campus lesioned rats. These data are consistent with the hypothesis that cell damage in frontal and temporal cortex increases the sensitivity to the sensorimotor gating-disruptive effects of dopamine receptor activation.

Neonatal excitotoxic hippocampal damage in rats causes post-pubertal changes in prepulse inhibition of startle and its disruption by apomorphine

Neonatal excitotoxic hippocampal damage in the rat results in postpubertal onset of a variety of abnormal behaviors related to excessive dopaminergic transmission in the mesolimbic/nigrostriatal system, and thus may be considered an animal model of some aspects of schizophrenia. Because sensorimotor gating is impaired in adult patients with schizophrenia and in rats with experimentally induced mesolimbic dopamine hyperactivity, the present experiments investigated the effects of neonatal (postnatal day 7, PD7) ibotenic acid (3 micrograms) lesions of the ventral hippocampus (VH) on the amplitude and prepulse inhibition (PPI) of acoustic startle in prepubertal (PD35) and postpubertal (PD56) rats. Startle was elicited using 105 and 118-dB pulses alone or preceded by 4, 8, or 16 dB above-background prepulses in rats treated with vehicle or apomorphine (APO; 0.025 or 0.1 mg/kg SC). At PD35, PPI in VH-lesioned rats did not differ significantly from these measures in sham operated rats. Apomorphine significantly increased startle amplitude and reduced PPI in both sham operated and VH-lesioned rats at PD35. At PD56, startle amplitude in VH-lesioned rats was not significantly different from controls, but PPI was reduced significantly compared to controls. Ventral hippocampus lesioned rats also exhibited an exaggerated reduction in PPI after treatment with APO. These findings provide further evidence of postpubertal impairments that may be related to increased mesolimbic dopamine transmission and receptor sensitivity in rats with neonatal hippocampal damage, and provide further support for the fidelity of this animal model of schizophrenia.

Effects of single and repeated exposure to apomorphine on the acoustic startle reflex and its inhibition by a visual prepulse

The acoustic startle reflex (ASR) is inhibited by startle-irrelevant stimuli that briefly precede reflex elicitation. This effect, prepulse inhibition (PPI), is reduced in strength for animals that have received dopamine agonists, such as apomorphine (APO). Reduction in PPI is most evident for weak masked noise prepulses, thus suggesting that APO disrupts the reception of stimuli to the extent that they present a low signal-to-noise ratio. Here we examine the effect of APO on PPI produced by non-masked visual prepulses. Light flashes were given at two intensities, 40, 70, 110, or 220 ms before ASR elicitation. In phase 1 (5 weeks in duration) half of the animals received one weekly injection of APO (0.5 mg/kg, IP) and one of vehicle (VEH), while the other half received two injections of VEH. Within these groups, half were tested 30 min after the injections, the other half kept test naive (four groups total). In phase 2, following a 4-week rest, all groups were tested after a low dose of APO (0.1 mg/kg) and VEH, 1 week apart. APO eliminated PPI for a dim flash and reduced PPI for a brighter flash to a level normally obtained with the dim flash, while increasing both ASR control values and activity. The bright light was maximally effective at a lead time of 70 ms and APO did not alter this value. Because in general the time of maximal inhibition varies with prepulse intensity for visual stimuli, the finding that the time of the peak remained constant reveals that APO has its effect on inhibition rather than on effective stimulus intensity. In phase 2, APO reduced PPI with no sign of sensitization from past drug exposure. However, APO increased the ASR only in groups previously exposed to APO, indicating behavioral sensitization. The differential effects of repeated exposure on these response measures suggest that neural substrates for the several behavioral effects of APO function at least in part independently.

Prepulse inhibition of the acoustic startle reflex using visual and auditory prepulses: disruption by apomorphine

The amplitude of the acoustic startle reflex can be reduced reliably when preceded at short intervals by a weak stimulus (prepulse) which itself does not elicit startle. The magnitude of this prepulse inhibition effect is attenuated by several dopamine agonists, such as apomorphine, especially when there is a relatively small difference between the intensity of the prepulse and the intensity of the background noise over which the prepulse is superimposed. One goal of the present experiment was to test the generality of this disruptive effect of apomorphine on prepulse inhibition by using either an auditory prepulse that included both a change in intensity and a change in frequency relative to the background noise or a visual prepulse stimulus. Apomorphine reduced auditory prepulse inhibition when induced by a small change in stimulus intensity, but not when induced by a change in both intensity and frequency. Apomorphine consistently reduced visual prepulse inhibition with a complete blockade at 100-ms test interval. However, it did not fully block the usual reduction in startle onset latency or even attenuate the increase in startle amplitude when a visual prepulse was presented 5, 10 or 15 ms before the startle stimulus. Consistent with conclusions from other laboratories using auditory prepulse inhibition, these data suggest that apomorphine did not prevent the animal from detecting prepulse presentation under conditions where the drug completely blocked prepulse inhibition. Moreover, they indicate that the blockade of prepulse inhibition by apomorphine was independent of prepulse modality, adding generality to the original finding. Visual prepulse inhibition may be a useful alternative procedure for evaluating the effects of drugs on this attentional process.

Alterations in behavioral responses to stressors following excitotoxin lesions of dorsomedial hypothalamic regions

The dorsomedial hypothalamus is important for regulation of cardiovascular responses associated with emotional arousal. This region has also been identified as a component of neural circuitry involved in fear/anxiety, yet clear evidence as to the effects of lesioning on stress-related behaviors is missing. In this study, we lesioned the dorsomedial hypothalamic region with the neurotoxin, ibotenic acid (IBO; 2.0 micrograms in 0.2 microliter), and studied the impact on spontaneous and unlearned behavioral responses to stressors. In the open field test, we observed non-generalized increases in motility parameters in the IBO rats with the differences occurring in the latter two-thirds of the test. In the elevated plus-maze, the IBO rats displayed a classic anxiolytic response with a greater proportion of entries into (and greater time spent in) the open arms of the maze. In the environment-specific social interaction (SI) test, the IBO rats showed a normal familiar/unfamiliar environment discrimination with respect to Total SI; however, the composition of the behaviors ('curiosity' vs. physical contact) by the IBO rats was markedly altered, with there being a 2-fold increase in non-violent physical interactions. Additionally, the differences in these traditional indices of anxiety were associated with lesioned animals exhibiting greater acoustic startle responsiveness than controls as a function of prepulse intensity. Overall, the results following IBO lesions indicate an altered responsiveness to sudden stressors, particularly as relates to novelty or exploration-oriented behaviors. The hypothalamic lesion may, therefore, have resulted in a disinhibition of normally suppressed responding to innate fear or challenging stimuli. This study contributes to those that have begun to define neural interactions that are essential for integrated stress responses.

Effects of haloperidol and SCH 23390 on acoustic startle and prepulse inhibition under basal and stimulated conditions

1. Adult Sprague-Dawley rats underwent startle testing for assessment of baseline startle amplitude and prepulse inhibition (PPI) of the startle reflex. 2. Animals were tested after administration of either: saline, a selective D1 dopamine (DA) receptor antagonist, a relatively selective D2 DA antagonist, or combined low dose D1 and D2 antagonists. 3. Changes due to antagonists were assessed with and without administration of the D1/D2 agonist apomorphine. 4. Testing without apomorphine stimulation showed that both D1 and D2 antagonists reduce baseline startle and enhance PPI. Further, the two antagonists exhibited a synergistic interaction. 5. Testing with apomorphine showed that D1 and D2 antagonists reduce apomorphine-induced startle enhancement. Again, the two exhibited a synergistic interaction. 6. For PPI, the D2 but not D1 antagonist reduced the apomorphine effect. However, the D1 antagonist potentiated the effect of the D2 antagonist.

Concurrent assessment of acoustic startle and auditory P50 evoked potential measures of sensory inhibition

The acoustic startle response (ASR) and midlatency auditory evoked potentials (AEP) have been utilized in the measurement of sensory inhibition. Using these different paradigms, abnormalities suggesting a lack of normal inhibition have been noted in a number of psychiatric syndromes. To date, the most commonly used sensory inhibition paradigms have not been studied in the same individuals, making generalizations across studies tenuous. In this report, reduction of ASR over multiple trials (habituation), prepulse inhibition (PPI) of ASR (decrease in ASR caused by low intensity prepulses) and P50 suppression (P50 AEP amplitude reduction in a paired-click paradigm) were measured in the same individuals. Relationships between these measures of acoustic startle and AEP inhibition were then assessed. Twenty subjects with no personal history of psychiatric disorder were tested and exhibited significant habituation and PPI of ASR as well as P50 suppression. Habituation of ASR was significantly and positively correlated with P50 suppression early, but not late, in AEP testing. Only a modest trend for a positive association between PPI and P50 suppression was noted. Habituation and PPI of startle were both highly correlated (positively) with P50 AEP amplitude. Habituation of startle remained significantly predictive of P50 suppression after controlling for P50 amplitude, whereas the modest association between PPI and P50 suppression was removed when P50 amplitude was factored out. Results indicate that habituation of acoustic startle, but not PPI, is highly associated with P50 suppression in control subjects. An unexpected finding was a robust positive correlation between P50 amplitude and both measures of startle inhibition. These findings and methodologic issues are discussed in terms of possible neural substrates involved in different measures of sensory inhibition.