Neonatal hippocampal Tat injections: developmental effects on prepulse inhibition (PPI) of the auditory startle response

Intraneuronal β-Amyloid Accumulation: Aging HIV-1 Human and HIV-1 Transgenic Rat Brain

The prevalence of HIV-1 associated neurocognitive disorders (HAND) is significantly greater in older, relative to younger, HIV-1 seropositive individuals; the neural pathogenesis of HAND in older HIV-1 seropositive individuals, however, remains elusive. To address this knowledge gap, abnormal protein aggregates (i.e., β-amyloid) were investigated in the brains of aging (>12 months of age) HIV-1 transgenic (Tg) rats. In aging HIV-1 Tg rats, double immunohistochemistry staining revealed abnormal intraneuronal β-amyloid accumulation in the prefrontal cortex (PFC) and hippocampus, relative to F344/N control rats. Notably, in HIV-1 Tg animals, increased β-amyloid accumulation occurred in the absence of any genotypic changes in amyloid precursor protein (APP). Furthermore, no clear amyloid plaque deposition was observed in HIV-1 Tg animals. Critically, β-amyloid was co-localized with neurons in the cortex and hippocampus, supporting a potential mechanism underlying synaptic dysfunction in the HIV-1 Tg rat. Consistent with these neuropathological findings, HIV-1 Tg rats exhibited prominent alterations in the progression of temporal processing relative to control animals; temporal processing relies, at least in part, on the integrity of the PFC and hippocampus. In addition, in post-mortem HIV-1 seropositive individuals with HAND, intraneuronal β-amyloid accumulation was observed in the dorsolateral PFC and hippocampal dentate gyrus. Consistent with observations in the HIV-1 Tg rat, no amyloid plaques were found in these post-mortem HIV-1 seropositive individuals with HAND. Collectively, intraneuronal β-amyloid aggregation observed in the PFC and hippocampus of HIV-1 Tg rats supports a potential factor underlying HIV-1 associated synaptodendritic damage. Further, the HIV-1 Tg rat provides a biological system to model HAND in older HIV-1 seropositive individuals.

Both HIV and Tat expression decrease prepulse inhibition with further impairment by methamphetamine

HIV infection and methamphetamine (METH) use are highly comorbid and represent a significant public health problem. Both conditions are known to negatively impact a variety of brain functions. One brain function that may be affected by HIV and METH use is sensorimotor gating, an automatic, pre-conscious filtering of sensory information that is thought to contribute to higher order cognitive processes. Sensorimotor gating is often measured using prepulse inhibition (PPI), a paradigm that can be conducted in both humans and animals, thereby enabling cross-species translational studies. While previous studies suggest HIV and METH may individually impair PPI, little research has been conducted on the effects of combined HIV and METH on PPI. The goal of this cross-species study was to determine the effects of METH on PPI in the inducible Tat (iTat) mouse model of HIV and in people with HIV. PPI was measured in the iTat mouse model before, during, and after chronic METH treatment and after Tat induction. Chronic METH treatment decreased PPI in male but not female mice. PPI normalized with cessation of METH. Inducing Tat expression decreased PPI in male but not in female mice. No interactions between chronic METH treatment and Tat expression were observed in mice. In humans, HIV was associated with decreased PPI in both men and women. Furthermore, PPI was lowest in people with HIV who also had a history of METH dependence. Overall, these results suggest HIV and METH may additively impair early information processing in humans, potentially affecting downstream cognitive function.

Chronic HIV-1 Tat exposure alters anterior cingulate cortico-basal ganglia-thalamocortical synaptic circuitry, associated behavioral control, and immune regulation in male mice

HIV-1 selectively disrupts neuronal integrity within specific brain regions, reflecting differences in viral tropism and/or the regional differences in the vulnerability of distinct neuronal subpopulations within the CNS. Deficits in prefrontal cortex (PFC)-mediated executive function and the resultant loss of behavioral control are a particularly debilitating consequence of neuroHIV. To explore how HIV-1 disrupts executive function, we investigated the effects of 48 h, 2 and/or 8 weeks of HIV-1 Tat exposure on behavioral control, synaptic connectivity, and neuroimmune function in the anterior cingulate cortex (ACC) and associated cortico-basal ganglia (BG)-thalamocortical circuitry in adult, Tat transgenic male mice. HIV-1 Tat exposure increased novelty-exploration in response to novel food, flavor, and environmental stimuli, suggesting that Tat triggers increased novelty-exploration in situations of competing motivation (e.g., drive to feed or explore vs. fear of novel, brightly lit open areas). Furthermore, Tat induced adaptability in response to an environmental stressor and pre-attentive filtering deficits. The behavioral insufficiencies coincided with decreases in the inhibitory pre- and post-synaptic proteins, synaptotagmin 2 and gephyrin, respectively, in the ACC, and alterations in specific pro- and anti-inflammatory cytokines out of 23 assayed. The interaction of Tat exposure and the resultant time-dependent, selective alterations in CCL4, CXCL1, IL-12p40, and IL-17A levels in the PFC predicted significant decreases in adaptability. Tat decreased dendritic spine density and cortical VGLUT1 inputs, while increasing IL-1β, IL-6, CCL5, and CCL11 in the striatum. Alternatively, IL-1α, CCL5, and IL-13 were decreased in the mediodorsal thalamus despite the absence of synaptic changes. Thus, HIV-1 Tat appears to uniquely and systematically disrupt immune regulation and the inhibitory and excitatory synaptic balance throughout the ACC-BG-thalamocortical circuitry resulting in a loss of behavioral control.

Diagnostic and prognostic biomarkers for HAND

In 2007, the nosology for HIV-1-associated neurocognitive disorders (HAND) was updated to a primarily neurocognitive disorder. However, currently available diagnostic tools lack the sensitivity and specificity needed for an accurate diagnosis for HAND. Scientists and clinicians, therefore, have been on a quest for an innovative biomarker to diagnose (i.e., diagnostic biomarker) and/or predict (i.e., prognostic biomarker) the progression of HAND in the post-combination antiretroviral therapy (cART) era. The present review examined the utility and challenges of four proposed biomarkers, including neurofilament light (NFL) chain concentration, amyloid (i.e., sAPPα, sAPPβ, amyloid β) and tau proteins (i.e., total tau, phosphorylated tau), resting-state functional magnetic resonance imaging (fMRI), and prepulse inhibition (PPI). Although significant genotypic differences have been observed in NFL chain concentration, sAPPα, sAPPβ, amyloid β, total tau, phosphorylated tau, and resting-state fMRI, inconsistencies and/or assessment limitations (e.g., invasive procedures, lack of disease specificity, cost) challenge their utility as a diagnostic and/or prognostic biomarker for milder forms of neurocognitive impairment (NCI) in the post-cART era. However, critical evaluation of the literature supports the utility of PPI as a powerful diagnostic biomarker with high accuracy (i.e., 86.7-97.1%), sensitivity (i.e., 89.3-100%), and specificity (i.e., 79.5-94.1%). Additionally, the inclusion of multiple CSF and/or plasma markers, rather than a single protein, may provide a more sensitive diagnostic biomarker for HAND; however, a pressing need for additional research remains. Most notably, PPI may serve as a prognostic biomarker for milder forms of NCI, evidenced by its ability to predict later NCI in higher-order cognitive domains with regression coefficients (i.e., r) greater than 0.8. Thus, PPI heralds an opportunity for the development of a brief, noninvasive diagnostic and promising prognostic biomarker for milder forms of NCI in the post-cART era.

Dose-dependent neurocognitive deficits following postnatal day 10 HIV-1 viral protein exposure: Relationship to hippocampal anatomy parameters

Despite the availability of antiretroviral prophylactic treatment, pediatric human immunodeficiency virus type 1 (HIV-1) continues to be a significant risk factor in the post-cART era. The time of infection (i.e., during pregnancy, delivery or breastfeeding) may play a role in the development of neurocognitive deficits in pediatric HIV-1. HIV-1 viral protein exposure on postnatal day (P)1, preceding the postnatal brain growth spurt in rats, had deleterious effects on neurocognitive development and anatomical parameters of the hippocampus (Fitting et al., 2008a,b). In the present study, rats were stereotaxically injected with HIV-1 viral proteins, including Tat1-86 and gp120, on P10 to further examine the role of timing on neurocognitive development and anatomical parameters of the hippocampus (Fitting et al., 2010). The dose-dependent virotoxin effects observed across development following P10 Tat1-86 exposure were specific to spatial learning and absent from prepulse inhibition and locomotor activity. A relationship between alterations in spatial learning and/or memory and hippocampal anatomical parameters was noted. Specifically, the estimated number of neurons and astrocytes in the hilus of the dentate gyrus explained 70% of the variance of search behavior in Morris water maze acquisition training for adolescents and 65% of the variance for adults; a brain-behavior relationship consistent with observations following P1 viral protein exposure. Collectively, late viral protein exposure (P10) results in selective alterations in neurocognitive development without modifying measures of somatic growth, preattentive processing, or locomotor activity, as characterized by early viral protein exposure (P1). Thus, timing may be a critical factor in disease progression, with children infected with HIV earlier in life being more vulnerable to CNS disease.

A Gap in Time: Extending our Knowledge of Temporal Processing Deficits in the HIV-1 Transgenic Rat

Approximately 50 % of HIV-1 seropositive individuals develop HIV-1 associated neurocognitive disorders (HAND), which commonly include alterations in executive functions, such as inhibition, set shifting, and complex problem solving. Executive function deficits in HIV-1 are fairly well characterized, however, relatively few studies have explored the elemental dimensions of neurocognitive impairment in HIV-1. Deficits in temporal processing, caused by HIV-1, may underlie the symptoms of impairment in higher level cognitive processes. Translational measures of temporal processing, including cross-modal prepulse inhibition (PPI), gap-prepulse inhibition (gap-PPI), and gap threshold detection, were studied in mature (ovariectomized) female HIV-1 transgenic (Tg) rats, which express 7 of the 9 HIV-1 genes constitutively throughout development. Cross-modal PPI revealed a relative insensitivity to the manipulation of interstimulus interval (ISI) in HIV-1 Tg animals in comparison to control animals, extending previously reported temporal processing deficits in HIV-1 Tg rats to a more advanced age, suggesting the permanence of temporal processing deficits. In gap-PPI, HIV-1 Tg animals exhibited a relative insensitivity to the manipulation of ISI in comparison to control animals. In gap-threshold detection, HIV-1 Tg animals displayed a profound differential sensitivity to the manipulation of gap duration. Presence of the HIV-1 transgene was diagnosed with 91.1 % accuracy using gap threshold detection measures. Understanding the generality and permanence of temporal processing deficits in the HIV-1 Tg rat is vital to modeling neurocognitive deficits observed in HAND and provides a key target for the development of a diagnostic screening tool.

Antiretroviral Treatment in HIV-1-Positive Mothers: Neurological Implications in Virus-Free Children

Since the worldwide introduction of antiretroviral therapy (ART) in human immunodeficiency virus type 1, HIV-1-positive mothers, together with HIV-1 testing prior to pregnancy, caesarian birth and breastfeeding cessation with replacement feeding, a reduction of HIV-1 mother-to-child transmission (MTCT) has been observed in the last few years. As such, an increasing number of children are being exposed in utero to ART. Several questions have arisen concerning the neurological effects of ART exposure in utero, considering the potential effect of antiretroviral drugs on the central nervous system, a structure which is in continuous development in the fetus and characterized by great plasticity. This review aims at discussing the possible neurological impairment of children exposed to ART in utero, focusing attention on the drugs commonly used for HIV-1 MTCT prevention, clinical reports of ART neurotoxicity in children born to HIV-1-positive mothers, and neurologic effects of protease inhibitors (PIs), especially ritonavir-“boosted” lopinavir (LPV/r) in cell and animal central nervous system models evaluating the potential neurotoxic effect of ART. Finally, we present the findings of a meta-analysis to assess the effects on the neurodevelopment of children exposed to ART in utero.

Temporal processsing demands in the HIV-1 transgenic rat: Amodal gating and implications for diagnostics

Despite the success of combination antiretroviral therapy (cART), approximately 50% of HIV-1 seropositive individuals develop HIV-1 associated neurocognitive disorders (HAND). Unfortunately, point-of-care screening tools for HAND lack sensitivity and specificity, especially in low-resource countries. Temporal processing deficits have emerged as a critical underlying dimension of neurocognitive impairments observed in HIV-1 and may provide a key target for the development of a novel point-of-care screening tool for HAND. Cross-modal prepulse inhibition (PPI; i.e., auditory, visual, or tactile prepulse stimuli) and gap-prepulse inhibition (gap-PPI; i.e., auditory, visual or tactile prepulse stimuli), two translational experimental paradigms, were used to assess the nature of temporal processing deficits in the HIV-1 transgenic (Tg) rat. Cross-modal PPI revealed a relative insensitivity to the manipulation of interstimulus interval (ISI) in HIV-1 Tg rats in comparison to controls, regardless of prestimulus modality. Gap-PPI revealed differential sensitivity to the manipulation of ISI, independent of modality, in HIV-1 Tg rats in comparison to control animals. Manipulation of context (i.e., concurrent visual or tactile stimulus) in auditory PPI revealed a differential sensitivity in HIV-1 Tg animals compared to controls. The potential utility of amodal temporal processing deficits as an innovative point-of-care screening tool was explored using a discriminant function analysis, which diagnosed the presence of the HIV-1 transgene with 97.4% accuracy. Thus, the presence of amodal temporal processing deficits in the HIV-1 Tg rat supports the hypothesis of a central temporal processing deficit in HIV-1 seropositive individuals, heralding an opportunity for the development of a point-of-care screening tool for HAND.

Progression of temporal processing deficits in the HIV-1 transgenic rat

The HIV-1 transgenic (Tg) rat, which expresses 7 of the 9 HIV-1 genes, was used to investigate the effect(s) of long-term HIV-1 viral protein exposure on chronic neurocognitive deficits observed in pediatric HIV-1 (PHIV). A longitudinal experimental design was used to assess the progression of temporal processing deficits, a potential underlying dimension of neurocognitive impairment in HIV-1. Gap prepulse inhibition (gap-PPI), a translational experimental paradigm, was conducted every thirty days from postnatal day (PD) 30 to PD 180. HIV-1 Tg animals, regardless of sex, displayed profound alterations in the development of temporal processing, assessed using prepulse inhibition. A differential sensitivity to the manipulation of interstimulus interval was observed in HIV-1 Tg animals in comparison to control animals. Moreover, presence of the HIV-1 transgene was diagnosed with 90.8% accuracy using measures of prepulse inhibition and temporal sensitivity. Progression of temporal processing deficits in the HIV-1 Tg rat affords a relatively untapped opportunity to increase our mechanistic understanding of the role of long-term exposure to HIV-1 viral proteins, observed in pediatric HIV-1, in the development of chronic neurological impairment, as well as suggesting an innovative clinical diagnostic screening tool.

Selective developmental alterations in The HIV-1 transgenic rat: Opportunities for diagnosis of pediatric HIV-1

Since the advent of combination antiretroviral therapy (cART), pediatric HIV-1 (PHIV) has evolved from a fatal disease to a chronic disease as children perinatally infected with HIV-1 survive into adulthood. The HIV-1 transgenic (Tg) rat, which expresses 7 of the 9 HIV-1 genes constitutively throughout development, was used to model the early development of chronic neurological impairment in PHIV. Male and female Fischer HIV-1 Tg and F344 N control rats, sampled from 35 litters, were repeatedly assessed during early development using multiple experimental paradigms, including somatic growth, locomotor activity, cross-modal prepulse inhibition (PPI) and gap-prepulse inhibition (gap-PPI). Later eye opening was observed in HIV-1 Tg animals relative to controls. HIV-1 Tg animals exhibited a shift in the development of locomotor activity implicating alterations in the maturation of the forebrain cholinergic inhibitory system. Alterations in the development of PPI and perceptual sharpening were observed in both auditory and visual PPI as indexed by a relative insensitivity to the dimension of time (msec for ISI; days of age for perceptual sharpening) as a function of the HIV-1 transgene. Presence of the HIV-1 transgene was diagnosed with 97.1 % accuracy using auditory and visual PPI measurements from PD 17 and 21. Early selective developmental alterations observed in the HIV-1 Tg rats provide an opportunity for the development of a point-of-care screening tool, which would permit the early diagnosis of PHIV and improve the long-term outcome for children perinatally infected with HIV-1.

The role of sensory modality in prepulse inhibition: An ontogenetic study

Deficits in prepulse inhibition (PPI), a measure of sensorimotor gating, are observed in neurodevelopmental and neuropsychiatric disorders. Despite the large PPI literature, the majority of studies characteristically employ tests with one interstimulus interval (ISI), of one modality, at one age. In the context of the auditory startle response (ASR), the present study examined (1) the profile for the ontogeny of PPI through adulthood in Long-Evans hooded rats with a reasonably comprehensive ISI function, (2) whether the ontogenetic profile for PPI is sensitive to modality of the prepulse stimulus, as a within-session variable, and (3) whether the maturation of PPI differs for males and females. Despite the basic effect of more pronounced PPI in adult relative to preweanling animals, each sensory modality displayed a unique ontogenetic profile for PPI, without any compelling evidence for major differences between males and females, in accordance with the known temporal course of peripheral and central maturational profiles of sensory systems in the rat. The context for assessing auditory PPI (auditory and tactile vs. auditory and visual prepulses) influenced the overall startle response, i.e., a shift in the height of the entire profile, but did not significantly impact the auditory PPI profile per se. The translational relevance of preclinical sensorimotor assessments to patients with neurodevelopmental and/or neuropsychiatric disorders depends partly on an understanding of the ontogeny of sensorimotor gating in different sensory systems, and can be strengthened with the use of a reasonably comprehensive number of ISIs to provide relatively precise and defined response functions.

Of mice and monkeys: can animal models be utilized to study neurological consequences of pediatric HIV-1 infection?

Pediatric human immunodeficiency virus (HIV-1) infection remains a global health crisis. Children are much more susceptible to HIV-1 neurological impairments than adults, which can be exacerbated by coinfections. Neurological characteristics of pediatric HIV-1 infection suggest dysfunction in the frontal cortex as well as the hippocampus; limited MRI data indicate global cerebral atrophy, and pathological data suggest accelerated neuronal apoptosis in the cortex. An obstacle to pediatric HIV-1 research is a human representative model system. Host-species specificity of HIV-1 limits the ability to model neurological consequences of pediatric HIV-1 infection in animals. Several models have been proposed including neonatal intracranial injections of HIV-1 viral proteins in rats and perinatal simian immunodeficiency virus (SIV) infection of infant macaques. Nonhuman primate models recapitulate the complexity of pediatric HIV-1 neuropathogenesis while rodent models are able to elucidate the role specific viral proteins exert on neurodevelopment. Nonhuman primate models show similar behavioral and neuropathological characteristics to pediatric HIV-1 infection and offer a stage to investigate early viral mechanisms, latency reservoirs, and therapeutic interventions. Here we review the relative strengths and limitations of pediatric HIV-1 model systems.

The HIV-1 transgenic rat model of neuroHIV

Despite the ability of current combination anti-retroviral therapy (cART) to limit the progression of HIV-1 to AIDS, HIV-positive individuals continue to experience neuroHIV in the form of HIV-associated neurological disorders (HAND), which can range from subtle to substantial neurocognitive impairment. NeuroHIV may also influence substance use, abuse, and dependence in HIV-positive individuals. Because of the nature of the virus, variables such as mental health co-morbidities make it difficult to study the interaction between HIV and substance abuse in human populations. Several rodent models have been developed in an attempt to study the transmission and pathogenesis of the HIV-1 virus. The HIV-1 transgenic (HIV-1Tg) rat is a reliable model of neuroHIV because it mimics the condition of HIV-infected patients on cART. Research using this model supports the hypothesis that the presence of HIV-1 viral proteins in the central nervous system increases the sensitivity and susceptibility of HIV-positive individuals to substance abuse.

Exposure to HIV-1 Tat in brain impairs sensorimotor gating and activates microglia in limbic and extralimbic brain regions of male mice

Human immunodeficiency virus (HIV) infection is associated with mood disorders and behavioral disinhibition. Impairments in sensorimotor gating and associated neurocognitive disorders are reported, but the HIV-proteins and mechanisms involved are not known. The regulatory HIV-1 protein, Tat, is neurotoxic and its expression in animal models increases anxiety-like behavior concurrent with neuroinflammation and structural changes in limbic and extra-limbic brain regions. We hypothesized that conditional expression of HIV-1 Tat1-86 in the GT-tg bigenic mouse model would impair sensorimotor gating and increase microglial reactivity in limbic and extralimbic brain regions. Conditional Tat induction via doxycycline (Dox) treatment (0-125 mg/kg, i.p., for 1-14 days) significantly potentiated the acoustic startle reflex (ASR) of GT-tg mice and impaired prepulse inhibition (PPI) of this response in a dose-dependent manner when Dox (100mg/kg) was administered for brief (1 day) or prolonged (daily for 7 days) intervals. A greater proportion of active/reactive Iba1-labeled microglia was seen in the anterior cingulate cortex (ACC), dentate gyrus, and nucleus accumbens core when Tat protein was induced under either brief or prolonged expression conditions. Other subregions of the medial prefrontal cortex, amygdala, hippocampal formation, ventral tegmental area, and ventral pallidum also displayed Tat-induced microglial activation, but only the activation observed in the ACC recapitulated the pattern of ASR and PPI behaviors. Tat exposure also increased frontal cortex GFAP. Pretreatment with indomethacin attenuated the behavioral effects of brief (but not prolonged) Tat-exposure. Overall, exposure to HIV-1 Tat protein induced sensorimotor deficits associated with acute and persistent neuroinflammation in limbic/extralimbic brain regions.

Neonatal intrahippocampal HIV-1 protein Tat(1-86) injection: neurobehavioral alterations in the absence of increased inflammatory cytokine activation

Pediatric AIDS caused by human immunodeficiency virus type 1 (HIV-1) remains one of the leading worldwide causes of childhood morbidity and mortality. HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the neurotoxicity of pediatric HIV-1 infection. Detrimental effects on development, behavior, and neuroanatomy follow neonatal exposure to the HIV-1 viral toxins Tat1-72 and gp120. The present study investigated the neurobehavioral effects induced by the HIV-1 neurotoxic protein Tat1-86, which encodes the first and second exons of the Tat protein. In addition, the potential effects of HIV-1 toxic proteins Tat1-86 and gp120 on inflammatory pathways were examined in neonatal brains. Vehicle, 25 μg Tat1-86 or 100 ng gp120 was injected into the hippocampus of male Sprague-Dawley pups on postnatal day 1 (PD1). Tat1-86 induced developmental neurotoxic effects, as witnessed by delays in eye opening, delays in early reflex development and alterations in prepulse inhibition (PPI) and between-session habituation of locomotor activity. Overall, the neurotoxic profile of Tat1-86 appeared more profound in the developing nervous system in vivo relative to that seen with the first exon encoded Tat1-72 (Fitting et al., 2008b), as noted on measures of eye opening, righting reflex, and PPI. Neither the direct PD1 CNS injection of the viral HIV-1 protein variant Tat1-86, nor the HIV-1 envelope protein gp120, at doses sufficient to induce neurotoxicity, necessarily induced significant expression of the inflammatory cytokine IL-1β or inflammatory factors NF-κβ and I-κβ. The findings agree well with clinical observations that indicate delays in developmental milestones of pediatric HIV-1 patients, and suggest that activation of inflammatory pathways is not an obligatory response to viral protein-induced neurotoxicity that is detectable with behavioral assessments. Moreover, the amino acids encoded by the second tat exon may have unique actions on the developing hippocampus.

The role of ephrin-A2 and ephrin-A5 in sensorimotor control and gating

Many factors influence neurodevelopment. However, their contribution to adult neural function is often unclear. This is often due to complex expression profiles, cell signalling, neuroanatomy, and a lack of effective tests to assess the function of neural circuits in vivo. Ephrin-A2 and ephrin-A5 are cell surface proteins implicated in multiple aspects of neurodevelopment. While the role of ephrin-As in visual, auditory and learning behaviours has been explored, little is known about their role in dopaminergic and neuromotor pathways, despite expression in associated brain regions. Here we probe the function of ephrin-A2 and ephrin-A5 in the development of the dopaminergic and neuromotor pathways using counts of tyrosine hydroxylase (TH) positive cells in the substantia nigra pars compacta (SNpc) and the ventral tegmental area (VTA), the acoustic startle reflex (ASR), and a measure of sensorimotor gating, prepulse inhibition (PPI). Analysis of the ASR and PPI in ephrin-A2 and/or ephrin-A5 knock-out mice revealed that both genes play distinct roles in mediating ASR circuits, but are unlikely to play a role in PPI. Knock-out of either gene resulted in robust changes in startle response magnitude and measures of startle onset and peak latencies. However, ephrin-A2 and ephrin-A5 regulate aspects of the ASR differently: ephrin-A2 KO mice have increased startle amplitude, increased sensitivity and reduced latency to startle, whilst ephrin-A5 KO mice show opposite effects. Neither of the gene knock outs affected PPI, despite ephrin-A5 KO mice showing changes in dopamine cell numbers in nuclei thought to regulate PPI. We propose that majority of the changes observed ephrin-A2 and ephrin-A5 KO mice appear to be mediated by the effects on motor neurons and their muscle targets, rather than changes in auditory sensitivity.

Prepulse inhibition in HIV-1 gp120 transgenic mice after withdrawal from chronic methamphetamine

HIV infection is frequently comorbid with methamphetamine (METH) dependence. Both factors are associated with impairment in inhibitory function that continues even after abstinence from the drug. Deficits in prepulse inhibition (PPI), a measure of sensorimotor gating, are induced by acute stimulant administration, but the combined effect of HIV and chronic METH exposure on PPI is not well characterized. We quantified baseline acoustic startle and PPI in mice expressing the HIV-1 gp120 envelope protein (gp120tg) and in wild-type (WT) littermates; thereafter, we administered a chronic regimen of METH or vehicle and tested startle and PPI after 7 days of drug withdrawal. We hypothesized that METH-treated gp120tg mice would exhibit PPI deficits compared with vehicle-treated WT or gp120tg animals. Before METH administration, drug-naive female gp120tg mice exhibited decreased PPI compared with female WT mice, whereas male gp120tg mice exhibited increased startle compared with other groups. After drug withdrawal, no consistent genotype effect was observed, but METH-treated mice exhibited increased PPI compared with vehicle, in contrast to previous reports of acute METH-induced PPI deficits. In summary, PPI impairment in HIV could depend on factors such as sex, whereas changes in PPI following METH withdrawal may depend on the quantity and duration of drug exposure.

Time and time again: temporal processing demands implicate perceptual and gating deficits in the HIV-1 transgenic rat

HIV-1-associated neurocognitive disorders (HAND) afflict up to 50 % of HIV-1+ individuals, despite the effectiveness of combination antiretroviral therapy (CART) in reducing the prevalence of more severe neurocognitive impairment. Alterations in brainstem auditory evoked potentials (BAEP), a measure of temporal processing, are one of the earliest neurological abnormalities of HIV-1-positive individuals. Prepulse inhibition (PPI) of the auditory startle response (ASR), a measure of sensorimotor gating, was studied in HIV-1 transgenic (Tg) rats, which express 7 of the 9 HIV-1 genes. Ovariectomized female Fischer HIV-1 Tg and control rats (ns = 41-42) were tested for PPI at three test periods, with at least 2 months separating each test period, using auditory and visual prepulses, an auditory startle stimulus, and interstimulus intervals (ISI) ranging from 0 to 4000 msec. Auditory and visual prepulse trial blocks were presented in counterbalanced order. For both auditory and visual prepulses, HIV-1 Tg animals exhibited a flatter ISI function, which did not sharpen with age, as it did in controls. Over time, auditory prepulses precipitated a temporal shift in peak inhibition in HIV-1 Tg animals relative to controls, whereas with visual prepulses, both groups displayed peak inhibition at the 40 msec ISI. A lack of perceptual sharpening with age and a relative insensitivity to the temporal dimension of sensorimotor gating are evident in the HIV-1 Tg rat prior to clinical signs of wasting. Deficits in sensorimotor gating may not only provide an early subtle diagnostic marker of HAND, but may also afford a key target for development of potential therapeutics.

Prenatal IV Cocaine: Alterations in Auditory Information Processing

One clue regarding the basis of cocaine-induced deficits in attentional processing is provided by the clinical findings of changes in the infants' startle response; observations buttressed by neurophysiological evidence of alterations in brainstem transmission time. Using the IV route of administration and doses that mimic the peak arterial levels of cocaine use in humans, the present study examined the effects of prenatal cocaine on auditory information processing via tests of the auditory startle response (ASR), habituation, and prepulse inhibition (PPI) in the offspring. Nulliparous Long-Evans female rats, implanted with an IV access port prior to breeding, were administered saline, 0.5, 1.0, or 3.0 mg/kg/injection of cocaine HCL (COC) from gestation day (GD) 8-20 (1×/day-GD8-14, 2×/day-GD15-20). COC had no significant effects on maternal/litter parameters or growth of the offspring. At 18-20 days of age, one male and one female, randomly selected from each litter displayed an increased ASR (>30% for males at 1.0 mg/kg and >30% for females at 3.0 mg/kg). When reassessed in adulthood (D90-100), a linear dose-response increase was noted on response amplitude. At both test ages, within-session habituation was retarded by prenatal cocaine treatment. Testing the females in diestrus vs. estrus did not alter the results. Prenatal cocaine altered the PPI response function across interstimulus interval and induced significant sex-dependent changes in response latency. Idazoxan, an α(2)-adrenergic receptor antagonist, significantly enhanced the ASR, but less enhancement was noted with increasing doses of prenatal cocaine. Thus, in utero exposure to cocaine, when delivered via a protocol designed to capture prominent features of recreational usage, causes persistent, if not permanent, alterations in auditory information processing, and suggests dysfunction of the central noradrenergic circuitry modulating, if not mediating, these responses.

Prenatal IV nicotine exposure produces a sex difference in sensorimotor gating of the auditory startle reflex in adult rats

Maternal smoking during pregnancy is associated with auditory processing deficits in children; these effects have been confirmed with animal models of continuous high-dose prenatal nicotine exposure. The present experiments utilized a novel, low-dose, intermittent, intravenous (IV) gestational nicotine exposure model to investigate potential deficits on the preattentive process of sensorimotor gating, as indexed by prepulse inhibition (PPI), in preweanling and adult rat offspring. Pregnant dams received bolus IV injections of nicotine (0.05 mg/kg/injection) 3×/day on gestational days 8-21. Auditory and tactile stimulus modalities were probed with tone and air puff prepulse stimuli, respectively. These prepulse stimuli preceded a 100 dB(A) startle tone by six different interstimulus intervals (ISIs; 0, 8, 40, 80, 120, 4000 ms) to define a curve of response inhibition. The magnitude of PPI increased with age, from 59 to 81% inhibition. Preweanlings (PNDs 14 and 18) and adults (PND 75) gestationally exposed to nicotine exhibited altered startle responding relative to controls, but the nature of the deficit became more localized at later ages. The entire curve of response inhibition in preweanlings exposed to prenatal nicotine (PND 14) was shifted up relative to controls, and notably, did not interact with prepulse stimulus modality, suggesting a generalized increased sensorimotor responsiveness as a function of prenatal nicotine. At PND 18, a shift in the response curve across all ISIs was again noted, but varied as a function of prepulse stimulus modality; the increased sensorimotor responsiveness was specific to the auditory, but not tactile, sensory modality. In adulthood, male and female animals prenatally exposed to nicotine were differentially sensitive to modulation by the ISIs, relative to control male and female animals. Specifically, despite robust PPI, adult females exposed to gestational nicotine were relatively insensitive to changes in ISI from 8 to 120 ms; in contrast, the robust PPI of nicotine-exposed males demonstrated a clear focal point of inhibition at 40 ms. These findings indicate that a low, daily dosing of IV prenatal nicotine produces long-lasting alterations in auditory PPI. An important implication of this research is that "chipping" with smoked-tobacco products during pregnancy may produce enduring changes in sensorimotor processing.

Dose-dependent long-term effects of Tat in the rat hippocampal formation: a design-based stereological study

The human immunodeficiency virus type 1 (HIV-1) protein transactivator of transcription (Tat) is believed to play a critical role in mediating central nervous system (CNS) pathology in pediatric HIV-1 infection. Long-term neurotoxicity was investigated in a design-based stereology study following intrahippocampal injection of Tat on postnatal day (P)10, a time period that approximates the peak in the rats' rate of brain growth and mimics clinical HIV-1 CNS infection at labor/delivery. The goal was to examine the impact of P10 intrahippocampal Tat injection on the anatomy of the adult hippocampus (5 month) to gain a better understanding about how timing of infection influences the rate of progression of pediatric HIV-1 infection [cf. Fitting et al. (2008a) Hippocampus 18:135-147]. Male P10 Sprague-Dawley rats were bilaterally injected with vehicle or one of three different doses of Tat (5, 25, or 50 mug). Unbiased stereological estimates were used to quantify total neuron number (Nissl stain) in five major subregions of the rat hippocampus: granular layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB). Glial cells (astrocytes and oligodendrocytes) were quantified in the DGH and SUB. No significant reduction of neuron number was noted for any of the five hippocampal subregions, in contrast to the very prominent reductions reported when Tat was administered on P1 [Fitting et al. (2008a) Hippocampus 18:135-147]. However, for glial cells, the number of astrocytes in the DGH and SUB as well as the number of oligodendrocytes in the DGH were linear dose dependently increased as a function of dose of Tat. In conjunction with previous stereological research [Fitting et al., (2008a) Hippocampus 18:135-147], the present data suggest that variability in the progression of pediatric HIV/acquired immunodeficiency syndrome (AIDS) may be better understood with the knowledge of the factor of timing of HIV-1 CNS infection.

Evidence for developmental dopaminergic alterations in the human immunodeficiency virus-1 transgenic rat

Neurologic impairments associated with human immunodeficiency virus (HIV) infection in pediatric patients may affect quality of life, and can develop despite antiretroviral therapy (ART). Behavioral changes observed in clinical studies of HIV-infected children suggest alterations in dopaminergic neurotransmission. Findings from our model of choice, the HIV-1 transgenic rat, reveal a significant increase in phosphorylated tyrosine hydroxylase protein expression and a decrease in dopamine transporter mRNA, without changes in tyrosine hydroxylase (TH) or dopamine transporter (DAT) protein or in more general markers of protein and gene expression levels in the HIV-1 transgenic rat midbrain. Thus these findings suggest selective vulnerability of the dopamine system in developing brains to HIV-1 infection.

Neonatal intrahippocampal injection of the HIV-1 proteins gp120 and Tat: differential effects on behavior and the relationship to stereological hippocampal measures

HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the central nervous system (CNS) pathology of acquired immune deficiency syndrome (AIDS). The present study sought to determine the potential role of Tat and/or gp120 on behavioral development and the relationship to the long-term effects of the HIV-1 proteins on the rat hippocampus. Male pups of 13 Sprague-Dawley litters were bilaterally injected on postnatal day (P)1. Every litter contributed an animal to each of four treatment condition: VEH (0.5 microl sterile buffer), gp120 (100 ng), Tat (25 microg) or combined gp120+Tat (100 ng+25 microg). Body weight was not affected by either protein treatment. Tat revealed a transient effect on many of the behavioral assessments early in development as well as on preattentive processes and spatial memory in adulthood. Gp120 had more selective effects on negative geotaxis (P8-P10) and on locomotor activity (P94-P96). Combined gp120+Tat effects were noted for eye opening with potential interactive effects of gp120 and Tat on negative geotaxis. Anatomical assessment at approximately 7 1/2 months of age was conducted by using design-based stereology to quantify the total cell number in five hippocampal subregions [granule layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB)] [Fitting, S., Booze, R.M., Hasselrot, U., Mactutus, C.F., 2007a. Differential long-term neurotoxicity of HIV-1 proteins in the rat hippocampal formation: a design-based stereological study. Hippocampus 18(2), 135-147]. A relationship between early reflex development and estimated cell number in the adult hippocampus was indicated by simple regression analyses. In addition, estimated number of neurons and astrocytes in the DGH explained 81% of the variance of the distribution of searching behavior in the probe test. Collectively, these data indicate that the DGH may participate in the spatial memory alterations observed in adulthood consequent to neonatal exposure to HIV-1 proteins.

Realistic expectations of prepulse inhibition in translational models for schizophrenia research

INTRODUCTION

Under specific conditions, a weak lead stimulus, or "prepulse", can inhibit the startling effects of a subsequent intense abrupt stimulus. This startle-inhibiting effect of the prepulse, termed "prepulse inhibition" (PPI), is widely used in translational models to understand the biology of brainbased inhibitory mechanisms and their deficiency in neuropsychiatric disorders. In 1981, four published reports with "prepulse inhibition" as an index term were listed on Medline; over the past 5 years, new published Medline reports with "prepulse inhibition" as an index term have appeared at a rate exceeding once every 2.7 days (n=678). Most of these reports focus on the use of PPI in translational models of impaired sensorimotor gating in schizophrenia. This rapid expansion and broad application of PPI as a tool for understanding schizophrenia has, at times, outpaced critical thinking and falsifiable hypotheses about the relative strengths vs. limitations of this measure.

OBJECTIVES

This review enumerates the realistic expectations for PPI in translational models for schizophrenia research, and provides cautionary notes for the future applications of this important research tool.

CONCLUSION

In humans, PPI is not "diagnostic"; levels of PPI do not predict clinical course, specific symptoms, or individual medication responses. In preclinical studies, PPI is valuable for evaluating models or model organisms relevant to schizophrenia, "mapping" neural substrates of deficient PPI in schizophrenia, and advancing the discovery and development of novel therapeutics. Across species, PPI is a reliable, robust quantitative phenotype that is useful for probing the neurobiology and genetics of gating deficits in schizophrenia.

Differential long-term neurotoxicity of HIV-1 proteins in the rat hippocampal formation: a design-based stereological study

The human immunodeficiency virus type 1 (HIV-1) proteins, gp120 and Tat, are believed to play a role in mediating central nervous system (CNS) pathology in HIV-1 infected patients. Using design-based stereology, we examined the role of neonatal intrahippocampal injections of gp120 and Tat on the adult hippocampus ( approximately 7(1/2) month). Postnatal day (P)1-treated Sprague-Dawley rats were bilaterally injected with vehicle (VEH, 0.5 microl sterile buffer), gp120 (100 ng), Tat (25 microg) or combined gp120 + Tat (100 ng + 25 microg). Using Nissl-stained tissue sections, we quantified total neurons in five subregions of the rat hippocampus [granual layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB)], and total glial cells (astrocytes and oligodendrocytes) in two subregions (DGH and SUB). Estimates of cell area and cell volume were taken in the DGH. There was a significant reduction of neuron number in the CA2/3 subfield by Tat and gp120, and a significant reduction in the DGH by Tat only. For glial cells, numbers of astrocytes in the DGH and SUB were increased by the Tat protein, whereas no effects were noted for gp120. Finally, for oligodendrocytes Tat increased cell number in the DGH but not in any other region; gp120 had no detectable effect in any brain region. Estimates of cell area and cell volume of the three different cell types revealed no significant differences between treatments. Collectively, these results suggest differential effects of gp120 and Tat on the estimated total number of neurons, as well as on the number of glial cells.

Neurotoxicity of HIV-1 Tat protein: involvement of D1 dopamine receptor

Neurotoxic viral proteins released from HIV-infected cells are believed to play a major role in the pathogenesis of the dementia displayed in a significant number of AIDS patients. HIV-1 associated neuropathology severely affects dopaminergic regions of the brain. Growing evidence indicates that HIV-1 neurotoxic proteins, such as Tat may affect the function of the dopamine transmission system. In turn, molecular components of dopamine neurotransmission may participate in a complex network of Tat-induced cell responses which result in neurodegeneration. In this study we investigated whether D1 dopamine receptors are involved in the mechanism of Tat neurotoxicity in primary rat neuronal cell cultures. We found that in rat midbrain cell cultures, which express significant levels of D1 dopamine receptors, the specific D1 antagonist SCH 23390 attenuates the cell death caused by HIV-1 Tat. In rat hippocampal cell cultures, where the expression of D1 receptors is low, SCH 23390 did not change the toxicity of Tat. Thus, the protective effect of SCH 23390 in rat primary neuronal cell cultures is a function of the level of D1 receptor protein expression. Our results provide further evidence for the involvement of the dopaminergic transmission system in the mechanism of HIV-1 Tat neurotoxicity.

Neonatal intrahippocampal gp120 injection: an examination early in development

The presence of human immunodeficiency virus type 1 (HIV-1) in the brain is believed to be responsible for mediating the pathogenesis of neurological abnormalities through the viral toxins gp120 and Tat. Numerous studies indicate neurotoxic effects of the HIV-1-protein Tat, with demonstrated neurobehavioral and cognitive alterations. However, less clear is the neurotoxic effect of gp120 on neurobehavior. This study was designed to characterize the potential deficits in sensory-motor and preattentive functions, following intrahippocampal administration of gp120. Using a randomized-block design, male and female pups of eight Sprague-Dawley litters were injected bilaterally with either vehicle (VEH) (1 microl volume) or one of the three gp120 doses (1.29, 12.9, or 129 ng/microl) at postnatal day (P)1. Sensory-motor functions were assessed at P3, as measured by the righting reflex and at P8, as measured by negative geotaxis. At P24 animals were tested on preattentive processes, as indexed by sensorimotor gating. Sensorimotor gating was measured by prepulse inhibition (PPI) of the auditory startle response (ASR) (ISIs of 0, 8, 40, 80, 120, and 4000 ms, six trial blocks, Latin-square design). Results indicated gp120-induced neurotoxicity on the righting reflex but not negative geotaxis. For sensorimotor gating, the PPI test demonstrated a reduced inhibition response on peak ASR latency as the dose of gp120 increased. No effect was noted for response inhibition on peak ASR amplitude. These data suggest that intrahippocampal injection of gp120 (0, 1.29, 12.9, or 129 ng/microl) had transient neurotoxic effects on sensory-motor function and limited effects on preattentive processes early in development.

Neonatal intrahippocampal glycoprotein 120 injection: the role of dopaminergic alterations in prepulse inhibition in adult rats

Following neonatal hippocampal administration on postnatal day 1, the dose-response effects of the human immunodeficiency virus 1 protein glycoprotein 120 (gp120) were studied in vivo on prepulse inhibition (PPI) in adulthood. Furthermore, the role of dopaminergic alterations was examined as a within-subject factor. Using a randomized-block design, male and female pups of eight Sprague-Dawley litters were injected bilaterally with either vehicle (1 microl volume) or gp120 (1.29, 12.9, or 129 ng/microl). At 9 months of age, rats were injected s.c. with saline (SAL) (0.1 ml/kg) and tested on preattentive processes, as indexed by sensorimotor gating. Sensorimotor gating was measured by PPI of the auditory startle response (ASR) [interstimulus intervals (ISIs) of 0, 8, 40, 80, 120, and 4000 ms, six trial blocks, Latin square design]. One month later, the animals were treated with a D(1)/D(2) agonist, apomorphine (APO) (0.1 mg/kg) and again tested for PPI. A significant attenuation of the baseline ASR by APO was noted. No significant effects were noted on control ASR trials (ISIs, 0 and 4000 ms). For the SAL condition, response inhibition was significantly reduced as a function of gp120 dose, and the inflection of the inhibition curve was significantly altered for the high-gp120 dose-treated animals. A gp120 treatment x APO drug interaction was evident on amplitude, but not latency, of the response inhibition, with an enhanced inhibition in the APO condition, collapsed across ISIs (08-120 ms) as the neonatal-injected gp120 dose increased. Use of APO to probe integrity of the dopaminergic system suggests long-lasting alterations in neuronal responses consequent to neonatal gp120 exposure.

Intrahippocampal injections of Tat: effects on prepulse inhibition of the auditory startle response in adult male rats

The presence of human immunodeficiency virus (HIV-1) in the brain mediates the pathogenesis of HIV-associated dementia complex (HAD), partially through the viral toxins gp120 and Tat. This study characterized potential deficits in sensorimotor gating, as measured by prepulse inhibition (PPI), following hippocampal administration of Tat. Adult, male Sprague-Dawley rats were bilaterally injected with 50 microg Tat or saline (1 microl volume), into the hippocampus. Following 7 weeks of recovery, all animals were tested using the auditory startle response (ASR) with habituation, control, and PPI trials. Assessment of ASR habituation [100dB(A) white noise stimulus, 70dB(A) background, 5-min acclimation period, 36 habituation trials with fixed interstimulus interval (ISI) of 10 s] demonstrated a significant approximately 50% reduction in the overall peak ASR amplitude, but no change in peak ASR latency, nor an effect on the rate of habituation. PPI measures demonstrated robust alterations in sensorimotor gating. The PPI test (ISI of 0, 8, 40, 80, 120, or 4000 ms, 6-trial blocks, Latin-square) showed an attenuated response on peak ASR amplitude during the control trials (0 and 4000 ms ISI), but not on the PPI trials (8-120 ms ISI). Most striking was the rightward shift in ISI for maximal inhibition of the response (chi2(1)=4.7, p Collapse

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MESH Headings

• Acoustic Stimulation
• Animals
• Gene Products, tat/pharmacology
• Habituation, Psychophysiologic/drug effects
• Hippocampus/drug effects
• Inhibition, Psychological
• Male
• Rats
• Rats, Sprague-Dawley
• Reflex, Startle/drug effects

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Grants

• R01 HD043680 NICHD NIH HHS
• DA014401 NIDA NIH HHS
• DA013137 NIDA NIH HHS
• R01 DA013137 NIDA NIH HHS
• HD043680 NICHD NIH HHS

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Affiliation(s)

Sylvia Fitting Department of Psychology, University of South Carolina, 1512 Pendleton Street, Columbia, South Carolina 29208, USA.
Rosemarie M Booze
Ulla Hasselrot
Charles F Mactutus

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Collaborators Collapse