Estrous cycle and HIV-1 Tat protein influence cocaine-conditioned place preference and induced locomotion of female mice

EcoHIV Infection Modulates the Effects of Cocaine Exposure Pattern and Abstinence on Cocaine Seeking and Neuroimmune Protein Expression in Male Mice

Cocaine use disorders (CUDs) and human immunodeficiency virus (HIV) remain persistent public health dilemmas throughout the world. One major hurdle for treating CUD is the increase in cocaine craving and seeking behavior that occurs over a protracted period of abstinence, an effect known as the incubation of craving. Little is known about how HIV may modulate this process. Thus, we sought to examine the impact of chronic HIV infection on the incubation of cocaine craving and associated changes in the central and peripheral immune systems. Here, mice were inoculated with EcoHIV, which is a chimeric HIV-1 construct that produces chronic HIV infection in mice. EcoHIV- and sham-infected mice were conditioned with cocaine daily or intermittently in a conditioned place preference (CPP) paradigm, followed by 1 or 21 days of forced abstinence prior to assessing preference for the cocaine-paired chamber. Under both conditioning regimens, sham mice exhibited incubation of cocaine CPP after 21 days of abstinence. EcoHIV-infected mice conditioned daily with cocaine showed enhanced cocaine seeking at both abstinence timepoints, whereas infected mice conditioned intermittently showed a reversal of the incubation effect, with higher cocaine seeking after 1 day of abstinence compared to 21 days. Analysis of corticolimbic CX3CL1-CX3CR1 and glutamate receptor expression revealed alterations in medial prefrontal cortex (mPFC) CX3CL1 and nucleus accumbens (NAc) GluN2A receptors that correlated with cocaine seeking following daily cocaine exposure. Moreover, examination of peripheral immune markers showed that the effect of abstinence and EcoHIV infection on these measures depended on the cocaine exposure regimen. Altogether, these results highlight the importance of cocaine abstinence and exposure pattern as critical variables that modulate HIV-associated neuroimmune outcomes and relapse vulnerability.

Neurological, Behavioral, and Pathophysiological Characterization of the Co-Occurrence of Substance Use and HIV: A Narrative Review

Combined antiretroviral therapy (cART) has greatly reduced the severity of HIV-associated neurocognitive disorders in people living with HIV (PLWH); however, PLWH are more likely than the general population to use drugs and suffer from substance use disorders (SUDs) and to exhibit risky behaviors that promote HIV transmission and other infections. Dopamine-boosting psychostimulants such as cocaine and methamphetamine are some of the most widely used substances among PLWH. Chronic use of these substances disrupts brain function, structure, and cognition. PLWH with SUD have poor health outcomes driven by complex interactions between biological, neurocognitive, and social factors. Here we review the effects of comorbid HIV and psychostimulant use disorders by discussing the distinct and common effects of HIV and chronic cocaine and methamphetamine use on behavioral and neurological impairments using evidence from rodent models of HIV-associated neurocognitive impairments (Tat or gp120 protein expression) and clinical studies. We also provide a biopsychosocial perspective by discussing behavioral impairment in differentially impacted social groups and proposing interventions at both patient and population levels.

Advancing the preclinical study of comorbid neuroHIV and substance use disorders: Current perspectives and future directions

Human immunodeficiency virus (HIV) remains a persistent public health concern throughout the world. Substance use disorders (SUDs) are a common comorbidity that can worsen treatment outcomes for people living with HIV. The relationship between HIV infection and SUD outcomes is likely bidirectional, making clear interrogation of neurobehavioral outcomes challenging in clinical populations. Importantly, the mechanisms through which HIV and addictive drugs disrupt homeostatic immune and CNS function appear to be highly overlapping and synergistic within HIV-susceptible reward and motivation circuitry in the central nervous system. Decades of animal research have revealed invaluable insights into mechanisms underlying the pathophysiology SUDs and HIV, although translational studies examining comorbid SUDs and HIV are very limited due to the technical challenges of modeling HIV infection preclinically. In this review, we discuss preclinical animal models of HIV and highlight key pathophysiological characteristics of each model, with a particular emphasis on rodent models of HIV. We then review the implementation of these models in preclinical SUD research and identify key gaps in knowledge in the field. Finally, we discuss how cutting-edge behavioral neuroscience tools, which have revealed key insights into the neurobehavioral mechanisms of SUDs, can be applied to preclinical animal models of HIV to reveal potential, novel treatment avenues for comorbid HIV and SUDs. Here, we argue that future preclinical SUD research would benefit from incorporating comorbidities such as HIV into animal models and would facilitate the discovery of more refined, subpopulation-specific mechanisms and effective SUD prevention and treatment targets.

HIV gp120 impairs nucleus accumbens neuroimmune function and dopamine D3 receptor-mediated inhibition of cocaine seeking in male rats

Cocaine Use Disorders (CUDs) are associated with an increased risk of human immunodeficiency virus (HIV) infection. Cocaine and the HIV envelope protein gp120 each induce distinct deficits to mesocorticolimbic circuit function and motivated behavior; however, little is known regarding how they interact to dysregulate these functions or how such interactions impact pharmacotherapeutic efficacy. We have previously shown that the selective, weak partial agonist of the dopamine D3 receptor (D3R), MC-25-41, attenuates cocaine-seeking behavior in male rats. Here, we sought to characterize changes in striatal neuroimmune function in gp120-exposed rats across abstinence from operant access to cocaine (0.75 mg/kg, i.v.) or sucrose (45 mg/pellet), and to examine the impact of gp120 exposure on MC-25-41-reduced cocaine seeking. After establishing a history of cocaine or sucrose self-administration, rats received intracerebroventricular gp120 infusions daily the first 5 days of abstinence and were sacrificed either on day 6 or after 21 days of forced abstinence and a cue-induced cocaine seeking test. We demonstrated that MC-25-41 treatment attenuated cue-induced cocaine seeking among control rats but not gp120-exposed rats. Moreover, postmortem analysis of nucleus accumbens (NAc) core neuroimmune function indicated cocaine abstinence- and gp120-induced impairments, and the expression of several immune factors within the NAc core significantly correlated with cocaine-seeking behavior. We conclude that cocaine abstinence dysregulates striatal neuroimmune function and interacts with gp120 to inhibit the effectiveness of a D3R partial agonist in reducing cocaine seeking. These findings highlight the need to consider comorbidities, such as immune status, when evaluating the efficacy of novel pharmacotherapeutics.

Opioid and Sucrose Craving Are Accompanied by Unique Behavioral and Affective Profiles after Extended Abstinence in Male and Female Rats

Incubation of craving refers to the intensification of drug-seeking behavior in response to reward-paired cues over the course of abstinence. In rodents, craving and drug-seeking behaviors have been measured by an increase in lever pressing in the absence of reinforcer availability in response to cue presentations. However, craving in rodents is difficult to define and little is known about the behavioral signatures that accompany increased drug-seeking behavior measured by lever pressing. The affective components of relapse are also important, but understudied in rodents. Hormonal fluctuations influence craving for psychostimulants, but little is known about the impact of the estrous cycle on opioid-seeking behavior. This study sought to delineate the behavioral and affective signatures associated with craving, and to examine the influence of the female estrous cycle on craving. Male and female rats underwent 10 d of intravenous opioid self-administration. Separate cohorts of control rats self-administered oral sucrose, a natural nondrug reward. Cue-induced seeking tests were conducted after 1 or 30d of forced abstinence. These sessions were recorded and scored for overall locomotion, instances of sniffing, grooming, or hyperactivity. Ultrasonic vocalizations (USVs) were also recorded to determine affective profiles that accompany opioid seeking. Although active lever presses and overall locomotion increased unanimously over extended abstinence from heroin and sucrose, a sex- and reinforcer-specific behavioral and affective signature of craving emerged. Furthermore, although the female estrous cycle did not affect taking or seeking, it appears to influence more granular behaviors.

HIV-1 Tat Protein Promotes Neuroendocrine Dysfunction Concurrent with the Potentiation of Oxycodone's Psychomotor Effects in Female Mice

Human immunodeficiency virus (HIV) is associated with neuroendocrine dysfunction which may contribute to co-morbid stress-sensitive disorders. The hypothalamic-pituitary-adrenal (HPA) or -gonadal (HPG) axes are perturbed in up to 50% of HIV patients. The mechanisms are not known, but we have found the HIV-1 trans-activator of transcription (Tat) protein to recapitulate the clinical phenotype in male mice. We hypothesized that HPA and/or HPG dysregulation contributes to Tat-mediated interactions with oxycodone, an opioid often prescribed to HIV patients, in females. Female mice that conditionally-expressed the Tat_1-86 protein [Tat(+) mice] or their counterparts that did not [Tat(-) control mice] were exposed to forced swim stress (or not) and behaviorally-assessed for motor and anxiety-like behavior. Some mice had glucocorticoid receptors (GR) or corticotropin-releasing factor receptors (CRF-R) pharmacologically inhibited. Some mice were ovariectomized (OVX). As seen previously in males, Tat elevated basal corticosterone levels and potentiated oxycodone's psychomotor activity in females. Unlike males, females did not demonstrate adrenal insufficiency and oxycodone potentiation was not regulated by GRs or CRF-Rs. Rather OVX attenuated Tat/oxycodone interactions. Either Tat or oxycodone increased anxiety-like behavior and their combination increased hypothalamic allopregnanolone. OVX increased basal hypothalamic allopregnanolone and obviated Tat or oxycodone-mediated fluctuations. Together, these data provide further evidence for Tat-mediated dysregulation of the HPA axis and reveal the importance of HPG axis regulation in females. HPA/HPG disruption may contribute vulnerability to affective and substance use disorders.

Neuroimmune Mechanisms as Novel Treatment Targets for Substance Use Disorders and Associated Comorbidities

Recent studies examining the neurobiology of substance abuse have revealed a significant role of neuroimmune signaling as a mechanism through which drugs of abuse induce aberrant changes in synaptic plasticity and contribute to substance abuse-related behaviors. Immune signaling within the brain and the periphery critically regulates homeostasis of the nervous system. Perturbations in immune signaling can induce neuroinflammation or immunosuppression, which dysregulate nervous system function including neural processes associated with substance use disorders (SUDs). In this review, we discuss the literature that demonstrates a role of neuroimmune signaling in regulating learning, memory, and synaptic plasticity, emphasizing specific cytokine signaling within the central nervous system. We then highlight recent preclinical studies, within the last 5 years when possible, that have identified immune mechanisms within the brain and the periphery associated with addiction-related behaviors. Findings thus far underscore the need for future investigations into the clinical potential of immunopharmacology as a novel approach toward treating SUDs. Considering the high prevalence rate of comorbidities among those with SUDs, we also discuss neuroimmune mechanisms of common comorbidities associated with SUDs and highlight potentially novel treatment targets for these comorbid conditions. We argue that immunopharmacology represents a novel frontier in the development of new pharmacotherapies that promote long-term abstinence from drug use and minimize the detrimental impact of SUD comorbidities on patient health and treatment outcomes.

Mini review: Promotion of substance abuse in HIV patients: Biological mediation by HIV-1 Tat protein

Despite successful viral suppression by combinatorial anti-retroviral therapy, HIV infection continues to negatively impact the quality of life of patients by promoting neuropathy and HIV-Associated Neurocognitive Disorders (HAND), where substance use disorder (SUD) is highly comorbid and known to worsen health outcomes. While substance abuse exacerbates the progression of HIV, emerging evidence also suggests the virus may potentiate the rewarding effect of abused substances. As HIV does not infect neurons, these effects are theorized to be mediated by viral proteins. Key among these proteins are HIV-1 Tat, which can continue to be produced under viral suppression in patients. This review will recap the behavioral evidence for HIV-1 Tat mediation of a potentiation of cocaine, opioid and alcohol reward, and explore the neurochemical dysfunction associated by Tat as potential mechanisms underlying changes in reward. Targeting rampant oxidative stress, inflammation and excitotoxicity associated with HIV and Tat protein exposure may prove useful in combating persistent substance abuse comorbid with HIV in the clinic.

Sex differences and Tat expression affect dopaminergic receptor expression and response to antioxidant treatment in methamphetamine-sensitized HIV Tat transgenic mice

Methamphetamine (Meth) abuse is a common HIV comorbidity. Males and females differ in their patterns of Meth use, associated behaviors, and responses, but the underlying mechanisms and impact of HIV infection are unclear. Transgenic mice with inducible HIV-1 Tat protein in the brain (iTat) replicate many neurological aspects of HIV infection in humans. We previously showed that Tat induction enhances the Meth sensitization response associated with perturbation of the dopaminergic system, in male iTat mice. Here, we used the iTat mouse model to investigate sex differences in individual and interactive effects of Tat and Meth challenge on locomotor sensitization, brain expression of dopamine receptors (DRDs) and regulatory adenosine receptors (ADORAs). Because Meth administration increases the production of reactive oxygen species (ROS), we also determined whether the effects of Meth could be rescued by concomitant treatment with the ROS scavenger N-acetyl cysteine (NAC). After Meth sensitization and a 7-day abstinence period, groups of Tat+ and Tat-male and female mice were challenged with Meth in combination with NAC. We confirmed that Tat expression and Meth challenge suppressed DRD mRNA and protein in males and females' brains, and showed that females were particularly susceptible to the effects of Meth on D1-like and D2-like DRD subtypes and ADORAs. The expression of these markers differed strikingly between males and females, and between females in different phases of the estrous cycle, in a Tat -dependent manner. NAC attenuated Meth-induced locomotor sensitization and preserved DRD expression in all groups except for Tat + females. These data identify complex interactions between sex, Meth use, and HIV infection on addiction responses, with potential implications for the treatment of male and female Meth users in the context of HIV, especially those with cognitive disorders.

[3H]Dopamine Uptake through the Dopamine and Norepinephrine Transporters is Decreased in the Prefrontal Cortex of Transgenic Mice Expressing HIV-1 Transactivator of Transcription Protein

Dysregulation of dopamine neurotransmission has been linked to the development of human immunodeficiency virus (HIV)-associated neurocognitive disorder (HAND). To investigate the mechanisms underlying this phenomenon, this study used an inducible HIV-1 transactivator of transcription (Tat) transgenic (iTat-tg) mouse model, which demonstrates brain-specific Tat expression induced by administration of doxycycline. We found that induction of Tat expression in the iTat-tg mice for either 7 or 14 days resulted in a decrease (∼30%) in the V _max of [³H]dopamine uptake via both the dopamine transporter (DAT) and norepinephrine transporter (NET) in the prefrontal cortex (PFC), which was comparable to the magnitude (∼35%) of the decrease in B _max for [³H]WIN 35,428 and [³H]nisoxetine binding to DAT and NET, respectively. The decreased V _max was not accompanied by a reduction of total or plasma membrane expression of DAT and NET. Consistent with the decreased V _max for DAT and NET in the PFC, the current study also found an increase in the tissue content of DA and dihydroxyphenylacetic acid in the PFC of iTat-tg mice after 7 days' administration of doxycycline. Electrophysiological recordings in layer V pyramidal neurons of the prelimbic cortex from iTat-tg mice found a significant reduction in action potential firing, which was not sensitive to selective inhibitors for DAT and NET, respectively. These findings provide a molecular basis for using the iTat-tg mouse model in the studies of NeuroHIV. Determining the mechanistic basis underlying the interaction between Tat and DAT/NET may reveal novel therapeutic possibilities for preventing the increase in comorbid conditions as well as HAND. SIGNIFICANCE STATEMENT: Human immunodeficiency virus (HIV)-1 infection disrupts dopaminergic neurotransmission, leading to HIV-associated neurocognitive disorders (HANDs). Based on our in vitro and in vivo studies, dopamine uptake via both dopamine and norepinephrine transporters is decreased in the prefrontal cortex of HIV-1 Tat transgenic mice, which is consistent with the increased dopamine and dihydroxyphenylacetic acid contents in this brain region. Thus, these plasma membrane transporters are an important potential target for therapeutic intervention for patients with HAND.

Combined HIV-1 Tat and oxycodone activate the hypothalamic-pituitary-adrenal and -gonadal axes and promote psychomotor, affective, and cognitive dysfunction in female mice

The majority of HIV⁺ patients present with neuroendocrine dysfunction and ~50% experience co-morbid neurological symptoms including motor, affective, and cognitive dysfunction, collectively termed neuroHIV. In preclinical models, the neurotoxic HIV-1 regulatory protein, trans-activator of transcription (Tat), promotes neuroHIV pathology that can be exacerbated by opioids. We and others find gonadal steroids, estradiol (E₂) or progesterone (P₄), to rescue Tat-mediated pathology. However, the combined effects of Tat and opioids on neuroendocrine function and the subsequent ameliorative capacity of gonadal steroids are unknown. We found that conditional HIV-1 Tat expression in naturally-cycling transgenic mice dose-dependently potentiated oxycodone-mediated psychomotor behavior. Tat increased depression-like behavior in a tail-suspension test among proestrous mice, but decreased it among diestrous mice (who already demonstrated greater depression-like behavior); oxycodone reversed these effects. Combined Tat and oxycodone produced apparent behavioral disinhibition of anxiety-like responding which was greater on diestrus than on proestrus. These mice made more central entries in an open field, but spent less time there and demonstrated greater circulating corticosterone. Tat increased the E₂:P₄ ratio of circulating steroids on diestrus and acute oxycodone attenuated this effect, but repeated oxycodone exacerbated it. Corticotropin-releasing factor was increased by Tat expression, acute oxycodone exposure, and was greater on diestrus compared to proestrus. In human neuroblastoma cells, Tat exerted neurotoxicity that was ameliorated by E₂ (1 or 10 nM) or P₄ (100, but not 10 nM) independent of oxycodone. Oxycodone decreased gene expression of estrogen and κ-opioid receptors. Thus, neuroendocrine function may be an important target for HIV-1 Tat/opioid interactions.

Pregnane steroidogenesis is altered by HIV-1 Tat and morphine: Physiological allopregnanolone is protective against neurotoxic and psychomotor effects

Pregnane steroids, particularly allopregnanolone (AlloP), are neuroprotective in response to central insult. While unexplored in vivo, AlloP may confer protection against the neurological dysfunction associated with human immunodeficiency virus type 1 (HIV-1). The HIV-1 regulatory protein, trans-activator of transcription (Tat), is neurotoxic and its expression in mice increases anxiety-like behavior; an effect that can be ameliorated by progesterone, but not when 5α-reduction is blocked. Given that Tat's neurotoxic effects involve mitochondrial dysfunction and can be worsened with opioid exposure, we hypothesized that Tat and/or combined morphine would perturb steroidogenesis in mice, promoting neuronal death, and that exogenous AlloP would rescue these effects. Like other models of neural injury, conditionally inducing HIV-1 Tat in transgenic mice significantly increased the central synthesis of pregnenolone and progesterone's 5α-reduced metabolites, including AlloP, while decreasing central deoxycorticosterone (independent of changes in plasma). Morphine significantly increased brain and plasma concentrations of several steroids (including progesterone, deoxycorticosterone, corticosterone, and their metabolites) likely via activation of the hypothalamic-pituitary-adrenal stress axis. Tat, but not morphine, caused glucocorticoid resistance in primary splenocytes. In neurons, Tat depolarized mitochondrial membrane potential and increased cell death. Physiological concentrations of AlloP (0.1, 1, or 10 nM) reversed these effects. High-concentration AlloP (100 nM) was neurotoxic in combination with morphine. Tat induction in transgenic mice potentiated the psychomotor effects of acute morphine, while exogenous AlloP (1.0 mg/kg, but not 0.5 mg/kg) was ameliorative. Data demonstrate that steroidogenesis is altered by HIV-1 Tat or morphine and that physiological AlloP attenuates resulting neurotoxic and psychomotor effects.

Selective Vulnerability of Striatal D2 versus D1 Dopamine Receptor-Expressing Medium Spiny Neurons in HIV-1 Tat Transgenic Male Mice

Despite marked regional differences in HIV susceptibility within the CNS, there has been surprisingly little exploration into the differential vulnerability among neuron types and the circuits they underlie. The dorsal striatum is especially susceptible, harboring high viral loads and displaying marked neuropathology, with motor impairment a frequent manifestation of chronic infection. However, little is known about the response of individual striatal neuron types to HIV or how this disrupts function. Therefore, we investigated the morphological and electrophysiological effects of HIV-1 trans-activator of transcription (Tat) in dopamine subtype 1 (D1) and dopamine subtype 2 (D2) receptor-expressing striatal medium spiny neurons (MSNs) by breeding transgenic Tat-expressing mice to Drd1a-tdTomato- or Drd2-eGFP-reporter mice. An additional goal was to examine neuronal vulnerability early during the degenerative process to gain insight into key events underlying the neuropathogenesis. In D2 MSNs, exposure to HIV-1 Tat reduced dendritic spine density significantly, increased dendritic damage (characterized by swellings/varicosities), and dysregulated neuronal excitability (decreased firing at 200-300 pA and increased firing rates at 450 pA), whereas insignificant morphologic and electrophysiological consequences were observed in Tat-exposed D1 MSNs. These changes were concomitant with an increased anxiety-like behavioral profile (lower latencies to enter a dark chamber in a light-dark transition task, a greater frequency of light-dark transitions, and reduced rearing time in an open field), whereas locomotor behavior was unaffected by 2 weeks of Tat induction. Our findings suggest that D2 MSNs and a specific subset of neural circuits within the dorsal striatum are preferentially vulnerable to HIV-1.SIGNIFICANCE STATEMENT Despite combination antiretroviral therapy (cART), neurocognitive disorders afflict 30-50% of HIV-infected individuals and synaptodendritic injury remains evident in specific brain regions such as the dorsal striatum. A possible explanation for the sustained neuronal injury is that the neurotoxic HIV-1 regulatory protein trans-activator of transcription (Tat) continues to be expressed in virally suppressed patients on cART. Using inducible Tat-expressing transgenic mice, we found that dopamine subtype 2 (D2) receptor-expressing medium spiny neurons (MSNs) are selectively vulnerable to Tat exposure compared with D1 receptor-expressing MSNs. This includes Tat-induced reductions in D2 MSN dendritic spine density, increased dendritic damage, and disruptions in neuronal excitability, which coincide with elevated anxiety-like behavior. These data suggest that D2 MSNs and specific circuits within the basal ganglia are preferentially vulnerable to HIV-1.

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.