Depressive-like Behavior Is Accompanied by Prefrontal Cortical Innate Immune Fatigue and Dendritic Spine Losses after HIV-1 Tat and Morphine Exposure.

Opioid use disorder (OUD) and HIV are comorbid epidemics that can increase depression. HIV and the viral protein Tat can directly induce neuronal injury within reward and emotionality brain circuitry, including the prefrontal cortex (PFC). Such damage involves both excitotoxic mechanisms and more indirect pathways through neuroinflammation, both of which can be worsened by opioid co-exposure. To assess whether excitotoxicity and/or neuroinflammation might drive depressive behaviors in persons infected with HIV (PWH) and those who use opioids, male mice were exposed to HIV-1 Tat for eight weeks, given escalating doses of morphine during the last two weeks, and assessed for depressive-like behavior. Tat expression decreased sucrose consumption and adaptability, whereas morphine administration increased chow consumption and exacerbated Tat-induced decreases in nesting and burrowing-activities associated with well-being. Across all treatment groups, depressive-like behavior correlated with increased proinflammatory cytokines in the PFC. Nevertheless, supporting the theory that innate immune responses adapt to chronic Tat exposure, most proinflammatory cytokines were unaffected by Tat or morphine. Further, Tat increased PFC levels of the anti-inflammatory cytokine IL-10, which were exacerbated by morphine administration. Tat, but not morphine, decreased dendritic spine density on layer V pyramidal neurons in the anterior cingulate. Together, our findings suggest that HIV-1 Tat and morphine differentially induce depressive-like behaviors associated with increased neuroinflammation, synaptic losses, and immune fatigue within the PFC.

HIV-1 Tat reduces apical dendritic spine density throughout the trisynaptic pathway in the hippocampus of male transgenic mice.

Nearly one-third of persons infected with HIV-1 (PWH) develop HIV-associated neurocognitive disorders (HAND), which can be exacerbated by exposure to opioids. The impact of opioids on HIV-induced alterations in neuronal plasticity is less well understood. Both morphine exposure and HIV have been shown to disrupt synaptic growth and stability in the hippocampus suggesting a potential site of convergence for their deleterious effects. In the present study, we examined the density of dendritic spines in CA1 and CA3 pyramidal neurons, and granule neurons within the dentate gyrus representing the hippocampal trisynaptic pathway after short-term exposure to the HIV transactivator of transcription (Tat) protein and morphine. We exposed inducible male, HIV-1 Tat transgenic mice to escalating doses of morphine (10-40 mg/kg, b.i.d.) and examined synaptodendritic structure in Golgi-impregnated hippocampal neurons. HIV-1 Tat, but not morphine, systematically reduced the density of apical, but not basilar, dendrites of CA1 and CA3 pyramidal neurons, and granule neuronal apical dendrites, suggesting the coordinated loss of specific synaptic interconnections throughout the hippocampal trisynaptic pathway.

HIV-1 Tat and morphine decrease murine inter-male social interactions and associated oxytocin levels in the prefrontal cortex, amygdala, and hypothalamic paraventricular nucleus.

Many persons infected with HIV-1 (PWH) and opioid-dependent individuals experience deficits in sociability that interfere with daily living. Sociability is regulated by the prefrontal cortico-hippocampal-amygdalar circuit. Within this circuit HIV-1 trans-activator of transcription (HIV-1 Tat) and opioids can increase dendritic pathology and alter neuronal firing. Changes in sociability are also associated with dysregulation of hypothalamic neuropeptides such as oxytocin or corticotropin releasing factor (CRF) in the prefrontal cortico-hippocampal-amygdalar circuit. Accordingly, we hypothesized that the interaction of HIV-1 Tat and morphine would impair inter-male social interactions and disrupt oxytocin and CRF within the PFC and associated circuitry. Male mice were exposed to HIV-1 Tat for 8 weeks and administered saline or escalating doses of morphine twice daily (s.c.) during the last 2 weeks of HIV-1 Tat exposure. Tat attenuated aggressive interactions with an unknown intruder, whereas morphine decreased both non-aggressive and aggressive social interactions in the resident-intruder test. However, there was no effect of Tat or morphine on non-reciprocal interactions in the social interaction and novelty tests. Tat, but not morphine, decreased oxytocin levels in the PFC and amygdala, whereas both Tat and morphine decreased the percentage of oxytocin-immunoreactive neurons in the hypothalamic paraventricular nucleus (PVN). In Tat(+) or morphine-exposed mice, regional levels of CRF and oxytocin correlated with alterations in behavior in the social interaction and novelty tests. Overall, decreased expression of oxytocin in the prefrontal cortico-hippocampal-amygdalar circuit is associated with morphine- and HIV-Tat-induced deficits in social behavior.

HIV-1 Tat and Morphine Differentially Disrupt Pyramidal Cell Structure and Function and Spatial Learning in Hippocampal Area CA1: Continuous versus Interrupted Morphine Exposure.

About half the people infected with human immunodeficiency virus (HIV) have neurocognitive deficits that often include memory impairment and hippocampal deficits, which can be exacerbated by opioid abuse. To explore the effects of opioids and HIV on hippocampal CA1 pyramidal neuron structure and function, we induced HIV-1 transactivator of transcription (Tat) expression in transgenic mice for 14 d and co-administered time-release morphine or vehicle subcutaneous implants during the final 5 d (days 9-14) to establish steady-state morphine levels. Morphine was withheld from some ex vivo slices during recordings to begin to assess the initial pharmacokinetic consequences of opioid withdrawal. Tat expression reduced hippocampal CA1 pyramidal neuronal excitability at lower stimulating currents. Pyramidal cell firing rates were unaffected by continuous morphine exposure. Behaviorally, exposure to Tat or high dosages of morphine impaired spatial memory Exposure to Tat and steady-state levels of morphine appeared to have largely independent effects on pyramidal neuron structure and function, a response that is distinct from other vulnerable brain regions such as the striatum. By contrast, acutely withholding morphine (from morphine-tolerant ex vivo slices) revealed unique and selective neuroadaptive shifts in CA1 pyramidal neuronal excitability and dendritic plasticity, including some interactions with Tat. Collectively, the results show that opioid-HIV interactions in hippocampal area CA1 are more nuanced than previously assumed, and appear to vary depending on the outcome assessed and on the pharmacokinetics of morphine exposure.

Effects of HIV gp120 on Neuroinflammation in Immunodeficient vs. Immunocompetent States.

HIV affects 37 million people worldwide, 25-69% of which develop HIV-associated neurocognitive disorders (HAND) regardless of antiviral treatment. HIV infection of the brain decreases cognitive function, disrupts/impairs learning and memory, and reduces quality of life for those affected. HIV-induced neuroinflammation has been associated with viral proteins such as gp120 and Tat, which remain elevated in the CNS even in patients with low peripheral viremia counts. In this study, we examined the effects of gp120 on neuroinflammation in immunodeficient vs. immunocompetent states by examining neuroinflammatory markers in gp120tg mice with or without systemic immunodeficiency caused by murine retroviral administration (LP-BM5 murine AIDS). Changes in inflammatory cytokine/chemokine mRNA expression was complex and dependent upon expression of gp120 protein, immunodeficiency status, brain region (hippocampus, frontal lobe, or striatum), and age. Gp120 expression reduced hippocampal synaptophysin expression but did not affect animals' learning/memory on the spontaneous T-maze test in our experimental conditions. Our results emphasize the critical role of the neuroinflammatory micro-environment and the peripheral immune system context in which gp120 acts. Multiple factors, particularly system-level differences in the immune response of different brain regions, need to be considered when developing treatment for HAND. Graphical Abstract.

Escalating morphine dosing in HIV-1 Tat transgenic mice with sustained Tat exposure reveals an allostatic shift in neuroinflammatory regulation accompanied by increased neuroprotective non-endocannabinoid lipid signaling molecules and amino acids.

BACKGROUND: Human immunodeficiency virus type-1 (HIV-1) and opiates cause long-term inflammatory insult to the central nervous system (CNS) and worsen disease progression and HIV-1-related neuropathology. The combination of these proinflammatory factors reflects a devastating problem as opioids have high abuse liability and continue to be prescribed for certain patients experiencing HIV-1-related pain. METHODS: Here, we examined the impact of chronic (3-month) HIV-1 transactivator of transcription (Tat) exposure to short-term (8-day), escalating morphine in HIV-1 Tat transgenic mice that express the HIV-1 Tat protein in a GFAP promoter-regulated, doxycycline (DOX)-inducible manner. In addition to assessing morphine-induced tolerance in nociceptive responses organized at spinal (i.e., tail-flick) and supraspinal (i.e., hot-plate) levels, we evaluated neuroinflammation via positron emission tomography (PET) imaging using the [18F]-PBR111 ligand, immunohistochemistry, and cytokine analyses. Further, we examined endocannabinoid (eCB) levels, related non-eCB lipids, and amino acids via mass spectrometry.  RESULTS: Tat-expressing [Tat(+)] transgenic mice displayed antinociceptive tolerance in the tail withdrawal and hot-plate assays compared to control mice lacking Tat [Tat(-)]. This tolerance was accompanied by morphine-dependent increases in Iba-1 ± 3-nitrotryosine immunoreactive microglia, and alterations in pro- and anti-inflammatory cytokines, and chemokines in the spinal cord and striatum, while increases in neuroinflammation were absent by PET imaging of [18F]-PBR111 uptake. Tat and morphine exposure differentially affected eCB levels, non-eCB lipids, and specific amino acids in a region-dependent manner. In the striatum, non-eCB lipids were significantly increased by short-term, escalating morphine exposure, including peroxisome proliferator activator receptor alpha (PPAR-α) ligands N-oleoyl ethanolamide (OEA) and N-palmitoyl ethanolamide (PEA), as well as the amino acids phenylalanine and proline. In the spinal cord, Tat exposure increased amino acids leucine and valine, while morphine decreased levels of tyrosine and valine but did not affect eCBs or non-eCB lipids. CONCLUSION: Overall results demonstrate that 3 months of Tat exposure increased morphine tolerance and potentially innate immune tolerance evidenced by reductions in specific cytokines (e.g., IL-1α, IL-12p40) and microglial reactivity. In contrast, short-term, escalating morphine exposure acted as a secondary stressor revealing an allostatic shift in CNS baseline inflammatory responsiveness from sustained Tat exposure.

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-1ß, 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.

A central role for glial CCR5 in directing the neuropathological interactions of HIV-1 Tat and opiates.

BACKGROUND: The collective cognitive and motor deficits known as HIV-associated neurocognitive disorders (HAND) remain high even among HIV+ individuals whose antiretroviral therapy is optimized. HAND is worsened in the context of opiate abuse. The mechanism of exacerbation remains unclear but likely involves chronic immune activation of glial cells resulting from persistent, low-level exposure to the virus and viral proteins. We tested whether signaling through C-C chemokine receptor type 5 (CCR5) contributes to neurotoxic interactions between HIV-1 transactivator of transcription (Tat) and opiates and explored potential mechanisms. METHODS: Neuronal survival was tracked in neuronal and glial co-cultures over 72 h of treatment with HIV-1 Tat ± morphine using cells from CCR5-deficient and wild-type mice exposed to the CCR5 antagonist maraviroc or exogenously-added BDNF (analyzed by repeated measures ANOVA). Intracellular calcium changes in response to Tat ± morphine ± maraviroc were assessed by ratiometric Fura-2 imaging (analyzed by repeated measures ANOVA). Release of brain-derived neurotrophic factor (BDNF) and its precursor proBDNF from CCR5-deficient and wild-type glia was measured by ELISA (analyzed by two-way ANOVA). Levels of CCR5 and µ-opioid receptor (MOR) were measured by immunoblotting (analyzed by Student's t test). RESULTS: HIV-1 Tat induces neurotoxicity, which is greatly exacerbated by morphine in wild-type cultures expressing CCR5. Loss of CCR5 from glia (but not neurons) eliminated neurotoxicity due to Tat and morphine interactions. Unexpectedly, when CCR5 was lost from glia, morphine appeared to entirely protect neurons from Tat-induced toxicity. Maraviroc pre-treatment similarly eliminated neurotoxicity and attenuated neuronal increases in [Ca2+]i caused by Tat ± morphine. proBDNF/BDNF ratios were increased in conditioned media from Tat ± morphine-treated wild-type glia compared to CCR5-deficient glia. Exogenous BDNF treatments mimicked the pro-survival effect of glial CCR5 deficiency against Tat ± morphine. CONCLUSIONS: Our results suggest a critical role for glial CCR5 in mediating neurotoxic effects of HIV-1 Tat and morphine interactions on neurons. A shift in the proBDNF/BDNF ratio that favors neurotrophic support may occur when glial CCR5 signaling is blocked. Some neuroprotection occurred only in the presence of morphine, suggesting that loss of CCR5 may fundamentally change signaling through the MOR in glia.

Morphine-potentiated cognitive deficits correlate to suppressed hippocampal iNOS RNA expression and an absent type 1 interferon response in LP-BM5 murine AIDS.

Opioid use accelerates neurocognitive impairment in HIV/AIDS patients. We assessed the effect of chronic morphine treatment and LP-BM5/murine AIDS (MAIDS) infection on cognition, cytokine production, and type 1 interferon (IFN) expression in the murine CNS. Morphine treatment decreased expression of pro-inflammatory factors (CCL5, iNOS) and reduced cognitive performance in LP-BM5-infected mice, correlating to increased hippocampal viral load and a blunted type 1 IFN response. In the striatum, morphine reduced viral load while increasing IFN-α RNA expression. Our results suggest that differentially regulated type 1 IFN responses may contribute to distinct regional outcomes in the hippocampus and striatum in LP-BM5/MAIDS.

CCR5 mediates HIV-1 Tat-induced neuroinflammation and influences morphine tolerance, dependence, and reward.

The HIV-1 regulatory protein, trans-activator of transcription (Tat), interacts with opioids to potentiate neuroinflammation and neurodegeneration within the CNS. These effects may involve the C-C chemokine receptor type 5 (CCR5); however, the behavioral contribution of CCR5 on Tat/opioid interactions is not known. Using a transgenic murine model that expresses HIV-1 Tat protein in a GFAP-regulated, doxycycline-inducible manner, we assessed morphine tolerance, dependence, and reward. To assess the influence of CCR5 on these effects, mice were pretreated with oral vehicle or the CCR5 antagonist, maraviroc, prior to morphine administration. We found that HIV-1 Tat expression significantly attenuated the antinociceptive potency of acute morphine (2-64 mg/kg, i.p.) in non-tolerant mice. Consistent with this, Tat attenuated withdrawal symptoms among morphine-tolerant mice. Pretreatment with maraviroc blocked the effects of Tat, reinstating morphine potency in non-tolerant mice and restoring withdrawal symptomology in morphine-tolerant mice. Twenty-four hours following morphine administration, HIV-1 Tat significantly potentiated (∼3.5-fold) morphine-conditioned place preference and maraviroc further potentiated these effects (∼5.7-fold). Maraviroc exerted no measurable behavioral effects on its own. Protein array analyses revealed only minor changes to cytokine profiles when morphine was administered acutely or repeatedly; however, 24 h post morphine administration, the expression of several cytokines was greatly increased, including endogenous CCR5 chemokine ligands (CCL3, CCL4, and CCL5), as well as CCL2. Tat further elevated levels of several cytokines and maraviroc pretreatment attenuated these effects. These data demonstrate that CCR5 mediates key aspects of HIV-1 Tat-induced alterations in the antinociceptive potency and rewarding properties of opioids.

Long-term morphine delivery via slow release morphine pellets or osmotic pumps: Plasma concentration, analgesia, and naloxone-precipitated withdrawal.

AIMS: Slow-release morphine sulfate pellets and osmotic pumps are common routes of chronic morphine delivery in mouse models, but direct comparisons of these drug delivery systems are lacking. In this study, we assessed the efficacy of slow-release pellets versus osmotic pumps in delivering morphine to adult mice. MAIN METHODS: Male C57BL/6NCr mice (8weeksold) were implanted subcutaneously with slow-release pellets (25mg morphine sulfate) or osmotic pumps (64mg/mL, 1.0µL/h). Plasma morphine concentrations were quantified via LC-MS/MS, analgesic efficacy was determined by tail flick assay, and dependence was assessed with naloxone-precipitated withdrawal behaviors (jumping) and physiological effects (excretion, weight loss). KEY FINDINGS: Morphine pellets delivered significantly higher plasma drug concentrations compared to osmotic pumps, which were limited by the solubility of the morphine sulfate and pump volume/flow rate. Within 96h post-implantation, plasma morphine concentrations were indistinguishable in pellet vs. pump-treated samples. While osmotic pump did not have an antinociceptive effect in the tail flick assay, pumps and pellets induced comparable dependence symptoms (naloxone-precipitated jumping behavior) from 24-72h post-implantation. SIGNIFICANCE: In this study, we compared slow-release morphine pellets to osmotic minipumps for morphine delivery in mice. We found that osmotic pumps and subcutaneous morphine sulfate pellets yielded significantly different pharmacokinetics over a 7-day period, and as a result significantly different antinociceptive efficacy. Nonetheless, both delivery methods induced dependence as measured by naloxone-precipitated withdrawal.

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.

Morphine increases hippocampal viral load and suppresses frontal lobe CCL5 expression in the LP-BM5 AIDS model.

Chronic opiate abuse accelerates the development of cognitive deficits in human immunodeficiency virus (HIV)-1 patients. To investigate morphine's effects on viral infection of the central nervous system, we applied chronic morphine treatment to the LP-BM5 murine acquired immunodeficiency syndrome (MAIDS) model. LP-BM5 infection induces proinflammatory cytokine/chemokine production, correlating to increased blood-brain barrier permeability. Morphine treatment significantly increased LP-BM5 viral load in the hippocampus, but not in the frontal lobe. Morphine reduced the chemokine CCL5 to non-infected levels in the frontal lobe, but not in the hippocampus. These data indicate a region-specific mechanism for morphine's effects on virally-induced neurocognitive deficits.

Alterations in osteopontin modify muscle size in females in both humans and mice.

PURPOSE: An osteopontin (OPN; SPP1) gene promoter polymorphism modifies disease severity in Duchenne muscular dystrophy, and we hypothesized that it might also modify muscle phenotypes in healthy volunteers. METHODS: Gene association studies were carried out for OPN (rs28357094) in the FAMuSS cohort (n = 752; mean ± SD age = 23.7 ± 5.7 yr). The phenotypes studied included muscle size (MRI), strength, and response to supervised resistance training. We also studied 147 young adults that had carried out a bout of eccentric elbow exercise (age = 24.0 ± 5.2 yr). Phenotypes analyzed included strength, soreness, and serum muscle enzymes. RESULTS: In the FAMuSS cohort, the G allele was associated with 17% increase in baseline upper arm muscle volume only in women (F = 26.32; P = 5.32 × 10), explaining 5% of population variance. In the eccentric damage cohort, weak associations of the G allele were seen in women with both baseline myoglobin and elevated creatine kinase. The sexually dimorphic effects of OPN on muscle were also seen in OPN-null mice. Five of seven muscle groups examined showed smaller size in OPN-null female mice, whereas two were smaller in male mice. The query of OPN gene transcription after experimental muscle damage in mice showed rapid induction within 12 h (100-fold increase from baseline), followed by sustained high-level expression through 16 d of regeneration before falling to back to baseline. CONCLUSION: OPN is a sexually dimorphic modifier of muscle size in normal humans and mice and responds to muscle damage. The OPN gene is known to be estrogen responsive, and this may explain the female-specific genotype effects in adult volunteers.

Invitro application of gold nanoprobes in live neurons for phenotypical classification, connectivity assessment, and electrophysiological recording.

Thermoregulatory neurons in the preoptic area of the anterior hypothalamus (POA) form synaptic networks, which affect responses that regulate body temperature. To characterize these pathways of activation, projections to effector control areas, like the dorsomedial hypothalamus (DMH), require labeling in live tissue slices. Traditional fluorescent dyes label axon terminals near an injection site, but unfortunately, also that of nearby fibers of passage. Here, we describe a novel methodology for retrograde labeling of neurons in vitro, which will allow for further electrophysiological recording. To determine if POA neurons project to the DMH, we have used nanometer-sized, gold nanoprobes, which provide for specific neuronal entry, via synapses in close proximity to the injection site. Upon neuronal entry, these nanoprobe complexes diffuse to the soma, where they are readily visualized and quantified. We found that conjugation of these gold nanoprobes with VGLUT-2 antibodies and polyethyleneimine (PEI) facilitates neuronal entry and a high level of labeling efficacy. This novel method, adapted from emerging cancer therapy technologies, is highly specific for determining axon terminal projections within particular neuronal populations, while maintaining neuronal viability for targeted live cell electrophysiological recording.