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

Abstract

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.

Keywords: Acoustic startle reflex; Glial fibrillary acidic protein (GFAP); Ionized calcium-binding adaptor protein 1; NeuroAIDS; Prepulse inhibition; Trans-activating transcriptor.

Figure 1

Acoustic startle reflex (arbitrary units ± SEM in response to 120 dB tone) among male GT-tg mice (n=10/group) administered saline (white circles) or doxycycline (black circles) in (A) increasing doses (25 – 125 mg/kg, i.p. for 7 days), or (B) increasing durations of exposure (100 mg/kg for 1 – 14 days) to induce HIV-1 Tat expression in brain. * indicates significant difference from saline-treated controls (one-way ANOVA, effect of treatment, p < 0.05).

Figure 2

Prepulse inhibition of the acoustic startle reflex (% ± SEM) among male GT-tg mice (n=10/group) administered saline (open symbols), (A) increasing doses of doxycycline (25 – 125 mg/kg, i.p. for 7 days), or (B) increasing durations of exposure (1 – 14 days) to doxycycline (100 mg/kg) to induce HIV-1 Tat expression in brain. * indicates doxycycline treatment group significantly differs from either saline-treatment group, irrespective of dB (repeated measures ANOVA, main effect of treatment); † indicates each prepulse dB significantly differs from each other, irrespective of doxycycline treatment (repeated measures ANOVA, main effect of dB amplitude, p < 0.05).

Figure 3

(A–D) Representative photomicrographs of Iba1-labeled microglia in anterior cingulate cortex. (E) Morphology rating (mean ± SEM; 1 = resting, 2 = early activated, 3 = activated/reactive, 4 = reactive) of microglia in subregions of the medial prefrontal cortex (n=8–12 observations/group): agranular insular cortex (white circles), anterior cingulate cortex (grey circles), and prelimbic cortex (black circles). ^ indicates significant difference from saline-treated controls in the anterior cingulate cortex (one-way ANOVA, main effect of treatment). * indicates significant difference from saline-treated controls in all depicted subregions (one-way ANOVAs, effects of treatment, p < 0.05). Scale bar, 100 µm.

Figure 4

Frontal cortex GFAP content (ng/mg ± SEM) among male GT-tg mice that were administered saline (white circle) or had HIV-1 Tat dose-dependently induced by doxycycline (7–8 observations/group). * indicates significant difference from saline-treated controls (one-way ANOVA, effect of treatment, p = 0.05).

Figure 5

Acoustic startle reflex (arbitrary units ± SEM in response to 120 dB tone) among male GT-tg mice (n=8–12/group) that received pretreatment with vehicle or indomethacin (10 mg/kg, i.p.) 20 h and again 30 min prior to inducing HIV-1 Tat (or not) via (A) a brief doxycycline exposure (100 mg/kg, 1d) or (B) a prolonged doxycycline exposure (100 mg/kg, 7d). * indicates significant difference from saline-treated controls (white circles; one-way ANOVA, effect of treatment, p < 0.05).

Figure 6

Prepulse inhibition of the acoustic startle reflex (% ± SEM) among male GT-tg mice (n=8–12/group) that received pretreatment with vehicle or indomethacin (10 mg/kg, i.p.) 20 h and again 30 min prior to having HIV-1 Tat induced (or not) via (A) a brief doxycycline exposure (100 mg/kg, 1d) or (B) a prolonged doxycycline exposure (100 mg/kg, 7d). * indicates treatment group significantly differs from saline-treatment (white symbols) group, irrespective of dB (repeated measures ANOVA, main effect of treatment); † indicates each prepulse dB significantly differs from each other, irrespective of treatment (repeated measures ANOVA, main effect of dB amplitude, p < 0.05).