Consistent with our primary hypothesis and with findings in PWH (Minassian et al, 2013; Walter et al., 2021), HIVtg rats exhibited PPI deficits relative to WT controls. Our secondary hypothesis was also supported, as an altered sensitivity to THC was observed in HIVtg vs WT rats. The nature of these effects contradicted our final hypothesis, however, with THC impairing PPI in WT rats without affecting PPI in HIVtg rats. CBD did not significantly affect primary measures of sensorimotor gating in HIVtg or WT rats. The present findings support the use of the HIVtg rat as a model of aberrant neural processing in PWH and the premise that cannabinoids exert differential effects based on HIV status. These results also suggest that chronic exposure to HIV proteins may reduce sensitivity to the acute psychotomimetic effects of THC. This relative insensitivity suggests that PWH may (1) require higher levels of THC than healthy individuals in order to experience the same effects of cannabis, and/or (2) experience fewer or less severe negative effects associated with fronto-striatal impairment. Either of these possibilities may serve to partially explain the higher rates of overall and heavy cannabis use observed among PWH relative to the general population (Compton et al., 2016).

In contrast with previous findings (Moran et al., 2013b), planned analysis of vehicle data alone during THC testing (Figure 3A)—and a main effect of genotype across all doses of CBD (Figure 4A)—revealed a previously undetected general PPI deficit of the HIVtg rat. Critically, this deficit was driven by the 68-dB prepulse condition, as indicated by a significant prepulse × genotype interaction during the THC assessment (Figure 3C) and a similar near-significant trend during the CBD assessment (Figure 4C). In this condition, prepulses were only 3 dB louder than background noise levels (65 dB), indicating that the PPI deficit of the HIVtg rat is specific to prepulse intensities that are considerably lower than those utilized in previous study (i.e., 15 dB above background; Moran et al., 2013b). Further comparisons between present and extant findings are difficult, given that the present study utilized a different startle paradigm than that typically used to assess HIVtg rats. To wit, the present paradigm manipulated prepulse intensity, while previous studies manipulated inter-stimulus interval (ISI; e.g., Moran et al., 2012). While our decision to eschew ISI manipulation precluded investigation of the robust timing deficits of the HIVtg rat (McLaurin et al., 2019a), the present paradigm was chosen because it was deemed more likely to recreate the specific PPI deficit observed in HAND patients (Walter et al., 2021; Minassian et al., 2013). This judgment was made for 2 reasons. First, timing deficits have yet to be observed in HAND—the PPI deficit reported in PWH was identified using a startle paradigm that used prepulse intensity, not ISI duration, as a variable and that had previously revealed deficits in patients with obsessive-compulsive disorder and schizophrenia (Minassian et al., 2007; Ahmari et al., 2012). Second, this HIV-mediated PPI deficit was observed not as a sub-standard decrease in overall startle reactivity to prepulse-preceded stimuli (the units typically reported in HIVtg rat studies), but as a reduction in the percent decrement of the ASR to prepulse-paired vs unpaired stimuli—a deficit not observed in HIVtg rats tested in ISI-centric paradigms (Moran et al., 2013b). Given that our group had previously identified PPI deficits in rodent models of psychiatric disease following prepulses of only certain volumes (e.g., Geyer et al., 1993), we had reason to believe that manipulating this variable would reveal a similar prepulse-specific deficit in the HIVtg rat. Indeed, by including a range of prepulse intensities, the present study was able to detect a PPI deficit in the HIVtg rat of a type that is directly comparable with clinical findings.

HIV genotype did not independently affect ASR during pulse-alone trials during either the THC or CBD assessments, thus enabling the interpretation that the observed reduction in percent PPI was due to a genuine sensorimotor gating deficit rather than generally reduced startle reactivity to both prepulse-paired and unpaired pulses (Swerdlow et al., 2000). Previous studies of the HIVtg rat have reported mixed findings on this measure, however (i.e., ASR during a “0-msec inter-stimulus interval” condition). To wit, male and female HIVtg rats exhibited alternately higher (Moran et al., 2012, 2013b), lower (Moran et al., 2013a; McLaurin et al., 2017a, b, 2018c, 2019b), and similar (Moran et al., 2013a; Roscoe et al., 2014) startle reactivity to unpaired pulses relative to WT controls at various ages, although statistical analyses of this variable were not reported in several of these publications. Of greatest relevance to the present study, in which rats were 3.5 months old at time of THC assessment, are potentially conflicting findings in 3-, 4-, and 5-month-old male and female rats. At 3 (McLaurin et al., 2018c) and 4 months of age (Moran et al., 2013a), male and female HIVtg rats apparently exhibited reduced ASR relative to controls (no statistical analyses reported), while a different cohort of female HIVtg rats demonstrated significantly elevated startle reactivity at 5 months (Moran et al., 2013b). An explanation for this inconsistency may be a natural inter-individual (and possibly inter-cohort) neural heterogeneity within nuclei relevant to the startle response, as is revealed in the HIVtg medial prefrontal cortex (mPFC) by chronic psychostimulant self-administration (McLaurin et al., 2018b). While it cannot be determined whether this inter-individual cortical heterogeneity directly accounts for the variability in ASR, it is possible that similar variability exists in other centers linked to this reflex. Regardless of underlying mechanisms, however, it is apparent from the heterogeneity of extant reports of HIVtg ASR that the absence of effect reported herein is not out of line with the established phenotype of the HIVtg rat and does not indicate any gross deviation from the norm.

Contrary to our hypothesis, the PPI deficit of the HIVtg rat was not ameliorated by THC. Meanwhile, THC significantly lowered PPI in WT rats (Figure 3A), consistent with near-significant trends reported by the only 2 extant studies of acute subcutaneous THC of which we are aware (Hložek et al., 2017; Uttl et al., 2018). Critically, this reduction of PPI in WT rats confirms the biological activity of subcutaneous THC at the doses investigated, enabling the interpretation that chronic expression of HIV proteins reduced the sensitivity of HIVtg rats to otherwise PPI-disruptive doses of THC. Thus, it may be that PWH use cannabis at higher rates than the general population not only because of its therapeutic effects (e.g., antiemesis, anti-inflammation, etc.; de Jong et al., 2005; Manuzak et al., 2018), but because they require higher doses to receive the same subjective effects or, indeed, because they are less sensitive to the negative consequences that may be predicted by impaired PPI. Such a lack of deleterious side effects of THC would support its continued therapeutic use in this population.

THC meanwhile potentiated the ASR of HIVtg rats, but not WT rats, to unpaired pulses (Figure 3B). This lack of effect on WT rats is consistent with previous reports of acute subcutaneous THC failing to alter ASR at doses that arithmetically reduced PPI (Hložek et al., 2017); however, the effect on HIVtg rats is without precedent. Notably, no genotype-specific effects of THC were observed on ASR during the 5 trial startle periods at the beginning and end of the session (supplementary Table 1). The HIVtg ASR data can therefore be interpreted in 2 ways: (1) THC specifically affected ASR only during the middle of startle testing, altering the rate of intra-session habituation of HIVtg rats so subtly that both genotypes still displayed the same overall startle reactivity at the end of the session; or (2) the effect of THC on HIVtg ASR was too subtle to be observed within the narrow 5-trial windows and could only emerge across the 14 main session pulse-alone trials. Critically, either way, this effect verifies the biological activity of THC in HIVtg rats at the given doses and therefore helps validate the interpretation that the lack of effect on HIVtg PPI reflects a genotype-specific reduction in THC responsivity.

CBD did not exert any significant deleterious main effects on ASR or PPI in either WT or HIVtg rats, nor did it exert any PPI-restorative effects in HIVtg rats (Figure 4A–B). Near-significant trends of CBD were observed on these measures, however, with 30 mg/kg reducing ASR and 1 mg/kg reducing PPI. These findings are consistent with 1 of 2 extant studies of acute intraperitoneal CBD on rat startle, although the dose-response dynamics appear to be different; to wit, CBD reduced the ASR of Sprague-Dawley rats at 3 and 10 mg/kg and reduced PPI at 10 mg/kg (Gururajan et al., 2011). No such reductions to either measure were observed in Wistar rats, meanwhile (Levin et al., 2014). This minor discrepancy between past and present studies may reflect an influence of rat strain on the outcome of what is apparently a subtle and dose-dependent action of CBD on startle circuitry. Indeed, given that the trends of CBD on ASR and PPI detected by the present study were (1) not significant at the P <.05 level and (2) not observable at all following separation of genotypes and consequent reduction of sample sizes, it can be concluded that CBD does not exert meaningful effects on sensorimotor gating in either HIVtg or WT Fischer 344 rats (minor sex- and dose-dependent effects on HABIT1 ASR notwithstanding) (supplementary Table 2).

Although the underlying mechanisms of THC and CBD were not investigated, the neural circuitry mediating ASR and PPI has been sufficiently characterized to allow for speculation. The nucleus accumbens (NAc), for example, is a critical region in the regulation of sensorimotor gating (Swerdlow et al., 2001). The primary cell type of the NAc, the medium spiny neuron (MSN) (Kemp and Powell, 1971), integrates ascending dopaminergic input from the ventral tegmental area (VTA) with descending glutamatergic signals from forebrain and limbic structures (Swerdlow et al., 2001). In general, pharmacological manipulations that enhance NAc dopamine tone induce PPI deficits in rats (Swerdlow et al., 2001). Acute systemic THC increases extracellular dopamine concentration in the NAc shell (Chen et al., 1990; Tanda et al., 1997)—a potentially similar action to that of PPI-disruptive doses of intra-shell d-amphetamine (Wan and Swerdlow, 1996). Notably, this THC-mediated increase in accumbal dopamine is blocked by both systemic cannabinoid CB1 receptor (CB1R) blockade and opioid receptor antagonism in the VTA (Tanda et al., 1997), the primary source of NAc dopamine (Groenewegen et al., 1991). This latter finding, in combination with reports of systemic THC increasing firing rates of dopaminergic VTA neurons projecting to the NAc shell (Secci et al., 2019), suggests that (1) the VTA is likely a mediator of THC-induced NAc dopamine release, and (2) the effects of THC on mesoaccumbal dopamine transmission are not directly realized by CB1R binding on mesencephalic/striatal dopaminergic neurons. The role of mesoaccumbal CB1R activation in PPI disruption is likely nominal, as infusion of the full CB1R agonist WIN 55,212-2 into the VTA or the NAc does not induce PPI alterations (Wegener et al., 2008). PPI is, however, reduced when WIN 55,212-2 is infused into the ventral hippocampus and mPFC (Wegener et al., 2008), both of which significantly enhance VTA excitation (and ostensibly mesoaccumbal dopamine release; Kalivas et al., 1989) following THC administration (Loureiro et al., 2015; Hudson et al., 2019; Secci et al., 2019). The PPI-disruptive effect of THC in WT rats may therefore have been partially mediated by CB1R activation in the mPFC and/or ventral hippocampus enhancing VTA glutamate transmission, and thereby stimulating dopamine release in the NAc shell. Critically, MSNs of the HIVtg NAc core exhibit functionally reduced synaptic connectivity and consequently receive less dopaminergic input from the VTA (Roscoe et al., 2014; Javadi-Paydar et al., 2017; McLaurin et al., 2018c). Assuming that this synaptic dysfunction extends into the NAc shell, such neuropathology may have hindered any THC-mediated increase in mesoaccumbal dopamine transmission and thereby prevented any reduction of PPI. Importantly, both THC and HIV genotype independently produced mere 20% reductions of PPI relative to WT baseline—relatively minor effects vs those of other experimental manipulations, for example, apomorphine (Mansbach et al., 1988). This relative subtlety of drug and genotype effects suggests, respectively, (1) only modest alteration of PPI-relevant neurocircuitry by THC, and (2) sub-maximally impaired PPI in HIVtg rats at baseline. It is therefore unlikely that the absence of drug-induced PPI alteration in HIVtg rats was due to a floor effect.

No 3-way interactions were observed between THC, genotype, and sex on PPI. This lack of sex-specificity is somewhat surprising given that THC has been reported to differentially affect female vs male rats, with females being more susceptible to THC-induced hypothermia (Borgen et al., 1973) and antinociception (Tseng and Craft, 2001). Of greater relevance to the present study are sex differences in MSN morphology among HIVtg rats. While accumbal synaptic connectivity is reduced in both male and female rats, the specific morphological alterations that produce this reduction vary across sexes; males exhibit generally reduced dendritic spine volume (McLaurin et al., 2018c), while a population shift toward reduced spine length (Roscoe et al., 2014; Javadi-Paydar et al., 2017), dendritic branching, and arbor complexity (McLaurin et al., 2018c) is observed in females. Given the tendency for dopaminergic vs glutamatergic afferents to synapse on MSNs at specific points along the spines (i.e., neck vs head; Freund et al., 1984; Zahm, 1992), these morphological differences may be predicted to give rise to different ratios of dopaminergic vs glutamatergic innervation of MSNs across sexes, possibly resulting in differential responsivity to pharmacological manipulation. Unfortunately, since the present study made no attempt to quantify MSN dendritic morphology, it cannot be determined whether any such differences in accumbal connectivity between genotypes or sexes had any influence on THC responsivity. It should be noted, however, that significant sex × genotype interactions did eventually emerge in the present cohort of rats, with HIVtg males demonstrating lower ASR than WT males during the CBD assessment. Meanwhile, the genotype-mediated PPI deficit persisted only in females, possibly the result of a floor effect on male startle reactivity driven by an inter-time point reduction in overall ASR. This emergence of sex effects may reflect a sex-specific progression of HIV-mediated neuronal insult during the 4-month washout period, highlighting the need for evaluation of the effects of THC in older HIVtg rats.

The 4-month washout period must also be considered in the context of the progression of clinical pathologies associated with the HIV transgene. Critically, while the original HIVtg line developed by Reid and colleagues (2001) presented with a syndrome of severe dermatological, cardiac, renal, and neurological pathology beginning at 5–9 months of age, the derivation of HIVtg rat utilized by the present study expresses a far more moderate phenotype. Consistent with previous characterizations of the contemporary HIVtg rat (in which the transgene is restricted to chromosome 9) (Peng et al., 2010; Moran et al., 2013a; McLaurin et al., 2018a), the cohort assessed herein demonstrated no overt signs of wasting or accelerated mortality between 3 and 8 months of age, and cataracts were present throughout the study. The absence of main and interactive effects of genotype on inter-trial movement during both the THC and CBD assessments (“no stimulus”; supplementary Tables 1 and 2) indicated a minimal contribution of any genotype-mediated motor alteration to PPI. Indeed, previous longitudinal assessment of HIVtg locomotor activity revealed no gross motor impairment between 90 and 480 days of age, although PPI assessment of these animals identified a change in optimal prepulse-pulse interval between 90 and 240 days of age (McLaurin et al., 2018a). Interestingly, within-subjects analysis of viral mRNA levels in male HIVtg rats at 2–3 and 10–11 months of age revealed a shift toward higher striatal and prefrontocortical HIV protein expression over time (Peng et al., 2010). This increase in viral protein expression in PPI-relevant brain regions may partially account for the inter-time-point variability observed in past and present assessments; however, it is difficult to directly attribute the emergence of sex effects to this phenomenon as age-specific viral expression has yet to be evaluated in female HIVtg rats. Direct comparison of the above THC and CBD effects is complicated as well, as CB1R expression appears to be similarly age dependent in the rat (Liu et al., 2003). Furthermore, the HIV protein Tat reduces CB1R function in rat hippocampal cultures (Wu and Thayer, 2020); this action may conceivably impact other PPI-regulatory nuclei in vivo and may potentiate over time with increased expression. This likelihood of significant inter-time point differences in HIVtg neuropathology necessitates interpretation of the results of the 2 drug assessments separately and in the context of age. Caution must be taken in generalizing the present findings across time points.

Given that group-level PPI deficits were first observed among only those PWH diagnosed with HAND (Minassian et al., 2013) and that lower baseline PPI correlated to poorer learning and memory in HIV⁺ men (Walter et al., 2021), PPI deficits may be an indicator of cognitively relevant frontal system dysfunction in HIV. Following this interpretation, the present finding that acute THC does not exacerbate the PPI deficit of HIVtg rats suggests that acute cannabis use may not produce the same level of cognitive impairment in HAND patients as in healthy subjects or even PWH who have not yet developed significant frontal systems damage. This interpretation is, however, severely limited by 3 main factors, all of which merit consideration in future studies of cannabinoids and HIV. The first limitation of the present study is that acute subcutaneous and/or i.p. THC and/or CBD cannot be expected to produce effects representative of those of conventional human cannabis use, which is typically characterized by repeated smoking, vaping, or ingestion. For example, repeated administration of THC creates long-term changes to dendritic spine morphology in the NAc (Kolb et al., 2006, 2018), which may be relevant to PPI in the context of the mechanisms described above. Such chronicity may account for the more severe effects of moderate-to-heavy cannabis consumption on learning and memory in PWH vs healthy individuals (Thames et al., 2016), which would not have been predicted by the results of the present study. The pharmacokinetic differences between routes of THC/CBD administration must also be taken into account, given that subcutaneous administration produces far less rapid increases in brain and serum cannabinoid levels than pulmonary administration in rats and lower peak brain cannabinoid levels than oral administration (Hložek et al., 2017). Secondly, the present study is limited in that it only characterized the effects of individual cannabinoids on PPI. Cannabis sativa contains more than 60 unique cannabinoids (Mechoulam, 2005) that may potentially interact with each other in a manner relevant to sensorimotor gating. Indeed, such an interaction has been reported between subcutaneous THC and CBD in rats, whereby their co-administration mitigated their individual PPI-disruptive effects (Hložek et al., 2017). Finally, it is important to note that none of the rats used in the present study were maintained on cART, the primary medication regimen for the suppression of HIV. It is currently unknown whether cART affects sensorimotor gating, although chronic administration does induce cognitive impairments in healthy mice (Pistell et al., 2010). Each of these limitations—treatment chronicity/route, potential inter-cannabinoid interactions, and cART effects—need to be addressed when extrapolating the present results to the potential consequences of cannabis use by PWH.

In conclusion, the present study identified a PPI deficit in the HIVtg rat that is (1) qualitatively similar to that observed among PWH and (2) not exacerbated by acute administration of otherwise PPI-disruptive doses of THC at approximately 3.5 months of age, or by CBD at approximately 8 months. Given that PPI deficits may be associated with cognitive decline in the HIV disease state (Minassian et al., 2013; Walter et al., 2021), these results may be tentatively interpreted as indicative of an HIV-conferred resistance to the deleterious effects of THC on overlapping cognition- and PPI-relevant neurocircuitry. Future studies are needed to validate this conclusion, however, which may include clinical investigation of the interactions between HIV and cannabis on PPI and cognition as well as between HIV and the subjective and therapeutic effects of THC.