Tamoxifen produces conditioned taste avoidance in male rats: an analysis of microstructural licking patterns and taste reactivity

Refined tamoxifen administration in mice by encouraging voluntary consumption of palatable formulations

Drug administration in preclinical rodent models is essential for research and the development of novel therapies. Compassionate administration methods have been developed, but these are mostly incompatible with water-insoluble drugs such as tamoxifen or do not allow for precise timing or dosing of the drugs. For more than two decades, tamoxifen has been administered by oral gavage or injection to CreERT2-loxP gene-modified mouse models to spatiotemporally control gene expression, with the numbers of such inducible models steadily increasing in recent years. Animal-friendly procedures for accurately administering tamoxifen or other water-insoluble drugs would, therefore, have an important impact on animal welfare. On the basis of a previously published micropipette feeding protocol, we developed palatable formulations to encourage voluntary consumption of tamoxifen. We evaluated the acceptance of the new formulations by mice during training and treatment and assessed the efficacy of tamoxifen-mediated induction of CreERT2-loxP-dependent reporter genes. Both sweetened milk and syrup-based formulations encouraged mice to consume tamoxifen voluntarily, but only sweetened milk formulations were statistically noninferior to oral gavage or intraperitoneal injections in inducing CreERT2-mediated gene expression. Serum concentrations of tamoxifen metabolites, quantified using an in-house-developed cell assay, confirmed the lower efficacy of syrup- as compared to sweetened milk-based formulations. We found dosing with a micropipette to be more accurate than oral gavage or injection, with the added advantage that the method requires little training for the experimenter. The new palatable solutions encourage voluntary consumption of tamoxifen without loss of efficacy compared to oral gavage or injections and thus represent a refined administration method.

Genetic and environmental influences on one-trial conditioned context aversion in mice

Anticipatory nausea (AN) is caused by an association between contextual cues and the experience of nausea (the side effects of chemotherapy or radiation treatment) and it develops predominantly in female patients undergoing chemotherapy. Preclinical studies in rodents show that the administration of an illness-inducing agent in the presence of novel contextual cues can cause conditioned context aversion (CCA) and this has been proposed to model AN. The literature also suggests that brief pre-exposure to a novel context prior to shock delivery is critical in the development of contextual fear conditioning in rodents (a phenomenon known as Immediate Shock Deficit), but this has not been assessed in CCA. The aim of present study was to develop a CCA paradigm to assess this in outbred (CD1) and inbred (C57BL/6J) mice and evaluate potential sex differences. The results revealed that a single conditioning trial in which a distinctive context was paired with LiCl-induced illness was sufficient to elicit a conditioned response in both female and male CD1 outbred mice, but not in C57BL/6J inbred mice. In addition, CCA was facilitated when animals had prior experience with the context. Finally, outbred female mice showed longer and more robust retention of CCA than male mice, which parallels clinical findings. The results indicate the importance of using CD1 outbred mice as an animal model of AN as well as examining sex differences in the CCA paradigm. Similar findings in humans encourage the future use of this novel CCA preclinical mouse model.

The Palatability of Lopinavir and Ritonavir Delivered by an Innovative Freeze-Dried Fast-Dissolving Tablet Formulation

Negative hedonic sensory qualities of HIV antiretroviral drugs often reduce patient adherence particularly in pediatric populations requiring oral consumption. This study examines the palatability of an innovative delivery mechanism utilizing a freeze-drying-in-blister approach to create fast-dissolving tablets (FDTs) containing a fixed-dose combination of lopinavir and ritonavir (LPV/r). Consumption patterns of solutions during brief-access and long-term testing and baby foodstuff consumption were analyzed to evaluate the orosensory detection and avoidance of placebo FDTs containing no LPV/r (FDT-) and FDTs containing LPV/r (FDT+). Rats showed no change in consumption patterns for the placebo FDT- compared with control solutions. Rats can detect but do not avoid FDT+ at body-weight-adjusted dosages in both brief-access (30-s) and long-term (23 h) consumption tests. There is an aversive response to concentrated doses of FDT+ during brief-access tests that cannot be masked by 25% sucrose. However, the strongest FDT+ concentration was not rejected when mixed with 50 g of applesauce, banana sauce, or rice cereal baby foodstuffs. The averseness of the FDT+ was associated with the presence of LPV/r and not the FDT- formulation itself. The novel FDT formulation appears to be a palatable delivery mechanism for oral antiretroviral pharmaceuticals especially when mixed with baby foodstuffs.

Temporal and qualitative dynamics of conditioned taste aversions in C57BL/6J and DBA/2J mice self-administering LiCl

Self-administration of LiCl solution has been shown to result in the formation of a conditioned taste aversion (CTA) that generalizes to NaCl in rats. This paradigm may have considerable ecological validity as it models CTA learning in natural settings, and also allows for the investigation of drinking microstructure as an assay of potential shifts in stimulus palatability. We used this paradigm to examine possible mouse strain differences in CTA acquisition, generalization, and extinction. In the first experiment, C57BL/6J (B6) and DBA/2J (D2) mice self-administered LiCl (or control NaCl) over a 20-minute free access acquisition period and were tested on the following day with a panel of taste solutions available in brief (5-s) trials delivered in random order. In the second experiment, mice again self-administered LiCl or NaCl (at low, 0.12 M, or high, 0.24 M concentrations) in a 20-minute session, and on the following day received a 20-minute free access period to equimolar NaCl. Strain differences were found for aspects of ingestive behavior, with B6 mice showing greater consumption of all stimuli, including water, while D2 mice lick faster, in less frequent but longer bursts. We did not, however, find evidence of a robust strain difference in taste aversion learning. Both strains demonstrated profound alterations in licking microstructure in the generalization session relative to controls. We suggest that a decrease in "lick efficiency" (the percentage of inter-lick intervals within a burst of short duration vs. longer duration) reflects avoidance behavior, and signals a shift in palatability of a stimulus following CTA.

The cancer drug tamoxifen: a potential therapeutic treatment for spinal cord injury

Tamoxifen (TMX) is a selective estrogen receptor modulator that can mimic the neuroprotective effects of estrogen but lacks its systemic adverse effects. We found that TMX (1 mg/day) significantly improved the motor recovery of partially paralyzed hind limbs of male adult rats with thoracic spinal cord injury (SCI), thus indicating a translational potential for this cancer medication given its clinical safety and applicability and the lack of currently available treatments for SCI. To shed light on the mechanisms underlying the beneficial effects of TMX for SCI, we used proteomic analyses, Western blots and histological assays, which showed that TMX treatment spared mature oligodendrocytes/increased myelin levels and altered reactive astrocytes, including the upregulation of the water channels aquaporin 4 (AQP4), a novel finding. AQP4 increases in TMX-treated SCI rats were associated with smaller fluid-filled cavities with borders consisting of densely packed AQP4-expressing astrocytes that closely resemble the organization of normal glia limitans externa (in contrast to large cavities in control SCI rats that lacked glia limitans-like borders and contained reactive glial cells). Based on our findings, we propose that TMX is a promising candidate for the therapeutic treatment of SCI and a possible intervention for other neuropathological conditions associated with demyelination and AQP4 dysfunction.

Effects of estrogen and estrus cycle on pharmacokinetics, absorption, and disposition of genistein in female Sprague-Dawley rats

Genistein is an active soy isoflavone with anticancer activities, but it is unknown why it has a higher oral bioavailability in female than in male rats. Our study determined the effects of estrus cycle on genistein's oral bioavailability. Female rats with various levels of estrogen were orally administered with genistein or used in a four-site rat intestinal perfusion experiment. Rats in "proestrus" group (with elevated estrogen) had significantly reduced (57% decrease, p < 0.05) oral bioavailability of total genistein (aglycone + conjugates) than those in "metoestrus" group (with basal level of estrogen). Female ovariectomized rats, due to lack of estrogen, showed oral bioavailability of total genistein similar to the "metoestrus" group but higher (155% increase, p < 0.05) than the "proestrus" group. On the basis of intestinal perfusion studies, the increased bioavailability was partially attributed to the higher (>100% increase, p < 0.05) hepatic disposition via glucuronidation and possibly more efficient enterohepatic recycling of genistein in the "metoestrus" group. Furthermore, chronic exogenous supplementation of estradiol in ovariectomized rats significantly reduced (77%, p < 0.05) the oral bioavailability of total genistein, mostly via increased sulfation (>10-fold) in liver, to a level comparable to those in the "proestrus" group. In conclusion, the oral bioavailability of total genistein was inversely proportional to elevated estrogen levels in female rats, which is partially mediated through the regulation of hepatic enzymes responsible disposition of genistein.

Estrogenic involvement in social learning, social recognition and pathogen avoidance

Sociality comes with specific cognitive skills that allow the proper processing of information about others (social recognition), as well as of information originating from others (social learning). Because sociality and social interactions can also facilitate the spread of infection among individuals the ability to recognize and avoid pathogen threat is also essential. We review here various studies primarily from the rodent literature supporting estrogenic involvement in the regulation of social recognition, social learning (socially acquired food preferences and mate choice copying) and the recognition and avoidance of infected and potentially infected individuals. We consider both genomic and rapid estrogenic effects involving estrogen receptors α and β, and G-protein coupled estrogen receptor 1, along with their interactions with neuropeptide systems in the processing of social stimuli and the regulation and expression of these various socially relevant behaviors.

Impact of Sex and Gonadal Hormones on Cocaine and Food Reinforcement Paradigms

Men and women express sexually dimorphic patterns of cocaine abuse, such that women progress faster from initially trying cocaine to becoming dependent upon the drug and display a greater incidence of relapse. Sex differences in response to cocaine are also seen in the laboratory in both humans and animal models. In this review, animal models of cocaine abuse that have reported sex differences in appetitive reinforcement are discussed. In both human and animal studies, sex differences in the subjective and behavioral effects of cocaine are often related to the female reproductive cycle and ovarian hormones. As a comparison, food reinforcement studies have shown the opposite profile of sex differences and the impact of sex steroids on food intake and response rate. In contrast, limited attention has been given to "choice" models in rodents of either sex, however, our recent studies have indicated a role of sex and estrogen in cocaine choice over food with intact females, and OVX females treated with estrogen, choosing cocaine significantly more than males. Interestingly, estrous cycle phase does not seem to impact cocaine choice as it does response rate in single-reinforcer studies, suggesting that genomic rather than neurosteroid effects of estrogen modulate sex differences in this model. Future studies should more fully explore the impact of sex hormones on concurrent reinforcement and discrete choice models of addiction.

Effect of the acute and chronic estrogen on anxiety in the elevated T-maze

Despite the extensive studies on the influences of estrogen (E(2)) on anxiety-like behaviors, there is still conflicting evidence regarding the specific effects of E(2) on anxiety. These discrepancies may be a result of different replacement regimens. The goals of this study were to evaluate anxiety-like behavior in ovariectomized rats (Ovx) using the elevated T-maze (ETM) test for the following variables: (1) the effects of acute versus chronic E(2) dosing, (2) the effects of chronic E(2) at different doses and, (3) the effects of Tamoxifen (Tam) co-administered with E(2). Rats in the acute E(2) dosing group (aE(2)) showed reduced inhibitory avoidance responses with prolong escape latencies compared to Ovx; while rats in the chronic E(2) dosing group (cE(2)) showed reduced inhibitory avoidance responses only. These results suggest that E(2) contains anxiolytic effects when given once or repeatedly. Moreover, when various doses of E(2) (1-100 μg/kg) were chronically given to the Ovx rats, all doses produced impaired inhibitory avoidance responses compared to Ovx, suggesting that chronic replacement of E(2) had no dose-dependent effect on anxiety-like behavior. Interestingly, in the 3-week delay replacement regimen, the low dose E(2) (1 μg/kg, s.c.) group displayed no anxiolytic effects as their inhibitory avoidance responses in the ETM were not different from their Ovx counterparts. On the contrary, the Ovx group that received Tam+E(2) (Tam 1 mg/kg, PO and E(2) 1 μg/kg, s.c.) had reduced inhibitory avoidance responses compared to other groups. These findings indicate that when Tam is co-administered with chronic low dose estrogen, it can act as an estrogen receptor agonist and result in anti-anxiety effects. Therefore, it is likely that the anxiolytic-like behavior relative to generalized anxiety disorder can be conserved when estrogen is given acutely or chronically; while the anxiolytic-like behavior relative to panic disorder can be conserved only when estrogen is given acutely.