Available online 8 August 2022, 101020

Abstract

Research on hormonal contraceptives (HC) in animal models is lacking, and as a result, so is our understanding of the impact of HC on the brain and behavior. Here, we provide a review of the pharmacology of HC, as well as the methodology and best practices for designing a model of HC in female rats. We outline specific methodological considerations regarding dosing, route of administration, exposure time/timing, and selecting a control group. We also provide a framework outlining important levels of analysis for thinking about the impact of HC on behavioral and neurobiological outcomes. The purpose of this review is to equip researchers with foundational knowledge, and some basic elements of experimental design for future studies investigating the impact of HC on the brain and behavior of female rats.

Introduction

Over 150 million women use hormonal contraceptives (HC) globally, and this number is growing (United Nations, 2019). Despite the widespread use of HC, we understand very little about their effects on the brain and behavior. A fundamental reason for this lack of understanding is that experiments in animals are limited. Researchers interested in studying HC in animal models may have questions about how to design an animal model of HC. Thus, the purpose of this review is to outline important considerations when designing experimental models of HC, specifically in female rats.

Here, we outline methodological considerations for the selection of dose, route of administration (ROA), exposure time, and selection of control groups as it relates to studying the effects of HC in female rats. Moreover, we provide a framework for designing experiments in rats with the intention of translating findings from studies in humans. The guidelines provided here are aimed to help navigate the sometimes complicated methodology of research on HC.

Section snippetsHow HC work

In order to understand the primary mechanism of action of HC, it is important to first describe the human menstrual cycle. The menstrual cycle is approximately 28 days long but can range anywhere between 21 to 35 days in length (Stanczyk et al., 2013). The 28-day cycle is divided into three broad phases: follicular, ovulatory, and luteal. Some separate the follicular phase into early-follicular (menses) and late-follicular, which are broadly separated by either low or high levels of

Selecting a dose

While there are detailed guidelines on how to translate drug doses between humans and rats (Nair and Jacob, 2016), these recommendations may fall short when studying HC. Free steroid hormones (endogenous or synthetic) have similarly low molecular weights, <400g/mol, and are non-polar and hydrophobic organic compounds (Payus et al., 2017). They readily permeate across phospholipid bilayers of cell membranes and the blood-brain barrier (Banks, 2012, Oren et al., 2004). EE is known to be active in

Routes of Administration

When it comes to route of administration (ROA) there are several options to consider, each of which has a cost and a benefit.

Exposure time

The number of days the rat is exposed to HC, once again, depends on the aim of the experiment. Most oral HC provide 21 days of hormonally active pills (pills which contain the synthetic hormones), followed by 7 days of hormone-free pills (Dickey, 2021; Fig. 2). Thus, one approach has been to expose female rats to HC daily for 21 consecutive days (Follesa et al., 2002, Simone et al., 2015, Wusu et al., 2021). However, as we will see in the next section (5.2.), 21 days of chronic exposure

Validating models of contraception.

After selecting the dose, ROA, and exposure time for the experiment, it is important to verify whether these specifications succeed in inhibiting ovulation or reaching a state of acyclicity. Vaginal cytology and HPG suppression are two converging lines of evidence that the model of HC is effective at producing this result.

Control Group.

The decision of what control group to include will largely depend on which component of HC are being modeled. If controlling for some of the known hormonal effects of HC, then the following control groups may be useful to consider.

Framework for modeling HC in female rats.

Above we have outlined the essential components for designing a model of HC in female rats. As we have shown, there are several important decisions that must be made regarding dosing, ROA, exposure time/timing, and control groups. However, these elements are the most basic parts of designing a model of HC in female rats. Here, we reference findings in humans that inform us on what components of HC may be appropriate to examine when using rat models.

Conclusion.

Here, we have outlined the most basic and essential methodological considerations for designing experiments on HC in female rats. We present the pharmacology of HC (Table 1, Table 2) which should help in designing future experiments. In addition, we have provided an overview of past research (Table 3) and a framework for designing future experiments. What is clear is that there is no one perfect way to model HC in female rats. The elements of each model (vis. dose, ROA, exposure time/timing,

Uncited references

Lacasse et al., 2022a, Pletzer et al., 2016, Sundström et al., 2014.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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