If You Give a Mouse a Cookie

Cecelia Ky-Lan Do
May 5
17 min read

by Artemis Christoforatos

art by Kai Yara

Hallucinogens, stimulants, dissociatives—what place do these drugs have in the lab? What can we learn from giving a mouse amphetamine or psychedelic mushrooms? Neuroscientists often give various substances to mice because their responses can elucidate the biology behind basic brain functions and in form possible treatments for diseases. Mouse research has been an essential instrument in revealing how our brains work, and continues to be integral to modern science. What happens if you give a mouse…?

AMPHETAMINE: a longest-standing tool for understanding dopamine Dopamine is a neurotransmitter necessary for movement, reward, learning, and memory [1]. As early as the mid-20th century, scientists found that rodents can respond to amphetamine with behaviors like excessive locomotion [2, 3]. As dosages increased to very high levels, the rodents’ locomotion was replaced by repetitive movements [4–6]. Instead of running around like before, they stayed in place, repeatedly sniffing the same spot, grooming themselves, and twitching [5, 6].

Based on this information and investigations into its molecular structure, scientists determined that amphetamine is a substrate of the dopamine transporter (DAT) protein. DAT is found in the outer membrane of dopamine-producing neurons [7]. After dopamine release, DAT works to transport dopamine that was not received by other neurons from the extracellular space back into the cell it was released from, hence controlling the amount of dopamine available for use by other cells [8, 9]. However, when amphetamine is also present, it competes with dopamine for transport via DATs, ultimately allowing less dopamine to be reabsorbed [10]. At the same time, when amphetamine enters the cell, it reverses the direction of the flow of dopamine to instead favor its release out into the synapse [11].

As a result, there is more dopamine available than usual to be used by the brain when amphetamine is present [11, 12]. While some dopamine is necessary for essential behaviors, from instinct and movement to cognitive tasks like decision-making, too much dopamine can impair them [13, 14].

Currently, amphetamine is often used to identify potential differences in the dopamine system of different mouse genotypes. For example, in one new study, a research group at the University of Copenhagen studied mice with and without a genetic mutation leading to DAT deficiency and lack of locomotive control [1]. They found that mutant mice responded to amphetamine in different brain regions than control mice, and also showed fewer locomotive issues. These individual differences in dopamine transport and its resulting effects on movement provide a possible explanation for why some people may respond unexpectedly to amphetamine medications like Adderall, with less instead of more hyperactivity [15].

PSILOCYBIN: found in various “magic mushrooms”

Psychedelic mushrooms have been used in numerous cultures throughout history for recreational, spiritual, and therapeutic purposes, but it is only within the past several years that neuroscientists have begun extensively exploring their potential for enhancing neuroplasticity and treating conditions such as depression [16]. Psilocybin is the active substance in many naturally occurring psychedelic plants [17, 18]. While researchers do not know exactly how or to what extent psilocybin can treat mental health conditions, so far it appears that psilocybin produces fast-acting and long-lasting reductions in depressive and anxiety-like behavior. This is paired with increased neuronal plasticity—the ability of neurons to adjust their structure and function over time [16, 17].

Depression is a condition that encompasses a variety of emotional and behavioral features. We can induce many of these depressive behaviors, such as through repeated stress [19, 20]. Many studies show that giving even a single dose of psilocybin to a mouse with these behaviors will reduce said behaviors for up to weeks afterwards [16–18, 21].

For example, a commonly used assay to observe perseverance in the context of depression is the forced swim test (FST), which involves placing a mouse in a pool of water and observing how hard it will try to escape before giving up [22]. Mice with depressive phenotypes tend to become immobile faster, that is, give up more quickly [23]. After psilocybin, the same mouse that might have originally immediately gone immobile may continue to persevere for a much longer time [17, 18].

Another facet of depression is anhedonia, or the reduced ability to feel pleasure. Anhedonia can be evaluated via the sucrose preference test, which compares a mouse’s preference between sugar and regular water [20]. Mice normally display a strong preference for sugar water, but mouse models of depression show lower or no preference for the sugar water, indicating the presence of anhedonic-like behaviors [20, 24]. Treatment with psilocybin results in the original depressed phenotype matching non-depressed controls in sucrose preference tests [16].

Sometimes, cognitive flexibility and the ability to learn can be another indicator of depressive symptomatology. Scientists often assess these abilities in mice using fear conditioning and extinction. In fear conditioning, a mouse may be taught to associate a neutral stimulus, such as an audible tone, with an aversive stimulus like a mild electric shock, until it freezes in fear every time it hears the tone, expecting a shock [25]. Fear extinction involves training the mouse to disassociate the tone and the shock by repeatedly playing the tone unpaired from the shock [21, 26]. Mice that score high in measures of anxiety or depression tend to continue to freeze for longer in fear extinction assays [27]. However, after psilocybin exposure, these mice may learn more quickly to stop fearing the tone due to increased neuroplasticity [17, 18, 21].

Using mice allows us to examine both biological and behavioral impacts of psilocybin. For instance, Sholl analysis of neurons in mouse brains, which involves analyzing the structures of dyed cells, has shown that psilocybin can make neurons in the hippocampus and cerebral cortex grow more branches from their axons, which means they can communicate with an increased number of neurons. It also triggers increased formation of little protrusions called spines on dendrites, which is where neurons receive inputs from other cells. Both are markers of learning [18, 28, 29]. Whole-cell recordings of electrical activity in carefully preserved brain slices further suggest that psilocybin changes how neurons communicate, and which neurons they communicate with [18, 30]. Even more excitingly, these neuronal changes can last for months [17, 18]. This evidence of plasticity, or the ability of neurons to change in response to stimuli, is essential to our mental resilience and central to our current understanding of how psilocybin works.

Of course, if you give anyone a hallucinogenic agent, they are probably going to experience hallucinations. Mice under psilocybin often exhibit a head-twitching response, which researchers believe to be evidence that they are hallucinating [16, 31]. Experimental protocols vary, but behavioral testing often happens several hours after head-twitching and assumed hallucinations stop [32].

OPIOIDS: pharmaceutical compounds based on our body’s neural pain-relief mechanisms

Much like research on psychedelic mushrooms, modern opioid research, which began in the 19th century, was predated by the consumption of opiates. These natural substances from poppy flowers have been known for thousands of years to have pain-relieving, sedative, and euphoric effects [33]. Our bodies naturally contain opioid receptors in parts of the brain, spinal cord, and the peripheral nervous system, such as the digestive tract [34–36]. Opioid receptors respond to natural opioid peptides that our bodies make to induce pain relief [37, 38]. When neurotransmitters called endorphins bind to opioid receptors, they trigger a cascade of neurons signalling to each other to release a large amount of dopamine, decreasing emotional and physical responses to pain while increasing a sense of reward [37]. The opioid system is the basis of many emotions and experiences, such as the runner's high, where mood-boosting endorphins are released during exercise [39].

Opiates derived from poppy plants, such as morphine and codeine, or developed synthetically, can be harmful because they are more potent than the endorphins we naturally produce. Endorphins cannot compete with the pain relief and euphoria they offer, making them addictive [40]. After long-term use, the body produces fewer endorphins and releases less dopamine naturally because it expects the artificial opioids instead [41]. Eventually, many people who were prescribed opioids developed addictions, while opioid dependence worsened as recreational opioids such as heroin and fentanyl were developed and distributed [33].

It is medically harmful to stop prescribing opioids altogether [42]. Opioids are an immensely helpful tool in palliative care and pain management [43]. For people with terminal conditions nearing the end of their lives, healthcare professionals generally agree that it is beneficial to prioritize effective pain management over any potential risks of opioid addiction [43]. Short-term prescriptions, after painful surgeries, for example, also aid in recovery [44]. Moreover, for people with intense chronic pain, a medication regimen that includes opioids can be life-changing [33].

Unfortunately, the continued necessary medical use of opioids means the risk of their misuse will persist. Given this, along with the current opioid addiction crisis, it is especially important to perform research on how to treat addiction to opioids and how to reduce the potential for addiction in the first place: this is where mouse models are valuable.

As with other research, mice are essential to opioid studies because their biology and behavior can both be studied and are similar to those of humans [45, 46]. Furthermore, in mouse experiments, we can manipulate and control experimental variables, which is more difficult to accomplish in humans.

Early research on rodent brains identified that opioid receptors are found in major dopamine-producing centers, such as the ventral-tegmental area in the midbrain, and that these receptors release enormous amounts of dopamine when activated by opioids [47]. Moreover, testing non-drug-related reward behaviors in these rodents demonstrated that reward centers were related to the interplay of dopamine and opioids [47]. Behavioral tests where mice were allowed to self-administer opioids mirrored addiction patterns seen in humans, and genetic testing revealed that gene expression in these mice changed due to long-term effects of lower endorphin release [48, 49].

All of this research has been happening against a backdrop of drug addiction crises during the past decades. What we have learned has informed therapeutics, such as recent investigations exploring naloxone (Narcan) treatment for opioid overdose [50, 51]. This has been made possible through advanced techniques, such as selective activation and suppression of receptors and neuron subtypes, to directly compare how different cell populations influence addictive behavior [51, 52]. Such methods provide significant insight into the biological mechanism behind addiction and the efficacy of the options we have for medical intervention in addiction [51, 53].

ADDENDUM: If you give a mouse a cookie…

The thing you need to know about giving a mouse anything is that you can’t just do it without preparation. There are rules and regulations that come into play when you bring a living being into research, whether it is a pig, a human, or a mouse [54]:

1. Make sure you are trained and licensed to work with mice.

Mice are living organisms that should be handled with care [55]. Research institutions, including Columbia University, provide in-person and online training on mouse care and handling that is available to undergraduates: subjects include mouse anatomy, illness identification, colony maintenance, best practices for housing, and how to scruff, handle, and euthanize animals [54]. You need to have passed any relevant training before you can touch a mouse.

In large research facilities, there are dedicated veterinary staff to ensure that animals are healthy and well cared for [54]. In addition to health requirements such as bedding and freely available food and water, mice are intelligent and social animals, and thus require enrichment and socialization. They are therefore usually housed in groups of up to five [54]. Aside from being unethical, limited access to these conditions can impact the results of your experiments since it alters these organisms’ normal behavior and brain functioning [56]. For example, several studies show that mice in social isolation experience stress, which can reduce their food intake, increase anxiety and avoidance, and alter performance in cognitive tasks [56–58].

2. Justify your animal model.

If you want to work with mice, you need to prove that you cannot use a model with a lower level of awareness of pain or suffering, like worms or fruit flies. Likewise, you would have to prove that working with mice can give you answers you want, and if answering your research question is worth the harm you may cause the animal [59]. If you want to investigate decision-making responses to amphetamine, fruit flies may be a poor animal model to use because it is hard to judge their level of awareness of the choices you may be presenting them with, not to mention measuring exactly how much amphetamine they may have in their system.

In a similar vein, you need to plan out your experiment in detail, explaining precisely what experiments you might perform on your mice, what distress the mice may experience in the process, and why those experiments are necessary to be done with the number of mice you plan to use [54]. Say you want to feed a mouse a cookie: before starting, you need to know why you are giving cookies to mice, what types of cookies you are giving, whether those mice may experience distress due to eating cookies, and how giving these mice cookies benefits humanity and science in the long term.

3. Submit your experimental protocol to your governing body’s animal review board.

Once you have your plan in place, provided you are in the United States, you must submit a report of that plan to your institution’s animal review board. Every formal research institution has an ethics board called the Institutional Animal Care and Use Committee (IACUC), which is adjacent to the human Institutional Review Board (IRB). The IACUC approves protocols that meet established ethical standards [60]. Today, without IACUC approval, your research cannot be peer reviewed and faces major barriers to publishing.

4. Perform your experiment and share your results.

When you have received the green light, collected your data, and analyzed your results, share what you have learned. Do mice that eat cookies complete mazes faster? Does eating cookies cause an increase in anxiety in mice, as measured via behavioral tasks? Other researchers need to know if they should expand on your research question, or maybe not perform that particular experiment again, given that someone else did it, which would minimize the total number of mice exposed to distressing conditions.

If you give a mouse a cookie, it might enter your home, make a mess, take a nap, and ask for a cup of milk—and then a cookie to go along with it. Questions beget more questions; there will always be more to research. For now, for better or for worse, that means more mice. And, if you have a mouse, you might as well give it a cookie and see what happens…

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If You Give a Mouse a Cookie

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