An underlying concern in our lab has been to maximize the ethological validity of our research. One aspect of our ethological approach is to provide animals with the opportunity to develop species typical behaviors through environmental enrichment. Rats go through a six-week schedule of ‘opportunities’ and ‘events’, which are provided by introducing certain features at particular times. The schedule of enhanced-enrichment begins on the first Monday after weanling rats have been in the Animal Care Facility for one week (i.e., at approximately 28 days of age).

Floor substrates

Different floor substrates provide a range of potent stimuli (including tactile, olfactory, and proprioceptive). There are four different types of floor substrates -- sand, wood shavings (beta-chip), gravel, and stones. Most levels have two different substrate areas in order to increase the variety of transitions.

Suet Blocks

A suet block provides a slowly-depleted food for ‘browsing’.
It is not easy for the rats to reach the suet through the mesh walls of the environment and the suet holder. In doing so, rats learn skilled use of their forelimbs.

Foraging

Substrate-forage areas provide opportunities to search for different types of food hidden in a variety of substrates. There are 3 types of food -- sunflower seeds, peanuts, and raisins -- occasionally scattered within the floor substrate. Each food type appears twice in the forage areas; thus, there are a total of six days on which rats have an opportunity to forage for food. A particular day of the week is designated the foraging day. The same food is scattered in all three foraging areas on foraging days.

Different cohorts receive different sequences of food types:

Sequence 1 = seeds, beans, raisins (then repeat)
Sequence 2 = beans, raisins, seeds (then repeat)
Sequence 3 = raisins, seeds, beans (then repeat)

Sexual Interaction

Rats have occasional access to sexually-receptive and non-receptive females (and probably also pregnant females), which provides them with a range of potent stimuli and opportunities for heterosexual behaviour. On one day during the 2nd, 4th, and 5th week, the males of a particular cohort are able to access the females by connecting the passageway to the ‘female-environment’. On those days, rats are able to pass freely among all five levels of the female-environment.

Aversive Stimuli

Encounters with aversive stimuli provide important stimulation and opportunities to learn about things that signal danger.

Examples of aversive stimuli:

A piece of wood that is lightly scented with the odour of a predator (domestic cat) should stimulate innate fear processes.

A shock-probe provides an opportunity to learn that a specific discrete object is dangerous.

LiCl injections simulate the consequences of ingesting a mildly-toxic food, and it should induce a conditioned taste aversion.

Schedule of Aversive Events and Access to Female

Week 1

On one day, the rats are given access to the lower two levels of the female environment, while females are confined to the upper three levels. There are no aversive events scheduled during Week 1.

Week 2

On one day, rats receive exposure to the entire female environment. This is the rats’ first exposure to female rats. (The females also have access to the all five levels of the environment). There are no aversive events scheduled during Week 2.

Week 3

On one day, rats have access to the lower two levels of the female environment, while females are confined to the upper three levels. Predator-odour and shock-probe “events” occur on this day.

Each rat receives an injection of LiCl immediately after being removed from the environments on the foraging day of Week 3. Note that one cohort will have tasted peanuts for the first time on this day, another will have tasted raisins for the first time, and the other will have tasted sunflower seeds for the first time.

Week 4

On one day, rats have access to the entire female environment; this is their second exposure to females. Predator-odour and shock-probe “events” also occur on this day.

Week 5*

On one day, rats have access to the entire female environment; this is their third exposure to females. Predator-odour and shock-probe “events” also occur on this day. This occurs on the foraging day of Week 5.

Week 6

On one day, the rats have access to the entire female environment, but no females will be present. No aversive events are scheduled during Week 6.

 

* With the exception of Week 5, predator-odour, shock-probe, and access to females do not occur on foraging days.

Loading and Unloading rats

Loading Rats

A cohort of rats is taken from their home cages and placed into a large loading cage, which is then placed on top of the environments. A hole in the bottom of the loading cage is over a passageway leading to Level 1 of the environments. The loading cage stays in place until the end of the enrichment session.

There are at least two important reasons for using this system of loading rats:

1. Rats are allowed to disperse into the environments on their own volition. In nature, rat pups are weaned by their mothers at around 3-weeks of age. They do not immediately leave the nest to make for themselves, but instead, they begin to take brief forays out into the world around them. At first, they return back to the huddle of siblings before going very far, but over the course of a few days, they stay out longer and longer. During this time, the rats are allowed to engage in this natural behaviour of huddling interspersed with bouts of exploration.

2. When rats enter a particular environment (either in nature, or in the laboratory), they establish a home base from which they conduct subsequent exploration and foraging. This home base is a key element of a rat’s environment. In a given environment, an individual rat will tend to use a particular location as its home base over and over again. Rats, therefore, consistently enter the enriched environments from a particular location.

Unloading Rats

A few minutes before the end of the session, we place an unloading cage on top of the environment (next to the loading cage). The unloading cage contains rat chow, which is intended to lure the rats through a one-way passage. Once a rat enters the unloading cage to get food, he is not able to leave. So, once the rats are all in the unloading cage, it can be lifted down from on top of the environment and rats are returned to their home cages.