Assessments of Cognitive Function Following Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 218

Because this protocol has not been established across different
injury models, changes may be needed to ensure a suffi cient differ-
ence in freezing times between sham and injured animals.

A conditioning cage or box (approx. 17.8 cm × 17.8 × 30.5 cm,
Plexiglass walls, and a metal grid fl oor) housed inside a soundproof
chamber is required for training. A computer-controlled HabiTest
system is available commercially from Colbourn instruments
(Allentown, PA). A house light supplies dim illumination inside
the chambers. Foot shocks are administered through a metal
floor grid using a precision feedback, current-regulated shocker.
A removable tray is located below the metal fl oor grid to move
animal waste. The cage is cleaned using 70% ethanol before each
experiment and before each animal enters. FreezeFrame Control
and Data Acquisition Software and FreezeView Data analysis
software (Colbourne Instruments) can be used to administer fear
conditioning protocols and analyze data. A computer-controlled
infrared thermal activity monitor is used to determine freezing.
A camera can be placed above the cage to record the session.

2.2.1. Fear Conditioning
Materials and Instruments

Each animal should be handled for 2 days prior to testing. During
acquisition, animals are placed in the cage with a dim house light
on. During acquisition, animals are allowed to move freely within
the cage for 3 min before receiving a foot shock (1 mA for 0.5 s).
The number of foot shocks over the trial, the length of the shock
and the current can be adjusted to allow for stronger association
conditioning. The exact protocol for investigating hippocampal-
dependant contextual fear following injury may need to be changed
depending on the injury model and other factors. Following con-
ditioning, animals are left in the box for a specifi ed amount of time
(2-5 min is the most common). Animals are then removed from
the cage and returned to their home cage. Between animals, the
entire cage is cleaned with 70% ethanol. Animals can then be tested
at either 24 or 48 h after the conditioning phase by being returned
to the same context (e.g., same cage, with no changes) for 5 min.
No shock is administered within the test phase. The amount of
time in inactivity (freezing) and number of freezing incidents
is determined by the activity monitor and computer software.
An observer (blinded) can also record the time and number of freezing
incidents, to ensure no differences between the observer and com-
puter scoring of freezing. Using a variation of the above protocol,
Lifshitz et al. ( 18 ) found impairments in hippocampal-dependant
contextual fear conditioning in mice at 7 days following FPI, but
not at 30 days post-injury. Using variations in the task, this study
demonstrated defi cits related to memory consolidation, rather than
acquisition or recall, following FPI.

2.2.2. Contextual Fear
Conditioning Procedure