- Fear can be defined as an emotional state that is induced by the perception of any danger that threatens the well being of the
individual. Fear producing stimuli can be either natural (no previous exposure to or training with the stimulus is necessary for it to
induce a fear state) or learned (prior conditioning is necessary in which this learned stimulus is paired with a naturally producing fear
stimulus and therefore comes to elicit a fear response as well).
- The natural stimuli can be broken down into four categories. First, the stimulus can be specific to dangers that are part of the
evolutionary history of the species. For instance, it may be a fear of darkness in vision-dependent species like humans. Second, the
stimulus can be exposure to novelty. This is a very potent condition for leading to a negative emotional response. Third, the physical
characteristics of the stimulus, such as intensity and duration, can affect not only whether or not a fear response is produced, but also
the amount of fear behavior or physiological response displayed. Finally, signals arising from interactions with conspecifics can induce
fear related responses. Some species have even shown avoidance to certain situations based on experiences of conspecifics, such as birds
and mammals (Boissy, 1995).
- Frequently, fear behaviors seem to contradict/compete with one another. For instance, if an animal is freezing, it can't press a lever to
escape foot-shock. According to Bolles and Fanselow (1980), in their perceptual-defensive-recuperative model of fear and pain, there are
three ways in which motivational systems might inhibit each other. One way is through response competition, in which one motivational
system calls forth a response that is so strong that it prevents other behaviors, even those from other motivational systems, from
occurring. Another way is through competition that is more central, meaning that it occurs at a single motivational level. For instance, a
hungry animal that is presented with a CS signalling foot-shock stops pressing the bar, not because the CR is inhibiting the animal
from pressing the lever for food, but because the animal suddenly becomes less hungry. Finally, it may be a perceptual inhibition. For
instance, the sudden introduction of a new motivational state, such as fear, might alter the perception of the old environment, making
the bar which used to stand for providing food no longer related to food reward.
- This paper focuses on the physiological and behavioral changes that occur within a fear state as well as the most prominent tests for
assessing fear related behaviors and anxiety reducing pharmacological agents (anxiolytics). There is an array of behavioral techniques to
study these behaviors; however, only those frequently used in current literature, or those that have provided a foundation for a particular
aspect of the study of fear, will be discussed.
- Multiple Indices of Fear :
When an animal is exposed to an aversive stimulus or has come to associate a non-threatening stimulus with an aversive one, specific
physiological as well as behavioral responses can be observed. For instance, the animal may experience decreased salivation, increased
heart rate, respiration change and stomach ulcers. Behaviorally, the animal may express freezing, grooming, increased startle, scanning
and vigilance, social interaction changes, and urination and defecation (Davis, 1992).
- This indicates that there are numerous methods for producing a fear state in which to study certain behavioral and pharmacological
phenomena. However, one recent distinction made by Blanchard and colleagues (1993) suggests that the tests may actually be divided
into two subparts: those that measure defensive behaviors to present, approaching danger, and those that measure reactions to potential
threat. They used two test batteries, the Fear/Defense Test Battery (F/DTB), for those present threats, and the Anxiety/Defense Test
Battery (A/DTB), for those potential threats. The results suggest that F/DTB behaviors are not systematically responsive to anxiolytics,
whereas risk assessment activities (contained in A/DTB) are significantly affected by anxiolytic benzodiazepines. This suggests that
more research should be conducted to take a closer look at the distinction between fear and anxiety and the behavioral, pharmacological,
and possibly physiological components of each.
- In this test(Davis, 1979), rats are assessed for the magnitude of their startle responses.
This response is measured by movement sensors which are located under the testing cage.
Rats are first trained to associate a neutral stimulus, such as a light or tone, with the onset of
foot shock. Then when this "conditioned" stimulus is presented with a sudden loud noise, the
magnitude of the startle is increased.
- Animals treated with anxiolytic drugs fail to exhibit the potentiated startle response,
yet the natural startle response remains unaffected
(Feldman, Meyer, & Quenzer, 1997).
- Apparatus : Startle response systems
- see : Startle relfex (Response Meter)
- Active avoidance, as opposed to passive avoidance, requires the animal to perform a particular behavior
in order to avoid an impending threat, such as shock. The animal can be trained to associate some
signal, such as a light or tone, with a shock that follows. If the animal performs the appropriate
behavior, such as pressing a lever or retreating to another room in the apparatus, the animal will
not receive the shock. However, if the animal fails to perform this behavior, it will receive the
shock (Feldman et al., 1997).
- Apparatus : Shuttle box, Avoidance systems
- see : Avoidance system
- The apparatus consists of a chamber with a shock prod extending in from one of the walls. The floor
of the chamber is lined with bedding. Once the rat is placed in the chamber, the following such
behaviors may be recorded: latency to first defensive burying (DB) response, the number of DB
responses, and the duration of each DB response. A DB response is defined as a response whereby,
following prod shock, the rat quickly retreats to the back of the cage and, while facing the prod,
starts to push the bedding toward the prod using the snout and forepaws (Pare, 1994).
- Apparatus :
- see :
- This is a test designed to measure the fear of isolation observed in rat pups until about postnatal day
14. When rat pups are removed from their mothers (or littermates) they exhibit high frequency
vocalizations which can be terminated by placing the pups back with the mother or littermates.
- This test is primarily used to test the anxiolytic effects of certain drugs. When isolated rat pups are
treated with benzodiazepines, their distress vocalizations were significantly reduced
(Feldman et al., 1997)
- Apparatus : cage, Sound
- see : Amplifier-audio
- This maze, developed by Pellow and colleagues (1985), is a cross-shaped maze with two open arms and
two arms enclosed by sides. The entire maze is elevated approximately 50 cm off the floor and relies
on rodents' natural fear of heights and open spaces. The number of entries into open versus closed
arms and the amount of time in each is recorded. A greater amount of fear-related behaviors, as
mentioned in the previous section, are observed while animals are in the open arms as opposed
to the closed arms.
- An increase in the open arm exploration is observed when animals are treated with anxiolytic agents,
while a decrease is observed if animals are treated with either caffeine or amphetamine (anxiogenic agents) (Feldman et al., 1997).
- Apparatus : Elevated plus maze
- see : maze
- "Freezing" can be defined as a motionless behavior and can be observed in animals who have just been
exposed to an aversive stimulus or have been fear conditioned to a signal, such as a light or tone,
or to the environment.
- Apparatus : Freezing monitor
- see : Freezing monitor
- The apparatus consists of a circular water tank in which the animals are individually placed and
behaviors such as time spent floating, the number of headshakes, and the number of bobbings can be
recorded. During the testing, there is no escape from the water.
- This test also is most often used to analyze the affects of anxiolytic agents (Feldman et al., 1997).
- Apparatus : Water pool
- see : activity
- The Geller-Seifter paradigm, an incremental conflict procedure, is a conflict test in which rats are trained to bar press to receive sweetened milk
(a very desirable reinforcement for a rat).
- This method uses a multiple VI-2/continuous reinforcement (CRF) schedule. Rats are trained to press a
lever on a VI-2 minute schedule for liquid reinforcement. After 15 minutes, a tone of 3 minutes in
duration signals a change to a CRF schedule during which every lever press is followed by liquid
reinforcement, but these reinforcements are accompanied by foot shock which are capable of
suppressing responding to very low levels (Gerald T. et al.,1994).
- When the rats are treated with an anxiolytic agent, responding during the VI-2 segment decreases slightly,
but responding during the CRF segment shows a robust increase (Feldman et al., 1997).
- Apparatus : operant chambers inside sound-attenuating enclosures
- see : Operant Test
- A typical learned helplessness paradigm consists of the animal being individually placed into a two-way
shuttle box and presented with a predetermined number of inescapable shocks. Approximately 24 hours
later, the animal is tested for escape performance in the shuttle box. The schedule for escape may
vary. For example, the animal may be placed on a FI-1 schedule, in which a single crossing from one
side of the shuttle box is required for escape, or a FI-2, in which the animal is required to cross
to the other side of the shuttle box and then back again (Pare, 1994).
- It has been shown that some drugs may disrupt one schedule, while leaving the other relatively unaffected.
For instance, when animals are given DMCM, a benzodiazepine receptor inverse agonist, interfered
with FR-2 (relatively unnatural) shuttle box escape, but not FR-1 (natural) (Maier, Busch, Maswood,
Grahn, & Watkins, 1995).
- Apparatus : Shuttle box
- see : Avoidance systems
- This is a simple test which takes advantage of rodents' natural tendency to avoid bright lighting.
The apparatus consists of two compartments, with one side brightly lit and the other side dark.
The animal is placed in the bright side and the number of crossings as well as the amount of time
spent on each side are recorded.
- This test is used to assess the effects of anxiolytics, which typically produce a dose-dependent increase
in the number of crossings, motor activity, and amount of time spent in the light (Feldman et al., 1997).
- Apparatus : shuttle box
- see : Avoidance systems
- The open-field arena consists of a circular outer wall (approximately 30 cm high) with the inside being
divided into three concentric circles. Each of the inner circles are then divided into essentially
equal areas, although disproportionate in number. A ceiling light is situated above the arena floor
and a drape hangs from the ceiling to the floor around the outside of the arena. This provides a
diffusion of the light and acts as a one-way screen. Rats are individually placed in the inner
circle and the desired behaviors are recorded for approximately 5 minutes, such as latency to leave
the inner circle, number of field segments entered, number of rearings, and the number of fecal boluses (Pare, 1994).
- Apparatus : Open field cage
- see : Activity
- In passive avoidance conditioning, the animal is trained not to perform a highly typical response in
order to avoid punishment, such as foot-shock. Although this is often used as a test of memory
in general, some have asserted that passive avoidance only evaluates memory that involves the
learning and storage of information about emotional events (Feldman et al., 1997).
- Apparatus : Shuttle box
- see : Avoidance systems
- This is another anxiety test which also relies on rodents' natural tendency to avoid bright light and
unfamiliar surroundings. It is based on the notion that by placing the animal in a novel or brightly
lit chamber, anxiety is increased and social interaction is decreased.
- Anxiolytics increase social interactions in these situations (Feldman et al., 1997).
- Apparatus : Shuttle box
- see : Avoidance systems
- In this test, animals are deprived of water and then learn to lick the tip of a bottle to drink.
The rats are given the bottle for 3 minutes, but after the 20th lick, the rat receives an
accompanying tongue shock with each lick which suppresses drinking behavior.
- When rats are pre-treated with anxiolytics, there is a decrease in this suppression (Feldman et al., 1997).
- Apparatus : Lick sensor
- see : Food & Dringking Monitor
- Blanchard, R. J., Yudko, E. B., Rodgers, R. J., & Blanchard, D. C. (1993). Defense system
psychopharmacology: An ethological approach to the pharmacology of fear and anxiety. Behavioural
Brain Research, 58, 155-165.
- Boissy, A. (1995). Fear and fearfulness in animals. The Quarterly Review of Biology, 70(2), 165-191.
- Bolles, R. C. & Fanselow, M. S. (1980). A perceptual-defensive-recuperative model of fear and pain.
The Behavioral and Brain Sciences, 3, 291-323.
- Davis, M. (1992). The role of the amygdala in fear and anxiety. Annual Review of Neuroscience, 15, 353-376.
- Feldman, R. S., Meyer, J. S., & Quenzer, L. F. (1997). Principles of Neuropsychopharmacology. Sunderland, MA: Sinauer Associates, Inc.
- Maier, S. F., Busch, C. R., Maswood, S., Grahn, R. E., & Watkins, L. R. (1995). The dorsal raphe nucleus
is a site of action mediating the behavioral effects of the benzodiazepine receptor inverse agonist
DMCM. Behavioral Neuroscience, 109(4), 759-766.
- Gerald T. Pollard and James L. Howard (1994). Comparison of Chlordiazepoxide and Food Deprivation on Punished and Unpunished Responding
Maintained by Food. Experimental and Clinical Psychopharmacology 2(1), 37-42
- Pare, W. P. (1994). Open field, learned helplessness, conditioned defensive burying, and forced-swim tests
in WKY rats. Physiology & Behavior, 55(3), 433-439.
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