Incubation of fear: experiments of the "Kamin Effect" in rats

Role of bed nucleus of the stria terminalis and amygdala AMPA receptors in the development and expression of context conditioning and sensitization of startle by prior shock

A core symptom of post-traumatic stress disorder is hyper-arousal-manifest in part by increases in the amplitude of the acoustic startle reflex. Gewirtz et al. (Prog Neuropsychopharmacol Biol Psychiatry 22:625-648, 1998) found that, in rats, persistent shock-induced startle increases were prevented by pre-test electrolytic lesions of the bed nucleus of the stria terminalis (BNST). We used reversible inactivation to determine if similar effects reflect actions on (a) BNST neurons themselves versus fibers-of-passage, (b) the development versus expression of such increases, and (c) associative fear versus non-associative sensitization. Twenty-four hours after the last of three shock sessions, startle was markedly enhanced when rats were tested in a non-shock context. These increases decayed over the course of several days. Decay was unaffected by context exposure, and elevated startle was restored when rats were tested for the first time in the original shock context. Thus, both associative and non-associative components could be measured under different conditions. Pre-test intra-BNST infusions of the AMPA receptor antagonist NBQX (3 μg/side) blocked the non-associative (as did infusions into the basolateral amygdala) but not the associative component, whereas pre-shock infusions disrupted both. NBQX did not affect baseline startle or shock reactivity. These results indicate that AMPA receptors in or very near to the BNST are critical for the expression and development of non-associative shock-induced startle sensitization, and also for context fear conditioning, but not context fear expression. More generally, they suggest that treatments targeting the BNST may be clinically useful for treating trauma-related hyper-arousal and perhaps for retarding its development.

What does it take to demonstrate memory erasure? Theoretical comment on Norrholm et al. (2008)

An issue of increasing interest in Pavlovian conditioning is to identify ways to facilitate the development and persistence of extinction. Both behavioral and molecular lines of evidence demonstrate that learning during extinction can be enhanced. Similar evidence has been offered to support the idea that extinction causes the original association to be unlearned, or erased. Differentiating between extinction and erasure accounts is extremely difficult and requires many assumptions about the fundamental nature of how memory storage maps into memory expression. In this issue of Behavioral Neuroscience, Norrholm et al. (see record 2008-13280-002) describe a study of extinction with humans that has the potential to serve as a translational bridge between rodent work and clinical applications. They find less recovery of a conditioned fear response when extinction occurs 10 min compared with 72 hr after conditioning; however, the recovery of subjects' expectancies of the fearful stimulus is independent of when extinction occurred. These findings and others discussed in this article demonstrate some of the challenges in making inferences about memory erasure during extinction.

Motivational properties of ethanol in naive rats as studied by place conditioning

The reinforcing properties of ethanol were examined in naive adult male rats by means of a place conditioning paradigm that has previously demonstrated the positive reinforcing properties of food, water and some drugs, and the aversive properties of punishers such as electric shock and lithium chloride. Only doses of 0.8-1.0 g/kg and higher produced clear place conditioning, and this was only conditioned place aversion; rats spent significantly more time on the side of the place conditioning box in which they received the vehicle than on the side in which they received ethanol. Doses between 0.1 g/kg and 0.8 g/kg produced increases in general activity, but did not produce any place conditioning. Control experiments indicated that the pattern of effects was not specific to the route of ethanol administration (intravenous or intragastric), rate of infusion, concentration, or vehicle. It was concluded that ethanol, in the doses used here, has only punishing or neutral motivational effects in naive rats and does not serve as a primary positive reinforcer in this model. The conclusions are discussed in relation to the relative difficulty encountered in attempts to produce ethanol self-administration, and the findings are viewed as consistent with a proposal that prolonged training and experience with ethanol are important for ethanol self-administration by the rat.

The suppression of behaviour in rats by previous experience and electric shock and its antagonism by atropine

Behavioural suppression was induced in rats by either previous experience alone or by a combination of previous experience and electric footshock. The effects of atropine (10 mg/kg) were compared with saline on a number of behavioural parameters suppressed by the above treatments. Atropine antagonised habituation and shock-induced suppression, which suggests that behavioural suppression however induced, may depend upon a common neuropharmacological mechanism.

Multiphasic retention deficits at periodic intervals after passive-avoidance learning

Aftetr one-trial passive-avoidance training, indepelenet groups of rats tested promptly after training or at successive 6-hour intervals displayed a repetitive pattern of high then low retention scores. These results suggest that some physiological rhythm may interact with retention performance.

Effects of fear on exploratory behaviour in rats

It has been suggested that mild fear may evoke rather than inhibit exploratory responses in rats. The relationship between conditioned fear and exploratory behaviour was analysed in three experiments and there was no evidence that mild fear increased exploration. Conditioned fear was found to be under relatively precise stimulus control and its magnitude was related to the intensities and durations of the unconditioned stimulus, inescapable electric shock.