Renewal and reinstatement of the conditioned but not the unconditioned response following habituation of the unconditioned stimulus

Research on the inhibition of learned fear currently relies almost exclusively on one specific procedure, namely extinction of the conditioned stimulus (CS). Importantly, however, learned fear responses can be reduced by a number of other procedures, including habituation of the unconditioned stimulus (US). We recently demonstrated that reductions in learned fear following US habituation, like CS extinction, were subject to both renewal and reinstatement (Storsve et al., 2010). The present study further investigates the associative and non-associative processes shared between habituation and extinction. Given that habituation is typically context-independent (Mackintosh, 1987), in the present study we directly compared renewal and reinstatement of both a conditioned response (CR; freezing) and an unconditioned response (UR; startle) following habituation. It was found that the reduction in conditioned freezing resulting from habituation was context specific (i.e., a change in context led to a renewal of the conditioned fear response; Experiment 1) and was attenuated when a pre-test shock was given (i.e., reinstatement of conditioned fear was observed; Experiment 2). In contrast, habituation of an unconditioned response elicited by the US (i.e., a startle response) was unaffected by either a change in test context or administration of a pre-test shock. This dissociation in the effects of habituation on learned and unlearned responses is discussed in relation to theories of fear extinction.     

Nicotine-evoked conditioned responding is dependent on concentration of sucrose unconditioned stimulus

Previous studies have shown that the interoceptive nicotine conditional stimulus (CS) functions similarly to exteroceptive CSs such as lights or environments. For instance, the appetitive conditioned response (CR) evoked when nicotine is repeatedly paired with sucrose presentations (the unconditioned stimulus; US) is sensitive to changes in training dose (CS salience) and the contiguity between the CS effects and sucrose. The current study was conducted to extend this research by examining the possible role of US intensity in CR acquisition and maintenance. Rats were trained using one of four sucrose concentrations: 0, 4, 16, or 32% (w/v). On nicotine sessions (0.4 mg base/kg), rats received 36 deliveries (4 s each) of their assigned concentration intermittently throughout the session; sucrose was withheld on saline sessions. In all groups, an appetitive goal-tracking CR was acquired at a similar rate. However, the asymptotic CR level varied with sucrose concentration. The magnitude of the CR was increased in rats trained with higher sucrose US concentrations. These findings are consistent with previous Pavlovian conditioning research, and extend the conditions under which the nicotine state functions as an interoceptive conditional stimulus.     

Comparison of the effect of serotonin administered into the dorsal hippocampus on conditioned passive and active avoidance in rats

It has been established that intrahippocampal bilateral injection of 5-HT in a dose of 2 micrograms prolonged the time of passive avoidance reflex retention, inhibiting the retrieval of active avoidance reflex and facilitating its extinction. The data obtained show the dependence of 5-HT effect on reinforcing and triggering properties of spatial-environmental and phasic conditioned stimuli. It is suggested that intrahippocampal 5-HT administration was responsible for the retrieval of traces of conditioned reinforced stimuli.     

Evaluation of the effect of arginine vasopressin on the formation of the conditioned reaction of active avoidance in rats

The effect was studied of arginine-vasopressin (AVP) intraperitoneal administration in a doze of 0.001 mg/kg on elaboration of conditioned reaction (CR) of active avoidance in rats. Estimation of reaction dynamics by dynamic delta index allowed to increase the informational value of the experimental data with taking into account the direction of reaction shift and individual characteristic of the animal. AVP improved the elaboration of active avoidance CR, lowered variability of individual learning ability and did not influence the change of the number of intersignal reactions and short-latency escapes in the process of learning. It is suggested that the main site of AVP effect is the animal's reaction to conditioned sound stimulus.     

Memory effect of caerulein and its analogs in active and passive avoidance responses in the rat

The memory effects of caerulein (CER) and its analogs ([des-Gln2]-CER and [Leu5,Nle8]-CER) were compared with that of cholecystokinin octapeptide (CCK-8) using active and passive avoidance responses in rats. In the active avoidance test, single subcutaneous (SC) injection of CER and its analogs immediately after the learning trials at doses of 10 and 100 ng/kg prevented extinction of learned task for 10 days, and at a dose of 1000 ng/kg for at least 15 days, but the effect of CCK-8 was somewhat weaker. In the saline control group, the number of responses decreased after 5 days. In the passive avoidance response, electroconvulsive shock (ECS)-induced amnesia was partially prevented by CCK-8 at doses of 100 and 1000 ng/kg SC, while CER and its analogs at doses of more than 100 ng/kg totally prevented the ECS-induced amnesia. Intraperitoneal administration of scopolamine caused complete amnesia which was also partially prevented by CCK-8, while CER could totally prevent the amnesia following SC injection of 2 micrograms/kg. These results indicate that CER and its analogs are more effective than CCK-8 for preventing experimental amnesia.