E A Antoniadis | PubMed Health

PubMed2001

Circadian phase-shifted rats show normal acquisition but impaired long-term retention of place information in the water task.

It is thought that circadian rhythms may influence learning and memory processes. However, research supporting this view does not dissociate a mnemonic impairment from other performance deficits. Furthermore, published reports do not specify the type of memory system influenced by the circadian system. The present study assessed the effects of phase shifting on acquisition and expression of place navigation in the water maze, a task sensitive to hippocampal dysfunction. The results showed that phase-shifting circadian rhythms in rats impaired the expression of place information on a retention test but not initial acquisition or encoding of place information. These results suggest that disruption of circadian rhythms may impair consolidation of previously encoded hippocampal place information.

Animals↗

PubMed2001

Amygdala, hippocampus, and unconditioned fear.

Embedded within contemporary views of emotional learning is a well-founded agreement that the amygdala plays a pivotal role in the formation and consolidation of aversive memories formed during fear conditioning. However, it is important to determine whether observed deficits are reflective of a memory impairment or whether they are simply attributable to a deficit in the performance of unconditioned fear responses such as freezing. Within the neurobiology of learning and memory literature, there is an ongoing debate concerning the potential role of the amygdala in the performance of unconditioned fear responses. A view put forth by Vazdarjanova and McGaugh (1998) suggests that the amygdala is not required for the formation and consolidation of the aversive memories formed during fear conditioning, but is essential in the performance of unconditioned fear responses. Data provided by Maren (1999) counter this view by positing that the amygdala is not required for the performance of fear responses, but its role is of a mnemonic nature in the conditioning of fear to neutral cues. To clarify the amygdala's participation in these two processes, a useful approach would involve a situation where animals with amygdala damage were examined for their unconditioned fear responses in reaction to footshock as well as the conditioning of these reactions to previously neutral cues paired with the aversive event. We have previously reported that rats with amygdala or hippocampal damage are impaired in discriminative fear conditioning to context. In the present experiment, we report the initial unconditioned fear responses to footshock by these same animals as well as the conditioned responses during testing. In both groups, the fear responses assessed (freezing, urination, defecation, and locomotion) were not impaired and did not differ from those expressed by the sham animals. The impairment of discriminative fear conditioning to context, in combination with the present experiment, represents a dissociation where damage to specific memory structures (amygdala or hippocampus) debilitates the mnemonic processes involved in fear conditioning, but not the performance of the fear responses per se.

Amygdala↗

PubMed2000

Circadian rhythms, aging and memory.

In human beings and animal models, cognitive performance is often impaired in natural and experimental situations where circadian rhythms are disrupted. This includes a general decline in cognitive ability and fragmentation of behavioural rhythms in the aging population of numerous species. There is some evidence that rhythm disruption may lead directly to cognitive impairment; however, this causal link has not been made for effects due to aging. We have tested this link by examining rhythms and performance on contextual conditioning with the conditioned place preference task, in elderly, age-matched hamsters. Young healthy hamsters developed a preference for a context that is paired with the opportunity to engage in wheel-running (experiment 1). Aged animals with consolidated locomotor rhythms developed similar degrees of preference, whereas the age-matched hamsters with fragmented rhythms did not (experiment 2). The degree of preference was also correlated with activity amplitude. These results support the notion that age-related rhythm fragmentation contributes to the age-related memory decline.

Aging↗

PubMed2000

Circadian rhythms, aging and memory.

In human beings and animal models, cognitive performance is often impaired in natural and experimental situations where circadian rhythms are disrupted. This includes a general decline in cognitive ability and fragmentation of behavioural rhythms in the aging population of numerous species. There is some evidence that rhythm disruption may lead directly to cognitive impairment; however, this causal link has not been made for effects due to aging. We have tested this link by examining rhythms and performance on contextual conditioning with the conditioned place preference task, in elderly, age-matched hamsters. Young healthy hamsters developed a preference for a context that is paired with the opportunity to engage in wheel-running (experiment 1). Aged animals with consolidated locomotor rhythms developed similar degrees of preference, whereas the age-matched hamsters with fragmented rhythms did not (experiment 2). The degree of preference was also correlated with activity amplitude. These results support the notion that age-related rhythm fragmentation contributes to the age-related memory decline.

Aging↗

PubMed2000

Amygdala, hippocampus and discriminative fear conditioning to context.

Various measures of fear have been shown to condition to a fearful context with different acquisition rates (Antoniadis EA, McDonald RJ. Fear conditioning to context expressed by multiple measures of fear in the rat, Behav Brain Res 1999;101(1):1-14). Freezing, locomotion, urination and preference are 'fast' measures of fear in that they discriminatively condition to context after a single training session, while ultrasonic vocalizations and defecation are 'slow' measures of fear given that they condition following three training sessions. In the present experiment we sought to assess the contribution of the amygdala and the hippocampus in this form of learning. Existing views differ on the degree of involvement of each memory structure. This discord probably emerges from the common use of non-discriminative paradigms and the assessment of a single measure of fear. With the use of a discriminative paradigm and the assessment of multiple measures of fear, results indicate that the amygdala is a memory structure that selectively mediates the conditioning of heart rate, and the hippocampus selectively mediates the conditioning of defecation and body temperature. The conditioning of preference, locomotion, freezing and ultrasonic vocalizations, necessitate the participation of both memory structures while the conditioning of urination does not seem to require the participation of either the hippocampus or the amygdala. The proposed view ascribes an equal role in fear conditioning to both the amygdala and the hippocampus.

Amygdala↗

PubMed1999

Discriminative fear conditioning to context expressed by multiple measures of fear in the rat.

There has been a renewed interest in the neural basis of fear conditioning to context. These current approaches are accompanied by some limitations including the use of short testing windows, non-discriminative paradigms, and unitary fear response assessment. In an attempt to circumvent these limitations, a discriminative context procedure assessing multiple response measures of fear was used in the present study. Conditioning consisted of three training sessions and each session consisted of 2 days. On day one, the animals were placed in the paired context and received three foot shocks. On the other day, they were placed in the unpaired chamber in the absence of any aversive event. Animals were tested after each training session and the response measures of fear recorded included: preference, freezing, heart rate, ultrasonic vocalizations, defecation, body temperature, urination and locomotion. The results suggest that behavioral, as well as physiological changes evoked by fearful stimuli become associated with the context in which the aversive event occurred. In general these findings also suggest that there are different learning parameters for the measures of fear examined in this paradigm.

Animals↗