The Influence Of The Locus Coeruleus On The Amygdaloid Complex And Its Significance In Learning And Memory

As a key structure of the limibic system, a large body of literature indicates that the amygdaloid complex plays an important role in emotional information processing and memory consolidation. Further studies suggest that the different subnuclei of the amygdala have different functions in emotional memory consolidation. It is thought that the basal lateral nucleus of amygdala (BLA) plays a pivotal role in emotional memory consolidation.The basal lateral nucleus of amygdala is a heterogeneous nuclear complex. It receives neuronal innervations from allocotica! sensory regions, has reciprocal connections with frontal cortex and hippocampus and is targeted by projections from the cholinergic nucleus basalis and the brainstem catecholaminergic nuclei. More recent studies indicate that the BLA receives extensive noradrenergic innervations from the pontine nucleus locus coeruleus (LC). Behavioural, lesion and pharmacological studies suggest that this pathway, LC-BLA, has an important role in mediating responses to emotional stimuli and in the formation of long term memory. Moreover, effects of many different memory modulators including opioids, stress hormones, and GABA appear to be mediated through the beta receptor in the BLA. Although extensive studies indicate that the beta receptror within BLA plays a critical role in memory consolidation, while LC has direct projections to BLA as well, little is known about the in vivo effect of LC activation on activity of BLA. Thus, our questions are: (1) based on the neuromodulatory effect of noradrenaline on BLA in learning and memory, what is the effect of LC activation on the activity of BLA neurons in vivo? (2) Given that LC activation has significant effect on BLA neuronal activity, what is the functional significance of this effect in learning and memory? Whether the neuronal activity of BLA will change in according to the learning and memory and what is the biological sense of this change?The experiment was performed on the anesthetized rat using electrophysiological technique. The study was focused on the influence of locus coeruleus activation by foot shock or electrical stimulation on the neuronal activity of the amygdala neurons, and the receptor type which mediated this influence was identified as well. Furthermore, the inhibitory avoidance conditioning was adopted to study the neuronal activity change of BLA and LC during the behavioural task. Meanwhile, the sleep was scored as well.1. LC influence on BLA neuronal activityThe experiment was performed on 54 urethane-anesthetized SD rats (1.2g/kg) using extracellular unit recording technique. A total number of 83 LC units and 356 BLA units were recorded during the rat receiving foot shock. The LC unit displays a low frequency discharging rate with an average firing rate of 1.60±0.11Hz. LC response was homogeneous and exhibited a multiphasic response consisting of a burst of 2-3 action potentials around 20 ms latency followed by a short inhibition then a more sustained excitatory phase, followed by sustained inhibition. The response of BLA unit to foot shock is heterogeneous. There are 44% recorded BLA units displayed an inhibitory response to foot shock with a latency of 41.4±2.4 ms and average firing rate of 2.4±0.2 Hz; while 17% recorded BLA units showed excitatory response with a latency of 41.9±3.5 ms and average firing rate of 4.9±0.7 Hz; the other 39% recorded BLA units did not respond to foot shock.In part of the experiment, the influence of direct LC stimulation on BLA was studied. 238 BLA units were recorded during LC stimulation. The results revealed that inhibitory response and excitatory response are 54% and 5% respectively with a latency of 34.4±1.7 ms and 35.0±6.2 ms respectively, while the average firing rates are 2.2±0.1 Hz and 4.4±1.4 Hz respectively. The other 41% recorded unit did not respond to LC stimulation.The results indicate that the effect of foot shock and LC stimulation on BLA activity is predominantly inhibitory. The following study was aimed to investigate the receptor type mediated this effect. 18 BLA units which showed inhibitory response to LC stimulation were recorded. After beta-adrenoceptor antagonist timolol infusion into the BLA, 67% response was blocked. 65% inhibitory response of BLA to foot shock was block as well (20 BLA units). The pharmacological experiment suggested that noradrenaline modulates BLA neuronal activity partially through beta receptor.The present study provides the first in vivo evidence for effects of direct activation of the noradrenergic LC on neuronal activity in the BLA. The results revealed that influence of noradrenaline on BLA was mainly inhibitory and this influence was partially through beta receptor.2. The neuronal activity alteration of BLA and LC during inhibitory avoidance conditioningThe experiment was performed on 19 SD rats and focused on investigating the neuronal activity alteration of BLA and LC by adopting inhibitory avoidance conditioning in freely moving rats. Three sessions, habituation, training and test, occurred over two days. The baseline, habituation and test lasted for 5 minutes respectively. Six LC units were recorded with an average firing rate of 1.1±0.2 Hz. The results revealed that the LC activity elevated both during habituation (1.8±0.2 Hz) and test (1.7±0.7 Hz). Interestingly, the LC activity sustained increase in post-test session for at least 5 minutes. For the recorded 68 BLA unit, the neuronal activity increased during habituation (2.0±0.3 Hz) compared with its baseline (1.4±0.2 Hz). This result is consistent with the previous reports. The latencies of entering the black box in habituation, training and test are 2.1 ±3.4 sec, 10.4±2.5 sec and 279.9±15.0 sec respectively. The latency of test session is significantly higher than the habituation session (p<0.01) which indicated conditioning was well-established in the test session. Further analysis for the post-habituation, post-training and post-test revealed that the neuronal activity of BLA increased significantly in the 100-minute post-test session, which did not appear in the post-habituation and post-test session. These results indicate when the rat was re-exposed to the previous training context, the consolidated information might return to a liable state and needed to be consolidated again, which also called reconsolidation. So, the BLA might play an important role in the memory reconsolidation for this inhibitory avoidance conditioning.3. Sleep pattern alteration during inhibitory avoidance conditioningLots of literature indicates that rapid eye movement (REM) sleep plays a critical role in memory consolidation. This part of study was performed on 7 SD rats and the sleep was analyzed in the inhibitory avoidance conditioning. The sleep state was monitored for 2 hours before and after each stage of the inhibitory avoidance conditioning (habituation, training and test). The predominant sleep pattern alteration appeared in the 2-hour post-test session. Two-tail paired t-test revealed that the REM latency of post-test was significantly shorter than its baseline (t(6)=3.37, P<0.05), while the number of REM episode in post-test was significantly higher than its baseline (t(6)=-4.25, P<0.01) and the percent of REM sleep in the total sleep time of post-test is significantly increased (p<0.05). These results revealed that REM sleep might play an important role in memory reconsolidation. In all, the present study provides the first in vivo evidence for effects of direct activation of the noradrenergic LC on neuronal activity in the BLA. The results revealed that influence of noradrenaline on BLA was predominantly inhibitory and mediated by beta-adrenoceptor. This implies that the LC-BLA circuit may play a pivotal role in emotional learning and memory. The behavioural study provides supplemental support for the BLA in memory processing which furthers our understanding of emotional memory processing. The sleep data imply that the REM sleep might have an important role in the memory reconsolidation of this inhibitory avoidance conditioning.