Acute administration of nicotine into the higher order auditory Te2 cortex specifically decreases the fear-related charge of remote emotional memories

Acute Nicotine Treatment Alleviates LPS-Induced Impairment of Fear Memory Reconsolidation Through AMPK Activation and CRTC1 Upregulation in Hippocampus

BACKGROUND

Fear memory is a fundamental capability for animals and humans to survive. Its impairment results in the disability to avoid danger. When memory is reactivated, a reconsolidation process, which can be disrupted by various stimuli, including inflammation, is required to become permanent. Nicotine has been shown to improve cognitive deficits induced by inflammation and other stimuli. Therefore, in the present study, we investigated the effect of nicotine on lipopolysaccharide (LPS)-induced impairment of fear memory reconsolidation and the underlying mechanism.

METHODS

Step-through inhibitory avoidance task was recruited to study fear memory of rat, i.p. LPS (0.5 mg/kg) treatment was used to induce inflammation, and western blot and immunostaining were applied to detect protein expression and distribution in medial prefrontal cortex and hippocampus.

RESULTS

Our data showed that LPS induced fear memory reconsolidation impairment without affecting retrieval. In addition, LPS significantly increased inflammation factors tumor necrosis factor-α and interleukin-1 beta and decreased CREB-regulated transcription coactivator 1 (CRTC1) expression and adenosine monophosphate-activated protein kinase (AMPK) activation in hippocampus. More importantly, LPS significantly decreased CRTC1 expression and AMPK activation in neurons by activating microglia cells. Of note, either nicotine treatment or activation of AMPK by intracerebroventricular infusion of metformin reduced LPS-induced impairment of fear memory reconsolidation and ameliorated inflammation factor tumor necrosis factor-α and interleukin-1 beta as well as the expression of CRTC1.

CONCLUSIONS

In conclusion, our results showed that acute nicotine treatment alleviates LPS-induced impairment of fear memory reconsolidation through activation of AMPK and upregulation of CRTC1 in hippocampus.

Basolateral amygdala, nicotinic cholinergic receptors, and nicotine: Pharmacological effects and addiction in animal models and humans

The amygdala is involved in processing incoming information about rewarding stimuli and emotions that denote danger such as anxiety and fear. Bi-directional neural connections between basolateral amygdala (BLA) and brain regions such as nucleus accumbens, prefrontal cortex, hippocampus, and hindbrain regions regulate motivation, cognition, and responses to stress. Altered local regulation of BLA excitability is pivotal to the behavioral disturbances characteristic of posttraumatic stress disorder, and relapse to drug use induced by stress. Herein, we review the physiological regulation of BLA by cholinergic inputs, emphasizing the role of BLA nicotinic receptors. We review BLA-dependent effects of nicotine on cognition, motivated behaviors, and emotional states, including memory, taking and seeking drugs, and anxiety and fear in humans and animal models. The alterations in BLA activity observed in animal studies inform human behavioral and brain imaging research by enabling a more exact understanding of altered BLA function. Converging evidence indicates that cholinergic signaling from basal forebrain projections to local nicotinic receptors is an important physiological regulator of BLA and that nicotine alters BLA function. In essence, BLA is necessary for behavioral responses to stimuli that evoke anxiety and fear; reinstatement of cue-induced drug seeking; responding to second-order cues conditioned to abused drugs; reacquisition of amplified nicotine self-administration due to chronic stress during abstinence; and to promote responding for natural reward.

Region- and Layer-Specific Activation of the Higher Order Auditory Cortex Te2 after Remote Retrieval of Fear or Appetitive Memories

The auditory cortex is involved in encoding sounds which have acquired an emotional-motivational charge. However, the neural circuitry engaged by emotional memory processes in the auditory cortex is poorly understood. In this study, we investigated the layers and regions that are recruited in the higher order auditory cortex Te2 by a tone previously paired to either fear or appetitive stimuli in rats. By tracking the protein coded by the immediate early gene zif268, we found that fear memory retrieval engages layers II-III in most regions of Te2. These results were neither due to an enhanced fear state nor to fear-evoked motor responses, as they were absent in animals retrieving an olfactory fear memory. These layers were also activated by appetitive auditory memory retrieval. Strikingly, layer IV was recruited by fear, but not appetitive memories, whereas layer V activity was related to the behavioral responses displayed to the CS. In addition to revealing the layers and regions that are recruited in the Te2 by either fear or appetitive remote memories, our study also shows that the neural circuitry within the Te2 that processes and stores emotional memories varies on the basis of the affective motivational charge of tones.

Reversible Inactivation of the Higher Order Auditory Cortex during Fear Memory Consolidation Prevents Memory-Related Activity in the Basolateral Amygdala during Remote Memory Retrieval

Recent findings have shown that the auditory cortex, and specifically the higher order Te2 area, is necessary for the consolidation of long-term fearful memories and that it interacts with the amygdala during the retrieval of long-term fearful memories. Here, we tested whether the reversible blockade of Te2 during memory consolidation may affect the activity changes occurring in the amygdala during the retrieval of fearful memories. To address this issue, we blocked Te2 in a reversible manner during memory consolidation processes. After 4 weeks, we assessed the activity of Te2 and individual nuclei of the amygdala during the retrieval of long-term memories. Rats in which Te2 was inactivated upon memory encoding showed a decreased freezing and failed to show Te2-to-basolateral amygdala (BLA) synchrony during memory retrieval. In addition, the expression of the immediate early gene zif268 in the lateral, basal and central amygdala nuclei did not show memory-related enhancement. As all sites were intact upon memory retrieval, we propose that the auditory cortex represents a key node in the consolidation of fear memories and it is essential for amygdala nuclei to support memory retrieval process.

Differential Recruitment of Auditory Cortices in the Consolidation of Recent Auditory Fearful Memories

Memories of frightening events require a protracted consolidation process. Sensory cortex, such as the auditory cortex, is involved in the formation of fearful memories with a more complex sensory stimulus pattern. It remains controversial, however, whether the auditory cortex is also required for fearful memories related to simple sensory stimuli. In the present study, we found that, 1 d after training, the temporary inactivation of either the most anterior region of the auditory cortex, including the primary (Te1) cortex, or the most posterior region, which included the secondary (Te2) component, did not affect the retention of recent memories, which is consistent with the current literature. However, at this time point, the inactivation of the entire auditory cortices completely prevented the formation of new memories. Amnesia was site specific and was not due to auditory stimuli perception or processing and strictly related to the interference with memory consolidation processes. Strikingly, at a late time interval 4 d after training, blocking the posterior part (encompassing the Te2) alone impaired memory retention, whereas the inactivation of the anterior part (encompassing the Te1) left memory unaffected. Together, these data show that the auditory cortex is necessary for the consolidation of auditory fearful memories related to simple tones in rats. Moreover, these results suggest that, at early time intervals, memory information is processed in a distributed network composed of both the anterior and the posterior auditory cortical regions, whereas, at late time intervals, memory processing is concentrated in the most posterior part containing the Te2 region.

SIGNIFICANCE STATEMENT

Memories of threatening experiences undergo a prolonged process of "consolidation" to be maintained for a long time. The dynamic of fearful memory consolidation is poorly understood. Here, we show that 1 d after learning, memory is processed in a distributed network composed of both primary Te1 and secondary Te2 auditory cortices, whereas, at late time intervals, memory processing is concentrated in the most posterior Te2 cortex. Together, our data reveal that the consolidation of fearful memories related to simple auditory stimuli requires the auditory cortex, provided that the inactivation encompasses both the primary and the secondary components of the cortex, and that different regions of the auditory cortex play complementary but different roles in these processes.