Hippocampal Inputs in the Prelimbic Cortex Curb Fear after Extinction.

In contrast to easily formed fear memories, fear extinction requires prolonged training. The prelimbic cortex (PL), which integrates signals from brain structures involved in fear conditioning and extinction such as the ventral hippocampus (vHIP) and the basolateral amygdala (BL), is necessary for fear memory retrieval. Little is known, however, about how the vHIP and BL inputs to the PL regulate the display of fear after fear extinction. Using functional anatomy tracing in male rats, we found two distinct subpopulations of neurons in the PL activated by either the successful extinction or the relapse of fear. During the retrieval of fear extinction memory, the dominant input to active neurons in the PL was from the vHIP, whereas the retrieval of fear memory, regardless of the age of a memory and testing context, was associated with greater BL input. Optogenetic stimulation of the vHIP-PL pathway after one session of fear extinction increased conditioned fear, whereas stimulation of the vHIP inputs after several sessions of extinction decreased the conditioned fear response. This latter effect was, however, transient, as stimulation of this pathway 28 d after extinction increased conditioned fear response again. The results show that repeated fear extinction training gradually changes vHIP-PL connectivity, making fear suppression possible, whereas in the absence of fear suppression from the vHIP, signals from the BL can play a dominant role, resulting in high levels of fear.SIGNIFICANCE STATEMENT Behavioral therapies of fear are based on extinction learning. As extinction memories fade over time, such therapies produce only a temporary suppression of fear, which constitutes a clinical and societal challenge. In our study, we provide a framework for understating the underlying mechanism by which extinction of fear memories fade by demonstrating the existence of two subpopulations of neurons in the prelimbic cortex associated with low and high levels of fear. Insufficient extinction and exposure to the context in which fear memory was formed promoted high fear neuronal activity in the prelimbic cortex, leading to fear retrieval. Extensive extinction training, on the other hand, boosted low fear neuronal activity and, as a result, extinction memory retrieval. This effect was, however, transient and disappeared with time.

Impact of the Usher syndrome on olfaction.

Usher syndrome is a genetically and clinically heterogeneous disease in humans, characterized by sensorineural hearing loss, retinitis pigmentosa and vestibular dysfunction. This disease is caused by mutations in genes encoding proteins that form complex networks in different cellular compartments. Currently, it remains unclear whether the Usher proteins also form networks within the olfactory epithelium (OE). Here, we describe Usher gene expression at the mRNA and protein level in the OE of mice and showed interactions between these proteins and olfactory signaling proteins. Additionally, we analyzed the odor sensitivity of different Usher syndrome mouse models using electro-olfactogram recordings and monitored significant changes in the odor detection capabilities in mice expressing mutant Usher proteins. Furthermore, we observed changes in the expression of signaling proteins that might compensate for the Usher protein deficiency. In summary, this study provides novel insights into the presence and purpose of the Usher proteins in olfactory signal transduction.

Sex differences in social modulation of learning in rats.

In its simplest form, empathy can be characterized as the capacity to share the emotional experiences among individuals, a phenomenon known as emotional contagion. Recent research shows that emotional contagion and its adaptive role can be studied in rodents. However, it is not known whether sex differences observed in human empathy extend to its more primitive forms. In the present study, we used a rat model of emotional contagion to compare the behavioral consequences of social transfer of information about threat, and the subsequent neural activation patterns in male and female rats. We found that: (1) males and females display a similar behavioral pattern during the interaction with either a fear-conditioned or a control rat; (2) interaction with a fear-conditioned conspecific positively modulates two-way avoidance learning in male and diestral female rats but not in estral females; and (3) such interaction results in increased c-Fos expression in the central and lateral nuclei of the amygdala and the prelimbic and infralimbic cortex in males, whereas in females no such changes were observed. Collectively, our results point to the occurrence of sex and estrus cycle phase differences in susceptibility to emotional contagion and underlying neuronal activation in rodents.

Reward learning requires activity of matrix metalloproteinase-9 in the central amygdala.

Learning how to avoid danger and pursue reward depends on negative emotions motivating aversive learning and positive emotions motivating appetitive learning. The amygdala is a key component of the brain emotional system; however, an understanding of how various emotions are differentially processed in the amygdala has yet to be achieved. We report that matrix metalloproteinase-9 (MMP-9, extracellularly operating enzyme) in the central nucleus of the amygdala (CeA) is crucial for appetitive, but not for aversive, learning in mice. The knock-out of MMP-9 impairs appetitively motivated conditioning, but not an aversive one. MMP-9 is present at the excitatory synapses in the CeA with its activity greatly enhanced after the appetitive training. Finally, blocking extracellular MMP-9 activity with its inhibitor TIMP-1 provides evidence that local MMP-9 activity in the CeA is crucial for the appetitive, but not for aversive, learning.

Matrix metalloproteinase (MMP) 9 transcription in mouse brain induced by fear learning.

Memory formation requires learning-based molecular and structural changes in neurons, whereas matrix metalloproteinase (MMP) 9 is involved in the synaptic plasticity by cleaving extracellular matrix proteins and, thus, is associated with learning processes in the mammalian brain. Because the mechanisms of MMP-9 transcription in the brain are poorly understood, this study aimed to elucidate regulation of MMP-9 gene expression in the mouse brain after fear learning. We show here that contextual fear conditioning markedly increases MMP-9 transcription, followed by enhanced enzymatic levels in the three major brain structures implicated in fear learning, i.e. the amygdala, hippocampus, and prefrontal cortex. To reveal the role of AP-1 transcription factor in MMP-9 gene expression, we have used reporter gene constructs with specifically mutated AP-1 gene promoter sites. The constructs were introduced into the medial prefrontal cortex of neonatal mouse pups by electroporation, and the regulation of MMP-9 transcription was studied after contextual fear conditioning in the adult animals. Specifically, -42/-50- and -478/-486-bp AP-1 binding motifs of the mouse MMP-9 promoter sequence have been found to play a major role in MMP-9 gene activation. Furthermore, increases in MMP-9 gene promoter binding by the AP-1 transcription factor proteins c-Fos and c-Jun have been demonstrated in all three brain structures under investigation. Hence, our results suggest that AP-1 acts as a positive regulator of MMP-9 transcription in the brain following fear learning.

Functional anatomy of neural circuits regulating fear and extinction.

The memory of fear extinction is context dependent: fear that is suppressed in one context readily renews in another. Understanding of the underlying neuronal circuits is, therefore, of considerable clinical relevance for anxiety disorders. Prefrontal cortical and hippocampal inputs to the amygdala have recently been shown to regulate the retrieval of fear memories, but the cellular organization of these projections remains unclear. By using anterograde tracing in a transgenic rat in which neurons express a dendritically-targeted PSD-95:Venus fusion protein under the control of a c-fos promoter, we found that, during the retrieval of extinction memory, the dominant input to active neurons in the lateral amygdala was from the infralimbic cortex, whereas the retrieval of fear memory was associated with greater hippocampal and prelimbic inputs. This pattern of retrieval-related afferent input was absent in the central nucleus of the amygdala. Our data show functional anatomy of neural circuits regulating fear and extinction, providing a framework for therapeutic manipulations of these circuits.

Transient brain ischemia due to cardiac arrest causes irreversible long-lasting cognitive injury.

Herein, we used a clinically-relevant model of 10 min cardiac arrest (CA) in Wistar rats. Histological analyses of the ischemic brains of old rats showed significant atrophy of CA(1) sector of hippocampus (Nissl and NeuN stainings) corresponding with increase of glial fibrillary acidic protein expression. The long-term behavioral consequences of above manipulation producing global brain ischemia were assessed in young, middle-aged and old rats, i.e., 3-, 6- and 18-months post-treatment, respectively. In young animals no differences were found in the context-dependent memory in Fear Conditioning test. The most striking behavioral abnormalities were found in middle-aged rats (6 months post-ischemia). Ischemic rats showed hyperactivity and decreased level of anxiety in Open Field and problems with spatial learning and memory in a Novel Object Location test, T-maze and Morris Water Maze. In old animals, a decline of motor and cognitive functions was found not only in ischemic but also in sham/control ones. This study describes consequences of global brain ischemia in aging animals.

Social modulation of learning in rats.

It is well known that emotions participate in the regulation of social behaviors and that the emotion displayed by a conspecific influences the behavior of other animals. In its simplest form, empathy can be characterized as the capacity to be affected by and/or share the emotional state of another. However, to date, relatively little is known about the mechanisms by which animals that are not in direct danger share emotions. In the present study, we used a model of between-subject transfer of fear to characterize the social interaction during which fear is transmitted, as well as the behavioral effects of socially transmitted fear. We found that (1) during social interaction with a recently fear-conditioned partner, observers and demonstrators exhibit social exploratory behaviors rather than aggressive behaviors; (2) learning and memory in a shock-motivated shuttle avoidance task are facilitated in rats that underwent a social interaction with a partner that had been fear conditioned; and (3) a brief social interaction with a recently fear-conditioned partner immediately before fear conditioning increases conditioned freezing measured on the next day. The observed effects were not due to a stress-induced increase in pain sensitivity or analgesia. Collectively, these data suggest that a brief social interaction with a cage mate that has undergone an aversive learning experience promotes aversive learning in an otherwise naïve animal. We argue that socially transferred fear is an adaptation that promotes defensive behavior to potentially dangerous situations in the environment.