Midazolam reduces the selective activation of the rhinal cortex by contextual fear stimuli

Activation of CB1 pathway in the perirhinal cortex is necessary but not sufficient for destabilization of contextual fear memory in rats

According to the reconsolidation theory, memories can be modified through the destabilization-reconsolidation process. The rodent perirhinal cortex (PER; Brodmann areas 35 and 36) critically participates in the process of fear conditioning. Previous studies showed that some of the parahippocampal regions are critical for contextual fear memory reconsolidation. In our research, through a three-day paradigm of CFC, we showed that protein synthesis in PER of rats is required for memory reconsolidation, and activation of CB1 pathway is necessary but not sufficient in inducing memory destabilization. This result underlines parahippocampal regions in destabilization and reconsolidation process of fear memory besides amygdala and hippocampus.

The Influence of Stomach Back-Shu and Front-Mu Points on Insular Functional Connectivity in Functional Dyspepsia Rat Models

Functional Dyspepsia (FD) is a common functional gastrointestinal disease, which can reduce the quality of life in patients. Prior research has indicated that insula is closely related to FD and that acupuncture can regulate the functional connectivity (FC) of FD. Therefore, we hypothesized that acupuncture on FD was effected through the insular pathway. To test our hypothesis, we performed electroacupuncture (EA) on FD rat models and then examined the FC between insula and other brain regions through resting-state functional magnetic resonance imaging (rs-fMRI). Seven-day-old male infant Sprague-Dawley (SD) rats were randomly divided into control group, FD model group, and FD acupuncture group, with twelve rats per group (n = 36). Upon establishing successful models, the FD acupuncture group was subjected to EA intervention using Stomach back-shu (BL-21) and front-mu (RN-12) points for ten consecutive days for durations of 20 minutes each day. After intervention, each group was subject to rs-fMRI. The digital image data obtained were analyzed using FC analysis methods. Subsequently, gastric ligation was performed to measure gastric emptying rates. Before EA intervention, the FD model group exhibited decreased functional connections between the insula and a number of brain regions. After EA intervention, FD acupuncture group exhibited increasing FC between insula and regions when compared to the FD model group, such as the primary somatosensory cortex (S1), hippocampal CA3 (CA3), polymorphic layer of dentate gyrus (PoDG), caudate putamen (CPu), and oral pontine reticular nuclei (PnO) (P < 0.05); decreasing FC was also exhibited between insula and regions such as the bilateral primary and secondary motor cortexes (M1/2), paraventricular hypothalamic nucleus (PVA), and limbic cortex (LC). These findings indicate that the effective treatment of FD using EA may be through regulating the abnormal FC between insula and several brain regions, in particular CA3, PoDG, and PVA.

Dopamine D2 receptors in the expression and extinction of contextual and cued conditioned fear in rats

Dopamine seems to mediate fear conditioning through its action on D2 receptors in the mesolimbic pathway. Systemic and local injections of dopaminergic agents showed that D2 receptors are preferentially involved in the expression, rather than in the acquisition, of conditioned fear. To further examine this issue, we evaluated the effects of systemic administration of the dopamine D2-like receptor antagonists sulpiride and haloperidol on the expression and extinction of contextual and cued conditioned fear in rats. Rats were trained to a context-CS or a light-CS using footshocks as unconditioned stimuli. After 24 h, rats received injections of sulpiride or haloperidol and were exposed to the context-CS or light-CS for evaluation of freezing expression (test session). After another 24 h, rats were re-exposed to the context-CS or light-CS, to evaluate the extinction recall (retest session). Motor performance was assessed with the open-field and catalepsy tests. Sulpiride, but not haloperidol, significantly reduced the expression of contextual and cued conditioned fear without affecting extinction recall. In contrast, haloperidol, but not sulpiride, had cataleptic and motor-impairing effects. The results reinforce the importance of D2 receptors in fear conditioning and suggest that dopaminergic mechanisms mediated by D2 receptors are mainly involved in the expression rather than in the extinction of conditioned freezing.

Network supporting contextual fear learning after dorsal hippocampal damage has increased dependence on retrosplenial cortex

Hippocampal damage results in profound retrograde, but no anterograde amnesia in contextual fear conditioning (CFC). Although the content learned in the latter have been discussed, alternative regions supporting CFC learning were seldom proposed and never empirically addressed. Here, we employed network analysis of pCREB expression quantified from brain slices of rats with dorsal hippocampal lesion (dHPC) after undergoing CFC session. Using inter-regional correlations of pCREB-positive nuclei between brain regions, we modelled functional networks using different thresholds. The dHPC network showed small-world topology, equivalent to SHAM (control) network. However, diverging hubs were identified in each network. In a direct comparison, hubs in both networks showed consistently higher centrality values compared to the other network. Further, the distribution of correlation coefficients was different between the groups, with most significantly stronger correlation coefficients belonging to the SHAM network. These results suggest that dHPC network engaged in CFC learning is partially different, and engage alternative hubs. We next tested if pre-training lesions of dHPC and one of the new dHPC network hubs (perirhinal, Per; or disgranular retrosplenial, RSC, cortices) would impair CFC. Only dHPC-RSC, but not dHPC-Per, impaired CFC. Interestingly, only RSC showed a consistently higher centrality in the dHPC network, suggesting that the increased centrality reflects an increased functional dependence on RSC. Our results provide evidence that, without hippocampus, the RSC, an anatomically central region in the medial temporal lobe memory system might support CFC learning and memory.

Medial temporal pathways for contextual learning: Network c-fos mapping in rats with or without perirhinal cortex lesions

BACKGROUND

In the rat brain, context information is thought to engage network interactions between the postrhinal cortex, medial entorhinal cortex, and the hippocampus. In contrast, object information is thought to be more reliant on perirhinal cortex and lateral entorhinal cortex interactions with the hippocampus.

METHOD

The 'context network' was explored by mapping expression of the immediate-early gene, c-fos, after exposure to a new spatial environment.

RESULTS

Structural equation modelling of Fos counts produced networks of good fit that closely matched prior predictions based on anatomically-grounded functional models. These same models did not, however, fit the Fos data from home-cage controls nor did they fit the corresponding data from a previous study exploring object recognition. These additional analyses highlight the specificity of the context network. The home-cage controls, meanwhile, showed raised levels of inter-area Fos correlations between the many sites examined, i.e., their changes in Fos levels lacked anatomical specificity. Two additional groups of rats received perirhinal cortex lesions. While the loss of perirhinal cortex reduced lateral entorhinal c-fos activity, it did not affect mean levels of hippocampal c-fos expression. Similarly, overall c-fos expression in the prelimbic cortex, retrosplenial cortex and nucleus reuniens of the thalamus appeared unaffected by the perirhinal cortex lesions.

CONCLUSION

The perirhinal cortex lesions disrupted network interactions involving the medial entorhinal cortex and the hippocampus, highlighting ways in which perirhinal cortex might affect specific aspects of context learning.

Limbic system development underlies the emergence of classical fear conditioning during the third and fourth weeks of life in the rat

Classical fear conditioning creates an association between an aversive stimulus and a neutral stimulus. Although the requisite neural circuitry is well understood in mature organisms, the development of these circuits is less well studied. The current experiments examine the ontogeny of fear conditioning and relate it to neuronal activation assessed through immediate early gene (IEG) expression in the amygdala, hippocampus, perirhinal cortex, and hypothalamus of periweanling rats. Rat pups were fear conditioned, or not, during the third or fourth weeks of life. Neuronal activation was assessed by quantifying expression of FBJ osteosarcoma oncogene (FOS) using immunohistochemistry (IHC) in Experiment 1. Fos and early growth response gene-1 (EGR1) expression was assessed using qRT-PCR in Experiment 2. Behavioral data confirm that both auditory and contextual fear continue to emerge between PD 17 and 24. The IEG expression data are highly consistent with these behavioral results. IHC results demonstrate significantly more FOS protein expression in the basal amygdala of fear-conditioned PD 23 subjects compared to control subjects, but no significant difference at PD 17. qRT-PCR results suggest specific activation of the amygdala only in older subjects during auditory fear expression. A similar effect of age and conditioning status was also observed in the perirhinal cortex during both contextual and auditory fear expression. Overall, the development of fear conditioning occurring between the third and fourth weeks of life appears to be at least partly attributable to changes in activation of the amygdala and perirhinal cortex during fear conditioning or expression. (PsycINFO Database Record

Further evidence for involvement of the dorsal hippocampus serotonergic and γ-aminobutyric acid (GABA)ergic pathways in the expression of contextual fear conditioning in rats

Intra-dorsal hippocampus (DH) injections of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a serotonin-1A (5-hydroxytryptamine (5-HT)-1A) receptor agonist, were previously shown to inhibit the expression of contextual fear when administered six hours after conditioning. However, further understanding of the consolidation and expression of aversive memories requires investigations of these and other mechanisms at distinct time points and the regions of the brain to which they are transferred. Thus, the purpose of the present study was to investigate the role of DH serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the expression of contextual fear 24 h after conditioning, reflected by fear-potentiated startle (FPS) and freezing behavior. The recruitment of the amygdala and medial prefrontal cortex (mPFC) in these processes was also evaluated by measuring Fos protein immunoreactivity. Although intra-DH injections of 8-OH-DPAT did not produce behavioral changes, muscimol reduced both FPS and the freezing response. Fos protein immunoreactivity revealed that contextual fear promoted wide activation of the mPFC, which was significantly reduced after intra-DH infusions of muscimol. The present findings, together with previous data, indicate that in contrast to 5-HT, which appears to play a role during the early phases of contextual aversive memory consolidation, longer-lasting GABA-mediated mechanisms are recruited during the expression of contextual fear memories.

Dopamine D2 receptors modulate the expression of contextual conditioned fear: role of the ventral tegmental area and the basolateral amygdala

Although dopaminergic systems are more commonly associated with the reinforcing effects of various stimuli, numerous reports have demonstrated a relationship between changes in dopaminergic transmission and aversive situations. In the present study, we examined the involvement of D1-like and D2-like receptors in the expression of conditioned freezing using the context as the conditioned stimulus. Intraperitoneal injections of the D1 agonist SKF38393 or the D1 antagonist SCH23390 did not change the conditioned freezing in rats subjected to the contextual fear paradigm. In contrast, intraperitoneal injections of the D2 agonist quinpirole and the D2 antagonist sulpiride caused a significant dose-dependent reduction in the expression of contextual conditioned freezing. As these data may reflect that the systemic manipulations acted on dopaminergic receptors in different brain areas, the effects of administration of quinpirole and sulpiride into the ventral tegmental area (VTA) and the basolateral amygdala complex (BLA) on the expression of contextual conditioned freezing were also evaluated. Intra-VTA quinpirole and intra-BLA sulpiride injections reduced the conditioned freezing response; intra-VTA sulpiride and intra-BLA quinpirole injections had no significant effects. These data suggest that D2-like receptors, but not D1-like receptors, play an important role in the expression of contextual conditioned freezing. Quinpirole may act at D2 presynaptic receptors located in the VTA, decreasing dopamine levels in the terminal fields of the mesolimbic pathway. The effects of sulpiride, in contrast, appear to be triggered by an action on postsynaptic dopaminergic receptors located in the BLA. However, it cannot be totally excluded that the injected solutions did not also affect neighboring amygdalar regions. Together with previous findings, the present data suggest the need to consider dopaminergic mechanisms in the mesolimbic circuit as novel targets for the pharmacological treatment of fear-related disorders, especially post-traumatic stress disorder.

D(1)-like receptors in the nucleus accumbens shell regulate the expression of contextual fear conditioning and activity of the anterior cingulate cortex in rats

Although dopamine-related circuits are best known for their roles in appetitive motivation, consistent data have implicated this catecholamine in some forms of response to stressful situations. In fact, projection areas of the ventral tegmental area, such as the amygdala and hippocampus, are well established to be involved in the acquisition and expression of fear conditioning, while less is known about the role of the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in these processes. In the present study, we initially investigated the involvement of the mPFC and NAc in the expression of conditioned fear, assessing freezing behaviour and Fos protein expression in the brains of rats exposed to a context, light or tone previously paired with footshocks. Contextual and cued stimuli were able to increase the time of the freezing response while only the contextual fear promoted a significant increase in Fos protein expression in the mPFC and caudal NAc. We then examined the effects of specific dopaminergic agonists and antagonists injected bilaterally into the posterior medioventral shell subregion of the NAc (NAcSh) on the expression of contextual fear. SKF38393, quinpirole and sulpiride induced no behavioural changes, but the D1-like receptor antagonist SCH23390 increased the freezing response of the rats and selectively reduced Fos protein expression in the anterior cingulate cortex and rostral NAcSh. These findings confirm the involvement of the NAcSh in the expression of contextual fear memories and indicate the selective role of NAcSh D1-like receptors and anterior cingulate cortex in this process.

Dual functions of perirhinal cortex in fear conditioning

The present review examines the role of perirhinal cortex (PRC) in Pavlovian fear conditioning. The focus is on rats, partly because so much is known, behaviorally and neurobiologically, about fear conditioning in these animals. In addition, the neuroanatomy and neurophysiology of rat PRC have been described in considerable detail at the cellular and systems levels. The evidence suggests that PRC can serve at least two types of mnemonic functions in Pavlovian fear conditioning. The first function, termed "stimulus unitization," refers to the ability to treat two or more separate items or stimulus elements as a single entity. Supporting evidence for this perceptual function comes from studies of context conditioning as well as delay conditioning to discontinuous auditory cues. In a delay paradigm, the conditional stimulus (CS) and unconditional stimulus (US) overlap temporally and co-terminate. The second PRC function entails a type of "transient memory." Supporting evidence comes from studies of trace cue conditioning, where there is a temporal gap or trace interval between the CS offset and the US onset. For learning to occur, there must be a transient CS representation during the trace interval. We advance a novel neurophysiological mechanism for this transient representation. These two hypothesized functions of PRC are consistent with inferences based on non-aversive forms of learning.

Role of amygdala-prefrontal cortex circuitry in regulating the expression of contextual fear memory

The basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) are inter-connected regions involved in fear memory expression. The reciprocal nature of projections between these areas differs along the rostrocaudal extent of BLA. This study investigated the role of functional interactions between BLA and the prelimbic (PL) subregion of mPFC in mediating contextual fear memory. Freezing served as the measure of conditioned fear. Experiments 1-3 examined the effects of left, right or bilateral infusion of bupivacaine into anterior BLA (aBLA), posterior BLA (pBLA) or PL on fear memory expression. Reversible inactivation of left, right or bilateral aBLA impaired fear memory expression. Bilateral inactivation of pBLA or PL also disrupted the expression of fear memory, although left or right inactivation alone had no significant effects in either region. Experiment 4 examined the effects of functionally disconnecting pBLA and PL on contextual fear memory by infusing bupivacaine unilaterally into pBLA and PL in the ipsilateral or contralateral hemisphere. Fear memory expression was impaired by asymmetric inactivation of pBLA and PL; however, a similar effect was also observed with symmetric inactivation of these regions. Bupivacaine infusion did not affect behavior in the open field, likely ruling out non-specific effects of inactivation on innate fear and locomotor activity. These results demonstrate different roles for rostral and caudal BLA in mediating the expression of contextual fear memory. They also raise the possibility that pBLA-PL circuitry is involved in subserving fear memory expression via complex processing mechanisms, although further research is needed to confirm this preliminary finding.