Jociane de Carvalho Myskiw

Modulation of the extinction of two different fear-motivated tasks in three distinct brain areas.

The hippocampus, basolateral amygdala and ventromedial prefrontal cortex participate in the extinction of inhibitory avoidance and contextual fear conditioning. We studied the effect of drugs acting on receptors involved in synaptic modulation on extinction of both tasks. The drugs were given bilaterally right after the first of two sessions of extinction in each task through cannulae implanted into the mentioned areas. The doses used are known to influence memory consolidation of the original tasks. Their effects were evaluated on a second extinction session 24h later, and assumed to result from influences on the consolidation of extinction. The glutamate NMDA receptor stimulant d-serine (50 μg/side) and the histamine methyl-transferase inhibitor SKF9188 (12.5 μg/side) enhanced, and the NMDA antagonist amino-phosphonopentanoate (5 μg/side) and the H2 histamine receptor antagonist ranitidine (17.5 μg/side) inhibited, extinction of both tasks regardless of the region into which they were administered. Thus, glutamate NMDA receptors are involved in the consolidation of extinction of both tasks, and histamine H2 receptors modulate that process in all areas studied. Norepinephrine (1 μg/side), the β-adrenoceptor antagonist timolol (1 μg/side), the D1 dopamine receptor agonist SKF38393 (12.5 μg/side) and the D1 antagonist SCH23390 (1.5 μg/side) also affected extinction of both tasks, but their effects varied with the task and with the site of infusion, suggesting that extinction modulation by β- and D1 receptors is more complex. In conclusion, extinction of two different aversive tasks is modulatable by various systems, which bears upon the behavioral and pharmacological treatment of fear-motivated brain disorders.

The learning of fear extinction.

Recent work on the extinction of fear-motivated learning places emphasis on its putative circuitry and on its modulation. Extinction is the learned inhibition of retrieval of previously acquired responses. Fear extinction is used as a major component of exposure therapy in the treatment of fear memories such as those of the posttraumatic stress disorder (PTSD). It is initiated and maintained by interactions between the hippocampus, basolateral amygdala and ventromedial prefrontal cortex, which involve feedback regulation of the latter by the other two areas. Fear extinction depends on NMDA receptor activation. It is positively modulated by d-serine acting on the glycine site of NMDA receptors and blocked by AP5 (2-amino-5-phosphono propionate) in the three structures. In addition, histamine acting on H2 receptors and endocannabinoids acting on CB1 receptors in the three brain areas mentioned, and muscarinic cholinergic fibers from the medial septum to hippocampal CA1 positively modulate fear extinction. Importantly, fear extinction can be made state-dependent on circulating epinephrine, which may play a role in situations of stress. Exposure to a novel experience can strongly enhance the consolidation of fear extinction through a synaptic tagging and capture mechanism; this may be useful in the therapy of states caused by fear memory like PTSD.

Hippocampal molecular mechanisms involved in the enhancement of fear extinction caused by exposure to novelty.

Significance Within a restricted time window, a brief exposure to a novel environment enhances the extinction of contextual fear. This can be explained by a hippocampal process of behaviorally induced synaptic tagging and capture. Here, we report that the effect requires glutamate NMDA receptors and L-voltage–dependent calcium channels and involves the activation of calcium/calmodulin-dependent protein kinase II, in addition to both ribosomal and nonribosomal protein synthesis. All these mechanisms operate only when the proteasomal-ubiquitin protein degradation system is intact, which suggests that they depend on synaptic protein turnover. Extinction enhancement by novelty is of great potential importance in the treatment of fear memories, such as those of posttraumatic stress disorder; the treatments of choice for such conditions are based on extinction procedures. Exposure to a novel environment enhances the extinction of contextual fear. This has been explained by tagging of the hippocampal synapses used in extinction, followed by capture of proteins from the synapses that process novelty. The effect is blocked by the inhibition of hippocampal protein synthesis following the novelty or the extinction. Here, we show that it can also be blocked by the postextinction or postnovelty intrahippocampal infusion of the NMDA receptor antagonist 2-amino-5-phosphono pentanoic acid; the inhibitor of calcium/calmodulin-dependent protein kinase II (CaMKII), autocamtide-2–related inhibitory peptide; or the blocker of L-voltage–dependent calcium channels (L-VDCCs), nifedipine. Inhibition of proteasomal protein degradation by β-lactacystin has no effect of its own on extinction or on the influence of novelty thereon but blocks the inhibitory effects of all the other substances except that of rapamycin on extinction, suggesting that their action depends on concomitant synaptic protein turnover. Thus, the tagging-and-capture mechanism through which novelty enhances fear extinction involves more molecular processes than hitherto thought: NMDA receptors, L-VDCCs, CaMKII, and synaptic protein turnover.

D1 and D5 dopamine receptors participate on the consolidation of two different memories.

Memory consolidation is the process by which recently acquired information becomes stable and is modulated by different neurotransmitters depending on the structure involved and the nature of the memory. Here we evaluate the participation of both D1 and D5 dopamine receptors in the CA1 region of the hippocampus in the consolidation of the memory of two different tasks, object recognition (OR) and inhibitory avoidance (IA). For this, male rats with infusion cannulae stereotaxically implanted in the CA1 region of the dorsal hippocampus were trained in an OR task involving exposure to two different objects, or in a one-trial step-down IA task. At different times after the training, some of the animals received intrahippocampal infusions of the D1-family receptor antagonist SCH-23390. In a test session carried out 24h later, the animals that received infusions immediately or 60 min but not 180 min after the training showed impaired long-term memory. Since D1- and D5-subtypes engage different signaling pathways involving cAMP-dependent protein kinase (PKA) and protein kinase C (PKC), respectively, we assessed whether they participate distinctively in consolidation. The animals that received intra-CA1 infusions of the PKA inhibitor, Rp-cAMP, or the PKC inhibitor, Gö6976, immediately after OR or IA training had a long-term memory impairment and the amnesic effect caused by SCH-23390 was reversed when co-infused with activators of PKA (8Br-cAMP) or PKC (PMA). These results indicate that both D1 and D5 dopamine receptors are required in the CA1 region of the hippocampus for consolidation of the two tasks. This supports the notion of a commonality of consolidation mechanisms across tasks.

Facilitation of fear extinction by novelty depends on dopamine acting on D1-subtype dopamine receptors in hippocampus

Significance A brief exposure to a novel environment was recently shown to enhance the extinction of contextual fear probably through a protein synthesis-dependent process of synaptic tagging and capture in the hippocampus. Here we report that this finding can be generalized to the extinction of another fear-motivated task, one-trial inhibitory avoidance. This generalization is important because extinction is used in exposure therapy to treat posttraumatic stress disorder in humans, and this may derive from various forms of fear-related stress. In addition, here we show that the effect of novelty on fear extinction is dependent on dopamine D1 but not D5 receptors in the hippocampus. These findings could be applicable to the exposure therapy of fear memory disorders. Extinction is the learned inhibition of retrieval. Recently it was shown that a brief exposure to a novel environment enhances the extinction of contextual fear in rats, an effect explainable by a synaptic tagging-and-capture process. Here we examine whether this also happens with the extinction of another fear-motivated task, inhibitory avoidance (IA), and whether it depends on dopamine acting on D1 or D5 receptors. Rats were trained first in IA and then in extinction of this task. The retention of extinction was measured 24 h later. A 5-min exposure to a novel environment 30 min before extinction training enhanced its retention. Right after exposure to the novelty, animals were given bilateral intrahippocampal infusions of vehicle (VEH), of the protein synthesis inhibitor anisomycin, of the D1/D5 dopaminergic antagonist SCH23390, of the PKA inhibitor Rp-cAMP or of the PKC inhibitor Gö6976, and of the PKA stimulator Sp-cAMP or of the PKC stimulator PMA. The novelty increased hippocampal dopamine levels and facilitated the extinction, which was inhibited by intrahippocampal protein synthesis inhibitor anisomysin, D1/D5 dopaminerdic antagonist SCH23390, or PKA inhibitor Rp-cAMP and unaffected by PKC inhibitor Gö6976; additionally, the hippocampal infusion of PKA stimulator Sp-cAMP reverts the effect of D1/D5 dopaminergic antagonist SCH 23390, but the infusion of PKC stimulator PMA does not. The results attest to the generality of the novelty effect on fear extinction, suggest that it relies on synaptic tagging and capture, and show that it depends on hippocampal dopamine D1 but not D5 receptors.

Histamine in the basolateral amygdala promotes inhibitory avoidance learning independently of hippocampus

Significance Integrity of the brain histaminergic system is necessary for long-term memory (LTM) but not short-term memory of step-down inhibitory avoidance (IA). Histamine depletion in hippocampus or basolateral amygdala (BLA) impairs LTM of that task. Histamine infusion into either structure restores LTM in histamine-depleted rats. The restoring effect in BLA occurs even when hippocampal activity was impaired. Cyclic adenosine monophosphate (cAMP) responsive-element-binding protein phosphorylation correlates anatomically and temporally with histamine-induced memory recall. Thus, histaminergic neurotransmission appears critical to provide the brain with the plasticity necessary for IA through recruitment of alternative circuits. Our findings indicate that the histaminergic system comprises parallel, coordinated pathways that provide compensatory plasticity when one brain structure is compromised. Recent discoveries demonstrated that recruitment of alternative brain circuits permits compensation of memory impairments following damage to brain regions specialized in integrating and/or storing specific memories, including both dorsal hippocampus and basolateral amygdala (BLA). Here, we first report that the integrity of the brain histaminergic system is necessary for long-term, but not for short-term memory of step-down inhibitory avoidance (IA). Second, we found that phosphorylation of cyclic adenosine monophosphate (cAMP) responsive-element-binding protein, a crucial mediator in long-term memory formation, correlated anatomically and temporally with histamine-induced memory retrieval, showing the active involvement of histamine function in CA1 and BLA in different phases of memory consolidation. Third, we found that exogenous application of histamine in either hippocampal CA1 or BLA of brain histamine-depleted rats, hence amnesic, restored long-term memory; however, the time frame of memory rescue was different for the two brain structures, short lived (immediately posttraining) for BLA, long lasting (up to 6 h) for the CA1. Moreover, long-term memory was formed immediately after training restoring of histamine transmission only in the BLA. These findings reveal the essential role of histaminergic neurotransmission to provide the brain with the plasticity necessary to ensure memorization of emotionally salient events, through recruitment of alternative circuits. Hence, our findings indicate that the histaminergic system comprises parallel, coordinated pathways that provide compensatory plasticity when one brain structure is compromised.

Extinction learning, which consists of the inhibition of retrieval, can be learned without retrieval

Significance Blockade of the retrieval of contextual fear conditioning by intrahippocampal muscimol administration does not impede extinction of the task measured up to 1 wk later, its eventual spontaneous recovery at 14 d, or its inhibition by two different protein synthesis inhibitors given into the hippocampus. These results show that extinction and retrieval are separate processes and strongly suggest that extinction is triggered or gated by the conditioned stimulus even in the absence of retrieval. In the present study we test the hypothesis that extinction is not a consequence of retrieval in unreinforced conditioned stimulus (CS) presentation but the mere perception of the CS in the absence of a conditioned response. Animals with cannulae implanted in the CA1 region of hippocampus were subjected to extinction of contextual fear conditioning. Muscimol infused intra-CA1 before an extinction training session of contextual fear conditioning (CFC) blocks retrieval but not consolidation of extinction measured 24 h later. Additionally, this inhibition of retrieval does not affect early persistence of extinction when tested 7 d later or its spontaneous recovery after 2 wk. Furthermore, both anisomycin, an inhibitor of ribosomal protein synthesis, and rapamycin, an inhibitor of extraribosomal protein synthesis, given into the CA1, impair extinction of CFC regardless of whether its retrieval was blocked by muscimol. Therefore, retrieval performance in the first unreinforced session is not necessary for the installation, maintenance, or spontaneous recovery of extinction of CFC.

Major neurotransmitter systems in dorsal hippocampus and basolateral amygdala control social recognition memory

Significance The hippocampus and basolateral amygdala—modulated by β-noradrenergic, D1/D5 dopaminergic, and H2-histaminergic receptors—control memory processing of many memories, but their role in social recognition memory (SRM) has been little studied. SRM is fundamental for the establishment of social relationships and, consequently, for the formation and stability of social groups. The social deficits of psychiatric disorders, such as autism and schizophrenia, are believed to be caused by alterations in SRM processing by the hippocampus and amygdala. Here we examine the involvement of the hippocampus and basolateral amygdala—and β-noradrenergic, D1/D5 dopaminergic, and H2-histaminergic receptors therein—in SRM consolidation. The results suggest an important and complex modulation of this process, which may help to elucidate the basis of inappropriate social behavior in psychiatric patients. Social recognition memory (SRM) is crucial for reproduction, forming social groups, and species survival. Despite its importance, SRM is still relatively little studied. Here we examine the participation of the CA1 region of the dorsal hippocampus (CA1) and the basolateral amygdala (BLA) and that of dopaminergic, noradrenergic, and histaminergic systems in both structures in the consolidation of SRM. Male Wistar rats received intra-CA1 or intra-BLA infusions of different drugs immediately after the sample phase of a social discrimination task and 24-h later were subjected to a 5-min retention test. Animals treated with the protein synthesis inhibitor, anisomycin, into either the CA1 or BLA were unable to recognize the previously exposed juvenile (familiar) during the retention test. When infused into the CA1, the β-adrenoreceptor agonist, isoproterenol, the D1/D5 dopaminergic receptor antagonist, SCH23390, and the H2 histaminergic receptor antagonist, ranitidine, also hindered the recognition of the familiar juvenile 24-h later. The latter drug effects were more intense in the CA1 than in the BLA. When infused into the BLA, the β-adrenoreceptor antagonist, timolol, the D1/D5 dopamine receptor agonist, SKF38393, and the H2 histaminergic receptor agonist, ranitidine, also hindered recognition of the familiar juvenile 24-h later. In all cases, the impairment to recognize the familiar juvenile was abolished by the coinfusion of agonist plus antagonist. Clearly, both the CA1 and BLA, probably in that order, play major roles in the consolidation of SRM, but these roles are different in each structure vis-à-vis the involvement of the β-noradrenergic, D1/D5-dopaminergic, and H2-histaminergic receptors therein.

PACAP modulates the consolidation and extinction of the contextual fear conditioning through NMDA receptors

Pituitary adenylate cyclase-activating polypeptide (PACAP) has a broad spectrum of biological functions including neurotransmitter, neurotrophic and neuroprotective. Moreover, it has been suggested that PACAP plays a role in the modulation of learning and memory as well as on the modulation of glutamate signaling. Thus, in the current study we investigated in the CA1 region of hippocampus and in the basolateral amygdala (BLA) the role of PACAP in the consolidation and extinction of contextual fear conditioning (CFC) and the interaction between PACAP and NMDA receptors. Male rats with cannulae implanted in the CA1 region of the hippocampus or in the BLA received immediately after the training or extinction training of the CFC infusions of the Vehicle, PACAP-38 (40 pg/side), PACAP 6-38 (40 pg/side) or PACAP 6-38 plus D-serine (50 μg/side). After 24h, the animals were subjected to a 3-min retention test. The results indicated that in the CA1 region of hippocampus, PACAP participates in the consolidation and extinction of the CFC, and in the BLA, PACAP participates only in the consolidation of the CFC. Additionally, the results suggest that the action of PACAP on the consolidation and extinction of the CFC is mediated by the glutamate NMDA receptors.

Memory retrieval of inhibitory avoidance requires histamine H1 receptor activation in the hippocampus.

Significance Several neurotransmitters contribute to memory formation by modulating selectively acquisition, consolidation, and/or retrieval. Integrity of the brain histamine system is necessary for the consolidation of inhibitory avoidance (IA) memory. Here, we report that cerebral histamine depletion also impairs retrieval of IA in rats and blunts retrieval-induced c-Fos activation and cAMP-responsive element binding protein phosphorylation in the CA1 region of the hippocampus. Histamine infusion into the CA1 restores IA retrieval in histamine-depleted rats by targeting brain histamine H1 receptors. Our study uncovers previously unidentified mechanisms involved in memory retrieval and may offer possible targets for eventual pharmacotherapies to treat dysfunctional aversive memories, including phobias, panic attacks, and posttraumatic stress disorders, as well as improve the efficacy of exposure psychotherapies. Retrieval represents a dynamic process that may require neuromodulatory signaling. Here, we report that the integrity of the brain histaminergic system is necessary for retrieval of inhibitory avoidance (IA) memory, because rats depleted of histamine through lateral ventricle injections of α-fluoromethylhistidine (a-FMHis), a suicide inhibitor of histidine decarboxylase, displayed impaired IA memory when tested 2 d after training. a-FMHis was administered 24 h after training, when IA memory trace was already formed. Infusion of histamine in hippocampal CA1 of brain histamine-depleted rats (hence, amnesic) 10 min before the retention test restored IA memory but was ineffective when given in the basolateral amygdala (BLA) or the ventral medial prefrontal cortex (vmPFC). Intra-CA1 injections of selective H1 and H2 receptor agonists showed that histamine exerted its effect by activating the H1 receptor. Noteworthy, the H1 receptor antagonist pyrilamine disrupted IA memory retrieval in rats, thus strongly supporting an active involvement of endogenous histamine; 90 min after the retention test, c-Fos–positive neurons were significantly fewer in the CA1s of a-FMHis–treated rats that displayed amnesia compared with in the control group. We also found reduced levels of phosphorylated cAMP-responsive element binding protein (pCREB) in the CA1s of a-FMHis–treated animals compared with in controls. Increases in pCREB levels are associated with retrieval of associated memories. Targeting the histaminergic system may modify the retrieval of emotional memory; hence, histaminergic ligands might reduce dysfunctional aversive memories and improve the efficacy of exposure psychotherapies.