Cristiane Regina Guerino Furini

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.

β‐Adrenergic receptors link NO/sGC/PKG signaling to BDNF expression during the consolidation of object recognition long‐term memory

The nitric oxide (NO)/soluble guanylyl cyclase (sGC)/protein kinase G (PKG) pathway is important for memory processing, but the identity of its downstream effectors as well as its actual participation in the consolidation of nonaversive declarative long‐term memory (LTM) remain unknown. Here, we show that training rats in an object recognition (OR) learning task rapidly increased nitrites/nitrates (NOx) content in the CA1 region of the dorsal hippocampus while posttraining intra‐CA1 microinfusion of the neuronal NO synthase (nNOS) inhibitor L‐NN hindered OR LTM retention without affecting memory retrieval or other behavioral variables. The amnesic effect of L‐NN was not state dependent, was mimicked by the sGC inhibitor LY83583 and the PKG inhibitor KT‐5823, and reversed by coinfusion of the NO donor S‐nitroso‐N‐acetylpenicillamine (SNAP) and the PKG activator 8‐bromoguanosine 3′,5′‐cyclic monophosphate (8Br‐cGMP). SNAP did not affect the amnesic effect of LY83583 and KT‐5823. Conversely, 8Br‐cGMP overturned the amnesia induced by LY83583 but not that caused by KT‐5823. Intra‐CA1 infusion of the β‐adrenergic receptor blocker timolol right after training hindered OR LTM and, although coadministration of noradrenaline reversed the amnesia caused by L‐NN, LY83583, and KT5823, the amnesic effect of timolol was unaffected by coinfusion of 8Br‐cGMP or SNAP, indicating that hippocampal β‐adrenergic receptors act downstream NO/sGC/PKG signaling. We also found that posttraining intra‐CA1 infusion of function‐blocking anti‐brain‐derived neurotrophic factor (BDNF) antibodies hampered OR LTM retention, whereas OR training increased CA1 BDNF levels in a nNOS‐ and β‐adrenergic receptor‐dependent manner. Taken together, our results demonstrate that NO/sGC/PKG signaling in the hippocampus is essential for OR memory consolidation and suggest that β‐adrenergic receptors link the activation of this pathway to BDNF expression during the consolidation of declarative memories.

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.

Infusion of protein synthesis inhibitors in the entorhinal cortex blocks consolidation but not reconsolidation of object recognition memory

Memory consolidation and reconsolidation require the induction of protein synthesis in some areas of the brain. Here, we show that infusion of the protein synthesis inhibitors anisomycin, emetine and cycloheximide in the entorhinal cortex immediately but not 180 min or 360 min after training in an object recognition learning task hinders long-term memory retention without affecting short-term memory or behavioral performance. Inhibition of protein synthesis in the entorhinal cortex after memory reactivation involving either a combination of familiar and novel objects or two familiar objects does not affect retention. Our data suggest that protein synthesis in the entorhinal cortex is necessary early after training for consolidation of object recognition memory. However, inhibition of protein synthesis in this cortical region after memory retrieval does not seem to affect the stability of the recognition trace.

A link between the Hippocampal and the Striatal Memory Systems of the Brain

Two major memory systems have been recognized over the years (Squire 1987): the declarative memory system, which is under the control of the hippocampus and related temporal lobe structures, and the procedural or habit memory system, which is under the control of the striatum and its connections. Most if not all learning tasks studied in animals, however, involve either the performance or the suppression of movement; this, if learned well, may be viewed as having become a habit. It is agreed that memory rules change from their first association to those that take place when the task is mastered. Does this change of rules involve a switch from one memory system to another? Here we will comment on: 1) reversal learning in the Morris water maze (MWM), in which the declarative or spatial component of a task is changed but the procedural component (to swim to safety) persists and needs to be re-linked with a different set of spatial cues; and 2) a series of observations on an inhibitory avoidance task that indicate that the brain systems involved change with further learning.

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.