The role of hippocampal estradiol in synaptic plasticity and memory: A systematic review.

The consolidation of long-term memory is influenced by various neuromodulators. One of these is estradiol, a steroid hormone that is synthesized both in peripheral endocrine tissue and in the brain, including the hippocampus. Here, we examine the evidence regarding the role of estradiol in the hippocampus, specifically, in memory formation and its effects on the molecular mechanisms underlying synaptic plasticity. We conclude that estradiol improves memory consolidation and, thereby, long-term memory. Previous studies have shown that it does this in three, interconnected ways: (1) via functional changes in excitatory activity, (2) signaling changes in calcium dynamics, protein phosphorylation and protein expression, and (3) structural changes to synaptic morphology. Through a functional network analysis of proteins affected by estradiol, we identify potential protein-protein interactions that further support a role for estradiol in modulating synaptic plasticity as well as highlight signaling pathways that may be involved in these changes within the hippocampus.

Oxytocin increases inhibitory synaptic transmission and blocks development of long-term potentiation in the lateral amygdala.

Oxytocin (OT) is a neuroactive peptide that influences the processing of fearful stimuli in the amygdala. In the central nucleus of the amygdala, the activation of OT receptors alters neural activity and ultimately suppresses the behavioral response to a fear conditioned stimulus. Receptors for OT are also found in the lateral amygdala (LA), and infusion of OT into the basolateral amygdala complex affects the formation and consolidation of fear memories. Yet, how OT receptor activation alters neurons and neural networks in the LA is unknown. In this study we used whole cell electrophysiological recordings to determine how OT-receptor activation changes synaptic transmission and synaptic plasticity in the LA of Sprague-Dawley rats. Our results demonstrate that OT-receptor activation results in a 200% increase in spontaneous inhibitory transmission in the LA that leads to the activation of presynaptic GABAB receptors. The activation of these receptors inhibits excitatory transmission in the LA, blocking long-term potentiation of cortical inputs onto LA neurons. Hence, this study provides the first demonstration that OT influences synaptic transmission and plasticity in the LA, revealing a mechanism that could explain how OT regulates the formation and consolidation of conditioned fear memories in the amygdala.NEW & NOTEWORTHY This study investigates modulation of synaptic transmission by oxytocin (OT) in the lateral amygdala (LA). We demonstrate that OT induces transient increases in spontaneous GABAergic transmission by activating interneurons in the basolateral amygdala. The resultant increase in GABA release in the LA activates presynaptic GABAB receptors on both inhibitory and excitatory inputs onto LA neurons, reducing release probability at these synapses. We subsequently demonstrate that OT modulates synaptic plasticity at cortical inputs to the LA.

The effect of standard laboratory diets on estrogen signaling and spatial memory in male and female rats.

Phytoestrogens are plant-derived compounds that can modulate estrogen activity in the brain and periphery. Laboratory rodent diets are typically high in soy-based phytoestrogens and therefore may influence neurophysiological and behavioural measures that are sensitive to estrogen signaling. Here we assessed such measures in rats (males and females) fed Australian made diets that varied in their soy levels. We found that a low-soy diet promoted greater weight, and lower levels of plasma estradiol, particularly in male rats. It also produced sex-specific effects on estrogen receptor gene expression in the brain, increasing ESR2 expression in the hippocampus and prefrontal cortex in female rats, and decreasing dopamine D1 receptor gene expression in the striatum of both male and female rats. We also found a dietary effect on short-term place recognition memory, but this was independent of soy levels in the diet. These results demonstrate that the choice of rodent laboratory diet can influence physiology, neurobiology and behavior, particularly on measures related to estrogen signaling.

Fear conditioning and long-term potentiation in the amygdala: what really is the connection?

The cellular mechanisms that underlie learning and memory formation remain one of the most intriguing unknowns about the mammalian brain. A plethora of experimental evidence over the last 30 years has established that long-term synaptic plasticity at excitatory synapses is the most likely mechanism that underlies learning and memory formation. Experiments done largely in acute brain slices maintained in vitro have revealed many of the molecular mechanisms in the induction and maintenance of long-term potentiation (LTP). However, evidence directly liking LTP with learning and memory formation has not been established. Pavlovian fear conditioning is a good candidate to provide such evidence. The relations between events that produce fear conditioning are simple; these relations and their fear products involve circuits in the amygdala that are well understood, as are those circuits in the amygdala that underlie LTP. The evidence that links LTP in the amygdala with fear conditioning is reviewed.

Baclofen acts in the central amygdala to reduce synaptic transmission and impair context fear conditioning.

The two main sub-divisions of the Central amygdala (CeA), the lateral-capsular (CeA-LC) and the medial (CeA-M), contain extensive networks of inhibitory interneurons. We have previously shown that activation of GABAB-receptors reduces excitatory transmission between axons of the pontine parabrachial nucleus and neurons of the CeA-LC by inhibiting glutamate release from presynaptic terminals13. Here we have characterised GABAB-receptor activation on other excitatory and inhibitory projections within the CeA. Using whole-cell, patch-clamp recordings, we found that the GABAB-receptor agonist baclofen significantly reduced excitatory and inhibitory transmission from all tested inputs into the CeA-LC and CeA-M. In all but one of the inputs, reductions in transmission were accompanied by an increase in paired pulse ratio, indicating that presynaptic GABAB-receptors acted to reduce the release probability of synaptic vesicles. To examine the impact of GABAB-receptors in the CeA on contextual fear-conditioning, we infused baclofen into the CeA immediately prior to training. Compared to vehicle-infused rats, baclofen-infused rats displayed significantly less freezing both during the final stages of the training period and at test 24 hours later. The results of this study demonstrate that, by suppressing excitatory and inhibitory transmission, activation of presynaptic GABAB-receptors in the CeA inhibits the development of context conditioned fear.

α2-adrenoceptor-mediated inhibition in the central amygdala blocks fear-conditioning.

The central amygdala is critical for the acquisition and expression of fear memories. This region receives a dense innervation from brainstem noradrenergic cell groups and has a high level of α2-adrenoceptor expression. Using whole-cell electrophysiological recordings from rat brain slices, we characterise the role of pre-synaptic α2-adrenoceptor in modulating discrete inhibitory and excitatory connections within both the lateral and medial division of the central amygdala. The selective α2-adrenoceptor agonist clonidine blocked the excitatory input from the pontine parabrachial neurons onto neurons of the lateral central amygdala. In addition, clonidine blocked inhibitory connections from the medial paracapsular intercalated cell mass onto both lateral and medial central amygdala neurons. To examine the behavioural consequence of α2-adrenoceptor-mediated inhibition of these inputs, we infused clonidine into the central amygdala prior to contextual fear-conditioning. In contrast to vehicle-infused rats, clonidine-infused animals displayed reduced levels of freezing 24 hours after training, despite showing no difference in freezing during the training session. These results reveal a role for α2-adrenoceptors within the central amygdala in the modulation of synaptic transmission and the formation of fear-memories. In addition, they provide further evidence for a role of the central amygdala in fear-memory formation.

Carbachol injections into the nucleus accumbens disrupt acquisition and expression of fear-potentiated startle and freezing in rats.

The nucleus accumbens is involved in different types of emotional learning, ranging from appetitive instrumental learning to Pavlovian fear conditioning. In previous studies, we found that temporary inactivation of the nucleus accumbens blocked both the acquisition and expression of conditioned fear. This was not due to altered dopaminergic activity as we have also found that intra-nucleus accumbens infusions of the dopamine agonist amphetamine do not affect either the acquisition or the expression of conditioned fear. Therefore, in the present study we examined whether cholinergic activity in the nucleus accumbens is involved in the acquisition and expression of conditioned fear. Specifically, the effect of intra-nucleus accumbens infusions of the unselective cholinergic agonist carbachol on the acquisition and expression of conditioned fear was assessed. Across several experiments, we measured fear to visual and acoustic conditioned stimuli and to the experimental context. Further, two different measures of conditioned fear were recorded: fear potentiation of startle and freezing. Intra-nucleus accumbens carbachol infusions disrupted acquisition as well as expression of conditioned fear, regardless of the modality of the fear-eliciting stimulus or of the specific measure of conditioned fear. This disruption of conditioned fear was not simply a by-product of enhanced motor activity which also occurred after intra-nucleus accumbens carbachol infusions. Interestingly, despite the substantial effect of intra-nucleus accumbens carbachol on expression of conditioned fear, the results of the final experiment suggest that these rats extinguish similarly to control rats. Taken together, the present results indicate that acetylcholine within the nucleus accumbens is important for the learning and retrieval of conditioned fear.

Daily Exposure to Sucrose Impairs Subsequent Learning About Food Cues: A Role for Alterations in Ghrelin Signaling and Dopamine D2 Receptors.

The prevalence of hedonic foods and associated advertising slogans has contributed to the rise of the obesity epidemic in the modern world. Research has shown that intake of these foods disrupt dopaminergic systems. It may be that a disruption of these circuits produces aberrant learning about food-cue relationships. We found that rodents given 28 days of intermittent access to sucrose exhibited a deficit in the ability to block learning about a stimulus when it is paired in compound with food and another stimulus that has already been established as predictive of the food outcome. This deficit was characterized by an approach to a cue signaling food delivery that is usually blocked by prior learning, an effect dependent on dopaminergic prediction-error signaling in the midbrain. Administering the D2 agonist quinpirole during learning restored blocking in animals with a prior history of sucrose exposure. Further, repeated central infusions of ghrelin produced a deficit in blocking in the same manner as sucrose exposure. We argue that changes in dopaminergic systems resulting from sucrose exposure are mediated by a disruption of ghrelin signaling as rodents come to anticipate delivery of the highly palatable sucrose outside of normal feeding schedules. This suggestion is supported by our finding that both sucrose and ghrelin treatments resulted in increases in amphetamine-induced locomotor responding. Thus, for the first time, we have provided evidence of a potential link between alterations in D2 receptors caused by the intake of hedonic foods and aberrant learning about cue-food relationships capable of promoting inappropriate feeding habits. In addition, we have found preliminary evidence to suggest that this is mediated by changes in ghrelin signaling, a finding that should stimulate further research into modulation of ghrelin activity to treat obesity.

Integration of reward signalling and appetite regulating peptide systems in the control of food-cue responses.

Understanding the neurobiological substrates that encode learning about food-associated cues and how those signals are modulated is of great clinical importance especially in light of the worldwide obesity problem. Inappropriate or maladaptive responses to food-associated cues can promote over-consumption, leading to excessive energy intake and weight gain. Chronic exposure to foods rich in fat and sugar alters the reinforcing value of foods and weakens inhibitory neural control, triggering learned, but maladaptive, associations between environmental cues and food rewards. Thus, responses to food-associated cues can promote cravings and food-seeking by activating mesocorticolimbic dopamine neurocircuitry, and exert physiological effects including salivation. These responses may be analogous to the cravings experienced by abstaining drug addicts that can trigger relapse into drug self-administration. Preventing cue-triggered eating may therefore reduce the over-consumption seen in obesity and binge-eating disorder. In this review we discuss recent research examining how cues associated with palatable foods can promote reward-based feeding behaviours and the potential involvement of appetite-regulating peptides including leptin, ghrelin, orexin and melanin concentrating hormone. These peptide signals interface with mesolimbic dopaminergic regions including the ventral tegmental area to modulate reactivity to cues associated with palatable foods. Thus, a novel target for anti-obesity therapeutics is to reduce non-homeostatic, reward driven eating behaviour, which can be triggered by environmental cues associated with highly palatable, fat and sugar rich foods.

Effects of a microinjection of morphine into the amygdala on the acquisition and expression of conditioned fear and hypoalgesia in rats.

A unilateral microinjection of morphine into the amygdala impaired the acquisition of fear and hypoalgesic responses in rats exposed to a heated floor in a hot-plate apparatus. This impairment was dose dependent, receptor specific, and not observed in rats microinjected with morphine into the caudal basolateral amygdala. A microinjection of morphine into the amygdala reduced the expression of fear responses and of naloxone-sensitive hypoalgesic responses, but did not reduce the expression of naloxone-insensitive hypoalgesic responses. The results document an involvement of opioidergic mechanisms in the amygdala in learned danger and of the amygdala in the control of opioid hypoalgesic responses. They also suggest that learned danger can activate antinociceptive mechanisms independently of the amygdala.

Effects of benzodiazepine microinjection into the amygdala or periaqueductal gray on the expression of conditioned fear and hypoalgesia in rats.

Four experiments studied the effects of an intracranial microinjection of a benzodiazepine (midazolam) on the expression of conditioned fear (measured as passive avoidance) and conditioned hypoalgesia in rats. Unilateral microinjection of midazolam into the basolateral amygdala reduced both hypoalgesic and avoidance responses, whereas unilateral microinjection of midazolam into the ventrolateral region of the periaqueductal gray (vlPAG) reduced the hypoalgesic response but not the avoidance response. The results are discussed in terms of gamma-aminobutyric acid-ergic inhibition of antinociceptive mechanisms in the vlPAG and of the activation of these mechanisms by amygdala-based fear processes.

Effects of systemic, intracerebral, or intrathecal administration of an N-methyl-D-aspartate receptor antagonist on associative morphine analgesic tolerance and hyperalgesia in rats.

A flavor paired with morphine shifted to the right the function relating morphine dose to tail-flick latencies and provoked hyperalgesic responses when rats were tested in the absence of morphine. These learned increases in nociceptive sensitivity were not mediated by alterations in tail-skin temperature. Microinjection of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP-5) into the lateral ventricle reversed the hyperalgesic responses but spared the tolerance to morphine analgesia. By contrast, systemic administration of the noncompetitive NMDA receptor antagonist MK-801 or intrathecal infusion of AP-5 reversed the hyperalgesic responses as well as the tolerance to morphine analgesia. The results demonstrate that associatively mediated tolerance to morphine analgesia can co-occur with hyperalgesic responses and are discussed relative to learned activation of endogenous pronociceptive mechanisms.

An infusion of bupivacaine into the nucleus accumbens disrupts the acquisition but not the expression of contextual fear conditioning.

An infusion of the local anesthetic bupivacaine into the nucleus accumbens (Acb) impaired the acquisition but not the expression of fear responses (freezing) to a shocked context but spared both the acquisition and expression of these responses to an auditory conditioned stimulus (CS) paired with the shock. In contrast, an infusion of bupivacaine into the amygdala impaired the acquisition and the expression of fear responses to both the CS and the context. The results demonstrate a critical role for the Acb in the acquisition but not the expression of contextual fear conditioning and are consistent with the view that this structure is involved in the processes by which rats represent a context.

Benzodiazepine-induced amnesia in rats: reinstatement of conditioned performance by noxious stimulation on test.

A benzodiazepine (midazolam), injected either systemically or directly into the basolateral amygdala (BLA), differentially affected the acquisition of fear responses to a shocked context: Administration of the drug before conditioning impaired subsequent freezing to the context but spared analgesic responses in rats tested there for sensitivity to formalin pain. Moreover, the pain test not only revealed evidence for analgesic responses but also served to reinstate conditioned freezing that was otherwise absent in rats conditioned under midazolam. The results were interpreted as showing that the presence of noxious stimulation on test serves either (a) to assist in retrieval of the context-shock association whose storage had been modified by midazolam's action in the BLA, or (b) to enable performance of the context-shock association whose affective properties had been blocked by midazolam's action in the BLA.

A peripheral, intracerebral, or intrathecal administration of an opioid receptor antagonist blocks illness-induced hyperalgesia in the rat.

We used the tail-flick response of rats to study the role of opioid receptors in illness-induced hyperalgesia. An intraperitoneal injection of lithium chloride (LiCl) produced hyperalgesia that was blocked in a dose-dependent manner by subcutaneous injection of the opioid antagonist naloxone. Neither hyperalgesia nor its blockade by naloxone were due to variations in tail-skin temperature induced by LiCl. Hyperalgesia was also blocked when opioid receptor antagonism was restricted to (a) the periphery, by intraperitoneal administration of the quaternary opioid receptor antagonist naloxone methiodide; (b) the brain, by intracerebroventricular microinjection of naloxone; or (c) the spinal cord, by intrathecal microinjection of naloxone. These results document a pain facilitatory role of opioid receptors in both the peripheral and central nervous systems and are discussed with reference to their analgesic and motivational functions.

Microinjection of morphine into the nucleus accumbens impairs contextual learning in rats.

A unilateral microinjection of morphine into the amygdala impaired fear conditioning to both a conditioned stimulus (CS) paired with shock and the context where shock occurred, whereas a microinjection of morphine into the nucleus accumbens (NA) spared fear conditioning to the CS but impaired, in a dose-dependent and receptor-specific manner, fear conditioning to the context. Morphine in the NA also spared extinction and latent inhibition of a CS but abolished the context specificity of these effects and eliminated the increase in discriminability that results from preexposure to a to-be-shocked context. The results identify a role for the NA in the processes by which rats learn about a context and are discussed in terms of an opioid disruption of either within-context associations or of attentional processes that contribute to such associations.

Fos expression in the spinal cord is suppressed in rats displaying conditioned hypoalgesia.

Two experiments used c-fos expression as a marker of spinal nociceptive processing to study the neural correlates of hypoalgesic responses to conditioned stimuli (CSs) paired with an aversive event. Immunoreactive (ir) neuronal labeling of Fos, the nuclear protein encoded by the c-fos gene, was examined in the spinal cords of rats killed 2 hr after injection of dilute formalin into a hind paw. Compared with control rats either not conditioned or conditioned in one environment but tested elsewhere, there were significantly fewer Fos-ir neurons in the spinal cords of rats displaying hypoalgesic responses when tested in the presence of aversive CSs. Naloxone abolished hypoalgesic responses and reinstated spinal Fos expression, indicating that aversive CSs activated opioid-based antinociceptive mechanisms. The results confirm that aversive CSs produce hypoalgesia by inhibiting the transmission of ascending nociceptive information.

Effects of midazolam and naloxone in rats tested for sensitivity/reactivity to formalin pain in a familiar, novel or aversively conditioned environment.

Rats tested for sensitivity/reactivity to formalin-induced pain in either an aversively conditioned or a novel environment displayed immediate but transient hypoalgesic responses that were insensitive to either a benzodiazepine (midazolam) or an opioid antagonist (naloxone). Exposure to the aversively conditioned, but not to the novel environment also provoked a more enduring hypoalgesic response that was abolished by either midazolam or naloxone. The results were taken to mean that fear is sufficient but not necessary for the production of hypoalgesic responses to environmental stimuli.

Contextual control over the expression of fear in rats conditioned under a benzodiazepine.

RATIONALE: Benzodiazepines disrupt fear conditioning, but this disruption is context-specific; if rats have been conditioned under a benzodiazepine, their fear is recovered if they are tested in a different context. The present experiments investigated how the conditioning context controls fear in rats conditioned under a benzodiazepine. OBJECTIVES: The experiments had three aims: (1) to replicate the finding that fear is recovered when rats are tested in a different context, (2) to test whether the conditioning context reduces fear generally or only for the specific stimulus conditioned in that context and (3) to test whether latent inhibition of the conditioning context reduces its control over fear. METHODS: Rats were injected with the benzodiazepine midazolam (1.25 mg/kg) or saline and exposed to a conditioned stimulus (CS) and shock in a distinctive chamber. Latent inhibition of the chamber was induced by extensively preexposing the rats to the chamber. The day after conditioning, fear was assessed by presenting the CS while rats were in either the conditioning chamber or a different chamber. RESULTS: The midazolam-induced reduction of fear was reversed (i.e. fear was partially recovered) if rats were tested in the different context, and was completely prevented if the conditioning context had been latently inhibited. These two effects were not additive since, when the conditioning context had been latently inhibited, rats showed less fear in the different context than in the conditioning context. CONCLUSIONS: We argue that midazolam does not disrupt conditioning, but imbues the conditioning context with control over retrieval of the CS-shock association. In this regard, the effects of midazolam closely parallel those of extinction.

Effects of hypophysectomy and adrenalectomy on naloxone-induced analgesia.

Experiment 1 demonstrated that pairings of the opiate antagonist, naloxone, with a heated floor came to induce analgesia, as indexed by the latencies with which rats licked their paws. This analgesia appears to be neurally mediated because it is unaffected by either hypophysectomy (experiment 2) or adrenalectomy (experiment 3). However, there was evidence for a pituitary involvement, as its removal potentiated the analgesic effect accruing from naloxone-stressor pairings.