Limbic progesterone receptors regulate spatial memory.

Progesterone and its receptors (PRs) participate in mating and reproduction, but their role in spatial declarative memory is not understood. Male mice expressed PRs, predominately in excitatory neurons, in brain regions that support spatial memory, such as the hippocampus and entorhinal cortex (EC). Furthermore, segesterone, a specific PR agonist, activates neurons in both the EC and hippocampus. We assessed the contribution of PRs in promoting spatial and non-spatial cognitive learning in male mice by examining the performance of mice lacking this receptor (PRKO), in novel object recognition, object placement, Y-maze alternation, and Morris-Water Maze (MWM) tasks. In the recognition test, the PRKO mice preferred the familiar object over the novel object. A similar preference for the familiar object was also seen following the EC-specific deletion of PRs. PRKO mice were also unable to recognize the change in object position. We confirmed deficits in spatial memory of PRKO mice by testing them on the Y-maze forced alternation and MWM tasks; PR deletion affected animal's performance in both these tasks. In contrast to spatial tasks, PR removal did not alter the response to fear conditioning. These studies provide novel insights into the role of PRs in facilitating spatial, declarative memory in males, which may help with finding reproductive partners.

Suppression of Neuroinflammation Attenuates Persistent Cognitive and Neurogenic Deficits in a Rat Model of Cardiopulmonary Bypass.

Post-operative cognitive dysfunction (POCD) can be a serious surgical complication, and patients undergoing cardiac procedures are at particular risk for POCD. This study examined the effect of blocking neuroinflammation on behavioral and neurogenic deficits produced in a rat model of cardiopulmonary bypass (CPB). Minocycline, a drug with established anti-inflammatory activity, or saline was administered daily for 30 days post-CPB. Treatment with minocycline reduced the number of activated microglia/macrophages observed in the dentate gyrus of the hippocampus at 6 months post-CPB, consistent with an anti-inflammatory action in this CPB model. Behavioral testing was conducted at 6 months post-CPB utilizing a win-shift task on an 8-arm radial maze. Minocycline-treated animals performed significantly better than saline-treated animals on this task after CPB. In addition, the CPB-induced reduction in adult neurogenesis was attenuated in the minocycline-treated animals. Together, these findings indicate that suppressing neuroinflammation during the early post-surgical phase can limit long-term deficits in both behavioral and neurogenic outcomes after CPB.

Mechanism of seizure-induced retrograde amnesia.

Seizures cause retrograde amnesia, but underlying mechanisms are poorly understood. We tested whether seizure activated neuronal circuits overlap with spatial memory engram and whether seizures saturate LTP in engram cells. A seizure caused retrograde amnesia for spatial memory task. Spatial learning and a seizure caused cFos expression and synaptic plasticity overlapping set of neurons in the CA1 of the hippocampus. Recordings from learning-labeled CA1 pyramidal neurons showed potentiated synapses. Seizure-tagged neurons were also more excitable with larger rectifying excitatory postsynaptic currents than surrounding unlabeled neurons. These neurons had enlarged dendritic spines and saturated LTP. A seizure immediately after learning, reset the memory engram. Seizures cause retrograde amnesia through shared ensembles and mechanisms.

Contributions of the Nucleus Accumbens Shell in Mediating the Enhancement in Memory Following Noradrenergic Activation of Either the Amygdala or Hippocampus.

The nucleus accumbens shell is a site of converging inputs during memory processing for emotional events. The accumbens receives input from the nucleus of the solitary tract (NTS) regarding changes in peripheral autonomic functioning following emotional arousal. The shell also receives input from the amygdala and hippocampus regarding affective and contextual attributes of new learning experiences. The successful encoding of affect or context is facilitated by activating noradrenergic systems in either the amygdala or hippocampus. Recent findings indicate that memory enhancement produced by activating NTS neurons, is attenuated by suppressing accumbens functioning after learning. This finding illustrates the significance of the shell in integrating information from the periphery to modulate memory for arousing events. However, it is not known if the accumbens shell plays an equally important role in consolidating information that is initially processed in the amygdala and hippocampus. The present study determined if the convergence of inputs from these limbic regions within the nucleus accumbens contributes to successful encoding of emotional events into memory. Male Sprague-Dawley rats received bilateral cannula implants 2 mm above the accumbens shell and a second bilateral implant 2 mm above either the amygdala or hippocampus. The subjects were trained for 6 days to drink from a water spout. On day 7, a 0.35 mA footshock was initiated as the rat approached the spout and was terminated once the rat escaped into a white compartment. Subjects were then given intra-amygdala or hippocampal infusions of PBS or a dose of norepinephrine (0.2 µg) previously shown to enhance memory. Later, all subjects were given intra-accumbens infusion of muscimol to functionally inactivate the shell. Muscimol inactivation of the accumbens shell was delayed to allow sufficient time for norepinephrine to activate intracellular cascades that lead to long-term synaptic modifications involved in forming new memories. Results show that memory improvement produced by infusing norepinephrine in either the amygdala or hippocampus is attenuated by interrupting neuronal activity in the shell 1 or 7 7 h following amygdala or hippocampus activation. These findings suggest that the accumbens shell plays an integral role modulating information initially processed by the amygdala and hippocampus following exposure to emotionally arousing events. Additionally, results demonstrate that the accumbens is involved in the long-term consolidation processes lasting over 7 h.

Differential activation of amygdala Arc expression by positive and negatively valenced emotional learning conditions.

Norepinephrine is released in the amygdala following negatively arousing learning conditions. This event initiates a cascade of changes including the transcription of activity-regulated cytoskeleton-associated protein (Arc) expression, an early-immediate gene associated with memory encoding. Recent evidence suggests that the valence of emotionally laden encounters may generate lateralized, as opposed to symmetric release of this transmitter in the right or left amygdala. It is currently not clear if valence-induced patterns of selective norepinephrine output across hemispheres are also reproduced in downstream pathways of cellular signaling necessary for memory formation. This question was addressed by determining if Arc expression is differentially distributed across the right and left amygdala following exposure to positively or negatively valenced learning conditions respectively. Male Sprague Dawley rats were randomly assigned to groups exposed to the Homecage only, five auditory tones only, or five auditory tones paired with footshock (0.35 mA) during Pavlovian fear conditioning. Western blot analysis revealed that Arc expression in the right amygdala was elevated significantly above that observed in the left amygdala 60 and 90 min following fear conditioning. Similarly, subjects exposed to a negatively valenced outcome consisting of an unexpected reduction in food rewards showed a greater level of Arc expression in only the right, but not left basolateral amygdala. Presenting a positively valenced event involving an unexpected increase in food reward magnitude following bar pressing, resulted in significantly greater Arc expression in the left, but not right basolateral amygdala (p < 0.01). These findings indicate that the valence of emotionally arousing learning conditions is reflected at later stages of synaptic plasticity involving the transcription of immediate early genes such as Arc.

Contribution of serotonin type 3 receptors in the successful extinction of cued or contextual fear conditioned responses: interactions with GABAergic signaling.

The changes in serotonin type 3 (5-HT) receptor activity influence memory and emotional regulation,the two essential components underlying the successful extinction of conditioned fear. These studies determined if blocking 5-HT3 receptors with granisetron influences the extinction of fear in male Sprague-Dawley rats. In Experiment 1, preextinction granisetron (0.5 or 1.0 mg/kg intraperitoneally) d~d not affect cued extinction learning but enhanced memory for the extinction session on retention test given 24 and 48 h later. In Experiment 2, granisetron injections given on days 1 and 4 during 6 days of extinction training reduced fear produced by contextual fear conditioning. Experiments 3 and 4 examined if 5-HT3 antagonists influence extinction memory by interactions with y-aminobutyric acid (GABA). The expression of the GABA receptor clustering protein gephyrin was significantly A elevated in the amygdala after cued fear extinction training and a subsequent extinction retention test given 24 h later. Gephyrin expression in the hippocampus but not in the amygdala or the medial prefrontal cortex was significantly reduced after contextual fear extinction sessions given 1 or 5 days after training. The current studies reveal the beneficial effects of 5-HT3 receptor activity in improving new learning associated with extinction of fearful memories and suggest that these actions could be mediated through influences on central GABAergic systems.

Interacting brain systems modulate memory consolidation.

Emotional arousal influences the consolidation of long-term memory. This review discusses experimental approaches and relevant findings that provide the foundation for current understanding of coordinated interactions between arousal activated peripheral hormones and the brain processes that modulate memory formation. Rewarding or aversive experiences release the stress hormones epinephrine (adrenalin) and glucocorticoids from the adrenal glands into the bloodstream. The effect of these hormones on memory consolidation depends upon binding of norepinephrine to beta-adrenergic receptors in the basolateral complex of the amygdala (BLA). Much evidence indicates that the stress hormones influence release of norepinephrine in the BLA through peripheral actions on the vagus nerve which stimulates, through polysynaptic connections, cells of the locus coeruleus to release norepinephrine. The BLA influences memory storage by actions on synapses, distributed throughout the brain, that are engaged in sensory and cognitive processing at the time of amygdala activation. The implications of the activation of these stress-activated memory processes are discussed in relation to stress-related memory disorders.

Interactions between brainstem noradrenergic neurons and the nucleus accumbens shell in modulating memory for emotionally arousing events.

The nucleus accumbens shell (NAC) receives axons containing dopamine-ß-hydroxylase that originate from brainstem neurons in the nucleus of the solitary tract (NTS). Recent findings show that memory enhancement produced by stimulating NTS neurons after learning may involve interactions with the NAC. However, it is unclear whether these mnemonic effects are mediated by norepinephrine (NE) release from NTS terminals onto NAC neurons. The present studies approached this question by examining the contribution of NAC α-noradrenergic receptors in mediating this effect and assessed whether glutamatergic activation of the NTS alters NE concentrations in the NAC. Rats were trained for 6 d to drink from a water spout located at the end of an inhibitory avoidance chamber. On day 7, a 0.35-mA footshock was initiated once the rat approached the spout and remained active until it escaped into the neutral compartment. Blockade of α-noradrenergic receptors in the NAC with phentolamine (0.5 µg/0.5 µL) attenuated memory enhancement produced by glutamatergic (50 ng/0.5 µL) infusion on NTS neurons (P < 0.01). Experiment 2 used in vivo microdialysis to assess whether glutamate activation of NTS alters NAC NE concentrations. NE levels were unchanged by NTS infusion of phosphate-buffered saline (PBS) or low dose glutamate (50 ng/0.5 µL) but elevated significantly (P < 0.05) by combining the same dose with the footshock (0.35 mA, 2 sec) given in Study 1 or infusion of (100 ng/0.5 µL) glutamate alone. Findings demonstrate that NE released from NTS terminals enhances representations in memory by acting on α-noradrenergic receptors within the NAC.

Valence dependent asymmetric release of norepinephrine in the basolateral amygdala.

Emerging evidence from literature on humans suggests the valence of emotionally laden environmental stimuli may dictate whether amygdala activation is greater in one hemisphere relative to the contralateral side. However, only a paucity of animal studies attempt to unravel the mechanisms underlying the selective, valence-dependent initiation of activity in the amygdala of opposing hemispheres. The present studies assessed whether exposure to positive or negative appetitive conditions in an operant learning task differentially impacts norepinephrine activation of the left versus right amygdala, respectively. Dialysate samples of norepinephrine were collected from the basolateral nucleus of male Sprague-Dawley rats. Fluctuations in norepinephrine activity were sampled during training conditions involving an abrupt shift (i.e., increase or decrease) in the magnitude of food rewards normally provided for bar press responses. In a subsequent study, dialysate samples of norepinephrine were collected before, during, and after presentation of a negatively valenced stimulus involving footshock delivery during Pavlovian fear conditioning. Norepinephrine concentrations in the left but not right basolateral amygdala were elevated in groups presented with a positive experience of an unexpected increase in food reward after bar pressing (p < .01), relative to respective controls. In contrast, exposure to negatively valenced events involving a reduction in expected food rewards after bar pressing or presentation of a footshock during fear conditioning produced significant increases in norepinephrine output sampled from only the right but not left basolateral amygdala. These findings demonstrate that the valence of a learning event is selective in initiating asymmetric activation of the basolateral amygdala.

Novelty-induced arousal enhances memory for cued classical fear conditioning: interactions between peripheral adrenergic and brainstem glutamatergic systems.

Exposure to novel contexts produce heightened states of arousal and biochemical changes in the brain to consolidate memory. However, processes permitting simple exposure to unfamiliar contexts to elevate sympathetic output and to improve memory are poorly understood. This shortcoming was addressed by examining how novelty-induced changes in peripheral and/or central arousal modulates memory for Pavlovian fear conditioning. Male rats were either exposed to the conditioning chamber for 5-min or given no exposure 24 h before conditioning with five tone-shock (0.35 mA) pairings. Retention was assessed 48 h later in a different context. Non-pre-exposed animals exhibited significantly greater freezing during conditioned stimulus (CS) presentations than did pre-exposed animals (P < 0.05). The improvement in retention produced by novelty was attenuated by pretraining a blockade of peripheral beta-adrenergic receptors with sotalol (6 mg/kg, i.p.). Study 2 revealed that novelty-induced increases in peripheral autonomic output are conveyed to the brain by visceral afferents that synapse upon brainstem neurons in the nucleus tractus solitarius (NTS). Blocking AMPA receptor activity in the NTS with CNQX (1.0 microg) significantly reduced freezing to the CS in non-pre-exposed animals (P < 0.01). Study 3 showed that elevating epinephrine levels in habituated animals influences learning through mechanisms similar to those produced by novelty-induced arousal. Pre-exposed animals given epinephrine (0.1 mg/kg) froze significantly more than saline controls (P < 0.01), and this effect was attenuated by intra-NTS infusion of CNQX. The findings demonstrate that novelty-induced arousal or increasing sympathetic activity with epinephrine in pre-exposed animals enhances memory through adrenergic mechanisms initiated in the periphery and transmitted centrally via the vagus/NTS complex.

Functional interactions between the nucleus tractus solitarius (NTS) and nucleus accumbens shell in modulating memory for arousing experiences.

The shell division of the nucleus accumbens receives noradrenergic input from neurons in the nucleus of the solitary tract (NTS) that transmit information regarding fluctuations in peripheral hormonal and autonomic activity. Accumbens shell neurons also receive converging inputs from limbic areas such as the hippocampus and amygdala that process newly acquired information. However, few studies have explored whether peripheral information regarding changes in emotional arousal contributes to memory processing in the accumbens. The beneficial effects on memory produced by emotional arousal and the corresponding activation of NTS neurons may be mediated through influences on neuronal activity in the accumbens shell during memory encoding. To explore this putative relationship, Experiment 1 examined interactions between the NTS and the accumbens shell in modulating memory for responses acquired after footshock training in a water-motivated inhibitory avoidance task. Memory for the noxious shock was significantly improved by posttraining excitation of noradrenergic NTS neurons. The enhanced retention produced by activating NTS neurons was attenuated by suppressing neuronal activity in the accumbens shell with bupivacaine (0.25%/0.5 microl). Experiment 2 examined the direct involvement of accumbens shell noradrenergic activation in the modulation of memory for psychologically arousing events such as a reduction in perceived reward value. Noradrenergic activation of the accumbens shell with phenylephrine (1.0 microg/0.5 microl) produced an enhancement in memory for the frustrating experience relative to control injections as evidenced by runway performance on an extended seven-day retention test. These findings demonstrate a functional relationship between NTS neurons and the accumbens shell in modulating memory following physiological arousal and identifies a role of norepinephrine in modulating synaptic activity in the accumbens shell to facilitate this process.

Peripheral arousal-related hormones modulate norepinephrine release in the hippocampus via influences on brainstem nuclei.

The peripheral hormone epinephrine (EPI) is known to modulate memory for arousing experiences. The mnemonic effects of EPI are attributed almost exclusively to actions on amygdala noradrenergic (NE) systems. EPI also increases neuronal activity in the locus coeruleus (LC), the primary source of NE to other limbic structures that process memory such as the hippocampus (HIPP). The actions of EPI on the LC suggest that its mnemonic properties may also be mediated by influencing NE output in the HIPP. To test this hypothesis, dialysate levels of NE were collected from the HIPP of male rats given an i.p. injection of saline that was followed 100 min later by i.p. EPI (0.3 mg/kg). NE levels sampled 20 min after EPI injection were significantly larger than baseline and continued to show significant peaks for 60 min. Experiment 2 examined whether peripheral signals initiated by EPI influence the HIPP via the nucleus of the solitary tract (NTS) by inactivating this nucleus with lidocaine prior to EPI injection. EPI injection did not increase NE levels sampled from the HIPP of rats given lidocaine into the NTS. EPI injection did produce significant elevations in HIPP NE levels in animals given a control solution into the NTS prior to the EPI injection. These findings indicate that the mnemonic effects of EPI reported in a wide range of learning conditions may not be mediated solely by NE release in the amygdala, but may also involve coactivation of the HIPP NE system.

Enhancement of noradrenergic neurotransmission in the nucleus of the solitary tract modulates memory storage processes.

These studies examined whether posttraining activation of alpha1-noradrenergic receptors in the nucleus tractus solitarius (NTS) influences neural processes that are involved in encoding information into memory. Different groups of male Sprague-Dawley rats were trained in two separate learning tasks. In experiment 1, rats were given either a control solution or the alpha1-noradrenergic agonist phenylephrine (0.5, 1.0, 5.0, or 10 microg/0.5 microl) directly into the NTS immediately after they were given a footshock (0.35 mA, 0.5 s) in the dark compartment of an inhibitory apparatus. In a retention test given 48 h later, groups that received either 5.0 or 10.0 microg of phenylephrine avoided the dark compartment for a significantly longer period of time than the PBS control group (P<0.05 and P<0.01, respectively). In experiment 2, identical doses of phenylephrine were infused in the NTS following footshock delivery in one alley of a Y-maze. Animals given either 1.0 or 5.0 microg of phenylephrine performed significantly better than PBS controls on several different measures that served as indices of retention. The results indicate that activation of alpha1-noradrenergic receptors in the NTS plays a critical role in the transmission of signals from the periphery to brain systems that process memory for emotionally significant experiences.

Glutamatergic transmission in the nucleus of the solitary tract modulates memory through influences on amygdala noradrenergic systems.

The authors examined whether glutamate release from the vagus nerve onto the nucleus of the solitary tract (NTS) is one mechanism by which the vagus influences memory and neural activity in limbic structures. Rats trained to drink from a spout were given a footshock (0.35 mA) on Day 5 after approaching the spout. Phosphate-buffered saline or 5.0, 50.0, or 100.0 nmol/0.5 microl glutamate was then infused into the NTS. Glutamate (5.0 or 50.0 nmol) significantly enhanced memory on the retention test. In Experiment 2, this effect was attenuated by blocking noradrenergic receptors in the amygdala with propranolol (0.3 microg/0.5 microl). Experiment 3 used in vivo microdialysis to determine whether footshock plus glutamate (50.0 nmol) alters noradrenergic output in the amygdala. These treatments caused a significant and long-lasting increase in amygdala noradrenergic concentrations. The results indicate that glutamate may be one transmitter that conveys the effects of vagal activation on brain systems that process memory.