Activation of Fyn tyrosine kinase in the mouse dorsal hippocampus is essential for contextual fear conditioning

Fyn-tyrosine-kinase-deficient mice exhibit defects in the Morris water maze test and long-term potentiation (LTP) induction in the hippocampus, and given that LTP has been postulated as the neural basis for memory formation, Fyn may be required for hippocampus-dependent memory formation. However, how Fyn is involved in the process of memory formation is unclear. To investigate the role of Fyn in hippocampal memory formation, we first tested the behavior of Fyn-deficient mice by contextual fear conditioning. A mouse was placed in a context and a foot shock was delivered, so that the mouse associated the context with the shock. We found that the freezing response of Fyn-deficient mice to the context was impaired at 24 h after conditioning. We then measured freezing at 1 h after conditioning, and found that their short-term contextual fear memory was also impaired. We used Western blotting to examine the mode of Fyn activation in dorsal hippocampal tissue following contextual fear conditioning. Fyn activation peaked as early as 5–10 min after contextual fear conditioning and persisted for at least 40 min. Concomitant increases in tyrosine phosphorylation of several proteins, including NR2B, were also observed, but no increases in tyrosine phosphorylation were observed in Fyn-deficient mice. Thus, both short-term and long-term (24-h) contextual fear memory were impaired in Fyn-deficient mice, and Fyn activation in the dorsal hippocampus transiently increased after contextual fear conditioning. These findings strongly suggest that activation of the Fyn signaling pathway is involved in hippocampus-dependent formation of contextual fear memory.     

Noradrenergic changes and memory loss in aged mice

The present paper addresses the question of whether a decline of central noradrenergic activity is associated with aging and memory loss in mice. Receptor binding techniques were utilized to compare alpha 2-adrenoceptor density in the brains of aged and young mice. Using [3H]rauwolscine, a selective alpha 2-adrenoceptor antagonist, two membrane binding sites were identified which were differentially affected by age. Whereas the density of high-affinity binding sites was unchanged in aged brain as compared to young controls, there was a significant decrease in the number of low-affinity sites. In a separate study, animals were tested for performance on a step-through inhibitory avoidance task, prior to sacrifice and morphological analysis of the brainstem noradrenergic nucleus locus coeruleus (LC). Aged mice exhibited a significant decrease in task retention as compared to young controls; a small, though non-significant, decline was also observed in the numbers of cells within LC. While young mice exhibited low within-group variance, individual aged animals differed greatly in both LC cell number and behavioral performance. Within the aged population, there was a highly significant correlation between the extent of LC cell loss and the degree of memory impairment. These results provide further evidence for an age-related decline in central noradrenergic function which may contribute to an associated memory loss.     

Axonal projection‐specific differences in somatodendritic α2 autoreceptor function in locus coeruleus neurons

The locus coeruleus (LC) contains the majority of central noradrenergic neurons sending wide projections throughout the entire CNS. The LC is considered to be essential for multiple key brain functions including arousal, attention and adaptive stress responses as well as higher cognitive functions and memory. Electrophysiological studies of LC neurons have identified several characteristic functional features such as low‐frequency pacemaker activity with broad action potentials, transient high‐frequency burst discharges in response to salient stimuli and an apparently homogeneous inhibition of firing by activation of somatodendritic α2 autoreceptors (α2AR). While stress‐mediated plasticity of the α2AR response has been described, it is currently unclear whether different LC neurons projecting to distinct axonal targets display differences in α2AR function. Using fluorescent beads‐mediated retrograde tracing in adult C57Bl6/N mice, we compared the anatomical distributions and functional in vitro properties of identified LC neurons projecting either to medial prefrontal cortex, hippocampus or cerebellum. The functional in vitro analysis of LC neurons confirmed their mostly uniform functional properties regarding action potential generation and pacemaker firing. However, we identified significant differences in tonic and evoked α2AR‐mediated responses. While hippocampal‐projecting LC neurons were partially inhibited by endogenous levels of norepinephrine and almost completely silenced by application of saturating concentrations of the α2 agonist clonidine, prefrontal‐projecting LC neurons were not affected by endogenous levels of norepinephrine and only partially inhibited by saturating concentrations of clonidine. Thus, we identified a limited α2AR control of electrical activity for prefrontal‐projecting LC neurons indicative of functional heterogeneity in the LC‐noradrenergic system.     

Dorsal hippocampal administration of triiodothyronine enhances long-term memory for trace cued and delay contextual fear conditioning in rats

Thyroid hormones play critical roles in brain maturation and cognitive functions. The present study investigated the role of thyroid hormone in emotional learning and memory using trace and delay contextual and cued fear conditioning tasks, respectively. Rats were administered triiodothyronine (T3) into the dorsal hippocampal area 10 min before training or immediately after training, and were scored for freezing behaviour in the same context and in a novel context with and without an auditory cue that had been paired previously with an aversive stimulus, a foot shock. Rats administered T3 before and after training both exhibited significantly increased long-term fear memory in the trace cued and the delay contextual fear conditioning procedures compared to their control groups. The T3-administered rats were not significantly different from their respective controls on the acquisition and short-term fear memory in the trace and delay fear conditioning tasks. No significant difference on long-term trace contextual and delay cued fear memory, respectively, was found. These results indicate that the observed T3-induced enhancement of long-term contextual and cued fear memory was specific to the hippocampus-dependent conditioning tasks. These findings are the first to demonstrate that infusion of T3 into the dorsal hippocampus can improve performance on an emotional memory task.     

Inbred mouse strain differences in the establishment of long-term fear memory

Studies describing variations in fear-related memory in inbred mouse strains typically focus upon 24 h retention. As a consequence, little is known about strain differences in the establishment of longer lasting memories of aversive events. In the present study, male mice from the strains A/Ibg, AKR/J, BALB/cByJ, CBA/J, C3H/HeIbg, C57BL/6J, DBA/2J, LP/J, SJL/J and 129/SvevTac were tested 24 h, 14, or 60 days after contextual and auditory-cued fear conditioning. Consistent with previous data, 24 h after conditioning these strains exhibited substantial variation in levels of memory for the context and the auditory cue as measured by freezing scores. Sixty days after training, most strains exhibited some forgetting of the context and auditory cue, and again there was significant strain variation. Strain rankings at 60-day retention were similar to that at 24 h with a significant genetic correlation between freezing values for the two time periods. Fourteen days following training, nearly all strains exhibited generalized freezing, a behavioral phenotype originally observed in C57BL/6 but not DBA/2 mice. These data confirm that cognitive differences exist between several popular inbred mouse strains during 24 h contextual fear recall. In addition, they extend these differences into retention time frames longer than those typically used and reveal several unique learning profiles of mouse strains that may be useful in furthering our understanding of how memories are formed. Emotionally arousing situations are often recalled a great deal of time after an event. Therefore, a more complete picture of the biochemical and genetic underpinnings of learning and memory will benefit from studies using time points that assess time points beyond 24 h retention. The utility of the 14-day hyper responsiveness phenotype as a potential model for fear-related psychopathology is also discussed.     

Sex differences in the effect of cannabinoid type 1 receptor deletion on locus coeruleus‐norepinephrine neurons and corticotropin releasing factor‐mediated responses

Cannabinoids are capable of modulating mood, arousal, cognition and behavior, in part via their effects on the noradrenergic nucleus locus coeruleus (LC). Dysregulation of LC signaling and norepinephrine (NE) efflux in the medial prefrontal cortex (mPFC) can lead to the development of psychiatric disorders, and CB1r deletion results in alterations of α2‐ and β1‐adrenoceptors in the mPFC, suggestive of increased LC activity. To determine how CB1r deletion alters LC signaling, whole‐cell patch‐clamp electrophysiology was conducted in LC‐NE neurons of male and female wild type (WT) and CB1r‐knock out (KO) mice. CB1r deletion caused a significant increase in LC‐NE excitability and input resistance in male but not female mice when compared to WT. CB1r deletion also caused adaptations in several indices of noradrenergic function. CB1r/CB2r‐KO male mice had a significant increase in cortical NE levels and tyrosine hydroxylase and CRF levels in the LC compared to WT males. CB1r/CB2r‐KO female mice showed a significant increase in LC α2‐AR levels compared to WT females. To further probe actions of the endocannabinoid system as an anti‐stress neuromediator, the effect of CB1r deletion on CRF‐induced responses in the LC was investigated. The increase in LC‐NE excitability observed in male and female WT mice following CRF (300 nM) bath application was not observed in CB1r‐KO mice. These results indicate that cellular adaptations following CB1r deletion cause a disruption in LC‐NE signaling in males but not females, suggesting underlying sex differences in compensatory mechanisms in KO mice as well as basal endocannabinoid regulation of LC‐NE activity.     

Sex differences in fear memory and extinction of mice with forebrain‐specific disruption of the mineralocorticoid receptor

Previous studies showed that the mineralocorticoid receptor (MR) is needed for behavioral flexibility in a fear conditioning paradigm. Female mice with forebrain‐specific deletion of the MR gene (MR^CaMKCre) were unable to show extinction of contextual fear, and could not discriminate between cue and context fear unlike control mice. In the present study, male and female (MR^CaMKCre) mice and control littermates were used to study sex‐specific fear conditioning, memory performance and extinction. The fear conditioning paradigm assessed both context‐ and cue‐related fear within one experimental procedure. We observed that at the end of the conditioning all mice acquired the fear‐motivated response. During the first minutes of the memory test, both male and female MR^CaMKCre mice remembered and feared the context more than the control mice. Furthermore, female MR^CaMKCre mice were not able to extinguish this memory even on the second day of memory testing. The female mutants also could not discriminate between cue (more freezing) and context periods (less freezing). In contrast, male MR^CaMKCre mice and the controls showed extinction and were capable to discriminate, although the MR^CaMKCre mice needed more time before they started extinction. These findings further support the relevance of MR for behavioral flexibility and extinction of fear‐motivated behavior. In conclusion, the loss of MR in the forebrain results in large differences in emotional and cognitive behaviors between female and male mice, which suggests a role of this receptor in the female prevalence of stress‐ and anxiety‐regulated disorders.     

Prolonged fear responses in mice lacking dopamine D1 receptor

Dopamine is an important neurotransmitter involved in learning and memory including emotional memory. The involvement of dopamine in conditioned fear has been widely documented. However, little is known about the molecular mechanisms that underlie contextual fear conditioning and memory consolidation. To address this issue, we used dopamine D₁-deficient mice (D₁−/−) and their wild-type (D₁+/+) and heterozygote (D₁+/−) siblings to assess aversive learning and memory. We quantified two different aspects of fear responses to an environment where the mice have previously received unsignaled footshocks. Using one-trial step-through passive avoidance and conditioned freezing paradigms, mice were conditioned to receive mild inescapable footshocks then tested for acquisition, retention and extinction of conditioned fear responses 5 min after and up to 45–90 days post-training. No differences were observed among any of the genotypes in the acquisition of passive avoidance response or fear-induced freezing behavior. However, with extended testing, D₁−/− mice exhibited prolonged retention and delayed extinction of conditioned fear responses in both tasks, suggesting that D₁−/− mice are capable of acquiring aversive learning normally. These findings demonstrate that the dopamine D₁ receptor is not important for acquisition or consolidation of aversive learning and memory but has an important role in modulating the extinction of fear memory.     

Dorsal hippocampus involvement in delay fear conditioning depends upon the strength of the tone-footshock association

The hippocampus is important for the formation of spatial, contextual, and episodic memories. For instance, lesions of the dorsal hippocampus (DH) produce demonstrable deficits in contextual fear conditioning. By contrast, it is generally agreed that the DH is not important for conditioning to a discrete cue (such as a tone or light) that is paired with footshock in a temporally contiguous fashion (delay conditioning). There are, however, some reports of hippocampus involvement in delay conditioning. The present series of experiments was designed to assess the conditions under which the hippocampus-dependent component of delay fear conditioning performance may be revealed. Here, we manipulated the number of conditioning trials and the intensity of the footshock in order to vary the strength of conditioning. The results indicate that the DH contributes to freezing performance to a delay conditioned tone when the conditioning parameters are relatively weak (few trials or low footshock intensity), but not when strong parameters are used. The results are discussed in terms of two parallel memory systems: a direct tone-footshock association that is independent of the hippocampus and a hippocampus-dependent memory for the conditioning session.     

Cellular imaging with zif268 expression in the rat nucleus accumbens and frontal cortex further dissociates the neural pathways activated following the retrieval of contextual and cued fear memory

Quantitative in situ hybridization revealed that the expression of the plasticity-associated gene zif268 was increased in specific regions of the rat frontal cortex and nucleus accumbens following fear memory retrieval. Increased expression of zif268 was observed in neurons in the core of the nucleus accumbens during the retrieval of contextual and discrete cued fear associations. In contrast, zif268 expression was additionally induced in neurons of the nucleus accumbens shell and the anterior cingulate cortex during the retrieval of contextual but not cued fear memories. No changes in the expression of this gene were seen in the ventral medial prefrontal cortex or ventral and lateral regions of the orbitofrontal cortex that were correlated specifically with the retrieval of fear memory. These experiments demonstrate the specific and dissociable activation of limbic cortical-ventral striatal regions that accompanies cued and contextual fear. These data, together with those previously published by our laboratory (Hall, J., Thomas, K.L. & Everitt, B.J. (2001) J. Neurosci., 21, 2186-2193), suggest that retrieval of contextual fear memories activates a wider limbic cortical-ventral striatal neural circuitry than does retrieval of cued fear memories. Moreover, the expression of zif268 may contribute to plasticity and reconsolidation of fear memory in these dissociable pathways.     

Prolonged fear responses in mice lacking dopamine D1 receptor

Dopamine is an important neurotransmitter involved in learning and memory including emotional memory. The involvement of dopamine in conditioned fear has been widely documented. However, little is known about the molecular mechanisms that underlie contextual fear conditioning and memory consolidation. To address this issue, we used dopamine D1-deficient mice (D1-/-) and their wild-type (D1+/+) and heterozygote (D1+/-) siblings to assess aversive learning and memory. We quantified two different aspects of fear responses to an environment where the mice have previously received unsignaled footshocks. Using one-trial step-through passive avoidance and conditioned freezing paradigms, mice were conditioned to receive mild inescapable footshocks then tested for acquisition, retention and extinction of conditioned fear responses 5 min after and up to 45-90 days post-training. No differences were observed among any of the genotypes in the acquisition of passive avoidance response or fear-induced freezing behavior. However, with extended testing, D1-/- mice exhibited prolonged retention and delayed extinction of conditioned fear responses in both tasks, suggesting that D1-/- mice are capable of acquiring aversive learning normally. These findings demonstrate that the dopamine D1 receptor is not important for acquisition or consolidation of aversive learning and memory but has an important role in modulating the extinction of fear memory.     

Contextual learning increases dendrite complexity and EphrinB2 levels in hippocampal mouse neurons

Although the role of hippocampus in memory processing is well assessed, an association of experience-dependent behavioural modifications with hippocampal neuron morphological and biochemical changes deserves further characterisation. Here, we present evidence of dendritic alterations together with rapid accumulation of EphrinB2, a factor known to influence cell plasticity, in pyramidal neurons of the CA1 area of mouse hippocampus, during the formation of recent contextual fear memory. Male C57BL/6N mice exhibited a robust fear response 24 h after contextual and cued fear conditioning. At this time and in the absence of the memory test, conditioned mice showed morphological alterations in hippocampal and lateral amygdala neurons. Western blot analysis of extracts from conditioned but not pseudoconditioned or naive mice showed a specific increase in the amount of EphrinB2 in the hippocampus but not the cortex. However, levels of EphA4 receptor, known to interact trans-synaptically with EphrinB2, did not change upon conditioning in extracts from the same structures. Finally, immunohistochemical analysis of the hippocampus and amygdala of conditioned mice showed increased levels of EphrinB2 in pyramidal neurons of the CA1 area, when compared to pseudoconditioned and control mice. Such increase was not observed in other hippocampal areas or the amygdala. These results suggest that rapid accumulation of EphrinB2 in hippocampal CA1 neurons is involved in the behavioural and cellular modifications induced by contextual fear conditioning. A similar mechanism does not appear to occur in lateral amygdala neurons, in spite of the robust behavioural and cellular modifications induced in such structure by cued fear conditioning.     

Temporal factors control hippocampal contributions to fear renewal after extinction

Fear can be extinguished by repeated exposure to a cue that signals threat. However, extinction does not erase fear, as an extinguished cue presented in a context distinct from that of extinction results in renewed fear of that cue. The hippocampus, which is involved in the formation of contextual representations, is a natural candidate structure for investigations into the neural circuitry underlying fear renewal. Thus far, studies examining the necessity of the hippocampus for fear renewal have produced mixed results. We isolated the conditions under which the hippocampus may be required for renewal. Rats received lesions of the dorsal hippocampus either prior to tone fear conditioning or following extinction. Fear renewal was measured using discrete tone presentations or a long, continuous tone. The topography of fear responding at test was assessed by comparing "early" and "sustained" renewal, where early fear was determined by freezing to the first discrete tone or the equivalent initial segment of a continuous tone and sustained fear was determined by freezing averaged across all discrete tones or the entire continuous tone. We found that following pretraining damage of the hippocampus, early renewal remained intact regardless of lesion condition. However, sustained renewal only persisted in discrete, but not continuous, tone-tested animals. A more extensive analysis of the topography of fear responding revealed that the disruption of renewal was generated when the tone duration at test began to violate that used during extinction, suggesting that the hippocampus is sensitive to mismatches in CS-duration. Postextinction lesions resulted in an overall reduction of fear renewal. This pattern of results is consistent with those observed for contextual fear conditioning, wherein animals display a resistance to anterograde amnesia despite the presence of a strong retrograde amnesia for the same contextual information. Furthermore, the data support a role for the hippocampus in sustaining renewal when the CS duration at test does not match that used during extinction.     

Pontine control of rapid eye movement sleep and fear memory

Aims

We often experience dreams of strong irrational and negative emotional contents with postural muscle paralysis during rapid eye movement (REM) sleep, but how REM sleep is generated and its function remain unclear. In this study, we investigate whether the dorsal pontine sub-laterodorsal tegmental nucleus (SLD) is necessary and sufficient for REM sleep and whether REM sleep elimination alters fear memory.

Methods

To investigate whether activation of SLD neurons is sufficient for REM sleep induction, we expressed channelrhodopsin-2 (ChR2) in SLD neurons by bilaterally injecting AAV1-hSyn-ChR2-YFP in rats. We next selectively ablated either glutamatergic or GABAergic neurons from the SLD in mice in order to identify the neuronal subset crucial for REM sleep. We finally  investigated the role of REM sleep in consolidation of fear memory using rat model with complete SLD lesions.

Results

We demonstrate the sufficiency of the SLD for REM sleep by showing that photo-activation of ChR2 transfected SLD neurons selectively promotes transitions from non-REM (NREM) sleep to REM sleep in rats. Diphtheria toxin-A (DTA) induced lesions of the SLD in rats or specific deletion of SLD glutamatergic neurons but not GABAergic neurons in mice completely abolish REM sleep, demonstrating the necessity of SLD glutamatergic neurons for REM sleep. We then show that REM sleep elimination by SLD lesions in rats significantly enhances contextual and cued fear memory consolidation by 2.5 and 1.0 folds, respectively, for at least 9 months. Conversely, fear conditioning and fear memory trigger doubled amounts of REM sleep in the following night, and chemo-activation of SLD neurons projecting to the medial septum (MS) selectively enhances hippocampal theta activity in REM sleep; this stimulation immediately after fear acquisition reduces contextual and cued fear memory consolidation by 60% and 30%, respectively.

Conclusion

SLD glutamatergic neurons generate REM sleep and REM sleep and SLD via the hippocampus particularly down-regulate contextual fear memory.     

Effects of dorsal striatum lesions in tone fear conditioning and contextual fear conditioning

It has been suggested that the striatum mediates hippocampus-independent memory tasks. Classical fear conditioning to a discrete stimulus such as a tone is not affected by hippocampal lesion, whereas contextual fear conditioning is an hippocampus dependent task. The purpose of the present study was to verify the effect of dorsal striatal lesions on tone and contextual fear conditioning. The lesioned rats were not impaired in contextual fear conditioning but in tone fear conditioning both electrolytically and neurotoxically lesioned animals showed less freezing compared with controls. The lesion effect was observed after a postoperative recovery period of 14 days but not after 2 months. The results support the hypothesis that the dorsal striatum is involved in hippocampus-independent memory tasks, but, in spite of this involvement, it does not seem to be a critical structure.     

Trace fear conditioning detects hypoxic-ischemic brain injury in neonatal mice

Trace fear conditioning is a well-established test for the assessment of learning deficits in rodents. The aim of this study was to determine whether hypoxia-ischemia (HI) on postnatal day 9 (P9) in mice prevents the acquisition and expression of cued and contextual fear learning in early adulthood. Brain injury was induced in mice on P9 by 30 min of HI. On P49 and P50, animals were tested for: (1) trace fear conditioning with a short delay (2 s) between a shock-paired tone plus light and shock, (2) trace fear conditioning with a longer delay (20 s) between a shock-paired tone and shock, and (3) trace fear conditioning with a 2-second delay between a shock-paired tone and shock with additional visual, olfactory and tactile contextual cues in the fear conditioning apparatus. Outcome was assessed as percent of time spent freezing during a 2-min test. Histological assessment of the hippocampus and amygdala was performed on P51 to determine the extent of HI injury. Both shock-paired tone plus light with a short delay and shock-paired tone with a short delay plus additional contextual cues enhanced tone-induced freezing behavior in a nonhandled control group, but not in the HI group. For trace fear conditioning with a 20-second delay between the tone and the shock, freezing behavior did not differ significantly between nonhandled control and HI animals. Dorsal hippocampal and amygdala volumes were smaller in the ischemic hemispheres of the HI mice that displayed impaired fear memory with shock-paired tone plus light. In summary, we have shown that trace fear conditioning is a sensitive method for detecting memory impairments in adolescent mice following mild HI injury during the neonatal period. Combining a discrete conditioned stimulus (shock-paired tone plus light) with a short trace delay was the most sensitive method for using the fear conditioning paradigm to detect mild HI damage to the hippocampus and amygdala.     

The hippocampus integrates context and shock into a configural memory in contextual fear conditioning

Contextual fear conditioning involves forming a representation for the context and associating it with a shock, which were attributed by the prevailing view to functions of the hippocampus and amygdala, respectively. Yet our recent evidence suggested that both processes require integrity of the dorsal hippocampus (DH). In view of the DH involvement in uniting multiple stimuli into a configuration, this study examined whether the DH would integrate context and shock into a shocked‐context representation. Male Wistar rats were trained on a two‐phase training paradigm of contextual fear conditioning. They explored a novel context on the first day to acquire a contextual representation, and received a shock in that context on the second day to form the context–shock memory. Tests of conditioned freezing given on the following days revealed two properties of configural memory—direct and mediated pattern completion: First, the contextual fear memory was retrieved in a novel context by a cue embedded in the configural set—a shock that did not elicit significant freezing on its own. Second, freezing was also elicited in a novel context by a transportation chamber that was not directly paired with the shock but could activate the fear memory inferentially. The effects were specific to the cue and not due to context generalization. Infusion of lidocaine into the DH, but not the amygdala, immediately after context–shock training impaired conditioned freezing elicited through either type of pattern completion. Our data suggest that the DH in contextual fear conditioning associates context and shock in parallel with the amygdala by incorporating the shock into an otherwise neutral context representation and turning it into a shocked‐context representation. © 2016 Wiley Periodicals, Inc.     

Different alpha(2) adrenoceptor subtypes control noradrenaline release and cell firing in the locus coeruleus of wildtype and monoamine oxidase-A knockout mice

In this study, we investigated which subtype(s) of alpha(2)-adrenoceptor control stimulated noradrenaline (NA) release and noradrenergic cell firing in the locus coeruleus (LC) of monoamine oxidase-A knockout (MAO-A KO) and C3H/HeJ wildtype mice. On short stimulus trains (10 pulses, 200 Hz), the alpha(2) agonist dexmedetomidine (10 nm) reduced NA efflux by 78 +/- 8% and 51 +/- 8% in wildtype and MAO-A KO mice, respectively. In both strains, BRL 44408 (100 nm) and ARC 239 (100 nm) each partially blocked the effect of dexmedetomidine. In MAO-A KO mice, BRL 44408 (100 nm) increased evoked NA efflux on short trains while ARC 239 (100 nm) had no effect. The two antagonists in combination increased NA efflux (by 81 +/- 34%, P < 0.001), significantly more than by BRL 44408 alone. Conversely, in wildtype mice, the alpha2-adrenoceptor antagonists did not significantly increase LC NA efflux. On long stimuli (30 pulses, 10 Hz), NA efflux was increased by BRL 44408 (P < 0.001) but not by ARC 239. The effect of BRL 44408 was significantly greater in MAO-A KO than wildtype mice (208 +/- 43% vs. 113 +/- 31% increase, P < 0.001). When we examined noradrenergic cell firing, we found that dexmedetomidine inhibited LC cell firing in both strains with comparable EC(50) values (2-5 nm), although E(max) was significantly lower in MAO-A KO mice (P < 0.001). The agonist effect was antagonized by BRL 44408 (P < 0.001) in wildtype but not in MAO-A KO mice, with a pK(B) of 7.75. ARC 239 had no effect on the agonist response in either strain. A combination of the antagonists was no more effective than BRL 44408 alone (in wildtypes) and had no effect in MAO-A KO mice. Neither BRL 44408 nor ARC 239 affected basal LC cell firing in wildtype or MAO-A KO mice. Collectively, these results suggest that, analogous to other monoamine cell groups, there are differences in the autoreceptor populations controlling NA efflux and LC cell firing and that important differences exist between MAO-A KO and wildtype mice.     

Fear conditioning in C57/BL/6 and DBA/2 mice: variability in nucleus accumbens function according to the strain predisposition to show contextual- or cue-based responding

The contribution of the nucleus accumbens shell, the dorsal hippocampus, and the basolateral amygdala to contextual and explicit cue fear conditioning was assessed in C57BL/6 (C57) and DBA/2 (DBA) mice showing differences in processing contextual information associated with consistent but non-pathological variations in hippocampal functionality. Mice from both strains with bilateral ibotenic acid or sham lesions located in each area were introduced in a conditioning chamber and exposed twice to the pairing of a tone (2 x 8 s, 2000 Hz, 80 dB) with a shock (2 s, 0.7 mA). On the following day, mice were first exposed to the training context then to the tone in a different context. Freezing behaviour was scored in all situations. C57 showed more freezing to the context than to the tone whereas DBA showed more freezing to the tone than to the context. In C57, both nucleus accumbens and hippocampal lesions impaired acquisition of contextual fear conditioning but paradoxically improved acquisition of cue fear conditioning, whereas amygdala lesions disrupted performance in every task. In DBA, nucleus accumbens lesions, like amygdala lesions, impaired acquisition of both contextual and cue fear conditioning, whereas hippocampal lesions did not produce any effect. The parallelism between the effect of nucleus accumbens and hippocampus lesions in C57, and between the effect of nucleus accumbens and amygdala lesions in DBA points to a variability in nucleus accumbens function according to the strain specialization to develop context- or cue-based responding.