[Show abstract][Hide abstract] ABSTRACT: Much attention has been paid to the associative processes that are necessary to fuse together representations of the various components of an episodic memory. In the present study, we focus on the processes involved in the formation of lasting representations of the individual components that make up a fear-conditioning episode. In one-trial contextual fear conditioning experiments, weak conditioning to context occurs if the shock is delivered immediately following placement of the animal in a novel conditioning apparatus, a phenomenon known as the immediate shock deficit. We show that the immediate shock deficit in mice may be alleviated by pre-exposure to either the context or shock. In using this approach to temporally dissect a contextual fear-conditioning task into its constituent representational and associative processes, we are able to examine directly the processes that are important for formation of lasting representations of the context conditioned stimulus (CS) or unconditioned stimulus (US). Our data indicate that the formation of a lasting representation of the context or shock engages protein synthesis-dependent processes. Furthermore, genetic disruption of cAMP-responsive element binding protein (CREB), a transcription factor that regulates the synthesis of new proteins required for long-term memory, disrupts the formation of lasting context memories. We go on to show that the stress hormone epinephrine modulates the consolidation of a context memory, and reverses consolidation deficits in the CREB-deficient mice. Finally we show that disrupting either NMDA or calcium/calmodulin-dependent kinase II (CaMKII) function impairs consolidation of context memories. Together, these data suggest that this approach is particularly suited for the characterization of molecular and cellular processes underlying the formation of stimulus representations.
[Show abstract][Hide abstract] ABSTRACT: The cAMP-responsive element binding protein (CREB) family of transcription factors is thought to be critical in memory formation. To define the role of CREB in distinct memory processes, we derived transgenic mice with an inducible and reversible CREB repressor by fusing CREBS133A to a tamoxifen (TAM)-dependent mutant of an estrogen receptor ligand-binding domain (LBD). We found that CREB is crucial for the consolidation of long-term conditioned fear memories, but not for encoding, storage or retrieval of these memories. Our studies also showed that CREB is required for the stability of reactivated or retrieved conditioned fear memories. Although the transcriptional processes necessary for the stability of initial and reactivated memories differ, CREB is required for both. The findings presented here delineate the memory processes that require CREB and demonstrate the power of LBD-inducible transgenic systems in the study of complex cognitive processes.
[Show abstract][Hide abstract] ABSTRACT: The ability to learn and remember individuals is critical for the stability of social groups. Social recognition reflects the ability of mice to identify and remember conspecifics. Social recognition is assessed as a decrease in spontaneous investigation behaviors observed in a mouse reexposed to a familiar conspecific. Our results demonstrate that group-housed mice show social memory for a familiar juvenile when tested immediately, 30 min, 24 h, 3 days, and 7 days after a single 2-min-long interaction. Interestingly, chronic social isolation disrupts long-term, but not 30-min, social memory. Even a 24-h period of isolation disrupts long-term social memory, a result that may explain why previous investigators only observed short-term social memory in individually housed rodents. Although it has no obvious configural, relational, or spatial characteristics, here we show that social memory shares characteristics of other hippocampus-dependent memories. Ibotenic acid lesions of the hippocampus disrupt social recognition at 30 min, but not immediately after training. Furthermore, long-term, but not short-term social memory is dependent on protein synthesis and cyclic AMP responsive element binding protein (CREB) function. These results outline behavioral, systems, and molecular determinants of social recognition in mice, and they suggest that it is a powerful paradigm to investigate hippocampal learning and memory.
[Show abstract][Hide abstract] ABSTRACT: Fear-potentiated startle was assessed in mice with a targeted disruption of the alpha and delta isoforms of the transcription factor cAMP response element binding protein (CREB) 24 hr after 5 tone + shock training trials. Whereas wild-type mice showed fear-potentiated startle that persisted up to 45 days after training, CREBαΔ−/− mice failed to show fear-potentiated startle. However, CREBαΔ−/− and wild-type mice had similar startle amplitudes and similar magnitudes of prepulse inhibition of startle, suggesting that CREBαΔ−/− mice have no obvious sensory or motor deficits. These results add to the literature indicating that CREB-activated transcription plays a critical role in the formation of long-term memory and illustrate the utility of the fear-potentiated startle paradigm for assessing cognition in genetically altered mice.
[Show abstract][Hide abstract] ABSTRACT: Learning and remembering the location of food resources, predators, escape routes, and immediate kin is perhaps the most essential form of higher cognitive processing in mammals. Two of the most frequently studied forms of place learning are spatial learning and contextual conditioning. Spatial learning refers to an animal's capacity to learn the location of a reward, such as the escape platform in a water maze, while contextual conditioning taps into an animal's ability to associate specific places with aversive stimuli, such as an electric shock. Recently, transgenic and gene targeting techniques have been introduced to the study of place learning. In contrast with the abundant literature on the neuroanatomical substrates of place learning in rats, very little has been done in mice. Thus, in the first part of this article, we will review our studies on the involvement of the hippocampus in both spatial learning and contextual conditioning. Having demonstrated the importance of the hippocampus to place learning, we will then focus attention on the molecular and cellular substrates of place learning. We will show that just as in rats, mouse hippocampal pyramidal cells can show place specific firing. Then, we will review our evidence that hippocampal-dependent place learning involves a number of interacting physiological mechanisms with distinct functions. We will show that in addition to long-term potentiation, the hippocampus uses a number of other mechanisms, such as short-term-plasticity and changes in spiking, to process, store, and recall information. Much of the focus of this article is on genetic studies of learning and memory (L&M). However, there is no single experiment that can unambiguously connect any cellular or molecular mechanism with L&M. Instead, several different types of studies are required to determine whether any one mechanism is involved in L&M, including (i) the development of biologically based learning models that explain the involvement of a given mechanism in L&M, (ii) lesion experiments (genetics and pharmacology), (iii) direct observations during learning, and (iv) experiments where learning is triggered by turning on the candidate mechanism. We will show how genetic techniques will be key to unraveling the molecular and cellular basis of place learning.
Neurobiology of Learning and Memory 07/1998; 70(1-2):44-61. DOI:10.1006/nlme.1998.3837 · 3.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The cAMP responsive element binding protein (CREB) is a nuclear protein that modulates the transcription of genes with cAMP responsive elements in their promoters. Increases in the concentration of either calcium or cAMP can trigger the phosphorylation and activation of CREB. This transcription factor is a component of intracellular signaling events that regulate a wide range of biological functions, from spermatogenesis to circadian rhythms and memory. Here we review the key features of CREB-dependent transcription, as well as the involvement of CREB in memory formation. Evidence from Aplysia, Drosophila, mice, and rats shows that CREB-dependent transcription is required for the cellular events underlying long-term but not short-term memory. While the work in Aplysia and Drosophila only involved CREB function in very simple forms of conditioning, genetic and pharmacological studies in mice and rats demonstrate that CREB is required for a variety of complex forms of memory, including spatial and social learning, thus indicating that CREB may be a universal modulator of processes required for memory formation.
[Show abstract][Hide abstract] ABSTRACT: Background: The cAMP responsive element binding protein (CREB) is a transcription factor the activity of which is modulated by increases in the intracellular levels of cAMP and calcium. Results from studies with Aplysia, Drosophila and mice indicate that CREB-activated transcription is required for long-term memory. Furthermore, a recent study found that long-term memory for olfactory conditioning can be induced with a single trial in transgenic Drosophila expressing a CREB activator, whereas in normal flies, with presumably lower CREB-mediated transcription levels, conditioning requires multiple spaced trials. This suggests that CREB-mediated transcription is important in determining the type of training required for long-term memory of olfactory conditioning in Drosophila. Interestingly, studies with cultured Aplysia neurons indicated that removing a CREB repressor promoted the formation of long-term facilitation, a cellular model of non-associative memory.
Current Biology 02/1997; 7(1):1-11. DOI:10.1016/S0960-9822(06)00022-4 · 9.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The glutamate receptor subtype that mediates the morphine withdrawal-induced activation of locus coeruleus (LC) neurons was examined in this study using in vitro and in vivo single-unit electrophysiologic recordings. For LC neurons recorded in vitro in rat brain slices, the selective alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) antagonist, LY293558, showed a greater than 10-fold selectivity for inhibiting the excitatory effects of AMPA vs kainate, and a greater than 30-fold selectivity for inhibiting the excitatory effects of AMPA vs NMDA. LY293558 also greatly reduced the response of LC neurons to glutamate in a concentration-dependent manner. In in vivo recordings in anesthetized rats, pretreatment with LY293558 (0.1 to 10 mg/kg, i.p.) dose dependently suppressed the morphine withdrawal-induced activation of LC neurons. In unanesthetized, morphine-dependent animals, pretreatment with LY293558 (1 to 30 mg/kg, i.p.) dose dependently suppressed naltrexone-precipitated morphine withdrawal signs. These results indicate: (1) AMPA receptors mediate a large component of the excitatory effects of glutamate on LC neurons; (2) activation of AMPA receptors plays an important role in the morphine withdrawal-induced activation of LC neurons; (3) AMPA antagonists can suppress many signs of morphine withdrawal in awake animals; and (4) AMPA antagonists may have therapeutic effects in humans for the treatment of opiate withdrawal.
[Show abstract][Hide abstract] ABSTRACT: Nitric oxide (NO) and carbon monoxide (CO) have been identified as two diffusible signaling messengers in the brain, capable of stimulating soluble guanylate cyclase. Locus coeruleus (LC) is rich in the alpha 1 and beta 1 subunits of soluble guanylate cyclase. Therefore, the possible role of the cGMP pathway in the regulation of LC neurons was investigated with electrophysiological techniques in rat brain slices. Bath application of various NO donors or CO-containing solutions increased the firing rate of most LC neurons. This activation was reversed by the NO scavenger hemoglobin, but not by methemoglobin. Bath or intracellular application of selective activators of cGMP-dependent protein kinase also caused increases in LC cell firing rate. The actions of NO donors and kinase activators were mutually occlusive and reversed by H8, an inhibitor of the cGMP-dependent protein kinase. Hemoglobin and H8 reduced the firing rate of LC neurons, but no change was found with inhibitors or activators of the NO synthase. In intracellular and whole-cell recordings, NO effect was associated with an inward current and an increase in the input conductance (mean reversal potential = -27 mV); these effects were abolished using a low-sodium buffer. Spontaneous EPSCs of LC cells were not modified with the NO donor administration. Taken together, these data suggest that NO and CO activate noradrenergic neurons of LC via a cGMP-dependent protein kinase and a nonselective cationic channel. It also is proposed that these effects occur at the postsynaptic level and that there may be a tonic regulation of LC neuronal firing by the cGMP pathway.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 03/1996; 16(4):1389-99. · 6.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During opiate withdrawal, there is an elevated and prolonged efflux of glutamate and aspartate in the locus coeruleus (LC). The enhanced excitatory amino acid (EAA) release is thought to contribute to the withdrawal-induced activation of LC neurons and to the expression of the physical withdrawal syndrome. In this study, prolonged bath applications of glutamate to LC neurons in brain slices resulted in a slowly developing long-term glutamate desensitization (LTGD). LTGD was observed during extracellular recordings or in neurons voltage-clamped to -60mV, in both cases reaching a maximum of about a 50% reduction in the glutamate response. Responses in the desensitized cells gradually recovered within 3 h. Cyclothiazide, an inhibitor of rapid glutamate receptor desensitization did not prevent LTGD. LTGD could not be induced by prolonged applications of EAA agonists other than glutamate, either alone or in various combinations. However, after induction by glutamate, there was cross-desensitization to quisqualate but not to AMPA or NMDA. LTGD was blocked by either lowering extracellular Ca2+ concentrations or by treatment with the protein kinase C inhibitor chelerythrine but not by inhibitors of calcium/calmodulin-dependent kinase or nitric oxide synthase. Applications of the protein kinase C activator phorbol diacetate did not cause a decrease in glutamate responses indicating that an activation of protein kinase C may not be sufficient for desensitization to occur. A decrement of the glutamate response resembling LTGD occurred after treatment by the protein phosphatase inhibitors okadaic acid or calyculin A. LC neurons in brain slices prepared from opiate-withdrawn rats exhibited glutamate responses that were initially desensitized and recovered within 3 h after withdrawal. These results suggest that LTGD in LC neurons may occur during opiate withdrawal and could contribute to the time course of LC hyperactivity and the associated behavioral withdrawal syndrome.
Brain Research 09/1995; 689(1):111-21. DOI:10.1016/0006-8993(95)00545-2 · 2.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electrophysiological studies suggest that an increase in excitatory amino acid release may occur in the locus coeruleus during opiate withdrawal. The present study examined directly by microdialysis in anesthetized rats the effect of naltrexone-precipitated opiate withdrawal on the efflux of excitatory amino acids in the locus coeruleus. A withdrawal-induced increase in glutamate and aspartate efflux was found when the microdialysis probe was located in the core of the locus coeruleus; no increase was seen in adjacent regions.
Brain Research 03/1994; 636(1):126-30. DOI:10.1016/0006-8993(94)90186-4 · 2.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In previous studies, we demonstrated that tyrosine hydroxylase and neurofilament proteins are regulated by chronic morphine and chronic cocaine treatments in the ventral tegmental area in Sprague-Dawley rats and that the imbred Lewis and Fischer 344 rat strains, under drug-naive conditions, show different levels of these proteins specifically in this brain region. In the current study, we compared Lewis and Fischer rats with respect to levels of adenylate cyclase, cyclic AMP-dependent protein kinase and G-proteins in the nucleus accumbens (NAc) and locus coeruleus (LC), brain regions in Sprague-Dawley rats where these proteins are regulated by chronic exposure to morphine or to cocaine. We found that levels of adenylate cyclase and cyclic AMP-dependent protein kinase activity are higher in the NAc and LC of Lewis rats compared to Fischer rats, whereas levels of Giα and Gβ were lower. These strain differences were not seen in several other brain regions analyzed and no strain differences were detected in levels of other G-protein subunits. Lewis and Fischer rats also differed in the ability of chronic morphine to regulate adenylate cyclase and cyclic AMP-dependent protein kinase in the NAc and LC. In the NAc, chronic morphine increased levels of the two enzymes in the Fischer strain only, whereas in the LC chronic morphine increased levels of the enzymes in both strains, with more robust effects seen in the Lewis rat. To understand possible physiological consequences of these strain differences in the cyclic AMP pathway, we studied LC neuronal activity under basal and chronic morphine-treated conditions. LC neurons of Lewis rats showed higher spontaneous firing rates in brain slices in vitro than those of Fischer rats and also showed greater morphine-induced increases in responsiveness to bath-applied 8-bromo-cyclic AMP. These electrophysiological findings are generally consistent with the biochemical observations. Moreover, Lewis and Fischer rats displayed very different opiate withdrawal syndromes, with different types of behaviors elicited upon precipitation of opiate withdrawal with the opiate receptor antagonist, naltrexone. The possible relationship between these behavioral findings and the biochemical and electrophysiological data is discussed. These studies provide further support for the possibility that Lewis and Fischer rat strains provide a useful model system in which some of the genetic factors that contribute to drug-related behaviors can be investigated.
Brain Research 06/1993; 611(1-611):7-17. DOI:10.1016/0006-8993(93)91770-S · 2.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Extracellular, single unit activity was recorded in noradrenergic neurons of the nucleus locus coeruleus (LC) in brain slices prepared from rats treated chronically with morphine. In contrast to previous reports, basal firing rates of LC neurons were 2-fold higher in slices from opiate-dependent animals compared to controls and they remained elevated for at least 7 h. In neurons from dependent animals the maximal excitation in response to 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cAMP), but not the EC50, was found to be substantially greater than in controls. This result parallels biochemical evidence of an up-regulation of the cAMP pathway in the LC of opiate-dependent animals. There was no difference in the response to glutamate between cells from dependent and control animals. We conclude that an increase in basal firing rate, possibly mediated by an up-regulation of the intrinsic cAMP pathway, contributes to the hyperactivity of the LC during opiate withdrawal in vivo.
European Journal of Pharmacology 02/1992; 211(1):47-53. DOI:10.1016/0014-2999(92)90261-2 · 2.53 Impact Factor