Alterations in time-place learning induced by lesions to the rat medial prefrontal cortex

This experiment examined the effect of medial prefrontal lesions on time-place learning in the rat. During the first phase, prior to lesioning, rats received training on an interval time-place task. Food was available on each of four levers for 3 consecutive min of a 12-min session. The levers provided food in the same sequence on all trials. Rats restricted the majority of their presses on each lever to the time in each session when it provided food and were able to anticipate when a lever was going to provide food. During the second phase some rats received lesions that were restricted to the medial prefrontal cortex. Following these very restricted lesions, rats continued pressing a lever after it stopped providing food (i.e. perseverated, as if their internal clock was running slow). The third phase involved changing the order in which the levers provided food. Lesions had no discernable effect on the rats' ability to learn the correct sequence of food availability. However, this change made the rats' timing perseveration even more noticeable. Our results suggest the medial prefrontal cortex is not necessary for acquisition of time-place sequencing information. However, lesions do appear to produce perseveration on components of the sequence.     

Dendritic reorganization in pyramidal neurons in medial prefrontal cortex after chronic corticosterone administration.

Chronic stress produces deficits in cognition accompanied by alterations in neural chemistry and morphology. For example, both stress and chronic administration of corticosterone produce dendritic atrophy in hippocampal neurons (Woolley C, Gould E, McEwen BS. 1990. Exposure to excess glucocorticoids alters dendritic morphology of adult hippocampal pyramidal neurons. Brain Res 531:225-231; Watanabe Y, Gould E, McEwen BS, 1992b. Stress induces atrophy of apical dendrites of hippocampal CA3 pyramidal neurons. Brain Res 588:341-345). Prefrontal cortex is also a target for glucocorticoids involved in the stress response (Meaney MJ, Aitken DH. 1985. [(3)H]Dexamethasone binding in rat frontal cortex. Brain Res 328:176-180); it shows neurochemical changes in response to stress (e.g., Luine VN, Spencer RL, McEwen BS. 1993. Effect of chronic corticosterone ingestion on spatial memory performance and hippocampal serotonergic function. Brain Res 616:55-70; Crayton JW, Joshi I, Gulati A, Arora RC, Wolf WA. 1996. Effect of corticosterone on serotonin and catecholamine receptors and uptake sites in rat frontal cortex. Brain Res 728:260-262; Takao K, Nagatani T, Kitamura Y, Yamawaki S. 1997. Effects of corticosterone on 5-HT(1A) and 5-HT(2) receptor binding and on the receptor-mediated behavioral responses of rats. Eur J Pharmacol 333:123-128; Sandi C, Loscertales M. 1999. Opposite effects on NCAM expression in the rat frontal cortex induced by acute vs. chronic corticosterone treatments. Brain Res 828:127-134), and mediates many of the behaviors that are altered by chronic corticosterone administration (e.g., Lyons DM, Lopez JM, Yang C, Schatzberg AF. 2000. Stress-level cortisol treatment impairs inhibitory control of behavior in monkeys. J Neurosci 20:7816-7821). To determine if glucocorticoid-induced morphological changes also occur in medial prefrontal cortex, the effects of chronic corticosterone administration on dendritic morphology in this corticolimbic structure were assessed. Adult male rats received s.c. injections of either corticosterone (10 mg in 250 microL sesame oil; n = 8) or vehicle (250 microL; n = 8) daily for 3 weeks. A third group of rats served as intact controls (n = 4). Brains were stained using a Golgi-Cox procedure and pyramidal neurons in layer II-III of medial prefrontal cortex were drawn; dendritic morphology was quantified in three dimensions. Sholl analyses demonstrated a significant redistribution of apical dendrites in corticosterone-treated animals: the amount of dendritic material proximal to the soma was increased relative to intact rats, while distal dendritic material was decreased relative to intact animals. Thus, chronic glucocorticoid administration dramatically reorganized apical arbors in medial prefrontal cortex. This reorganization likely reflects functional changes and may contribute to stress-induced changes in cognition.     

Activity-dependent changes of the hippocampal CA3–CA1 synapse during the acquisition of associative learning in conscious mice

Contemporary neuroscientists are paying increasing attention to subcellular, molecular and electrophysiological mechanisms underlying learning and memory processes. Recent efforts have addressed the development of transgenic mice affected at different stages of the learning process, or emulating pathological conditions involving cognition and motor-learning capabilities. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice, in order to grasp activity-dependent neural changes taking place during the very moment of the process. These in vivo models should integrate the fragmentary information collected by different molecular and in vitro approaches. In this regard, long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity. Moreover, N -methyl- d -aspartate (NMDA) receptors are accepted as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors with the plastic changes taking place, in selected neural structures, during actual learning. Here, we review data on the involvement of the hippocampal CA3–CA1 synapse in the acquisition of classically conditioned eyelid conditioned responses (CRs) in behaving mice. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of CRs and the physiological changes that occur at the CA3–CA1 synapse during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo , but also to hinder the formation of both CRs and functional changes in strength of the CA3–CA1 synapse. Thus, there is experimental evidence relating activity-dependent synaptic changes taking place during actual learning with LTP mechanisms and with the role of NMDA receptors in both processes.     

Lateralized learning and memory effects of angiotensin II microinjected into the rat CA1 hippocampal area

The effects of angiotensin II (ANG II) microinjected unilaterally (left or right) and bilaterally (left and right) at a dose of 0.5 microg (0.5 nmol) into the CA1 hippocampal area of male Sprague Dowley rats on learning and memory (shuttle box) were studied. Bilateral microinjections of ANG II improved learning, i.e. increased the number of avoidances during the two training days as compared to the respective controls microinjected with saline. ANG II facilitated learning and memory, especially when microinjected into the left CA1 hippocampal area as compared to the respective controls microinjected with saline. Left-side microinjection of ANG II increased the number of avoidances on the first and second training day as compared to the right-side microinjection of ANG II. These findings suggest asymmetric effects of ANG II on cognitive processes in hippocampus.     

Activity-dependent regulation of h channel distribution in hippocampal CA1 pyramidal neurons

The hyperpolarization-activated cation current, I(h), plays an important role in regulating intrinsic neuronal excitability in the brain. In hippocampal pyramidal neurons, I(h) is mediated by h channels comprised primarily of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel subunits, HCN1 and HCN2. Pyramidal neuron h channels within hippocampal area CA1 are remarkably enriched in distal apical dendrites, and this unique distribution pattern is critical for regulating dendritic excitability. We utilized biochemical and immunohistochemical approaches in organotypic slice cultures to explore factors that control h channel localization in dendrites. We found that distal dendritic enrichment of HCN1 is first detectable at postnatal day 13, reaching maximal enrichment by the 3rd postnatal week. Interestingly we found that an intact entorhinal cortex, which projects to distal dendrites of CA1 but not area CA3, is critical for the establishment and maintenance of distal dendritic enrichment of HCN1. Moreover blockade of excitatory neurotransmission using tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione, or 2-aminophosphonovalerate redistributed HCN1 evenly throughout the dendrite without significant changes in protein expression levels. Inhibition of calcium/calmodulin-dependent protein kinase II activity, but not p38 MAPK, also redistributed HCN1 in CA1 pyramidal neurons. We conclude that activation of ionotropic glutamate receptors by excitatory temporoammonic pathway projections from the entorhinal cortex establishes and maintains the distribution pattern of HCN1 in CA1 pyramidal neuron dendrites by activating calcium/calmodulin-dependent protein kinase II-mediated downstream signals.     

Integrative properties of radial oblique dendrites in hippocampal CA1 pyramidal neurons

Although radial oblique dendrites are a major synaptic input site in CA1 pyramidal neurons, little is known about their integrative properties. We have used multisite two-photon glutamate uncaging to deliver different spatiotemporal input patterns to single branches while simultaneously recording the uncaging-evoked excitatory postsynaptic potentials and local Ca2+ signals. Asynchronous input patterns sum linearly in spite of the spatial clustering and produce Ca2+ signals that are mediated by NMDA receptors (NMDARs). Appropriately timed and sized input patterns ( approximately 20 inputs within approximately 6 ms) produce a supralinear summation due to the initiation of a dendritic spike. The Ca2+ signals associated with synchronous input were larger and mediated by influx through both NMDARs and voltage-gated Ca2+ channels (VGCCs). The oblique spike is a fast Na+ spike whose duration is shaped by the coincident activation of NMDAR, VGCCs, and transient K+ currents. Our results suggest that individual branches can function as single integrative compartments.     

羟基马桑毒素所致的大鼠海马锥体细胞痫样放电活动

本研究采用离体海马脑片电生理研究技术,细胞外记录海马锥体细胞群体锋电位(population spike,PS),观察羟基马桑毒素(tutin)对大鼠海马脑片CA1区锥体细胞电活动的影响,探讨tutin是否具有致痛作用及其致痫机制。结果如下:(1)用40、30和20μg/ml浓度的tutin灌流海马脑片,可显著增高由顺向刺激Schaffer侧支所诱发的PS的幅度,灌流tutin 30min时,PS第一个波的幅度分别为对照的(388.7±20.1)%、(317.2±19.1)%和(180.9±11.6)%(各组n=5,P<0.05)。(2)伴随PS波幅的增高,可出现成串痫样放电波,波数4～11个不等。(3)灌流tutin后的部分脑片(n=9/34),在未刺激Schaffer侧支时也出现自发的成串、高幅痫样放电。(4)灌流CNQX阻断非NMDA受体后,再灌流tutin,PS幅度和放电波数均无显著性变化,即CNQX可完全抑制tutin所致的痫样放电;灌流AP-5阻断NMDA受体后,tutin仍可使PS幅度增高但放电波数无显著性增加,即AP-5可部分抑制tutin所致的痫样放电。上述结果表明,tutin可使海马脑片锥体细胞兴奋活动增强,具有致痫作用;兴奋性谷氨酸受体尤其是非NMDA受体可能介导tutin的致痫作用。     

Loss of hippocampal CA3 pyramidal neurons in mice lacking STAM1

STAM1, a member of the STAM (signal transducing adapter molecule) family, has a unique structure containing a Src homology 3 domain and ITAM (immunoreceptor tyrosine-based activation motif). STAM1 was previously shown to be associated with the Jak2 and Jak3 tyrosine kinases and to be involved in the regulation of intracellular signal transduction mediated by interleukin-2 (IL-2) and granulocyte-macrophage colony-stimulating factor (GM-CSF) in vitro. Here we generated mice lacking STAM1 by using homologous recombination with embryonic stem cells. STAM1(-/-) mice were morphologically indistinguishable from their littermates at birth. However, growth retardation in the third week after birth was observed for the STAM1(-/-) mice. Unexpectedly, despite the absence of STAM1, hematopoietic cells, including T- and B-lymphocyte and other hematopoietic cell populations, developed normally and responded well to several cytokines, including IL-2 and GM-CSF. However, histological analyses revealed the disappearance of hippocampal CA3 pyramidal neurons in STAM1(-/-) mice. Furthermore, we observed that primary hippocampal neurons derived from STAM1(-/-) mice are vulnerable to cell death induced by excitotoxic amino acids or an NO donor. These data suggest that STAM1 is dispensable for cytokine-mediated signaling in lymphocytes but may be involved in the survival of hippocampal CA3 pyramidal neurons.     

Early stress and chronic methylphenidate cross-sensitize dopaminergic responses in the adolescent medial prefrontal cortex and nucleus accumbens

Methylphenidate (MP) is widely used to treat attention deficit/hyperactivity disorder in children. However, basic research has been mainly focused on MP treatment in adult, behaviorally normal rodents. Here we analyzed MP-evoked changes of dopamine (DA) release in the limbic system of juvenile rodents with hyperactive and attention deficit-like symptoms. Using dual probe in vivo microdialysis, DA levels were quantified in the medial prefrontal cortex and nucleus accumbens of juvenile and adolescent degus ( Octodon degus ). Acute stress- and acute MP-evoked dopaminergic responses in normal juvenile and adolescent animals were compared with (i) animals showing symptoms of hyperactivity and attention deficits induced by early life stress, i.e. repeated parental separation during the first 3 weeks of life, and (ii) animals chronically treated with MP during pre-adolescence. Our main results revealed that (i) early life stress and (ii) chronic MP treatment during pre-adolescence cross-sensitize limbic dopaminergic functions in adolescent animals. Furthermore, we demonstrated a unique pattern of acute MP-evoked DA release in the juvenile compared with the adolescent medial prefrontal cortex and nucleus accumbens. Our findings that the functional maturation of dopaminergic limbic function is significantly altered by early life experience, i.e. repeated parental separation and chronic MP treatment, allow novel insights into the etiology of attention deficit/hyperactivity disorder and into the long-term consequences of MP treatment on brain development.     

Inhibitory processes of hippocampal CA1 pyramidal neurons following kindling-induced epilepsy in the rat

To determine the alterations in cellular function which may contribute to the chronic predisposition of neuronal tissue to epileptiform activity, the membrane properties and inhibitory processes of hippocampal CA1 pyramidal cells were investigated using in vitro slices prepared from commissural-kindled rats. No changes were observed in resting membrane potential, input resistance, spike amplitude, and membrane time constant of kindled CA1 pyramidal neurons when compared with controls. There were also no differences between control and kindled preparations in the amplitude of recurrent inhibitory postsynaptic potentials (IPSP) and in the duration of inhibition produced by either alvear (Alv) or stratum radiatum (SR) stimulation. Irrespective of group, repetitive stimulation of the Alv reduced the amplitude of the recurrent IPSP but failed to induce seizurelike activity. On the other hand, repetitive stimulation of SR frequently produced a neuronal burst discharge even though the duration and to some extent the amplitude of orthodromic inhibition was increased. On the basis of these data, it may be suggested that chronic changes in CA1 pyramidal cell membrane properties and transient reductions of inhibitory processes do not underlie the enhanced sensitivity of these neurons to seizure activity associated with kindling.     

Monoamines and self-stimulation of the medial prefrontal cortex in the rat

The participation of noradrenaline (NE) and serotonine (5-HT) in self-stimulation (SS) of the medial prefrontal cortex (MPC) in the rat has been studied. Three groups of rats with bilateral electrodes implanted into the MPC were used in these experiments. In one of the groups, electrodes were also implanted into the locus coeruleus. In the first group, the rats received systemic injections of the following drugs: clonidine (alpha-agonist), phenoxybenzamine (alpha-antagonist), isoproterenol (beta-agonist) and propranolol (beta-antagonist). In the second group, p-chlorophenylalanine (a 5-HT synthesis inhibitor) was administered intragastrically and SS measured during the following 16 days. In these two groups of rats and previous to every SS session, spontaneous motor activity (SM) was measured as control for non specific effects of the drugs. In a third group of rats, lesions of the locus coeruleus were performed unilaterally and SS measured in both prefrontal cortex during the following 16 days post-lesion. SS contralateral to the lesioned side served as control for non-specific effects of the lesions. After all these treatments, SS of the MPC was not specifically affected. Our results suggest the non participation of NE and 5-HT terminals in the neural substrates underlying SS of the MPC.     

Retrosplenial and hippocampal projections of structures of the thalamoparietal associative system in rats

Afferent connections of the nucleus lateralis posterior (NLP) of the thalamus and area 7 of the parietal cortex with the retrosplenial region of the limbic cortex and hippocampus were studied in rats with retrograde axon transport of horseradish peroxidase. It was shown that the NLP receives ipsilateral projections from area 29d neurons, while area 7 receives ipsilateral axons from area 29d and 29c neurons. It was found that associations of the retrosplenial region with associative cortex are far more pronounced than with associative thalamus. Moreover, the afferent connections of area 7 with area 29d are more numerous than with area 29c. We disclosed no projections of areas 29a and 29b to thalamoparietal system structures. In addition to neocortical input from the limbic cortex, area 7 receives afferent fibers from the archicortex; neurons situated in hippocampus area CA1 are the source of these projections.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy Academy of Sciences, Leningrad. Translated from Neirofiziologiya, Vol. 23, No. 6, pp. 647–655, November–December, 1991.     

Facilitating action of medial prefrontal cortex upon the noxious thermally-evoked responses in thalamic centralis lateralis nucleus.

This paper shows a medial prefrontal cortex (CxAP9) facilitating influence upon the unit activity of the centralis lateralis (Cl) nucleus of the thalamus, in rats anesthetized with urethane. Cortical influences were studied using both cortical cooling and cortical spreading depression (CSD) procedures. Both spontaneous and noxious thermally evoked activities were considered. When CSD was propagated and affected the CxAP9, as well as during the cooling of this area, both spontaneous activity and the responses evoked in Cl cells by noxious stimulation were blocked. This effect was interpreted as a cortical disfacilitation upon Cl cells. During the cortical silent period we tested the excitability of a few Cl cells, provoking their activation by passing electrical current across the same Cl recording electrode. No changes were observed in their excitable response threshold during CSD or cortical cooling. Our results are in agreement with the proposition of a tonic cortical facilitatory action upon the spontaneous and noxious-evoked responses recorded in the Cl cells.     

成年大鼠海马CA1区锥体细胞KATP通道的特性

为了解成年大鼠海马CA1区锥体细胞KATP通道的特性,实验采用膜片钳技术的内面向外式记录法,在急性分离的CA1区锥体神经元上,研究了可被胞浆侧ATP所抑制的钾离子单通道的特性,当细胞膜内外两侧的K^ 浓度均为140mmol/L时,通道的电导为63pS,翻转电位为1．71mV,通道呈弱向内向整流性,在负钳制电位时,通道开放时常被短时的关闭所打断,而在正钳制电位时,这种短时程的关闭状态明显少于负钳制电位时,但通道开放概率未见明显的电压依赖性,ATP对通道活动的抑制作用呈浓度依赖性,抑制通道活动50％的ATP浓度为0．1mmol/L.KATP通道的特异性阻断剂tolbutamide(甲糖宁,1mmol/L)可完全阻断通道的活动,而KATP通道开放剂diazoxide(二氮嗪,1mmol/L）则不增强通道的活动。     

Gating properties of single SK channels in hippocampal CA1 pyramidal neurons.

The activation of small-conductance calcium-activated potassium channels (SK) has a profound effect on membrane excitability. In hippocampal pyramidal neurons, SK channel activation by Ca2+ entry from a preceding burst of action potentials generates the slow afterhyperpolarization (AHP). Stimulation of a number of receptor types suppresses the slow AHP, inhibiting spike frequency adaptation and causing these neurons to fire tonically. Little is known of the gating properties of native SK channels in CNS neurons. By using excised inside-out patches, a small-amplitude channel has been resolved that was half-activated by approximately 0.6 microM Ca2+ in a voltage-independent manner. The channel possessed a slope conductance of 10 pS and exhibited nonstationary gating. These properties are in accord with those of cloned SK channels. The measured Ca2+ sensitivity of hippocampal SK channels suggests that the slow AHP is generated by activation of SK channels from a local rise of intracellular Ca2+.     

The role of the medial prefrontal cortex in achieving goals

Achieving goals in changing environments requires the course of action to be selected on the basis of goal expectation and memory of action-outcome contingency. It is often also essential to evaluate action on the basis of immediate outcomes and the discrimination of early action steps from the final step towards the goal. Recently, in single-cell recordings in monkeys, the neuronal activity that appears to underlie these processes has been noted in the medial part of the prefrontal cortex. Medial prefrontal cells were also active when the subjects extracted the rules of a task in a novel environment. The processes described above might play important roles in rule learning.     

The role of prefrontal dopamine D1 receptors in the neural mechanisms of associative learning

Dopamine is thought to play a major role in learning. However, while dopamine D1 receptors (D1Rs) in the prefrontal cortex (PFC) have been shown to modulate working memory-related neural activity, their role in the cellular basis of learning is unknown. We recorded activity from multiple electrodes while injecting the D1R antagonist SCH23390 in the lateral PFC as monkeys learned visuomotor associations. Blocking D1Rs impaired learning of novel associations and decreased cognitive flexibility but spared performance of already familiar associations. This suggests a greater role for prefrontal D1Rs in learning new, rather than performing familiar, associations. There was a corresponding greater decrease in neural selectivity and increase in alpha and beta oscillations in local field potentials for novel than for familiar associations. Our results suggest that weak stimulation of D1Rs observed in aging and psychiatric disorders may impair learning and PFC function by reducing neural selectivity and exacerbating neural oscillations associated with inattention and cognitive deficits.     

Chaotic stimulus dependent long-term potentiation in the hippocampal CA1 area

In our previous report [Tsukada, M., Aihara, T., Saito, H., Kato, H., 1996. Neural Netw. 9, 1357-1365], the temporal pattern sensitivity of long-term potentiation (LTP) in hippocampal CA1 neurons was estimated by using Markov chain stimuli (MS) with different values of the serial correlation coefficient rho1 between successive interstimulus-intervals. In this paper, the effect of chaotic stimuli (CS) on induction of LTP in the hippocampal CA1 area was investigated in comparison with that of MS and periodic pattern stimuli (PS). The CS were produced by a modified Bernoulli map, so that interstimulus sequences with various values of rho1 can be generated by changing the parameter B. These stimuli had an identical first order statistics (mean interstimulus-interval), but their higher order statistics such as the serial correlation coefficients were different. The LTP induced by CS at B = 2 was significantly larger in magnitude than that of PS and MS, and also depended on the initial value of CS at B = 2 and 3. These results suggest that chaotic signals play an important role for memory coding in the hippocampal CA1 network.