Developmental changes in long-term potentiation in CA1 of rat hippocampal slices

Long-term potentiation (LTP) was examined in the CA1 region of rat hippocampal slices at postnatal day 9 (P9), P15, P30, P60, P90, P120, and P300. A single 100 Hz × 1 sec tetanus failed to induce LTP in P9 slices, while similar degrees of LTP were observed at P15, P30, and P60. At P30, changes in population spike (PS) amplitudes were accurately predicted by changes in dendritic excitatory postsynaptic potentials (EPSPs). However, at P15, the predicted increase in PS calculated from corresponding changes in dendritic EPSPs was significantly less than the observed increase, suggesting that EPSP-PS dissociation (ES-dissociation) plays a substantial role in LTP at P15. Additionally, the corresponding changes in somatic EPSP height measured in the CA1 cell layer did not predict the E-S dissociation at P15, suggesting that the E-S dissociation arises largely from changes in the excitability of the soma. Using a single 100 Hz × 1 sec tetanus, LTP proved difficult to induce in slices from rats ≥ P90, with slices showing initial enhancement that faded over 60 min of monitoring. © 1995 Wiley-Liss, Inc.     

Dorsoventral differences in intrinsic properties in developing CA1 pyramidal cells

The dorsoventral and developmental gradients of entorhinal layer II cell grid properties correlate with their resonance properties and with their hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channel current characteristics. We investigated whether such correlation existed in rat hippocampal CA1 pyramidal cells, where place fields also show spatial and temporal gradients. Resonance was absent during the first postnatal week, and emerged during the second week. Resonance was stronger in dorsal than ventral cells, in accord with HCN current properties. Resonance responded to cAMP in ventral but not in dorsal cells. The dorsoventral distribution of HCN1 and HCN2 subunits and of the auxiliary protein tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b) could account for these differences between dorsal and ventral cells. The analogous distribution of the intrinsic properties of entorhinal stellate and hippocampal cells suggests the existence of general rules of organization among structures that process complementary features of the environment.     

Slow intrinsic spikes recorded in vivo in rat CA1-CA3 hippocampal pyramidal neurons

The membrane potential of 45 CA1-CA3 hippocampal pyramidal cells was recorded in curarized and urethanized rats. Two slow spike types were observed together with the usual Na+ type action potential. Slow spikes were termed HTS and LTS because they were essentially identical to the high and low threshold Ca2+ spikes observed in vitro and probably represent the same kinds of activities. LTS and HTS (22 and 29 cells, respectively) were triggered at potentials greater than or equal to 65 mV and less than or equal to 55 mV, had mean durations of 23.7 and 90.4 ms and mean amplitudes of 11.5 and 39.3 mV, respectively, and fired an overriding burst of the action potentials. LTS and HTS were sometimes present in the same neuron (n = 16). Depolarizing pulses triggered rhythmic HTS at rates that increased with depolarization up to a 5 impulses/s maximum. The same limit was found with imposed membrane potential sine currents at frequencies within the theta rhythm range. With spontaneous or imposed hyperpolarizations LTS were evoked by depolarizing, at the break of hyperpolarizing pulses, or spontaneously. They were also evoked at the depolarizing, or recovery, slopes of inhibitory postsynaptic potentials. HTS and LTS supply pyramidal neurons with different firing patterns which enhance the system's integrative possibilities. Evidence is provided that theta is not exclusively generated by network properties, since rhythmic HTS may participate.     

Temperature dependence of intrinsic membrane properties and synaptic potentials in hippocampal CA1 neurons in vitro

The temperature dependence of intrinsic membrane conductances and synaptic potentials in guinea pig hippocampal CA1 pyramidal neurons were examined in vitro as they were cooled from 37 degrees C to between 33 and 27 degrees C. Cooling reversibly increased resting input resistance in a voltage-independent manner (Q10 = 0.58 to 0.75). The amplitude and duration of orthodromically evoked action potentials were increased by cooling (Q10 = 0.87 and 0.52 to 0.53, respectively), whereas the maximum rates of rise and fall were reduced (Q10 = 1.27 to 1.49 and 2.19 to 2.44, respectively). The amplitude and duration of the afterhyperpolarization which follows a directly evoked train of action potentials were substantially increased at low temperatures. It is possible to attribute this increase to an augmentation of Ca2+ influx during the train and also to a slowing of Ca2+ removal from the cytoplasm. Spike frequency adaptation during prolonged depolarizing pulses was enhanced at low temperatures. In addition, there was a decrement in spike amplitude during the train of action potentials. These observations all suggest an increase in Ca2+-activated K+ conductance at low temperature. A late, slow, hyperpolarizing synaptic potential in response to orthodromic stimulation became apparent at low temperature. This potential had an apparent reversal potential more negative than the early inhibitory postsynaptic potential, suggesting that it was mediated by a K+ conductance, possibly activated by Ca2+ influx. We conclude that reductions in temperature of as little as 5 to 10 degrees C from normal can significantly alter the intrinsic and synaptic physiology of hippocampal neurons and should, therefore, be considered an important variable in in vitro brain slice experiments.     

脑缺血后大鼠海马CA1区神经元持续钠电流变化的研究

目的研究脑缺血后大鼠海马CA1区神经元持续钠电流的变化,探讨钠通道阻滞剂对缺血脑组织保护的机理。方法酶消化法急性分离SD大鼠(10~14d)海马CA1区神经元,全细胞膜片钳技术记录脑缺血对持续钠电流的影响。结果缺血3 min时持续钠电流增加到正常时的1.59±0.26倍(P﹤0.05),缺血5 min时增加到正常时的2.92±0.46倍(P﹤0.05)。结论脑缺血时钠通道开放,持续钠电流增加。     

Persistent changes in action potential broadening and the slow afterhyperpolarization in rat CA1 pyramidal cells after febrile seizures

Febrile (fever-induced) seizures (FS) are the most common form of seizures during childhood and have been associated with an increased risk of epilepsy later in life. The relationship of FS to subsequent epilepsy is, however, still controversial. Insights from animal models do indicate that especially complex FS are harmful to the developing brain and contribute to a hyperexcitable state that may persist for life. Here, we determined long-lasting changes in neuronal excitability of rat hippocampal CA1 pyramidal cells after prolonged (complex) FS induced by hyperthermia on postnatal day 10. We show that hyperthermia-induced seizures at postnatal day 10 induce a long-lasting increase in the hyperpolarization-activated current I(h). Furthermore, we show that a reduction in the amount of spike broadening and in the amplitude of the slow afterhyperpolarization following FS are also likely to contribute to the hyperexcitability of the hippocampus long term.     

Intracellular records of theta rhythm in hippocampal CA1 cells of the rat

In the urethane-anesthetized rat, intracellular recordings from hippocampal CA1 cells, some of them identified as projection (probably pyramidal) cells, showed oscillations of the resting membrane potential in the theta frequency range ('intracellular theta rhythm') which is phase-locked to the extracellularly recorded theta rhythm. Current injection or acetate ion diffusion, which reversed an inhibitory postsynaptic potential (IPSP) evoked by alvear stimulation, inverted the phase relationship between intracellular and extracellular theta rhythms. A high correlation was also found between amplitudes of the intracellular theta and the evoked IPSP at different membrane potentials. These results indicate that hippocampal theta rhythm in the urethane-anesthetized rat is predominantly caused by a rhythmic modulation of impinging IPSPs on pyramidal cells.     

Chronic LPS exposure produces changes in intrinsic membrane properties and a sustained IL-beta-dependent increase in GABAergic inhibition in hippocampal CA1 pyramidal neurons

Chronic inflammation has been reported to be a significant factor in the induction and progression of a number of chronic neurological disorders including Alzheimer's disease and Down syndrome. It is believed that inflammation may promote synaptic dysfunction, an effect that is mediated in part by pro-inflammatory cytokines such as interleukin-1beta (IL-1beta). However, the role of IL-1beta and other cytokines in synaptic transmission is still poorly understood. In this study, we have investigated how synaptic transmission and neuronal excitability in hippocampal pyramidal neurons are affected by chronic inflammation induced by exposing organotypic slices to the bacterial cell-wall product lipopolysaccharide (LPS). We report that CA1 pyramidal neurons recorded in whole cell from slices previously exposed to LPS for 7 days had resting membrane potential and action potential properties similar to those of the controls. However, they had significantly lower membrane resistance and a more elevated action potential threshold, and displayed a slower frequency of action potential discharge. Moreover, the amplitude of pharmacologically isolated postsynaptic gamma-aminobutyric acid (GABA)ergic potentials, but not excitatory glutamatergic postsynaptic potentials, was significantly larger following chronic LPS exposure. Interestingly, co-incubation of the IL-1 receptor antagonist (IL-1Ra) concurrently with LPS prevented the increase in GABAergic transmission, but not the reduction in intrinsic neuronal excitability. Finally, we confirmed that LPS dramatically increased IL-1beta, and IL-1beta-dependent IL-6 levels in the culture medium for 2 days before returning to baseline. We conclude that CA1 pyramidal neurons in slices chronically exposed to LPS show a persistent decrease in excitability due to a combined decrease in intrinsic membrane excitability and an enhancement in synaptic GABAergic input, the latter being dependent on IL-1beta. Therefore, chronic inflammation in hippocampus produces IL-1beta-dependent and -independent effects in neuronal and synaptic function that could contribute significantly to cognitive disturbances.     

Long‐lasting intrinsic persistent firing in rat CA1 pyramidal cells: A possible mechanism for active maintenance of memory

The hippocampus is critical for memory tasks which require an active maintenance of memory for a short period of time; however, the underlying neural mechanisms remain unknown. Most theoretical and computational models, which date back to the classic proposals by Donald Hebb in 1949 , have been self‐constrained by anatomy, as most models rely on the recurrent connectivity in region CA3 to support “reverberating activity” capable of memory maintenance. However, several physiological and behavioral studies have specifically implicated region CA1 in tasks which require an active maintenance of memory. Here, we demonstrate that despite limited recurrent connectivity, CA1 contains a robust cellular mechanism for active memory maintenance in the form of self‐sustained persistent firing. Using in vitro whole‐cell recordings, we demonstrate that brief stimulation (0.2–2 s) reliably elicits long‐lasting (> 30 s) persistent firing that is supported by the calcium‐activated non‐selective cationic current. In contrast to more traditional ideas, these data suggest that the hippocampal region CA1 is capable of active maintenance of memory. © 2013 Wiley Periodicals, Inc.     

Coupling in rat hippocampal slices: Dye transfer between CA1 pyramidal cells

Intracellular injections of Lucifer Yellow-CH (LY) into CA1 pyramidal cells were made in rat hippocampal slices to study dye transfer between neurons as evidence that these cells are electrotonically coupled. Extensive control procedures were performed which substantially reduced inadvertent staining. Over half of the neurons were dye-coupled after injections in stratus pyramidale. Dye coupling occurred even when spike amplitudes were greater than or equal to 70 mV throughout the impalement and was still present after chemical synapses were blocked with a low Ca2+ solution containing Mn2+. Somata of dye-coupled cells were usually located within 35 micrometers (post-fixation) of the injected cell and showed no preferred orientation. Fast prepotentials and dye coupling occurred independently. Neurons in superior cervical ganglia, which were sliced and injected using similar procedures, showed no dye coupling. Intradendritic injections of LY in stratum radiatum also yielded dye coupling between CA 1 pyramidal cells, although the dye coupling was less frequent. Within stratum radiatum, neither extracellular ejections nor intracellular injections of interneurons were associated with multiple staining. Thus, injection of LY into the soma or dendrite of a single CA1 pyramidal cell often resulted in multiple staining, and in many ensembles the somata were well spaced. Control experiments suggested that such dye transfer is not by an extracellular route. This implied that some CA1 cells are electrotonically coupled. Further electrophysiological and morphological studies are required to resolve the discrepancies among various techniques used to evaluate the amount of coupling in the hippocampus.     

Layer-specific potentiation of evoked IPSCs in rat hippocampal CA1 pyramidal cells by lanthanum

Lanthanum (La3+) potentiates or depresses GABAA receptors (GABAAR) in a subunit-dependent manner. Such differential modulators may help to discriminate between-layer-specific inhibitory synaptic inputs to individual neurons, which use different molecular GABAAR isoforms. Here, we show that inhibitory postsynaptic currents (IPSCs) in CA1 pyramidal cells are potentiated by La3+ (100 microM) when they are evoked by stimulation in stratum oriens. In contrast, stimulation in stratum radiatum yields IPSCs which are not sensitive towards La3+. These data point towards an input-specific molecular and functional diversity of inhibitory synapses at CA1 pyramidal cells.     

Thiopental attenuates hypoxic changes of electrophysiology, biochemistry, and morphology in rat hippocampal slice CA1 pyramidal cells.

BACKGROUND AND PURPOSE: Thiopental has been shown to protect against cerebral ischemic damage; however, it has undesirable side effects. We have examined how thiopental alters histological, physiological, and biochemical changes during and after hypoxia. These experiments should enable the discovery of agents that share some of the beneficial effects of thiopental. METHODS: We made intracellular recordings and measured ATP, sodium, potassium, and calcium concentrations from CA1 pyramidal cells in rat hippocampal slices subjected to 10 minutes of hypoxia with and without 600 micromol/L thiopental. RESULTS: Thiopental delayed the time until complete depolarization (21+/-3 versus 11+/-2 minutes for treated versus untreated slices, respectively) and attenuated the level of depolarization at 10 minutes of hypoxia (-33+/-6 versus -12+/-5 mV). There was improved recovery of the resting potential after 10 minutes of hypoxia in slices treated with thiopental (89% versus 31% recovery). Thiopental attenuated the changes in sodium (140% versus 193% of prehypoxic concentration), potassium (62% versus 46%), and calcium (111% versus 197%) during 10 minutes of hypoxia. There was only a small effect on ATP (18% versus 8%). The percentage of cells showing clear histological damage was decreased by thiopental (45% versus 71%), and thiopental improved protein synthesis after hypoxia (75% versus 20%). CONCLUSIONS: Thiopental attenuates neuronal depolarization, an increase in cellular sodium and calcium concentrations, and a decrease in cellular potassium and ATP concentrations during hypoxia. These effects may explain the reduced histological, protein synthetic, and electrophysiological damage to CA1 pyramidal cells after hypoxia with thiopental.     

Examination of persistent effects of repeated administration of pentylenetrazol on rat hippocampal CA1: evidence from in vitro study on hippocampal slices

The early and long-lasting effects of pentylenetetrazol-kindling on hippocampal CA1 synaptic transmission were investigated. Experiments were carried out in the hippocampal slices from control and kindled rats at two post-kindling periods, i.e. 48–144 h (early phase) and 30–33 days (long-lasting phase). Field potentials, i.e. population excitatory postsynaptic potential (pEPSP) and population spike (PS) were recorded at the stratum pyramidale following stimulation of the stratum radiatum. Kindling-induced changes in synaptic transmission were assessed by stimulus-response functions and paired-pulse responses. The results showed that 48–144 h after kindling, the PS amplitude in the CA1 of kindled slices enhanced, and a second PS appeared compared to control slices. But at 30–33 days after kindling, the pEPSP slope in the CA1 of kindled slices enhanced without any change in the PS compared with those in the control slices. Evaluation of paired-pulse responses showed a significant reduction in paired-pulse inhibition for PS 48–144 h after kindling and a significant increase in paired-pulse inhibition for pEPSP 30–33 days after kindling. Our results suggest that pentylenetetrazol-kindling is accompanied by enhanced excitability and a reduction of paired-pulse inhibition in hippocampal CA1. The increased paired-pulse inhibition one month after kindling, may be interpreted as an adaptive process to cope with subsequent seizures.     

Heat-shock pretreatment prevents suppression of long-term potentiation induced by scopolamine in rat hippocampal CA1 synapses

We examined the effect of heat-shock pretreatment on long-term potentiation (LTP) in the CA1 hippocampal slices of the rat using the muscarinic blocker scopolamine as the LTP (memory) suppressor. Time course study using immunohistochemical techniques indicated peak expression of HSP70 16 h after heat-shock treatment. Focusing on that time point we found tetanic stimulation (at 100 Hz) induced LTP of 191.1+/-12.2% in control slices (n=7), which was suppressed by scopolamine to 114.5+/-2.8 %. Heat-shock pretreatment successfully prevented such suppression (216.6+/-38.2% and 190.2+/-10.6% with and without scopolamine, respectively, n=7). Both HSP expression and LTP responses were relatively small taken either 2 or 48 h after heat-shock or sham pretreatment. These results suggest that the induction of HSPs is time-dependent and can prevent scopolamine-mediated LTP suppression.     

Metabolic changes induced in rat hippocampal slices by norepinephrine

The oxidative metabolic activity of restricted regions of hippocampal slices was assessed by a continuous measurement of the fluorescence of intramitochondrial nicotinamide-adenine dinucleotide (NADH). A large increase in NADH fluorescence was triggered by substituting the oxygen supply to the slice by nitrogen gas. A large and transient increase in NADH fluorescence was also produced by superfusion of the the slice with a high (50 mM) potassium-containing medium. Addition of norepinephrine (NE) to the superfusion medium caused a propranolol-inhibited increase in NADH fluorescence. Furthermore, ouabain, which inhibits the Na-K pump, blocked the effects of NE. An analog of cyclic adenosine monophosphate (cAMP), 8-bromo cAMP, mimicked the effect of NE. Finally, effects of NE could still be produced in a kainic acid-treated hippocampus, where most neurons were previously destroyed by the drug. It is suggested that NE activates a Na-K-ATPase, that this effect might be mediated by cAMP, and that these interrelations may underly the physiological action of NE in the brain.     

Persistent firing supported by an intrinsic cellular mechanism in hippocampal CA3 pyramidal cells

Short‐term information retention is crucial for information processing in the brain. It has long been suggested that the hippocampal CA3 region is able to support short‐term information retention through persistent neural firing. Theoretical studies have shown that this persistent firing can be supported by abundant excitatory recurrent connections in CA3. However, it remains unclear whether individual cells can support persistent firing. In this study, using in vitro whole‐cell patch‐clamp recordings in a rat hippocampal slice preparation, we show that hippocampal CA3 pyramidal cells support persistent firing under perfusion of the cholinergic agonist carbachol (10 μm ). Furthermore, in contrast to earlier theoretical studies, this persistent firing is independent of ionotropic glutamatergic synaptic transmission and is supported by the calcium‐activated non‐selective cationic current. Because cholinergic receptor activation is crucial for short‐term memory tasks, persistent firing in individual cells may support short‐term information retention in the hippocampal CA3 region.     

Spatial organization of NG2 glial cells and astrocytes in rat hippocampal CA1 region

Similar to astrocytes, NG2 glial cells are uniformly distributed in the central nervous system (CNS). However, little is known about the interspatial relationship, nor the functional interactions between these two star‐shaped glial subtypes. Confocal morphometric analysis showed that NG2 immunostained cells are spatially organized as domains in rat hippocampal CA1 region and that each NG2 glial domain occupies a spatial volume of ～178, 364 μm³. The processes of NG2 glia and astrocytes overlap extensively; each NG2 glial domain interlaces with the processes deriving from 5.8 ± 0.4 neighboring astrocytes, while each astrocytic domain accommodates processes stemming from 4.5 ± 0.3 abutting NG2 glia. In CA1 stratum radiatum, the cell bodies of morphologically identified glial cells often appear to make direct somatic‐somata contact, termed as doublets. We used dual patch recording and postrecording NG2/GFAP double staining to determine the glial identities of these doublets. We show that among 44 doublets, 50% were NG2 glia–astrocyte pairs, while another 38.6% and 11.4% were astrocyte–astrocyte and NG2 glia–NG2 glia pairs, respectively. In dual patch recording, neither electrical coupling nor intercellular biocytin transfer was detected in astrocyte–NG2 glia or NG2 glia–NG2 glia doublets. Altogether, although NG2 glia and astrocytes are not gap junction coupled, their cell bodies and processes are interwoven extensively. The anatomical and physiological relationships revealed in this study should facilitate future studies to understand the metabolic coupling and functional communication between NG2 glia and astrocytes. © 2013 Wiley Periodicals, Inc.     

Changes in rat hippocampal CA1 synapses following imipramine treatment

Neuronal plasticity in hippocampus is hypothesized to play an important role in both the pathophysiology of depressive disorders and the treatment. In this study, we investigated the consequences of imipramine treatment on neuroplasticity (including neurogenesis, synaptogenesis, and remodelling of synapses) in subregions of the hippocampus by quantifying number of neurons and synapses. Adult male Sprague-Dawley rats were injected with imipramine or saline (i.p.) daily for 14 days. Unbiased stereological methods were used to quantify the number of neurons and synapses. No differences in the volume and number of neurons of hippocampal subregions following imipramine treatment were found. However, the number and percentage of CA1 asymmetric spine synapses increased significantly and, conversely, the percentage of asymmetric shaft synapses significantly decreased in the imipramine treated group. Our results indicate that administration of imipramine for 14 days in normal rats could significantly increase the excitatory spine synapses, and change the relative distribution of spine and shaft synapses. We speculate that the present findings may be explained by the establishment of new synaptic connections and by remodelling or transformation of existing synapses.     

Reduced potassium currents in old rat CA1 hippocampal neurons

Potassium currents are an important factor in repolarizing the membrane potential and determining the level of neuronal excitability. We compared potassium currents in CA1 hippocampal neurons dissociated from young (2-3 months old) and old (26-30 months old) Sprague-Dawley rats. Whole-cell patch-clamp techniques were used to measure the delayed rectifier (sustained) and the A-type (transient) potassium currents. The delayed rectifier current was smaller in old (548 +/- 57 pA) than in young (1193 +/- 171 pA) neurons. In the absence of extracellular calcium, the delayed rectifier current was also smaller in old (427 +/- 41 pA) than in young (946 +/- 144 pA) neurons. The cell membrane capacitance was unchanged in old (13.3 +/- 1.2 pF) compared to young (13.6 +/- 1.2 pF). Therefore, the reduction in the delayed rectifier current was not due to a change in membrane surface area. Moreover, activation and inactivation of the delayed rectifier current were unchanged in old compared to young neurons. The slope of the current-voltage relation, however, was smaller in old (B = 5.03) than in young (B = 9.62) neurons. Similarly, the A-current was smaller in old (100 +/- 16 pA) than in young (210 +/- 44 pA) neurons in the presence of extracellular calcium. This reduction of potassium currents could account for the prolongation of action potentials reported previously for old rat CA1 hippocampal neurons. The age-related reduction in potassium current indicates plasticity in neuronal function that can impact communication in the hippocampal neural network during aging.     

A physiological test for electrotonic coupling between CA1 pyramidal cells in rat hippocampal slices

Antidromic stimulation in slices of rat hippocampal cortex was used to test for electrotonic coupling between CA1 pyramidal neurons. The slices were incubated in low Ca2+ medium containing Mn2+, in order to block chemical synapses. In a small percentage of intracellular recordings, short-latency depolarizations could be observed after chemical transmission had been blocked. Preceding somatic action potentials did not block the depolarizations. This indirect test provides electrophysiological evidence for coupling between some CA1 pyramidal cells.