Dendrites Of Hippocampal CA1 Pyramidal Cells Research Articles

Neuron‐restrictive silencer factor‐mediated hyperpolarization‐activated cyclic nucleotide gated channelopathy in experimental temporal lobe epilepsy

Enduring, abnormal expression and function of the ion channel hyperpolarization-activated cyclic adenosine monophosphate gated channel type 1 (HCN1) occurs in temporal lobe epilepsy (TLE). We examined the underlying mechanisms, and investigated whether interfering with these mechanisms could modify disease course. Experimental TLE was provoked by kainic acid-induced status epilepticus (SE). HCN1 channel repression was examined at mRNA, protein, and functional levels. Chromatin immunoprecipitation was employed to identify the transcriptional mechanism of repressed HCN1 expression, and the basis for their endurance. Physical interaction of the repressor, NRSF, was abolished using decoy oligodeoxynucleotides (ODNs). Video/electroencephalographic recordings were performed to assess the onset and initial pattern of spontaneous seizures. Levels of NRSF and its physical binding to the Hcn1 gene were augmented after SE, resulting in repression of HCN1 expression and HCN1-mediated currents (I(h) ), and reduced I(h) -dependent resonance in hippocampal CA1 pyramidal cell dendrites. Chromatin changes typical of enduring, epigenetic gene repression were apparent at the Hcn1 gene within a week after SE. Administration of decoy ODNs comprising the NRSF DNA-binding sequence (neuron restrictive silencer element [NRSE]), in vitro and in vivo, reduced NRSF binding to Hcn1, prevented its repression, and restored I(h) function. In vivo, decoy NRSE ODN treatment restored theta rhythm and altered the initial pattern of spontaneous seizures. Acquired HCN1 channelopathy derives from NRSF-mediated transcriptional repression that endures via chromatin modification and may provide insight into the mechanisms of a number of channelopathies that coexist with, and may contribute to, the conversion of a normal brain into an epileptic one.

Kv4.2 knockout mice demonstrate increased susceptibility to convulsant stimulation

Kv4.2 subunits contribute to the pore-forming region of channels that express a transient, A-type K(+) current (A-current) in hippocampal CA1 pyramidal cell dendrites. Here, the A-current plays an important role in signal processing and synaptic integration. Kv4.2 knockout mice show a near elimination of the A-current in area CA1 dendrites, producing increased excitability in this region. In these studies, we evaluated young adult Kv4.2 knockout mice for spontaneous seizures and the response to convulsant stimulation in the whole animal in vivo and in hippocampal slices in vitro. Electroencephalogram electrode-implanted Kv4.2 knockout and wild-type mice were observed for spontaneous behavioral and electrographic seizures. The latency to seizure and status epilepticus onset in Kv4.2 knockout and wild-type mice was assessed following intraperitoneal injection of kainate. Extracellular field potential recordings were performed in hippocampal slices from Kv4.2 knockout and wild-type mice following the bath application of bicuculline. No spontaneous behavioral or electrographic seizures were observed in Kv4.2 knockout mice. Following kainate, Kv4.2 knockout mice demonstrated a decreased seizure and status epilepticus latency as well as increased mortality compared to wild-type littermates. The background strain modified the seizure susceptibility phenotype in Kv4.2 knockout mice. In response to bicuculline, slices from Kv4.2 knockout mice exhibited an increase in epileptiform bursting in area CA1 as compared to wild-type littermates. These studies show that loss of Kv4.2 channels is associated with enhanced susceptibility to convulsant stimulation, supporting the concept that Kv4.2 deficiency may contribute to aberrant network excitability and regulate seizure threshold.

H channel-dependent deficit of theta oscillation resonance and phase shift in temporal lobe epilepsy

I h tunes hippocampal CA1 pyramidal cell dendrites to optimally respond to theta inputs (4–12 Hz), and provides a negative time delay to theta inputs. Decreased I h activity, as seen in experimental temporal lobe epilepsy (TLE), could significantly alter the response of dendrites to theta inputs. Here we report a progressive erosion of theta resonance and phase lead in pyramidal cell dendrites during epileptogenesis in a rat model of TLE. These alterations were due to decreased I h availability, via a decline in HCN1/HCN2 subunit expression resulting in decreased h currents, and altered kinetics of the residual channels. This acquired HCN channelopathy thus compromises temporal coding and tuning to theta inputs in pyramidal cell dendrites. Decreased theta resonance in vitro also correlated with a reduction in theta frequency and power in vivo. We suggest that the neuronal/circuitry changes associated with TLE, including altered I h-dependent inductive mechanisms, can disrupt hippocampal theta function.

Intracellular calcium increases in hippocampal CA1 pyramidal cell dendrites during the induction of long-term depression (LTD)

Intracellular calcium increases in hippocampal CA1 pyramidal cell dendrites during the induction of long-term depression (LTD)

Arachidonic Acid Reciprocally Alters the Availability of Transient and Sustained Dendritic K+Channels in Hippocampal CA1 Pyramidal Neurons

The dendrites of hippocampal CA1 pyramidal cell dendrites express a high density of transient A-type K(+) channels, which play a critical role in the back-propagation of action potentials and in the determination of dendritic excitability. Recently, arachidonic acid and its nonmetabolizable analogue 5,8,11,14-eicosatetraynoic acid (ETYA) were shown to block transient K(+) channels in the somata of these cells (), but to have little effect on the somatic action potential. In the present study we have investigated the effects of arachidonic acid and ETYA on the gating of channels and the excitability of the apical dendrites of CA1 pyramidal neurons. We found not only a block of transient K(+) channels, but also an enhancement of sustained outward currents. The sustained currents consisted of at least two distinct channel types. The larger conductance channel (>50 pS) was identified as a K(+) channel. Arachidonic acid greatly enhanced the amplitude of back-propagating dendritic action potentials (>200 micrometer from the soma) but did not result in sustained depolarizations of the dendrites similar to those seen with 4-aminopyridine (4-AP) application. In fact, arachidonic acid reduced dendritic excitability when applied after 4-AP. Thus, arachidonic acid appears to cause a shift of available channels from the fast, transient type to the slower, sustained types. The net effect appears to be an enhancement of dendritic action potential amplitude that occurs without compromising the electrical stability of the dendrites.

Modulation of mammalian dendritic GABA(A) receptor function by the kinetics of Cl- and HCO3- transport.

1. During prolonged activation of dendritic GABAA receptors, the postsynaptic membrane response changes from hyperpolarization to depolarization. One explanation for the change in direction of the response is that opposing HCO3- and Cl- fluxes through the GABAA ionophore diminish the electrochemical gradient driving the hyperpolarizing Cl- flux, so that the depolarizing HCO3- flux dominates. Here we demonstrate that the necessary conditions for this mechanism are present in rat hippocampal CA1 pyramidal cell dendrites. 2. Prolonged GABAA receptor activation in low-HCO3- media decreased the driving force for dendritic but not somatic Cl- currents. Prolonged GABAA receptor activation in low-Cl- media containing physiological HCO3- concentrations did not degrade the driving force for dendritic or somatic HCO3- gradients. 3. Dendritic Cl- transport was measured in three ways: from the rate of recovery of GABAA receptor-mediated currents between paired dendritic GABA applications, from the rate of recovery between paired synaptic GABAA receptor-mediated currents, and from the predicted vs. actual increase in synaptic GABAA receptor-mediated currents at progressively more positive test potentials. These experiments yielded estimates of the maximum transport rate (vmax) for Cl- transport of 5 to 7 mmol l-1 s-1, and indicated that vmax could be exceeded by GABAA receptor-mediated Cl- influx. 4. The affinity of the Cl- transporter was calculated in experiments in which the reversal potential for Cl- (ECl) was measured from the GABAA reversal potential in low-HCO3- media during Cl- loading from the recording electrode solution. The calculated KD was 15 mM. 5. Using a standard model of membrane potential, these conditions are demonstrated to be sufficient to produce the experimentally observed, activity-dependent GABA(A) depolarizing response in pyramidal cell dendrites.

Pairing the cholinergic agonist carbachol with patterned Schaffer collateral stimulation initiates protein synthesis in hippocampal CA1 pyramidal cell dendrites via a muscarinic, NMDA-dependent mechanism

Effects of afferent stimulation on local synthesis of protein in CA1 pyramidal cell dendrites were studied using light microscope autoradiography. Tissue was fixed with paraformaldehyde immediately after 3 min exposure to 3H-leucine in order to trap 3H associated with macromolecules. The rate of 3H-leucine incorporation into dendrites of resting hippocampal slices was 10% the rate of incorporation into cell somata. Ninety percent of the incorporation into the somata was inhibited by cycloheximide (300 microM); none of the incorporation into dendrites was blocked by cycloheximide. Thus, there is no measurable extramitochondrial synthesis of protein in the dendrites of the resting slice. Slices were exposed to 50 microM carbachol and the Schaffer collateral afferents to the CA1 pyramidal cells were stimulated intermittently at 10 Hz over a 20 min period. In this case, 3H incorporation into dendrites was increased almost threefold over resting levels, with no effect on label over the cell somata. There was no associated increase in uptake of free 3H-leucine, and the increase in label was completely blocked by cycloheximide. Thus, associating carbachol and afferent stimulation appears to activate de novo protein synthesis in the dendrites. Neither the carbachol alone nor the Schaffer collateral stimulation alone increased synthesis. The activation of dendrite synthesis was completely blocked by 5 microM atropine, and also by 50 microM D-aminophosphonovalerate. It did not occur when carbachol was paired with steady stimulation of the Schaffer collaterals at 1 Hz for 20 min, rather than with the patterned high-frequency stimulation. Thus, associating a cholinergic agonist with a level of neural activity that occurs in CA3 and CA1 pyramidal cells during exploratory behavior (Muller et al., 1987) initiates local protein synthesis in target dendrites. This effect is dependent on muscarinic cholinergic receptors and NMDA-type glutamate receptors. The possible relationship of this phenomenon to mechanisms of learning is discussed.

Morphological alterations in rat CA1 hippocampal pyramidal cell dendrites resulting from chronic ethanol consumption and withdrawal.

Hippocampal CA1 pyramidal cell dendrites were studied in rats after 5 months of consumption of an ethanol liquid diet and 5 months of ethanol diet followed by 2 months of withdrawal. Morphometric data were compared with those obtained from matched littermate, yoke -fed control animals. Dendritic branching in Golgi-Cox-stained tissues was assessed by standard and modified Sholl analysis techniques and basilar dendrites were analysed three-dimensionally by computer. Five months of chronic ethanol consumption caused a significant decrease in the number of second-order basilar dendrites, 60-90 micron from the apical border of the cell layer. No significant changes in the neuronal density of CA1 or CA3 cells were found; however, the thickness of the strata oriens and radiatum of the CA1 field was significantly decreased in the ethanol-fed group. After 5 months of chronic ethanol consumption and 2 months of withdrawal, the thickness of the strata returned to control sizes and the frequency of proximal basilar branching recovered. Evidence of lengthening and new branching of distal basilar dendrites occurred in the third-, fourth-, and fifth-order segments when control animals 6 and 8 months of age were compared. During the 2-month period of withdrawal, the number and length of third-, fourth-, and fifth-order segments of basilar dendrites increased when compared to the nonwithdrawn ethanol group while the number and length of second- and third-order segments decreased. This is comparable to the changes seen during normal aging and suggests that withdrawal may interact with aging to produce enhanced dendritic growth in "compensation" for the developmental retardation induced by chronic ethanol intake.