Biomedical subjects

Darrin H Brager

Publications and source records attributed to Darrin H Brager.

6 recordsLinked to original sources

PubMed2005

Activity-dependent decrease of excitability in rat hippocampal neurons through increases in I(h).

Hippocampal long-term potentiation (LTP) induced by theta-burst pairing of Schaffer collateral inputs and postsynaptic firing is associated with localized increases in synaptic strength and dendritic excitability. Using the same protocol, we now demonstrate a decrease in cellular excitability that was blocked by the h-channel blocker ZD7288. This decrease was also induced by postsynaptic theta-burst firing alone, yet it was blocked by NMDA receptor antagonists, postsynaptic Ca2+ chelation, low concentrations of tetrodotoxin, omega-conotoxin MVIIC, calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitors and a protein synthesis inhibitor. Increasing network activity with high extracellular K+ caused a similar reduction of cellular excitability and an increase in h-channel HCN1 protein. We propose that backpropagating action potentials open glutamate-bound NMDA receptors, resulting in an increase in I(h) and a decrease in overall excitability. The occurrence of such a reduction in cellular excitability in parallel with synaptic potentiation would be a negative feedback mechanism to normalize neuronal output firing and thus promote network stability.

2-Amino-5-phosphonovalerate↗

PubMed2005

Endocannabinoid signaling dynamics probed with optical tools.

Intercellular signaling dynamics critically influence the functional roles that the signals can play. Small lipids are synthesized and released from neurons, acting as intercellular signals in regulating neurotransmitter release, modulating ion channels on target cells, and modifying synaptic plasticity. The repertoire of biological effects of lipids such as endocannabinoids (eCBs) is rapidly expanding, yet lipid signaling dynamics have not been studied. The eCB system constitutes a powerful tool for bioassaying the dynamics of lipid signaling. The eCBs are synthesized in, and released from, postsynaptic somatodendritic domains that are readily accessible to whole-cell patch electrodes. The dramatic effects of these lipid signals are detected electrophysiologically as CB1-dependent alterations in conventional synaptic transmission, which therefore serve as a sensitive reporter of eCB actions. We used electrophysiological recording, photolytic release of caged glutamate and a newly developed caged AEA (anandamide), together with rapid [Ca2+]i measurements, to investigate the dynamics of retrograde eCB signaling between CA1 pyramidal cells and GABAergic synapses in rat hippocampus in vitro. We show that, at 22 degrees C, eCB synthesis and release must occur within 75-190 ms after the initiating stimulus, almost an order of magnitude faster than previously thought. At 37 degrees C, the time could be < 50 ms. Activation of CB1 and downstream processes constitute a significant fraction of the total delay and are identified as major rate-limiting steps in retrograde signaling. Our findings imply that lipid messenger dynamics are comparable with those of metabotropic neurotransmitters and can modulate neuronal interactions on a similarly fast time scale.

Animals↗

PubMed2003

Regulation of exocytosis from single visualized GABAergic boutons in hippocampal slices.

Regulation of GABA release is crucial for normal brain functioning, and GABAA-mediated IPSCs are strongly influenced by repetitive stimulation and neuromodulation. However, GABA exocytosis has not been examined directly in organized tissue. Important issues remain outside the realm of electrophysiological techniques or are complicated by postsynaptic factors. For example, it is not known whether all presynaptic modulators affect release from all boutons in the same way, or whether modulator effects depend on the presence of certain types of voltage-gated calcium channels (VGCCs). To address such issues, we used confocal imaging and styryl dyes to monitor exocytosis from identified GABAergic boutons in organotypic hippocampal slice cultures. Repetitively evoked IPSCs declined more rapidly and completely than exocytosis, suggesting that depletion of filled vesicles cannot fully account for IPSC depression and underscoring the usefulness of directly imaging exocytosis. Stimulation at 10 Hz produced a transient facilitation of exocytosis that was dependent on L-type VGCCs. Using specific toxins, we found that release mediated via N-type and P-type VGCCs had similar properties. Neither baclofen nor a cannabinoid receptor agonist, CP55940, affected all boutons uniformly; they slowed release from some but completely prevented detectable release from others. Increasing stimulus frequency overcame this blockade of release. However, baclofen and CP55940 did not act identically, because only baclofen reduced facilitation and affected bouton releasing via P/Q-type VGCCs. Direct observation thus revealed novel features of GABAergic exocytosis and its regulation that would have been difficult or impossible to detect electrophysiologically. These features advance the understanding of the regulation of synapses and networks by presynaptic inhibition.

Animals↗

PubMed2003

Activity-dependent activation of presynaptic protein kinase C mediates post-tetanic potentiation.

Vesicle exocytosis is mediated by the complex interaction between synaptic vesicle and plasma membrane proteins, many of which are substrates for protein kinases. Exogenous protein kinase activators increase release probability at several mammalian CNS synapses, but the physiological conditions under which presynaptic protein kinases become activated are not known. We report here that calcium/phospholipid-dependent protein kinase C (PKC) is activated by high-frequency stimulation and mediates post-tetanic potentiation (PTP) in the rat hippocampus.

Animals↗

PubMed2003

Activity-dependent release of adenosine contributes to short-term depression at CA3-CA1 synapses in rat hippocampus.

High-frequency stimulation results in a transient, presynaptically mediated decrease in synaptic efficacy called short-term depression (STD). Stimulation of Schaffer-collateral axons at 10 Hz for 5 s resulted in approximately 75% depression of excitatory postsynaptic current (EPSC) slope recorded from CA1 cells in rat organotypic slice cultures. An adenosine A(1) receptor antagonist decreased the magnitude of STD elicited with 10-Hz stimulation by approximately 30%. The A(1) receptor antagonist had no effect on STD elicited with 3-Hz stimulation. The activation of A(1) receptors during 10-Hz stimulation was not due to the extracellular conversion of released ATP to adenosine, because block of 5'-ectonucleotidases did not significantly affect STD. The adenosine transport inhibitor dipyridamole did not reduce STD, indicating that adenosine was not released by facilitated transport. We conclude that 10-Hz, but not 3-Hz, stimulation causes the vesicular release of adenosine and the rapid (<3 s) activation of presynaptic inhibitory A(1) receptors, which account for approximately 40% of homosynaptic EPSC depression.

Adenosine↗

PubMed2002

Short-term synaptic plasticity, simulation of nerve terminal dynamics, and the effects of protein kinase C activation in rat hippocampus.

Phorbol esters are hypothesised to produce a protein kinase C (PKC)-dependent increase in the probability of transmitter release via two mechanisms: facilitation of vesicle fusion or increases in synaptic vesicle number and replenishment. We used a combination of electrophysiology and computer simulation to distinguish these possibilities. We constructed a stochastic model of the presynaptic contacts between a pair of hippocampal pyramidal cells that used biologically realistic processes and was constrained by electrophysiological data. The model reproduced faithfully several forms of short-term synaptic plasticity, including short-term synaptic depression (STD), and allowed us to manipulate several experimentally inaccessible processes. Simulation of an increase in the size of the readily releasable vesicle pool and the time of vesicle replenishment decreased STD, whereas simulation of a facilitation of vesicle fusion downstream of Ca(2+) influx enhanced STD. Because activation of protein kinase C with phorbol ester enhanced STD of EPSCs in rat hippocampal slice cultures, we conclude that an increase in the sensitivity of the release process for Ca(2+) underlies the potentiation of neurotransmitter release by PKC.

Algorithms↗