• Activity-dependent short-term depression of inhibitory synaptic currents in the hippocampus

      Whittington, Kevin C.; Alger, Bradley Eugene (2010)
      An understanding of the regulation of GABAergic synaptic inhibition is important because of its influence on neuronal excitability, synaptic plasticity, and disease processes in the central nervous system. Aberrant inhibition is believed to be the cause of pathophysiological processes, such as epilepsy and mood disorders (e.g. anxiety). I seek to determine what regulates inhibitory transmission at short time scales (milliseconds - minutes) in the hippocampus, the structure that provides a cognitive map of the physical environment and, in addition, is the locus of explicit memory formation. I use electrophysiological and pharmacological tools to study inhibitory transmission at certain interneuron-pyramidal neuron synapses in the CA1 field of the rat hippocampus. Certain interneurons, those that contain cholecystokinin, also express the presynaptic cannabinoid receptor (CB1) as well as the GABAB autoreceptor, both key mediators of presynaptic inhibition. Hippocampal interneurons can fire in rhythmic, brief bursts. However, the roles of endogenous cannabinoids (endocannabinoids; eCBs) and GABAB autoreceptors in regulating inhibitory postsynaptic currents (IPSCs) elicited by such bursts has not been described. The fundamental hypothesis is that certain features of the short-term depression induced by these bursts are mediated by eCBs and others by GABA acting on GABAB autoreceptors. I find that eCB-mediated depression of pyramidal cell IPSCs develops slowly (tau ~ 30 s). IPSC depression mediated by the GABAB autoreceptor occurs in parallel with the eCB-mediated depression, but it is restricted to the time domain of hundreds of milliseconds (tau ~ 100 ms). Interestingly, although CB1 and GABAB receptors are expressed on the same nerve terminals, have the same effector (the Gi/o G protein), and the same molecular targets (Ca2+ and K+ channels), they reduce inhibitory transmission via non-interacting and distinct mechanisms. I also discovered that eCBs are released from cells via a novel mechanism. I conclude that a new form of short-term depression mediated by eCBs is present at certain inhibitory synapses of the hippocampus, and may help understand the functions of hippocampal neuronal circuits.
    • Cholinergic involvement in the GABAergic regulation of inhibition in the hippocampus

      Martin, Laura Ann; Alger, Bradley Eugene (1997)
      Synaptic inhibition modulates neuronal excitability. In addition, synaptic inhibition itself can be modulated. Hippocampal pyramidal cells undergo a process called depolarization-induced suppression of inhibition (DSI), in which a brief depolarization of the pyramidal cell results in a transient decrease in GABA A-receptor-mediated inhibition in that cell. DSI is initiated in the pyramidal cell, but is expressed presynaptically as a decrease in GABA release from the interneurons synapsing with that cell. DSI of spontaneous, action-potential-dependent, inhibitory postsynaptic currents (sIPSCs) seldom occurs in the absence of carbachol, a cholinergic agonist. The goal of this thesis is to identify the mechanism by which cholinergic stimulation facilitates DSI, using whole-cell, voltage-clamp recordings in CA1 pyramidal cells in the rat acute hippocampal slice preparation. Muscarinic, but not nicotinic, receptor activation increased the frequency and amplitude of sIPSCs and induced DSI. Pharmacological data suggest that M1 or M3 muscarinic receptors mediate this effect. Although cholinergic stimulation greatly facilitated DSI of sIPSCs, atropine-insensitive DSI occurred in the absence of carbachol, under certain circumstances (e.g., DSI of evoked IPSCs (eIPSCs)), suggesting that biochemical effectors activated by mAChR are not essential for induction of the DSI mechanism. Several lines of evidence suggest that carbachol facilitates DSI by increasing interneuronal activity. For example, DSI is not observed until large amplitude sIPSCs occur, and when the large amplitude sIPSCs are eliminated, DSI is no longer present. Norepinephrine and elevated extracellular potassium also increased sIPSC activity and induced DSI, which, in some cells, was comparable to that produced with carbachol. Moreover, baclofen, a GABA B receptor agonist which inhibits only a subset of IPSCs, reversibly blocked DSI of sIPSCs in carbachol, as well as DSI of eIPSCs in the presence of atropine. Therefore, DSI-susceptible IPSCs, whether activated by carbachol or electrical stimulation, originate from a subpopulation of interneurons possessing GABA B receptors. Additional experiments using localized electrical stimulation confirmed that only certain IPSCs are susceptible to DSI. We conclude that a subpopulation of interneurons, activated by cholinergic synaptic input, is particularly sensitive to DSI. Muscarinic receptor activation appears to facilitate DSI by increasing the occurrence of IPSCs susceptible to it.
    • Optogenetic Dissection of Interneuron Microcircuits Driving Cholinergically-Activated Rhythms in CA1 Hippocampus

      Nagode, Daniel A.; Alger, Bradley Eugene (2012)
      Perisomatic GABAergic inhibition is thought to play a prominent role in hippocampal oscillations associated with the release of acetylcholine (ACh) from septal cholinergic afferents. A prominent hypothesis states that parvalbumin (PV)-containing basket cells, activated concurrently by ACh and fast glutamatergic input, drive these rhythms. However, this model - generated solely from in vitro studies using bath application of cholinergic agonists - has never been tested under conditions of endogenous ACh release. To investigate the selective effects of endogenous ACh on inhibitory microcircuits in slices, we have taken an optogenetic approach by injecting choline acetyltransferase (ChAT)-Cre mice with a Cre-dependent AAV vector carrying the light-activated cation channel Channelrhodopsin2 (ChR2). In acute hippocampal slices taken from these animals, brief trains of blue light generate sustained low-frequency rhythmic IPSCs in CA1 pyramidal cells, and inhibitory local field potentials (LFPs), even in the presence of ionotropic glutamate receptor antagonists. These rhythms are almost entirely blocked by the muscarinic ACh receptor (mAChR) antagonist atropine, and are potently suppressed by the endocannabinoid-mediated process of depolarization-induced suppression of inhibition (DSI). This suggests that the IPSCs are driven type 1 cannabinoid receptor (CB1R)-expressing interneurons, which in the hippocampus comprise the cholecystokinin (CCK)-positive basket cells, not PV+ basket cells. Surprisingly, the oscillations are also greatly inhibited by activation of mu-opioid receptors, which are indeed highly concentrated on the terminals of PV+ basket cells, but not CCK+ cells. However, selective pharmacological inhibition as well as optogenetic silencing of PV+ cells using the light-driven chloride pump Halorhodopsin (NpHR), or the proton pump Archaerhodopsin (Arch), had no significant effect on the rhythms. Surprisingly, IPSCs evoked through direct optogenetic stimulation of CCK+ cells were sensitive to a mu-opioid receptor agonist. This finding is inconsistent with previous anatomical studies, and suggests the presence of an inhibitory oscillator in CA1 which is activated by endogenous ACh, driven solely by CCK+ basket cells, and regulated by both endogenous cannabinoids and opioids.
    • Partial inhibition of sodium,potassium-ATPase activity modulates neuronal excitability in area CA1 of rat hippocampus

      Mason, Susanne Elizabeth; Alger, Bradley Eugene (2001)
      Because of its physiological role in maintaining cellular homeostasis, regulation of the Na+,K+-ATPase is an ideal candidate mechanism for modulating excitability. Cardiac glycosides, which are specific ligands of the Na+,K+-ATPase, induce epileptiform burst firing in rat hippocampus that resembles interictal burst potentials observed between epileptic seizures. We investigated a role for shifts in transmembrane ion gradients, intrinsic membrane properties of principal neurons, coupling of postsynaptic dendritic inputs and firing properties, and the balance between excitatory and inhibitory synaptic inputs in burst firing induced by bath application of the cardiac glycoside dihydroouabain (DHO) in area CA1 of in vitro rat hippocampus.;Simultaneous field potential and K+-ion sensitive microelectrode recordings established that epileptiform burst firing induced by DHO was temporally correlated with inhibition of the Na+,K+-ATPase, but independent of increases in resting [K+]o of <1 mM or changes in intrinsic membrane properties of pyramidal neurons. DHO did enhance the ability of a given EPSP to generate an action potential. This enhanced EPSP-spike (E-S) coupling was associated with a prolonged depolarizing potential, presumably due to activation or enhancement of dendritic conductances. Enhanced E-S coupling, but not activation of the prolonged depolarizing potential, was occluded by the GABAA receptor antagonist, picrotoxin. DHO caused partial global suppression of excitatory and inhibitory synaptic transmission. Therefore, we conclude that partial inhibition of Na+,K +-ATPase activity causes epileptiform burst firing by creating an imbalance between excitatory and inhibitory inputs, in which overall excitation predominates. Partial Na+,K+-ATPase inhibition also caused a long-term suppression of fEPSPs that was of a similar magnitude as that induced by a chemical LTD protocol. Like the conventional LTD process, the DHO-induced long-term suppression could reverse a previously induced state of long-term potentiation (LTP). Unlike conventional LTD, however, the reversal of LTP by DHO did not require activation of NMDA or metabotropic glutamate receptors, suggesting that DHO activates signaling pathways downstream of the receptors. Therefore, in addition to a short-term effect on neuronal excitability, partial inhibition of Na+,K+-ATPase activity also mediates long-term synaptic plasticity in rat hippocampus.