Dudman Lab: Publications

Many computations in neural circuits rely upon appropriate patterning of action potentials. The patterns of action potentials a neuron fires are shaped by its complement of membrane ion channels. However, while the total ionic current across a neuron’s membrane is determined by stochastic transitions between open and closed states of many individual ions channels, most neuronal models are deterministic and consider only the average behavior of populations of ion channels. We show that a model of stellate neurons found in layer II of the entorhinal cortex, when implemented using stochastically gating ion channels, accounts well for experimentally described properties of these neurons, including firing of clustered patterns of action potentials and functional changes caused by deletion of HCN1 channels. Analysis of the model shows that clustered patterns of action potentials arise through transient increases in the probability of spike initiation during a brief time window following recovery from a preceding action potential. This model provides an example of a general mechanism for patterning of neuronal activity through brief activity-dependent changes in spike probability and may help explain the patterns of spikes fired by entorhinal neurons that encode spatial location in behaving animals.

Distal dendrites of hippocampal CA1 pyramidal neurons receive direct sensory information that is relevant to spatial learning and memory through the perforant path synapses from the entorhinal cortex. The HCN1 hyperpolarization-activated cation channels are targeted to these distal dendrites where they act as an inhibitory constraint of synaptic integration and long-term plasticity. However, since HCN channels contribute a depolarizing current, the mechanism of their inhibitory action remains unclear. Here we report that HCN1 constrains synaptically-evoked distal dendritic Ca2+ spikes, which have previously been implicated in the induction of LTP at the perforant path synapses. Experimental and computational results indicate that resting HCN channels provide a steady-state depolarization that increases resting inactivation of T-type and N-type voltage-gated Ca2+ channels, thereby limiting distal Ca2+ influx. This mechanism by which Ih constrains voltage-gated Ca2+ channel availability may represent a general means by which these channels regulate dendritic excitability and calcium electrogenesis at distal compartments of pyramidal neurons where high densities of h-channels are found.

Synaptic potentials originating at distal dendritic locations are severely attenuated when they reach the soma and, thus, are poor at driving somatic spikes. Nonetheless, such inputs often convey essential information, suggesting that distal inputs may be important for compartmentalized dendritic signaling. Here we report a new plasticity rule in which stimulation of the distal perforant path inputs to hippocampal CA1 pyramidal neurons induces long-term potentiation at proximal Schaffer collateral synapses when the two inputs are paired at a precise interval. This subthreshold form of heterosynaptic plasticity occurs in the absence of somatic spiking but requires activation of both NMDA receptors and release of Ca2+ from internal stores. Our results suggest that direct sensory information arriving at distal CA1 synapses through the perforant path may serve a novel function: providing compartmentalized, instructive signals that assess the saliency of mnemonic information propagated through the hippocampal circuit to proximal synapses.

Whereas recent studies have elucidated principles for representation of information within the entorhinal cortex, less is known about the molecular basis for information processing by entorhinal neurons. The HCN1 gene encodes ion channels that mediate hyperpolarization-activated currents (Ih) that control synaptic integration and influence several forms of learning and memory. We asked if HCN1 channels may control processing of information by stellate shaped cells found within layer II of the entorhinal cortex. Axonal projections from this non-pyramdial class of principle neuron form a major component of the synaptic input to the dentate gyrus of the hippocampus. To investigate the influence of HCN1 channels on resting and active properties of stellate neurons we carried out whole-cell recordings in horizontal brain slices prepared from adult wild-type and HCN1 knockout mice. We find that HCN1 channels are required for rapid and full activation of hyperpolarization-activated currents in stellate neurons. HCN1 channels dominate the membrane conductance at rest and suppress low frequency (< 4 Hz) components of spontaneous and evoked membrane potential activity. In addition, we find that when stellate cells receive continuous depolarizing input sufficient for firing of repeated action potentials, HCN1 channels control the pattern of spike output by promoting recovery of the spike afterhyperpolarization. These data suggest that HCN1 channels may control the input to the hippocampal dentate gyrus by distinct actions on integration by entorhinal stellate cells during resting and active states.

Ligand-gated ion channels involved in the modulation of synaptic strength are the AMPA, kainate, and NMDA glutamate receptors. Small molecules that potentiate AMPA receptor currents relieve cognitive deficits caused by neurodegenerative diseases such as Alzheimer's disease and show promise in the treatment of depression. Previously, there has been limited understanding of the molecular mechanism of action for AMPA receptor potentiators. Here we present cocrystal structures of the glutamate receptor GluR2 S1S2 ligand-binding domain in complex with aniracetam [1-(4-methoxybenzoyl)-2-pyrrolidinone] or CX614 (pyrrolidino-1,3-oxazino benzo-1,4-dioxan-10-one), two AMPA receptor potentiators that preferentially slow AMPA receptor deactivation. Both potentiators bind within the dimer interface of the nondesensitized receptor at a common site located on the twofold axis of molecular symmetry. Importantly, the potentiator binding site is adjacent to the "hinge" in the ligand-binding core "clamshell" that undergoes conformational rearrangement after glutamate binding. Using rapid solution exchange, patch-clamp electrophysiology experiments, we show that point mutations of residues that interact with potentiators in the cocrystal disrupt potentiator function. We suggest that the potentiators slow deactivation by stabilizing the clamshell in its closed-cleft, glutamate-bound conformation.

Molecular adaptations are believed to contribute to the mechanism of action of antipsychotic drugs (APDs). We attempted to establish common gene regulation patterns induced by chronic treatment with APDs. METHODS: Gene expression analysis was performed with the Affymetrix U34A array in the frontal cortex (FC) and the striatum of rats chronically treated with two concentrations of either clozapine or haloperidol. Key data were verified with real-time quantitative polymerase chain reaction. RESULTS: Many genes in the FC affected by APD-treatment contribute to similar functions. mRNAs coding for synaptic vesicle docking- and microtubule-associated proteins were upregulated; mRNAs for serine-threonine protein phosphatases were downregulated, whereas the serine-threonine kinases protein kinase A, protein kinase C, and calcium/calmodulin kinase II alpha and IV were upregulated, indicating increased potential for protein phosphorylation. In the striatum, altered gene expression was less focused on genes of particular function or location, and the high concentration of haloperidol had a different gene expression profile than any of the other APD treatments. CONCLUSION: We found an increase in the transcription of genes coding for proteins involved in synaptic plasticity and synaptic activity in the FC. We furthermore found that the gene expression profile of APDs is different between FC and striatum.

Responses to threat-related stimuli are influenced by conscious and unconscious processes, but the neural systems underlying these processes and their relationship to anxiety have not been clearly delineated. Using fMRI, we investigated the neural responses associated with the conscious and unconscious (backwardly masked) perception of fearful faces in healthy volunteers who varied in threat sensitivity (Spielberger trait anxiety scale). Unconscious processing modulated activity only in the basolateral subregion of the amygdala, while conscious processing modulated activity only in the dorsal amygdala (containing the central nucleus). Whereas activation of the dorsal amygdala by conscious stimuli was consistent across subjects and independent of trait anxiety, activity in the basolateral amygdala to unconscious stimuli, and subjects' reaction times, were predicted by individual differences in trait anxiety. These findings provide a biological basis for the unconscious emotional vigilance characteristic of anxiety and a means for investigating the mechanisms and efficacy of treatments for anxiety.

The importance of long-term synaptic plasticity as a cellular substrate for learning and memory is well established. By contrast, little is known about how learning and memory are regulated by voltage-gated ion channels that integrate synaptic information. We investigated this question using mice with general or forebrain-restricted knockout of the HCN1 gene, which we find encodes a major component of the hyperpolarization-activated inward current (Ih) and is an important determinant of dendritic integration in hippocampal CA1 pyramidal cells. Deletion of HCN1 from forebrain neurons enhances hippocampal-dependent learning and memory, augments the power of theta oscillations, and enhances long-term potentiation (LTP) at the direct perforant path input to the distal dendrites of CA1 pyramidal neurons, but has little effect on LTP at the more proximal Schaffer collateral inputs. We suggest that HCN1 channels constrain learning and memory by regulating dendritic integration of distal synaptic inputs to pyramidal cells.

In contrast to our increasingly detailed understanding of how synaptic plasticity provides a cellular substrate for learning and memory, it is less clear how a neuron's voltage-gated ion channels interact with plastic changes in synaptic strength to influence behavior. We find, using generalized and regional knockout mice, that deletion of the HCN1 channel causes profound motor learning and memory deficits in swimming and rotarod tasks. In cerebellar Purkinje cells, which are a key component of the cerebellar circuit for learning of correctly timed movements, HCN1 mediates an inward current that stabilizes the integrative properties of Purkinje cells and ensures that their input-output function is] independent of the previous history of their activity. We suggest that this nonsynaptic integrative function of HCN1 is required for accurate decoding of input patterns and thereby enables synaptic plasticity to appropriately influence the performance of motor activity.

Addictive drugs such as amphetamine and cocaine stimulate the dopaminergic system, activate dopamine receptors and induce gene expression throughout the striatum. The signal transduction pathway leading from dopamine receptor stimulation at the synapse to gene expression in the nucleus has not been fully elucidated. Here, we present evidence that D1 receptor stimulation leads to phosphorylation of the transcription factor Ca2+ and cyclic AMP response element binding protein (CREB) in the nucleus by means of NMDA receptor-mediated Ca2+ signaling. Stimulation of D1 receptors induces the phosphorylation of Ser897 on the NR1 subunit by protein kinase A (PKA). This phosphorylation event is crucial for D1 receptor-mediated CREB phosphorylation. Dopamine cannot induce CRE-mediated gene expression in neurons transfected with a phosphorylation-deficient NR1 construct. Moreover, stimulation of D1 receptors or increase in cyclic AMP levels leads to an increase in cytosolic Ca2+ in the presence of glutamate, but not in the absence of glutamate, indicating the ability of dopamine and cyclic AMP to facilitate NMDA channel activity. The recruitment of the NMDA receptor signal transduction pathway by D1 receptors may provide a general mechanism for gene regulation that is fundamental for mechanisms of drug addiction and long-term memory.