Modulatory Influences on the Hippocampus and Entorhinal Cortex
The functional regulation of cortical circuits depends on neuromodulators such as acetylcholine, norepinephrine, serotonin, and dopamine to alter the information processing mediated by fast transmitters such as glutamate and GABA. The primary mechanisms s
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Modulatory Influences on the Hippocampus and Entorhinal Cortex Kishan Gupta and Michael E. Hasselmo
Abstract
The functional regulation of cortical circuits depends on neuromodulators such as acetylcholine, norepinephrine, serotonin, and dopamine to alter the information processing mediated by fast transmitters such as glutamate and GABA. The primary mechanisms shared by the neuromodulators include altering the dynamics of excitatory and inhibitory synaptic transmission, altering synaptic modification properties, and changing the resting membrane potential. Aside from synaptic modulation, the same neuromodulators can also affect spike frequency with prominent examples including pyramidal cell spike frequency adaptation. Though the cellular effects of neuromodulators may be similar, a diversity of upstream systems regulates the influence of competing neuromodulators both in the spatial distribution and temporal dynamics of release. The diversity of neuromodulator receptor subtypes also influences the nature of the cellular effects of neuromodulation. The purpose of this chapter is not to differentiate between competing neuromodulatory systems, but to survey evidence for how different neuromodulators affect change in information transmission in the hippocampal system. This chapter also highlights physiological examples of acetylcholine effects in active research areas of entorhinal persistent spiking, subthreshold properties of stellate cells, and theta modulation of hippocampal and entorhinal networks.
K. Gupta • M.E. Hasselmo (*) Center for Memory and Brain, Department of Psychology, Graduate Program for Neuroscience, Boston University, 2 Cummington St., Boston, MA 02215, USA e-mail: [email protected] D. Derdikman and J.J. Knierim (eds.), Space, Time and Memory in the Hippocampal Formation, DOI 10.1007/978-3-7091-1292-2_7, # Springer-Verlag Wien 2014
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K. Gupta and M.E. Hasselmo
Introduction
A number of neuromodulatory systems influence the functional properties of the hippocampus and entorhinal cortex (EC) and their role in memory function. Neuromodulators including acetylcholine, norepinephrine, serotonin, and dopamine alter the processing of information within these cortical circuits by altering the intrinsic properties of neurons, by causing presynaptic inhibition of glutamatergic and GABAergic synaptic transmission, by altering synaptic modification properties within these circuits, or by altering the spiking properties of cortical neurons. Physiological and behavioral data indicate that neuromodulators play an important functional role in these circuits. For example, the encoding of episodic memory appears to depend upon the circuit dynamics induced by the neuromodulatory effects of acetylcholine. Patients administered scopolamine prior to encoding a word list perform worse in subsequent recall of those words compared to administration post-encoding (Ghoneim and Mewaldt 1975; Petersen 1977). This effect may be mitigated if test subjects are highly familiar with the task (Sarter et al. 2003) p
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