The organization of hippocampal networks (Fig 1) leads to several distinct kinds of oscillation, which can be considered as 'emergent' properties of the network. (2)
Perhaps the most prominent rhythm in the hippocampus is theta (3-7 Hz) which, at least in rats, is associated with locomotion. Theta results from interactions of the hippocampus with two other limbic structures, the septum and the entorhinal cortex. Often superimposed on theta is a faster rhythm known as gamma (30-100 Hz). The best evidence we have now is that gamma is generated by local circuits in the hippocampus and that inhibitory neurones play a crucial role. (4) The role of gamma in the hippocampus remains unclear; in the neocortex it has been implicated in higher cognitive processes such as the 'binding' of individual sensory features into coherent perceived objects.
Networks for gamma rhythms(4)
The first strong clue that inhibitory neurones played a central role in gamma rhythms came from hippocampal and neocortical slices in which fast excitatory postsynaptic potentials had been blocked by drugs. Excitation by pulses of glutamate or agonist drugs acting at metabotropic (i.e. G-protein-coupled) glutamate receptors resulted in rhythmic inhibitory postsynaptic potentials in the gamma frequency band. A series of experimental tests of predictions from realistic computer simulations showed that this gamma rhythm was generated by the mutual inhibition of inhibitory neurones which produced a synchronous interruption of the fast discharge the metabotropic glutamate receptor activation would otherwise have evoked. These interruptions lasted for a time, of the order of 25 ms, that depended on the time course of the inhibitory postsynaptic potentials in interneurones. We named this phenomenon 'interneuronal network gamma'.
During interneuronal network gamma pyramidal cells generate rhythmic inhibitory postsynaptic potentials, but do not reach threshold unless they are driven by some other input. If pyramidal cells are brought close to threshold at the same time as the inhibitory neurones, the properties of gamma change. This happens when slices are subject to short trains of electrical stimuli (typically 20 shocks over a period of 200 ms), in the absence of drugs to block excitatory postsynaptic potentials. An intermediate kind of gamma rhythm occurs when slices are exposed to cholinergic drugs such as carbachol, or to non-desensitizing glutamate agonist drugs such as kainic acid. Here each pyramidal cell fires on some cycles of the rhythm, so that on average some fluctuating fraction of pyramidal cells fires on each cycle.
New properties appear in tetanic gamma, including the ability to synchronize rhythms at pairs of sites separated by long distances (several millimetres) with phase lags shorter than predicted by conduction delays. The details are complex, but one theory argues that the long-range synaptic connections from pyramidal cells to interneurones at the remote site result in action potential doublets which play an error-correction role. A second intriguing aspect of this theory concerns the slowing of gamma to beta (15-30 Hz) frequencies, which is attributed partly to the recovery of the after-hyperpolarization and partly to potentiation of synapses between the pyramidal cells (this differs from long-term potentiation, which mainly affects incoming 'afferent' synapses rather than local associational synapses). These ideas on tetanic gamma are controversial, with recent evidence pointing to an alternative non-synaptic model, which may be more closely related to epileptic seizures than to cognition. Whatever the outcome of this debate, the concepts developed for tetanic gamma are proving very valuable in understanding the carbachol/kainate gamma mentioned above, and in time will play a major role in unravelling the mechanisms of gamma rhythms in vivo.
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