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1 Department of Biology and Biochemistry, University of Houston, 4800 Calhoun Road, Houston, TX 77204-5513, USA E-mail: ccolbert{at}uh.edu
Although the basic trisynaptic hippocampal circuit (Andersen et al. 1966) is one of the most extensively studied circuits in the brain, there remain classes of hippocampal neurons that are poorly understood. Most of these cells have been identified as interneurons by electrophysiological or immunohistochemical methods. However, a heterogeneous class of projection neurons in hippocampal CA1, containing both pyramidal and non-pyramidal morphologies, has been described that appears to be excitatory on grounds of ultrastructure and axonal labelling (radiatum giant cells, Gulyas et al. 1998).
A paper by Bullis et al. (2007) in this issue of The Journal of Physiology extends our knowledge of a putative subset of these neurons in CA1 that the authors describe as pyramidal-like principal (PLP) neurons. These neurons display pyramidal morphologies with apical trunks and arborizations that extend into s. radiatum and s. lacunosum-moleculare, suggesting that the source of their input is similar to that of pyramidal cells. Only their somata, which lie in s. radiatum, suggest a difference from typical CA1 pyramids. Bullis et al. demonstrate using retrograde axonal labelling that PLPs project to the septum and olfactory bulb, demonstrating their role as principal cells. These subcortical projections of CA1, particularly the olfactory bulb, suggest a circuit distinct from the classic entorhinal–hippocampal–subicular–entorhinal loop. The relatively scarce PLPs, while embedded within CA1 and perhaps receiving similar input, may provide a direct top-down feedback signal to the bulb and other subcortical structures.
Bullis et al. report that a major difference between PLPs and pyramidal cells is the density gradient of Ih channels along the apical trunk. Ih contributes to input resistance, resting membrane potential, resonant and oscillatory behaviour, and temporal integration (reviewed in Robinson & Siegelbaum, 2003). Input at distal sites is normalized by Ih, reducing the relative attenuation of distal EPSPs compared to proximal EPSPs. In large pyramidal neurons, such as those in CA1 and neocortical layer V, the density of Ih channels increases dramatically with increasing distance from the soma (Magee, 1998; Berger et al. 2001). Remarkably, the density of Ih channels in PLP dendrites decreases with distance from the soma. The other properties of Ih currents the authors examined were similar to those of CA1 pyramids. The reverse gradient of Ih suggests that, while PLPs likely receive input from similar sources to CA1 pyramids, the appropriate integration and normalization of synaptic input for neurons of this ‘circuit within a circuit’ differ qualitatively from the much more abundant pyramidal neurons.
References
Berger T, Larkum ME & Luscher HR (2001). J Neurophysiol 85, 855–868.
Bullis JB, Jones TD & Poolos NP (2007). J Physiol 579, 431–443.
Gulyas AI, Toth K, McBain CJ & Freund TF (1998). Eur J Neurosci 10, 3813–3822.[CrossRef][Medline]
Magee JC (1998). J Neurosci 18, 7613–7624.
Robinson RB & Siegelbaum SA (2003). Annu Rev Physiol 65, 453–480.[CrossRef][Medline]
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