Abstract

Spike timing-dependent plasticity (STDP) is a causal form of Hebb’s law of synaptic plasticity, where the precise timing of the presynaptic and postsynaptic action potentials determines the sign and magnitude of synaptic modifications (Bell et al., 1997; Bi and Poo, 1998; Magee and Johnston, 1997; Markram et al., 1997; Debanne et al., 1998; Sjostrom et al., 2001; Yao and Dan, 2001; Zhang et al., 1998). In their pioneering study, Bi and Poo (1998) showed that the shape of the STDP curve in the in-vitro hippocampal network has an asymmetric shape with the largest LTP/LTD value at Δτ = tpost - tpre = +/-10 ms, respectively. New experimental evidence has shown that the STDP asymmetry can sometimes change with the target and the location of the synapse (Tzounopoulos et al., 2004; Froemke et al., 2005; Letzkus et al., 2006; Caporale and Dan, 2009) and can be dynamically regulated by the activity of adjacent synapses (Harvey and Svoboda et al., 2007; Caporale and Dan, 2009) or by the action of neuromodulators (Seol et al., 2007; Caporale and Dan, 2009). Nishiyama and colleagues (2000) reported that "...the profile of STDP induced in the hippocampal CA1 network with inhibitory interneurons is symmetrical for the relative timing of pre- and postsynaptic activation". Optical imaging studies in CA1 revealed that the shape of the STDP curve depends on the location on the stratum radiatum (SR) dendrite. A symmetric STDP profile was observed in the proximal-to-the-soma SR dendrite and an asymmetric STDP profile in the distal-to-the-soma one (Tsukada et al., 2005; Aihara et al., 2007). They suggested that this change in the shape of the STDP curve (i.e. from symmetry to asymmetry and vice versa) may be due to inhibition in the proximal SR dendrites (Tsukada et al., 2005). The functional consequences of such a change in the STDP temporal kernel dynamics are of great importance in neural network dynamics. A symmetrical STDP profile with short temporal windows may serve as a coincidence detector between the incoming inputs and plays a functional role in heteroassociation of memories (Cutsuridis et al., 2010). On the other hand an asymmetric STDP profile with broad temporal windows may play a role in chunking of ordered items in sequence learning (Hayashi and Igarashi, 2009).
Up-to-now very few studies (Cutsuridis, 2010, 2011, 2012, 2013) have investigated the inhibitory factors (frequency, strength, timing, etc.) that are responsible for such a change in the shape in the STDP temporal kernel and the conditions under which a transition from asymmetrical STDP kernel to a symmetrical STDP kernel is possible. In this chapter, I will present two simplified compartmental models of CA1 pyramidal cells in order to investigate the role of theta-modulated inhibition on the shape, sign and magnitude of the STDP kernel in CA1 pyramidal cell proximal dendrites.