Event:

The neural mechanisms for spatial memory formation are believed to comprise an integration of processes mediated by hippocampal synaptic plasticity in the form of long-term potentiation (LTP) and long-term depression (LTD) (Morris, Eur J Neurosci 2006; Kemp and Manahan-Vaughan, TINS 2007). Novel space consists of several types of information, however, that may evoke differential synaptic responses in individual hippocampal subregions. We have observed that in all hippocampal subregions (CA1, CA3, dentate gyrus), and at multiple types of hippocampal synapses (perforant path- dentate gyrus, mossy fiber-CA3, Schaffer collateral-CA1) exploration of a novel spatial environment facilitates the expression of robust LTP (>24h) in freely behaving rats (Kemp and Manahan-Vaughan, PNAS, 2004; Hagena and Manahan-Vaughan, Cereb Cortex 2011).
By contrast, LTD facilitation is subregion- and synapse- specific and dependent on the nature of the spatial content. For example, In the CA1 region, partially concealed, small, positional cues have a facilitatory effect on LTD. On the other hand, LTD in the dentate gyrus is facilitated by large directional cues. Thus, although LTP is facilitated uniformly (but synapse-specifically) in both areas by the same novel environment, LTD is facilitated in a region-specific manner, and is based on type of novel spatial cue presented (Kemp and Manahan-Vaughan, Cereb Cortex 2008). Strikingly, active exploration is not required: facilitation of LTD by spatial contextual information also occurs when the information is presented on a computer screen whilst the animal is stationary (Kemp and Manahan-Vaughan, Cereb Cortex 2012).
Patterned afferent stimulation is not a prerequisite for the appearance of synaptic plasticity in association with declarative learning- plasticity also occurs in an input- specific manner when merely test-pulse stimulation is given (Manahan-Vaughan & Braunewell, PNAS 1999; Goh & Manahan-Vaughan, Cereb Cortex 2013). Furthermore, effects are also not species-specific, as mice also respond to spatial learning events by expressing synaptic plasticity (Goh & Manahan-Vaughan, Cereb Cortex 2013).
Taken together, these data suggest that LTP and LTD contribute to the encoding and storage of different components of a spatial representation and that LTD may be specifically involved in storing information about spatial context. The direction of change of synaptic strength is determined by the behavioural relevance of the spatial exerience and the level of arousal of the animal. Here, noradrenaline and dopamine release from the locus coeruleus play a critical role (Lemon et al, Cereb Cortex 2009).