**3.3.3 Cooperativity**

298 Neuroscience – Dealing with Frontiers

Even though plasticity events can be distinguished as either LTP or LTD, a huge variation in forms of LTP has also been observed. Several factors contribute to these different types of LTP. LTP varies with the type of molecular pathway involved in its induction as can be seen in the case of LTP in hippocampal CA1 region which is either N-methyl-D-aspartate receptor (NMDAR) dependent or independent. Different regions of brain show different forms of LTP. The age of the organism also contributes to the variation in LTP. LTP in neonatal (<9 postnatal days) rodent hippocampal CA1 region is different from that in mature animals (Yasuda et al., 2003). Based on the pre and postsynaptic activity patterns required for induction, LTP can be classified as Hebbian, Non-Hebbian, anti-Hebbian and neo-Hebbian. Hebbian LTP requires both pre and post synaptic activity at the same time for its induction (Hebb, 1949). Non-Hebbian LTP requires activation of either pre or post synaptic compartment and does not need simultaneous depolarization of pre- and postsynaptic cells; an example of this occurs in the mossy fibre hippocampal pathway. Anti-Hebbian LTP can be formed by a conjunction of presynaptic depolarization and postsynaptic hyperpolarization (Lamsa et al., 2007). This can be induced in several classes of interneurons in strata oriens and pyramidale by high or low frequency stimulation patterns applied to axon collaterals of local pyramidal neurons, as long as the postsynaptic membrane potential is kept negative to the action potential threshold (Kullmann & Lamsa, 2008). In addition to these three forms of LTP, neo-Hebbian LTP has been described recently which is related with the late phase of LTP induced by an NMDAR dependent process (Lisman et al., 2011). This form of LTP not only depends on the two factors of the Hebbian condition (glutamate release and postsynaptic depolarization), but also on a third factor, dopamine release. Dopamine will enhance the protein synthesis within the dendrites of

Analyses of LTP decay rates and biochemical mechanisms have revealed three different forms of LTP (LTP-1, 2 and 3) in the hippocampus. LTP-1, induced by a single train of conditioning stimulation, is short lasting (2–3 h *in vitro*) and is dependent on post-translational modifications of existing synaptic proteins. LTP-2, induced by several repetitions of a conditioning train, is an intermediate form of LTP that depends on protein synthesis but not transcription of new mRNA. Finally, LTP-3, induced by multiple, spaced repetitions of conditioning stimulation, is very durable (perhaps even permanent in some cases, e.g.

Abraham et al.*,* 2002) and requires gene transcription and translation (Raymond, 2008).

explained using NMDA receptor dependent LTP mechanisms.

adjacent synapses. But this is not valid for closely located synapses.

Three key properties of LTP were elucidated in the 1970s and early 1980s. All of these can be

LTP can be generated only in those synapses which had undergone activation and not in

One of the interesting properties of LTP mechanism is its associativity. If a weak non-LTP inducing stimulation in one afferent is paired with a strong LTP inducing stimulation in

**3.2 Types of LTP** 

hippocampal neurons (Smith, 2005).

**3.3 Properties of LTP** 

**3.3.1 Input specificity** 

**3.3.2 Associativity** 

LTP can also be generated by weaker stimulation of a crucial number of presynaptic fibres to achieve a threshold stimulation to activate a postsynaptic neuron to induce LTP. This property is called cooperativity because different presynaptic fibres are cooperatively eliciting LTP.
