Question

9. Long term potentiation lasting between 30 minutes and an hour doesn't require new protein synthesis. Please explain. 10. Explain the statement: "The level and timing of Ca²+ rise in spine determines LTD or LTP".

Answer 1

Answer 9. Earlier by people, memories were supposed to be of two type: Long term memory and short term memory. But as science advanced, molecular mechanisms were explored and nowadays memory is considered to be encoded in the neuronal netwoks in brain by its special characterstic, synaptic plasticity i.e. changes in the strength of synapse. Synapse is a chemical process in which neuronal axon end release neurotransmitters which are further recepted by the dendrites of another neuron and message is delivered. In synaptic plasiticity the most common is LTP (Long-term potentiation) carried out in CA1 region of Hippocampus in brain.

It involves two process in relation with memory:

• LTP consists of two different temporal phases, S-LTP and L-LTPand each requires a different type of triggering stimulation. The short-lasting LTP is for about 1-3 hours and is induced with a single high-frequency stimulation i.e.100 Hz for1 s, whereas long-lasting LTP (L-LTP) exists for more than 4 h and requires repeated stimulation at different time interavl as 3 or 4, 5 or 10 min apart.
• The late phase of L-LTP depends on protein synthesis process, triggered by the stimulus which induces LTP. But the early phase of L-LTP and the S-LTP are induced by a single stimulus would depend on the post-translational modifications of pre-existing proteins and on attachment of AMPA receptors into the postsynaptic density of the dendritic spines,

So, Long lasting potential for short period (30 minutes to 1 hour) doesnot require new protein synthesis as one trigger is enough.

Answer 10.  Long-term depression (LTD) is an activity-dependent reduction of neuronal synapses efficiency which can last for hours or longer and followed by a long patterned stimulus. LTD can occurs in many region of the brain Central nervous system with different mechanisms depending upon the brain area and its developmental progress. Calcium plays an important role and its high concentration is a neccessity in LTD induction.

Calcium signaling in the post-synaptic neuronal cell involves both spatial and temporal overlap of climbing fibre and induces the release of calcium in dendrites. Similarly, parallel fibre calcium release induces mGluRs and IP3 signalling cascade. In the climbing fibres, due to voltage-gated calcium channels calcium release, a regenerative action potential is induced by the AMPAR-mediated depolarization which spreads to the dendrites. It is paired with PF-mediated mGluR1 activation and ultimately results in LTD induction. Whereas, in the parallel fibres, constant GluRs activation indirectly induces the binding of IP3 to its receptor (IP3) and results in activate calcium release from intracellular storage. During this calcium induction, a positive feedback loop is induced which regenerates calcium for long-term depression. Both parallel and climbing fibres should be active together to depolarize the Purkinje cells and activate mGlur1s. The calcium release involves Parallel Fibre activation just before CF activity.