Neuronal and Cellular Signalling Lecture 4 Notes: The Role of Calcium in the Nervous System Calcium and Neurotransmitter Release - Soluble Ca2+ can act as a: (a) Charge carrier (e.g. cardiac action potential) (b) Signalling molecule (e.g. energy output, cellular metabolism) - Most intracellular Ca2+ is buffered (i.e. bound to other molecules). The concentration of free calcium is very low to maintain a large electrochemical gradient and prevent interference with biological processes (e.g. signalling cascades). - Insoluble Ca2+ (majority of calcium in the body) is the major structural constituent of bones and teeth - The equilibrium (Nernst/reversal) potential is the voltage at which there is no net movement of ions across the membrane. If a membrane was selectively permeable to K+ alone, its membrane potential would be Ex, however the presence of other ions (e.g. calcium) has some effect on the equilibrium potential (influence of Ca2+ changes depending on cell type). To calculate the equilibrium potential of Ca2+ the following formula can be used: I 2+¿ Cai ¿ ¿ out ¿ 2+¿ Ca ¿ ? ? ¿ ¿ ¿ ¿ RT log i Ca2+= .2+¿ zF E; - The valence (z) of Ca2+ is 2, RT is 58, [Ca]out is 2 and [Ca]in is 0.0001. Therefore, the equation can be solved (for Eca) as +125mV. - At the NMJ, Ca2+ facilitates action potential-triggered neurotransmitter release from the presynaptic cell. Low extracellular Ca2+ concentration reduces the likelihood of end plate potential (EPP, gives rise to action potentials) generation in the postsynaptic cell. Ca2+ serves as a second messenger during neurotransmitter release. - If a presynaptic cell is clamped at -70mV (i.e. close to physiological membrane potential) whilst Na+ and K+ channels are inactivated, and it is depolarised to -25mV, an AP is triggered. Moderate depolarisation 50 presynaptic potential (mV) 0 Cell 1 -25 mV -70 0 presynaptic Ca2+ current (LA) -3 -6 od do is Cell 2 +50 mV A 'tail' Ca2+ current -25 postsynaptic (HA) current -50 -75 5 ms time ->
results in Ca2+ entry into the presynaptic terminal (i.e. presynaptic Ca2+ current) followed by a postsynaptic EPP. - However, if the cell is depolarised to +50mV (i.e. strong depolarisation) a presynaptic Ca2+ current/postsynaptic EPP is not generated. 1 Ca2+ is necessary and sufficient for neurotransmitter release, as demonstrated by experiments using fluorescent dyes bound to calcium which visualises Ca2+ entry into cells. Alternately, if Ca2+ channels are blocked in the postsynaptic cell, no membrane potential will be generated in the postsynaptic cell. - Neurotransmitter release is aided by proteins (most are anchored to a membrane via transmembrane helices) such as synaptotagmin (attached to the vesicle, senses and binds to calcium), syntaxin and SNAP-25. This complex is known as the SNARE complex. - Firstly, free SNARES on the vesicle membrane (synaptobrevin and synaptotagmin) are not connected to proteins on the plasma membrane. These SNARE proteins complex (i.e. with the syntaxin and SNAP-25 on the postsynaptic membrane), shortly followed by synaptotagmin binding (i.e. is not initially part of the SNARE