well as synaptic vesicles in the presynaptic axonal compartment can provide even stronger evidence. Morphology alone does not prove the existence of functional synapses, but unambiguous evidence of synaptic transmission can be obtained using electrophysiological tools (Regehr and Stevens, 2001). A prerequisite for synaptic transmission is the expression of certain ionotropic neurotransmitter receptors, which can be tested by exposing the cells to specific receptor agonists such as glutamate or GABA and determining the current responses in the presence and absence of channel blockers to show specificity. Obviously, the expression of neurotransmitter receptors does not prove synaptic function. The presence of functional synapses can be demonstrated by the recording of typical spontaneous postsynaptic currents (PSCs), which display as distinct waveforms (Figure 4D,E) with sharp rise and slow decay phases caused by the dynamics of presynaptic vesicle exocytosis, and biophysical properties of postsynaptic receptors. PSCs can be recorded spontaneously or evoked by stimulation of presynaptic terminals (e.g. by extracellular stimulation). Application of specific neurotransmitter receptor blockers demonstrates specificity of the responses and distinguish inhibitory (IPSC) from excitatory postsynaptic activity (EPSC). Sometimes, noise caused by fluctuation of membrane channels during the recording can cause membrane potential deflections that appear to be bewildering synaptic
