Apamin is the smallest neurotoxin polypeptide known, and the only one that passes the blood-brain barrier. Apamin thus reaches its target organ, the central nervous system. Here it inhibits small-conductance Ca-activated K channels (SK channels) in neurons. These channels are responsible for the afterhyperpolarizations that follow action potentials, and therefore regulate the repetitive firing frequency.
Three different types of SK channels show different characteristics. Only SK2 and SK3 are blocked by apamin, whereas SK1 is apamin insensitive. SK channels function as a tetramer of subunits. Heteromers have intermediate sensitivity. SK channels are activated by the binding of intracellular Ca to the protein calmodulin, which is constitutively associated to the channel. Transport of potassium ions out of the cell along their concentration gradient causes the membrane potential to become more negative. The SK channels are present in a wide range of excitable and non-excitable cells, including cells in the central nervous system, intestinal myocytes, endothelial cells, and hepatocytes.
Binding of apamin to SK channels is mediated by amino acids in the pore region as well as extracellular amino acids of the SK channel. It is likely that the inhibition of SK channels is caused by blocking of the pore region, which hinders the transport of potassium ions. This will increase the neuronal excitability and lower the threshold for generating an action potential. Other toxins that block SK channels are tamapin and scyllatoxin.