Postnatal decrease of sodium current density in rat pituitary melanotropes following the onset of dopaminergic innervation

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Expression of tetrodotoxin-sensitive and resistant sodium channels by rat melanotrophs

Rat melanotrophs fire Na+ and Ca2(+)-dependent action potentials. Whereas the molecular identity of Ca2+ channels expressed by these cells is well documented, less is known about Na channels. We characterize the expression of seven sodium channel alpha-subunit and the beta1- and beta2-subunit mRNAs. The tetrodotoxin-resistant Nav1.8 and Nav1.9 alpha subunit mRNAs are detected in the newborn intermediate lobe and in cultured melanotrophs. Electrophysiological recordings further demonstrate the expression of both tetrodotoxin-sensitive and tetrodotoxin-resistant currents by dissociated melanotrophs. Moreover, activated sodium channels are able to elicit intracellular calcium waves, both in the absence or in the presence of tetrodotoxin. This work shows that rat melanotrophs express functional tetrodotoxin-resistant sodium channels, whose activation can lead to the generation of intracellular calcium waves.