Distinct rat neurohypophysial nerve terminal populations identified by size, electrophysiological properties and neuropeptide content

Abstract

Voltage-gated ion channels are critical to excitation-secretion coupling in nerve terminals. We have identified two distinct populations of rat neurohypophysial (NHP) terminals distinguished by size, neuropeptide content and electrophysiological properties, including resting membrane potential, action potential (AP) properties, and K + current and Na + current characteristics. In large terminals (10–16 μm diameter), resting membrane potential was more negative than in small terminals (5–9.9 μm; −61±4 mV vs. −55±3 mV; p<0.01), action potential amplitude was larger (69±4 mV vs. 53±3 mV; p<0.01), peak I K was larger (1460±90 pA vs. 1140±70 pA; p<0.05) with a more negative V 1/2 for activation (−3.1 mV vs. −0.6 mV; p<0.05), and Na + current density was greater (∼470 pA/pF vs. ∼300 pA/pF; p<0.01) with more negative V 1/2 values for activation from −70 or −90 mV holding potentials (−44 mV vs. −24 mV; p<0.01). A positive linear correlation between I Na amplitude and terminal size showed an inflection at a diameter of 9.2 μm. Neuropeptide content was generally segregated into a population of small terminals (<10 μm diameter) containing predominantly vasopressin and a population of large terminals (≥10 μm diameter) containing predominantly oxytocin (OT); a small fraction of terminals in each group contained both peptides. These findings suggest that electrophysiological differences between small vasopressin-containing and large oxytocin-containing neurohypophysial terminals may contribute to their observed differential firing and peptide release patterns.