Do Action Potentials in Vasopressin Magnocellular Neurons Back‐propagate from Soma to Dendrites to Evoke Neuropeptide Dendritic Release?

Abstract

In addition to releasing their peptidergic cargo into the systemic circulation from their axons in the posterior pituitary, magnocellular neurosecretory vasopressin (VP) and oxytocin (OT) neurons in the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei also release these neuropeptides from their dendrites. Dendritic release of VP and OT play important functional roles, including recruitment and coordination of neurohumoral responses to an osmotic stimulation. Dendritic release of neuropeptides involves Ca2+‐dependent exocytosis, which in other neuronal types, including substantia nigra dopaminergic neurons, has been shown to be triggered by backpropagating action potentials (bAPs). However, it is at present unknown whether action potentials in VP neurons can back‐propagate from soma to dendrites, and whether they participate in dendritic release of VP. To directly test this, we performed simultaneous dual patch‐clamp electrophysiological recordings from somata (whole‐cell, current‐clamp) and dendrites (loose patch, voltage clamp) in identified VP neurons of eGFP‐VP rats. Various degrees and patters of somatic firing activity were evoked by direct current injection through the somatic patch pipette. Surprisingly, we found evidence for bAPs only in a very small proportion of recorded VP neurons (~20%, 91/435 recordings). When present, bAPs appeared as fast inward deflections that occurred within less than 1 millisecond from the evoked somatic AP. bAPs displayed high fidelity, even at relatively high somatic firing frequencies (up to 10 Hz), with almost no propagation failures observed within the measured distance from the soma. The presence or absence of bAPs was not dependent on neuronal input resistance, dendritic access resistance or dendritic distance from the soma (15–100 μM) Similarly, neither the number, frequency nor pattern (continuous or bursting) of somatic action potential had an influence on the incidence or properties of bAPs (p> 0.5 in all cases). Blockade of K+ channels with 4AP (5 mM n=11) or TEA (30 mM, n=7) did not enabled somatic action potentials to back‐propagate to the patched dendrite. Ongoing morphometric analysis of patched neurons aim at determining whether back‐propagation of action potential correlates with specific morphological features of these neurons. Taken together, our results indicate that despite being highly electrotonically compact, the vast majority of neurosecretory VP neurons do not show evidence for bAPS. These findings raise important questions regarding mechanisms underlying activity‐dependent dendritic release of neuropeptides in hypothalamic neurons.Support or Funding InformationNIH R01NS094640 to JES