The flounder is capable of full osmoregulatory adaptation to both freshwater (< 10 mosmol l^-1) and seawater (1000 mosmol l^-1) environments as part of its circannual migration. This involves major functional changes in osmoregulatory organs such as gut, gills, bladder and kidney. The caudal neurosecretory system (CNSS), located in the terminal segments of the spinal cord, plays an important role in this process. Magnocellular Dahlgren cells synthesise and secrete neuropeptide hormones, including urotensin I, urotensin II and possibly parathyroid hormone-related peptide, which are involved in this osmoregulatory adaptation. This study describes the electrophysiological properties underlying the repetitive firing activity of Dahlgren cells.

Isolated CNSS taken from humanely killed fish fully adapted to seawater conditions were mounted in a cooled (9-11 °C) interface recording chamber and continuously superfused (0.5 ml min^-1) with aerated flounder Ringer solution. Intracellular sharp microelectrode current clamp recordings were made in the presence of 1 µM tetrodotoxin (TTX). Other agonists and antagonists were superfused over the CNSS for at least 15 min before taking further recordings. Statistics are presented as means ± S.E.M. (n = number of neurons); P values were calculated using Student’s two-tailed paired t test.

The rising phase of Dahlgren cell action potentials (ca 80 mV amplitude) consists of TTX-sensitive Na⁺ (ca 50 mV) and nifedipine- (10 µM) sensitive Ca²⁺ (ca 30 mV) components. A sag potential (1-4 mV) was generated in response to hyperpolarising square wave current injection (-0.3 to -1.2 nA), which was followed by a depolarising after potential (DAP, 2-9 mV). Both of these responses were voltage dependent (Brierley et al. 2001) and may contribute to repetitive firing activity. The hyperpolarisation-activated sag potential was absent during perfusion with tetraethylammonium (TEA, 1 mM; n = 3) or Cs⁺ (1 mM; n = 3) ions, indicating that it is mediated by a potassium conductance. Nifedipine (10 µM) significantly reduced DAP amplitude (n = 3, V_M = -45 mV, -1.2 nA pulses, 95.0 ± 5.0 % reduction, P < 0.0001) suggesting it was mediated by L-type Ca²⁺ channels. DAPs were not generated in nominally Ca²⁺ free Ringer solution, and were maintained when Ba²⁺ (1 mM) was substituted for Ca²⁺ (n = 2) as the charge carrying cation. Sag potentials persisted in the presence of nifedipine or Ba²⁺, and in nominally Ca²⁺-free Ringer solution. Since peptide secretion is likely to depend on firing pattern and rate in Dahlgren cells, modulation of sag potential and DAP may contribute to changes in secretory activity during seawater-freshwater adaptation.

This work was this work was supported by a BBSRC research grant.