Migration between tidal estuaries and open sea depends on a wide-ranging osmoregulatory capacity. This is achieved by few fish species (e.g. the flounder, Platichthys flesus) and provides increased access to rich feeding grounds. The caudal neurosecretory system (CNSS), located in the terminal eight vertebral segments of the spinal cord, contributes to the neurohormonal control of osmoregulation (Winter et al. 2000). Dahlgren cells secrete two osmoregulatory peptides (urotensins I and II) while acetylcholine (ACh) is synthesised within the CNSS in large quantities (Conlon & Balment, 1996). Dahlgren cells generate four types of electrical activity pattern (Brierley et al. 2001; unpublished data from this group). Transitions between quiescence, tonic, phasic and bursting activity patterns are likely to be important in maintaining appropriate circulating levels of neuropeptide. Here we examined the role of ACh in triggering such transitions.

Isolated CNSS dissected from fish (UK Home Office protocol), fully adapted to either seawater (SWA) or fresh water (FWA), were mounted in a cooled (9-11 °C) interface chamber and continuously superfused (0.5 ml min^-1) with aerated Ringer solution. Intra- and extracellular (multi-unit) recordings were made for up to 6 h. The selective ACh receptor agonists, nicotine (NCT, nicotinic AChR) and oxotremorine (OXT, muscarinic AChR) were superfused (100 µM in Ringer) over the CNSS for 10 min. Spontaneous transitions between firing patterns occurred in 20 % of Dahlgren cells. Superfusion with NCT or OXT triggered transitions in 44 % of cells (32 % of SWA cells and 63 % of FWA cells). Superfusion with NCT had no effect on the activity pattern of SWA bursting cells (n = 11) but triggered transitions to bursting activity in 8/17 non-bursting cells. In contrast, NCT triggered bursting in only 3/13 FWA cells and inhibited all activity in 7/9 spontaneously bursting neurones. Superfusion with OXT led to short-term inhibition of all cells leading to cessation of firing activity in 6/18 SWA and 7/11 FWA cells; the remaining cells were unaffected. Intracellular recordings showed a marked increase in membrane potential in SWA and FWA Dahlgren cells in response to OXT, leading to hyperpolarisation by 21.8 ± 5.7 mV (n = 5 cells, mean ± S.E.M.). In summary responses to muscarinic activation are largely inhibitory. However, Dahlgren cells show differential responses to nicotinic stimulation depending on their ongoing activity and adaptation state.

This work was supported by BBSRC and NERC.