Inhibitory effect of 5-hydroxytryptamine receptor activation on caudal neurosecretory cells of the flounder, Platichthys flesus

Immunocytochemical evidence suggests that the neuroendocrine Dahlgren cells of the teleost caudal neurosecretory system (CNSS) are innervated by descending serotonergic fibres, However, the modulatory effect(s) of 5-hydroxytryptamine (5-HT) on the activity of the CNSS are not known, The present study investigates the effect of superfusion of 5-HT and the selective 5-HT, receptor agonist 5-carboxamidotryptamine (5-CT) on the electrophysiological properties of Dahlgren cells recorded intracellularly in an isolated CNSS preparation from the flounder, Superfusion of 5-HT (10(-7)-10(-3)M) caused a concentration-dependent, reversible hyperpolarization of the resting membrane potential (E-m) of cells previously identified as 'Type 1' (putative urotensin I-secreting) cells (control = -63.5+/-1.5 mV; 10(-4)M 5-HT = -95.0+/-0.9 mV, n=6, P<0.01). The EC50 was 7.6+/-4.1 mu M (n=6). Hyperpolarization resulted in a reduction or cessation of firing of these cells, suggesting an inhibitory role for the serotonergic input to the CNSS. Hyperpolarization was accompanied by a concomitant decrease in the membrane input resistance (control =16.6+/-2.8 M Omega; 10(-4)M 5-HT=6.4+/-1.3 M Omega; n=6, P<0.05) and time constant (control = 60.3+/-13.1 ms; 10(-4)M 5-HT = 16.0+/-4.4 ms, n=6, P<0.05). These effects were mimicked by the superfusion of much lower concentrations of 5-CT (EC50=47.1+/-7.1 nM, n=4) suggesting that they are possibly mediated by a 5-HT1 receptor subtype, if the teleost 5-HT1 receptor has a markedly higher affinity for 5-CT than 5-HT1 in common with mammalian 5-HT1 receptors. In contrast to the findings in Type 1 cells, cells identified as 'Type 2' (putative urotensin It-secreting) did not respond to either 5-HT or 5-CT, suggesting that the serotonergic input into the CNSS plays no role in the modulation of activity of this sub-population of neuroendocrine cells, Accordingly, these data suggest a functional difference between Type 1 and Type 2 Dahlgren cells, previously differentiated only on electrophysiological criteria and spatial distribution within the CNSS.