Abstract
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-HT1 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 (Em) 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 microM (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 Momega; 10(-4) M 5-HT = 6.4 +/- 1.3 MD; 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-HT, in common with mammalian 5-HT1 receptors. In contrast to the findings in Type 1 cells, cells identified as 'Type 2' (putative urotensin II-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.
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