Electrophysiological characterization of cells of the caudal neurosecretory system in the teleost, Platichthys flesus

Abstract

The physiological function of the caudal neurosecretory system (CNSS) and the factors regulating its activity are poorly understood. This paper describes a novel protocol for intracellular recording from the neurosecretory Dahlgren cells in an isolated preparation from the flounder, Platichthys flesus . Using electrophysiological criteria two types of cell were identified, designated “Type 1” and “Type 2.” Type 1 cells frequently showed spontaneous action potentials (0.5–8 Hz) and subthreshold excitatory post-synaptic potentials, while these were rarely present in Type 2 cells. Both cell types showed outward rectification in response to injected depolarizing current, which was more pronounced in Type 2 cells than in Type 1 cells. Electrophysiological characteristics of Type 1 cells were E m (resting membrane potential) −58.8 ± 0.9 mV (mean ± SEM); R i (input resistance) 23.6 ± 3.0 MΩ; τ (time constant) 37.8 ± 5.6 msecs; action potential amplitude 91.8 ± 2.2 mV; action potential width 4.3 ± 1.0 msecs; threshold potential −48.8 ± 0.8 mV (n = 34). For Type 2 cells the equivalent parameters were E m −605 ± 1.8 mV; R i 29.7 ± 6.8 MΩ; τ 38.5 ± 10.4 msecs; action potential amplitude 75.5 ± 5.0 mV; action potential width 5.1 ± 0.7 msecs; threshold potential −45.5 ± 2.4 mV (n = 13). The action potential amplitude differed significantly ( P < 0.01) between the two cell populations, as did the amplitude ( P < 0.05) and duration of the after-hyperpolarization ( P < 0.0001). The morphology of the cells as revealed by Lucifer Yellow injection was broadly similar in the two types. Both had a characteristic looped axon leaving the perikaryon, as well as ipsilateral and contralateral dendritic trees with extensive blebbing. However, the dendritic trees of Type 1 cells were more complex, and ramified further within the spinal cord, than those of Type 2 cells. Using electrophysiological criteria, cells defined as Type 1 were located predominantly in preterminal vertebral segments IV–VI of the spinal cord and Type 2 cells mainly in preterminal segments I–III. This spatial distribution is consistent with earlier immunocytochemical studies and strongly suggests that these cells represent urotensin I and urotensin II secreting populations respectively. The isolated preparation of the CNSS is clearly a valuable model for further investigation of the mechanisms controlling neurosecretory cell activity.