title: Spontaneous spiking activities of porcine pars intermedia cells: effects of thyrotropin-releasing hormone.
abstract: The electrophysiological properties of porcine melanotrophs in primary culture were studied with patch clamp techniques. In the cell-attached (c/a) configuration, extracellular spikes were recorded but in only 47 of the 259 cells examined; active cells were more frequently found in short-term cultures (48 h, 22 out of 71). In the whole-cell (WCR) configuration, the mean resting potential was -46.6 mV and the mean input resistance 2.41 G omega. In the current clamp mode, of the 99 cells recorded, 39.4% displayed spontaneous spiking activities, 14.1% spiked when a depolarizing current was applied, and 46.5% were silent. Two patterns of spontaneous activity were recorded. The first consisted of rapid membrane depolarizations (mean duration: 16.0 ms) which reached a mean value of +8.9 mV. These action potentials occurred either at random intervals or at a mean frequency of 1.19 Hz. Tetrodotoxin (TTX), a sodium channel blocker, completely abolished these action potentials. The second pattern consisted of regular bursts (mean duration: 1.69 s and mean frequency: 0.15 Hz) of spikes with smaller amplitudes (culminating at -9.5 mV) and greater durations (79.7 ms). This pattern could be recorded in the presence of TTX in the external medium. In the c/a configuration, thyrotropin-releasing hormone (TRH) and (3Me-His2)-TRH enhanced spike frequency, whereas histidyl-proline-diketopiperazine, a metabolite of TRH, had no effect. In WCR, out of the 27 cells tested, TRH (5.10(-9)-5.10(-8) M) induced firing in 4 quiescent cells and increased the frequency of action potentials in 4 spontaneously active cells. This was usually, but not necessarily, preceded by a hyperpolarization (n = 5). TRH (5.10(-8) M) enhanced the secretion of melanocyte-stimulating hormone (alpha MSH) from perfused isolated melanotrophs by 62.8 +/- 9.8% (n = 9) over basal levels. (3Me-His2)-TRH and (Pro3)-TRH mimicked the TRH response, whereas histidyl-proline-diketopiperazine was without effect.
authors.affiliation: Laboratoire de Neuroendocrinologie Comparée, Université Louis Pasteur, Strasbourg, France.
authors.forename: J
authors.initials: J
authors.lastname: Trouslard
authors.forename: B A
authors.initials: BA
authors.lastname: Demeneix
authors.forename: P
authors.initials: P
authors.lastname: Feltz
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D005260:Female; D007033:Hypothalamus, Middle / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D008297:Male; D009074:Melanocyte-Stimulating Hormones / Q000097:blood / Q000276:immunology; D013552:Swine; D013779:Tetrodotoxin / Q000494:pharmacology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
chemical_list: D015220:Calcium Channels; D013779:Tetrodotoxin; D013973:Thyrotropin-Releasing Hormone; D009074:Melanocyte-Stimulating Hormones