Proceedings of The Physiological Society
University of Bristol (2005) J Physiol 567P, PC58
5-HT modulates mouse hypothalamic arcuate RipCre neuronal excitability in a 5-HT2 receptor-independent manner
Hisadome, Kazunari; Smith, Mark A; Ashford, Michael L.J.; Withers, Dominic J;
1. Pathology and Neuroscience, University of Dundee, Dundee, United Kingdom. 2. Centre for Diabetes and Endocrinology, University College London, London, United Kingdom.
Hypothalamic arcuate neurones that express the gene for pro-opiomelanocortin (POMC) form a distinct neuronal population that controls energy homeostasis. Many neuromodulators affect the excitability of these neurones and specifically, serotonin (5-HT) may modulate these neurones via activation of 5-HT2C receptors (Heisler et al. 2002). Recently we have identified a novel arcuate neuronal population in mice, which regulate energy homeostasis, by the expression of green fluorescent protein (GFP) induced by the rat insulin 2 promoter (RipCre) transgene (Choudhury et al. 2005). To determine if 5-HT could modulate these neurones, whole cell current-clamp recordings were made from hypothalamic arcuate neurones (GFP positive) identified by epifluorescence and differential interference contrast microscopy, as previously described (Choudhury et al. 2005). Mice (8-16 weeks) were humanely killed. All data are expressed as means ± S.E.M. RipCre neurons had a mean resting potential of -50 ± 1 mV (n = 32) and spontaneously fired sodium-mediated action potentials. Application of 5-HT to the perfusion solution (1 μM) or ejection (2 μM) directly above the recording neurone had a mixed affect on neuronal excitability. In the majority of recordings (44 %) 5-HT hyperpolarized RipCre neurones from -52 ± 2 mV to -69 ± 2 mV (n = 14/32). Alternatively 5-HT either failed to induce a response (34 %), or depolarized neurones from -48 ± 2 mV to -43 ± 2 mV (n = 7/32). It has been suggested that 5-HT2C receptors play an important role in hypothalamic control of energy homeostasis. However, the non-specific 5-HT2 receptor agonist, α-methyl-5-HT (α -me-5-HT; 1 μM) failed to induce any changes in neuronal excitability (n = 5). At higher concentrations (5 - 10 μM), α-Me-5-HT mimicked the effects of 5-HT (n = 9); yet these, and 5-HT, responses could not be inhibited by the 5-HT2 antagonist ketanserin (10 - 100 nM; n = 4), or by selective 5-HT2B (SB204741, 10 nM; n = 2) or 5-HT2C (SB242084, 100nM; n = 2) receptor antagonists. These data strongly suggest that the effects of high concentrations of α-me-5-HT are probably non-specific. In contrast, 5-carboxamidotryptamine (5-CT, 1 μM) hyperpolarized RipCre neurones from -49 ± 1 to -68 ± 8 mV (n = 3/4). These data suggest that 5-HT 1, 5, 6 or 7 receptors may modulate the 5-HT response in RipCre neurons. However, the 5-HT1A receptor agonist, 8-hydroxy-di-n-propylamino tetralin (8-OH DPAT, 1 μM), failed to alter excitability (n = 4).
Where applicable, experiments conform with Society ethical requirements