Properties of a T-Type Ca Channel–Activated Slow Afterhyperpolarization in Thalamic Paraventricular Nucleus and Other Thalamic Midline Neurons

Zhang L, Renaud LP, Kolaj M. Properties of a T-type Ca channel– activated slow afterhyperpolarization in thalamic paraventricular nucleus and other thalamic midline neurons. J Neurophysiol 101: 2741–2750, 2009. First published March 25, 2009; doi:10.1152/jn.91183.2008. Burst firing mediated by a low-threshold spike (LTS) is the hallmark of many thalamic neurons. However, postburst afterhyperpolarizations (AHPs) are relatively uncommon in thalamus. We now report data from patch-clamp recordings in rat brain slice preparations that reveal an LTS-induced slow AHP (sAHP) in thalamic paraventricular (PVT) and other midline neurons, but not in ventrobasal or reticular thalamic neurons. The LTS-induced sAHP lasts 8.9 0.4 s and has a novel pharmacology, with resistance to tetrodotoxin and cadmium and reduction by Ni or nominally zero extracellular calcium concentration, which also attenuate both the LTS and sAHP. The sAHP is inhibited by 10 mM intracellular EGTA or by equimolar replacement of extracellular Ca with Sr , consistent with select activation of LVA T-type Ca channels and subsequent Ca influx. In control media, the sAHP reverses near EK , shifting to 78 mV in 10.1 mM [K ]o and is reduced by Ba 2 or tetraethylammonium. Although these data are consistent with opening of Ca -activated K channels, this sAHP lacks sensitivity to specific Ca -activated K channel blockers apamin, iberiotoxin, charybdotoxin, and UCL-2077. The LTS-induced sAHP is suppressed by a -adrenoceptor agonist isoproterenol, a serotonin 5-HT7 receptor agonist 5-CT, a neuropeptide orexin-A, and by stimulation of the cAMP/protein kinase A pathway with 8-Br-cAMP and forskolin. The data suggest that PVT and certain midline thalamic neurons possess an LTS-induced sAHP that is pharmacologically distinct and may be important for information transfer in thalamic–limbic circuitry during states of attentiveness and motivation.