Regulation of thyrotropin-releasing hormone-expressing neurons in paraventricular nucleus of the hypothalamus by signals of adiposity - PubMed (original) (raw)
Regulation of thyrotropin-releasing hormone-expressing neurons in paraventricular nucleus of the hypothalamus by signals of adiposity
Masoud Ghamari-Langroudi et al. Mol Endocrinol. 2010 Dec.
Abstract
Fasting-induced suppression of thyroid hormone levels is an adaptive response to reduce energy expenditure in both humans and mice. This suppression is mediated by the hypothalamic-pituitary-thyroid axis through a reduction in TRH levels expressed in neurons of the paraventricular nucleus of the hypothalamus (PVN). TRH gene expression is positively regulated by leptin. Whereas decreased leptin levels during fasting lead to a reduction in TRH gene expression, the mechanisms underlying this process are still unclear. Indeed, evidence exists that TRH neurons in the PVN are targeted by leptin indirectly via the arcuate nucleus, whereas correlative evidence for a direct action exists as well. Here we provide both in vivo and in vitro evidence that the activity of hypothalamic-pituitary-thyroid axis is regulated by both direct and indirect leptin regulation. We show that both leptin and α-MSH induce significant neuronal activity mediated through a postsynaptic mechanism in TRH-expressing neurons of PVN. Furthermore, we provide in vivo evidence indicating the contribution of each pathway in maintaining serum levels of thyroid hormone.
Figures
Figure 1
Single-plane confocal images from 35-μm-thick brain sections indicate that a large fraction of TRH-GFP neurons in the PVN are labeled by ip injections of FG and not immunoreactive to neurophysin (NPH). Sections were cut frozen by a sledge microtome and stained free floating. A, Panels show the same portion of the PVN indicating immunoreactivity to GFP, FG, and overlay; B, frequency and distribution of staining for TRH-GFP, FG, and fraction of GFP-expressing neurons that labeled with FG in mouse PVN along the anterior-posterior axis; C, panels show the same portion of the PVN indicating immunoreactivity to GFP, FG, NPH, and overlay; D, fractions of TRH-GFP neurons that do not co-stain with FG (green), co-stain with FG (yellow), and co-stain with FG and NPH (white) and the fraction of NPH-, FG-positive neurons that do not express TRH-GFP (magenta) in mouse PVN.
Figure 2
α-MSH activates firing activity of TRH-GFP neurons in the PVN through MC4-R signaling. A, Voltage trace of a whole-cell recording from a TRH-GFP neuron firing spontaneous action potentials. Note that bath application of 250 n
m
α-MSH generates reversible increases in frequency of firing associated with a depolarization. B, Frequency histogram of this recording. C, Traces obtained from different time points of the same recording (indicated by a–c on the histogram) expanded in time scale to clearly demonstrate changes in firing rate and membrane potential in association with applications of the peptide. D and E, Means ±
sem
of action potential firing frequency (D) and membrane potentials (E) in control and in 250–350 n
m
α-MSH. *, P < 0.0001. F, Voltage trace of whole-cell recording indicates that bath application of 250 n
m
α-MSH fails to increase firing activity in neurons pretreated with 50 n
m
HS014. G, Frequency histogram of this effect. H, Bar graphs indicate the means ±
sem
of effects of 250 n
m
α-MSH on five neurons pretreated with 50 n
m
HS014 (n = 5; P > 0.05 by paired t test).
Figure 3
The excitatory effects of α-MSH on TRH-GFP neurons is mediated by postsynaptic mechanisms. A, Voltage trace recording from a spontaneously firing neuron indicates that effects of 250 n
m
α-MSH persist in a hypothalamic slice pretreated with 1 m
m
kynurenic acid (KYN) and 200 μ
m
picrotoxin (PIC) to block ionotropic glutamate and GABA(A) neurotransmission. Note that α-MSH increases frequency of firing as well as duration of bursts of firing. B, Frequency histogram quantifies these effects. C and D, Means ±
sem
of frequency of firing (C) and membrane potentials (D) in control and during applications of 250–350 n
m
α-MSH recorded from neurons pretreated with KYN and PIC. *, P < 0.001 (C) and P < 0.01 (D), paired t test. E, Voltage trace recording of a TRH-GFP neuron indicates that application of 250 n
m
α-MSH generates depolarization of membrane potential in the presence of 0.5 μ
m
TTX. Note that the depolarization elicited a few Ca2+-mediated action potentials. F, Bar graphs show means ±
sem
of the effects of 250–350 n
m
α-MSH on membrane potentials in TTX-pretreated neurons (n = 19). *, P < 0.0001.
Figure 4
NPY potently and reversibly inhibits firing activity of TRH-GFP neurons. A, Voltage trace recording from a spontaneously firing TRH-GFP-expressing neuron indicates that 100 n
m
NPY potently and reversibly inhibits its firing activity. Note that decrease in firing frequency is associated with hyperpolarization of membrane potentials. B and C, Bar graphs indicate means ±
sem
of inhibitory effects of 100 n
m
NPY on action potential firing frequency (B) and membrane potentials (C) of eight TRH-GFP neurons tested. *, P < 0.001.
Figure 5
Leptin activates action potential firing activity of TRH-GFP neurons. A, Voltage trace of whole-cell recording from a spontaneously firing TRH-GFP neuron indicates that bath application of 25 n
m
leptin induces an increase in firing activity associated with depolarization of membrane potential. B, Frequency histogram of this effect. C and D, Bar graphs indicate means ±
sem
of effects of 25–50 n
m
leptin on action potential firing frequency (C) and membrane potentials (D) of 30 neurons tested. *, P < 0.005 (C) and P < 0.1 (D). E–H, Leptin inhibits action potential firing activity of non-TRH-GFP-expressing neurons; E, voltage trace of whole-cell recording from a non-TRH-GFP-expressing neuron indicates that bath application of 50 n
m
leptin induces inhibition of firing activity associated with hyperpolarization of membrane potential; F, frequency histogram of this effect; G and H, bar graphs indicate means ±
sem
of effects of 25–50 n
m
leptin on action potential firing frequency (G) and membrane potentials (H) of nine neurons tested. *, P < 0.001 (G) and P < 0.05 (H).
Figure 6
The excitatory effects of leptin on TRH-GFP neurons are mediated through postsynaptic mechanisms. A, Voltage trace recording indicates that 35 n
m
leptin generates an increase in firing activity associated with depolarization of membrane potential in a TRH-GFP neuron pretreated with 1 m
m
KYN and 200 μ
m
PIC. B, Frequency histogram of this effect. C, Insets of the voltage trace (shown in A) expanded in time scale obtained from different time points (indicated by a–c) to clearly demonstrate the effects of leptin on firing activity and membrane potential. D and E, Bar graphs indicate means ±
sem
of effects of 25–50 n
m
leptin on frequency of firing (D) and membrane potentials (E) of 14 neurons pretreated with KYN and PIC. *, P < 0.001. F, Voltage trace of recording from a TRH-GFP neuron indicates that 25 n
m
leptin induces significant depolarization of membrane potential in the presence of TTX. Note that leptin-induced depolarization generated Ca2+-mediated action potentials. G, Bar graphs of means ±
sem
of depolarizing effects of 25–50 n
m
leptin on 34 neurons tested. *, P < 0.0001.
Figure 7
Regulation of levels of serum total T4 by both melanocortin-dependent and -independent pathways of leptin action. The bar graphs indicate serum total T4 levels from 10- to 12-wk-old male C57B/6J mice at the end of 36 h of different treatments as indicated below each column. *, P < 0.05 (one-way ANOVA, Bonferroni’s multiple-comparison test, n values are indicated above each column).
References
- Lechan RM, Fekete C 2006 The TRH neuron: a hypothalamic integrator of energy metabolism. Prog Brain Res 153:209–235 - PubMed
- Segerson TP, Kauer J, Wolfe HC, Mobtaker H, Wu P, Jackson IM, Lechan RM 1987 Thyroid hormone regulates TRH biosynthesis in the paraventricular nucleus of the rat hypothalamus. Science 238:78–80 - PubMed
- Kakucska I, Rand W, Lechan RM 1992 Thyrotropin-releasing hormone gene expression in the hypothalamic paraventricular nucleus is dependent on feedback regulation by both triiodothyronine and thyroxine. Endocrinology 130:2845–2850 - PubMed
- Tapia-Arancibia L, Arancibia S, Astier H 1985 Evidence for α1-adrenergic stimulatory control of in vitro release of immunoreactive thyrotropin-releasing hormone from rat median eminence: in vivo corroboration. Endocrinology 116:2314–2319 - PubMed
Publication types
MeSH terms
Substances
LinkOut - more resources
Full Text Sources
Molecular Biology Databases