The one (one of a few) disadvantage of c-Fos as a marker of recent neuronal activity is that it only shows neurons activated by a certain response in question and not those inhibited by the said response.
One solution -- at least in terms of neuronal activation in response to leptin -- could be SOCS-3.
SOCS-3
- is a suppressor of cytokine signaling-3
- ''an intracellular protein induced by activation of cytokine receptors such as the leptin signaling by blocking phosphorylation of the receptor and downstream STAT proteins involved in leptin receptor-activated signal transduction [45,46].'' (Baskin et al., 2000)
- ''In CHO cells transfected with ObRb, expression of SOCS-3 inhibits leptin-induced tyrosine phosphorylation of JAK2 proteins, suggesting that SOCS-3 is a leptin-regulated inhibitor of proximal leptin signaling in vivo [47].'' (Baskin et al., 2000)
So if we use c-Fos as a marker of a response to leptin, only neurons that are activated by leptin (such as ARC POMC neurons) will be shown (Elias et al., 1999). But is we use SOCS-3 as a marker of a response to leptin, we could identify neurons that are activated by leptin (ARC POMC neurons) AND those that are inhibited by leptin (such as ARC NPY/AgRP neurons) (Elias et al., 1999).
references:
- Baskin DG et al. (2000). SOCS-3 expression in leptin-sensitive neurons of the hypothalamus of fed and fasted rats. Regulatory Peptides;92:9-15.
- Elias CF, Aschkenasi C, Lee C, Kelly J, Ahima RS, Bjorbaek C et al. (1999). Leptin differentially regulates NPY and POMC neurons projecting to the lateral hypothalamic area. Neuron;23:775–86.
The co-expression of POMC, NPY and 5-HTRs in the ARC
Have you ever been wondering about what the are the co-expression patterns of pro-opiomelanocortin (POMC) and the neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus (ARC) with what serotonin receptors (5-HTRs)? Yes, I thought so... Here is little info for you!
I have included the methods used to label the peptide (to label a protein: immunireactivity = IR; to label protein's mRNA: in situ hybridisation = ISH) and the animal model with the validation of the model where relevant.
POMC & 5-HT2CRs
wt RAT
50% to 80% of POMC cells (POMC-IR) (from rostral to caudal ARC respectively) express 2Cs (2C-ISH) in a wt rat (Heisler et al. 2002)
POMC-tau/lacZ+/- MOUSE
75% of POMC cells (beta-galactosidase-IR) express 5-HT2CRs (5-HT2CR-IR - but uses a dodgy antibody) in a POMC-tau/lacZ+/- (Lam et al., 2008)
28% of 5-HT2CR cells (5-HT2CR-IR - but uses a dodgy antibody) express POMCs (beta-galactosidase-IR) in a POMC-tau/lacZ+/- (Lam et al. 2008)
validation:
100% of beta-galactosidase-IR express POMC-ISH in a POMC-tau/lacZ+/- (Lam et al., 2008)
NPY & 5-HT1BRs
NPY-GFP MOUSE
16.5% (1/6) of NPY cells (GFP-IR) express 5-HT1BRs (5-HT1BR-ISH) in a NPY-GFP mouse (Heisler et al., 2006)
33% (1/3) of 5-HT1BR cells (5-HT1BR-ISH) express NPY (GFP-IR) in a NPY-GFP mouse (Heisler et al., 2006)
validation:
99.5% of GFP-IR express AgRP-ISH in a NPY-GFP mouse (Heisler et al., 2006)
Of note, apparently there are differences in expression of POMC in mice and in rats (Cowley et al., 2001) so that might explain the differences (in addition to the different genetic model).
I have included the methods used to label the peptide (to label a protein: immunireactivity = IR; to label protein's mRNA: in situ hybridisation = ISH) and the animal model with the validation of the model where relevant.
POMC & 5-HT2CRs
wt RAT
50% to 80% of POMC cells (POMC-IR) (from rostral to caudal ARC respectively) express 2Cs (2C-ISH) in a wt rat (Heisler et al. 2002)
POMC-tau/lacZ+/- MOUSE
75% of POMC cells (beta-galactosidase-IR) express 5-HT2CRs (5-HT2CR-IR - but uses a dodgy antibody) in a POMC-tau/lacZ+/- (Lam et al., 2008)
28% of 5-HT2CR cells (5-HT2CR-IR - but uses a dodgy antibody) express POMCs (beta-galactosidase-IR) in a POMC-tau/lacZ+/- (Lam et al. 2008)
validation:
100% of beta-galactosidase-IR express POMC-ISH in a POMC-tau/lacZ+/- (Lam et al., 2008)
NPY & 5-HT1BRs
NPY-GFP MOUSE
16.5% (1/6) of NPY cells (GFP-IR) express 5-HT1BRs (5-HT1BR-ISH) in a NPY-GFP mouse (Heisler et al., 2006)
33% (1/3) of 5-HT1BR cells (5-HT1BR-ISH) express NPY (GFP-IR) in a NPY-GFP mouse (Heisler et al., 2006)
validation:
99.5% of GFP-IR express AgRP-ISH in a NPY-GFP mouse (Heisler et al., 2006)
Of note, apparently there are differences in expression of POMC in mice and in rats (Cowley et al., 2001) so that might explain the differences (in addition to the different genetic model).
The ARC is full of neurons other than POMC/CART and AgRP/NPY
I have long suspected that the neurons of the arcuate nucleus of the hypothalamus (ARC) express much more then the anorexigenic POMC/CART and the orexigenic AgRP/NPY peptides. And in the last two days I learned what else can be found in the ARC. It's quite a selection...
POMC and CART... mostly co-expressed, essential for energy balance regulation
AgRP and NPY... co-expressed, essential for energy balance regulation
kisspeptin... assumed to connect reproduction to energy balance
neurokinin B
dynorphin... apparently the last three are co-expressed together within one neuron
dopamine
SOC3... co-expressed with both POMC and AgRP, associated with energy balance
TMEM18... found generally in neurons, associated with energy balance
and the list continues. It makes it very interesting to study this nucleus and the complex neural networks.
POMC and CART... mostly co-expressed, essential for energy balance regulation
AgRP and NPY... co-expressed, essential for energy balance regulation
kisspeptin... assumed to connect reproduction to energy balance
neurokinin B
dynorphin... apparently the last three are co-expressed together within one neuron
dopamine
SOC3... co-expressed with both POMC and AgRP, associated with energy balance
TMEM18... found generally in neurons, associated with energy balance
and the list continues. It makes it very interesting to study this nucleus and the complex neural networks.
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