Paiva I. et al., Journal of Clinical Investigation, 2022
Original article: https://doi.org/10.1172%2FJCI149371
Contact authors:
David Blum, UMR-S1172 Lille Neuroscience & Cognition (LilNCog), Lille, France.
Anne-Laurence Boutillier, LNCA, UMR7364 CNRS Unistra, Strasbourg

Abstract (text and graphic) by Margot FITA

Revised by

• David Blum, UMR-S1172 Lille Neuroscience & Cognition (LilNCog), Lille, France.
• Anne-Laurence Boutillier, LNCA, UMR7364 CNRS Unistra, Strasbourg

Caffeine is regularly consumed through coffee, tea or sodas, making it the most consumed psychoactive substance. Increasing alertness, attention or hampering fatigue are all effects caffeine consumers are experimenting. Additionally, experimental evidence confirmed the beneficial contribution of caffeine in memory, cognitive abilities and life expectancy. However, most of these studies were led on acute consumption of caffeine while it is commonly consumed chronically. Through a pipeline of omics techniques, Paiva I. et al. (2022) looked forward to uncover the effects of chronic caffeine consumption at the epigenetic, metabolic and proteomic levels in mice hippocampus.

Epigenomic analysis in hippocampus bulk tissues revealed an overall significant depletion of two active histone acetylation marks in mice subjected to chronic caffeine treatment. Gene ontology analysis showed that most of the associated gene regions were mainly involved in metabolic and lipidic functions, which was further emphasized by MALDI-MS analysis of hippocampus, confirming that about 90% of metabolic components were found in a decreased amount under chronic caffeine treatment with respect to control animals. Thus, the hippocampal metabolome appeared mostly negatively affected by the treatment; however, proteomic analysis revealed three quarters of altered proteins were enriched under chronic caffeine treatment, with a significant portion of enriched proteins associated to functions such as glutamatergic transmission, and more generally with the modulation of chemical synaptic transmission. This is in agreement with the literature, in which benefits of caffeine on neuronal activity are widely referenced. The results of proteomic analysis concerning proteins involved in synaptic function led researchers to repeat the epigenomic analysis of the effect of chronic caffeine treatment exclusively on sorted hippocampal neurons. While initial epigenomic data revealed a depletion of acetylated histone marks in the whole hippocampus of mice under chronic caffeine treatment, in sorted hippocampal neurons, these marks showed however a significant enrichment on genes mostly involved in synaptic function, synaptic signalling and long-term potentiation. Together, these results suggest chronic caffeine intake has a neuron-specific epigenetic modulation which is thought to enhance the expression of genes related to synaptic and glutamatergic functions while metabolic processes are being dampened. So far, results have been obtained only from mice in home cage. Authors therefore looked forward to compare transcriptional regulation induced by learning processes between mice chronically consuming caffeine and control animals.  Chronic caffeine intake altered gene expression five times more in response to learning when compared to control group. Strikingly, the amplitude of expression between home cage and learning condition was higher when animals were chronically treated with caffeine. Thus, when the hippocampal system is stimulated by experience, the transcriptomic response seems to be enhanced in caffeinated-animals.

Collectively, these observations suggest chronic caffeine consumption triggers different epigenomic responses on neuronal and non-neuronal hippocampus cell populations. Globally, epigenomic changes in the hippocampus due to chronic caffeine consumption are thought to support neuronal activity while metabolic processes supplied by glial components are dampened. This mechanism might serve to maximize metabolic endeavours during learning phases and to favour synaptic processes underlying learning, such as ion transport or glutamatergic transmission and ultimately long term potentiation. If at first sight the dual mechanism of chronic caffeine consumption sounds profitable during learning, the long-term impact on non-neuronal tissue remains however quite unknown. Besides, some alterations were persistent since they remained two weeks after caffeine withdrawal. For this reason, the effect of chronic caffeine on different glial cells should be further studied to complete these data.

Bailly et al., Biological Psychiatry, 2022
Original article: https://doi.org/10.1016/j.biopsych.2022.09.013
Contact author: Emmanuel Darcq, INSERM U1114, Department of Psychiatry, University of Strasbourg

Abstract (text and graphic) by Lise HERMANT

Mu opioid receptors (MOR) are expressed throughout peripheral and central nervous systems. Those receptors are both activated by endogenous peptides (such as endorphins, enkephalins and dynorphins) produced in response to noxious stimulation. They can also be stimulated by exogenous opioids such as morphine which is a benchmark painkiller.
MORs are Gi/Go protein coupled receptors. Thus, once activated by their ligands, they will inhibit the cell activity on which they are expressed. (see graphical abstract, panel 1).

Those receptors are highly expressed in the habenula (Hb). Indeed, some hybridization in situ, immunolabelling assays and genetic modified mice in which MOR was fused with a fluorescent protein reporter (mCherry or Venus) have shown that this small epithalamic structure, and more particularly the medial habenula (MHb), is the site of the densest expression of MOR in the central nervous system.
The MHb is composed of excitatory neurons such as glutamatergic and cholinergic ones. In a previous study, authors conditionally knocked out the MOR gene specifically in the habenula and showed that place aversion conditioned by naloxone (an antagonist of MORs) is less efficient in mutant mice. This was the first genetic evidence that the aversive effect of naloxone is mediated, at least in part, by MOR at the level of the habenula circuitry. As MORs are inhibitory receptors, authors hypothesized that its signaling normally acts as a brake on habenular neurons expressing the receptor in order to limit aversive states. Therefore, habenular neurons expressing MOR (Hb-MOR neurons) might promote negative affect such as aversion that would be inhibited by MOR activation.

To further investigate this question, authors used a knock-in mouse line targeting the Oprm1 gene that is encoding the mu opioid receptor. This mouse line permits to co-express MOR and
eGFP/Cre proteins in the same neuron and allows the targeting and manipulation of opioidresponsive neurons. Thanks to anterograde tracing, authors identified the two main projection sites of Hb-MOR neurons : the interponduncular nucleus (IPN) and the dorsal raphe nucleus (DRN).

In order to see which kind of behaviors are related to the activity of Hb-MOR neurons, authors expressed the ChannelRhodopsine2 specifically in those neurons by using a Cre dependant vector. KI MOR/Cre mice went through a panel of behavioral tests (as shown in the middle part of the graphical abstract) while having the optical fiber implanted either in the habenula, in the interpedoncular nucleus or in the dorsal raphe nucleus allowing real-time optostimulation of Hb-MOR neurons projecting to those structures. First, the real time place preference, to highlight aversive affect of the stimulation of Hb-MOR neurons, was realized. Anxiety was then assessed thanks to the elevated plus maze and the marble burying test. Finally, the tail suspension test was used to highlight despair-like behaviors.

Authors showed that the optogenetic stimulation of Hb-MOR neurons directly in the habenula induces real time place avoidance confirming the aversive character of those neurons. The optogenetic stimulation of Hb-MOR neurons projecting to the interpedoncular nucleus produced real-time place avoidance and despair-like behaviors as the immobility time in the tail
suspension test was increased whereas the optogenetic stimulation of Hb-MOR neurons projecting to the dorsal raphe nucleus increased anxiety-related behaviors as the time spent in closed arms in the elevated plus maze and the number of marbles buried were significantly increased. (see graphical abstract, pannel 3).

Altogether, Hb-MOR neurons encode aversion and their activation in two distinct pathways leads to different behavioral effects. This study reveals an « aversion-limiting » function of MOR activity in the habenula that was not known before.
As Hb-MOR neurons are heterogeneous, further experiments will aim to describe which specific neuronal populations are implicated in such effects.