Prolonged High Fat Diet Worsens the Cellular Response to a Small, Covert-like Ischemic Stroke.

Obesity is associated with worse neurological outcomes following overt ischemic strokes. The majority of strokes however, are covert, small strokes that often evade detection. How obesity impacts the cellular response to covert strokes is unclear. Here, we used a diet-induced obesity model by feeding mice a high fat diet (HFD) and examining its impact on the behavioral and cellular responses to either an Endothelin-1-induced focal ischemic stroke or a saline injection (control). Specifically, we examined cells in regions with different levels of blood perfusion: the non-perfused core, the hypo-perfused surround and the perfused region around the infarct. We show that HFD selectively exacerbated the response to stroke but not to saline injections. Stroke affected the composition of microglia/macrophages, astrocytes and neurons within each region of perfusion. In the non-perfused core, the majority of cells were Iba-1+ microglia and macrophages. HFD resulted in a greater infiltration of CD68+ macrophages into the infarct core while CD68+ /TMEM119+ microglia were reduced. Furthermore, there was a trend towards an increased spread of the astrogliosis scar from the infarct border in the HFD condition. Within the hypo-perfused region, significantly fewer neurons survived in HFD-fed mice than Chow-fed mice, suggesting that neurons in the HFD condition have an increased vulnerability. In summary, diet-induced obesity exacerbates covert-like stroke injuries by worsening the cellular responses in the varying levels of perfusion across the infarct.

Dopamine modulates excitatory transmission to orexin neurons in a receptor subtype-specific manner.

Dopamine (DA) can promote or inhibit consummatory and reward-related behaviors by activating different receptor subtypes in the lateral hypothalamus and perifornical area (LH/PF). Because orexin neurons are involved in reward and localized in the LH/PF, DA may modulate these neurons to influence reward-related behaviors. To determine the cellular mechanism underlying dopaminergic modulation of orexin neurons, the effect of DA on excitatory transmission to these neurons was investigated using in vitro electrophysiology on rat brain slices. We found that low concentrations (0.1-1 µM) of DA increased evoked excitatory postsynaptic current amplitude while decreasing paired-pulse ratio. In contrast, high concentrations (10-100 µM) of DA did the opposite. The excitatory effect of low DA was blocked by the D1 receptor antagonist SCH-23390, whereas the inhibitory effect of high DA was blocked by the D2 receptor antagonist sulpiride. These results indicate distinct roles of D1 and D2 receptors in bidirectional presynaptic modulation of excitatory transmission. DA had stronger effects on isolated synaptic activity than repetitive ones, suggesting that sensitivity to dopaminergic modulation depends on the level of network activity. In orexin neurons from high-fat diet-fed rats, a high concentration of DA was less effective in suppressing repetitive synaptic activity compared with chow controls. Therefore, in diet-induced obesity, intense synaptic inputs may preferentially reach orexin neurons while intermittent signals are inhibited by high DA levels. In summary, our study provides a cellular mechanism by which DA may exert opposite behavioral effects in the LH/PF through bidirectional modulation of orexin neurons via different DA receptors.

Concentration-dependent activation of dopamine receptors differentially modulates GABA release onto orexin neurons.

Dopamine (DA) and orexin neurons play important roles in reward and food intake. There are anatomical and functional connections between these two cell groups: orexin peptides stimulate DA neurons in the ventral tegmental area and DA inhibits orexin neurons in the hypothalamus. However, the cellular mechanisms underlying the action of DA on orexin neurons remain incompletely understood. Therefore, the effect of DA on inhibitory transmission to orexin neurons was investigated in rat brain slices using the whole-cell patch-clamp technique. We found that DA modulated the frequency of spontaneous and miniature IPSCs (mIPSCs) in a concentration-dependent bidirectional manner. Low (1 µM) and high (100 µM) concentrations of DA decreased and increased IPSC frequency, respectively. These effects did not accompany a change in mIPSC amplitude and persisted in the presence of G-protein signaling inhibitor GDPßS in the pipette, suggesting that DA acts presynaptically. The decrease in mIPSC frequency was mediated by D2 receptors whereas the increase required co-activation of D1 and D2 receptors and subsequent activation of phospholipase C. In summary, our results suggest that DA has complex effects on GABAergic transmission to orexin neurons, involving cooperation of multiple receptor subtypes. The direction of dopaminergic influence on orexin neurons is dependent on the level of DA in the hypothalamus. At low levels DA disinhibits orexin neurons whereas at high levels it facilitates GABA release, which may act as negative feedback to curb the excitatory orexinergic output to DA neurons. These mechanisms may have implications for consummatory and motivated behaviours.