Interaction of Mechanisms Involving Epoxyeicosatrienoic Acids, Adenosine Receptors, and Metabotropic Glutamate Receptors in Neurovascular Coupling in Rat Whisker Barrel Cortex

Abstract

Adenosine, astrocyte metabotropic glutamate receptors (mGluRs), and epoxyeicosatrienoic acids (EETs) have been implicated in neurovascular coupling. Although A 2A and A 2B receptors mediate cerebral vasodilation to adenosine, the role of each receptor in the cerebral blood flow (CBF) response to neural activation remains to be fully elucidated. In addition, adenosine can amplify astrocyte calcium, which may increase arachidonic acid metabolites such as EETs. The interaction of these pathways was investigated by determining if combined treatment with antagonists exerted an additive inhibitory effect on the CBF response. During whisker stimulation of anesthetized rats, the increase in cortical CBF was reduced by approximately half after individual administration of A 2B , mGluR and EET antagonists and EET synthesis inhibitors. Combining treatment of either a mGluR antagonist, an EET antagonist, or an EET synthesis inhibitor with an A 2B receptor antagonist did not produce an additional decrement in the CBF response. Likewise, the CBF response also remained reduced by B50% when an EET antagonist was combined with an mGluR antagonist or an mGluR antagonist plus an A 2B receptor antagonist. In contrast, A 2A and A 3 receptor antagonists had no effect on the CBF response to whisker stimulation. We conclude that (1) adenosine A 2B receptors, rather than A 2A or A 3 receptors, play a significant role in coupling cortical CBF to neuronal activity, and (2) the adenosine A 2B receptor, mGluR, and EETs signaling pathways are not functionally additive, consistent with the possibility of astrocytic mGluR and adenosine A 2B receptor linkage to the synthesis and release of vasodilatory EETs.

Pyramidal Neurons Are "Neurogenic Hubs" in the Neurovascular Coupling Response to Whisker Stimulation

“…The selective P450 epoxygenases inhibitor MS-PPOH significantly diminished the perfusion response (Ϫ39.0 Ϯ 4.5%, p Ͻ 0.001) (Fig. 4 B), a reduction remarkably similar to that reported after superfusion of the same compound for 1 or 2 h over the sensory cortex (Ϫ38%; Shi et al, 2008). Considering that astrocytes also express glutamate and GABA-A receptors (Conti et al, 1997;Cahoy et al, 2008), which activation by selective agonists induces vasodilatation independently of spiking activity (Fergus and Lee, 1997;Zonta et al, 2003;Lovick et al, 2005), we examined the relationship between EETs and excitatory and inhibitory neurotransmission.…”

“…4C, 6C). These observations pointed to NMDA and GABA-A receptors contributing to the evoked CBF response, at least partly through distinct pathways, notwithstanding the glutamate effects mediated through receptors other than NMDA, such as mGluRs and AMPA/kainate receptors (Norup Nielsen and Lauritzen, 2001;Zonta et al, 2003;Lovick et al, 2005;Gsell et al, 2006;Shi et al, 2008; this study). We similarly combined COX-2 inhibition and GABA-A receptor blockade, but this failed to further impair the hyperemic response compared with COX-2 inhibition alone (Ϫ51.9 Ϯ 9.4% vs Ϫ51.2 Ϯ 8.0%) (Figs.…”

“…5B). We then assessed the implication of group I mGluRs, identified as a required signaling cascade in the hyperemic response to sensory stimulation (Zonta et al, 2003;Shi et al, 2008). We found that combined blockade of mGluR1, restricted to a subset of cortical GABA interneurons (Baude et al, 1993) including those that contain VIP (Cauli et al, 2000) and mGluR5 that are expressed in most cortical neurons and astrocytes (Cauli et al, 2000;Cahoy et al, 2008), with MPEP and LY367385 significantly reduced the evoked CBF response (Ϫ40.3 Ϯ 5.3%, p Ͻ 0.01) (Fig.…”

IGF‐1 deficiency impairs neurovascular coupling in mice: implications for cerebromicrovascular aging

“…Changes in CBF ( n  = 7–8 mic in each group) were assessed in three trials (5‐ to 10‐min intervals). CBF responses to whisker stimulation were repeated in the presence of the following inhibitors administered topically onto the brain surface of separate groups of animals: HET0016 (inhibitor of 20‐hydroxyeicosatrienoic acid (20‐HETE) production, 10 −6  mol L −1 for 30 min; Cayman Chemicals, Ann Arbor, MI, USA) (Liu et al ., 2008), MS‐PPOH (inhibitor of EET production, 20 × 10 −6  mol L −1 for 30 min; Cayman Chemicals) (Shi et al ., 2008), L‐NAME (N ω ‐Nitro‐L‐arginine methyl ester, inhibitor of nitric oxide synthase, 10 −4  mol L −1 for 20 min; Sigma‐Aldrich, St. Louis, MO, U.S.A.), apocynin (inhibitor of NADPH oxidases, 3 × 10 −4  mol L −1 for 30 min; Cayman Chemicals), fluoroacetate sodium (inhibitor of the tricarboxylic acid cycle predominantly in glial cells, 10 −4  mol L −1 min; Sigma‐Aldrich, St. Louis, MO, U.S.A.) (Fonnum et al ., 1997; Lecrux et al ., 2012), indomethacin (cyclooxygenase inhibitor, 5 × 10 −4  mol L −1 ; Sigma‐Aldrich, St. Louis, MO, U.S.A.) (Kitaura et al ., 2007), MPEP (6‐Methyl‐2‐(phenylethynyl)pyridine hydrochloride, group I metabotropic glutamate receptors (mGluR) subtype 5 antagonist, 5 × 10 −5  mol L −1 ) (Zonta et al ., 2003), and the NMDA (N‐methyl‐D‐aspartate) receptor antagonist D‐APV (D‐2‐Amino‐5‐Phosphonovaleric acid, 5 × 10 −5  mol L −1 ; Cayman Chemicals) (Stobart et al ., 2013). In a separate series of experiments ( n  = 8 in each group), CBF responses to topical administration of L‐glutamate (500 μmol L −1 ) (Hall et al ., 2014) were determined in the absence and presence of MPEP (5 × 10 −5  mol L −1 ) and D‐APV (5 × 10 −5  mol L −1 ) (Stobart et al ., 2013).…”

TRPV4 channels stimulate Ca 2 + -induced Ca 2+ release in astrocytic endfeet and amplify neurovascular coupling responses

“…Also in cultured cortical astrocytes, TRPV4 channels form complexes with aquaporin 4 to regulate cell volume responses to hypoosmotic stress (12). Notably, it has been shown that TRPV4 channels in other preparations are activated by epoxyeicosatrienoic acids (EETs) (13)(14)(15), which have been implicated in NVC (16)(17)(18) and reportedly stimulate Ca 2+ influx in rat cortical astrocytes (19,20). TRPV4 channels are thermosensitive, osmosensitive, and mechanosensitive, and thereby mediate processes by which cells sense and respond to environmental cues (21)(22)(23)(24) (25)(26)(27)(28)(29).…”