NMDAR blockade-induced neonatal brain injury: Reversal by the calcium channel agonist BayK 8644

Abstract

We have previously shown that P7 rat pups injected with the N-methyl-D-aspartate receptor (NMDAR) blockerMK801 displayed robust apoptotic injury within hours after injection. Further studies from our lab suggest that loss of calcium cannot be compensated for when vulnerable neurons lack calcium buffering capabilities. Thus, to elevate calcium in these neurons prior to MK801 exposure, we injected P7 rats with the calcium channel agonist BayK 8644. Whereas BayK 8644 did not induce apoptosis by itself, it was found to block MK801-induced injury in a dose-dependent manner. Reversal of MK801 toxicity was complete in the caudate-putamen, partial in the somatosensory cortex but was not observed in the retrosplenial cortex. These results suggest that postnatal brain injury resulting from agents that block the NMDAR, which include commonly used anesthetics as well as drugs of abuse, may be prevented in vulnerable neurons by compensatory increases in calcium prior to exposure to these antagonists. KeywordsRat; Postnatal; Apoptosis; Set point hypothesis; MK801; Dizocilpine We and others have previously shown that blockade of the Nmethyl-D-aspartate receptor (NMDAR) in postnatal day 7 (P7) rat pups causes profound apoptotic injury [2,12]. In contrast, at P21, agents that block this receptor no longer promote cell death. Numerous mechanisms have been proposed for this age-dependent injury such as loss of neuronal calcium [10][11][12], disruption of synaptic circuitry, up-regulation of NMDAR protein or absence of trophic support [1,8,9,13,14].We proposed some time ago that at P7 the regulatory mechanisms that determine optimal calcium levels may not have matured in some neuronal populations [12]. In its simplest form, this calcium set point (CSP) hypothesis [3] states that excessive gain or loss of calcium will result in neuronal injury. In the case of NMDAR blockade, an inability to adapt to loss of calcium entry pushes vulnerable neurons towards cell death [3,[10][11][12]. Indeed, we were the first to show that MK801-induced expression of the pro-apoptotic marker activated caspase-3 (AC3) in P7 rat brains was almost exclusively found in cells that lacked the calcium binding proteins calbindin (CB), calretinin (CR), or parvalbumin (PV) [5], and that gain of these proteins (in particular PV) coincided with loss of MK801 sensitivity [6,7].

Is age-dependent, ketamine-induced apoptosis in the rat somatosensory cortex influenced by temperature?

“…The time of the first injection was defined as time = 0 (t 0 ). For comparison with this and previous studies, we also injected MK801 at 1 mg/kg (one injection only at t 0 ), as this dose produces robust AC3 induction that is reliable and widespread (Turner et al, 2002, Turner et al, 2007b, Turner et al, 2009a). Sterile PBS (1 injection at 0, 1, 2 and 3 hours; 4 total) served as vehicle controls.…”

“…We referred to this sampling territory as the SSC ANT . Numbers of AC3-positive neurons were estimated within lamina IV and V of the SSC, as previous studies have reported reliable and robust injury in these layers (Turner et al, 2007b, Turner et al, 2009a, Turner et al, 2009b). For comparison, we also qualitatively evaluated AC3-positive profiles in the superficial portion of layer II/III in the same sections we sampled layers IV and V.…”

“…Blockade of the NMDAR can lead to induction of apoptosis throughout the CNS that is age-dependent and observed in many species, including non-human primates (Ikonomidou et al, 1999a, Wang et al, 2001, Olney et al, 2002, Turner et al, 2002, Bittigau et al, 2003, Wang et al, 2004, Slikker et al, 2007, Turner et al, 2007b, Wang and Johnson, 2007, Turner et al, 2009a). Since this discovery, many groups have sought to understand mechanisms or expand on the pathological outcomes (McInnis et al, 2002, Olney et al, 2004, Wang et al, 2005, Yon et al, 2005, Kaindl et al, 2006, Lu et al, 2006, Turner et al, 2007b, Stefovska et al, 2008, Turner et al, 2009a).…”

“…Since this discovery, many groups have sought to understand mechanisms or expand on the pathological outcomes (McInnis et al, 2002, Olney et al, 2004, Wang et al, 2005, Yon et al, 2005, Kaindl et al, 2006, Lu et al, 2006, Turner et al, 2007b, Stefovska et al, 2008, Turner et al, 2009a). Indeed, in rodents, injury from agents such as MK801, ketamine, or phencyclidine, leads to short-term and long-term molecular changes in functionally diverse protein families (Wang et al, 2004, Lema Tomé et al, 2006, Turner et al, 2007b, Kaindl et al, 2008, Ringler et al, 2008, Turner, 2009, Turner et al, 2009c) that suggest the still developing CNS has been permanently altered (Abekawa et al, 2007, Wang et al, 2007, Lunardi et al, 2009).…”

Postnatal Expression of GAD67

“…Although morphology can guide us in the SSC ANT , where dying cells are typically pyramidal in shape [20,26], in other brain regions (Cg, CP) the phenotype is more varied [1,2,7,20]. Regardless, our data here and elsewhere suggest that a closer examination of the relationship between the GABA terminals and these dying cells might tell us more about why these cells are so vulnerable.…”