N-Type Calcium Channels and Their Regulation by GABA_BReceptors in Axons of Neonatal Rat Optic Nerve

Abstract

Axons of neonatal rat optic nerves exhibit fast calcium transients in response to brief action potential stimulation. In response to one to four closely spaced action potentials, evoked calcium transients showed a fast-rising phase followed by a decay with a time constant of approximately 2-3 sec. By selective staining of axons or glial cells with calcium dyes, it was shown that the evoked calcium transient originated from axons. The calcium transient was caused by influx because it was eliminated when bath calcium was removed. Pharmacological profile studies with calcium channel subtype-specific peptides suggested that 58% of the evoked calcium influx was accounted for by N-type calcium channels, whereas L- and P/Q-type calcium channels had little, if any, contribution. The identity of the residual calcium influx remains unclear. GABA application caused a dramatic reduction of the amplitude of the action potential and the associated calcium influx. When GABAA receptors were blocked by bicuculline, the inhibitory effect of GABA on the action potential was eliminated, whereas that on the calcium influx was not, indicating involvement of GABAB receptors. Indeed, the calcium influx was inhibited by the GABAB receptor agonist baclofen. This baclofen effect was occluded by a previous block of N-type calcium channels and was unaffected by the broad-spectrum K+ channel blocker 4-AP. We conclude that neonatal rat optic nerve axons express N-type calcium channels, which are subjected to regulation by G-protein-coupled GABAB receptors. We suggest that receptor-mediated inhibition of axonal calcium channels plays a protective role in neonatal anoxic and/or ischemic injury.

Neurotransmitter signaling in white matter

“…For example, the effects of GABA-A receptor activation are associated with elevated extracellular [K 1 ] that may originate from glia, but experimentally elevating [K 1 ] does not duplicate the effects of GABA upon excitability leading to the conclusion that receptor expression is axonal (Sakatani et al, 1994). In general, it may be assumed that the effects of neurotransmitters on glial Ca 21 and membrane properties appear to be mediated largely by glial expression of neurotransmitter receptors (Butt and Jennings, 1994;Hamilton et al, 2008), whereas effects upon axonal excitability appear to be mediated largely by axolemma expression, rather than (Constantinou and Fern, 2009;Sakatani et al, 1991Sakatani et al, , 1992Sun and Chiu, 1999) (Sun and Chiu, 1999) Increase ischemia-tolerance RON (Fern et al, 1994(Fern et al, , 1995 Glycine Reduced axon excitability nRON (Constantinou and Fern, 2009 glial responses that subsequently modify excitability (Nikolaeva et al, 2009;Sun and Chiu, 1999;Zhang et al, 2004).…”

“…In addition, electrophysiological criteria have been used to distinguish glial cell types, allowing the first descriptions of functional glutamate, GABA-A and glycine receptors in identified astrocytes, oligodendrocytes, glioblasts and OPCs in situ (Berger et al, 1992;Butt and Jennings 1994;Pastor et al, 1995 ] does not duplicate the effects of GABA upon excitability leading to the conclusion that receptor expression is axonal (Sakatani et al, 1994). In general, it may be assumed that the effects of neurotransmitters on glial Ca 21 and membrane properties appear to be mediated largely by glial expression of neurotransmitter receptors (Butt and Jennings, 1994;Hamilton et al, 2008), whereas effects upon axonal excitability appear to be mediated largely by axolemma expression, rather than glial responses that subsequently modify excitability (Nikolaeva et al, 2009;Sun and Chiu, 1999;Zhang et al, 2004). …”

“…Most evidence indicates that astrocyte signaling spreads as a circular wave in all directions from a focal source, through gap junctions and by the release of ATP to activate glial receptors in a "spillover" or volume transmission manner. The ATP-mediated rise in astrocyte calcium may stimulate them to release glutamate or other neurotransmitters, such as GABA and acetylcholine (ACh) (see below), which have been shown to act on axonal receptors to modulate their conduction properties (Sakatani et al, 1994;Sasaki et al, 2011;Sun and Chiu, 1999;Zhang et al, 2004). In addition, intercellular Ca 21 waves in astrocytes have been shown to trigger microglial Ca 21 responses through the release of ATP (Schipke et al, 2002;Verderio and Matteoli, 2001), which is central to their injury response (Maeda et al, 2010;Tsuda et al, 2010).…”

Presynaptic GABABReceptors Regulate Retinohypothalamic Tract Synaptic Transmission by Inhibiting Voltage-Gated Ca2+Channels

“…The component of the EPSC (13%) that was not inhibited by these blockers was inhibited by Cd 2ϩ and might reflect the contribution of SNX-482-resistant R-type Ca 2ϩ channels (Tottene et al 2000). In axons of the rat optic nerve and RGCs nearly 40 -60% of the evoked Ca 2ϩ influx accounted for N-type Ca 2ϩ channels, whereas P/Q-type Ca 2ϩ channels make little, if any, contribution (Guenther et al 1994;Sun and Chiu 1999). In RGCs, L-type Ca 2ϩ channels mediate 25% of the Ca 2ϩ current (Guenther et al 1994).…”

“…Activation of GABA B receptors suppresses GABA release from the terminals of cultured SCN neurons through a G-protein-mediated inhibition of N-and P/Q-type Ca 2ϩ channels (Chen and Van den Pol 1998). Also baclofen-induced inhibition of N-type Ca 2ϩ channels was found in axons of neonatal rat optic nerve (Sun and Chiu 1999). In addition, baclofen activates fast inactivating A-type K ϩ channels and Ca 2ϩ -activated K ϩ channels (SK channels) (Bettler et al 2004;Saint et al 1990).…”