Optogenetic analysis of GABA_Breceptor signaling inCaenorhabditis elegansmotor neurons

Abstract

Schultheis C, Brauner M, Liewald JF, Gottschalk A. Optogenetic analysis of GABA B receptor signaling in Caenorhabditis elegans motor neurons. J Neurophysiol 106: 817-827, 2011. First published May 25, 2011 doi:10.1152/jn.00578.2010.-In the nervous system, a perfect balance of excitation and inhibition is required, for example, to enable coordinated locomotion. In Caenorhabditis elegans, cholinergic and GABAergic motor neurons (MNs) effect waves of contralateral muscle contraction and relaxation. Cholinergic MNs innervate muscle as well as GABAergic MNs, projecting to the opposite side of the body, at dyadic synapses. Only a few connections exist from GABAergic to cholinergic MNs, emphasizing that GABA signaling is mainly directed toward muscle. Yet, a GABA B receptor comprising GBB-1 and GBB-2 subunits, expressed in cholinergic MNs, was shown to affect locomotion, likely by feedback inhibition of cholinergic MNs in response to spillover GABA. In the present study, we examined whether the GBB-1/2 receptor could also affect short-term plasticity in cholinergic MNs with the use of channelrhodopsin-2-mediated photostimulation of GABAergic and cholinergic neurons. The GBB-1/2 receptor contributes to acute body relaxation, evoked by photoactivation of GABAergic MNs, and to effects of GABA on locomotion behavior. Loss of the plasma membrane GABA transporter SNF-11, as well as acute photoevoked GABA release, affected cholinergic MN function in opposite directions. Prolonged stimulation of GABA MNs had subtle effects on cholinergic MNs, depending on stimulus duration and gbb-2. Thus GBB-1/2 receptors serve mainly for linear feedback inhibition of cholinergic MNs but also evoke minor plastic changes. locomotion; metabotropic GABA receptor; plasticity; channelrhodopsin-2; excitatory-inhibitory balance IN MAMMALS, GABA B receptors are extrasynaptic, high-affinity G protein-coupled receptors (GPCRs) that either act as presynaptic autoreceptors on GABAergic neurons or detect spillover GABA, released at nearby synapses (Bettler et al. 2004). GABA B receptors are obligate heterodimers of B1 and B2 subunits (Jones et al. 1998;White et al. 1998) that together with auxiliary subunits (KCTD proteins) appear to form tetramers or even higher order oligomers (Schwenk et al. 2010). GABA B receptors can modulate the function of excitatory neurons by heterosynaptic inhibition, via signaling through heterotrimeric G proteins (through "released" G␤␥ subunits), to inhibit presynaptic voltage-gated Ca 2ϩ -channels (Herlitze et al. 1996;Ikeda 1996). Alternatively, they can trigger postsynaptic G protein-activated inward-rectifying potassium (GIRK) channels, again via G␤␥ subunits, to induce a slow inhibitory current (Luscher et al. 1997;Schwenk et al. 2010 Cholinergic motor neurons (MNs) in the Caenorhabditis elegans ventral nerve cord activate muscles and GABAergic neurons at dyadic synapses/neuromuscular junctions (NMJs) to coevoke a contralateral inhibition of muscles, thus allowing a bend of the body to occur (Schuske et al. 2004;White e...

An Evolutionarily Conserved Switch in Response to GABA Affects Development and Behavior of the Locomotor Circuit of Caenorhabditis elegans

“…The GABA A receptor UNC-49 is expressed on body wall muscles and directly mediates muscle relaxation in adults (McIntire et al 1993a). Pevious studies have shown that a GABA B receptor, consisting of two obligate subunits GBB-1 and GBB-2, is expressed on the cholinergic motor neurons that excite the body wall muscles, and by inhibiting these excitatory neurons the GABA B receptor indirectly relaxes the muscles (Dittman and Kaplan 2008;Schultheis et al 2011). Thus GABA leads to body wall muscle relaxation in adult animals via both the GABA A and GABA B receptors.…”

“…We simply repeated the optogenetic assays of Schultheis et al (2011), except using early L1 larvae instead of adult animals, to determine whether GABA A and GABA B signaling inhibit or excite muscles in early L1s. We obtained strains of C. elegans carrying a chromosomally integrated transgene that expresses channel rhodopsin 2 (ChR2), a blue-lightactivated cation channel, specifically in GABAergic motor neurons, and that additionally contain various combinations of mutations for the GABA A receptor UNC-49 and the GABA B receptor subunits GBB-1 and GBB-2 (Schultheis et al 2011).…”

“…Optogenetic assays and automated video analysis for the extraction of worm body lengths as shown in Figure 7 were adapted from previous studies (Liewald et al 2008;Schultheis et al 2011). All-trans retinal plates were made by adding 4 ml of 100 mM stock all-trans retinal (Spectrum Chemical, no.…”

Optogenetic analyses of neuronal network function and synaptic transmission in Caenorhabditis elegans

“…Since these early experiments, quite a number of optogenetic tools have been established in C. elegans. These include a number of variants of ChR2 which have different properties that are useful for specific applications [10,26,37,38,49]. Potent inhibitory rhodopsins have also been established which act as outward-directed proton pumps [15,16,18].…”

“…Mechanisms of synaptic transmission can also be analyzed in C. elegans using optogenetic methods (optogenetic investigation of neurotransmission, OptIoN; Liewald et al [25]). Since C. elegans neurons usually generate graded potentials and no action potentials, the strength of the light-induced depolarization largely correlates in a linear manner with the amount of neurotransmitter released [26,37,38]. Postsynaptic effects can be quantified on the basis of the triggered behavior (e.g., depolarization of cholinergic neurons leads to a strong contraction of the entire animal), or on the basis of electrophysiological recordings of the induced postsynaptic currents [25].…”