Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin

Abstract

Galanin modulates spinal nociceptive processing by interacting with two receptors, GalR1 and GalR2. The underlying neurophysiological mechanisms were examined by whole-cell recording from identified neurons in the substantia gelatinosa of young adult rats. GalR1 was activated with a 'cocktail' containing the GalR1/2 agonist, AR-M 961 (0.5 microM), in the presence of the GalR2 antagonist, M871 (1.0-2.5 microM). GalR2 was activated with the selective agonist, AR-M 1896 (0.5-1.0 microM). Application of the 'GalR1 agonist cocktail' often activated an inwardly-rectifying conductance in delay firing (excitatory) and tonically firing (inhibitory) neurons. This conductance was not activated by AR-M 1896 which instead decreased or increased an outwardly-rectifying conductance at voltages positive to -70 mV. Despite this variability in its actions on current-voltage relationships, AR-M 1896 very consistently decreased membrane excitability, as measured by cumulative action potential latency in response to a depolarizing current ramp. This strong GalR2-mediated effect was seen in neurons where membrane conductance was decreased, and where membrane excitability might be predicted to increase. GalR2 was also located presynaptically, as AR-M 1896 increased the interevent interval of spontaneous EPSCs in both delay and tonic cells. By contrast, the 'GalR1 agonist cocktail' had little effect on spontaneous EPSCs, suggesting that presynaptic terminals do not express GalR1. These diverse actions of GalR1 and GalR2 activation on both inhibitory and excitatory neurons are discussed in relation to the known spinal antinociceptive and pro-nociceptive actions of galanin, to the possible association of GalR1 with the inhibitory G-protein, G(i/o) and to report that GalR2 activation suppresses Ca2+ channel currents.

Biphasic modulation by galanin of excitatory synaptic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

“…Galanin also reduced the peak amplitudes of the monosynaptic A␦-fiber and C-fiber EPSCs evoked by stimulating the dorsal root in Ͼ70% of the SG neurons examined. This result is consistent with the observation that galanin reduces the peak amplitudes of EPSCs evoked in SG neurons by stimulating the dorsal root entry zone in young adult rats (Alier et al 2008). This primary afferentevoked EPSC amplitude reduction is presynaptic in origin, because the sEPSC amplitude is unaffected by galanin.…”

“…This result may be consistent with the presence of GalR2 and GalR3 mRNAs in the rat spinal cord and DRG (O'Donnell et al 1999;Waters and Krause 2000). Alier et al (2008) have reported that galanin 2-11 dominantly increases the inter-event interval of sEPSC (decreases sEPSC frequency). Although a reason for the discrepancy between their study and our study is unknown, this may be due to a difference in the age of the rats used or distinct types of the neurons tested.…”

“…The presynaptic effects of galanin, i.e., sEPSC frequency increase and EPSC amplitude decrease, were mimicked by the GalR2/R3 agonist galanin 2-11 [this drug is also known as AR-M1896 and was used as a selective GalR2 agonist by Alier et al (2008), but it also binds to GalR3; see Hökfelt 2005], indicating an involvement of GalR2/R3. This result may be consistent with the presence of GalR2 and GalR3 mRNAs in the rat spinal cord and DRG (O'Donnell et al 1999;Waters and Krause 2000).…”

“…Furthermore, the concentration dependency for this action and whether galanin affects inhibitory transmission in SG neurons have not been investigated. Alier et al (2008) have also found that galanin inhibits excitatory transmission evoked by stimulating the dorsal root entry zone. …”

“…The SG neurons receive not only glutamatergic excitatory synaptic transmission but also (GABAergic and glycinergic) inhibitory synaptic transmission (Willis and Coggeshall 1991), the modulation of which may also play a role in regulating nociceptive transmission (Coull et al 2003;Moore et al 2002; for review see Kohno 2007). Although Alier et al (2008) have reported that GalR1 and GalR2 activation have an effect on excitatory transmission in SG neurons of young adult rats, how galanin itself acts on excitatory transmission in SG neurons has not been examined. Furthermore, the concentration dependency for this action and whether galanin affects inhibitory transmission in SG neurons have not been investigated.…”