AR-M 1896

Binding IC50: 1.76 nM for rGalR2; 879 nM for hGalR1

Galanin is a 29-aa neuropeptide with a complex role in pain processing. Galanin receptor subtypes are present in dorsal root ganglia and spinal cord with a differential distribution. Three galanin receptors, GalR1, GalR2 and GalR3, have been identified and cloned. AR-M1896 is a specific galanin R2 (GalR2) agonist.

In vitro: Additional removal of the glycine residue in position 1 resulted in AR-M1896 with almost unchanged GalR2 affinity and functional activity, and 500-fold selectivity for GalR2-Rs over GalR1-Rs. This compound represents a truly GalR2-selective galanin analog and, thus, could be used as a pharmacological tool to differentiate between these two receptors [1].

In vivo: In normal rats mechanical and cold allodynia of the hindpaw are induced by intrathecal infusion of low-dose galanin. The same effect is seen with equimolar doses of AR-M1896 or AR-M961 (an agonist both at GalR1 and GalR2 receptors). In allodynic Bennett model rats, the mechanical threshold dose-dependently increased after intrathecal injection of a high AR-M961dose, whereas no effect was observed in the control or AR-M1896 group. No effect of either compounds was observed in nonallodynic Bennett model rats [1].

Clinical trial: Up to now, AR-M1896 is still in the preclinical development stage.

Reference:
[1] Liu HX, Brumovsky P, Schmidt R, Brown W, Payza K, Hodzic L, Pou C, Godbout C, Hökfelt T. 
Receptor subtype-specific pronociceptive and analgesic actions of galanin in the spinal cord: selective actions via GalR1 and GalR2 receptors.  Proc Natl Acad Sci U S A. 2001 Aug 14;98(17):9960-4.

Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin.[Pubmed:17910903]

Pain. 2008 Jul;137(1):138-46.

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.

Regulation of kindling epileptogenesis by hippocampal galanin type 1 and type 2 receptors: The effects of subtype-selective agonists and the role of G-protein-mediated signaling.[Pubmed:16699066]

J Pharmacol Exp Ther. 2006 Aug;318(2):700-8.

The search for antiepileptic drugs that are capable of blocking the progression of epilepsy (epileptogenesis) is an important problem of translational epilepsy research. The neuropeptide galanin effectively suppresses acute seizures. We examined the ability of hippocampal galanin receptor type 1 (GalR1) and type 2 (GalR2) to inhibit kindling epileptogenesis and studied signaling cascades that mediate their effects. Wistar rats received 24-h-long intrahippocampal infusion of a GalR1/2 agonist galanin(1-29), GalR1 agonist M617 [galanin(1-13)-Gln14-bradykinin(2-9)-amide], or GalR2 agonist galanin(2-11). The peptides were administered alone or combined with an inhibitor of Gi protein pertussis toxin (PTX), Gi-protein activated K+ channels (GIRK) inhibitor tertiapin Q (TPQ), G(q/11) protein inhibitor [D-Arg1,D-Trp(5,7,9),Leu11]-substance P (dSP), or an inhibitor of intracellular Ca2+ release dantrolene. Sixteen hours into drug delivery, the animals were subjected to rapid kindling-60 electrical trains administered to ventral hippocampus every 5 min. M617 delayed epileptogenesis, whereas galanin(1-29) and galanin(2-11) completely prevented the occurrence of full kindled seizures. TPQ abolished anticonvulsant effect of M617 but not of galanin(2-11). PTX blocked anticonvulsant effects of M617 and inversed the action of galanin(1-29) and galanin(2-11) to proconvulsant. dSP and dantrolene did not modify seizure suppression through GalR1 and GalR2, but eliminated the proconvulsant effect of PTX + galanin(1-29) and PTX + galanin(2-11) combinations. We conclude that hippocampal GalR1 exert their disease-modifying effect through the Gi-GIRK pathway. GalR2 is antiepileptogenic through the Gi mechanism independent of GIRK. A secondary proconvulsant pathway coupled to GalR2 involves G(q/11) and intracellular Ca2+. The data are important for understanding endogenous mechanisms regulating epileptogenesis and for the development of novel antiepileptogenic drugs.

Galanin (2-11) binds to GalR3 in transfected cell lines: limitations for pharmacological definition of receptor subtypes.[Pubmed:15944007]

Neuropeptides. 2005 Jun;39(3):165-7.

The neuropeptide galanin regulates a variety of physiological and pathophysiological processes through three G protein coupled receptors, GalR1, GalR2, and GalR3. The studies on galanin receptor subtype specific effects have been hampered by the lack of high affinity subtype selective antagonist and/or agonist to any of these three galanin receptor subtypes. Since its recent introduction in 2003, galanin (2-11) has been widely used as a GalR2 selective agonist in several in vitro and in vivo studies. In the present paper, we demonstrate that galanin (2-11) binds to rat GalR3 receptors in transfected cell lines with a similar affinity as it binds to GalR2. As none of the available antagonists are galanin receptor subtype selective, as shown here for M35 and M40, more work is needed to confirm whether a galanin (2-11) effect is GalR2 mediated and there is an urgent need for high affinity galanin receptor subtype selective ligands. For now one needs to interpret the data obtained at lower galanin (2-11) concentrations as effects mediated by non-GalR1 type galanin receptors, i.e., GalR2 and/or GalR3.