Structure-activity relationships of BnOCPA derivatives at native pre- and postsynaptic adenosine A1Rs.

The addition of a benzyloxy group to the adenosine A1R agonist cyclopentyladenosine (CPA) produces a compound (BnOCPA) that selectivity activates A1R signalling via the G protein alpha subunit Gob. In rat hippocampus, BnOCPA inhibits CA3-CA1 synaptic transmission (IC50 65 nM) likely via Gob, with no effect on Goa-mediated postsynaptic membrane electrophysiology at concentrations up to 1 μM. We have investigated how modifications of the benzyloxy group of BnOCPA affects signalling: addition of methoxy group, MBnOCPA, substituting a phenoxy group, PhOCPA; and addition of bromide to the phenoxy group, BPhOCPA. These derivatives, previously shown to be selective A1R agonists, were assessed through inhibition of hippocampal CA3-CA1 synaptic transmission and postsynaptic effects on CA1 pyramidal neurons. All compounds inhibited synaptic transmission and increased paired-pulse facilitation, indicative of presynaptic actions. At a concentration that inhibited synaptic transmission (>75 %), none of the compounds hyperpolarised the resting membrane potential and thus retained the pre- vs postsynaptic selectivity of BnOCPA. Increasing their concentration allowed a measure of the selectivity range. BnOCPA hyperpolarised ∼ 40 % of neurons at 3 μM (∼ 50x the IC50 for synaptic inhibition). For PhOCPA and MBnOCPA, hyperpolarisation, occurred for some neurons, at 1 μM. For BPhOCPA, no hyperpolarisation occurred at concentrations up to 10 μM. Pre-application of the derivatives (at 300 nM) reduced adenosine-mediated hyperpolarisation, showing they bind but do not activate postsynaptic A1Rs. This data shows that modifying the structure of BnOCPA changes the discrimination between different native A1Rs, likely based upon selective activation of G-protein coupling.