Opposite to α2-adrenergic agonists, an imidazoline I1 selective compound does not influence reflex bradycardia in rabbits

This work aimed to study the respective effects of central α2-adrenergic receptors (α2-ARS) and I1 imidazoline receptors (I1Rs) in the facilitatory effects of imidazoline-like drugs on the reflex bradycardia (RB). Experiments were performed in anaesthetized rabbits. The reflex bradycardic response was induced by phenylephrine injected i.v. LNP 509, rilmenidine and dexmedetomidine were administered intracisternally (i.c.). LNP509 (1 mg/kg, i.c.), a ligand highly selective for I1Rs, induced hypotension (54 ± 3 vs. 93 ± 2 mm Hg) and bradycardia (260 ± 13 vs. 322 ± 13 beats/min) (p < 0.05, n = 5) but did not affect RB. Rilmenidine (1 μg/kg, i.c.), a hybrid ligand which binds to both I1 and α2-ARS, also decreased arterial pressure (61 ± 2 vs. 101 ± 2 mm Hg) and heart rate (260 ± 4 vs. 308 ± 8) (p < 0.01, n = 5); it potentiated the RB (maximum R–R interval: 284 ± 17 vs. 196 ± 6 ms) (p < 0.05, n = 5). Dexmedetomidine (1 μg/kg, i.c.), a ligand selective for α2-ARs, reduced blood pressure (53 ± 3 vs. 104 ± 2 mm Hg) and heart rate (246 ± 4 vs. 312 ± 8 beats/min) (p < 0.05, n = 5) and potentiated the RB (maximum R–R interval: 518 ± 38 vs. 194 ± 4 ms) (p < 0.05, n = 5). The potentiation of RB was much greater than that observed with rilmenidine and was significantly prevented by L-NNA injected centrally. This study shows that: (i) an exclusive action on I1Rs which decreases arterial pressure, does not potentiate the RB ii) activation of α2-ARs potentiates the RB (iii) the R–R prolongation caused by α2-ARs stimulation is prevented by central NOS inhibition.