4-acetic acid-ribotide (IAA-RP), an endogenous agonist at imidazoline receptors (I-Rs), is a putative neurotransmitter/regulator in mammalian brain. We studied the effects of IAA-RP on excitatory transmission by performing extracellular and whole cell recordings at Schaffer collateral-CA1 synapses in rat hippocampal slices. Bathapplied IAA-RP induced a concentration-dependent depression of synaptic transmission that, after washout, returned to baseline within 20 min. Maximal decrease occurred with 10 M IAA-RP, which reduced the slope of field extracellular postsynaptic potentials (fEPSPs) to 51.2 Ϯ 5.7% of baseline at 20 min of exposure. Imidazole-4-acetic acid-riboside (IAA-R; 10 M), the endogenous dephosphorylated metabolite of IAA-RP, also produced inhibition of fEPSPs. This effect was smaller than that produced by IAA-RP (to 65.9 Ϯ 3.8% of baseline) and occurred after a further 5-to 8-min delay. The frequency, but not the amplitude, of miniature excitatory postsynaptic currents was decreased, and paired-pulse facilitation (PPF) was increased after application of IAA-RP, suggesting a principally presynaptic site of action. Since IAA-RP also has low affinity for ␣ 2 -adrenergic receptors (␣ 2 -ARs), we tested synaptic depression induced by IAA-RP in the presence of ␣ 2 -ARs, I 1 -R, or I 3 -R antagonists. The ␣ 2 -AR antagonist rauwolscine (100 nM), which blocked the actions of the ␣ 2 -AR agonist clonidine, did not affect either the IAA-RP-induced synaptic depression or the increase in PPF. In contrast, efaroxan (50 M), a mixed I 1 -R and ␣ 2 -AR antagonist, abolished the synaptic depression induced by IAA-RP and abolished the related increase in PPF. KU-14R, an I 3 -R antagonist, partially attenuated responses to IAA-RP. Taken together, these data support a role for IAA-RP in modulating synaptic transmission in the hippocampus through activation of I-Rs. presynaptic inhibition; hippocampal slice preparation; Schaffer collaterals