The magnocellular neurons of the hypothalamic supraoptic nucleus (SON) synthesize and secrete oxytocin (OXT) and vasopressin (AVP) from their dendrites. These peptides, and several other neurotransmitters, have been shown to modulate afferent glutamatergic neurotransmission in the SON. The neuropeptide, galanin (GAL) is also localized in SON magnocellular neurons and in afferent fibers in the nucleus. We show that GAL dose-dependently reduces evoked excitatory postsynaptic currents (eEPSCs), alters paired pulse ratio and decreases mEPSC frequency, but not amplitude or decay kinetics in both OXT and AVP neurons. GAL therefore modulates excitatory neurotransmission at a likely presynaptic receptor. Neither OXT/AVP, GABA(B) nor cannabinoid antagonists blocked this effect. A GAL2/3 agonist mimicked GAL's action while GAL1 antagonist did not block GAL's effect, suggesting that GAL2/3 receptors mediate the presynaptic effect. In nondehydrated rats GAL causes a small postsynaptic response, as assessed by input resistance measurements. When the rats were water deprived for 2 days the presynaptic response to GAL was unaltered; however, the postsynaptic decrease in input resistance and hyperpolarization was increased, an effect consistent with a previously described increase in GAL1 receptor expression in dehydration. A GAL1 receptor antagonist blocked the postsynaptic effects. Last, when a train of eEPSCs was elicited, GAL was found to inhibit the earlier events in a train but not the latter. This indicates that GAL may modulate a single synaptic event more effectively than trains of synaptic inputs, thereby acting as a high-pass filter.