Physiological evidence for local excitatory synaptic circuits in the rat hypothalamus

We conducted whole cell voltage-clamp and current-clamp recordings in slices of rat hypothalamus to test for local excitatory synaptic circuits. Local excitatory inputs to neurons of the paraventricular nucleus (PVN) and supraoptic nucleus (SON) were studied with the use of electrical and chemical stimulation. Extracellular electrical stimulation provided indirect evidence of local excitatory circuits. Single stimuli evoked multiple excitatory postsynaptic potentials (EPSPs) or excitatory postsynaptic currents (EPSCs) in some PVN and SON cells, invoking polysynaptic excitatory inputs. Repetitive stimulation (10-20 Hz, 2-10 s) elicited long afterdischarges of EPSPs/EPSCs, suggesting a potentiation of upstream synapses in a polysynaptic circuit. Bath application of metabotropic glutamate receptor agonists provided more conclusive evidence for local excitatory circuits. Metabotropic receptor activation caused an increase in the frequency of EPSPs/EPSCs that was blocked by tetrodotoxin, suggesting that it was mediated by activation of local presynaptic excitatory neurons. The local excitatory inputs to SON and PVN neurons were mediated by glutamate release, because the EPSPs/EPSCs elicited with electrical stimulation and metabotropic receptor activation were blocked by ionotropic glutamate receptor antagonists. Finally, glutamate microstimulation furnished the most direct demonstration of local excitatory synaptic circuits. Glutamate microstimulation of perinuclear sites elicited an increase in the frequency of EPSPs/EPSCs in 13% of the PVN and SON neurons tested. Two sites provided most of the local excitatory synaptic inputs to PVN neurons, the dorsomedial hypothalamus and the perifornical region. These experiments provide converging physiological evidence for local excitatory synaptic inputs to hypothalamic neurons, inputs that may play a role in pulsatile hormone release.