Title: Frequency-dependent Release of Substance P Mediates Heterosynaptic 1 Potentiation of Glutamatergic Synaptic Responses in the Rat Visual Thalamus 2

29 30 To investigate the interaction between peptides and glutamatergic synapses in the 31 dorsal thalamus, we compared the frequency-dependent plasticity of excitatory 32 postsynaptic potentials (EPSPs) in the tectorecipient zone of rodent lateral posterior 33 nucleus (LPN), which is densely innervated by axons that contain the neuromodulator 34 substance P (SP). Immunocytochemistry and confocal and electron microscopy revealed 35 that neurokinin 1 (NK1) receptors are distributed on the dendrites of LPN cells, while SP 36 is contained in axons originating from the superior colliculus (SC) and is reduced 37 following SC lesions. In vitro whole cell recordings in parasagittal slices revealed that 38 stimulation of the SC or optic radiations (CTX) evoked LPN EPSPs that increased in 39 amplitude with increasing stimulation intensity, suggesting convergence. With 0.5-10Hz 40 stimulus trains, CTX EPSP amplitudes displayed frequency-dependent facilitation, while 41 SC EPSP amplitudes were unchanged. High frequency SC stimulation (100Hz for 0.5 42 seconds), or bath application of SP, resulted in gradual increases in both SC and CTX 43 EPSP amplitudes to 2 fold or greater above baseline within 15-20 minutes post 44 stimulation/application. This enhancement correlated with increases in input resistance, 45 and both the potentiation and resistance change were abolished in the presence of the 46 NK1 antagonist L-703,606. These results indicate that SP is released when SC-LPN 47 neurons fire at high frequency, and SP acts postsynaptically via NK1 receptors to 48 potentiate subsequent rLPN responses to both cortical and tectal inputs. We suggest that 49 the SP-mediated potentiation of synaptic responses may serve to amplify responses to 50 threatening objects that move across large regions of the visual field. 51 52