Characterization of sensory and corticothalamic excitatory inputs to rat thalamocortical neurones in vitro

1. Using an in vitro slice preparation of the rat dorsal lateral geniculate nucleus (dLGN), the properties of retinogeniculate and corticothalamic inputs to thalamocortical (TC) neurones were examined in the absence of GABAergic inhibition. 2. The retinogeniculate EPSP evoked at low frequency (≤ 0.1 Hz) consisted of one or two fast-rising (0.8 ± 0.1 ms), large-amplitude (10.3 ± 1.6 mV) unitary events, while the corticothalamic EPSP had a graded relationship with stimulus intensity, owing to its slower-rising (2.9 ± 0.4 ms), smaller-amplitude (1.3 ± 0.3 mV) estimated unitary components. The retinogeniculate EPSP exhibited a paired-pulse depression of 60.3 ± 5.6% at 10 Hz, while the corticothalamic EPSP exhibited a paired-pulse facilitation of > 150%. This frequency-dependent depression of the retinogeniculate EPSP was maximal after the second stimulus, while the frequency-dependent facilitation of the corticothalamic EPSP was maximal after the fourth or fifth stimulus, at interstimulus frequencies of 1-10 Hz. 4. There was a short-term enhancement of the ≤ 0.1 Hz corticothalamic EPSP (64.6 ± 9.2%), but not the retinogeniculate EPSP, following trains of stimuli at 50 Hz. 5. The ≤ 0.1 Hz corticothalamic EPSP was markedly depressed by the non-NMDA antagonist 1-(4-amino-phenyl)-4-methyl-7,8-methylene-dioxy-5H-2,3-benzodiazepine (GYKI 52466), but only modestly by the NMDA antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid ((RS)-CPP), and completely blocked by the co-application of GYKI 52466, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), (RS)-CPP and (5R,10S)-(±)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801). Likewise, the corticothalamic responses to trains of stimuli (1-500 Hz) were greatly reduced by this combination of ionotropic glutamate receptor antagonists. 6. In the presence of GYKI 52466, CNQX, (RS)-CPP and MK-801, residual corticothalamic responses and slow EPSPs, with a time to peak of 2-10 s, could be generated following trains of five to fifty stimuli. Neither of these responses were occluded by 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), suggesting they are not mediated via group I and II metabotropic glutamate receptors.