The thalamocortical (TC) neurones of the sensory thalamic nuclei receive three excitatory inputs: two are the ascending input from the sensory pathway and the descending input from the cortex, and the third is the modulatory input from the brainstem activating system (Jones, 1985). The sensory and cortical inputs form synapses at different locations on the dendritic arbors of TC neurones, with the sensory input more proximal to the soma and the cortical input more distal. The sensory input to auditory, somatosensory and visual thalamic nuclei has been well characterized as being mediated by both non-NMDA and NMDA ionotropic glutamate receptors both in vitro (Scharfman, Lu, Guido, Adams & Sherman, 1990; Turner, Leresche, Guyon, Soltesz & Crunelli, 1994;Hu, Senatorov & Mooney, 1994;Kao & Coulter, 1997) and in vivo (Salt, 1986;Sillito, Murphy & Salt, 1990a;Sillito, Murphy, Salt & Moody, 1990b;Salt & Eaton, 1991). However, while the contribution of these ionotropic glutamate receptors to corticothalamic synaptic transmission has been established both in vitro and in vivo (Deschênes & Hu, 1990;Scharfman et al. 1990;Eaton & Salt, 1996;Kao & Coulter, 1997) 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 (û 0·1 Hz) consisted of one or two fastrising (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. 3. 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 û 0·1 Hz corticothalamic EPSP (64·6 ± 9·2 %), but not the retinogeniculate EPSP, following trains of stimuli at 50 Hz. 5. The û 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, res...

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1. We have examined the possibility that N-methyl-D-aspartate (NMDA) receptors may be involved in the visual response of relay cells in the cat dorsal lateral geniculate nucleus (dLGN). The selective NMDA receptor antagonists D-2-amino-5-phosphonovalerate (APV) and 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) have been iontophoretically applied to X and Y cells in the dLGN and their effects on the visual response to a light spot flashed within the receptive field center determined. 2. The antagonist effects were assessed at ejection current levels producing a selective blockade of the responses to iontophoretically applied NMDA with respect to those elicited by the non-NMDA receptor agonists quisqualate and kainate. These selective effects were determined in an experimental paradigm where the visual response and responses to NMDA and the non-NMDA receptor agonists were compared in the same test run. The data refer to a total population of 52 cells (28 X, 24 Y). 3. Application of APV abolished or greatly reduced the visual responses of both X and Y cells. The mean percentage reduction in the visual response for the X cells studied was 59 +/- 10% (SE; n = 7) and for the Y cells 66 +/- 8% (SE; n = 11). Both the early onset transient and the sustained component of the visual response to the flashed stimulus were equally affected. 4. The antagonist CPP produced a similar pattern of effect to APV, substantially reducing or abolishing the visual response in both X and Y cells.(ABSTRACT TRUNCATED AT 250 WORDS)

1. Many sensory events contain multimodal information, yet most sensory nuclei are devoted to the analysis of single-modality information. In the deep superior colliculus (DSC), visual, auditory, and somatosensory information converges on individual multimodal neurons. The responses of multimodal neurons are determined by the temporal and spatial correspondence properties of the converging inputs such that stimuli arising from the same event elicit a facilitated multimodal response. 2. N-methyl-D-aspartate (NMDA) receptors may underlie the detection of spatial and temporal coincidence and could be involved in the generation of multimodal facilitatory responses because of the nonlinear properties of NMDA-receptor-mediated events. To assess the role of NMDA receptors in multimodal integration, we made extracellular recordings from single multisensory neurons in the DSC of the cat. 3. The responses to visual, auditory, and somatosensory stimuli alone and to multimodal combinations of stimuli were challenged with iontophoretically applied D-2-amino-5-phosphonovalerate (AP5), an NMDA receptor antagonist. All responses to visual stimuli presented alone (n = 9) were greatly reduced. Somatosensory responses (n = 25) were usually decreased. In contrast, the responses to auditory stimulation were decreased (n = 9), unaffected (n = 3), or enhanced (n = 5). 4. Responses to multimodal stimulus presentations were consistently reduced during iontophoretic application of AP5, irrespective of the modalities that made up the stimulus. The reductions of multimodal responses were significantly greater than the sum of the reductions of responses to single-modality stimuli. 5. The data suggest that for unimodal stimuli, the importance of NMDA receptors in synaptic transmission of sensory responses in DSC may be dependent on the stimulus modality. Furthermore, NMDA receptors are of major importance in the integration of input from different modalities for the generation of multimodal responses.

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