Visual input to the mouse lateral posterior and posterior thalamic nuclei: photoreceptive origins and retinotopic order

Abstract

Key points: The lateral posterior and posterior thalamic nuclei have been implicated in aspects of visually guided behaviour and reflex responses to light, including those dependent on melanopsin photoreception. Here we investigated the extent and basic properties of visually evoked activity across the mouse lateral posterior and posterior thalamus. We show that a subset of retinal projections to these regions derive from melanopsin-expressing retinal ganglion cells and find many cells that exhibit melanopsin-dependent changes in firing. We also show that subsets of cells across these regions integrate signals from both eyes in various ways and that, within the lateral posterior thalamus, visual responses are retinotopically ordered.

Abstract: In addition to the primary thalamocortical visual relay in the lateral geniculate nuclei, a number of other thalamic regions contribute to aspects of visual processing. Thus, the lateral posterior thalamic nuclei (LP/pulvinar) appear important for various functions including determining visual saliency, visually guided behaviours and, alongside dorsal portions of the posterior thalamic nuclei (Po), multisensory processing of information related to aversive stimuli. However, despite the growing importance of mice as a model for understanding visual system organisation, at present we know very little about the basic visual response properties of cells in the mouse LP or Po. Prompted by earlier suggestions that melanopsin photoreception might be important for certain functions of these nuclei, we first employ specific viral tracing to show that a subset of retinal projections to the LP derive from melanopsin-expressing retinal ganglion cells. We next use multielectrode electrophysiology to demonstrate that LP and dorsal Po cells exhibit a variety of responses to simple visual stimuli including two distinct classes that express melanopsin-dependent changes in firing (together comprising ∼25% of neurons we recorded). We also show that subgroups of LP/Po cells integrate signals from both eyes in various ways and that, within the LP, visual responses are retinotopically ordered. Together our data reveal a diverse population of visually responsive neurons across the LP and dorsal Po whose properties align with some of the established functions of these nuclei and suggest new possible routes through which melanopsin photoreception could contribute to reflex light responses and/or higher order visual processing.

Figure 1. Melanopsin ganglion cell projections to the lateral posterior thalamus

Anterograde tracing of melanopsin ganglion cells was achieved by injecting a virus containing floxed eYFP and BFP sequences (AAV‐ hEF1a.lox.TagBFP.lox.eYFP.lox.WPRE.hGH‐InvBYF) into the vitreous of Opn4^cre/+ mice. A–C, representative example of eYFP expression in LP and LGN at ∼−2.18 mm from bregma. D–F, representative example of eYFP expression in LP and LGN at ∼−2.30 mm from bregma. eYFP signal shown as black. A and D, whole thalamus; B and E, magnifications of orange box in A and D; C and F, magnifications of orange box in B and E. Scale bars = 200 μm. G, regions of mRGC terminal labelling identified in five Opn4^cre/+ mice receiving unilateral intravitreal viral injections.

Figure 2. Fast and slow visual responses across the lateral posterior and posterior thalamus

A, histological reconstruction of a multi‐electrode probe placement within the lateral posterior thalamus (LP). Probe shanks are marked with DiI (red fluorescence), light microscopic image is pseudocoloured green. Scale bar = 200 μm, Po = posterior thalamus, LGN = lateral geniculate nuclei. B, multi‐unit firing responses (at probe placement shown in A) evoked by a 5 s, 405 nm, light step producing an effective corneal irradiance of 14.4 log photons cm^–2 s^–1 at the contralateral eye. C, normalised mean (±SEM) change in firing for three classes of wild‐type (WT) LP/Po neuronal response to full field light steps as in B (n = 115, 74 and 58 for transient, sustained and delayed cells, respectively). D, response latency distributions for LP/Po cells from WT and Opn1mw^R animals (n = 26, 29 and 16 for Opn1mw^R transient, sustained and delayed cells). E, anatomical distribution of transient, sustained and delayed cells, showing the % of cells of each class binned using a moving circular window (240 μm diameter). Analysis based on a total of 526 WT and 162 Opn1mw^R neurons (including 279 and 91 cells, respectively, that did not show overt responses to full field light steps). APT = anterior pretectum.

Figure 3. Melanopsin contributions to lateral posterior and posterior thalamic visual responses

A and B, normalised mean (±SEM) change in firing for classes of LP/Po neuronal response to full field 460 nm (blue line; 30 s 14.9 log melanopsin‐effective photons cm^–2 s^–1, 14.4 log L‐cone‐effective photons cm^–2 s^–1) or 655 nm (red line; 30 s 11.2 log melanopsin‐effective photons cm^–2 s^–1, 14.4 log L‐cone‐effective photons cm^–2 s^–1) steps in Opn1mw^R mice (n = 15 and 10 for sustained and delayed cells, respectively). C and D, spectral composition of cone opsin and melanopsin isolating stimuli; traces show background spectra, and purple/green lines show stimulus spectra (respectively). Steps from background to stimulus spectra result in 78% Michelson contrast for L‐ and S‐ cone opsins (C) or 93% Michelson contrast for Melanopsin (D). E–G, normalised mean (±SEM) change in firing for three classes of LP/Po neuronal response to 5 s full field cone‐ (upper panel) or melanopsin‐ (lower panel) isolating steps in Opn1mw^R mice (n = 14, 17 and 6 for sustained, transient and delayed cells, respectively).

Figure 4. Delayed cells integrate visual signals from both eyes

A and B, example responses of two delayed cells following full field light steps applied to the contralateral, ipsilateral or both eyes (5 s, 405 nm LED, 13.4–15.4 log photons cm^–2 s^–1). C, distribution of contralateral vs. ipsilateral eye‐evoked response amplitude (change in mean spike rate during 5 s light step relative to 5 s preceeding) for delayed cells in WT mice (n = 58). All cells exhibited responses to both eyes, with variable preference towards contra‐ or ipsilateral eye. D, distribution of both vs. dominant eye‐only‐evoked responses (measured as in C). Binocular responses were reliably larger than those evoked by the dominant eye alone. E, normalised mean (±SEM) change in firing for delayed cells in WT mice evoked by 9.4–15.4 log photons cm^–2 s^–1 light steps applied to the dominant, non‐dominant or both eyes. Note that across the population, sensitivity to light is extremely low and that binocular facilitation is associated with an increase in maximal response with no change in sensitivity (four‐parameter sigmoid curve fits with F‐tests for difference in EC₅₀/Hill slope or maxima, P = 0.397 and < 0.0001, respectively).

Figure 5. Subsets of lateral posterior thalamic cells combine eye‐specific signals in a sub‐additive manner

A–D, example responses of LP cells following full field light steps applied to the contralateral, ipsilateral or both eyes (5 s, 405 nm LED, 10.4–14.4 log photons cm^–2 s^–1). Subsets of sustained and transient LP cells (n = 18/67 and 10/89, respectively) exhibited excitatory responses to stimulation of either eye (A, B), or lacked clear responses to ipsilateral eye stimuli but showed a marked reduction in response under binocular vs. contralateral eye only stimulation (C, n = 7 and 4 for sustained and transient). Remaining sustained and transient cells exhibited purely monocular responses driven by the contralateral eye (D) – no cells were observed that exhibited purely ipsilateral eye driven responses. E, distribution of contralateral vs. ipsilateral eye‐evoked response amplitude (peak spike response observed 0–500 ms after start of light steps between 9.4 and 15.4 log photons cm^–2 s^–1) for sustained/transient, binocular or antagonistic cells. F, distribution of both vs. dominant eye‐only‐evoked responses (measured as in E). G and H, normalised mean (±SEM) initial change in firing (0–500 ms after light step) for ‘binocular’ cells (G, n = 28), ‘antagonistic’ cells (H, n = 11) and monocular cells (I, n = 117) evoked by 9.4–15.4 log photons cm^–2 s^–1 light steps applied to the dominant, non‐dominant or both eyes. Note that, for binocular cells (G), irradiance response relationships for dominant eye‐only and both eye stimuli were statistically identical whereas for antagonistic cells (H) both eye responses were suppressed (four‐parameter sigmoid curve fits with F‐tests, P = 0.218 and P < 0.0001, respectively). J, projected anatomical locations of binocular and antagonistic cells, exhibiting pronounced clustering around the lateral portions of the LP. K, proportion of sustained or transient LP cells exhibiting monocular responses as a function of anatomical location, showing highest density in ventral and medial regions.

Figure 6. Retinotopic organisation and RF properties of lateral posterior thalamic neurons

A and B, spatiotemporal receptive field (RF) maps for four neurons responding to vertical (top panels) or horizontal (bottom panels) flashing bars (250 ms duration), presented to the upper lateral (A) or central (B) visual field. Colour scale represents the difference in spike rate observed during the presentation of a white vs. black bar. Visual angles are expressed relative to the midpoint between the eyes. We observed neurons exhibiting both ON (A, Bi, ii) and OFF RFs (Biii), some of which exhibited clear centre surround organisation (Bii, iii). C, two‐dimensional RF map for neuron in Biii, in response to sparse noise stimuli (flashing white and black spots), showing good correspondence to results obtained using bars. D, mean RF positions in the azimuthal and elevation plane for neurons recorded across the LP and surrounding brain regions, revealing a clear retinotopic order. Data are based on 56 cells (39 WT and 17 Opn1mw^R), binned using a moving circular window (240 μm diameter). E, relationship between anatomical separation and RF centre separation for simultaneously recorded cells (linear distance in medial–lateral and dorsal–ventral or azimuth and elevation, respectively). Despite some scatter, there was a clear positive correlation between anatomical separation and difference in RF centre position (r = 0.25, P = 0.007). F, estimated RF size for neurons recorded across the LP region (n = 48).