Relative numbers of cortical and brainstem inputs to the lateral geniculate nucleus - PubMed (original) (raw)

Relative numbers of cortical and brainstem inputs to the lateral geniculate nucleus

A Erişir et al. Proc Natl Acad Sci U S A. 1997.

Abstract

Terminals of a morphological type known as RD (for round vesicles and dense mitochondria, which we define here as the aggregate of types formerly known as RSD and RLD, where "S" is small and "L" is large) constitute at least half of the synaptic inputs to the feline lateral geniculate nucleus, which represents the thalamic relay of retinal input to cortex. It had been thought that the vast majority of these RD terminals were of cortical origin, making the corticogeniculate pathway by far the largest source of input to geniculate relay cells. However, another source of RD terminals recently identified derives from cholinergic cells of the brainstem parabrachial region. (These cells also contain NO.) We used techniques of electron microscopy to determine quantitatively the relative contribution of cortex and brainstem to the population of RD terminals. We identified corticogeniculate terminals by orthograde transport of biocytin injected into the visual cortex and identified brainstem terminals by immunocytochemical labeling for choline acetyltransferase or brain NO synthase (the synthesizing enzymes for acetylcholine and NO, respectively). We estimated the relative numbers of corticogeniculate and brainstem terminals with a two-step algorithm: First, we determined the relative probability of sampling each terminal type in our material, and then we calculated what mixture of identified corticogeniculate and brainstem terminals was needed to recreate the size distribution of the parent RD terminal population. We conclude that brainstem terminals comprise roughly one-half of the RD population. Thus, the cortical input is perhaps half as large and the brainstem input is an order of magnitude larger than had been thought. This further suggests that the brainstem inputs might play a surprisingly complex and subtle role in the control of the geniculocortical relay.

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Figures

Figure 1

Figure 1

Examples of RD terminals (asterisks) and their synaptic contact zones (arrowheads) in the A laminae of the cat lateral geniculate nucleus. (A) Unlabeled RD terminal. (B) Corticogeniculate terminal labeled with biocytin. (C) Brainstem terminal labeled with antibody directed against ChAT. (Bar = 0.5 μm for A_–_C.)

Figure 2

Figure 2

Measurements of synaptic terminals for parent RD terminals (i.e., the larger population from which cholinergic brainstem and corticogeniculate terminals are drawn), brainstem terminals labeled for ChAT or BNOS, and corticogeniculate terminals labeled with biocytin. (A_–_C) Frequency histograms indicating the extent of the synaptic contact zones. For each of the terminals, the contact zone was serially reconstructed to derive the number of serial sections needed to contain the reconstructed synaptic contact zone. (D_–_F) Frequency histograms for each terminal type showing their cross-sectional areas. The numbers of terminals in each sample are indicated, and the terminals represented in A_–_C are a subset of those in D_–_F.

Figure 3

Figure 3

Determination of the mixture of labeled brainstem and corticogeniculate terminals needed to reconstruct the size distribution of parent RD terminals. (A) Means (squares) and SD (circles) of the cross-sectional areas of the terminal populations constructed from various mixtures of corticogeniculate and brainstem terminals (see text for details). Each point represents the average of three independent sampling algorithms whereby corticogeniculate and brainstem terminals were randomly selected for the mixtures. The percentages of corticogeniculate terminals indicated on the abscissa have been corrected for the sampling bias favoring them over brainstem terminals, and thus these values represent X′ as described in the text. The horizontal lines represent the mean (solid line) and SD (dashed line) of the target population of parent RD terminals. Completely filled squares and circles indicate that the mixtures of corticogeniculate and brainstem terminals are significantly different from the parent RD population in terms of cross-sectional area, open symbols indicate that each of the three mixtures is statistically indistinguishable from the parent RD population, and half-filled symbols indicate that some of the mixtures are different from the parent RD population whereas others are not. (B) Terminal size distribution of parent RD population (redrawn from Fig. 2_D_). (C) Size distribution of combination of all six mixtures of 49% and 53% corticogeniculate terminals, mixtures that were statistically indistinguishable from the parent RD population.

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