Thalamocortical Synapses
Authors: Anthony Zador
Journal: Neuron (1999)

Nearly all of the sensory information that enters the cortex passes through the thalamus, and the most important thalamocortical (TC) projection is onto spiny neurons in layer 4. These TC synapses thus represent the main conduit through which information from the periphery flows into the cortex for further processing. One might imagine that this conduit would be correspondingly wide, but in fact it is remarkably narrow, comprising only about a tenth of all synapses onto a typical neuron in layer 4. The vigorous and rapid responses of layer 4 neurons to sensory stimulation indicate that TC inputs nevertheless exert a powerful influence, leading to speculation that TC inputs are proportionately more powerful than intracortical (IC) inputs. This prediction was confirmed by Stratford and colleagues (1996), who reported that the TC connection was on average more than twice as strong as its IC counterpart. Now, a study by 2Gil Z. Connors B.W. Amitai Y. Neuron. 1999; 23 (this issue,): 385-397Abstract Full Text Full Text PDF PubMed Scopus (277) Google Scholar( [this issue of Neuron]) takes these observations one step further by probing the mechanism underlying this extra strength. Our basic framework for understanding synapses comes from the classic work of Katz and colleagues (Katz, 1968) on the neuromuscular junction. Katz proposed that when an action potential invades the synaptic terminal, it triggers the release of neurotransmitter from membrane-bound vesicles into the synaptic cleft. Katz developed a simple mathematical model in which he described the size of the postsynaptic response in terms of three quantities: the number (n) of sites from which a vesicle might be released, the probability (p) that a vesicle is released from each site, and the size (q) of the response due to each vesicle. The mean response size, or efficacy (E), is then just the product of these three quantities: E = q·n·p. How do q, n, and p conspire to yield a mean efficacy at TC connections that is more than double that at IC connections? Gil and colleagues conclude that both the number n of release sites and the probability p of release are higher at the TC connection, but that the mean quantal size q is the same. Key to these experiments was the thalamocortical brain slice preparation, in which thalamic and cortical afferents to the same layer 4 neuron can be activated independently. In these special slices, Gil and colleagues first compared the quantal size q in the two pathways by substituting extracellular Sr2+ for Ca2+ to enhance the asynchronous component of release (3Goda Y. Stevens C.F. Proc. Natl. Acad. Sci. USA. 1994; 91: 12942-12946Crossref PubMed Scopus (434) Google Scholar). Following a stimulus, this asynchronous component can be seen as a gentle rain of vesicles released by synapses in the stimulated pathway. Because the asynchronous component of release persists at a low rate for a few hundred milliseconds, the currents caused by individual vesicles can be resolved and the size of the individual quanta estimated. The quantal estimates obtained in this way were the same at the TC and IC pathways and were in close agreement with estimates obtained using another approach, in which release probability at individual boutons was reduced to such a low level that, on average, no more than a single vesicle was released. If the quantal size q is the same, then the increased efficacy of the TC pathway must be due to differences in either the release probability p, the number of release sites n, or both. Even within a single population of synapses, there is a wide distribution of release probabilities. For example, at the well-studied Schaeffer collateral input from hippocampal region CA3 to region CA1, release probability ranges from <0.01 to nearly 1. An estimate of this distribution for the TC and IC pathways was obtained using the NMDA open channel blocker MK-801, which revealed that the TC pathway contained more high-probability synapses than the IC pathway. Only part of the enhanced efficacy of the TC pathway is due to the increased release probability. To assess the number n of release sites in the two pathways, Gil and colleagues compared the size of the single-axon evoked response using minimal stimulation. In this technique, the extracellular stimulating current is reduced until only a single fiber is activated. If a connection consists of only a single release site (n = 1), as it often does at the hippocampal connection from region CA3 to region CA1, then the quantal size is equal to the single-axon evoked response. If instead the connection consists of more than one release site, as for example at the neuromuscular junction (where n is on the order of a thousand), then the single-axon response may be much greater. This experiment suggested that the average number of release sites at IC connections (n = 2) is much smaller than at TC connections (n = 7). These results provide strong evidence for important differences between the TC and IC inputs. However, due to the nature of the preparation, not all desired tests to confirm this can be performed. At the hippocampal CA3–CA1 pathway, where minimal stimulation has become a standard technique, the most rigorous studies apply three independent tests to determine whether stimulation is minimal (4Stevens C.F. Wang Y. Nature. 1994; 371: 704-707Crossref PubMed Scopus (318) Google Scholar, 1Dobrunz L.E. Stevens C.F. Neuron. 1997; 18: 995-1008Abstract Full Text Full Text PDF PubMed Scopus (924) Google Scholar). First, the relationship between response size and stimulus intensity must show a plateau—a region where an increase in stimulus intensity causes no increase in response size, presumably because one fiber is so much closer to the stimulating electrode that there is a broad safety factor before other fibers are recruited. Second, the response must have a fixed shape and latency. Both of these tests were applied in the study by Gil and colleagues. However, at the CA3–CA1 pathway, a third test can be performed that acts as post facto confirmation that in fact only a single synapse contributed to the observed response. Since CA3 Schaeffer collateral axons typically make only a single synapse onto their CA1 target, the average response size when release failures are excluded (the potency) should be independent of release probability. If this condition is met—if, for example, the potency is the same on both pulses during paired-pulse facilitation—then one can be more confident that only a single axon is being stimulated. At cortical synapses, however, this third internal check cannot be performed, since high synapse multiplicity is indistinguishible from supraminimal stimulation. There remains therefore the formal possibility that the apparently high multiplicity really reflects something else—for example, a propensity for cortical axons to travel in bundles and fire in groups. While there is no evidence to support this alternative interpretation, it remains a logical possibility until other techniques can be brought to bear. The present findings provide a cellular mechanism that helps reconcile the apparently dominant functional role of the TC input in vivo with its relative sparseness. The next step will be to provide a physical interpretation for this increased number of release sites. There are at least three possibilities. First, they could represent distinct boutons made by a single axon, as has been described between pairs of layer 5 neurons (5Markram H. Lubke J. Frotscher M. Roth A. Sakmann B. J. Physiol. 1997; 500: 409-440Crossref PubMed Scopus (732) Google Scholar). Second, they could represent multiple active zones at a single bouton. Finally, they could represent multivesicular release at a single active zone. Further work will distinguish these possibilities.