Department of Psychology, The University of Connecticut, Storrs, CT 06269, USA
Address correspondence to H.A. Swadlow, Department of Psychology (U-20), The University of Connecticut, Storrs, CT 06269, USA. Email: swadlow{at}psych.psy.uconn.edu.
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Abstract |
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Introduction |
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Although GABAergic interneurons are relatively abundant within the cortex [they comprise 1525% of the neurons in many cortical areas (Hendry et al., 1987; Meinecke and Peters, 1987
; Prieto et al., 1994
)], these neurons have received little attention in studies of receptive field and other response properties of cortical neurons in intact subjects. This may, in part, be due to the difficulty in identifying these neurons in the extracellular record. Whereas cortical efferent neurons may be unambiguously identified by antidromic activation, the identification of interneurons is more problematic. This chapter will review evidence indicating that one class of inhibitory interneurons can be identified in the extracellular record. The receptive fields and other response properties of these neurons will be described, and the relationship of these receptive fields to a highly divergent/convergent functional network that links these neurons to their thalamocortical afferents will be explored.
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The Identification of GABAergic Interneurons in the Extracellular Record |
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It has been recognized for some time that some interneurons have distinctive response properties. A high-frequency burst of spikes elicited by electrical stimulation of convergent inputs was described in Renshaw cells of the spinal cord (Eccles, 1957) and in many other central interneurons (Andersen et al., 1964a
,b
). In sensory neocortex, Mountcastle et al. (Mountcastle et al., 1969
) speculated that cells in somatosensory cortex with thin spikes may be interneurons, but added the caveat that such cells could actually be thalamic afferents. Similarly, Simons (Simons, 1978
) identified such fast-spike neurons in rat somato-sensory barrel cortex, and showed that they have receptive field properties that differ from those of regular spike neurons. The suggestion that these fast-spike cortical elements may be interneurons gained considerable support from intracellular studies showing that a class of cortical neurons with short-duration action potentials had an aspinous or sparsely spinous non-pyramidal morphology (McCormick et al., 1985
; Connors and Kriegstein, 1986
). In addition, these cells were shown to respond to a depolarizing intracellular current pulse with a very high-frequency, non-adapting discharge of action potentials and, subsequently, to stain positively for GAD. It was soon shown, however, that fast-spike interneurons comprise only a subset of the population of cortical inhibitory interneurons. Other identified GABAergic interneurons were shown to lack the physiological signature of the fast-spike population. Moreover, this latter class of inhibitory interneurons did not stain for parvalbumin, a calcium-binding protein that is associated with fast-spike interneurons (Kawaguchi, 1993
; Kawaguchi and Kubota, 1993
).
These latter results showed that not all cortical GABAergic interneurons have action potentials of very short duration. Conversely, it has also become clear that not all short-duration action potentials are generated by fast-spike, parvalbumin-expressing interneurons. Intracellular studies have documented short-duration action potentials in a number of neurons that were subsequently labeled and shown to be of pyramidal morphology (Dykes et al., 1988; Gray and McCormick, 1996
) [also see (Takahashi, 1965)], and the spikes of a small but significant number of cortical efferent neurons, recorded extracellularly and identified by antidromic activation, are of very short duration (Swadlow, 1988
,1989
,1990
). We can conclude, then, that although a majority of cortical neurons with short-duration spikes are fast-spike GABAergic interneurons, other cortical populations may also have short-duration action potentials.
To reduce this ambiguity, criteria other than spike-duration can be added to the procedure for identifying fast-spike inter-neurons in the extracellular record. As noted above, these cells can emit very high-frequency bursts of action potentials, and this characteristic has been useful in identifying putative inhibitory interneurons (Swadlow, 1988,1989
,1990
,1991
,1994
,1995
). Thus, suspected inhibitory interneurons (SINs) in visual and somatosensory cortices were identified by a burst of three or more spikes elicited by electrical stimulation of afferent pathways, where peak frequencies were required to exceed 600 Hz. As expected, SINs identified by the above criteria also had action potentials that were much briefer (approximately half the duration) than those of efferent populations. In addition, SINs in these cortical regions respond vigorously at short latencies to electrical stimulation of multiple cortical sites, which alleviates concerns that they may be thalamic afferents (above). Figure 1A1,A2
shows the extracellular spikes elicited by ventrobasal (VB) thalamic stimulation in one such neuron in rabbit barrel cortex. Intracellular recordings were obtained from a small number of such neurons (five) in S1 barrel cortex that met the above extracellular criteria for classification as a SIN. Each of these cells, recorded in fully awake rabbits, responded to a depolarizing current pulse with the high-frequency, nonadapting discharge of action potentials that is characteristic of fast-spike, GABAergic interneurons. Figure 1B
shows results from one of these neurons.
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Response Properties of Fast-spike Interneurons in Sensory Cortex |
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SINs have very low thresholds to sensory stimulation. Figure 2A shows that SINs of S1 barrel cortex had much lower angular thresholds for detecting a whisker displacement than did any of the efferent populations of this region (Swadlow, 1989
). More than 70% of SINs responded to a deflection of 0.8° in their principle vibrissa. In contrast, most efferent neurons of all classes required a deflection of >2°. Similarly, SINs in the vibrissae and forepaw representations of S1, as well as in S2 and in motor cortex (Swadlow, 1989
,1990
,1991
,1994
) responded much more faithfully to high-frequency peripheral stimulation (Fig. 2B
) than did the efferent populations. Another remarkable difference between SINs and the efferent populations of S1 barrel cortex was in their responses to different directions of whisker deflections. Most efferent neurons of all classes responded selectively to a narrow range of displacement angles (Fig. 2C
). This is, perhaps, not surprising since most VB thalamocortical neurons also display strong directional selectivity (Simons and Carvell, 1989
). However, the great majority of SINs, found in the same microelectrode penetrations as the efferent neurons, showed little or no selectivity for the direction of whisker displacement. This is surprising, because most SINs receive strong thalamocortical input, much of which originates in thalamocortical neurons that show strong directional selectivity. I will argue (below) that this results from a non-selective convergence of thalamocortical input onto SINs of layer 4. Similar results are seen in rabbit visual cortex, where most corticocortical and corticofugal (corticotectal and corticothalamic) efferent neurons show both orientation and directional selectivity, but SINs are very widely tuned for both orientation and direction of motion (Swadlow, 1988
).
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It is not yet clear whether the above results in rabbit and rat, showing a high sensitivity, but low degree of specificity in the responses of SINs (or fast-spike neurons), hold for these elements in sensory cortex of other species. A small number of GABAergic interneurons have been identified in in vivo intracellular studies of cat visual cortex. Both Martin et al. (Martin et al., 1983) and Azouz et al. (Azouz et al., 1997
) report that these neurons are orientation selective, and these results are consistent with results showing orientation selectivity in the IPSPs onto pyramidal neurons of cat V1 (Ferster, 1986
). However, recent intracellular studies (Hirsch et al., 2000
) have revealed inhibitory interneurons that lack orientation selectivity in layer 4 of cat V1. It would be useful to know whether these latter cells receive significant monosynaptic input from LGNd and could, thereby, mediate a broadly tuned feedforward inhibition onto spiny stellate neurons (Troyer et al., 1998
; Ferster and Miller, 2000
). Surprisingly, putative fast-spike inhibitory interneurons have not been identified with any regularity in extracellular analyses of feline visual cortex. This is curious, given the great number of studies and intense interest in this cortical region. It may be that these neurons are more difficult to isolate in cat or simply that they have not been sought with sufficient effort.
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Thalamocortical Connectivity of Fast-spike Interneurons |
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Convergence and Divergence of Thalamocortical Input to S1 SINs |
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Recently, we provided a more direct demonstration of this high degree of functional convergent and divergent connectivity by obtaining simultaneous recordings of multiple VB neurons and multiple, topographically aligned S1 SINs (Swadlow and Gusev, 2002). This strategy allowed us to compare the impact of several different VB neurons on the same SIN and, conversely, to examine the influence of a single thalamocortical neuron on multiple post-synaptic targets. Our results confirmed the highly convergent/divergent functional thalamocortical input to S1 SINs. SINs that lacked any directionality were shown to receive a strong convergent functional input from multiple VB neurons that each showed a strong directional selectivity, but in widely varying directions. Figure 5
shows one such case, in which three VB neurons with widely varying directional selectivities (range = 135°) were shown to provide input to an S1 SIN that lacked any directional preference.
Remarkable divergence from single VB neurons to multiple SINs of a barrel was also demonstrated (Swadlow and Gusev, 2002). One VB neuron (dubbed Hercules because of potent functional contacts with many SINs) was shown to make a functional contact with each of nine SINs that were studied over a 4 day period in the aligned S1 barrel. The VBSIN pairs shown in Figure 4
show two of the contacts made by Hercules on one of these days.
Such a high degree of divergence/convergence is suggestive of the complete transmission line between successive nodes of a network that was described by Griffith (Griffith, 1963) [also (Abeles, 1991
)]. These networks are characterized by very reliable transmission between the input and output nodes, but the cost of this reliability is a sacrifice in complexity of task (Griffith, 1963
). This description is very consistent with the response properties of S1 SINs. As described above, these neurons are exquisitely sensitive to very low amplitude peripheral stimulation and they respond faithfully to high stimulus frequencies. Enhanced sensitivity is an expected consequence of a high degree of convergence because spikes in a large number of input neurons each have an opportunity to elicit a response in the target neuron. SINs have sacrificed complexity of task, however, in that they lack the directional selectivity that is seen in most of their thalamocortical inputs. In this view, SINs obtain their multidirectional receptive fields by pooling the input from a large number of thalamocortical neurons with widely differing directional preferences.
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Sharp Synchrony Among Fast-spike Interneurons of S1 |
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These predictions were tested by recording from pairs of SINs within an S1 barrel (Swadlow et al., 1998). The cross-correlogram presented in Figure 6
shows results generated by two SINs of the same barrel, each of which gave strong evidence of receiving monosynaptic thalamic input (latencies of <1.7 ms to electrical stimulation of VB and <7.5 ms to air-puff stimulation, above). Here, and in nearly all pairs of such thalamocortically driven SINs that were found within the same barrel, a sharp increase in spike frequency occurred in each SIN nearly simultaneously (±1 ms) with a spike in the other SIN. This effect did not depend upon peripheral stimulation, as it occurred whether action potentials were spontaneous or stimulus driven. Moreover, it was seen at horizontal inter-electrode distances of up to 350 µm, as long as the two SINs were found within the same barrel. For pairs of SINs recorded within a single barrel,
4% of the spikes of each SIN were sharply synchronous with the spikes of the other. As expected, sharp synchrony between SINs of neighboring barrels was minimal or absent, even when inter-electrode distances were <300 mm. We found that sharply synchronous activity in layer-4 SINs was not oscillatory. Autocorrelograms generated by the action potentials of individual SINs and by the synchronous events occurring between two SINs showed no signs of side-bands that are indicative of oscillatory activity. Moreover, sharp synchrony between SINs was present in both fully awake and anesthetized states. Sharp synchrony was not seen between SINs and other populations of the same barrel that showed no evidence of monosynaptic thalamic input. Recent preliminary evidence based on recordings of triads of SINs within a barrel (unpublished) indicate that sharply synchronous activity between two SINs does not reflect a communal synchrony among all SINs of the barrel. Instead, synchronous events among the SINs of a barrel are roughly independent.
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Functional Thalamocortical Connectivity, Sharp Synchrony and Electrical Coupling Among Fast-spike, GABAergic Interneurons |
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It is important to note that we do not yet know the extent to which fast-spike interneurons are electrically coupled in intact adults, and how general this phenomenon is among various mammalian lines. Given outstanding questions concerning the orientation tuning of GABAergic interneurons in layer 4 of cat (Azouz et al., 1997; Hirsch et al., 2000
) and the putative role of such neurons in controlling the receptive fields of excitatory neurons (Troyer et al., 1998
), it would be very useful to know whether these elements are electrically coupled in adult cats, the spatial extent of any such coupling, and the relationship of this coupling to orientation tuning and other receptive field properties of these neurons. If fast-spike GABAergic interneurons of feline visual cortex are electrically coupled over distances similar to those seen in rodents [±
200 µm (Amitai et al., 2001
)], one would expect this to broaden any orientation preference in these neurons that was directly generated by feedforward (thalamocortical) connectivity. This effect could be especially prominent near pinwheel centers, where the distance between orientation columns is reduced (Maldonado et al., 1997
). Clearly, further experimental work and computational studies are required to unravel the relative contributions of diverging/converging thalamocortical synaptic input and electrical dendritic coupling to (i) the observed sharp synchrony among these interneurons, (ii) their highly divergent/convergent functional thalamocortical connectivity and (iii) their receptive field properties.
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Fast-spike Interneurons of Layer 4: a Substrate for Fast, Potent Feedforward Inhibition |
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Footnotes |
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