Acetylcholine is a ubiquitous cortical neuromodulator implicated in cognition. In order to understand the potential for acetylcholine to play a role in visual attention, we studied nicotinic acetylcholine receptor (nAChR) localization and function in area V1 of the macaque. We found nAChRs presynaptically at thalamic synapses onto excitatory, but not inhibitory, neurons in the primary thalamorecipient layer 4c. Furthermore, consistent with the release enhancement suggested by this localization, we discovered that nicotine increases responsiveness and lowers contrast threshold in layer 4c neurons. We also found that nAChRs are expressed by GABAergic interneurons in V1 but rarely by pyramidal neurons, and that nicotine suppresses visual responses outside layer 4c. All sensory systems incorporate gain control mechanisms, or processes which dynamically alter input/output relationships. We demonstrate that at the site of thalamic input to visual cortex, the effect of this nAChR-mediated gain is an enhancement of the detection of visual stimuli.

Cholinergic neuromodulation, a candidate mechanism for aspects of attention, is complex and is not well understood. Because structure constrains function, quantitative anatomy is an invaluable tool for reducing such a challenging problem. Our goal was to determine the extent to which m1 and m2 muscarinic acetylcholine receptors (mAChRs) are expressed by inhibitory vs. excitatory neurons in the early visual cortex. To this end, V1 and V2 of macaque monkeys were immunofluorescently labelled for gamma-aminobutyric acid (GABA) and either m1 or m2 mAChRs. Among the GABA-immunoreactive (ir) neurons, 61% in V1 and 63% in V2 were m1 AChR-ir, whereas 28% in V1 and 43% in V2 were m2 AChR-ir. In V1, both mAChRs were expressed by fewer than 10% of excitatory neurons. However, in V2, the population of mAChR-ir excitatory neurons was at least double that observed in V1. We also examined m1 and m2 AChR immunoreactivity in layers 2 and 3 of area V1 under the electron microscope and found evidence that GABAergic neurons localize mAChRs to the soma, whereas glutamatergic neurons expressed mAChRs more strongly in dendrites. Axon and terminal labelling was generally weak. These data represent the first quantitative anatomical study of m1 and m2 AChR expression in the cortex of any species. In addition, the increased expression in excitatory neurons across the V1/V2 border may provide a neural basis for the observation that attentional effects gain strength up through the visual pathway from area V1 through V2 to V4 and beyond.

Orientation and spatial frequency tuning are highly salient properties of neurons in primary visual cortex (V1). The combined organization of these particular tuning properties in the cortical space will strongly shape the V1 population response to different visual inputs, yet it is poorly understood. In this study, we used two-photon imaging in macaque monkey V1 to provide the first data demonstrating the 3D cell-by-cell layout of both spatial frequency and orientation tuning in large mammals. We first show that spatial frequency tuning is organized into highly structured maps that remain consistent across the depth of layer II/III, similar to orientation. Next, we show that orientation and spatial frequency maps are intimately related at the fine spatial scale observed with two-photon imaging. We find that not only do the map gradients have a striking tendency toward orthogonality, but they also co-vary negatively from cell-to-cell at the spatial scale of cortical columns.

Acetylcholine (ACh) is believed to underlie mechanisms of arousal and attention in mammals. ACh also has a demonstrated functional effect in visual cortex that is both diverse and profound. We have reported previously that cholinergic modulation in V1 of the macaque monkey is strongly targeted toward GABAergic interneurons. Here we examine the localization of m1 and m2 muscarinic receptor subtypes across subpopulations of GABAergic interneurons-identified by their expression of the calcium-binding proteins parvalbumin, calbindin, and calretinin-using dualimmunofluorescence confocal microscopy in V1 of the macaque monkey. In doing so, we find that the vast majority (87%) of parvalbumin-immunoreactive neurons express m1-type muscarinic ACh receptors. m1 receptors are also expressed by 60% of calbindin-immunoreactive neurons and 40% of calretinin-immunoreactive neurons. m2 AChRs, on the other hand, are expressed by only 31% of parvalbumin neurons, 23% of calbindin neurons, and 25% of calretinin neurons. Parvalbuminimmunoreactive cells comprise ≈75% of the inhibitory neuronal population in V1 and included in this large subpopulation are neurons known to veto and regulate the synchrony of principal cell spiking. Through the expression of m1 ACh receptors on nearly all of these PV cells, the cholinergic system avails itself of powerful control of information flow through and processing within the network of principal cells in the cortical circuit. Keywords cholinergic; neuromodulation; GABAergic; striate cortex; immunofluorescence; dual-labeling; calcium-binding proteins; calbindin; calretinin; parvalbumin Acetylcholine (ACh) is a ubiquitous neuromodulator in the mammalian central nervous system and is implicated in many brain processes, including the sleep/wake cycle and arousal (Jasper and Tessier, 1971;Jimenez-Capdeville and Dykes, 1996;Vazquez and Baghdoyan, 2001), reward and addiction (Maskos et al., 2005), attention (Sarter et al., 2005), learning and memory (Everitt and Robbins, 1997;Rezvani and Levin, 2001;Hasselmo and McGaughy, 2004), as well as a number of neuropathologies including Alzheimer's disease (Gallagher and Colombo, 1995). We have previously reported that muscarinic acetylcholine receptors (mAChRs) are expressed by a larger proportion of GABAergic interneurons than non-GABAergic (putatively excitatory) neurons in the primary visual cortex (V1) of the macaque monkey (Disney et al., 2006). However, cortical inhibitory neurons are not a homogeneous population. GABAexpressing neurons display considerable functional diversity, which is reflected in their varying dendritic and axonal morphology and in the cells and cellular compartments their axons target. While anatomical classification of GABAergic neurons has traditionally been based on these NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript morphological characteristics (Lund, 1987;Van Brederode et al., 1990;Lund and Yoshioka, 1991;Meskenaite, 1997;Lund and Wu, 1997; De-Felipe et al., 1999), an alternative classificat...

Acetylcholine (ACh) has been implicated in selective attention. To understand the local circuit action of ACh, we iontophoresed cholinergic agonists into the primate primary visual cortex (V1) while presenting optimal visual stimuli. Consistent with our previous anatomical studies showing that GABAergic neurons in V1 express ACh receptors to a greater extent than do excitatory neurons, we observed suppressed visual responses in 36% of recorded neurons outside V1's primary thalamorecipient layer (4c). This suppression is blocked by the GABA(A) receptor antagonist gabazine. Within layer 4c, ACh release produces a response gain enhancement (Disney AA, Aoki C, Hawken MJ. Neuron 56: 701-713, 2007); elsewhere, ACh suppresses response gain by strengthening inhibition. Our finding contrasts with the observation that the dominant mechanism of suppression in the neocortex of rats is reduced glutamate release. We propose that in primates, distinct cholinergic receptor subtypes are recruited on specific cell types and in specific lamina to yield opposing modulatory effects that together increase neurons' responsiveness to optimal stimuli without changing tuning width.