Cholinergic deafferentation of the neocortex using 192 IgG-saporin impairs feature binding in rats

Abstract

The binding problem refers to the fundamental challenge of the CNS to integrate sensory information registered by distinct brain regions to form a unified neural representation of a stimulus. Although the human cognitive literature has established that attentional processes in frontoparietal cortices support feature binding, the neurochemical and specific downstream neuroanatomical contributions to feature binding remain unknown. Using systemic pharmacology in rats, it has been shown that the neuromodulator acetylcholine is essential for feature binding at encoding, but the neural source of such critical cholinergic neurotransmission has yet to be identified. Cholinergic efferents from the nucleus basalis magnocellularis (NBM) of the basal forebrain provide the majority of the cholinergic input to the neocortex. Accordingly, it was hypothesized that the NBM is the neural source that provides the critical neuromodulatory support for feature binding. To test this hypothesis, rats received bilateral 192 IgG-saporin lesions of the NBM, and their feature binding performance was tested using a forced-choice digging paradigm. Relative to sham-lesioned rats, NBM-lesioned rats were significantly impaired at acquiring a crossmodal feature conjunction (FC) stimulus set that required feature binding, whereas their ability to retrieve an FC stimulus set and to acquire two crossmodal feature singleton stimulus sets, one of greater difficulty than the other but neither requiring feature binding, remained intact. These behavioral findings, along with histological analyses demonstrating positive relationships between feature-binding acquisition and markers of cholinergic activity in frontoparietal regions, reveal the importance of neocortical cholinergic input from the NBM to feature binding at encoding.

Figure 1.

A, Illustration of the two different trial types, target and distractor, of the forced-choice digging tasks. On target trials, the rewarded (+) stimulus was the odor–texture bowl, whereas on distractor trials, the rewarded stimulus was the blank bowl. B, Illustration of a typical session. On every trial, rats were simultaneously presented with two digging bowls: an odor–texture bowl and the blank bowl. Half of the trials were target (T) trials, and the remaining half were distractor (D) trials presented in a pseudorandom order. Rats had to use the crossmodal features of the presented odor–texture bowl to determine the correct bowl choice.

Figure 2.

A–C, Illustration of the features defining the FC (A), FS (B), and FS enhanced-difficulty (C) stimuli. Solid lines indicate pairings of crossmodal features in target bowls, and dashed lines indicate pairings of crossmodal features in distractor bowls. For the FC stimuli, feature binding was required for correct bowl selection, as each individual odor and texture were associated with both a target and a distractor bowl. For both types of FS stimuli, feature binding was not required, as rats could rely on a single feature (odor or texture) for correct bowl selection. However, for the FS enhanced-difficulty stimuli, rats had the additional requirement of learning when to rely on odor and when to rely on texture to determine the correct bowl choice as one odor (odor 7) and one texture (texture 8) were associated with both a target and a distractor bowl.

Figure 3.

Choline acetyltransferase and parvalbumin immunohistochemistry of the NBM. A–D, Panels (10× magnification) depict ChAT-immunoreactive cells (A, B) and parvalbumin-immunoreactive cells (C, D) in the NBM of a typical sham-lesioned (A, C) and NBM-lesioned (B, D) rat. A loss of ChAT-immunoreactive cells (B), but not parvalbumin-immunoreactive cells (D), is apparent in the NBM-lesioned rat. The rectangular outlines superimposed on the rat brain coronal schematics delineate the NBM cell-counting frames, and the solid gray fill illustrates the typical extent of ChAT-immunoreactive cell loss. Rat brain schematics were adapted from Paxinos and Watson (2007) and displayed coordinates refer to the anterior–posterior plane.

Figure 4.

Choline acetyltransferase and parvalbumin immunohistochemistry of the MS/VDB. A–D, Panels (10× magnification) depict ChAT-immunoreactive cells (A, B) and parvalbumin-immunoreactive cells (C, D) in the MS/VDB of a typical sham-lesioned (A, C) and NBM-lesioned (B, D) rat. No loss of ChAT-immunoreactive cells (B) or parvalbumin-immunoreactive cells (D) is apparent in the NBM-lesioned rat. The rectangular outlines superimposed on the rat brain coronal schematics delineate the MS/VDB cell-counting frames. Rat brain schematics were adapted from Paxinos and Watson (2007) and displayed coordinates refer to the anterior/posterior plane.

Figure 5.

Acetylcholinesterase histochemistry. A–F, Panels (1.25× magnification) depict AChE staining in the frontal cortex (A, B), parietal cortex (C, D), and hippocampus (E, F) of a typical sham-lesioned (A, C, E) and NBM-lesioned (B, D, F) rat. Significant depletion of AChE-positive fibers in the frontal (B) and parietal (D) cortices, but not the hippocampus (F), is apparent in the NBM-lesioned rat. The rectangular outlines superimposed on the rat brain coronal schematics delineate the boundaries used for obtaining optical density values for AChE densitometry. Rat brain schematics were adapted from Paxinos and Watson (2007) and displayed coordinates refer to the anterior/posterior plane.

Figure 6.

FC performance during the last six sessions before surgery and the subsequent retrieval of these same FC stimuli during the six postsurgery sessions.

Figure 7.

A–C, Postsurgical acquisition of novel FC (A), FS (B), and FS enhanced-difficulty (C) stimuli. Accuracy has been binned into four-session blocks. The boxes highlight the final level of performance attained by rats on the three tasks.

Figure 8.

Scatterplots illustrating the relationships between FC acquisition and NBM ChAT immunoreactivity and neocortical AChE reactivity. A, B, Linear regression of the data revealed significant positive relationships between postsurgical performance during the final block of FC acquisition and the number of ChAT-positive cells in the NBM (A) and AChE optical density in the frontal cortex (B). C, A weak trend toward significance was found for the positive relationship between FC acquisition performance and AChE optical density in the parietal cortex. *p < 0.05; ⁺p = 0.10.