Activation of cholecystokinin neurons in the dorsal pallium of the telencephalon is indispensable for the acquisition of chick imprinting behavior

Abstract

Chick imprinting behavior is a good model for the study of learning and memory. Imprinting object is recognized and processed in the visual wulst, and the memory is stored in the intermediate medial mesopallium in the dorsal pallium of the telencephalon. We identified chicken cholecystokinin (CCK)-expressing cells localized in these area. The number of CCK mRNA-positive cells increased in chicks underwent imprinting training, and these cells expressed nuclear Fos immunoreactivity at high frequency in these regions. Most of these CCKpositive cells were glutamatergic and negative for parvalbumin immunoreactivity. Semi-quantitative PCR analysis revealed that the CCK mRNA levels were significantly increased in the trained chicks compared with untrained chicks. In contrast, the increase in CCK-and c-Fos-double-positive cells associated with the training was not observed after closure of the critical period. These results indicate that CCK cells in the dorsal pallium are activated acutely by visual training that can elicit imprinting. In addition, the CCK receptor antagonist significantly suppressed the acquisition of memory. These results suggest that the activation of CCK cells in the visual wulst as well as in the intermediate medial mesopallium by visual stimuli is indispensable for the acquisition of visual imprinting.

Demonstration of a Neural Circuit Critical for Imprinting Behavior in Chicks

“…Visual imprinting was performed with a device described previously (Maekawa et al, 2006(Maekawa et al, , 2007. Briefly, a running wheel connected to a custom-made computer system was used to record the chick's movements toward or away from the display (Muromachi Kikai).…”

“…We calculated the preference score (PS) (McCabe et al, 1982) using the following formula as an index of success of visual imprinting: PS ϭ SUM (training image)/{SUM (training image) ϩ SUM (new image)}, where SUM (training image) is the number of wheel revolutions toward the display during the presentation of the training stimulus, and SUM (new image) is the number of wheel revolutions toward the display during the presentation of the new image. The chicks that rotated the wheel Ͻ22.5 revolutions during the 15 min evaluation period were excluded from the analysis (Maekawa et al, 2007).…”

“…In situ hybridization was performed with freefloating sections according to procedures described previously (Maekawa et al, 2007). The zebra finch NR1, NR2A, and NR2B probes were kindly provided by Dr. Wada (Duke University, Durham, NC) .…”

“…The zebra finch NR1, NR2A, and NR2B probes were kindly provided by Dr. Wada (Duke University, Durham, NC) . Vesicular glutamate transporter 2 (vGlut2) and GAD65 probes were described previously (Maekawa et al, 2007). Control experiments were performed, and no specific signals were observed when the sections were processed with the digoxigenin-labeled sense RNA probes.…”

“…Previous studies have demonstrated that the visual Wulst (VW), bearing homologies to the mammalian visual cortex, and the intermediate medial mesopallium (IMM), which is partially similar to the mammalian associate cortex (Reiner et al, 2004), play crucial roles in the process of imprinting (Horn et al, 1979;Kohsaka et al, 1979;McCabe et al, 1981;Maekawa et al, 2006). We have shown recently that visual imprinting is accompanied by plastic changes in the VW that result in an increase of the region responding to the imprint stimulus as well as an increase in the number of cholecystokinin-positive neurons (Maekawa et al, 2006(Maekawa et al, , 2007. The processed visual information necessary for eliciting imprinting behavior should be transmitted to the IMM, which serves as the memory-storage region.…”