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. 2010 Feb 13;17(2):80-5.
doi: 10.1101/lm.1572710. Print 2010 Feb.

Ventral lateral geniculate input to the medial pons is necessary for visual eyeblink conditioning in rats

Hunter E Halverson  1 , John H Freeman
Affiliations

Ventral lateral geniculate input to the medial pons is necessary for visual eyeblink conditioning in rats

Hunter E Halverson et al. Learn Mem. .

Abstract

The conditioned stimulus (CS) pathway that is necessary for visual delay eyeblink conditioning was investigated in the current study. Rats were initially given eyeblink conditioning with stimulation of the ventral nucleus of the lateral geniculate (LGNv) as the CS followed by conditioning with light and tone CSs in separate training phases. Muscimol was infused into the medial pontine nuclei (MPN) after each training phase to examine conditioned response (CR) retention to each CS. The spread of muscimol infusions targeting the MPN was examined with fluorescent muscimol. Muscimol infusions into the MPN resulted in a severe impairment in retention of CRs with the LGNv stimulation and light CSs. A less severe impairment was observed with the tone CS. The results suggest that CS information from the LGNv and light CSs is relayed to the cerebellum through the MPN. Retrograde tracing with fluoro-gold (FG) showed that the LGNv and nucleus of the optic tract have ipsilateral projections to the MPN. Unilateral inputs to the MPN from the LGNv and nucleus of the optic tract may be part of the visual CS pathway that is necessary for visual eyeblink conditioning.

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Figures

Figure 1.
Figure 1.
(A) Coronal section of the visual thalamus showing a representative electrode placement in the ventral lateral geniculate (LGNv). (LGNd) Dorsal lateral geniculate; (IGL) intergeniculate leaf; (et) electrode tip. Magnification, 2.5×. (B) Coronal sections of the basilar pontine nuclei with the locations of cannula tips indicated by black dots. (C) Coronal section of the pontine nuclei showing a fluorescent muscimol infusion targeting the medial pontine nuclei (MPN). The cannula tip was in the right MPN. (LPN) Lateral pontine nuclei. Magnification, 4×. (D) Coronal section of the brain showing the largest (light gray) and smallest (dark gray) spread of muscimol.
Figure 2.
Figure 2.
Mean ± SE conditioned response (CR) percentage for rats given training with a ventral lateral geniculate stimulation (LGNv) CS (sessions 1–5), a light CS (sessions 6–10), and a tone CS (sessions 11–15) in different phases of training. Rats received muscimol infusions into the MPN (black arrows) on sessions 4, 9, and 14. Rats received saline infusions into the MPN (gray arrows) on sessions 5, 10, and 15. Muscimol blocked CRs with both LGNv stimulation (session 4) and light CSs (session 9) and had a partial effect on retention to the tone CS (session 14).
Figure 3.
Figure 3.
Mean ± SE conditioned response (CR) percentage in blocks of 10 trials during the first session of phase 1(session 1), phase 2 (session 6), and phase 3 (session 11) of training for rats given one of two training sequences: (1) LGNv stimulation→light→tone (SLT) or (2) LGNv stimulation→tone→light (STL).
Figure 4.
Figure 4.
(A) Coronal section of the pontine nuclei showing one of the smaller fluoro-gold infusion sites in the MPN. (B) Coronal section of the pontine nuclei showing one of the larger FG infusions in the MPN. (LPN) Lateral pontine nucleus; (MPN) medial pontine nucleus; (RtTg) reticulotegmental nucleus; (VPN) ventral pontine nucleus. Magnification, 4×.
Figure 5.
Figure 5.
(A,B) Coronal sections of the visual thalamus showing two examples of FG-labeled neurons in the ventral lateral geniculate (LGNv). Magnification, 4×.
Figure 6.
Figure 6.
Coronal section showing FG-labeled neurons in the anterior pretectal nucleus (APT) and nucleus of the optic tract (OT). (Inset) Higher magnification shows the OT. Labeling was exclusively ipsilateral to the injection site. Magnification, 4× and 20×.
Figure 7.
Figure 7.
Coronal section of the cerebellum showing FG-labeled neurons in the dentate nucleus (DN) and anterior interpositus nucleus (AIN). Labeling was primarily contralateral to the injection site. Magnification, 4× and 20×.
Figure 8.
Figure 8.
Hypothesized visual CS pathway necessary for delay eyeblink conditioning. Inputs from the retina to the ventral lateral geniculate (LGNv) and nucleus of the optic tract (OT) are relayed to the cerebellar cortex (CCTX) and anterior interpositus nucleus (AIN) via the medial pontine nuclei (MPN).
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