Medial auditory thalamic nuclei are necessary for eyeblink conditioning

Abstract

The auditory conditioned stimulus (CS) pathway that is necessary for delay eyeblink conditioning was investigated with induced lesions of the medial auditory thalamus contralateral to the trained eye in rats. Rats were given unilateral lesions of the medial auditory thalamus or a control surgery followed by twenty 100-trial sessions of delay eyeblink conditioning with a tone CS and then five sessions of delay conditioning with a light CS. Rats that had complete lesions of the contralateral medial auditory thalamic nuclei, including the medial division of the medial geniculate, suprageniculate, and posterior intralaminar nucleus, showed a severe deficit in conditioning with the tone CS. Rats with complete lesions also showed no cross-modal facilitation (savings) when switched to the light CS. The medial auditory thalamic nuclei may modulate activity in a short-latency auditory CS pathway or serve as part of a longer latency auditory CS pathway that is necessary for eyeblink conditioning.

Figure 1

Images of nissl stained sections of the contralateral medial auditory thalamus, which includes the medial division of the medial geniculate (MGm), the posterior intralaminar nucleus (PIN), and the suprageniculate nucleus (SG) in a control rat (A), two rats with incomplete lesions (B, C), and three rats with complete lesions (D, E, and F). The dorsal (MGd) and ventral (MGv) divisions of the medial geniculate are also labeled. Dotted lines indicate the borders of the lesions, and dashed lines indicate the borders of the nuclei.

Figure 2

Drawing of coronal sections of the rat brain depicting the smallest (black regions) and largest (gray regions) lesions in rats with complete (upper) or incomplete (lower) lesions of the medial auditory thalamus, which includes the medial division of the medial geniculate (MGm), the posterior intralaminar nucleus (PIN), and the suprageniculate (SG). APT = anterior pretectal nucleus; LPMC = lateral posterior thalamic nucleus, mediocaudal division; MGd = dorsal division of the medial geniculate; MGv = ventral division of the medial geniculate; OT = optic tract nucleus; PPT = posterior pretectal nucleus; RN = red nucleus; SN = substantia nigra. The number to the right of the drawings indicates the anterior–posterior stereotaxic coordinate relative to bregma. From The Rat Brain in Stereotaxic Coordinates (Figure 42) by G. Paxinos and C. Watson, 1998, New York: Academic Press. Copyright 1998 by Academic Press. Adapted with permission.

Figure 3

Mean (±) standard error of the mean conditioned response (CR) percentage over twenty 100-trial sessions of delay eyeblink conditioning with a tone-conditioned stimulus (CS) and five sessions of conditioning with a light CS in rats with compete lesions of the medial auditory thalamic nuclei (complete), incomplete lesions (incomplete), saline-infused controls (saline), and controls that were given training only with a light stimulus (light).

Figure 4

(A) The short-latency auditory CS pathway for eyeblink conditioning, which includes the cochlear nuclei (CN), the pontine nuclei (PN), and their mossy fiber inputs to the cerebellar cortex (CTX) and interpositus nucleus (IPN). (B) A possible long-latency auditory CS pathway for eyeblink conditioning, which includes the inferior colliculus (IC), medial auditory thalamic nuclei (MATN), and their projections to the PN.