Functional disconnection of the substantia nigra pars compacta from the pedunculopontine nucleus impairs learning of a conditioned avoidance task

Abstract

The pedunculopontine tegmental nucleus (PPTg) targets nuclei in the basal ganglia, including the substantia nigra pars compacta (SNc), in which neuronal loss occurs in Parkinson's disease, a condition in which patients show cognitive as well as motor disturbances. Partial loss and functional abnormalities of neurons in the PPTg are also associated with Parkinson's disease. We hypothesized that the interaction of PPTg and SNc might be important for cognitive impairments and so investigated whether disrupting the connections between the PPTg and SNc impaired learning of a conditioned avoidance response (CAR) by male Wistar rats. The following groups were tested: PPTg unilateral; SNc unilateral; PPTg-SNc ipsilateral (ipsilateral lesions in PPTg and SNc); PPTg-SNc contralateral (contralateral lesions in PPTg and SNc); sham lesions (of each type). SNc lesions were made with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine HCl (MPTP, 0.6micromol); PPTg lesions with ibotenate (24nmol). After recovery, all rats underwent 50-trial sessions of 2-way active avoidance conditioning for 3 consecutive days. Rats with unilateral lesions in PPTg or SNc learnt this, however rats with contralateral (but not ipsilateral) combined lesions in both structures presented no sign of learning. This effect was not likely to be due to sensorimotor impairment because lesions did not affect reaction time to the tone or footshock during conditioning. However, an increased number of non-responses were observed in the rats with contralateral lesions. The results support the hypothesis that a functional interaction between PPTg and SNc is needed for CAR learning and performance.

Fig. 1

Representative left side PPTg ibotenic acid lesion. Panels A, C, D show cresyl violet and NeuN immunostained tissue and panels B, E, F, G and H show ChAT immunostained tissue. The sham-lesioned tissue is shown in the panels D, F, and H, and the lesioned tissue is shown in the panels C, E, and G. PPTg, pedunculopontine tegmental nucleus; LDTg, laterodorsal tegmental nucleus. Scale bar represents 200 μm.

Fig. 2

Representative TH-immunostained left side SNc MPTP lesion. Panel A shows the lesion on the left side and sham on the right side. Panel B shows detail of the VTA and SNc of lesioned side, panel C the SNc and VTA of the sham side. SNc, substantia nigra compacta; VTA, ventral tegmental area. Scale bar represents 200 μm.

Fig. 3

Representative TH-immunostained coronal slices of the striatum of a rat bearing an MPTP lesion of the right SNc. The slices are ordered in a rostral to caudal orientation. CPu, caudoputamen; NAc, nucleus accumbens. Scale bar represents 200 μm.

Fig. 4

Effects of unilateral infusion of 0.6 μmol MPTP into the rat SNc or of 24 nmol ibotenic acid into the PPTg on striatal dopamine content. * P < 0.05 Newman-Keuls after two-way ANOVA.

Fig. 5

Turning behaviour of unilaterally (A), ipsilaterally and contralaterally (B) PPTg and SNc lesioned rats challenged with 0.1 mg/kg apomorphine (i.p.). Data are expressed as the mean ±SEM. number of ipsiversive (positive scale) and contraversive turns (negative scale) counted over 1 h after drug challenge (n = 7 - 18 animals per group). * P ≤ 0.05 compared to the sham group (Newman–Keuls test after one-way ANOVA).

Fig. 6

Effect of the PPTg and/or SNc lesion on latency to respond to the CS and US during learning of a 2-way active avoidance response. Bars represent the mean ± SEM latencies to cross to the opposite side of the shuttle box after the CS-US onset averaged by day. * P < 0.05 compared to sham rats in the same day; + P < 0.05 compared to the same group on day 1; # P ≤ 0.05 compared to the same group on day 2; two-way ANOVA, followed by the Newman-Keuls test.

Fig. 7

Effect of the PPTg and/or SNc lesion on learning of the 2-way active avoidance. The bars represent the mean ± SEM number of avoidance (A), escape (B), non-response (C) or inter-trial crossings (ITC, D) averaged by day; * P < 0.05 compared to sham in the same day; + P < 0.05 compared to the same group on day 1 ; # P ≤ 0.05 compared to the PPTg-SNc contralateral group in the same day; two-way ANOVA, followed by the Newman-Keuls test.