Nigrostriatal denervation changes the effect of cannabinoids on subthalamic neuronal activity in rats

Abstract

Rationale: It is known that dopaminergic cell loss leads to increased endogenous cannabinoid levels and CB1 receptor density.

Objective: The aim of this study was to evaluate the influence of dopaminergic cell loss, induced by injection of 6-hydroxydopamine, on the effects exerted by cannabinoid agonists on neuron activity in the subthalamic nucleus (STN) of anesthetized rats.

Results: We have previously shown that Δ(9)-tetrahydrocannabinol (Δ(9)-THC) and anandamide induce both stimulation and inhibition of STN neuron activity and that endocannabinoids mediate tonic control of STN activity. Here, we show that in intact rats, the cannabinoid agonist WIN 55,212-2 stimulated all recorded STN neurons. Conversely, after dopaminergic depletion, WIN 55,212-2, Δ(9)-THC, or anandamide inhibited the STN firing rate without altering its discharge pattern, and stimulatory effects were not observed. Moreover, anandamide exerted a more intense inhibitory effect in lesioned rats in comparison to control rats.

Conclusions: Cannabinoids induce different effects on the STN depending on the integrity of the nigrostriatal pathway. These findings advance our understanding of the role of cannabinoids in diseases involving dopamine deficits.

Fig. 1

Histograms illustrating the mean firing rate (a) and mean coefficient of variation (percent) (b) of STN neurons recorded in intact (white square) and 6-OHDA-lesioned (black square) rats; **p < 0.01; ***p < 0.001 vs. corresponding basal value (Student's t test). Distribution of firing patterns (c) of STN neurons in intact and lesioned rats; **p < 0.01 vs. intact group (Chi-squared (χ ²) test). d Samples of spontaneous spiking activity recorded in the STN of an intact (top) and a lesioned rat (bottom). Vertical lines represent spike events. Note the increase in the basal firing rate as well as in the bursting discharge in lesioned rats

Fig. 2

Representative firing rate histograms illustrating the effect of intravenous (i.v.) administration of cumulative doses of WIN 55,212-2 (31.25–250 μg/kg, i.v., doubling doses) on STN neuronal activity in intact (a) and 6-OHDA-lesioned rats (b). Both stimulatory and inhibitory effects were reversed by administration of the CB1 receptor antagonist rimonabant (250–2,000 μg/kg, i.v.). c The mean firing rate and (d) the mean value of the coefficient of variation of STN neurons following WIN 55,212-2 administration (31.25–250 μg/kg, i.v., doubling doses) in intact (white circle; n = 8) and lesioned rats (black circle; n = 8). Data are expressed as mean ± SEM; *p < 0.05; **p < 0.01; ***p < 0.001 vs. corresponding basal value (one-way repeated measures ANOVA followed by the Student–Newman–Keuls test)

Fig. 3

Effects of cumulative doses of Δ⁹-THC and anandamide on STN neurons in intact and 6-OHDA-lesioned rats. a The mean firing rate and (b) the mean value of the coefficient of variation of STN neurons following Δ⁹-THC administration (250–2,000 μg/kg, i.v., doubling doses) in lesioned rats (black circle; n = 9) and in intact rats, where two opposite effects were found: an increase (white circle; n = 10) and a decrease in firing rate (black square; n = 5). c The mean firing rate and (d) the mean value of the coefficient of variation of STN neurons following anandamide administration (50–150 μg, i.c.v.) in lesioned rats (black circle; n = 5) and in intact rats, where two opposite effects were also observed: an increase (white circle; n = 8) and a decrease in firing rate (black square; n = 8). Data are expressed as mean ± SEM. *p < 0.05; **p < 0.01; ***p < 0.001 vs. corresponding basal value (one-way repeated measures ANOVA followed by the Student–Newman–Keuls test)

Fig. 4

Histological verification of the recording site in the subthalamic nucleus (STN). a Schematic illustration of the recording location of STN neurons. Image modified from Paxinos and Watson (1986), with permission from Elsevier. b Pontamine Sky Blue dots in Neutral Red-stained STN sections at different anatomical levels are homogenously distributed within the nucleus (the examples correspond to recorded neurons in 6-OHDA-lesioned rats and in intact rats in which WIN 55,212-2 was administered). Scale bar, 40 μm

Fig. 5

Representative tyrosine hydroxylase (TH) immunostaining in the striatum (a, b) and substantia nigra (c, d) following 6-OHDA infusion into the right nigrostriatal pathway. In control rats (a, c), TH immunostaining was intense and comparable between both sides. In rats microinfused unilaterally with 6-OHDA into the right nigrostriatal pathway (b, d), a substantial loss of TH immunostaining was observed in each area (right side) compared with the contralateral side (left side)