Local administration of dopaminergic drugs into the ventral tegmental area modulates cataplexy in the narcoleptic canine

Abstract

Cataplexy in the narcoleptic canine may be modulated by systemic administration of monoaminergic compounds. In the present study, we have investigated the effects of monoaminergic drugs on cataplexy in narcoleptic canines when perfused locally via microdialysis probes in the amygdala, globus pallidus/putamen, basal forebrain, pontine reticular formation and ventral tegmental area of narcoleptic and control Doberman pinchers. Cataplexy was quantified using the Food-Elicited Cataplexy Test and analyzed by electroencephalogram, electroculogram and electromyogram. Local perfusion with the monoaminergic agonist quinpirole, 7-OH-DPAT and BHT-920, into the ventral tegmental area produced a dose-dependent increase in cataplexy without significantly reducing basal muscle tone. Perfusion with the antagonist raclopride in the same structure produced a moderate reduction in cataplexy. Local perfusion with quinpirole, 7-OH-DPAT and BHT-920 into the globus pallidus/putamen also produced an increase, while raclopride produced a decrease, in cataplexy in narcoleptic canines. In control animals, none of the above drugs produced cataplexy or muscle atonia when perfused into either the ventral tegmental area or the globus pallidus/putamen. Other monoaminergic drugs tested in these two brain areas; prazosin, yohimbine, amphetamine, SKF 38393 and SCH 23390 had no effects on cataplexy. Local perfusion with each of the above listed drugs had no effect on cataplexy in any of the other brain regions examined. These findings show that cataplexy may be regulated by D2/D3 dopaminergic receptors in the ventral tegmental area and perhaps the globus pallidus/ putamen. It is suggested that neurons in the mesolimbic dopamine system of narcoleptics are hypersensitive to dopaminergic autoreceptor agonists.

Fig. 1.

Effects of bilateral perfusion with various dopaminergic compounds in the GP/putamen (GP: globus pallidus) on cataplexy in narcoleptic canines. Local perfusion 7-OH-DPAT (10 M) (a), quinpirole (10⁻⁴–10⁻² M) BHT-920 (10⁻⁴–10⁻² M) (c) and raclopride M) in the narcoleptic canines is shown. Drugs were mixed into artificial cerebrospinal fluid and perfused through microdialysis probes at the indicated concentrations over the course of a 4-h experiment: none during the first hour,10⁻⁴ M during the second hour, 10⁻³ M during the third hour and 10⁻² M during the fourth hour. Each drug was tested with four narcoleptic canines, though the same animals were not always used. The mean number of cataplectic attacks and elapsed time for two food-elicited cataplexy tests (FECT) per test period is shown. For the purpose of figure presentation, status cataplecticus (complete muscle atonia) was arbitrarily designated as 15 attacks elapsed over 600 s. Each drug perfusion time point was compared with the basal time points using a Fisher PLSD post-hoc test; * indicates P < 0.05 satisfactory for comparison with either basal time point. Univariate analyses of number of cataplectic attacks over the tested dose range for each drug were; 7-OH-DPAT:F_4,20 = 9.851, P=0.001 Quinpirole: F_4,28=3.244, P = 0.048; BHT-920: F_4,28 = 1.322, P = 0.279; Raciopride: F_2,14=4.235, P = 0.028. Univariate analyses of elapsed FECT time over the tested dose range for each drug were; 7-OH-DPAT: F _4,20= 19.366, P = Quinpirole: F_4,28=3.975, P = 0.026; BHT-920: F_4,28=4.257, P = 0.006; Raciopride: F_2,14=1.112, P = 0.655.

Fig. 2.

Effects of bilateral perfusion with various dopaminergic compounds in the ventral tegmental area (VTA) on cataplexy in narcoleptic canines. Local perfusion 7-OH-DPAT (10⁻⁴–10⁻² M) (a), quinpirole (10⁻⁴–10⁻² M) (b), BHT-920 (10⁻⁴–10⁻² M) (c) and raclopride (10⁻³ M) in the narcoleptic canines is shown. The mean number of cataplectic attacks and elapsed time for two food-elicited cataplexy tests (FECT) per test period is shown. For the purpose of figure presentation, status cataplecticus (complete muscle atonia) was arbitrarily designated as 15 attacks elapsed over 600 s, thus, for these levels of cataplexy there was no statistical variability in the score. The same four narcoleptic canines were tested for each drug. Each drug perfusion time point before status cataplecticus was compared with the basal time points using a Fisher PLSD post-hoc test; * indicates P < 0.05 satisfactory for comparison with either basal time point. Univariate analyses of number of cataplectic attacks over the tested dose range for each drug were; 7-OH-DPAT: F_5,35 = 11 .923, P = 0.001; Quinpirole: 5.378, P = 0.001 ; BHT-920: 8.146, P = 0.001 ; Raciopride: F_4.28 = 1.264, P = 0.303. Univariate analyses of elapsed FECT time over the tested dose range for each drug were: 7-OH-DPAT: F_5.35 = 11.511, P = 0.001; Quinpirole: F_6.42 = 4.226, P = 0.002; BHT-920: F_6.42 = 11.715, P = 0.001 ; Raciopride: F_4.28 = 2.826, P = 0.039.

Fig. 3.

Polygraph recordings and spectral analysis of cataplexy in two narcoleptic canines receiving either quinpirole (10⁻³ M) (a,c,e,f) or 7-OH-DPAT (10⁻³ M) (b,d,g,h) in the ventral tegmental area (VTA). Polygraph recording of (a,c) baseline cataplexy, (b) bilateral quinpirole perfusion and (d) bilateral 7-OH-DPAT perfusion in the VTA are shown. Recordings in (a) and (c) were obtained from the same animal (animal no. 1). Recordings in (b) and (d) were obtained from the same animal (animal no. 2). All EEG recordings were bipolar, taken from fronto-parietal cortex leads, and were taken on the same day from each animal. Means and standard deviation of the EEG power density (V² per Hz) for each 0.2 Hz frequency band (0–20 Hz) were calculated by averaging all 10-s epochs which were free of artifact for each condition; baseline (e,g), quinpirole (10⁻³ M) (f) and 7-OH-DPAT (10⁻³ M) (h). Power spectra in (e,f) and (g,h) were taken from the same animal, respectively.

Fig. 3.

Polygraph recordings and spectral analysis of cataplexy in two narcoleptic canines receiving either quinpirole (10⁻³ M) (a,c,e,f) or 7-OH-DPAT (10⁻³ M) (b,d,g,h) in the ventral tegmental area (VTA). Polygraph recording of (a,c) baseline cataplexy, (b) bilateral quinpirole perfusion and (d) bilateral 7-OH-DPAT perfusion in the VTA are shown. Recordings in (a) and (c) were obtained from the same animal (animal no. 1). Recordings in (b) and (d) were obtained from the same animal (animal no. 2). All EEG recordings were bipolar, taken from fronto-parietal cortex leads, and were taken on the same day from each animal. Means and standard deviation of the EEG power density (V² per Hz) for each 0.2 Hz frequency band (0–20 Hz) were calculated by averaging all 10-s epochs which were free of artifact for each condition; baseline (e,g), quinpirole (10⁻³ M) (f) and 7-OH-DPAT (10⁻³ M) (h). Power spectra in (e,f) and (g,h) were taken from the same animal, respectively.

Fig. 4.

Behavioral tests on the extracellular levels of dopamine in the lateral GP/putamen (GP: globus pallidus) of narcoleptic (n=4) and control canines (n=2) measured by in vivo microdialysis. FECT treav ment represents data from seven bilateral microdialysis sessions in four narcoleptic canines (each animal tested 2 times = 14 total obervations), MOTOR treatment represents data from five bilateral microdialysis sessions in four narcoleptic canines (three animals tested I time, one animal tested 2 times =10 total observations) and CONTROL treatment represents data from three bilateral microdialysis sessions in two control canines (one animal tested onc time, one animal tested 2 times =6 total observations). During FECT treatment each animal performed two FECT trials per min, during MOTOR treatment each animal performed two MOTOR activity trails without having cataplexy per 10 min and during CONTROL treatment each animal performed two FECT trials per 10 min (no cataplexy occurred). Within each group the treatment and post-treatment time points were compared with pretreatment time points (Fishers PLSD post-hoc test). No significant differences were found.

Fig. 5.

Behavioral tests on the extracellular levels of dopamine in the ventral tegmental area (VTA) of narcoleptic (n = 4) and control canine group (3 controls, 1 heterozygous) (n = 4) measured by in vivo microdialysis. FECT treatment represents data from six bilateral microdialysis sessions in four narcoleptic canines (two animals tested 2 times, two animals tested 1 time = 11 total observations (one observation run lost due to sample contamination)). MOTOR treatment represents data from four bilateral microdialysis sessions in four narcoleptic canines (each animal tested 1 time = 6 total observations (2 observation runs lost due to sample contamination)). CONTROL treatment represents data from six bilateral microdialysis sessions in four control canines (two animals tested 2 times, two animals tested 1 time = 11 total observations (one observation run lost due to sample contamination)). During FECT treatment each animal performed two FECT trials per 10 min, during MOTOR treatment each animal performed two MOTOR activity trails without having cataplexy per 10 min and during CONTROL treatment each animal performed two FECT trials per 10 min (no cataplexy occurred). Within each group the treatment and post-treatment time points were compared with pretreatment time points (Fishers PLSD post-hoc test). No significant differences were found.

Fig. 6.

Histological analysis of microdialysis probe locations in the narcoleptic and control canines. Microphotographs showing typical probe locations in the (a) amygdala, (b) basal forebrain, (c,d) lateral globus pallidus/putamen (e,f) ventral tegmental area (VTA) are shown. Each photomicrograph in (a-d,f) shows a tract in one hemisphere made by lowering an injection cannula (same gauge as the microdialysis probes) into the brain at the same coordinates as the microdialsyis probes (done before sacrificing each animal). The photomicrograph in (e) shows tracts in both hemispheres. The coronal section with the ventral most tip of the tract is shown and the tracts are indicated by arrows. The injection tract shown in the amygdala is located within the central nucleus of the amygdaloid complex (a). The injection tract in the basal forebrain is located in the magnocellular regions of the BF near to the lateral preoptic region and the diagonal band of Broca (b). The injection tracts in the lateral globus pallidus/putamen were within the putamen, adjacent to the claustrum, and extended partially down to the lateral GP (abbreviations: GP-globus pallidus, IC-internal capsule, PI-Lputamen) (c,d). In (c) the arrows with filled triangles indicate a tract from a previously implanted microdialysis probe while the arrow with an open triangle shows the site of injection of 1.0 μl 2% cresyl violet made by the injection cannula used for the histological verification procedure. (c) and (d) were taken from different animals, narcoleptic and control, respectively. The injection tracts in the VTA were within the central portion of the VTA, just medial of the substantia nigra pars compacta (abbreviations: SNC-substantia nigra pars compacta, VTA-ventral tegmental area) (e,f). The sections in (e,f) were stained for tyrosine hydroxylase-immunoreactivity (TH-IR); (e) and (f) were taken from the same animal. The vertical bars in (a–c,e) indicate 5 mm and the vertical bars in (d,f) indicate 1 mm.