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Neuronal hyposensitivity to dopamine in the caudate nucleus depleted of biogenic amines by tegmental lesions
EXPERIMENTAL
42,703-706
NEUROLOGY
(1974)
RESEARCH
NOTE
Neuronal Hyposensitivity to Dopamine in the Caudate Nucleus Depleted of Biogenic Amines by Tegmental Lesions R.
SPEHLMANN
AND
STEPHEN
M.
STAHLI
Veterans Administra.tion Research Hospdtal, and Department of Neurology, Northwestern University Medical School, Chicago, Illinois 60611 Received
November
14,1973
Parkinson’s disease is characterized by deficiencies of dopamine, norepinephrine, and serotonin in the corpus striatu’m (i.e., caudate nucleus and putamen) (4). These deficiencies are probably the result of the degeneration of ascending monoaminergic aff erents : similar deficiencies are produced in animals by lesions in the mesencephalicventromedial tegmenturn (1, 6, 7). Here we have used this experimental model to study the effects of monoamine depletion on the sensitivity of striatal neurons to dopamine because this amine is thought to be of crucial importance for the pathophysiology and treatment of Parkinson’s disease(4). The action of microiontophoretically applied dopamine on neurons in the caudate nucleus of cats with long-standing (17-211 days) electrocoagulative lesions in the ipsilateral tegmentum (A 7.5 ; L 6, D -3 and -3.75) was compared with the action of dopamine in intact cats. In order to reduce experimental variability, each experiment on an animal with a lesion was preceded or followed, within a few days, by an experiment on an intact animal, and the same microelectrodes were used in both experiments. Both types of cats were prepared as encephale isolC under inhalation anesthesia and kept on local anesthesia during the recording. This preparation was chosen because general anesthesia reduces the number of spontaneously firing neurons in the caudate nucleus and also alters the neuronal sensitivity to drugs (2). The micropipettes had a total tip diameter of 3-6 pm and consisted
of seven
Dopamine (1
M,
drug
barrels
surrounding
a central
pH 4.O), gamma-aminobutyric acid (4
recording M,
1Supported by NIH Research Grant NS 06820. S.M.S. was a Medical Fellow of N.I.G.M.S. while working on this project. 703 Copyright All rights
@ 1974 by of reproduction
Academic Press. Inc. in any
form
reserved.
barrel.
pH 4.0, and Scientist
704
SPEHLMANN
q
unoperated
@
operated
5
side
0.5
0
DA
STAHL
side
0.2
0
FIG. nuclei lesions 5 HT,
AND
0
NE
5HT
1. Concentrations of the biogenic amines (mean + SE) in the caudate on the unoperated vs. the operated side of the 20 cats with chronic tegmental used for the microiontophoretic studies. DA, dopamine; NE, norepinephrine; serotonin.
L-glutamate (2 h!r, pH 8.0) were retained in or expelled from their barrelfis by iontophoretic currents of up to 10 X lo-* amp which were balanced at the tip of the pipette by passing an opposing current through a barrel filled with NaCl (3 M, pH 7.0). The micropipette was inserted stereotaxically into the head of the caudate nucleus (A 14.5-18.5; L 2-8; D l-8). In most instances, the correct location of the pipette tip was ascertained by inserting fine wires into each of the two or three stabs made in one experiment. Spectrofluorometric assays (3, 5) showed that in the 20 cats with chronic lesions, the levels of the biogenic amines in the caudate nuclei of the operated side were markedly reduced (Fig. 1) ; the concentration of dopamine in these nuclei amounted to 11-66ojo of that on the unoperated side. The microiontophoretic application of dopamine suppressed the spontaneous firing of 33 of 64 neurons (.%?%) in intact cats, but of only 15 of 53 neurons (28%) in cats with lesions (Fig. 2A). A chi-square test indicated that the likelihood for this distribution to occur by chance alone was less than 1%. The mean value of the minimum effective iontophoretic current intensity (“threshold”) for this su’ppressanteffect was significantly increased in cats with lesions (Fig. 2B). The effect of dopamine on silent neurons was evaluated by measuring the threshold for the production of spontaneous firing by microiontophoretic application of L-glutamate before, during, and after dopamine application. Dopamine increased the threshold for excitation by L-glutamate in 5 of 13 neurons (38%) in intact cats and in 14 of ZG neurons (54%) in cats with lesions. In contrast, gammaaminobutyric acid suppressed the spontaneous firing and increased the
CAUDATE
q
intact
jxJ
amine-depleted
DA Proport ne”rOnS
ion of suppressed
705
NUCLEUS
DA minimum
GABA
L-ah
effeqtive iontophoritic current Intensity
FIG. 2. The effects of dopamine (DA), gamma-aminobutyric acid (GABA), and L-glutamate on the firing of neurons in the caudate nuclei of intact cats and of neurons in the amine-depleted caudate nuclei of the 20 cats with chronic lesions. * t test significant at P < 0.02; ** Chi-square test significant at P < 0.01.
threshold for excitation by L-glutamate in virtually all neurons in both groups of cats. The threshold for the suppression of the spontaneous firing by gamma-aminobutyric acid was lower in cats with lesions than in intact cats (Fig. 2B), but the difference was not signifiant as judged by a t test. Similarly, L-glutamate excited practically all neurons; the thresholds for this effect were similar in both groups of cats (Fig. 2B). These results clearly suggest that lesions in the ventromedial tegmentum which reduce the level of dapamine and other biogenic amines in the caudate nucleus decrease the depressant effect of dopamine on spontaneously firing neurons in that structure. This apparent hyposensitivity to dopamine is not likely to be the result of degenerative changes altering the ability of dopamine to diffuse through the caudate tissue becausethe neuronal sensitivity to L-glutamate was not altered. The hyposensitivity is, therefore, probably due to a reduction in number or in sensitivity of the dopamine receptors or to a combination of these factors. REFERENCES 1. AND~N, N. E., A. DAHLSTR~M, STEDT. 1966. Ascending
K. FUXE, D. CARLSSON, L. OLSON, and U. UNGERmonoamine neurons to the telencephalon and diencephalon. Acta Physiol. Stand. 6’7 : 313-326. 2. BLOOM, F. E., E. COSTA, and G. C. SALMOIRAGHI. 1965. Anesthesia and the responsiveness of individual neurons in the caudate nucleus of the cat to
706
3. 4. 5.
6.
7.
SPEHLMANN
AND
STAHL
acetylcholine, norepinephrine and dopamine administered by microelectrophoresis. J. Pharmacol. Exp. Thcr. 150: 244-252. CHAXG, C. C. 1964. A sensitive method for spectrophotofluorometric assay of catecholamines. Int. J. Nezrropltarlrlacol. 3 : 643-649. HORNYKIEWICZ, 0. 1973. Parkinson’s disease: from brain homogenate to treatment. Fed. Proc. 32: 183-190. MAICKEL, R. P., R. H. Cox, J. SAILLANT, and F. B. MILLER. 1968. A method for the determination of serotonin and norepinephrine in discrete areas of rat brain. lnt. J. Neuropharmacol. I : 275-281. POIRIER, L. J., P. SINGH, R. BOUCHER, G. BOUVIER, A. OLIVIER, and P. LAROCHELLE. 1967. Effect of brain lesions on striatal monoamines in the cat. Arch. Neural. 17 : 601-608. POIRIER, L. J., and T. L. SOURKES. 1965. Influence of the substantia nigra on the catecholamine content of the striatum. Bruin 88: 181-192.
42,703-706
NEUROLOGY
(1974)
RESEARCH
NOTE
Neuronal Hyposensitivity to Dopamine in the Caudate Nucleus Depleted of Biogenic Amines by Tegmental Lesions R.
SPEHLMANN
AND
STEPHEN
M.
STAHLI
Veterans Administra.tion Research Hospdtal, and Department of Neurology, Northwestern University Medical School, Chicago, Illinois 60611 Received
November
14,1973
Parkinson’s disease is characterized by deficiencies of dopamine, norepinephrine, and serotonin in the corpus striatu’m (i.e., caudate nucleus and putamen) (4). These deficiencies are probably the result of the degeneration of ascending monoaminergic aff erents : similar deficiencies are produced in animals by lesions in the mesencephalicventromedial tegmenturn (1, 6, 7). Here we have used this experimental model to study the effects of monoamine depletion on the sensitivity of striatal neurons to dopamine because this amine is thought to be of crucial importance for the pathophysiology and treatment of Parkinson’s disease(4). The action of microiontophoretically applied dopamine on neurons in the caudate nucleus of cats with long-standing (17-211 days) electrocoagulative lesions in the ipsilateral tegmentum (A 7.5 ; L 6, D -3 and -3.75) was compared with the action of dopamine in intact cats. In order to reduce experimental variability, each experiment on an animal with a lesion was preceded or followed, within a few days, by an experiment on an intact animal, and the same microelectrodes were used in both experiments. Both types of cats were prepared as encephale isolC under inhalation anesthesia and kept on local anesthesia during the recording. This preparation was chosen because general anesthesia reduces the number of spontaneously firing neurons in the caudate nucleus and also alters the neuronal sensitivity to drugs (2). The micropipettes had a total tip diameter of 3-6 pm and consisted
of seven
Dopamine (1
M,
drug
barrels
surrounding
a central
pH 4.O), gamma-aminobutyric acid (4
recording M,
1Supported by NIH Research Grant NS 06820. S.M.S. was a Medical Fellow of N.I.G.M.S. while working on this project. 703 Copyright All rights
@ 1974 by of reproduction
Academic Press. Inc. in any
form
reserved.
barrel.
pH 4.0, and Scientist
704
SPEHLMANN
q
unoperated
@
operated
5
side
0.5
0
DA
STAHL
side
0.2
0
FIG. nuclei lesions 5 HT,
AND
0
NE
5HT
1. Concentrations of the biogenic amines (mean + SE) in the caudate on the unoperated vs. the operated side of the 20 cats with chronic tegmental used for the microiontophoretic studies. DA, dopamine; NE, norepinephrine; serotonin.
L-glutamate (2 h!r, pH 8.0) were retained in or expelled from their barrelfis by iontophoretic currents of up to 10 X lo-* amp which were balanced at the tip of the pipette by passing an opposing current through a barrel filled with NaCl (3 M, pH 7.0). The micropipette was inserted stereotaxically into the head of the caudate nucleus (A 14.5-18.5; L 2-8; D l-8). In most instances, the correct location of the pipette tip was ascertained by inserting fine wires into each of the two or three stabs made in one experiment. Spectrofluorometric assays (3, 5) showed that in the 20 cats with chronic lesions, the levels of the biogenic amines in the caudate nuclei of the operated side were markedly reduced (Fig. 1) ; the concentration of dopamine in these nuclei amounted to 11-66ojo of that on the unoperated side. The microiontophoretic application of dopamine suppressed the spontaneous firing of 33 of 64 neurons (.%?%) in intact cats, but of only 15 of 53 neurons (28%) in cats with lesions (Fig. 2A). A chi-square test indicated that the likelihood for this distribution to occur by chance alone was less than 1%. The mean value of the minimum effective iontophoretic current intensity (“threshold”) for this su’ppressanteffect was significantly increased in cats with lesions (Fig. 2B). The effect of dopamine on silent neurons was evaluated by measuring the threshold for the production of spontaneous firing by microiontophoretic application of L-glutamate before, during, and after dopamine application. Dopamine increased the threshold for excitation by L-glutamate in 5 of 13 neurons (38%) in intact cats and in 14 of ZG neurons (54%) in cats with lesions. In contrast, gammaaminobutyric acid suppressed the spontaneous firing and increased the
CAUDATE
q
intact
jxJ
amine-depleted
DA Proport ne”rOnS
ion of suppressed
705
NUCLEUS
DA minimum
GABA
L-ah
effeqtive iontophoritic current Intensity
FIG. 2. The effects of dopamine (DA), gamma-aminobutyric acid (GABA), and L-glutamate on the firing of neurons in the caudate nuclei of intact cats and of neurons in the amine-depleted caudate nuclei of the 20 cats with chronic lesions. * t test significant at P < 0.02; ** Chi-square test significant at P < 0.01.
threshold for excitation by L-glutamate in virtually all neurons in both groups of cats. The threshold for the suppression of the spontaneous firing by gamma-aminobutyric acid was lower in cats with lesions than in intact cats (Fig. 2B), but the difference was not signifiant as judged by a t test. Similarly, L-glutamate excited practically all neurons; the thresholds for this effect were similar in both groups of cats (Fig. 2B). These results clearly suggest that lesions in the ventromedial tegmentum which reduce the level of dapamine and other biogenic amines in the caudate nucleus decrease the depressant effect of dopamine on spontaneously firing neurons in that structure. This apparent hyposensitivity to dopamine is not likely to be the result of degenerative changes altering the ability of dopamine to diffuse through the caudate tissue becausethe neuronal sensitivity to L-glutamate was not altered. The hyposensitivity is, therefore, probably due to a reduction in number or in sensitivity of the dopamine receptors or to a combination of these factors. REFERENCES 1. AND~N, N. E., A. DAHLSTR~M, STEDT. 1966. Ascending
K. FUXE, D. CARLSSON, L. OLSON, and U. UNGERmonoamine neurons to the telencephalon and diencephalon. Acta Physiol. Stand. 6’7 : 313-326. 2. BLOOM, F. E., E. COSTA, and G. C. SALMOIRAGHI. 1965. Anesthesia and the responsiveness of individual neurons in the caudate nucleus of the cat to
706
3. 4. 5.
6.
7.
SPEHLMANN
AND
STAHL
acetylcholine, norepinephrine and dopamine administered by microelectrophoresis. J. Pharmacol. Exp. Thcr. 150: 244-252. CHAXG, C. C. 1964. A sensitive method for spectrophotofluorometric assay of catecholamines. Int. J. Nezrropltarlrlacol. 3 : 643-649. HORNYKIEWICZ, 0. 1973. Parkinson’s disease: from brain homogenate to treatment. Fed. Proc. 32: 183-190. MAICKEL, R. P., R. H. Cox, J. SAILLANT, and F. B. MILLER. 1968. A method for the determination of serotonin and norepinephrine in discrete areas of rat brain. lnt. J. Neuropharmacol. I : 275-281. POIRIER, L. J., P. SINGH, R. BOUCHER, G. BOUVIER, A. OLIVIER, and P. LAROCHELLE. 1967. Effect of brain lesions on striatal monoamines in the cat. Arch. Neural. 17 : 601-608. POIRIER, L. J., and T. L. SOURKES. 1965. Influence of the substantia nigra on the catecholamine content of the striatum. Bruin 88: 181-192.