- Email: [email protected]
Increased dopamine agonist sensitivity is accompanied by a decrease in striatal dopamine D2 receptor concentration in the hypothyroid rat
1410
Supported by MH45286.
References Glowinski, .L, and Cheramy, A., 1981, In L. Stjarne, P. Hedqvist, H. Lagercrantz and A. Wennmalm (Eds.) Chemical Neurotransmission: 75 Years, Academic Press, New York, 285. Grace, A.A., 1987, In L.A. Chiodo and A.S. Freeman (Eds.), Neurophysiology of Dopaminergic Systems-C,JTrent Status and Cfinical Perspectives, Lakeshore, Detroit, 1. Nedergaar~ S., Bolam, J.P. and Greenfield, S.A., 1988, Nature, 333, 174. P.we.282 1
Increased dopmnine agonist sensitivity is accompanied by a decrease in striatal dopamine D2 receptor concentration in the hypothyroid rat Cameron, D.L. and Crocker, A.D. Dept. of Clinical Pharmacology and Centre for Neuroscience, School of Medicine, The Flinders University of South Australia, Bedford Park, South Australia, 5042, Australia
The hypothyroid rat has been shown to exhibit increased behavioural sensitivity to dopamine receptor agonists in the face of unaltered dopamine receptor concentration as determined by figand binding in striatal homogenates (Cameron and Crocker, 1987). However, previous work from this laboratory (Cameron and Crocker, 1989) has demonstrated that the dopamine receptors responsible for one of the dopamine agonist-induced responses, stereotyped head-down sniffing (SHDS), are located in a discrete sub area of the striatal complex. Therefore it is possible that the altered agonist response observed in hypothyroid animals was due to a localised change in striatal dopamine receptor concentration that was masked in homogenate assays. The aim of this study was to assess the regional concentration of D1 and D2 receptors, using quantitative autoradiography, in the striata of hypothyroid rats. Rats were rendered hypothyroid by the addition of 0.01~ propylthiouracil to their drinking water. Hypothyroidism was confirmed by a drop in core body temperature and a reduction in serum thyroid hormone levels. Animals were subsequently challenged with the mixed D I / D 2 agonist; apomorphine, and the selective D2 agonist, LY171555, in order to confn'm an increase in the SHDS response of the hypothyroid rats. This increase in agonist sensitivity was manifested by a significant decrease in EC50 value for both drugs but no change in E o or Emax. Once the behavioural experiments were finished the animals were sacrificed by cervical dislocation and their brains rapidly removed and cooled in ice-cold saline. Portions of brain were embedded in OCT, snap frozen in isopentane-quenched liquid nitrogen and then 20/Am coronal sections cut and thaw mouated on gelatin coated slides. After desiccation the sections were incubated with eider [3HISCH23390 (D1 receptors) or [3H]Sulpiride (D2 receptors). Non-specific binding was determined with an excess of either cold SCH23390 or cold sulpiride, respectively. After washing and drying the sections were opposed, along with autoradiographic standards, to [3H[ sensitive film. After a three week exposure the fdm was developed using standard procedures and the density of the resulting images quantified using a computer-based image analysis system. The density of both D1 and D2 receptors was assessed in the caudate-putamen, nucleus accumbens, and the previously defined sensitive area. In addition, the density of D2 receptors was also assessed in the lateral and medial caudate-putamen as previous studies have pointed to a differential regulation of the D2 subtype in these areas (Savasta et al., 1988). No difference in D1 receptor concentration was detected between hypothyroid rats and controls in any of the areas studied. The D2 receptor concentratien, however was found to be reduced in all areas, with the greatest reduction in the dorsal str~atum (caudate-putamen). It was concluded that the i~crease in agonist sensitivity observed may be due to an amplification in the signal transduction pathway at a point dist-! "o the dopamine receptor. Indeed, it is likely that the decreased concentration of D2 receptors observed may reflect a compensation for the increased gain in the circuit which accounts for the lack of baseline behavicural difference. However, under the increased "load" of exogenously administered agonist stimulation, this compensation may no longer be adequate and the hypothyroid-induced amplification in another part of the circuit is ma~-~ifested as an apparent increase in dopamine agonist sensitivity.
1411
References Cameron, D.L. and A.D. Crocker, 1987, Pharmacol. Biochem. Behav. 28, 193. Cameron, D.L. and A.D. Crocker, 1989, Neuroscience (in press). Savatsa, M., A. Dubois, J. Benevides and B. Scatton, 1988, Neurosci. Lett. 85, 119.
P.we.283 [
Production of prolonged supersensitization of dopamine D2 receptors Kostrzewa, R.M., H a m d i , A. a n d Kostrzewa, F.P. Department of Pharmacology, .lames H. Quillen College of Medicine, East Tennessee State University, Johnson City, TN 37614, U.S.A.
Stereotypic and motor responses to dopamine (DA) agonists can be enhanced by repeated D1 agonist treatment of rats that received neonatal 6-hydroxydopamine (6-OHDA), a neurotoxin that destroys dopaminergic fibe~'s (Breese et al., 1985). This phenomenon, known as 'priming' is not produced by repeated D2 agonist treatments (Cfisweil et al., 1989). In an attempt to produce animals with sensitized D2 receptors, rats were treated daily from birth with a D1 agonist, SKF38393 (2.6 m g / k g / d × 32d, i.p.), or a D2 agnnist, quinpirole (2.6 m g / k g / d × 32d, i.p.), a n d / o r 6-OHDA (134/tg, i.c.v., 3d after birth). At 7 wk rats were observed for stereotypic and motor responses induced by acute quinpirole treatment (2.6 mg/kg, i.p.) - analogous to the method of Breese et al. (1985). Rats were scored for motor and stereotypic behaviors, once a rain every 10 min over a 90 min session. As shown in table 1, licking, grooming and digging responses were markedly increased in rats that had been treated during dewelopment with SKF38393 + 6-OHDA. This neonatal priming of D2 receptors by a D1 agonist is analogous to the adult priming of D2 receptors (Criswell et al., 1989), although behavioral sensitivities are different by each method. Digging responses were increased in the group treated in development with quinpirole + 6-OHDA. This D2 priming of D2 receptors represents a new phenomenon. Eating, but not gnawing, was increased in those rats treated in development with quinpirole alone. This prolonged D2 sensitization of one behavior would have occurred with an intact DA system, and therefore, would represent a sensitization different from priming. In vitro striatai [3H]spiroperidol binding was not different among the groups, indicating that sensitivity, and not number, of D2 receptors is the primary alteration. Increased D2 receptor sensitivity is first observed at 3 to 4 wk, when daily quinpirole injections produce jumping and enhanced locomotor activity in rats, regardless of whether rats are co-treated with 6-OHDA. The 3 new rat mo0e~,s, i~ which D2 ~'eceptors have been sensitized, may be useful in studying several clinical entities, including tardive dyskinesia, a neuroleptic-induced disorder in humans, in which receptor sensitization seems to occur. Supported by BRSG 2 S07RR05959 and the Scottish Rite Schizophrenia Research Program, N.M.J., U.S.A. Table 1 Numbers of stereotypic responses [Mean and (SEM)] to a challenge dose of nuinpirole (2.6 mg/kg, i.p.) in rats that were treated in development with SKF38393 or quinpirole, alone or in combination with 6-OHDA. Treatment group
Stereotypic behavior Licking
Grooming
Digging
Eating
Diluent SKF38395 Quinpirole Diluent + 6-OHDA SKF38393 + 6-OHDA Quinpirole + 6-OHDA
0.2 (0.2) 0.2 (0.2) 2.2 (1.3) 8.6 (5.9) 20.6 (6.5) * 2.2 (0.9)
0.0 (0.0) 0.0 (0.0) 1.2 (1.2) 8.0 (5.9) 20.0 (6.9) * 1.2 (1.2)
0.4 (0.2) 6.2 (2.4) 4.6 (1.4) 2.2 (1.0) 20.0 (3.2) ¢ 15.2 (6.2) ~*
4.6 (2.3) 9.4 (4.5) 15.0 (4.2) * 4.2 (2.1) 1.0 (0.8) 5.6 (1.4)
* P < 0.05, vs. all groups: analysis of variance and Newman-Keuls test. # P < 0.05, vs. 'Diluent +6-OHDA' group and groups not treated with 6-OHDA.
Supported by MH45286.
References Glowinski, .L, and Cheramy, A., 1981, In L. Stjarne, P. Hedqvist, H. Lagercrantz and A. Wennmalm (Eds.) Chemical Neurotransmission: 75 Years, Academic Press, New York, 285. Grace, A.A., 1987, In L.A. Chiodo and A.S. Freeman (Eds.), Neurophysiology of Dopaminergic Systems-C,JTrent Status and Cfinical Perspectives, Lakeshore, Detroit, 1. Nedergaar~ S., Bolam, J.P. and Greenfield, S.A., 1988, Nature, 333, 174. P.we.282 1
Increased dopmnine agonist sensitivity is accompanied by a decrease in striatal dopamine D2 receptor concentration in the hypothyroid rat Cameron, D.L. and Crocker, A.D. Dept. of Clinical Pharmacology and Centre for Neuroscience, School of Medicine, The Flinders University of South Australia, Bedford Park, South Australia, 5042, Australia
The hypothyroid rat has been shown to exhibit increased behavioural sensitivity to dopamine receptor agonists in the face of unaltered dopamine receptor concentration as determined by figand binding in striatal homogenates (Cameron and Crocker, 1987). However, previous work from this laboratory (Cameron and Crocker, 1989) has demonstrated that the dopamine receptors responsible for one of the dopamine agonist-induced responses, stereotyped head-down sniffing (SHDS), are located in a discrete sub area of the striatal complex. Therefore it is possible that the altered agonist response observed in hypothyroid animals was due to a localised change in striatal dopamine receptor concentration that was masked in homogenate assays. The aim of this study was to assess the regional concentration of D1 and D2 receptors, using quantitative autoradiography, in the striata of hypothyroid rats. Rats were rendered hypothyroid by the addition of 0.01~ propylthiouracil to their drinking water. Hypothyroidism was confirmed by a drop in core body temperature and a reduction in serum thyroid hormone levels. Animals were subsequently challenged with the mixed D I / D 2 agonist; apomorphine, and the selective D2 agonist, LY171555, in order to confn'm an increase in the SHDS response of the hypothyroid rats. This increase in agonist sensitivity was manifested by a significant decrease in EC50 value for both drugs but no change in E o or Emax. Once the behavioural experiments were finished the animals were sacrificed by cervical dislocation and their brains rapidly removed and cooled in ice-cold saline. Portions of brain were embedded in OCT, snap frozen in isopentane-quenched liquid nitrogen and then 20/Am coronal sections cut and thaw mouated on gelatin coated slides. After desiccation the sections were incubated with eider [3HISCH23390 (D1 receptors) or [3H]Sulpiride (D2 receptors). Non-specific binding was determined with an excess of either cold SCH23390 or cold sulpiride, respectively. After washing and drying the sections were opposed, along with autoradiographic standards, to [3H[ sensitive film. After a three week exposure the fdm was developed using standard procedures and the density of the resulting images quantified using a computer-based image analysis system. The density of both D1 and D2 receptors was assessed in the caudate-putamen, nucleus accumbens, and the previously defined sensitive area. In addition, the density of D2 receptors was also assessed in the lateral and medial caudate-putamen as previous studies have pointed to a differential regulation of the D2 subtype in these areas (Savasta et al., 1988). No difference in D1 receptor concentration was detected between hypothyroid rats and controls in any of the areas studied. The D2 receptor concentratien, however was found to be reduced in all areas, with the greatest reduction in the dorsal str~atum (caudate-putamen). It was concluded that the i~crease in agonist sensitivity observed may be due to an amplification in the signal transduction pathway at a point dist-! "o the dopamine receptor. Indeed, it is likely that the decreased concentration of D2 receptors observed may reflect a compensation for the increased gain in the circuit which accounts for the lack of baseline behavicural difference. However, under the increased "load" of exogenously administered agonist stimulation, this compensation may no longer be adequate and the hypothyroid-induced amplification in another part of the circuit is ma~-~ifested as an apparent increase in dopamine agonist sensitivity.
1411
References Cameron, D.L. and A.D. Crocker, 1987, Pharmacol. Biochem. Behav. 28, 193. Cameron, D.L. and A.D. Crocker, 1989, Neuroscience (in press). Savatsa, M., A. Dubois, J. Benevides and B. Scatton, 1988, Neurosci. Lett. 85, 119.
P.we.283 [
Production of prolonged supersensitization of dopamine D2 receptors Kostrzewa, R.M., H a m d i , A. a n d Kostrzewa, F.P. Department of Pharmacology, .lames H. Quillen College of Medicine, East Tennessee State University, Johnson City, TN 37614, U.S.A.
Stereotypic and motor responses to dopamine (DA) agonists can be enhanced by repeated D1 agonist treatment of rats that received neonatal 6-hydroxydopamine (6-OHDA), a neurotoxin that destroys dopaminergic fibe~'s (Breese et al., 1985). This phenomenon, known as 'priming' is not produced by repeated D2 agonist treatments (Cfisweil et al., 1989). In an attempt to produce animals with sensitized D2 receptors, rats were treated daily from birth with a D1 agonist, SKF38393 (2.6 m g / k g / d × 32d, i.p.), or a D2 agnnist, quinpirole (2.6 m g / k g / d × 32d, i.p.), a n d / o r 6-OHDA (134/tg, i.c.v., 3d after birth). At 7 wk rats were observed for stereotypic and motor responses induced by acute quinpirole treatment (2.6 mg/kg, i.p.) - analogous to the method of Breese et al. (1985). Rats were scored for motor and stereotypic behaviors, once a rain every 10 min over a 90 min session. As shown in table 1, licking, grooming and digging responses were markedly increased in rats that had been treated during dewelopment with SKF38393 + 6-OHDA. This neonatal priming of D2 receptors by a D1 agonist is analogous to the adult priming of D2 receptors (Criswell et al., 1989), although behavioral sensitivities are different by each method. Digging responses were increased in the group treated in development with quinpirole + 6-OHDA. This D2 priming of D2 receptors represents a new phenomenon. Eating, but not gnawing, was increased in those rats treated in development with quinpirole alone. This prolonged D2 sensitization of one behavior would have occurred with an intact DA system, and therefore, would represent a sensitization different from priming. In vitro striatai [3H]spiroperidol binding was not different among the groups, indicating that sensitivity, and not number, of D2 receptors is the primary alteration. Increased D2 receptor sensitivity is first observed at 3 to 4 wk, when daily quinpirole injections produce jumping and enhanced locomotor activity in rats, regardless of whether rats are co-treated with 6-OHDA. The 3 new rat mo0e~,s, i~ which D2 ~'eceptors have been sensitized, may be useful in studying several clinical entities, including tardive dyskinesia, a neuroleptic-induced disorder in humans, in which receptor sensitization seems to occur. Supported by BRSG 2 S07RR05959 and the Scottish Rite Schizophrenia Research Program, N.M.J., U.S.A. Table 1 Numbers of stereotypic responses [Mean and (SEM)] to a challenge dose of nuinpirole (2.6 mg/kg, i.p.) in rats that were treated in development with SKF38393 or quinpirole, alone or in combination with 6-OHDA. Treatment group
Stereotypic behavior Licking
Grooming
Digging
Eating
Diluent SKF38395 Quinpirole Diluent + 6-OHDA SKF38393 + 6-OHDA Quinpirole + 6-OHDA
0.2 (0.2) 0.2 (0.2) 2.2 (1.3) 8.6 (5.9) 20.6 (6.5) * 2.2 (0.9)
0.0 (0.0) 0.0 (0.0) 1.2 (1.2) 8.0 (5.9) 20.0 (6.9) * 1.2 (1.2)
0.4 (0.2) 6.2 (2.4) 4.6 (1.4) 2.2 (1.0) 20.0 (3.2) ¢ 15.2 (6.2) ~*
4.6 (2.3) 9.4 (4.5) 15.0 (4.2) * 4.2 (2.1) 1.0 (0.8) 5.6 (1.4)
* P < 0.05, vs. all groups: analysis of variance and Newman-Keuls test. # P < 0.05, vs. 'Diluent +6-OHDA' group and groups not treated with 6-OHDA.