Ž .Developmental Brain Research 100 1997 130–132

Short communication

Dopamine D1 and D2 antagonists block L-DOPA-induced air-stepping indecerebrate neonatal rats

Anne E. McCrea, Donald J. Stehouwer, Carol Van Hartesveldt )

Department of Psychology, UniÕersity of Florida, GainesÕille, FL 32611-2250, USA

Accepted 22 January 1997

Abstract

L-DOPA administered to neonatal rats suspended in air elicits stereotypic locomotor activation termed air-stepping; it can beŽ .dose-dependently blocked by a dopamine DA D1 or D2 antagonist. In order to determine whether the forebrain is the site for this

Ž .blockade, decerebrate 5-day-old rats were pretreated subcutaneously with either the DA D1 receptor antagonist SCH 23390 16 mgrkg ,Ž .the DA D2 receptor antagonist spiperone 6 mgrkg , or vehicle before receiving 100 mgrkg L-DOPA. Both antagonists blocked

L-DOPA-induced air-stepping in both decerebrate and intact pups.

Keywords: Dopamine; SCH 23390; Spiperone; Locomotion; Development; Air-stepping; Decerebrate

Systemic administration of L-3,4-dihydroxyphenyl-Ž .alanine L-DOPA increases locomotor activity in develop-w xing rats 10,14 . If the pups are suspended in air, locomotor

w xactivity results that is termed air-stepping 20 . L-DOPA isŽ . Ž .converted to both dopamine DA and noradrenaline NA

w xin the brain 11 , but the roles of each of these neurotrans-mitters in L-DOPA-induced air-stepping are not known.Previously, it was shown that both the DA D1 receptorantagonist SCH 23390 and the DA D2 receptor antagonistspiperone independently blocked L-DOPA-induced air-

w xstepping in 5-day-old rat pups 18 . Decerebration does notw xaffect L-DOPA-induced air-stepping 7 , but because the

forebrain is a major target for the projections of midbraindopaminergic neurons and has a dense concentration of

w xDA receptors even in neonatal rat 15 , dopamine antago-nists could act at this level to block L-DOPA-inducedlocomotion. For example, the DA antagonist haloperidol

w xinjected into the neostriatum elicits catalepsy 3 and po-w xtentiates the dorsal immobility response 13 ; haloperidol

injected into the nucleus accumbens or neostriatum in-w xcreases muscle rigidity in the limbs 6 . The present experi-

ment was designed to test whether DA antagonists blockL-DOPA-induced air-stepping in the absence of the fore-brain.

) Ž .C orresponding author. Fax: q 1 352 392-7985.vanhart@psych.ufl.edu

Ž . ŽThe DA D1 antagonist R q SCH 23390 Research.Biochemicals, Inc. was dissolved in distilled water. The

ŽDA D2 antagonist spiperone spiroperidol; Research Bio-.chemicals, Inc. was dissolved in glacial acetic acid. L-

Ž .DOPA L-b-3,4-dihydroxyphenylalanine; Sigma was dis-solved in 1 N hydrochloric acid and distilled water. Allwere adjusted to a pH of 6–7 using phosphate-bufferedsaline. Vehicle solutions were prepared similarly and ad-justed to the same pH. All solutions were injected s.c. at avolume of 0.01 mlrkg at the nape of the neck.

Five-day-old pups bred in this laboratory were anes-thetized by hypothermia induced by placing them in an icebath. Sham operations consisted of a longitudinal incisionmade in the skin above the tectum and a burr hole in theskull just rostral to lambda and lateral to the midline. Fordecerebration, a hand-held curved knife blade was insertedinto the hole and swept from one side to the other. Ratsrecovered in a warm container for at least 90 min. Onlythose that showed spontaneous or cutaneously elicitedlocomotion were used.

Subjects were first given an injection of either 16mgrkg SCH 23390, 6 mgrkg spiperone, or a vehiclesolution. They were then suspended in adhesive tape slingsin glass-enclosed incubators maintained at 338C illumi-nated from below with fluorescent lights. An injection ofL-DOPA was given 15 min after 16 mgrkg SCH 23390 orits vehicle, and 45 min after 6 mgrkg spiperone or itsvehicle. Doses of the antagonists were based on previous

0165-3806r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0165-3806 97 00027-8

( )A.E. McCrea et al.rDeÕelopmental Brain Research 100 1997 130–132 131

w xresults 18 . All groups had 6 subjects. Immediately afterthe injection of L-DOPA, subjects were videotaped for 60min and the following behaviors were measured:1. Inactive: no movement2. Active: laterally directed movements of the head, trunk,

or tail, andror limb movements lacking stereotypicalternation

3. Air-stepping: alternation of all four limbs in a diagonalprogression pattern, with dorsiflexed posture, alignmentof the head, trunk, and tail, and absence of laterallydirected head movements

4. Atypical air-stepping: stereotypic locomotor activitylacking one or more of the criteria for air-steppingPups were anesthetized using sodium pentobarbital and

transcardially perfused with heparinized saline and forma-lin. Brains were removed, refrigerated in formalin, andsectioned in the sagittal plane at 90 mm. Animals withevidence of significant bleeding were omitted. Only sub-jects with transections at the rostral edge of the superiorcolliculus dorsally and the rostral edge of the substantianigra ventrally were used.

Total durations of time spent in each behavioral cate-gory were analyzed using a 2-way ANOVA for each

Ž .antagonist 2 doses=2 types of surgery . Pairwise com-parisons were made using a Duncans Multiple Range Test.

Consistent with previous results, decerebration did notw xaffect either the locomotor pattern 7 or the mean dura-

tions of L-DOPA-induced typical or atypical air-steppingŽ .Figs. 1 and 2 , although it increased the variability of thedurations.

In both sham and decerebrate animals pre-treated withŽ .SCH 23390 and given L-DOPA Fig. 1 , there was aŽcomplete blockade of both typical Fs75.03, dfs1,23,

. Ž .P-0.001 and atypical Fs6.73, dfs1,23, P-0.02air-stepping relative to animals pre-treated with the vehi-cle. Pretreatment with SCH 23390 elicited more ‘active’

Ž . Žbehavior as defined above than with the vehicle Fs.17.72, dfs1,23, P-0.001 , but there was a significant

Fig. 1. Effects of 16 mgrkg SCH 23390 or its vehicle on L-DOPA-in-duced air-stepping, atypical air-stepping, and active behavior in sham-op-

Ž . Ž .erated SHAM and decerebrate DEC 5-day-old rat pups.

Fig. 2. Effects of 6 mgrkg spiperone or its vehicle on L-DOPA-inducedair-stepping, atypical air-stepping, and active behavior in sham-operatedŽ . Ž .SHAM and decerebrate DEC 5-day-old rat pups.

Ž .interaction with surgery Fs5.33, dfs1,23, Ps0.03 .Decerebrates given SCH 23390 had significantly more‘active’ behavior than decerebrates or sham operates given

Ž .the vehicle P ’s-0.01 .In both sham and decerebrate animals pre-treated with

Ž .spiperone and given L-DOPA Fig. 2 , typical air-steppingŽwas almost completely blocked Fs84.67, dfs1,23, P

.-0.001 . Atypical air-stepping was significantly althoughŽnot completely blocked in both groups Fs6.03, dfs

.1,23, P-0.03 . There were no significant differences in‘active’ behavior among the groups.

Whether L-DOPA elicits air-stepping via its action onDA receptors, NA receptors, or both, its effects can bemodulated by blockade of DA receptors below the level ofthe forebrain. DA antagonists might block L-DOPA-in-duced air-stepping at the level of the spinal cord, where

w xDA receptors have been identified 17 . DA applied toneonatal rat spinal cord in vitro with one hindlimb attached

w xelicits slow hindlimb stepping 12 . In a similar preparationŽ .without the hindlimb , rhythmic discharges in the ventralroot elicited by DA can be blocked with the DA D2

w xreceptor antagonist haloperidol 19 . However, L-DOPAadministered to rat pups with mid-thoracic spinal transec-

w xtions fails to elicit stepping in the hindlimbs 7 . This resultsuggests either that L-DOPA fails to elicit air-stepping byits action at the spinal cord level, or that following thetransection L-DOPA cannot be metabolized into sufficientdopamine andror noradrenaline to elicit the behavior.When an aromatic L-amino acid decarboxylase inhibitor isadministered to intact pups prior to L-DOPA, air-stepping

w xis not elicited 1 .DA antagonists might also block air-stepping at the

level of the brain stem. Both posture and limb movementhave been linked with DA and its receptors in the substan-tia nigra. Administration of L-DOPA increases the concen-

w xtration of DA in substantia nigra neurons 16 , which canbe released dendritically to both DA and non-DA neuronsw x2 . Inactivating both D1 and D2 receptors in the substantia

( )A.E. McCrea et al.rDeÕelopmental Brain Research 100 1997 130–132132

Ž .nigra pars reticulata but not the striatum results in anincrease in muscle tone measured as tonic electromyo-

w xgraphic activity 4,5 .Turning behavior elicited by L-DOPA in rats with a

unilateral 6-hydroxydopamine lesion is also dependent onnigral DA receptors. L-DOPA increases DA levels in the

Ž .substantia nigra on each side . Turning behavior elicitedby L-DOPA coincides temporally with increased DA in thesubstantia nigra rather than the striatum. Furthermore,injection of the DA D1 antagonist SCH 23390 into theDA-depleted substantia nigra pars reticulata blocks L-

w xDOPA-induced turning 16 .Other behavioral effects of DA outside the forebrain

have been found; for example, the DA agonist apomor-w xphine suppresses the intake of sucrose 8 and stimulates

w xpenile erection in decerebrate rats 9 . The present dataalso suggest that dopamine antagonists used in the treat-ment of schizophrenia or other disorders may produce sideeffects through actions below the level of the forebrain.

Acknowledgements

This research was supported by NS 28850 to D.J.S. andC.V.H.

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