The Journal of Neuroscience, May 1, 1999, 19(9):3594–3609 Striatal Neuronal Activity and Responsiveness to Dopamine and Glutamate after Selective Blockade of D1 and D2 Dopamine Receptors in Freely Moving Rats Eugene A. Kiyatkin and George V. Rebec Program in Neural Science, Department of Psychology, Indiana University, Bloomington, Indiana 47405 Although striatal neurons receive continuous dopamine (DA) a weak depressing effect on basal activity and was completely input, little information is available on the role of such input in ineffective in blocking the neuronal response to DA. Although regulating normal striatal functions. To clarify this issue, we eticlopride reduced the magnitude of the GLU response, the assessed how systemic administration of selective D1 and D2 response threshold was lower, and depolarization inactivation receptor blockers or their combination alters striatal neuronal occurred more often relative to control. The combined admin- processing in freely moving rats. Single-unit recording was istration of these drugs resembled the effects of SCH-23390, combined with iontophoresis to monitor basal impulse activity but whereas the change in basal activity and the GLU response of dorsal and ventral striatal neurons and their responses to was weaker, the DA blocking effect was stronger than SCH- glutamate (GLU), a major source of excitatory striatal drive, and 23390 alone. Our data support evidence for DA as a modulator DA. SCH-23390 (0.2 mg/kg), a D1 antagonist, strongly elevated of striatal function and suggest that under behaviorally relevant basal activity and attenuated neuronal responses to DA com- conditions tonically released DA acts mainly via D1 receptors to pared with control conditions, but GLU-induced excitations provide a continuous inhibiting or restraining effect on both were enhanced relative to control as indicated by a reduction in basal activity and responsiveness of striatal neurons to GLU- response threshold, an increase in response magnitude, and a mediated excitatory input. more frequent appearance of apparent depolarization inactiva- Key words: striatum; dopamine; glutamate; single-unit activ- tion. In contrast, the D2 antagonist eticlopride (0.2 mg/kg) had ity; iontophoresis; SCH-23390; eticlopride Striatal neurons integrate cognitive, motivational, and sensorimo- tion, the pattern of such modulation, its mechanisms, and its role tor information for behavioral output (Graybiel, 1995). Gluta- in regulating physiological and behavioral processes remain mate (GLU), which is released from both corticostriatal and unclear. thalamostriatal fibers, plays a crucial role in this integrative pro- An interesting feature of striatal DA transmission is that apart cess by controlling striatal neuronal excitability (Groves, 1983; from phasic fluctuations in release associated with specific envi- Parent and Hazrati, 1995; Wilson and Kawaguchi, 1996). Striatal ronmental and behavioral events, there appears to be a relatively neurons also receive dense input from dopamine (DA)- stable level of continuous DA release (Grace, 1991; Le Moal and containing afferents, which arise from midbrain nuclei and syn- Simon, 1991). Consistent with this view, midbrain DA neurons apse in close relation to GLU terminals (Smith and Bolam, 1990). discharge in vitro without any excitatory input (Bunney et al., Although DA is known to be important in regulating various 1991) and most, if not all, are tonically active in vivo (Dai and physiological and behavioral functions (Fibiger and Phillips, 1986; Tepper, 1998; Kiyatkin and Rebec, 1998). In fact, although the Grace, 1991; Le Moal and Simon, 1991), its essential role has activity of most DA neurons is phasically modulated by various been difficult to characterize. Ample in vitro and in vivo electro- somatosensory stimuli (Chiodo et al., 1980; Strecker and Jacobs, physiological data suggest that DA acts as a modulator altering 1985; Kiyatkin, 1988) and during behavior (Schultz, 1986), the the efficiency of neuronal responses to other inputs, particularly mean rate of activity remains relatively stable (Miller et al., 1981; to GLU (Calabresi et al., 1997; Grenhoff and Johnson, 1997; Steinfels et al., 1983; Strecker and Jacobs, 1985; Trulson, 1985). Cepeda and Levine, 1998). Further support for this modulatory Although a deficit in DA transmission results in profound role has emerged from work with awake, freely moving animals, changes in striatal functioning implicated in various cognitive and in which iontophoretic DA has been shown to attenuate neuronal motor diseases (Grace, 1991; Zigmond, 1994), the role of tonic excitations occurring during somatosensory stimulation or behav- DA input in regulating normal striatal functions remains unclear. ior (Rolls et al., 1984; Kiyatkin and Rebec, 1996) but to have The present study was designed to shed light on the role of bidirectional effects on the GLU response (Pierce and Rebec, endogenously released DA in regulating striatal neuronal func- 1995; Kiyatkin and Rebec, 1996). Although these data suggest tions and the receptor mechanisms involved in mediating this that DA modulates the processing of behavior-related informa- action. Single-unit recording was combined with iontophoresis to examine changes in spontaneous impulse activity and the re- Received Nov. 12, 1998; revised Jan. 29, 1999; accepted Feb. 8, 1999. sponses of dorsal and ventral striatal neurons to DA and GLU This work was supported by the National Institute on Drug Abuse (Grants DA induced by DA receptor blockade. Because the cellular effects of 02451 and DA 00335). We greatly appreciate Paul Langley for technical assistance and Faye Caylor for help in preparing this manuscript. DA are mediated via two distinct families of receptors (Cooper et Correspondence should be addressed to Eugene A. Kiyatkin at the above address. al., 1991; Jackson and Westlind-Danielsson, 1994), we used sys- Copyright © 1999 Society for Neuroscience 0270-6474/99/193594-16$05.00/0 temic administration of selective D1 (SCH-23390) and D2 (eti- Kiyatkin and Rebec • D1/D2 Regulation of Striatal Activity J. Neurosci., May 1, 1999, 19(9):3594–3609 3595 clopride) antagonists (Neve and Neve, 1997) or their combina- All iontophoretic applications used for statistical analysis were per- tion. To ensure behaviorally relevant conditions, all recordings formed when the animals rested quietly with no sign of overt movement. Data obtained during spontaneous movements were analyzed separately. were performed in awake, unrestrained rats. Pharmacological treatments. Approximately 2–3 hr after recording began, animals received one of three treatments: 0.2 mg/kg SCH-23390 [R(1)- MATERIALS AND METHODS 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride]; 0.2 mg/kg S(2)-eticlopride HCl (ETI); or their combi- Animals and surgery. Data were obtained from male, Sprague Dawley ; nation, 0.2 mg/kg SCH plus 0.2 mg/kg ETI. Both drugs have high rats, weighing 400 gm and bred in our animal colony from source antagonistic activity at either D1 or D2 DA receptor types (relative animals supplied by Harlan Industries (Indianapolis, IN). All animals D1:D2 affinity, SCH 5 2500:1 and ETI 5 1:514,000; Neve and Neve, were housed individually under standard laboratory conditions (12 hr 1997) and, at these doses, interactions with other receptors, particularly light/dark cycle beginning at 7:00 A.M.) with ad libitum access to food 5-HT2 receptors in the case of SCH, appear to be negligible (Bischoff et and water. Experiments were performed in compliance with the Guide al., 1986; Hjorth and Carlsson, 1988). SCH and ETI (Research Biochemi- for the Care and Use of Laboratory Animals (National Institutes of cals International, Natick, MA) were dissolved in saline immediately Health, Publication 865–23) and were approved by the Indiana Univer- before use and injected subcutaneously. Control animals received saline. sity Bloomington Institutional Animal Care and Use Committee. In most cases, the injections were performed during neuronal recording Rats were anesthetized with chloropent (0.33 ml/100 gm, i.p.), to permit predrug and postdrug comparisons of drug-induced changes in mounted in a stereotaxic apparatus, and prepared for subsequent single- impulse activity and responsiveness to DA and GLU. Further recordings unit recording as described in detail previously (Kiyatkin and Rebec, were made from stochastically sampled units tested at different times (up 1996). For recording in the striatum, a hole was drilled unilaterally to 180 min) after drug injection. The relatively long period of postinjec- through the skull (1.2–1.4 mm anterior and 2.0 mm lateral to bregma), tion neuronal sampling permitted a rough estimation of the time course and the dura was carefully retracted. A plastic, cylindrical hub, designed of drug action. to mate with the microelectrode holder on the recording day, was Histology. After completion of the last recording session, rats were centered over the hole and secured with dental cement to three stainless deeply anesthetized with chloropent, and an epoxy-insulated tungsten steel screws threaded into the skull. One screw served as both electrical electrode, inserted into one barrel of the pipette, was lowered into the ground and attachment for the head-mounted preamplifier. The hub was recording area. Current was passed through the electrode (50 mA for sealed with silicone rubber to prevent drying of the brain surface. After 15–20 sec) to produce microlesions at depths of 5.0 and 7.0 mm below the a 3–4 d recovery period, during which each animal was habituated to the skull surface. Rats were then perfused transcardially with formosaline, recording chamber for a total of 4–6 hr, the recording session began and and the brain was removed and stored for subsequent histological pro- resumed on each of the next 2–5 d. During the recovery period, most cessing. Brain tissue was frozen, cut (50 mm sections), mounted on slides, animals were also habituated to the injection procedure, in which saline and stained with cresyl violet. The location of each recording site was was injected subcutaneously once daily to the lower back area.