Dopamine Terminal Density Research Articles

Evaluation of Drug Concentrations Delivered by Microiontophoresis.

Microiontophoresis uses an electric current to eject a drug solution from a glass capillary and is often utilized for targeted delivery in neurochemical investigations. The amount of drug ejected, and its effective concentration at the tip, has historically been difficult to determine, which has precluded its use in quantitative studies. To address this, a method called controlled iontophoresis was developed which employs a carbon-fiber microelectrode incorporated into a multibarreled iontophoretic probe to detect the ejection of electroactive species. Here, we evaluate the accuracy of this method. To do this, we eject different concentrations of quinpirole, a D2 receptor agonist, into a brain slice containing the dorsal striatum, a brain region with a high density of dopamine terminals. Local electrical stimulation was used to evoke dopamine release, and inhibitory actions of quinpirole on this release were examined. The amount of drug ejected was estimated by detection of a coejected electrochemical marker. Dose response curves generated in this manner were compared to curves generated by conventional perfusion of quinpirole through the slice. We find several experimental conditions must be optimized for accurate results. First, selection of a marker with an identical charge was necessary to mimic the ejection of the cationic agonist. Next, evoked responses were more precise following longer periods between the end of the ejection and stimulation. Lastly, the accuracy of concentration evaluations was improved by longer ejections. Incorporation of these factors into existing protocols allows for greater certainty of concentrations delivered by controlled iontophoresis.

Chronic cocaine administration reduces striatal dopamine terminal density and striatal dopamine release which leads to drug-seeking behaviour

Drug addiction is associated with altered dopamine (DA) neurotransmission in the basal ganglia. We have previously shown that chronic stimulation of the dopamine D2 receptor (D 2R) with cocaine results in reduced striatal DA terminal density. The aims of this study were to establish whether this reduction in DA terminal density results in reduced striatal DA release and increased cocaine-seeking behaviour and whether D 2R antagonism can restore the cocaine-induced alterations in DA neurotransmission and drug-seeking behaviour. Rats were housed individually and either control, cocaine, haloperidol (D 2R antagonist), or cocaine and haloperidol was administered in the drinking water for 16 weeks. Chronic cocaine treatment, which reduced striatal DA terminal density by 20%, resulted in a reduction in basal (−34%) and cocaine-evoked (−33%) striatal DA release and increased cocaine-seeking behaviour. These cocaine-mediated effects on striatal DA terminal density, DA release and drug-seeking could be prevented by co-administration with haloperidol. Basal and cocaine-evoked DA release in the striatum directly correlated with DA terminal density and with preference for cocaine. We conclude that striatal DA terminal density and DA release is an important factor in maintaining drug preference and should be considered as a factor in drug-seeking behaviour and relapse.

Sprouting of dopamine terminals and altered dopamine release and uptake in Parkinsonian dyskinaesia

Failed storage capacity, leading to pulsatile delivery of dopamine (DA) in the striatum, is used to explain the emergence of 'wearing off' and dyskinaesia in Parkinson's disease. In this study, we show that surviving DA neurons in 6-OHDA lesioned rats sprout to re-innervate the striatum, and maintain terminal density until approximately 60% of neurons are lost. We demonstrate that DA terminal density correlates with baseline striatal DA concentration ([DA]). Electrochemical and synaptosome studies in 6-OHDA lesioned rats and primates suggest that impaired striatal DA re-uptake and increased DA release from medial forebrain bundle fibres contribute to maintaining striatal DA levels. In lesioned rats where terminal density fell by 60% or more, L-DOPA administration increased striatal DA levels markedly. The striatal [DA] produced by L-DOPA directly correlated with the extent of dyskinaesia, suggesting that dyskinaesia was related to high striatal [DA]. While sprouting and decreased dopamine uptake transporter function would be expected to contribute to the marked increase in L-DOPA induced [DA], the increased [DA] was most marked when DAergic fibres were >60% denervated, suggesting that other release sites, such as serotonergic fibres might be contributing. In conclusion, the extent of dyskinaesia was directly proportional to the extent of DA terminal denervation and levels of extra-synaptic striatal DA. We propose that sprouting of DA terminals and decreased dopamine uptake transporter function prevent the appearance of Parkinsonian symptoms until about 60% loss of nigral neurons, but also contribute to dysregulated striatal DA release that is responsible for the emergence of dyskinaesia and 'wearing off'.

Striatal Contributions to Reward and Decision Making

The striatum is the major input nucleus of the basal ganglia. It is thought to play a key role in learning on the basis of positive reinforcement and in action selection. One view of the striatum conceives it as comprising a reiterated matrix of processing units that perform common operations in different striatal regions, namely synaptic plasticity according to a three-factor rule, and lateral inhibition. These operations are required for reinforcement learning and selection of previously reinforced actions. Analysis of the behavioral effects of circumscribed lesions of the striatum, however, suggests regional specialization of learning and decision-making operations. We consider how a basic processing unit may be modified by regional variations in neurochemical parameters, for example, by the gradient in density of dopamine terminals from dorsal to ventral striatum. These variations suggest subtle differences between dorsolateral and ventromedial striatal regions in the temporal properties of dopamine signaling, which are superimposed on regional differences in connectivity. We propose that these variations make sense in relation to the temporal structure of activity in striatal inputs from different regions, and the requirements of different learning operations. Dorsolateral striatal (DLS) regions may be subject to brief, precisely timed pulses of dopamine, whereas ventromedial striatal regions integrate dopamine signals over a longer time course. These differences may be important for understanding regional variations in the contribution to reinforcement of habits, versus incentive processes that are sensitive to the value of expected rewards.

Alterations in expression of dopamine receptors and neuropeptides in the striatum of GTP cyclohydrolase-deficient mice

The hph-1 mice have defective tetrahydrobiopterin biosynthesis and share many neurochemical similarities with l-dopa-responsive dystonia (DRD) in humans. In both, there are deficiencies in GTP cyclohydrolase I and low brain levels of dopamine (DA). Striatal tyrosine hydroxylase (TH) levels are decreased while the number of DA neurones in substantia nigra (SN) appears normal. The hph-1 mouse is therefore a useful model in which to investigate the biochemical mechanisms underlying dystonia in DRD. In the present study, the density of striatal DA terminals and DA receptors and the expression of D-1, D-2, and D-3 receptors, preproenkephalin (PPE-A), preprotachykinin (PPT), and nitric oxide synthase (NOS) mRNAs in the striatum and nucleus accumbens and nigral TH mRNA expression were examined. Striatal DA terminal density as judged by specific [3H]mazindol binding was not altered while the levels of TH mRNA were elevated in the SN of hph-1 mice compared to control (C57BL) mice. Total and subregional analysis of the striatum and nucleus accumbens showed that D-2 receptor ([3H]spiperone) binding density was increased while D-1 receptor ([3H]SCH 23390) and D-3 receptor ([3H]7-OH-DPAT) binding density was not altered. In the striatum and nucleus accumbens, expression of PPT mRNA was elevated but PPE-A mRNA, D-1, D-2 receptor, and nNOS mRNA were not changed in hph-1 mice compared to controls. These findings suggest that an imbalance between the direct strionigral and indirect striopallidal output pathways may be relevant to the genesis of DRD. However, the pattern of changes observed is not that expected as a result of striatal dopamine deficiency and suggests that other effects of GTP cyclohydrolase I deficiency may be involved.

11C]DTBZ-PET correlates of levodopa responses in asymmetric Parkinson's disease.

Levodopa effectively improves motor symptoms of Parkinson's disease. However, the beneficial effects of levodopa often erode over time with the emergence of response fluctuations. Although these response changes have been recognized from the early levodopa era, their mechanisms remain poorly understood. We investigated the role of dopamine (DA) terminal loss in the development of motor fluctuations by employing PET with [11C](+/-)dihydrotetrabenazine ([11C]DTBZ) as an in vivo marker for DA nerve terminals. Levodopa response was characterized by analysing the time-response curve to a single dose of levodopa with a finger-tapping test. PET scans were performed in 11 patients with asymmetric Parkinson's disease (age: 61.12 +/- 7.97 years; duration of Parkinson's disease: 10.55 +/- 4.53 years; mean +/- SD). Each patient performed finger-tapping tests for up to 5 h after taking a therapeutic dose of levodopa. Results showed significantly lower [11C]DTBZ binding potential (BP; Bmax/Kd) and baseline tapping rates on the more affected putamen and corresponding body side, respectively, than on the other (P = 0.003 for the former, P = 0.013 for the latter). Among the variables describing the time-response curve, the duration and early decay time were significantly shorter on the more affected side (P = 0.051 and P = 0.021, respectively). Latency to the onset and latency to 50% Emax (the magnitude of the levodopa response) were significantly longer on the more-affected side (P = 0.013 and P = 0.004, respectively). Emax was not significantly different between the two sides. The asymmetry (difference from the more affected to less affected side) of [11C]DTBZ BP in the putamen showed a highly significant correlation with the corresponding asymmetry of the estimated EC50 (levodopa concentration producing 50% of the maximal response; P = 0.022; r = -0.727), a marginally significant correlation with that of latency to the onset (P = 0.065; r = -0.583) and no significant correlation with that of the magnitude, duration or early decay time. This pattern of changes in levodopa response from the less affected to more affected side was similar to that from stable to fluctuating responders except for the latency to onset. These findings suggest a pathogenetic role for DA terminal loss in the development of motor fluctuations. However, the absence of a significant correlation between the early decay of levodopa response and DA terminal density suggests that DA terminal loss alone cannot account for the development of motor fluctuations. Therefore, our study suggests that both levodopa treatment and DA terminal loss contribute to the pathogenesis of motor fluctuations.

The Role of Interleukin-1, Interleukin-6, and Glia in Inducing Growth of Neuronal Terminal Arbors in Mice

After injury to the substantia nigra pars compacta (SNpc), remaining neurons sprout to ensure normal dopamine delivery to the striatum. The consequent striatal reinnervation is highly regulated, with remaining cells sprouting so that density of dopamine terminals returns to normal. Sprouting as a result of injury is accompanied by a strong glial response; however, it is difficult to know whether this response is as a result of the injury or whether it is aiding in the sprouting. The two cytokines interleukin-1 (IL-1) and interleukin-6 (IL-6) are important modulators of the glia response. This study demonstrates their role in regulating the sprouting of dopaminergic neurons and the associated glia response as a means to examine the role of glia in sprouting. Sprouting was induced by 6-hydroxydopamine lesions of the SNpc and by haloperidol treatment (in the absence of injury). In wild-type animals, sprouting in association with microglial and astrocyte proliferation followed partial lesions of the SNpc and haloperidol treatment. Neither treatment evoked sprouting or glia proliferation in the type I IL-1 receptor-deficient mice, whereas in IL-6-deficient mice, both treatments resulted in glial proliferation but not sprouting. We conclude that IL-1 plays a role in modulating glia proliferation and thereby guidance and trophic factors for new fibers, whereas IL-6 may be important in triggering the outgrowth of new fibers. This study demonstrates that these cytokines play an important role in plasticity and regeneration that is separate from the inflammatory response associated with brain injury.

Effects of long-term treatment with dopamine receptor agonists and antagonists on terminal arbor size.

This study demonstrates that pharmacological manipulation of the dopamine (DA) receptors can modulate the size of the axonal tree of substantia nigra pars compacta (SNpc) neurons in mice. Pharmacological blockade or genetic ablation of the D2 receptor (D2R) resulted in sprouting of DA SNpc neurons whereas treatment with a D2 agonist resulted in pruning of the terminal arbor of these neurons. Agents such as cocaine, that indirectly stimulate D2R, also resulted in reduced terminal arbor. Specific D1 agonists or antagonists had no effect on the density of DA terminals in the striatum. We conclude that the D2 receptor has a central role in regulating the size of the terminal arbor of nigrostriatal neurons. These findings have implications relating to the use of dopaminergic agonists in the management of Parkinson's disease and in controlling plasticity following injury, loss or transplantation of DA neurons.

Timing: A critical determinant of the functional consequences of neonatal 6-OHDA lesions.

Previous data have indicated that intrastriatal (IS) lesions of the dopamine (DA) system early in development result in a selective effect on D1 receptor expression and sensitivity, which is not seen with adult lesions or lesions made later in development. The purpose of the present study was to test the hypothesis that the timing of the lesion is a critical determinant of the consequences of DA depletion during development. Rats received IS injections of 6-hydroxydopamine (6-OHDA) on day of birth/postnatal day 1 (P0/1) or P7, which resulted in similar decreases in the number of DA uptake sites (> or =70% loss), a measure of DA terminal density. As adults, lesioned rats were challenged with DA receptor agonists to examine the functional sensitivity of D1 and D2 receptors. In adulthood, P0/1-lesioned rats exhibited increases in oral dyskinesias and rearing behavior following treatment with the partial D1 receptor agonists, SKF38393 and SKF77434, whereas rats lesioned on P7 exhibited increases in grooming. P7-lesioned rats also exhibited increases in gnawing, explosive jumping, and self-biting behavior following treatment with the full D1 receptor agonist SKF82958, which were not observed in the other groups. The results support the hypothesis that the timing of DA denervation is of paramount importance for governing the functional consequences of neonatal lesions, as measured by the incidence of DA agonist-induced behaviors in adulthood.

On the distribution patterns of D 1, D 2, tyrosine hydroxylase and dopamine transporter immunoreactivities in the ventral striatum of the rat

The distribution of dopamine D 1 and D 2 receptor immunoreactivities in the nucleus accumbens and the olfactory tubercle of adult and postnatal male rats were compared with the distribution of tyrosine hydroxylase and dopamine transporter immunoreactivities. An overall co-distribution of D 1 and D 2 receptor immunoreactivities with tyrosine hydroxylase immunoreactivity was found in the nucleus accumbens and the olfactory tubercle. However, the major finding in this study was, following a more detailed analysis in coronal sections of the shell part of the nucleus accumbens, the existence of nerve cell patches of strong D 1 receptor immunoreactivity associated with low D 2 receptor, dopamine transporter and tyrosine hydroxylase immunoreactivities. These patches were mainly surrounded by areas of strong D 2 receptor, tyrosine hydroxylase and dopamine transporter immunoreactivities and could be found also in the olfactory tubercle. Similar observations were made in postnatal rats. Serial reconstructions of the patches of strong D 1 receptor immunoreactivity in the rostrocaudal direction were made. The patches formed a continuous tubular nerve cell system in the shell part of the nucleus accumbens. Since this nerve cell system was found to be surrounded by a high density of dopamine terminals, it may represent a compartment where dopamine transmission mainly acts on D 1 receptors via local diffusion (i.e. via volume transmission). However, it must be noted that the D 1 receptor rich patches constitute only a small fraction of the nucleus accumbens and the overall density of tyrosine hydroxylase immunoreactive terminals correlates with the density of both D 1 and D 2 receptors in the nucleus accumbens. In conclusion, the present paper gives new aspects on the chemical microarchitecture of the nucleus accumbens.

Paraquat elicited neurobehavioral syndrome caused by dopaminergic neuron loss

The herbicide paraquat, bearing structural similarity to the known dopaminergic neurotoxicant MPTP, has been suggested as a potential etiologic factor in Parkinson's disease. Consideration of paraquat as a candidate neurotoxicant requires demonstration that systemic delivery produces substantia nigra dopaminergic neuron loss and the attendant neurobehavioral syndrome reflecting depletion of dopamine terminals within the striatum. To address these issues paraquat was administered systemically into adult C57 bl/6 mice, ambulatory behavior monitored, substantia nigra dopamine neuron number and striatal dopamine terminal density quantified. The data indicate that paraquat like MPTP elicits a dose-dependent decrease in substantia nigra dopaminergic neurons assessed by a Fluoro-gold prelabeling method, a decline in striatal dopamine nerve terminal density assessed by measurement of tyrosine hydroxylase immunoreactivity; and neurobehavioral syndrome characterized by reduced ambulatory activity. Taken together, these data suggest that systemically absorbed paraquat crosses the blood–brain barrier to cause destruction of dopamine neurons in the substantia nigra, consequent reduction of dopaminergic innervation of the striatum and a neurobehavioral syndrome similar to the well characterized and bona fide dopaminergic toxin MPTP.

Effects of gestational exposure to monoamine oxidase inhibitors in rats: preliminary behavioral and neurochemical studies.

Monoamine neurotransmitters are important in the development of the immature mammalian brain, prior to assuming their role as neurotransmitters. The endogenous levels of these transmitters are highly regulated by the enzyme monoamine oxidase (MAO). Thus, any change in this enzyme should have a profound effect on brain development. In order to test this hypothesis, we treated developing rat pups with the monoamine oxidase inhibitors (MAO-Is), clorgyline (MAO-A, 3 mg/kg), and deprenyl (MAO-B, 3 mg/kg) throughout gestation (MAO-I-birth), or throughout gestation and to sacrifice (MAO-I-sac). The animals were analyzed for serotonin and dopamine terminal density, using 3H-paroxetine and 3H-GBR 12935, respectively. Whereas there were no changes in the development of the dopamine system, the serotonin system was severely affected, particularly in the cortex that showed a significant reduction of innervation at 30 days postnatal. The animals reached all normal development milestones on schedule, and had no changes in measures of anxiety (% light/dark); however, the animals showed increased open field activity and deficits in a passive avoidance paradigm, which may be a measure of impulsivity. The MAO-I-sac animals were severely impaired, showing stereotypic behavior, seizures, and eventually visual impairments. Our results are discussed in terms of relevance to human disease states, such as atypical Norrie's disease, impulsivity, and hyperactivity. As well, our results should be used to caution against the use of MAO-Is in women of child-bearing age.

Rostral-caudal differences in effects of nucleus accumbens amphetamine on VTA ICSS

The effects of amphetamine along the rostrocaudal axis of the nucleus accumbens (NAcc) on ventral tegmental area (VTA) intracranial self-stimulation (ICSS) were studied. Eighteen rats were trained to lever press for ICSS in the VTA. Rate-frequency functions were determined by logarithmically decreasing the frequency of cathodal pulses in a stimulation train from a value that induced maximal responding to one that induced no responding (thresholds). After ICSS thresholds stabilized, (+)-amphetamine (20.0 μg/0.5 μl) or its vehicle, distilled H 2O (0.5 μl), were injected directly into the rostral NAcc ( n = 6) or the caudal NAcc ( n = 8) or the caudate-putamen (CP) ( n = 5) just dorsal to the caudal NAcc. Results showed that amphetamine in the caudal NAcc significantly decreased ICSS thresholds without affecting asymptotic rates of responding, indicating a potentiation of the rewarding efficacy of VTA stimulation. Amphetamine in the rostral NAcc or CP produced smaller, non-significant, decreases in ICSS thresholds. Further analyses revealed a significant positive correlation ( r 13 = 0.51, P < 0.05) between the site of injection along the rostrocaudal axis of the NAcc and the size of the amphetamine-produced potentiation of VTA stimulation reward. Others have reported topographical differences, including dopamine terminal density and D 1 receptor density, in the NAcc. The present results indicate that these anatomical and neurochemical differences appear to be correlated with behavioural differences.

Dopamine uptake in five structures of the brain: Comparison of rate, sodium dependence and sensitivity to cocaine

The rate of uptake of dopamine (DA) per μg protein and the sensitivity of uptake to sodium ([Na] o) and cocaine, have been measured in synaptosomes from five structures in the brain of the rat: striatum, nucleus accumbens, neocortex, limbic cortex and thalamus. Probably reflecting the number of DA terminals, there was a wide variation in the rate of uptake in the different structures: uptake was far greater in the striatum and nucleus accumbens (10–20-fold) than in the neocortex, limbic cortex or thalamus. Uptake in all structures was inhibited by cocaine. With high [Na] o, the IC 50's varied from 1.10 μM for the thalamus to 3.32 μM for the nucleus accumbens. Maximum percentage inhibition varied from 76.0 for limbic cortex to 96.8 for nucleus accumbens and 97.7 for striatum; thus most uptake was carried-mediated. With low [Na] o, IC 50 for the nucleus accumbens was unchanged, while the IC 50's for the striatum and limbic cortex were less. Maximum percentage inhibition was similar to that found for high [Na] o. Although the sensitivity to [Na] varied, synaptosomes from all areas showed Na-dependent uptake; lowering [Na] in the incubation medium from 133.2 to 9.5 mM reduced the uptake by a minimum of 36.9% in the neocortex to a maximum of 78.0% in striatum. Neither the rate of uptake nor the Na-dependence correlated precisely with the sensitivity to cocaine but the two structures which showed the greatest rate of uptake (nucleus accumbens and striatum) also showed the greatest sensitivity to [Na] o. Generally, the rate of uptake correlated with the density of DA terminals and number of cocaine binding sites as reported by others. The results are consistent with the conclusion that the nucleus accumbens and striatum are particularly important in the cocaine abuse syndrome because of two factors: (1) the high rate of uptake as a result of the high density of DA terminals predisposes the functions of these structures to a greater impact by cocaine; (2) as a consequence, the overall cocaine abuse syndrome depends to a great extent on the specific functions that these structures subserve.

Neonatal 6-OHDA lesions differentially affect striatal D1 and D2 receptors.

Lesions to the dopamine (DA) system in early postnatal development have different behavioral consequences compared to lesions made in adulthood. Intrastriatal injections of the neurotoxin 6-hydroxydopamine (6-OHDA) on the day of birth (PO) or postnatal day 1 (P1) produce a selective supersensitivity to D1 receptor agonists and a subsensitivity to D1 antagonists (Neal and Joyce, 1991a). In this paper, we describe the long-term effects of early DA loss on DA receptor regulation. Pups received bilateral intrastriatal injections of the neurotoxin 6-OHDA (4 micrograms per striatum) on PO or P1. Adult rats were killed at 90 days of age and the brains were processed for quantitative autoradiography (QAR) or tyrosine hydroxylase (TH) immunocytochemistry. Cohorts were tested for the behavioral responses to the selective D1 receptor agonist SKF38393 (10 mg/kg). Neonatally lesioned rats exhibited increases in abnormal perioral movements in response to D1 receptor stimulation. There was a heterogenous and patchy loss (40-50%) of [3H]mazindol binding to high-affinity DA uptake sites (a marker of DA terminal density) and a similar loss of TH-like immunoreactivity within the striata of the neonatally lesioned rats. There was also a reduction in the number of mu-opioid receptor patches (labelled with [3H]naloxone), a marker for the striatal patch compartment, and a similar patchy loss of D1 binding sites (labeled with [3H]SCH23390). The binding of [3H]spiroperidol to D2 sites was not altered. This is in contrast to the changes observed following adult 6-OHDA lesions, wherein there is a significant increase in the number of D2 binding sites (Joyce, 1991a,b). The results are discussed with respect to the behavioral consequences of neonatal lesions and the differences between neonatal and adult lesions.

Dopamine D 1 receptor behavioral responsitivity following selective lesions of the striatal patch compartment during development

The behavioral effects of selective destruction of the dopamine (DA) input to the patch compartment of rat striatum early in development was investigated. Rat pups were given bilateral intrastriatal (i.s.) injections of the neurotoxin 6-hydroxydopamine (6-OHDA) on day of birth (P0) or postnatal day 1 (P1), which resulted in selective behavioral alterations following DA agonist treatment in adulthood. Neonatally-lesioned rats exhibited self-biting behavior following treatment with the DA precursor l-dihydroxyphenylalanine ( l-DOPA). In response to treatment with the selective D 1 agonist SKF38393, there was an increased incidence of abnormal perioral movements. The cataleptogenic effects of the D 1 antagonist SCH23390 and the D 2 antagonist haloperidol were also studied. Neonatally-lesioned rats were significantly less cataleptic compared to control rats following D 1 antagonist treatment, but not following D 2 antagonist treatment. Autoradiographs of [ 3H]mazindol binding to DA uptake sites (a measure of DA terminal density) showed a ‘patchy’ loss of approx. 40–50% in striatal tissue sections derived from the i.s. lesioned rats. These data suggest that injections of 6-OHDA into the striatum during this early postnatal period cause a DA lesion that results in long-term effects on a D 1 receptor system.

Postnatal development of striatal dopamine function. II. Effects of neonatal 6-hydroxydopamine treatments on D 1 and D 2 receptors, adenylate cyclase activity and presynaptic dopamine function

To evaluate the influence of patch and matrix ingrowth of DA terminals upon striatal DA (dopamine) receptor function, we performed bilateral intrastriatal (i.s.) or single intracisternal (i.c.) injections of 6-hydroxydopamine (6-OHDA) into rat pups at various postnatal ages and determined D 1 and D 2 receptor binding, adenylate cyclase activities and markers for presynaptic DA terminal density and turnover as the animals matured. All injection schedules yielded: (a) variable and partial loss of DA, (b) increased DA turnover, (c) small (15–40%) increases in D 1 receptor number but no change in affinity for antagonist ([ 3H]SCH 23390), (d) 2–3-fold increases in affinity of D 1 receptors for agonist (SKF 38393) with preserved regulation of agonist affinity by guanine nucleotide, (e) no significant changes in DA-, guanine-nucleotide-, manganese- and forskolin-stimulated AC (adenylate cyclase) activity. D 2 receptor binding was evaluated between 1 and 7 weeks of age in animals with i.s. treatment and 7 and 10 weeks of age in animals with i.c. treatment and was reduced by 40–50% with both treatment regimens. [ 3H]mazindol binding, a marker for presynaptic terminal DA transport sites, was reduced 30–40% by multiple i.s. or i.c. treatment regimens. In animals treated with one i.s. injection, [ 3H]mazindol binding was reduced 70% at 1 week of age, equal to control by 2 weeks and 14–46% greater than control between 3 and 7 weeks. We conclude that striatal D 1 receptor sites maintain their density and second messenger function independently of postsynaptic DA terminal ingrowth, whereas the development of D 2 receptor sites is sensitive to disruptions of DA terminal ingrowth.

Brain stimulation reward and dopamine terminal fields. II. Septal and cortical projections

The boundaries and relative sensitivities of the substrates of septal and cortical brain stimulation reward were mapped in relation to the dopamine terminal fields in these regions using a dorsal-ventral moveable electrode. Brain stimulation was rewarding at all levels of the posterior lateral septum and not just in the region of dopamine terminal innervation. Reward thresholds, ease of training, maximum response rates and stability of responding were all unrelated to the proximity of the stimulating electrode to the band of dopamine terminals revealed by glyoxylic acid-induced dopamine fluorescence. Stimulation was also rewarding in the interior lateral septum; the best sites were in the ventral portions of this region although dopamine terminal fluorescence was uniform throughout. Thus the anatomy of the brain stimulation reward substrate of the lateral septal nucleus does not bear a special relation to the anatomy of dopamine terminals within this region. Stimulation was also rewarding in each of the dopamine terminal fields of the cerebral cortex. The best self-stimulation was obtained with electrodes in the medial frontal cortex; sulcal frontal cortex was next best, entorhinal cortex was next, and pyriform cortex, though reliably positive, supported the weakest self-stimulation. Variations in self-stimulation threshold were seen as electrodes were moved through homogenous regions of dopamine terminal density in some regions, while stable thresholds were associated with movements through areas of varying dopamine terminal density in others; thus, again, there was no special relation between goodness of self-stimulation and density of dopaminergic innervation. These data suggest that rewarding brain stimulation in these regions is not due to direct activation of either the dopaminergic terminals or the cells that they innervate.

Brain stimulation reward and dopamine terminal fields. I. Caudate-putamen, nucleus accumbens and amygdala

The boundaries and relative sensitivity of brain stimulation reward were mapped in relation to the dopamine (DA) terminal fields of the striatum and adjacent limbic structures. Brain stimulation was rewarding throughout the caudate and nucleus accumbens and in portions of the amygdala and olfactory tubercle. The best striatal sites were anterior, ventral and medial; this correlated with an anterior-posterior gradient but not with a dorsal-ventral or a medial-lateral gradient of DA terminal density. No close correspondence was seen between the boundaries of the reward system and those of the DA terminal fields as revealed by glyoxylic acid-induced DA fluorescence. Reward sites in the olfactory tubercle and amygdala were found in DA-free as well as DA-rich regions of these structures; stimulation in DA-rich regions did not always support self-stimulation. These data go against the view that direct activation of dopamine terminals or their efferent targets accounts for the rewarding quality of stimulation in these regions.