Impaired ontogeny of striatal dopamine D1 and D2 binding sites after postnatal treatment of rats with SCH-23390 and spiroperidol

Dcc haploinsufficiency regulates dopamine receptor expression across postnatal lifespan

Adolescence is a period during which the medial prefrontal cortex (mPFC) undergoes significant remodeling. The netrin-1 receptor, deleted in colorectal cancer (DCC), controls the extent and organization of mPFC dopamine connectivity during adolescence and in turn directs mPFC functional and structural maturation. Dcc haploinsufficiency leads to increased mPFC dopamine input, which causes improved cognitive processing and resilience to behavioral effects of stimulant drugs of abuse. Here we examine the effects of Dcc haploinsufficiency on the dynamic expression of dopamine receptors in forebrain targets of C57BL6 mice. We conducted quantitative receptor autoradiography experiments with [³H]SCH-23390 or [³H]raclopride to characterize D1 and D2 receptor expression in mPFC and striatal regions in male Dcc haploinsufficient and wild-type mice. We generated autoradiograms at early adolescence (PND21±1), mid-adolescence (PND35±2), and adulthood (PND75±15). C57BL6 mice exhibit overexpression and pruning of D1, but not D2, receptors in striatal regions, and a lack of dopamine receptor pruning in the mPFC. We observed age- and region-specific differences in D1 and D2 receptor density between Dcc haploinsufficient and wild-type mice. Notably, neither group shows the typical pattern of mPFC dopamine receptor pruning in adolescence, but adult haploinsufficient mice show increased D2 receptor density in the mPFC. These results show that DCC receptors contribute to the dynamic refinement of D1 and D2 receptor expression in striatal regions across adolescence. The age-dependent expression of dopamine receptor in C57BL6 mice shows marked differences from previous characterizations in rats.

Effects of repeated and acute aripiprazole or haloperidol treatment on dopamine synthesis in the dorsal striatum of young rats: comparison to adult rats

The purpose of the present study was to determine whether repeated treatment with the D2 partial agonist aripiprazole or the D2 antagonist haloperidol alters dopamine (DA) synthesis characteristics in the dorsal striatum of young rats. To this end, rats received a daily pretreatment regimen of aripiprazole or haloperidol on postnatal days (PD) 10-20 and were tested 24 or 72 h later after an acute injection of vehicle, aripiprazole, haloperidol, or quinpirole (a D2 agonist). For comparison purposes, adult rats were pretreated with an 11-day regimen of saline or haloperidol on PD 70-80 and DA synthesis was measured after acute drug treatment on PD 83. Dorsal striatal DA synthesis was determined by measuring L-dihydroxyphenylalanine accumulation after NSD-1015 treatment. In a separate experiment, the ability of repeated drug treatment to up-regulate dorsal striatal D2 receptors was assessed in young and adult rats 72 h after drug discontinuation. The major findings of this study were that: (a) acute treatment with haloperidol and aripiprazole increased DA synthesis while quinpirole reduced it; (b) pretreatment with haloperidol and aripiprazole blunted the synthesis-modulating effects of acutely administered dopaminergic drugs; and (c) DA synthesis of young and adult rats was affected in a qualitatively similar manner by DA agonist, antagonist, and partial agonist drugs. In conclusion, results from the present study suggest that synthesis-modulating autoreceptors in the dorsal striatum are functionally mature by the end of the preweanling period and DA synthesis declines to near basal levels during the course of repeated aripiprazole treatment.

D2 antagonist during development decreases anxiety and infanticidal behavior in adult female prairie voles (Microtus ochrogaster)

On postnatal day 8, prairie vole pups were randomly assigned a treatment of 1mg/kg SKF38393 (D1 agonist), quinpirole (D2 agonist), SCH23390 (D1 antagonist), eticlopride (D2 antagonist), or saline vehicle. As adults, females treated with eticlopride exhibited reduced anxiety-like behavior in an elevated plus maze and a reduction in infanticidal behavior. These behavioral effects were not seen in males. These data demonstrate that a single exposure to a D2 antagonist during development can have persistent, sex-specific effects on behavior into adulthood.

A comparison of the binding profiles of dextromethorphan, memantine, fluoxetine and amitriptyline: Treatment of involuntary emotional expression disorder

We compared the binding profiles of medications potentially useful in the treatment of involuntary emotional expression disorder at twenty-six binding sites in rat brain tissue membranes. Sites were chosen based on likelihood of being target sites for the mechanism of action of the agents in treating the disorder or their likelihood in producing side effects experienced by patients treated with psychoactive agents. We used radioligand binding assays employing the most selective labeled ligands available for sites of interest. Concentrations of labeled ligand were used at or below the K(i) value of the ligand for the target site. Compounds were initially screened at 1 muM. For compounds that competed for greater than 20-30% of specific binding at target sites of interest, full concentration curves were constructed. Dextromethorphan, amitriptyline and fluoxetine competed for binding to sigma(1) receptors and to serotonin transporters with high to moderate affinity. Of the target sites tested, these are the most likely to contribute to the therapeutic benefit of the various agents. In addition, all three drugs showed some activity at alpha(2) and 5-HT(1B/D) sites. Of the drugs tested, dextromethorphan bound to the fewest sites unlikely to be target sites. Although the mechanism of action of dextromethorphan or any drug that has been used in the treatment of involuntary emotional expression disorder is currently unknown, our data support that the affinity of the drug for sigma(1) receptors is consistent with its possible action through this receptor type in controlling symptoms of the disorder.

Trajectories of brain development: point of vulnerability or window of opportunity?

Brain development is a remarkable process. Progenitor cells are born, differentiate, and migrate to their final locations. Axons and dendrites branch and form important synaptic connections that set the stage for encoding information potentially for the rest of life. In the mammalian brain, synapses and receptors within most regions are overproduced and eliminated by as much as 50% during two phases of life: immediately before birth and during the transitions from childhood, adolescence, to adulthood. This process results in different critical and sensitive periods of brain development. Since Hebb (1949) first postulated that the strengthening of synaptic elements occurs through functional validation, researchers have applied this approach to understanding the sculpting of the immature brain. In this manner, the brain becomes wired to match the needs of the environment. Extensions of this hypothesis posit that exposure to both positive and negative elements before adolescence can imprint on the final adult topography in a manner that differs from exposure to the same elements after adolescence. This review endeavors to provide an overview of key components of mammalian brain development while simultaneously providing a framework for how perturbations during these changes uniquely impinge on the final outcome.

Comparative postnatal development of dopamine D(1), D(2) and D(4) receptors in rat forebrain

Postnatal development of dopamine D(1), D(2) and D(4) receptors in the caudate-putamen, nucleus accumbens, frontal cortex and hippocampus was assessed in rat brain between postnatal days 7 and 60. In the caudate-putamen and nucleus accumbens, density of all three receptor subtypes increased to a peak at postnatal day 28, then declined significantly in both regions (postnatal days 35-60) to adult levels. In the frontal cortex and hippocampus, these receptors rose steadily and continuously to stable, maximal adult levels by postnatal day 60. Evidently, D(1), D(2) and D(4) receptors follow a similar course of development in several cortical, limbic and extrapyramidal regions of rat forebrain, with selective elimination of excess dopamine receptors at the time of puberty in the caudate-putamen and accumbens but not other brain regions.

Cocaine-induced alterations in the density of monoaminergic receptors in the embryonic guinea pig cerebral wall

Quantitative receptor autoradiography was used to examine the effect of chronic cocaine exposure on the density of alpha1-, alpha2- and beta-adrenergic, 5-HT1A- and 5-HT2-serotonergic, and D1- and D2-dopaminergic receptors in the fetal guinea pig cerebral wall which contained forming motor area of the cerebral cortex. The pregnant guinea pig received two daily subcutaneous injections of 20 mg/kg cocaine beginning on the 20th day of pregnancy (E20). The control animals received injections of equivalent volume of saline. The receptor densities were examined between days 5-30 of the treatment, which corresponds to E25-E50. By the fifth day of treatment (E25), cocaine produced downregulation of all receptors studied throughout the entire depth of the fetal cerebral wall. More extended treatment, however, resulted in recovery of receptor levels. Finally, from days 20-30 of treatment (E40-E50) there was a significant upregulation of noradrenergic and dopaminergic receptor sites. These findings demonstrate that exposure to cocaine in utero can influence adrenergic, serotonergic, and dopaminergic receptors in the embryonic cerebral wall, which may lead to alteration in corticogenesis. Furthermore, the present study reveals that, in the course of chronic treatment, cocaine may completely reverse its receptor regulatory activity in the fetal brain.

Distribution and postnatal ontogeny of adenosine A2A receptors in rat brain: comparison with dopamine receptors

In adult rat brain, adenosine A2A receptors and dopamine D2 receptors are known to be located on the same cells where they interact in an antagonistic manner. In the present study we wanted to examine when this situation develops and compared the postnatal ontogeny of the binding of the adenosine A2A receptor agonist [3H]CGS 21680, the binding of the dopamine D1 receptor antagonist [3H]SCH 23390 and the dopamine D2 receptor antagonist [3H]raclopride. All three radioligands bound to the striatum at birth and this binding increased several-fold during the postnatal period. [3H]SCH 23390 binding developed first (mostly during the first week), followed by [3H]raclopride binding (first to third week) and [3H]CGS 21680 binding (only during second and third week). For all three radioligands the binding tended to decrease between 21 days and adulthood. This occurred earlier and was more pronounced in the globus pallidus than in the other examined structures. The increase in [3H]CGS 21680 binding from newborn to adult was mainly due to four-fold increase in the number of binding sites. The pharmacology of [3H]CGS 21680 binding to caudate-putamen was similar in newborn, one-week-old and adult animals, and was indicative of A2A receptors. The binding was inhibited by guanylyl imidodiphosphate at all ages, indicating that A2A receptors are G-protein-coupled already at birth. In contrast to the large increase in [3H]CGS 21680 binding, there was a decrease in the levels of A2A messenger RNA during the postnatal period in the caudate-putamen. In cerebral cortex [3H]CGS 21680 bound to a different site than the A2A receptor. From birth to adulthood cortical binding of [3H]CGS 21680 increased four-fold and that of the adenosine A1 agonist [3H]cyclohexyladenosine 19-fold. During early postnatal development [3H]SCH 23390 binding was higher in deep than in superficial cortical layers, but this difference disappeared in adult animals. There was binding of both [3H]CGS 21680 and [3H]cyclohexyladenosine to the olfactory bulb, suggesting a role of the two adenosine receptors in processing of olfactory information. [3H]CGS 21680 binding was present in the external plexiform layer and glomerular layer, and increased during development, but the density of binding sites was about one tenth of that seen in caudate putamen. [3H]cyclohexyladenosine showed a very different labelling pattern, resembling that observed with [3H]SCH 23390. Postnatal changes in adenosine receptors may explain age-dependent differences in stimulatory caffeine effects and endogenous protection against seizures. Since A2A receptors show a co-distribution with D2 receptors throughout development, caffeine may partly exert such actions by regulating the activity of D2 receptor-containing striatopallidal neurons.

Functional and molecular differentiation of the dopamine system induced by neonatal denervation

The administration of the neurotoxin 6-hydroxydopamine (6-OHDA) to damage the mesostriatal dopamine (DA) system in the neonate results in different neurochemical and behavioral consequences as compared to lesions made in adulthood. There have been few direct data to support the conclusion that the behavioral changes following neonatal 6-OHDA lesions reflect plasticity of the DA system. It is our hypothesis that the plasticity of the developing DA system is fundamentally different from that of the adult. Responses to 6-OHDA lesions can only be understood within the context of the status of the mesostriatal DA system at the time of the lesion. There are stages of development in the early postnatal period when certain components of the mesostriatal DA system are differentially sensitive to 6-OHDA lesions. These "windows" of vulnerability can be predicted from an analysis of the developmental expression of DA receptors and the maturation of the subpopulation of the mesostriatal DA system that innervates them. We review the differences in the behavioral plasticity of the adult and neonate sustaining 6-OHDA lesions to the mesostriatal DA system, the mechanisms responsible for the behavioral plasticity in the adult, and our conceptualization of which mechanisms are affected in the neonate.

The effects of cocaine on cerebral metabolic function in periweanling rats: the roles of serotonergic and dopaminergic uptake blockade

This report examines the short-term effects of cocaine exposure during postnatal (PoN) days 11-21 on the metabolic function of major central neuronal systems in the rat. It also examines the effects of inhibition of serotonin and dopamine uptake during this period of development. By comparing the effects of fluoxetine, a serotonin uptake inhibitor, and GBR12909, a dopamine uptake inhibitor, to the effects of cocaine, the contributions of these pharmacologic actions to the neurochemical effects of cocaine were determined. Four groups of rats were injected subcutaneously: cocaine 25 (mg/kg), fluoxetine (25/kg), GBR12909 (25 mg/kg) and vehicle-injected. On day 21 all received their final dose of drug or vehicle 20 minutes prior to the deoxyglucose procedure. Glucose utilization in 43 of 56 brain regions selected for analysis showed a main effect of treatment (P < or = 0.05, ANOVA) and 7 showed significant treatment X gender interactions. Females demonstrated a markedly greater sensitivity to the effects of cocaine than did the males. Both males and females showed a negligible response to fluoxetine treatment. In the female cocaine-treated group, 10 of 13 motor structures, 7 of 12 sensory structures, 10 of 24 limbic structures, 2 of 2 association areas, and 3 of 5 hypothalamic structures demonstrated significantly increased rates of glucose utilization compared to the vehicle-injected group (P < or = 1 =0.05, Dunnett test). In the cocaine-treated males, only 3 of 56 regions were affected. The gender differences in response to RBR12909 were less apparent. In the females, 11 regions showed increased rates of glucose utilization, while in the males 7 regions were stimulated. Fluoxetine produced the smallest overall effect with 2 structures showing increases in metabolism in the females and 2 structures showing decreases in metabolism in the males. The present study therefore suggests at 21 days of age, that inhibition of dopamine uptake makes a more significant contribution to the metabolic effects of cocaine than inhibition of serotonin uptake and that females are more sensitive to the effects of cocaine than males. Furthermore, the sexual dichotomy seen in the long-term effects of cocaine; females show the greater effect; is also seen at the time of drug administration.

Developmental and age-related changes in the D2 dopamine receptor mRNA subtypes in rat brain

The influence of ontogeny and aging on the D2 dopamine receptor mRNA in rat brain were examined using in situ hybridization histochemistry and Northern analysis utilizing oligonucleotide probes complementary to the different D2 mRNA subtypes. At birth, there was a high level of D2 dopamine receptor mRNA in corpus striatum relative to that found in the cerebral cortex and other brain areas. The hybridization signal of striatum (using a probe that hybridizes to both the D2A and D2B mRNA) increased during the first two postnatal weeks, reached a peak at day 16, then declined slightly. The D2A mRNA showed a similar distribution and developmental pattern. Intracisternal injection of 6-hydroxydopamine into neonates did not significantly alter the increase of the D2 dopamine receptor mRNAs, suggesting that neuronal input does not influence the ontogenetic development of this mRNA. In striatum, olfactory tubercule and inferior colliculus, the D2A mRNA declined between 3 and 24 months of age. By contrast, there was an age-related increase in the D2A mRNA in the anterior and intermediate lobes of the pituitary. The mRNA for the D2B dopamine receptor showed very low but nevertheless detectable levels in striatum, olfactory tubercule and pituitary. Like with the D2A mRNA, in 24-month-old rats the D2B mRNA declined in striatum and olfactory tubercule and increased in pituitary. These results show that there are differential tissue-related changes in the mRNAs for the D2 dopamine receptor during both development and aging.

A2a adenosine receptor gene expression in developing rat brain

Adenosine is a neuromodulator in the adult central nervous system. Membrane-bound receptors for adenosine have been identified and cDNAs for A1, A2a, A2b, and A3 adenosine receptor subtypes have been cloned recently. The present study documents the developmental appearance of A2a adenosine receptor gene expression in the rat brain. In situ hybridization using 35S-labeled RNA probes generated from the rat A2a adenosine receptor cDNA revealed receptor gene expression in the striatum on gestational day (GD) 14. Developmental alterations in the pattern of receptor gene expression within the striatum suggest that this receptor mRNA is expressed by striatal neurons soon after they complete neurogenesis. Transient expression of the A2a adenosine receptor mRNA was observed in cerebral cortex, subiculum, parafascicularis nucleus of the thalamus, facial nucleus, trigeminal nucleus, locus coeruleus, area postrema, anterior pituitary gland and in the fetal cerebral vasculature. The ganglia of cranial nerves V, VII, VIII, IX and X expressed A2a adenosine receptor mRNA in fetuses; adults have not been examined. A2a adenosine receptor mRNA was expressed in the carotid body and intermediate lobe of the pituitary during development and also in adult rats. Northern blot analysis revealed that the A2a adenosine receptor transcript is consistent in size (ca 2.5 kb) across the developmental period examined (GD 14 through adult). Previous studies in adult rats have reported that A2a adenosine receptor gene expression is limited to a population of striatal medium spiny neurons. This study documents early developmental expression of the A2a adenosine receptor gene in the striatum and its transient expression elsewhere in the brain and cerebral vasculature. If the A2a adenosine receptor mRNA is translated into receptor protein shortly after the mRNA is expressed, adenosine could influence neuronal differentiation, migration, synaptogenesis, and angiogenesis. Expression of A2a adenosine receptor mRNA in cranial ganglia, carotid body, and intermediate lobe of the pituitary gland similarly suggests novel sites of adenosine action during development and in the adult.

MIF-1 fails to modify agonist-induced oral activity in neonatal 6-OHDA-treated rats

L-Prolyl-L-leucyl-glycinamide (MIF-1) is known to attenuate apomorphine-induced stereotypes in adult rats that are lesioned as neonates with 6-hydroxydopamine (6-OHDA). To test whether MIF-1 would affect dopamine (DA) agonist-induced and serotonin (5-HT) agonist-induced oral activity, both intact and neonatal 6-OHDA-treated rats were studied. Rats at 3 days from birth were injected with desipramine (20 mg/kg, IP), 1 h before 6-OHDA HBr (100 micrograms, salt form, in each lateral ventricle) or its vehicle, saline-ascorbic acid (0.1%). At approximately 6 months rats were treated with MIF-1 (0.1, 1.0, or 10.0 mg/kg, IP), 10 min before SKF 38393 HCl (1.0 mg/kg, IP) or m-chlorophenylpiperazine 2HCl (m-CPP 2HCl; 0.5 mg/kg, IP), DA D1 and 5-HT1C,2 receptor agonists, respectively. Although both agonists increased oral activity in control and neonatal 6-OHDA-treated rats, MIF-1 did not modify the response. In rats that received either of the three doses of MIF-1 for 21 consecutive days, there was still no observed effect of MIF-1 on the oral response of control and 6-OHDA-lesioned rats to SKF 38393 and m-CPP. These findings indicate that MIF-1 does not modify the oral activity response of supersensitized D1 and 5-HT1C receptors in adult rats that are lesioned neonatally with 6-OHDA.

D1 dopamine receptor binding and mRNA levels are not altered after neonatal 6-hydroxydopamine treatment: evidence against dopamine-mediated induction of D1 dopamine receptors during postnatal development

The role of dopaminergic innervation on the postnatal developmental expression of D1 dopamine receptors was investigated. Bilateral destruction of dopamine-containing neurons was achieved by treating rats intracisternally with 6-hydroxydopamine (6-OHDA) on postnatal day 3, and rats were killed on day 21. To ensure effective reduction of D1 receptor activation by residual dopamine, a group of 6-OHDA-lesioned rats was given twice daily injections of the D1 receptor antagonist SCH-23390, from day 4 to 20. D1 dopamine receptor binding was assessed in the caudate-putamen, nucleus accumbens, and olfactory tubercle by quantitative autoradiographic analysis of [3H]SCH-23390 binding. In addition, the relative amount of D1A receptor mRNA was assessed by in situ hybridization of a 35S-labeled riboprobe. In the developing rats, neither the amount of [3H]SCH-23390 binding nor the amount of D1A receptor mRNA was altered by 6-OHDA lesioning followed by chronic treatment with SCH-23390. Thus, bilateral destruction of dopamine-containing neurons and treatment with SCH-23390 in neonatal rats did not interfere with the developmental expression of D1 receptors or alter the levels of mRNA that code for this receptor protein. Treatment of intact rats with SCH-23390 from postnatal day 4 to 20 also did not alter [3H]SCH-23390 binding or levels of D1 receptor mRNA. However, adult rats treated chronically with SCH-23390 exhibited increased [3H]SCH-23390 binding but did not show a significant change in D1 receptor mRNA levels.

Fetal nicotine exposure alters ontogeny of M1-receptors and their link to G-proteins

Prenatal nicotine exposure has been shown to disrupt the development of cholinergic presynaptic tone and behaviors mediated through muscarinic cholinergic receptors. The current study examines nicotine's effects on ontogeny of postsynaptic muscarinic M1-receptors in rat striatum and hippocampus after continuous maternal infusions of 2 mg/kg/day or 6 mg/kg/day from gestational days 4 through 20. Although brain region weights were unaffected by nicotine exposure, significant alterations in receptor development and receptor regulation by G-proteins were found. Postnatal development of striatal M1-receptor binding, as identified with [3H]pirenzepine, was significantly impaired with either of the fetal nicotine regimens. Treatment with 2 mg/kg/day also produced alterations in striatal receptor affinity state, characterized by enhanced ability of an agonist (oxotremorine-M) to displace [3H]pirenzepine; raising the dose to 6 mg/kg/day masked the affinity shift by affecting G-protein regulatory mechanisms, such that addition of the GTP analog, GppNHp, produced a larger decrease in agonist affinity. In the hippocampus, no such effects on receptor binding, affinity state, or G-protein regulation were seen with either regimen. These data thus indicate that fetal nicotine exposure, even at doses that do not cause overt signs of maternal/fetal/neonatal toxicity or growth impairment, influences cholinergic receptor development and regulation of cell signaling mediated by G-proteins. The selectivity of effects toward M1-receptors in the striatum, a region with a prenatal peak of neuronal mitosis, as compared to hippocampus, where mitosis peaks postnatally, suggests that vulnerability to nicotine involves a critical phase of cell development, rather than being targeted toward receptors of a given subtype.

Effect of prenatal dopamine receptor blocking on somatostatin receptor binding in the developing rat brain

To date, the possible functional interaction between the dopaminergic and the somatostatinergic system during the development of the brain is unknown. This study examines whether blockage of brain dopamine receptors during fetal life might influence postnatal somatostatin (SS) receptor development. Pregnant Sprague-Dawley rats were injected intraperitoneally with either haloperidol (2.5 mg/kg/day), which blocks dopaminergic receptors, or saline. The injections were given for 16 days, commencing on the 4th or 5th day after conception (as counted from the appearance of spermatozoa in daily vaginal smear). The administration of haloperidol during gestation did not affect the levels of somatostatin-like immunoreactivity (SLI) in the two brain areas at any of the times studied. However, this treatment resulted in a decrease in the total number of receptors for 125I-Tyr11-SS in frontoparietal cortex and hippocampal plasma membranes in the 14-day-old offspring but not at 21, 35, or 60 days after birth. No significant differences in the apparent SS binding affinity values were seen after fetal exposure to haloperidol. These results suggest that the development of SS receptors in rat frontoparietal cortex and hippocampus can be transitorily delayed by fetal blockage of dopamine receptors.

Ontogenetic homologous sensitization to the antinociceptive action of quinpirole in rats

Repeated postnatal treatment of rats with the dopamine receptor agonist, quinpirole results in exaggeration of selected behaviors that are induced by quinpirole in adulthood. To determine whether the antinociceptive response to quinpirole could be similarly enhanced, rats were treated daily from birth with quinpirole HCl (3.0 mg/kg per day i.p. x 28 days) and their response time in the hot plate analgesia test was determined at 4 months. An acute dose of quinpirole HCl (100 or 1000 micrograms/kg i.p.) produced an analgesic response in the neonatally primed rats and in the vehicle controls. More significantly, the effect was substantially greater in the quinpirole-primed group at each of these two doses of quinpirole. This effect of quinpirole was fully attenuated in both groups by treatment with the dopamine receptor antagonist, spiperone HCl (0.30 mg/kg i.p., 1 h before quinpirole). The analgesic effect of morphine sulfate (6.0 mg/kg i.p.) was not greater in the quinpirole-primed group. These findings demonstrate that the ontogenetic sensitization of quinpirole receptors results in enhanced antinociceptive responses to quinpirole in adulthood. This animal model may be useful for studying the involvement of dopamine systems in algesia and analgesia.

Ontogenetic expression of D2 dopamine receptor mRNA in rat corpus striatum

The ontogenetic expression of the D2 dopamine receptor (D2R) mRNA has been characterized in rat corpus striatum by in situ hybridization histochemistry and Northern and slot blot analyses using oligonucleotide probes directed toward either the D2R-A subtype of the D2R mRNA or to both the D2R-A and D2R-B subtypes of the D2R mRNA. The results showed that both D2R mRNAs were detected in rat striatum at birth, gradually increased until day 16 postnatally (P16), then declined slightly. At early stages of development, the hybridization signal, when viewed under low magnification, was fairly evenly distributed throughout the striatum. However, later in development (P16) a cluster pattern became manifest. Autoradiographic studies using the mu-opiate receptor as an indication of striatal 'patches' in serial, adjacent sections of striatum indicated that the cluster pattern of the D2R mRNA was not associated solely with the patch or matrix compartments of the striatum. A cellular analysis showed that at early developmental stages the quantity of D2R mRNA per cell was very low in striatum. During the first two postnatal weeks, certain subpopulations of striatal neurons evidenced a marked increase in the expression of D2R mRNA per cell. Administration of 6-hydroxydopamine into neonatal rats failed to significantly change the developmental profile of D2R mRNA in the rat striatum of 16- and 32-day-old animals, although the same treatment caused a marked increase in proenkephalin mRNA. These results suggest that the postnatal development of the D2R mRNA in rat striatum correlates well with the ontogeny of the D2 dopamine receptor, that the developmental expression of the D2R mRNA is highly associated with the maturation and differentiation of striatal neurons, and that the development of the D2R mRNA in rat striatum, unlike that of proenkephalin mRNA, can proceed even with reduced dopaminergic afferent input from the substantia nigra.

Ontogenic homologous supersensitization of dopamine D1 receptors

To determine whether prolonged supersensitization of dopamine D-1 receptors could be produced during ontogeny, rats were treated daily, from birth, for 33 consecutive days with the D-1 receptor agonist, SKF 38393 HCl (3.0 mg/kg per day i.p.). These rats were additionally treated at 3 days after birth with the neurotoxin, 6-hydroxydopamine HBr (6-OHDA; 200 micrograms i.c.v., half in each lateral ventricle) or its vehicle. At 6 to 7 weeks from birth a challenge dose of SKF 38393 HCl (3.0 mg/kg i.p.) increased stereotypy scores for a number of behaviors in 6-OHDA-lesioned rats that were treated repeatedly during ontogeny with SKF 38393. These accentuated behaviors included licking, grooming, taffy pulling, jumping, paw treading and locomotion. Although the findings demonstrate an increased sensitivity of D-1 receptors to an agonist, there was no change in the Bmax or Kd for D-1 receptors in the striatum. In rats that were treated during postnatal development with SKF 38393, but not lesioned with 6-OHDA, SKF 38393-induced stereotyped behaviors were not substantially different from control. The neonatally primed rat model may be useful for probing mechanisms of receptor supersensitivity.

Supersensitized D1 receptors mediate enhanced oral activity after neonatal 6-OHDA

Enhanced oral responses have been observed in rats that are treated shortly after birth with 6-hydroxydopamine (6-OHDA). A series of studies was conducted to characterize this effect. A dose-response curve demonstrated that the dopamine D1 receptor agonist, SKF 38393, produced a maximal response in 6-OHDA-treated rats at a dose of 0.10 mg/kg (IP). With the D2 receptor antagonist, spiperone, a bell-shaped dose-response curve was seen, with a maximal effect in the 6-OHDA group occurring at 80 micrograms/kg. There were only slight increases in oral activity with different SKF 38393 or spiperone doses in the saline group, indicating that there was an overt supersensitization of D1 receptors in the 6-OHDA-treated rats. The D1 antagonist SCH 23390 (0.30 mg/kg, IP) attenuated the response to both SKF 38393 and spiperone. The oral response to the D2 agonist, quinpirole (0.10 mg/kg, IP) was not preferentially increased in the 6-OHDA group of rats. These findings indicate that the enhanced oral response in neonatal 6-OHDA-treated rats is mediated by supersensitive dopamine D1 receptors. The persistence of the enhanced oral response in 6-OHDA-treated rats at 8 months demonstrates that this sensitization of D1 receptors is a long-lived phenomenon.

Presynaptic input regulates development of beta-adrenergic control of rat brain ornithine decarboxylase: effects of 6-hydroxydopamine or propranolol

beta-Adrenergic control of ornithine decarboxylase (ODC) activity is exerted only during a critical period in central nervous system development, playing an important role in neurotransmitter modulation of cell replication and differentiation. The current study examines the effects of lesions caused by 6-hydroxydopamine administration to neonatal rats, or of gestational exposure to propranolol, on the subsequent development of the ODC response to beta-adrenergic stimulation elicited by an acute intracisternal challenge with isoproterenol. 6-Hydroxydopamine treatment severely attenuated the ability of isoproterenol to stimulate ODC in the cerebellum, a tissue that shows a postnatal peak of ODC reactivity. In contrast, much smaller effects were seen in the cerebral cortex, which has an earlier (pre/perinatal) peak of ODC, despite the fact that norepinephrine depletion was more persistent in the cortex. On the other hand, blockade of fetal beta-receptors by maternal propranolol infusions resulted in immediate postnatal attenuation of the ODC response in cerebral cortex, but not cerebellum. These data suggest that early exposure of beta-receptors to norepinephrine "programs" the subsequent efficiency of the receptor linkage to ODC during a critical ontogenetic period that occurs prenatally in the cerebral cortex and postnatally in the cerebellum.

Ontogenic homologous supersensitization of quinpirole-induced yawning in rats

Yawning in male rats is a behavior that may be induced by a group of dopamine receptors when low doses of dopamine-receptor agonists are administered. To determine whether agonist treatments during postnatal development could produce a long-lived supersensitization of these dopamine receptors, rats were treated daily for the first 28 days from birth with quinpirole HCl (3.0 mg/kg/day, IP), an agonist that acts at D2 and D3 receptors. At 8 to 10 weeks from birth the dose-effect curve for quinpirole-induced yawning demonstrated that a supersensitization of dopamine receptors for yawning behavior had occurred. Yawning at the optimal dose of quinpirole HCl (100 microgram/kg, IP) was increased 2-fold. The Bmax and Kd for D2 receptor binding in rat striatum were unaltered in this group of rats. These findings indicate that dopamine receptors can be ontogenically "primed" or supersensitized, and that the phenomenon apparently is not related to changes in striatal D2 receptor binding characteristics.

Dopamine D1 receptor development depends on endogenous dopamine

Profound depletion of forebrain dopamine by 6-hydroxydopamine in neonatal rats (day 3) was associated with up to 82% loss of D1 receptor sites labeled with [3H]SCH-23390 at day 21. Administration of the selective D1 agonist SKF-38393 (days 6-18) abolished the correlation between D1 receptor density and DA concentrations, even with greater than 99% depletion of DA. In intact control animals, there was an inverse correlation between spontaneous variation in levels of DA and D1 receptor site density in forebrain tissue (r = -0.79) which also was abolished by treatment with the D1 agonist. Thus, D1 receptor density may be regulated by reciprocal regulatory processes during normal development, but may fail to develop in the absence of an adequate level of stimulation.

Weanling rats exposed prenatally to cocaine exhibit an increase in striatal D2 dopamine binding associated with an increase in ligand affinity

Prenatal exposure to cocaine can result in abnormal neurobehavioral development. This study found an increase in D2 dopamine receptor binding, associated with an increase in ligand affinity, in striatum of weanling rats exposed prenatally to cocaine. There were no changes in D2 receptor binding in nucleus accumbens nor D1 receptor binding in either striatum or nucleus accumbens. Alterations in D2 dopamine receptors may be associated with neurobehavioral alterations following prenatal cocaine exposure.

Prenatal nicotine exposure impairs beta-adrenergic function: persistent chronotropic subsensitivity despite recovery from deficits in receptor binding

Gestational exposure to nicotine has been shown to interfere with biochemical markers of development of central and peripheral noradrenergic activity. The current study examines the development and function of cardiac beta-adrenergic receptors in the offspring of pregnant rats given nicotine infusions of 6 mg/kg/day from gestational days 4 through 20, administered by subcutaneously implanted osmotic minipumps. Prenatal nicotine exposure delayed the development of beta-adrenergic receptor binding capabilities, as assessed with [125I]pindolol in membrane preparations from heart and kidney. The deficits in receptor binding were associated with marked subsensitivity of chronotropic responses to administration of a beta-adrenergic agonist, isoproterenol. Although the effects on receptor binding resolved after weaning, functional deficiencies in responsiveness to isoproterenol or to preganglionic electrical stimulation of sympathetic nerves to the heart persisted into adulthood. These results indicate that prenatal exposure to nicotine produces long-term alterations in adrenergic responsiveness of sympathetic target tissues.

MIF-1 attenuates apomorphine stereotypies in adult rats after neonatal 6-hydroxydopamine

Since prolyl-leucyl-glycinamide (MIF-1) modifies the behavior of adult rats after treatment with neuroleptics, we examined whether MIF-1 would also modify adult behavior after treatment of neonatal rats with 6-hydroxydopamine (6-OHDA). Rats received 6-OHDA (100 micrograms i.c.v.) or diluent at 3 days after birth and either MIF-1 (2.0 mg/kg per day s.c. x 10 days) or diluent beginning at 28 or 29 days after birth. At 5 weeks, a low dose (0.1 mg/kg s.c.) of apomorphine increased the distance traveled, time in ambulation, number of stereotypic movements, and number of movements per time in stereotypy, but decreased the time in stereotypy in the 6-OHDA group. MIF-1 (x 7 or 8 days) showed a tendency to attenuate the increased number of movements and significantly (P less than 0.05) reduced all of the other effects of neonatal 6-OHDA. Behavior induced by higher doses of apomorphine in the 6-OHDA group (reduced licking and head nodding; increased paw treading, taffy pulling and self-biting) were not attenuated by MIF-1. At 38 or 39 days, total in vitro binding of [3H]SCH-23390 and [3H]spiroperidol to striatal homogenates was not altered in any of the groups. The findings demonstrate that specific early developmental alterations in apomorphine-induced behaviors can be modified by treatment of adult rats with MIF-1, even in the absence of overt changes in the binding of striatal dopamine D-1 and D-2 receptors.