The indirect basal ganglia pathway in dopamine D(2) receptor-deficient mice

Recent pathophysiological models of basal ganglia function in Parkinson's disease predict that specific neurochemical changes in the indirect pathway would follow the lack of stimulation of D(2) dopamine receptors. Post mortem studies of the basal ganglia in genetically modified mice lacking functional copies of the D(2) dopamine receptor gene allowed us to test these predictions. When compared with their congenic N(5) wild-type siblings, mice lacking D(2) receptors show an increased expression of enkephalin messenger RNA in the striatum, and an increased activity and expression of cytochrome oxidase I in the subthalamic nucleus, as expected. In addition, D(2) receptor-deficient mice display a reduced expression of glutamate decarboxylase-67 messenger RNA in the globus pallidus, as the basal ganglia model predicts. This reduction contrasts with the lack of change or increase in glutamate decarboxylase-67 messenger RNA expression found in animals depleted of dopamine after lesions of the mesostriatal dopaminergic system. Furthermore, D(2) receptor-deficient mice show a significant decrease in substance P messenger RNA expression in the striatonigral neurons which form the direct pathway. Finally, glutamate decarboxylase-67 messenger RNA expression in the basal ganglia output nuclei was not affected by mutations in the D(2) receptor gene, a fact that could probably be related to the absence of a parkinsonian locomotor phenotype in D(2) receptor-deficient mice. In summary, these findings provide compelling evidence demonstrating that the lack of endogenous stimulation of D(2) receptors is sufficient to produce subthalamic nucleus hyperactivity, as assessed by cytochrome oxidase I histochemistry and messenger RNA expression, and strongly suggest the existence of interactions between the basal ganglia direct and indirect pathways.

The role of the D(2) dopamine receptor (D(2)R) in A(2A) adenosine receptor (A(2A)R)-mediated behavioral and cellular responses as revealed by A(2A) and D(2) receptor knockout mice

The A(2A)R is largely coexpressed with D(2)Rs and enkephalin mRNA in the striatum where it modulates dopaminergic activity. Activation of the A(2A)R antagonizes D(2)R-mediated behavioral and neurochemical effects in the basal ganglia through a mechanism that may involve direct A(2A)R-D(2)R interaction. However, whether the D(2)R is required for the A(2A)R to exert its neural function is an open question. In this study, we examined the role of D(2)Rs in A(2A)R-induced behavioral and cellular responses, by using genetic knockout (KO) models (mice deficient in A(2A)Rs or D(2)Rs or both). Behavioral analysis shows that the A(2A)R agonist 2-4-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine reduced spontaneous as well as amphetamine-induced locomotion in both D(2) KO and wild-type mice. Conversely, the nonselective adenosine antagonist caffeine and the A(2A)R antagonist 8-(3-chlorostyryl)caffeine produced motor stimulation in mice lacking the D(2)R, although the stimulation was significantly attenuated. At the cellular level, A(2A)R inactivation counteracted the increase in enkephalin expression in striatopallidal neurons caused by D(2)R deficiency. Consistent with the D(2) KO phenotype, A(2A)R inactivation partially reversed both acute D(2)R antagonist (haloperidol)-induced catalepsy and chronic haloperidol-induced enkephalin mRNA expression. Together, these results demonstrate that A(2A)Rs elicit behavioral and cellular responses despite either the genetic deficiency or pharmacological blockade of D(2)Rs. Thus, A(2A)R-mediated neural functions are partially independent of D(2)Rs. Moreover, endogenous adenosine acting at striatal A(2A)Rs may be most accurately viewed as a facilitative modulator of striatal neuronal activity rather than simply as an inhibitory modulator of D(2)R neurotransmission.

Facilitated glutamatergic transmission in the striatum of D2 dopamine receptor-deficient mice

Dopamine (DA) receptors play an important role in the modulation of excitability and the responsiveness of neurons to activation of excitatory amino acid receptors in the striatum. In the present study, we utilized mice with genetic deletion of D2 or D4 DA receptors and their wild-type (WT) controls to examine if the absence of either receptor subtype affects striatal excitatory synaptic activity. Immunocytochemical analysis verified the absence of D2 or D4 protein expression in the striatum of receptor-deficient mutant animals. Sharp electrode current- and whole cell patch voltage-clamp recordings were obtained from slices of receptor-deficient and WT mice. Basic membrane properties were similar in D2 and D4 receptor-deficient mutants and their respective WT controls. In current-clamp recordings in WT animals, very little low-amplitude spontaneous synaptic activity was observed. The frequency of these spontaneous events was increased slightly in D2 receptor-deficient mice. In addition, large-amplitude depolarizations were observed in a subset of neurons from only the D2 receptor-deficient mutants. Bath application of the K+ channel blocker 4-aminopyridine (100 microM) and bicuculline methiodide (10 microM, to block synaptic activity due to activation of GABA(A) receptors) markedly increased spontaneous synaptic activity in receptor-deficient mutants and WTs. Under these conditions, D2 receptor-deficient mice displayed significantly more excitatory synaptic activity than their WT controls, while there was no difference between D4 receptor-deficient mice and their controls. In voltage-clamp recordings, there was an increase in frequency of spontaneous glutamate receptor-mediated inward currents without a change in mean amplitude in D2 receptor-deficient mutants. In WT mice, activation of D2 family receptors with quinpirole decreased spontaneous excitatory events and conversely sulpiride, a D2 receptor antagonist, increased activity. In D2 receptor-deficient mice, sulpiride had very little net effect. Morphologically, a subpopulation of medium-sized spiny neurons from D2 receptor-deficient mice displayed decreased dendritic spines compared with cells from WT mice. These results provide evidence that D2 receptors play an important role in the regulation of glutamate receptor-mediated activity in the corticostriatal or thalamostriatal pathway. These receptors may function as gatekeepers of glutamate release or of its subsequent effects and thus may protect striatal neurons from excessive excitation.

Dopamine-dependent synaptic plasticity in striatum during in vivo development

The neurotransmitters dopamine (DA) and glutamate in the striatum play key roles in movement and cognition, and they are implicated in disorders of the basal ganglia such as Parkinson's disease. Excitatory synapses in striatum undergo a form of developmental plasticity characterized by a decrease in glutamate release probability. Here we demonstrate that this form of synaptic plasticity is DA and DA D2 receptor dependent. Analysis of spontaneous synaptic responses indicates that a presynaptic mechanism involving inhibition of neurotransmitter release underlies the developmental plasticity. We suggest that a major role of DA in the striatum is to initiate mechanisms that regulate the efficacy of excitatory striatal synapses, producing a decrease in glutamate release.

Ethanol-conditioned place preference is reduced in dopamine D2 receptor-deficient mice

Pharmacological blockade studies have supported a role of the dopamine system in ethanol reward for many years, but receptor subtype specificity has been difficult to establish. Recently, genetically engineered mice lacking functional dopamine D2 receptors have been shown to drink less ethanol in a two-bottle choice task. To determine whether reduced ethanol intake reflects a reduction in ethanol reward, D2 receptor-deficient [knockout (KO)] mice were compared to heterozygous (HET) and wild-type (WT; C57BL/6xDBA/2 F2 hybrid) mice in a place conditioning task. Under conditions that produced reliable place preference in both WT and HET mice, KO mice showed no evidence of place conditioning, suggesting that D2 receptor gene inactivation reduced ethanol reward or the ability to learn about ethanol reward. Consistent with previous findings, this mutation also produced a gene dose-related reduction in basal activity levels. Moreover, KO and HET mice showed enhancement of ethanol-stimulated activity relative to WT mice. However, differences in basal and ethanol-stimulated activity did not explain the differences in place conditioning. Overall, this study strongly supports the conclusion that dopamine D2 receptors normally influence ethanol reward in mice.

Naltrexone and alcohol drinking in mice lacking beta-endorphin by site-directed mutagenesis

Alcohol-induced activation of the opioid system may contribute to the reinforcing properties of alcohol. This study investigated whether elimination of beta-endorphin (BE) synthesis via site-directed mutagenesis in embryonic stem cells would alter alcohol intake in mice. Both BE-deficient and wildtype (WT) mice generated from the targeted stem cells were backcrossed for nine generations onto a C57BL/6 background, and were maintained with ad libitum food and water. Mice had access to alcohol (10% v/v) under the following conditions: 24 h, scheduled access for 2 h/day, following acute (1 or 2 days) or chronic (5 weeks) alcohol deprivation, and scheduled access following six doses of naltrexone (0.125-16.0 mg/kg BW, ip) or saline treatment. Alcohol intake was similar in BE-deficient and WT mice given chronic access to alcohol, but greater in BE-deficient compared with WT mice during the first 10 days of scheduled access to alcohol, but not after more extensive experience with scheduled access. BE-deficient, but not WT mice, increased alcohol intake following 2 days, but not 1 day or 5 weeks, of deprivation. Naltrexone reduced alcohol drinking both in BE-deficient and WT mice, suggesting that drinking is mediated, in part, by activation of opioid receptors in both genotypes.

Lack of operant ethanol self-administration in dopamine D2 receptor knockout mice

RATIONALE

Dopamine D2 receptors are postulated to play an important role in modulating the reinforcing effects of abused drugs including ethanol.

OBJECTIVES

This experiment examined operant ethanol self-administration in dopamine D2 receptor knockout (KO) mice and wild-type (WT) mice using a continuous access procedure.

METHODS

Adult male KO and WT mice were trained in 30-min sessions to perform a lever press response for access to 10% v/v ethanol. After training, the mice were placed in test chambers on a continuous (23 h/day) basis with access to food (one lever press, i.e., FR1), 10% v/v ethanol (four lever presses, i.e., FR4), and water from a sipper tube (phase 1). After 30 consecutive sessions, response patterns were determined for 0, 5, 10, 20 and 30% v/v ethanol (phase 2). Saccharin (0.2% w/v) was subsequently added to the ethanol mixture and responding was examined for 0, 5, 10 and 20% ethanol (phase 3).

RESULTS

During phase 1, WT mice displayed higher ethanol-lever responding compared to KO mice. Food lever responding and water intake was the same in both genotypes. During phase 2, WT mice displayed concentration-dependent ethanol lever responding, whereas KO mice responded at low rates regardless of ethanol concentration. WT mice also responded more for food compared to KO mice. Each genotype showed similar water intakes except at the 20% ethanol concentration, where WT mice had lower intakes. During phase 3, WT mice continued to show higher responding for all concentrations including saccharin alone. WT mice also continued to respond more for food compared to KO mice, but drank less water. In each phase, WT mice displayed episodic (bout) responding on the ethanol lever. KO mice did not respond for ethanol in bouts.

CONCLUSIONS

Reduced responding in the KO mice for several reinforcers including ethanol indicates a more general role for dopamine D2 receptors in motivated responding rather than a specific role in ethanol reinforcement.

Effect of the mu-opioid agonist DAMGO on medial basal hypothalamic neurons in beta-endorphin knockout mice

The endogenous opioid neurotransmitter beta-endorphin (beta-END), a product of the proopiomelanocortin (POMC) gene, is strongly implicated in the control of the female reproductive cycle, stress responses, and antinociception. Using selective gene targeting, we have generated a strain of mice that do not express any beta-END. These mice exhibit both normal reproduction and normal basal and stress-induced hypothalamic-pituitary-axis activity, but exhibit a significantly attenuated opioid-mediated stress-induced analgesia. To further understand the cellular bases of these responses, we have studied mediobasal hypothalamic (MBH) neurons, including POMC neurons, using whole-cell patch recording in an in vitro slice preparation. Twenty-seven MBH cells were recorded in wild-type and 25 MBH cells were recorded in beta-END knockout mice. Neurons from both genotypes showed a significant positive correlation between DAMGO concentration (from 30 nM to 10 microM) and the induced outward K(+) current. The genotypes did not differ, however, in either the DAMGO-induced maximum outward current response or EC(50), or for the maximal response to the GABA(B) agonist baclofen. Furthermore, quantitative receptor autoradiography utilizing (3)H-DAMGO did not reveal any differences in total mu-opioid receptor binding between genotypes. Therefore, we conclude that the complete absence of beta-END throughout development did not alter either the expression of mu-opioid receptors or their coupling to K(+) channels in MBH neurons.

Ventilatory responses to acute and chronic hypoxia in mice: effects of dopamine D(2) receptors

We used genetically engineered D(2) receptor-deficient [D(2)-(-/-)] and wild-type [D(2)-(+/+)] mice to test the hypothesis that dopamine D(2) receptors modulate the ventilatory response to acute hypoxia [hypoxic ventilatory response (HVR)] and hypercapnia [hypercapnic ventilatory response (HCVR)] and time-dependent changes in ventilation during chronic hypoxia. HVR was independent of gender in D(2)-(+/+) mice and significantly greater in D(2)-(-/-) than in D(2)-(+/+) female mice. HCVR was significantly greater in female D(2)-(+/+) mice than in male D(2)-(+/+) and was greater in D(2)-(-/-) male mice than in D(2)-(+/+) male mice. Exposure to hypoxia for 2-8 days was studied in male mice only. D(2)-(+/+) mice showed time-dependent increases in "baseline" ventilation (inspired PO(2) = 214 Torr) and hypoxic stimulated ventilation (inspired PO(2) = 70 Torr) after 8 days of acclimatization to hypoxia, but D(2)-(-/-) mice did not. Hence, dopamine D(2) receptors modulate the acute HVR and HCVR in mice in a gender-specific manner and contribute to time-dependent changes in ventilation and the acute HVR during acclimatization to hypoxia.

Functional uncoupling of adenosine A(2A) receptors and reduced responseto caffeine in mice lacking dopamine D2 receptors

Dopamine D(2) receptors (Rs) and adenosine A(2A)Rs are coexpressed on striatopallidal neurons, where they mediate opposing actions. In agreement with the idea that D(2)Rs tonically inhibit GABA release from these neurons, stimulation-evoked GABA release was significantly greater from striatal/pallidal slices from D(2)R null mutant (D(2)R(-/-)) than from wild-type (D(2)R(+/+)) mice. Release from heterozygous (D(2)R(+/-)) slices was intermediate. However, contrary to predictions that A(2A)R effects would be enhanced in D(2)R-deficient mice, the A(2A)R agonist CGS 21680 significantly increased GABA release only from D(2)R(+/+) slices. CGS 21680 modulation was observed when D(2)Rs were antagonized by raclopride, suggesting that an acute absence of D(2)Rs cannot explain the results. The lack of CGS 21680 modulation in the D(2)R-deficient mice was also not caused by a compensatory downregulation of A(2A)Rs in the striatum or globus pallidus. However, CGS 21680 significantly stimulated cAMP production only in D(2)R(+/+) striatal/pallidal slices. This functional uncoupling of A(2A)Rs in the D(2)R-deficient mice was not explained by reduced expression of G(s), G(olf), or type VI adenylyl cyclase. Locomotor activity induced by the adenosine receptor antagonist caffeine was significantly less pronounced in D(2)R(-/-) mice than in D(2)R(+/+) and D(2)R(+/-) mice, further supporting the idea that D(2)Rs are required for caffeine activation. Caffeine increased c-fos only in D(2)R(-/-) globus pallidus. The present results show that a targeted disruption of the D(2)R reduces coupling of A(2A)Rs on striatopallidal neurons and thereby responses to drugs that act on adenosine receptors. They also reinforce the ideas that D(2)Rs and A(2A)Rs are functionally opposed and that D(2)R-mediated effects normally predominate.

Selective increase of Nurr1 mRNA expression in mesencephalic dopaminergic neurons of D2 dopamine receptor-deficient mice

The orphan nuclear receptor Nurr1 is critical for the survival of mesencephalic dopaminergic precursor neurons. Little is known about the mechanisms that regulate Nurr1 expression in vivo. Other members of this receptor family have been shown to be activated by dopamine. We sought to determine if Nurr1 expression is also regulated by endogenous dopamine through dopamine receptors. Consequently, we investigated the expression of Nurr1 mRNA in genetically modified mice lacking both functional copies of the D2 dopamine receptor gene and in their congenic siblings. Quantitative in situ hybridization demonstrated a significant increased expression of Nurr1 mRNA in the substantia nigra pars compacta and the ventral tegmental area of D2 dopamine receptor -/- mice. No change in Nurr1 expression was detected in other brain regions, such as the habenular nuclei and temporal cortex. Among the cell groups studied, mesencephalic dopaminergic neurons are unique in that they express both Nurr1 and the D2 dopamine receptor, and synthesize dopamine. Thus, it seems plausible that the selective increase in Nurr1 expression observed in D2 receptor-deficient mice is the consequence of an impaired dopamine autoreceptor function.

Abnormal adaptations to stress and impaired cardiovascular function in mice lacking corticotropin-releasing hormone receptor-2

The actions of corticotropin-releasing hormone (Crh), a mediator of endocrine and behavioural responses to stress, and the related hormone urocortin (Ucn) are coordinated by two receptors, Crhr1 (encoded by Crhr) and Crhr2. These receptors may exhibit distinct functions due to unique tissue distribution and pharmacology. Crhr-null mice have defined central functions for Crhr1 in anxiety and neuroendocrine stress responses. Here we generate Crhr2-/- mice and show that Crhr2 supplies regulatory features to the hypothalamic-pituitary-adrenal axis (HPA) stress response. Although initiation of the stress response appears to be normal, Crhr2-/- mice show early termination of adrenocorticotropic hormone (Acth) release, suggesting that Crhr2 is involved in maintaining HPA drive. Crhr2 also appears to modify the recovery phase of the HPA response, as corticosterone levels remain elevated 90 minutes after stress in Crhr2-/- mice. In addition, stress-coping behaviours associated with dearousal are reduced in Crhr2-/- mice. We also demonstrate that Crhr2 is essential for sustained feeding suppression (hypophagia) induced by Ucn. Feeding is initially suppressed in Crhr2-/- mice following Ucn, but Crhr2-/- mice recover more rapidly and completely than do wild-type mice. In addition to central nervous system effects, we found that, in contrast to wild-type mice, Crhr2-/- mice fail to show the enhanced cardiac performance or reduced blood pressure associated with systemic Ucn, suggesting that Crhr2 mediates these peripheral haemodynamic effects. Moreover, Crhr2-/- mice have elevated basal blood pressure, demonstrating that Crhr2 participates in cardiovascular homeostasis. Our results identify specific responses in the brain and periphery that involve Crhr2.

Quantitative analysis of the dopamine D4 receptor in the mouse brain

The D4 receptor (D4R), a member of the dopamine D2-like receptor family, has been implicated in the pathophysiology of several diseases and has been the target of various investigations regarding its distribution and quantification. The brain distribution of the D4R has been well described in various species, but the quantification is still an issue of controversy, because no specific ligand is commercially available. To circumvent this difficulty we have performed a biochemical and autoradiographical study in brain samples obtained from mice lacking D4Rs and their wild-type siblings; comparison of their binding parameters allows a more accurate quantification of the members of the D2-like receptor family (D2, D3, and D4 receptors). We found that the distribution of D2-like receptors in mouse brain is similar to that of rat brain, i.e., caudate putamen, nucleus accumbens, olfactory tubercle, and hippocampus. The contribution of the D4R to the overall population of D2-like receptors is 17% in nucleus accumbens, 21% in caudate putamen and olfactory tubercle, and 40% in hippocampus. Based on our study we conclude that nemonapride probably binds to nondopaminergic sites that if not properly blocked may lead to overestimations of D4R levels. We observed that the experimental condition that better estimates the density of D4 receptors is the displacement of D2 and D3 [3H]nemonapride binding sites with cold raclopride.

Dopamine D4 receptor-knock-out mice exhibit reduced exploration of novel stimuli

The involvement of dopamine neurotransmission in behavioral responses to novelty is suggested by reports that reward is related to increased dopamine activity, that dopamine modulates exploratory behavior in animals, and that Parkinson's disease patients report diminished responses to novelty. Some studies have reported that polymorphisms of the human dopamine D4 receptor (D4R) gene are associated with personality inventory measures of the trait called "novelty-seeking". To explore a potential role for the D4R in behavioral responses to novelty, we evaluated D4R-knock-out (D4R-/-) and wild-type (D4R+/+) mice in three approach-avoidance paradigms: the open field, emergence, and novel object tests. These three paradigms differ in the degree to which they elicit approach, or exploratory behavior, and avoidance, or anxiety-related behavior. Thus, we used these three tests to determine whether the D4R primarily influences the exploratory or the anxious component of responses to approach-avoidance conflicts. D4R-/- mice were significantly less behaviorally responsive to novelty than D4R+/+ mice in all three tests. The largest phenotypic differences were observed in the novel object test, which maximizes approach behavior, and the smallest phenotypic differences were found in the open field test, which maximizes avoidance behavior. Hence, D4R-/- mice exhibit reductions in behavioral responses to novelty, reflecting a decrease in novelty-related exploration.

Pituitary lactotroph adenomas develop after prolonged lactotroph hyperplasia in dopamine D2 receptor-deficient mice

Tuberoinfundibular dopamine tonically inhibits PRL expression and secretion from the pituitary gland by the activation of dopamine D2 receptors (D2R) localized on lactotrophs. Mutant female mice that lack D2Rs have persistent hyperprolactinemia but also develop extensive hyperplasia of pituitary lactotrophs and peliosis of the adenohypophysis at 9 to 12 months of age, while age-matched male D2R-deficient mice have no morphologic adenohypophysial lesion. We now report that both female and male D2R-deficient mice 17 to 20 months of age develop pituitary lactotroph adenomas. Of 12 aged female mice examined, all developed monohormonal PRL-immunoreactive neoplasms that had a characteristic juxtanuclear Golgi pattern of PRL staining and loss of the reticulin fiber network. Several of these adenomas were 50-fold larger than normal glands with marked suprasellar extension and invasion of brain but no gross evidence of distant metastases. They also had striking peliosis that was more marked than the lesion seen in the hyperplastic pituitaries of the younger females. These findings demonstrate that a chronic loss of neurohormonal dopamine inhibition promotes the hyperplasia-neoplasia sequence in adenohypophysial lactotrophs. Our results are analogous to previous data indicating that protracted stimulation of adenohypophysial cells by hormones or growth factors results in proliferation with initial hyperplasia followed by the development of neoplasia. Six aged male D2R-deficient mice had slightly enlarged anterior pituitaries similar in size to normal female glands. However, each case exhibited multifocal, microscopic lactotroph adenomas with strong nuclear immunoreactivity for estrogen receptors and Pit-1 transcription factor. The unexpected development of adenomas in males without preexisting or concomitant hyperplasia suggests that prolonged loss of dopamine inhibition may also cause neoplasia by distinct cellular mechanisms in male and female animals.

LbetaT2 gonadotroph cells secrete follicle stimulating hormone (FSH) in response to active A

Secretion of luteinizing hormone in response to gonadotropin releasing hormone (GnRH) has been described in the recently developed LbetaT2 gonadotroph cell line. We evaluated the expression of follicle stimulating hormone (FSH)beta mRNA and secretion of FSH from LbetaT2 cells in response to GnRH and activin A. LbetaT2 cells were treated with activin A in doses from 0 to 50 ng/ml, with or without a daily 10 nM GnRH pulse, or with GnRH alone. FSH secretion was stimulated over 6-fold by concomitant GnRH and activin A in a dose-responsive fashion at 72 h of treatment. FSHbeta mRNA was detectable by ribonuclease protection assay only in cells treated with activin A with or without GnRH. The demonstration of FSHbeta gene expression in LbetaT2 cells further validates these cells as mature, differentiated gonadotrophs and as an important tool for the study of gonadotroph physiology.

Ethanol oral self-administration is increased in mutant mice with decreased beta-endorphin expression

The relationship between ethanol (EtOH) administration and the endogenous opioid system has been studied for many years and a considerable body of evidence supports the contention that EtOH modulates the production and/or release of endogenous opioid peptides. However, substantially less is known about the converse influence: the effect that opioids have on EtOH sensitivity. In this study, we used the beta-endorphin deficient mutant mouse line C57BL/6-Pomc1(tm1Low) to investigate the possible role of a specific opioid peptide on EtOH consumption. Homozygous knockout mice (entirely lacking beta-endorphin), heterozygous mice (50% beta-endorphin expression) and sibling wildtype mice from the same strain were evaluated in a two-bottle free choice paradigm for oral self-administration of EtOH. Across varying EtOH concentrations only the heterozygous mice were found to consistently drink more than wildtype mice. These data support the hypothesis that beta-endorphin modulates the response to EtOH.

K+ channel modulation in rodent neurohypophysial nerve terminals by sigma receptors and not by dopamine receptors

1. Sigma receptors bind a diverse group of chemically unrelated ligands, including pentazocine, apomorphine (a dopamine receptor agonist) and haloperidol (a dopamine receptor antagonist). Although sigma binding sites are widely distributed, their physiological roles are poorly understood. Here, the whole-terminal patch-clamp technique was used to demonstrate that sigma receptors modulate K+ channels in rodent neurohypophysis. 2. Previous work suggested that dopamine type 4 (D4) receptors modulate neurohypophysial K+ current, so this study initially tested the role of dopamine receptors. Experiments using transgenic mice lacking D2, D3 or D4 receptors indicated that the reduction of K+ current by PPHT and U101958 (ligands thought to be selective for dopamine receptors) is not mediated by dopamine receptors. The sensitivity of the response to U101958 (a drug that binds to D4 receptors) was the same in both wild-type and D4 receptor-deficient mice. 3. Experiments with other ligands revealed a pharmacological signature inconsistent with any known dopamine receptor. Furthermore, dopamine itself (at 100 microM) had no effect. Thus, despite the activity of a number of putative dopamine receptor ligands, dopamine receptors play no role in the modulation of neurohypophysial K+ channels. 4. Because of the negative results regarding dopamine receptors, and because some of the dopamine receptors ligands used here are known to bind also to sigma receptors, experiments were conducted to test for the involvement of sigma receptors. In rat neurohypophysis the sigma receptor ligands SKF10047, pentazocine, and ditolylguanidine all reversibly inhibited K+ current in a concentration-dependent fashion, as did haloperidol and apomorphine (ligands that bind to both dopamine and sigma receptors). The activity of these and other ligands tested here matches the reported binding specificity for sigma receptors. 5. Fifteen candidate endogenous sigma receptor ligands, including biogenic amines (e.g dopamine and serotonin), steroids (e.g. progesterone), and peptides (e.g. neuropeptide Y), were screened for activity at the sigma receptor. All were without effect. 6. Haloperidol reduced K+ current proportionally at all voltages without shifting the voltage dependence of activation and inactivation. Sigma receptor ligands inhibited current through two distinct K+ channels, the A-channel and the Ca2+-dependent K+ channel. In rat, all drugs reduced current through both channels proportionally, suggesting that both channels are modulated by a single population of sigma receptors. In contrast, mouse peptidergic nerve terminals either have two receptors which are sensitive to these drugs, or a single receptor that is differentially coupled to ion channel function. 7. The inhibition of voltage-activated K+ current by sigma receptors would be expected to enhance the secretion of oxytocin and vasopressin from the neurohypophysis.

The dopamine D2, but not D3 or D4, receptor subtype is essential for the disruption of prepulse inhibition produced by amphetamine in mice

Brain dopamine (DA) systems are involved in the modulation of the sensorimotor gating phenomenon known as prepulse inhibition (PPI). The class of D2-like receptors, including the D2, D3, and D4 receptor subtypes, have all been implicated in the control of PPI via studies of DA agonists and antagonists in rats. Nevertheless, the functional relevance of each receptor subtype remains unclear because these ligands are not specific. To determine the relevance of each receptor subtype, we used genetically altered strains of "knock-out" mice lacking the DA D2, D3, or D4 receptors. We tested the effects of each knock-out on both the phenotypic expression of PPI and the disruption of PPI produced by the indirect DA agonist d-amphetamine (AMPH). No phenotypic differences in PPI were observed at baseline. AMPH significantly disrupted PPI in the D2 (+/+) mice but had no effect in the D2 (-/-) mice. After AMPH treatment, both DA D3 and D4 receptor (+/+) and (-/-) mice had significant disruptions in PPI. These findings indicate that the AMPH-induced disruption of PPI is mediated via the DA D2 receptor and not the D3 or D4 receptor subtypes. Uncovering the neural mechanisms involved in PPI will further our understanding of the substrates of sensorimotor gating and could lead to better therapeutics to treat gating disorders, such as schizophrenia.

Expression of IGF system genes during T-antigen driven pituitary tumorigenesis

Expression of IGF-I, IGF-II, the Type-I IGF receptor and six IGF binding proteins were examined in three different T-ag-driven mouse tumors. Unlike the widespread expression of IGF-II in pancreatic beta-cell tumors, IGF-II was not widely expressed in the two different pituitary tumors examined indicating that a mechanism independent of focal IGF-II expression can also drive T-antigen tumorigenesis. In addition, multiple IGF binding proteins were expressed in all three tumor types. This expression, however, was generally heterogeneous with no specific changes to indicate a required role for any IGF binding protein in T-antigen tumorigenesis.

Dopamine D2 receptor-deficient mice exhibit decreased dopamine transporter function but no changes in dopamine release in dorsal striatum

Presynaptic D2 dopamine (DA) autoreceptors, which are well known to modulate DA release, have recently been shown to regulate DA transporter (DAT) activity. To examine the effects of D2 DA receptor deficiency on DA release and DAT activity in dorsal striatum, we used mice genetically engineered to have two (D2+/+), one (D2+/-), or no (D2-/-) functional copies of the gene coding for the D2 DA receptor. In vivo microdialysis studies demonstrated that basal and K+-evoked extracellular DA concentrations were similar in all three genotypes. However, using in vivo electrochemistry, the D2-/- mice were found to have decreased DAT function, i.e., clearance of locally applied DA was decreased by 50% relative to that in D2+/+ mice. In D2+/+ mice, but not D2-/- mice, local application of the D2-like receptor antagonist raclopride increased DA signal amplitude, indicating decreased DA clearance. Binding assays with the cocaine analogue [3H]WIN 35,428 showed no genotypic differences in either density or affinity of DAT binding sites in striatum or substantia nigra, indicating that the differences seen in DAT activity were not a result of decreased DAT expression. These results further strengthen the idea that the D2 DA receptor subtype modulates activity of the striatal DAT.

Alcohol preference and sensitivity are markedly reduced in mice lacking dopamine D2 receptors

Although dopaminergic transmission has been strongly implicated in alcohol self-administration, the involvement of specific dopamine receptor subtypes has not been well established. We studied the ethanol preference and sensitivity of D2-receptor-deficient mice to directly evaluate whether dopamine D2 receptors contribute to alcohol (ethanol) consumption. We report a marked aversion to ethanol in these mice, relative to the high preference and consumption exhibited by wild-type littermates. Sensitivity to ethanol-induced locomotor impairment was also reduced in these mutant mice, although they showed a normal locomotor depressant response to the dopamine D1 antagonist SCH-23390. These data demonstrate that dopamine signaling via D2 receptors is an essential component of the molecular pathway determining ethanol self-administration and sensitivity.

Authentic cell-specific and developmentally regulated expression of pro-opiomelanocortin genomic fragments in hypothalamic and hindbrain neurons of transgenic mice

The pro-opiomelanocortin (POMC) gene is expressed in a subset of hypothalamic and hindbrain neurons and in pituitary melanotrophs and corticotrophs. POMC neurons release the potent opioid beta-endorphin and several active melanocortins that control homeostasis and feeding behavior. POMC gene expression in the CNS is believed to be controlled by distinct cis-acting regulatory sequences. To analyze the transcriptional regulation of POMC in neuronal and endocrine cells, we produced transgenic mice carrying POMC27*, a transgene containing the entire 6 kb of the POMC transcriptional unit together with 13 kb of 5' flanking regions and 8 kb of 3' flanking regions. POMC27* was tagged with a heterologous 30 bp oligonucleotide in the third exon. In situ hybridization studies showed an accurate cell-specific pattern of expression of POMC27* in the arcuate nucleus and the pituitary. Hypothalamic mRNA-positive neurons colocalized entirely with beta-endorphin immunoreactivity. No ectopic transgenic expression was detected in the brain. Deletional analyses demonstrated that neuron-specific expression of POMC transgenes required distal 5' sequences localized upstream of the pituitary-responsive proximal cis-acting elements that were identified previously. POMC27* exhibited a spatial and temporal pattern of expression throughout development that exactly paralleled endogenous POMC. RNase protection assays revealed that POMC27* expression mimicked that of POMC in different areas of the CNS and most peripheral organs with no detectable ectopic expression. Hormonal regulation of POMC27* and POMC was identical in the hypothalamus and pituitary. These results show that distal 5' sequences of the POMC gene located between -13 and -2 kb target expression into the CNS of transgenic mice in a precise neuron-specific, developmentally and hormonally regulated manner.