Dopamine actions on calcium currents, potassium currents and hormone release in rat melanotrophs

Dopamine agonists for the treatment of pituitary tumours: From ergot extracts to next generation therapies

Dopamine agonists are a key tool in the therapeutic arsenal of endocrinologists worldwide. They exert their effects by binding to dopamine-2 (D2) receptors expressed by pituitary tumour cells to modulate hormonal secretion and tumour size. They are the established first-line treatment for prolactinomas which express high levels of D2 receptors. Growing data support their use as an adjuvant treatment option for other pituitary tumours including growth hormone, adrenocorticotrophic hormones, thyroid hormone secreting adenomas and nonfunctional pituitary tumours, all of which have been shown to express D2 receptors as well, albeit to varying extents. For those pituitary tumours inadequately treated by dopamine agonist alone, combined agonism of D2 and somatostatin receptors represent a new frontier in clinical development. Here we review the development and role of dopamine agonist for the treatment of prolactinomas, the literature supporting their adjuvant use for the treatment of all other pituitary tumours, and recent progress in the development of the next generation of chimeric compounds that target D2 and other receptor subtypes highly expressed on pituitary tumour cells.

The MAPK Pathway-Based Drug Therapeutic Targets in Pituitary Adenomas

Mitogen-activated protein kinases (MAPKs) include ERK, p38, and JNK MAPK subfamilies, which are crucial regulators of cellular physiology, cell pathology, and many diseases including cancers. For the MAPK signaling system in pituitary adenomas (PAs), the activation of ERK signaling is generally thought to promote cell proliferation and growth; whereas the activations of p38 and JNK signaling are generally thought to promote cell apoptosis. The role of MAPK in treatment of PAs is demonstrated through the effects of currently used medications such as somatostatin analogs such as SOM230 and OCT, dopamine agonists such as cabergoline and bromocriptine, and retinoic acid which inhibit the MAPK pathway. Further, there are potential novel therapies based on putative molecular targets of the MAPK pathway, including 18beta-glycyrrhetinic acid (GA), dopamine-somatostatin chimeric compound (BIM-23A760), ursolic acid (UA), fulvestrant, Raf kinase inhibitory protein (RKIP), epidermal growth factor pathway substrate number 8 (Eps8), transmembrane protein with EGF-like and two follistatin-like domains (TMEFF2), cold inducible RNA-binding protein (CIRP), miR-16, and mammaliansterile-20-like kinase (MST4). The combined use of ERK inhibitor (e.g., SOM230, OCT, or dopamine) plus p38 activator (e.g., cabergoline, bromocriptine, and fulvestrant) and/or JNK activator (e.g., UA), or the development of single drug (e.g., BIM-23A760) to target both ERK and p38 or JNK pathways, might produce better anti-tumor effects on PAs. This article reviews the advances in understanding the role of MAPK signaling in pituitary tumorigenesis, and the MAPK pathway-based potential therapeutic drugs for PAs.

Common and diverse elements of ion channels and receptors underlying electrical activity in endocrine pituitary cells

The pituitary gland contains six types of endocrine cells defined by hormones they secrete: corticotrophs, melanotrophs, gonadotrophs, thyrotrophs, somatotrophs, and lactotrophs. All these cell types are electrically excitable, and voltage-gated calcium influx is the major trigger for their hormone secretion. Along with hormone intracellular content, G-protein-coupled receptor and ion channel expression can also be considered as defining cell type identity. While many aspects of the developmental and activity dependent regulation of hormone and G-protein-coupled receptor expression have been elucidated, much less is known about the regulation of the ion channels needed for excitation-secretion coupling in these cells. We compare the spontaneous and receptor-controlled patterns of electrical signaling among endocrine pituitary cell types, including insights gained from mathematical modeling. We argue that a common set of ionic currents unites these cells, while differential expression of another subset of ionic currents could underlie cell type-specific patterns. We demonstrate these ideas using a generic mathematical model, showing that it reproduces many observed features of pituitary electrical signaling. Mapping these observations to the developmental lineage suggests possible modes of regulation that may give rise to mature pituitary cell types.

Dopamine D2 receptor expression in the corticotroph cells of the human normal pituitary gland

The dopamine D₂ receptor is the main dopamine receptor expressed in the human normal pituitary gland. The aim of the current study was to evaluate dopamine D₂ receptor expression in the corticotroph cell populations of the anterior lobe and pars intermedia, as well as posterior lobe of the human normal pituitary gland by immunohistochemistry. Human normal pituitary gland samples obtained from routine autopsies were used for the study. In all cases, histology together with immunostaining for adrenocorticotropic hormone, melanocyte-stimulating hormone, prolactin, and neurofilaments were performed and compared to the immunostaining for D₂ receptor. D₂ receptor was heterogeneously expressed in the majority of the cell populations of the anterior and posterior lobe as well as in the area localized between the anterior and posterior lobe, and arbitrary defined as "intermediate zone". This zone, characterized by the presence of nerve fibers included the residual pars intermedia represented by the colloid-filled cysts lined by the remnant melanotroph cells strongly expressing D₂ receptors, and clusters of corticotroph cells, belonging to the anterior lobe but localized within the cysts and adjacent to the posterior lobe, variably expressing D₂ receptors. D₂ dopamine receptor is expressed in the majority of the cell populations of the human normal pituitary gland, and particularly, in the different corticotroph cell populations localized in the anterior lobe and the intermediate zone of the pituitary gland.

Polymorphisms in MEN1 and DRD2 genes are associated with the occurrence and characteristics of pituitary adenomas

OBJECTIVE

Although pituitary adenomas (PAs) affect a significant proportion of the population, only a fraction have the potential to become clinically relevant during an individual's lifetime, causing hormonal imbalance or complications due to mass effect. The overwhelming majority of cases are sporadic and without a clear familial history, and the genotype-phenotype correlation in PA patients is poorly understood. Our aim was to investigate the involvement of genes known for their role in familial cases on drug response and tumor suppression in the development and pathology of PAs in a patient group from Latvia.

DESIGN

The study included 143 cases and 354 controls, we investigated the role of single-nucleotide polymorphisms (SNPs) in seven genes (SSTR2, SSTR5, DRD2, MEN1, AIP, GNAS, and PRKAR1A) associated with pituitary tumor occurrence, phenotype, and clinical symptoms.

METHODS

Genotyping of 96 tag and nonsynonymous SNPs was performed in the genomic regions of interest.

RESULTS

We discovered a significant association (OR=17.8, CI 0.95=2.18-145.5, P=0.0002) between a rare MEN1 mutation (rs2959656) and clinically active adenoma in our patients. Additionally, rs7131056 at DRD2 was associated with a higher occurrence of extrasellar growth in patients with prolactinoma and somatotropinoma (OR=2.79, CI 0.95=1.58-4.95, P=0.0004).

CONCLUSIONS

rs2959656, a nonsynonymous variant in MEN1, is associated with the development of clinically active PA. Furthermore, rs7131056 in DRD2 contributes to either faster growth of the adenoma or reduced symptomatic presentation, allowing PAs to become larger before detection.

Advances in understanding pituitary tumors

Pituitary tumors are common in the general population. Since neuroimaging techniques have improved, pituitary tumors are more often diagnosed incidentally. About 16.7% of the general population show changes in the pituitary gland. Predominantly, pituitary tumors are benign pituitary adenomas. Pituitary carcinomas or aggressive pituitary tumors are extremely rare. They might develop from benign adenomas. New genetic and epigenetic abnormalities help us to understand pituitary tumorigenesis and might lead to therapeutical targeting drugs in the future. Macroadenomas (>1 cm) can lead to visual field disturbances, compression of cranial nerves, hypopituitarism, and infiltration of the cavernous sinuses. The functional status of the pituitary tumor is important. About half to one third of all pituitary tumors are non-functioning pituitary adenomas. The other pituitary tumors show a specific pattern of hormone secretion. About 25% to 41% of all pituitary tumors are prolactinomas, acromegaly with production of growth hormone represents 10% to 15% of adenomas, Cushing's disease with production of adrenocorticotropic hormone accounts for 10%, and other hormonal characteristics are less common. Transsphenoidal resection and total adenomectomy are desirable. Radiosurgery has enriched the surgical treatment options. Surgical treatment is the intervention of choice except for prolactinomas, where pharmaceutical treatment is recommended. Pharmaceutical treatment consists of dopamine agonists such as cabergoline and somatostatin analogues that include octreotide and pasireotide; retinoic acid is of theoretical interest while peroxisome proliferator-activated receptor-gamma-ligands are not clinically useful. In acromegaly, pegvisomant is a further treatment option. Temozolomide should be considered in aggressive pituitary tumors. In general, pharmaceutical options developed recently have extended the repertoire of treatment possibilities of pituitary tumors.

Somatostatin and dopamine receptors as targets for medical treatment of Cushing's Syndrome

Somatostatin (SS) and dopamine (DA) receptors are widely expressed in neuroendocrine tumours that cause Cushing's Syndrome (CS). Increasing knowledge of specific subtype expression within these tumours and the ability to target these receptor subtypes with high-affinity compounds, has driven the search for new SS- or DA-based medical therapies for the various forms of CS. In Cushing's disease, corticotroph adenomas mainly express dopamine receptor subtype 2 (D(2)) and somatostatin receptor subtype 5 (sst(5)), whereas sst(2) is expressed at lower levels. Activation of these receptors can inhibit ACTH-release in primary cultured corticotroph adenomas and compounds that target either sst(5) (pasireotide, or SOM230) or D(2) (cabergoline) have shown significant efficacy in subsets of patients in recent clinical studies. Combination therapy, either by administration of both types of compounds separately or by treatment with novel somatostatin-dopamine chimeric molecules (e.g. BIM-23A760), appears to be a promising approach in this respect. In selected cases of Ectopic ACTH-producing Syndrome (EAS), the sst(2)-preferring compound octreotide is able to reduce cortisol levels effectively. A recent study showed that D(2) receptors are also significantly expressed in the majority of EAS and that cabergoline may decrease cortisol levels in subsets of these patients. In both normal adrenal tissue as well as in adrenal adenomas and carcinomas that cause CS, sst and DA receptor expression has been demonstrated. Although selected cases of adrenal CS may benefit from sst or DA-targeted treatment, its total contribution to the treatment of these patients is likely to be low as surgery is effective in most cases.

Immunoreactivity to neurofilaments in the rodent anterior pituitary is associated with the expression of alpha 1A protein subunits of voltage-gated Ca2+ channels

We recently reported that rodent anterior pituitary (AP) cells (with the exception of corticotrophs and melanotrophs) express neuronal markers, including 68-kDa neurofilaments (NF68) in an oestrogen-dependent manner. The functional significance of neurofilament (NF) expression in the AP is unknown, but recent data in myelinated nerve fibres from NF-null mice suggest that NFs can regulate ion channel function. Because Ca(2+) influx through voltage-gated Ca(2+) channels is required for hormone secretion in AP cells, and oestrogen regulates the expression of Ca(2+) channels in AP cells, the present study examined the expression of alpha1 subunits of voltage gated Ca(2+) channels in relation to that of NF68. Using quantitative immunofluorescence, we demonstrate that alpha 1C and alpha 1D subunits are abundantly expressed in female AP cells, alpha 1A subunits are moderately expressed, and alpha 1G and alpha 1B subunits are expressed at the lowest levels. Double-immunostaining showed that NF68 expression is not correlated with that of alpha 1C, alpha 1D or alpha 1B. Expression of alpha 1G and NF68 appear to be mutually exclusive from each other. Moreover, alpha 1A subunit and NF68 expression are significantly correlated and alpha 1A immunoreactivity is sexually dimorphic (i.e. low in males and high in females) and its levels of expression vary during the oestrous cycle, similar to NF68. Finally, omega-agatoxin IVA, a specific blocker of P/Q type Ca(2+) currents that are a result of the activity of alpha 1A subunits, inhibited to a greater extent spontaneous [Ca(2+)](i) fluctuations in AP cells from females in oestrous and dioestrous, whereas cells from females in pro-oestrous and males were less affected by this toxin. These results suggest a preferential participation of P/Q-type Ca(2+) channels and hence alpha 1A subunits, in regulating spontaneous Ca(2+) transients in AP cells under conditions where the proportion of NF68-expressing cells is high. It remains to be determined whether the expression of NF68 affects that of alpha 1A Ca(2+) channel subunits or vice versa.

Activation of Dopamine D2 Receptors Linked to Voltage-Sensitive Potassium Channels Reduces Forskolin-Induced Cyclic AMP Formation in Rat Pituitary Cells

3,4-Dihydroxyphenylethylamine (dopamine) D2 receptor agonists, including BHT 920 and bromocriptine, and the potassium channel opener minoxidil share the property of hyperpolarizing the plasma membrane by activating voltage-dependent potassium channels. These drugs were tested for their ability to inhibit the cyclic AMP formation induced by forskolin either in intact or in broken pituitary cells. In contrast to bromocriptine, which was active in both experimental systems, BHT 920 and minoxidil inhibited the forskolin-induced cyclic AMP formation in intact-cell but not in broken-cell preparations. The effects of BHT 920 were (a) concentration dependent, with a calculated IC50 of 0.7 microM, (b) dopaminergic in nature, being specifically antagonized by sulpiride, (c) not additive with those induced by minoxidil, and (d) less effective in the presence of potassium channel blockers, such as 4-aminopyridine and tetraethylammonium. These data indicate that the inhibition of forskolin-induced cyclic AMP formation by BHT 920 in intact pituitary cells is not a primary consequence of receptor occupation, but a late event, possibly related to the opening of voltage-dependent potassium channels elicited by this drug through the activation of a subtype of dopamine D2 receptors uncoupled to adenylyl cyclase.

Voltage-activated Ca(2+) channels and their role in the endocrine function of the pituitary gland in newborn and adult mice

We have prepared fresh pituitary gland slices from adult and, for the first time, from newborn mice to assess modulation of secretory activity via voltage-activated Ca(2+) channels (VACCs). Currents through VACCs and membrane capacitance have been measured with the whole-cell patch-clamp technique. Melanotrophs in newborns were significantly larger than in adults. In both newborn and adult melanotrophs activation of VACCs triggered exocytosis. All pharmacologically isolated VACC types contributed equally to the secretory activity. However, the relative proportion of VACCs differed between newborns and adults. In newborn cells L-type channels dominated and, in addition, an exclusive expression of a toxin-resistant R-type-like current was found. The expression of L-type VACCs was up-regulated by the increased oestrogen levels observed in females, and was even more emphasized in the cells of pregnant females and oestrogen-treated adult male mice. We suggest a general mechanism modulating endocrine secretion in the presence of oestrogen and particularly higher sensitivity to treatments with L-type channel blockers during high oestrogen physiological states.

G-protein coupled receptor oligomerization in neuroendocrine pathways

Protein-protein interactions are fundamental processes for many biological systems including those involving the superfamily of G-protein coupled receptors (GPCRs). A growing body of biochemical and functional evidence supports the existence of GPCR-GPCR homo- and hetero-oligomers. In particular, hetero-oligomers can display pharmacological and functional properties distinct from those of the homodimer or oligomer thus adding another level of complexity to how GPCRs are activated, signal and traffick in the cell. Dimerization may also play a role in influencing the activity of agonists and antagonists. We are only beginning to unravel how and why such complexes are formed, the functional implications of which will have an enormous impact on GPCR biology. Future research that studies GPCRs as dimeric or oligomeric complexes will enhance not only our understanding of GPCRs in cellular function but will also be critical for novel drug design and improved treatment of the vast array of GPCR-related conditions.

Mortality in 7B2 null mice can be rescued by adrenalectomy: involvement of dopamine in ACTH hypersecretion

The serine protease prohormone convertase 2 (PC2), principally involved in the processing of polypeptide hormone precursors in neuroendocrine tissues, requires interaction with the neuroendocrine protein 7B2 to generate an enzymatically active form. 7B2 null mice express no PC2 activity and release large quantities of uncleaved ACTH, resulting in a lethal endocrine condition that resembles pituitary Cushing's (Westphal, C. H., Muller, L., Zhou, A., Bonner-Weir, S., Schambelan, M., Steiner, D. F., Lindberg, I. & Leder, P. (1999) Cell 96, 689). Here, we have compared the 7B2 and PC2 null mouse models to determine why the 7B2 null, but not the PC2 null, exhibits a lethal disease state. Both 7B2 and PC2 nulls contained highly elevated pituitary adrenocorticotropic hormone (ACTH); the neurointermediate lobe content of ACTH in 7B2 nulls was 13-fold higher than in WT mice; that of the PC2 null was 65-fold higher. However, circulating ACTH levels were much higher in the 7B2 null than in the PC2 null. Because hypothalamic inhibitory dopaminergic control represents the major influence on intermediate lobe proopiomelanocortin-derived peptide secretion, dopamine levels were measured, and they revealed that 7B2 null pituitaries contained only one-fourth of WT pituitary dopamine. Adrenalectomized 7B2 null animals survived past the usual time of death at 5 weeks; a month after adrenalectomy, they exhibited normal levels of pituitary dopamine, circulating ACTH, and corticosterone. Elevated corticosterone, therefore, seems to play a central role in the lethal phenotype of the 7B2 null, whereas a 7B2-mediated dopaminergic deficiency state may be involved in the actual ACTH hypersecretion phenomenon. Interestingly, adrenalectomized 7B2 nulls also developed unexpectedly severe obesity.

Dopamine modulates exocytosis independent of Ca(2+) entry in melanotropic cells

Dopamine is a known inhibitor of pituitary melanotropic cells. It reduces Ca(2+) influx by hyperpolarizing the cell membrane and by modulating high- and low-voltage-activated (HVA and LVA) Ca(2+) channels. As a result, dopamine reduces the hormonal output of the cell. However, it is unknown how dopamine affects each of the four different HVA Ca(2+) channel types individually. Moreover, it is unknown whether dopamine interacts with exocytosis independent of Ca(2+) channels. Here we show that dopamine differentially modulates the HVA Ca(2+) channels and that it affects the stimulus-secretion coupling through a direct effect on the exocytotic machinery. Sustained L- and P-type Ba(2+) currents are reduced in amplitude and inactivating N- and Q-type currents acquire different activation and inactivation kinetics in the presence of dopamine. The Q-type current shows slow activation, which is a hallmark for direct G-protein modulation. We used membrane capacitance measurements to monitor exocytosis. Surprisingly, we find that the amount of exocytosis per step depolarization is not diminished by dopamine despite the reduction in Ca(2+) current. To test whether dopamine affects the release machinery downstream of Ca(2+) entry, we stimulated exocytosis by dialyzing cells with buffered high-Ca(2+) solutions. Dopamine increased the amount and the rate of exocytosis. In the first 90 s, the rate of secretion was increased two- to threefold, but it was normalized again at 180 s, suggesting that predominantly vesicles that fuse early in the exocytotic phase are modulated by dopamine. Thus while Ca(2+) channels are inhibited by dopamine, the exocytotic machinery downstream of Ca(2+) influx is sensitized. As a result, release is more effectively stimulated by Ca(2+) influx during dopamine inhibition.

Monoaminergic synapses and schizophrenia: 45 years of neuroleptics

In 1952 Delay and Deniker introduced the first antipsychotic, chlorpromazine, into the treatment of mental patients. They subsequently defined the word 'neuroleptic' to describe drugs as different as reserpine and chlorpromazine which seemed to have similar effects on the mental life of patients. In the 1960s the hypothesis was developed, mainly due to Carlsson, that the principal mode of action of neuroleptics was to interfere with synaptic transmission mediated by dopamine (DA) in the brain. This concept was given substantial credence with the discovery by Seeman and Snyder in the 1970s that many of the neuroleptics acted as DA receptor blockers. Subsequently two different classes of DA receptor were defined on the basis of their coupling to adenylate cyclase by Kebabian. In the 1980s molecular biology led to the cloning of five different DA receptors, and at the end of this period vanTol and his colleagues cloned the D4 DA receptor, which has been of considerable interest in the 1990s as it is greatly elevated in the brains of schizophrenics. This historical review ends with a consideration of the possibility that in addition to DA receptors, serotonin and perhaps other transmitter receptors are involved in the aetiology of schizophrenia.

Presynaptic effect of 7-OH-DPAT on evoked [3H]-acetylcholine release in rat striatal synaptosomes

The objective of the present experiments was to study the presynaptic effect of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT, a D(2)-like dopamine receptor agonist) on [3H]-acetylcholine ([3H]-ACh) release induced by potassium (15 mM, 25 mM and 60 mM), potassium channel-blockers (4-aminopyridine, 4-AP; tetraethylammonium, TEA and quinine) and veratridine to gain insight into the mechanisms involved in the activation of the D(2) dopamine-receptor subtype located at striatal cholinergic nerve terminals. 7-OH-DPAT (1 microM) inhibited the evoked [3H]-ACh release induced by K(+) 15 mM in a similar percentage than that obtained during basal conditions (30% and 27%, respectively). Nevertheless, in the presence of 25 mM and 60 mM of K(+) the inhibitory effect of 7-OH-DPAT was completely abolished. 4-AP (1-100 microM) and TEA (1 and 5 mM) significantly enhanced [3H]-ACh release, showing 69.32%+/-7.60% (P<0.001) and 52.27%+/-5.64% (P<0.001), respectively, at the highest concentrations tested. In these conditions, 7-OH-DPAT (1 microM) inhibited the release induced by potassium channel-blockers approximately 25-27%. Quinine (0.1-1 microM) did not alter [3H]-ACh release either in the presence or absence of 7-OH-DPAT. Veratridine 10 microM evoked [3H]-ACh release in the presence of a low-calcium medium, but in such conditions 7-OH-DPAT (1 microM) did not modify the neurotransmitter release in the absence or presence of veratridine. Present data indicate that activation of the presynaptic D(2) dopamine receptor inhibits the [3H]-ACh release by increasing K(+) conductance, as high K(+) concentrations abolished the inhibitory control of 7-OH-DPAT on [3H]-ACh release. This effect could be mediated by potassium channels different from those sensitive to 4-AP, TEA and quinine. In addition, the presynaptic D(2) dopamine-receptor activation seems to not involve changes in intracellular Ca(2+).

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.

Regulation of exocytosis in neuroendocrine cells: spatial organization of channels and vesicles, stimulus-secretion coupling, calcium buffers and modulation

Neuroendocrine cells display a similar calcium dependence of release as synapses but a strongly different organization of channels and vesicles. Biophysical and biochemical properties of large dense core vesicle release in neuroendocrine cells suggest that vesicles and channels are dissociated by a distance of 100-300 nm. This distinctive organization relates to the sensitivity of the release process to mobile calcium buffers, the resulting relationship between calcium influx and release and the modulatory mechanisms regulating the efficiency of excitation-release coupling. At distances of 100-300 nm, calcium buffers determine the calcium concentration close to the vesicle. Notably, the concentration and diffusion rate of mobile buffers affect the efficacy of release, but local saturation of buffers, possibly enhanced by diffusion barriers, may limit their effects. Buffer conditions may result in a linear relationship between calcium influx and exocytosis, in spite of the third or fourth power relation between intracellular calcium concentration and release. Modulation of excitation-secretion coupling not only concerns the calcium channels, but also the secretory process. Transmitter regulation mediated by cAMP and PKA, as well as use-dependent regulation involving calcium, primarily stimulates filling of the releasable pool. In addition, direct effects of cAMP on the probability of release have been reported. One mechanism to achieve increased release probability is to decrease the distance between channels and vesicles. GTP may stimulate release independently from calcium. Thus, while in most cases primary inputs triggering these pathways await identification, it is evident that large dense core vesicle release is a highly controlled and flexible process.

Amphetamine blocks long-term synaptic depression in the ventral tegmental area

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.

All classes of calcium channel couple with equal efficiency to exocytosis in rat melanotropes, inducing linear stimulus-secretion coupling

1. The contribution of low voltage-activated (LVA) T-type Ca2+ channels and four different types of high voltage-activated (HVA) Ca2+ channel to exocytosis, and the relationship between calcium influx and exocytosis during action potentials (APs) were studied in pituitary melanotropes. 2. Selective HVA Ca2+ channel blockers reduced exocytosis, monitored by membrane capacitance measurements, proportional to the reduction in Ca2+ influx. The efficacy of Ca2+ in stimulating exocytosis did not change in the presence of the Ca2+ channel blockers, indicating that all HVA Ca2+ channels act together in stimulating exocytosis. 3. The relationship between Ca2+ influx and exocytosis during the AP was examined using APs recorded from spontaneously active melanotropes as command templates under voltage clamp. Under voltage clamp, multiphasic Ca2+ currents were activated over the entire duration of the APs, i.e. during the rising phase as well as the plateau phase. The maximum amplitude of the Ca2+ current coincided with the peak of the AP. 4. The relationship between Ca2+ entry and exocytosis was linear for the different phases of the AP. Also, the influx of Ca2+ through LVA T-type channels stimulated exocytosis with the same efficacy as through the HVA channels. 5. APs of increasing duration ( approximately 50 to approximately 300 ms) evoked increasing amounts of exocytosis. The number of entering Ca2+ ions and the capacitance change were linearly related to AP duration, resulting in a fixed relationship between Ca2+ entry and exocytosis. 6. The results show that Ca2+ ions, entering a melanotrope, couple with equal strength to exocytosis regardless of the channel type involved. We suggest that the linear relationship between Ca2+ entry and secretion observed under physiological conditions (during APs), results from the equal strength with which LVA and HVA channels in melanotropes couple to exocytosis. This guarantees that secretion takes place over the entire duration of the AP.

Dopamine D2 receptor isoforms expressed in AtT20 cells differentially couple to G proteins to acutely inhibit high voltage-activated calcium channels

The dopamine D2 receptor belongs to the serpentine superfamily of receptors, which have seven transmembrane segments and activate G proteins. D2 receptors are known to be linked, through Galpha(o)- and Galpha(i)-containing G proteins, to several signaling pathways in neuronal and secretory cells, including inhibition of adenylyl cyclase and high voltage-activated Ca2+ channels (HVA-CCs). The dopamine D2 receptor exists in two alternatively spliced isoforms, "long" and "short" (D2L, and D2S, respectively), which have identical ligand binding sites but differ by 29 amino acids in the third intracellular loop, the proposed site for G protein interaction. This has led to the speculation that the two isoforms may interact with different G proteins. We have transfected the AtT20 cell line with either D2L (KCL line) or D2S (KCS line) to facilitate experimentation on the individual isoforms. Both lines show dopamine agonist-dependent inhibition of Q-type HVA-CCs. We combined G protein antisense knock-down studies with multiwavelength fluorescence video microscopy to measure changes in HVA-CC inhibition to investigate the possibility of differential G protein coupling to this inhibition. The initial, rapid, K+ depolarization-induced increase in intracellular Ca2+ concentration is due to influx through HVA-CCs. Our studies reveal that both D2 isoforms couple to Galpha(o) to partially inhibit this influx. However, D2L also couples to Galpha(i)3, whereas D2S couples to Galpha(i)2. These data support the hypothesis of differential coupling of D2 receptor isoforms to G proteins.

Dopamine D2-receptor isoforms expressed in AtT20 cells inhibit Q-type high-voltage-activated Ca2+ channels via a membrane-delimited pathway

Dopamine D2 receptors both acutely and chronically inhibit high-voltage-activated Ca2+ channels (HVA-CCs). Two alternatively spliced isoforms, D2L (long) and D2S (short), are expressed at high levels in rat pituitary intermediate lobe melanotropes but are lacking in anterior lobe corticotropes. We stably transfected D2L and D2S into corticotrope-derived AtT20 cells. Both isoforms coupled to inhibition of Q-type calcium channels through pertussis toxin-sensitive G proteins. Thus, we have created a model system in which to study the kinetics of D2-receptor regulation of Ca2+ channels. Rapid inhibition of HVA-CCs was characterized using a novel fluorescence video imaging technique for the measurement of millisecond kinetic events. We measured the time elapsed (lag time) between the arrival of depolarizing isotonic 66 mM K+, sensed by fluorescence from included carboxy-X-rhodamine (CXR), and the beginning of increased intracellular Ca2+ levels (sensed by changes in indo 1 fluorescence ratio). The lag time averaged 350-550 ms, with no significant differences among cell types. Addition of the D2-agonist quinpirole (250 microM) to the K+/CXR solution significantly increased the lag times for D2-expressing cells but did not alter the lag time for AtT20 controls. The increased lag times for D2L- and D2S-transfected cells suggest that at least a fraction of the Ca2+ channels was inhibited within the initial 350-550 ms. As this inhibition time is too fast for a multistep second messenger pathway, we conclude that inhibition occurs via a membrane-delimited diffusion mechanism.

Dopamine modulates inwardly rectifying hyperpolarization-activated current (Ih) in cultured rat olfactory receptor neurons

The presence of dopamine receptors in olfactory receptor neurons (ORNs) suggests that odor sensitivity may be modulated by neurotransmitters at the level of primary sensory neurons. Using standard patch-clamp techniques on rat ORNs, we found that 1 microM dopamine, 500 microM SQ 22536 (SQ, an adenylyl cyclase inhibitor), 20 and 50 microM quinpirole (a selective dopamine D2 receptor agonist), and 1 mM adenosine 3', 5'-cyclic monophosphate (cAMP) modulate the hyperpolarization-activated current Ih. On hyperpolarizing from a holding potential of -58 mV, a small Cs+-sensitive inwardly rectifying current (Ih) was observed. Increases in extracellular K+ increased Ih amplitude without shifting its voltage dependence of activation, whereas increases in temperature produced an increase in Ih amplitude and a hyperpolarizing shift in the activation curve. Application of 1 microM dopamine reversibly shifted Ih activation to more negative potentials and decreased Ih current amplitudes. These effects were blocked by concomitant application of dopamine with sulpiride, a selective dopamine D2 receptor antagonist. The effects of dopamine were mimicked by quinpirole. Quinpirole (20 microM) decreased Ih current amplitude, but was without effect on Ih voltage dependence of activation. However, 50 microM quinpirole produced both a reduction of Ih peak currents and a hyperpolarizing shift in the activation curve for Ih. External application of the adenylyl cyclase inhibitor SQ 22536 produced a reversible decrease in peak currents but had no effect on Ih voltage dependence of activation, whereas internal application of cAMP shifted Ih activation to more depolarized potentials. Because Ih modulates cell excitability and spike frequency adaptation, our findings support a role for dopamine in modulating the sensitivity and output of rat ORNs to odorants.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Relationships between dopamine-induced changes in cytosolic free calcium concentration ([Ca2+]i) and rate of prolactin secretion. Elevated [Ca2+]i does not indicate prolactin release

This study was undertaken to investigate the relationship between dopamine (DA) induced changes in the cytosolic calcium concentration ([Ca2+]i) and the rate of prolactin secretion using GH4ZR7, a rat pituitary cell line, which express only one subtype of D2 receptor. GH4ZR7 cells were loaded with Fluo-3, a fluorescent Ca2+ indicator, and then perifused with two different doses of DA (10(-7) mol/L and 5 x 10(-4) mol/L). We monitored changes in [Ca2+]i and rate of prolactin release simultaneously by attaching a spectrofluorometer to a dynamic perifusion system. DA has stimulatory and inhibitory effect on prolactin secretion in GH4ZR7 cells; 10(-7) mol/LDA slightly increased [Ca2+]i and stimulated prolactin release, whereas 5 x 10(-4) mol/LDA decreased [Ca2+]i and inhibited prolactin secretion. When the cells were pretreated with pertussis toxin (PTX), 10(-7) mol/L DA had no significant change in [Ca2+]i while stimulating prolactin release, and 5 x 10(-4) mol/L DA reduced [Ca2+]i without having any significant effect on the rate of prolactin secretion. The results of this study demonstrate that changes in [Ca2+]i do not always correlate with the rate of prolactin release from lactotrophs. The dissociation between [Ca2+]i and prolactin release is somewhat expected considering the diverse role of [Ca2+]i and post-[Ca2+]i events, which can change the rate of prolactin release.

Dopamine inhibition: enhancement of GABA activity and potassium channel activation in hypothalamic and arcuate nucleus neurons

Dopamine (DA) decreases activity in many hypothalamic neurons. To determine the mechanisms of DA's inhibitory effect, whole cell voltage- and current-clamp recordings were made from primary cultures of rat hypothalamic and arcuate nucleus neurons (n = 186; 15-39 days in vitro). In normal buffer, DA (usually 10 microM; n = 23) decreased activity in 56% of current-clamped cells and enhanced activity in 22% of the neurons. In neurons tested in the presence of glutamate receptor antagonists D,L-2-amino-5-phosphonovalerate (AP5; 100 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), DA application (10 microM) revealed heterogeneous effects on electrical activity of cells, either hyperpolarization and decrease in activity (53% of 125) or depolarization and increase in spontaneous activity (22% of 125). The DA-mediated hyperpolarization of membrane potential was associated with a decrease in the input resistance. The reversal potential for the DA-mediated hyperpolarization was -97 mV, and it shifted in a positive direction when the concentration of K+ in the incubating medium was increased, suggesting DA activation of K+ channels. Because DA did not have a significant effect on the amplitude of voltage-dependent K+ currents, activation of voltage-independent K+ currents may account for most of the hyperpolarizing actions of DA. DA-mediated hyperpolarization and depolarization of neurons were found during application of the Na+ channel blocker tetrodotoxin (1 microM). The hyperpolarization was blocked by the application of DA D2 receptor antagonist eticlopride (1-20 microM; n = 7). In the presence of AP5 and CNQX, DA (10 microM) increased (by 250%) the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in 11 of 19 neurons and evoked IPSCs in 7 of 9 cells that had not previously shown any IPSCs. DA also increased the regularity and the amplitude (by 240%) of spontaneous IPSCs in 9 and 4 of 19 cells, respectively. Spontaneous and DA-evoked IPSCs and inhibitory postsynaptic potentials were blocked by the gamma-aminobutyrate A (GABA(A)) antagonist bicuculline (50 microM), verifying their GABAergic origin. Pertussis toxin pretreatment (200 ng/ml; n = 15) blocked the DA-mediated hyperpolarizations, but did not prevent depolarizations (n = 3 of 15) or increases in IPSCs (n = 6 of 10) elicited by DA. Intracellular neurobiotin injections (n = 21) revealed no morphological differences between cells that showed depolarizing or hyperpolarizing responses to DA. Immunolabeling neurobiotin-filled neurons that responded to DA (n = 13) showed that GABA immunoreactive neurons (n = 4) showed depolarizing responses to DA, whereas nonimmunoreactive neurons (n = 9) showed both hyperpolarizing (n = 6) and depolarizing (n = 3) responses. DA-mediated hyperpolarization, depolarization, and increases in frequency of postsynaptic activity could be detected in embryonic hypothalamic or arcuate nucleus neurons after only 5 days in vitro, suggesting that DA could play a modulatory role in early development. These findings suggest that DA inhibition in hypothalamic and arcuate nucleus neurons is achieved in part through the direct inhibition of excitatory neurons, probably via DA D2 receptors acting through a Gi/Go protein on K+ channels, and in part through the enhancement of GABAergic neurotransmission.

Anterior pituitary hypoplasia and dwarfism in mice lacking the dopamine transporter

Deletion of the dopamine transporter (DAT) results in increased dopaminergic tone, anterior pituitary hypoplasia, dwarfism, and an inability to lactate. DAT elimination alters the spatial distribution and dramatically reduces the numbers of lactotrophs and somatotrophs in the pituitary. Despite having normal circulating levels of growth hormone and prolactin in blood, hypoplastic glands from DAT-/- mice fail to respond to secretagog stimulation. The effects of DAT deletion on pituitary function result from elevated DA levels that down-regulate the lactotroph D2 DA receptors and depress hypothalamic growth hormone-releasing hormone content. These results reveal an unexpected and important role or DA in the control of developmental events in the pituitary gland and assign a critical role for hypothalamic DA reuptake in regulating these events.

Dopamine D2 receptor stimulation alters G-protein expression in rat pituitary intermediate lobe melanotropes

Stimulation of dopamine D2 receptors inhibits melanotrope pro-opiomelanocortin (POMC) biosynthesis and alpha-melanocyte-stimulating hormone (MSH) secretion. These effects are mediated by G-protein alpha i- and alpha o-subunits and are reversed by stimulating receptors linked to activation of G alpha s protein. Melanotrope activity is increased by haloperidol, a D2 receptor antagonist, and decreased by bromocriptine, a D2 receptor agonist. Both the short and long isoforms of the D2 receptor mRNA and protein increase following chronic haloperidol treatment. After chronic bromocriptine treatment the short isoform is downregulated, whereas the long isoform is upregulated. Our hypothesis is that specific G protein alpha- subunits alter in pattern of expression similarly to the receptor isoforms. Using immunohistochemistry and in situ hybridization, this study examined changes in G alpha i, G alpha o, and G alpha s protein and mRNA expression following chronic treatments with bromocriptine or haloperidol. G alpha i3 and G alpha o immunoreactivities increased following bromocriptine treatment, whereas G alpha s and G alpha i1/2 did not change. Gs immunoreactivity increased after haloperidol treatment, whereas G alpha i1/2, G alpha i3, and G alpha o did not change. G alpha i and G alpha o mRNA increased following bromocriptine and decreased following haloperidol treatments, whereas the inverse results were observed with G alpha s mRNA. These results suggest D2 receptor activation can specifically increase G alpha i3 and G alpha o expression, and D2 receptor blockade increases G alpha s expression.

Inhibition of the T-type Ca2+ current by the dopamine D1 receptor in rat adrenal glomerulosa cells: requirement of the combined action of the G betagamma protein subunit and cyclic adenosine 3',5'-monophosphate

Modulation of ionic Ca2+ currents by dopamine (DA) could play a pivotal role in the control of steroid secretion by the rat adrenal glomerulosa cells. In the present study, we report that DA decreases the T-type Ca2+ current amplitude in these cells. The use of pharmacological agonists and antagonists reveals that this effect is mediated by activation of the D1-like receptors. Modulation by cAMP is complex inasmuch as preincubation of the cells with 8-Br-cAMP or the specific adenylyl cyclase inhibitor, 2',3'-dideoxyadenosine, have no effect per se, but prevent the DA-induced inhibition. The inhibitory effect of DA was abolished by addition of GDPbetaS to the pipette medium but not by pertussis toxin. If a cell is dialyzed with medium containing G alpha(s)-GDP, the inhibitory effect is reduced and cannot be recovered by the addition of GTPgammaS, indicating that the alpha(s) is not involved, but rather the betagamma-subunit. Indeed, DA-induced inhibition was mimicked by G betagamma in the pipette and 8-Br-cAMP in the bath. Similarly, G betagamma release from the activation of the AT1 receptor of angiotensin II did affect the current amplitude only in the presence of 8-Br-cAMP in the bath. The mitogen-activated protein kinase cascade, which can be activated by receptors coupled to Gs, was not involved as shown by the lack of activation of p42mapk by DA and the absence of effect of the mitogen-activated protein kinase inhibitor, PD 098059, on the DA-induced inhibition. Because the binding of G betagamma-subunits to various effectors involves the motif QXXER, we therefore tested the effect of the QEHA peptide on the inhibition of the T-type Ca2+ current induced by DA. The peptide, added to the medium pipette (200 microM), abolished the effect of DA. We conclude that the presence of the G betagamma and an increase in cAMP concentration are both required to inhibit the T-type Ca2+ current in rat adrenal glomerulosa cells.

Two distinct Na+ currents control cytosolic Ca2+ pulsing in Xenopus laevis pituitary melanotrophs

Studies with the Na+ channel blocker tetrodotoxin (TTx) on Ca(2+)-dependent hormone release by mammalian and amphibian pituitary melanotrophs have suggested that the Na+ spikes these cells generate are not responsible for triggering Ca2+ influx and consequently secretion. In contrast, we found in Xenopus laevis melanotrophs that the spontaneously occurring elevations in cytosolic free Ca2+ concentration ("Ca2+ pulses') were dependent on the presence of extracellular Na+ and sensitive to TTx and the Na+ channel activator, veratridine. However, an inhibitory effect of TTx could only be demonstrated when the extracellular Na+ concentration was lowered to near-threshold levels. In voltage-clamp experiments, two distinct Na+ currents were recorded, one sensitive to TTx and the other insensitive to TTx but blocked by micromolar concentrations of Cd2+. Together they appeared to control action potential activity and spontaneous Ca2+ pulsing. These data strongly suggest that Na+ action potentials do regulate cytosolic free Ca2+ concentration in melanotrophs.

A calcium channel reversibly blocked by omega-conotoxin GVIA lacking the class D alpha 1 subunit

In acutely dissociated rat medium-spiny neostriatal neurons, a substantial fraction (approximately 40%) of the calcium current block by omega-conotoxin-GVIA (CgTx) reverses rapidly with washing. The reversibly blocked current differed in voltage dependence from the current irreversibly blocked by CgTx. To test the hypothesis that the reversibly blocked current was attributable to calcium channels with a class D alpha 1 subunit, acutely isolated neurons were subjected to combined patch clamp/reverse transcription-polymerase chain reaction analysis. These studies revealed no correlation between the reversibility of the CgTx block and the expression of class D alpha 1 subunit mRNA.

Co-culture of hypothalamic neurons and melanotrope cells: a model to study synaptogenesis between central neurons and endocrine cells

As a first step towards elucidating mechanisms involved in neuroendocrine synaptogenesis, we developed a model of co-culture based on hypothalamic-intermediate pituitary interactions. Dissociated hypothalamic neurons from fetal rats at embryonic day 15 were cultured in a defined medium together with melanotrope cells of the pituitary intermediate lobe from neonatal rats. In these co-cultures, establishment of synaptic contacts between GABAergic or dopaminergic neurons and an endocrine target cell the melanotrope cell, was studied by morphofunctional approaches. Using double immunostaining with antibodies directed against glutamate decarboxylase or tyrosine hydroxylase and alpha-melanocyte-stimulating hormone, we demonstrated morphological contacts between GABAergic or dopaminergic neurons and melanotrope cells as early as three days in vitro. Furthermore, using an antibody directed against synapsin I, we showed a modification of synapsin I immunoreactivity from diffuse to punctate distribution correlated with the establishment of contacts and the observation of characteristic neuroendocrine synapses by electron microscopy. These results were further confirmed by electrophysiological studies. Patch-clamp recordings demonstrated that, at six days in vitro, some melanotrope cells displayed GABAergic synaptic currents, which occurred either spontaneously and/or could be evoked chemically by 50 mM KCl or 100 microM kainate. The proportion of the melanotrope cells receiving functional synaptic inputs increased until 10 days in culture, a stage at which virtually all melanotrope cells in contact with neurons possessed functional synapses. The results presented here describe the establishment of neuroendocrine synapses in vitro, studied by combining morphofunctional and electrophysiological approaches.

Dopamine (D2) receptor regulation of intracellular calcium and membrane capacitance changes in rat melanotrophs

1. Indo-1 microfluorimetry and patch clamp techniques were used to study the decrease in cytosolic [Ca2+] ([Ca2+]i) caused by dopamine (D2) receptor activation and the calcium dependence of membrane capacitance changes in single rat melanotrophs. 2. [Ca2+]i decreased when extracellular calcium was removed or when the calcium channel blockers nickel (2 mM) or cadmium (100 microM) were applied by bath perfusion. 3. Quinpirole, a dopamine (D2) receptor agonist, reduced [Ca2+]i by 55 +/- 9 nM and hyperpolarized membrane potential by 29 +/- 9 mV simultaneously. 4. Quinpirole-induced [Ca2+]i decrease required deactivation of voltage-dependent calcium channels. Voltage clamping the membrane potential at -25 mV prevented the quinpirole-induced [Ca2+]i decrease. Nickel (2 mM) reduced [Ca2+]i without hyperpolarization and precluded additional [Ca2+]i decrease by quinpirole. 5. Membrane capacitance measurement of secretion rates in cells dialysed with buffered calcium solutions showed that secretion began at approximately 400 nM Cai2+. 6. Melanotrophs have IP3-sensitive calcium stores, but no caffeine-sensitive calcium stores. Calcium released from IP3-sensitive calcium stores also stimulated secretion. 7. Secretion in melanotrophs is modulated by protein kinase activators. cAMP (200 microM) enhanced secretion at [Ca2+]i > 1000 nM. Phorbol myristate acetate (PMA; 200 nM) enhanced secretion at [Ca2+]i < 400 nM, but not in the absence of calcium. 8. Dopamine receptor activation can reduce secretion by reducing the calcium influx through calcium channels with hyperpolarization of the membrane potential. However downregulation of either cAMP or protein kinase C activity may also contribute to the decrease in secretion.

Dihydropyridine block of omega-agatoxin IVA- and omega-conotoxin GVIA-sensitive Ca2+ channels in rat pituitary melanotropic cells

High voltage-activated Ca2+ currents in rat melanotropic cells consist of a sustained and an inactivating component. In this study the pharmacological properties of the high voltage-activated Ca2+ channels underlying these components are investigated with whole-cell recordings. We report that melanotropes express four pharmacologically distinct high voltage-activated Ca2+ channels. Non-inactivating L-type channels account for 35% of the total high voltage-activated channel population. These channels have a very high affinity for the dihydropyridine nimodipine (EC50 approximately 3 pM). The cone snail toxin omega-conotoxin GVIA irreversibly blocked an inactivating high voltage-activated component which accounted for 26% of the total whole-cell high voltage-activated Ca2+ current. The spider toxin omega-agatoxin IVA reversibly blocked an additional 31% of the total high voltage-activated current. The current blocked by omega-agatoxin IVA was not homogenous and consisted of a sustained component with a high affinity for omega-agatoxin IVA (< 10 nM) and an inactivating current with a low affinity for omega-agatoxin IVA (> 100 nM). Both the omega-agatoxin IVA and omega-conotoxin GVIA-blocked currents were very sensitive to nimodipine and nitrendipine with a half maximal block at 200-500 nM. 10 microM nimodipine blocked 70% of the omega-conotoxin GVIA-sensitive current and 90% of the omega-agatoxin IVA-sensitive current. Thus, omega-conotoxin GVIA- and omega-agatoxin IVA-sensitive high voltage-activated Ca2+ channels in melanotropes have an unusual high affinity for dihydropyridines compared to N-, P-, and Q-type channels in other preparations.

Ca2+ current expression in pituitary melanotrophs of neonatal rats and its regulation by D2 dopamine receptors

1. We have examined the voltage-dependent Ca2+ channel activity of rat melanotrophs during the early postnatal period. The cells were dissociated from pituitary intermediate lobes, kept in culture for 5-24 h and then subjected to whole-cell patch-clamp experiments. 2. Like their adult counterparts, neonatal melanotrophs were able to generate Na+ currents, K+ currents and Ca2+ currents in response to membrane depolarization. Ca2+ currents were carried by both low- and high-threshold Ca2+ channels. 3. High-threshold Ca2+ current density decreased sharply between postnatal day 4 (P4) and P12. This period coincides with the onset of dopaminergic innervation within the intermediate lobe. Accordingly, the developmental decrease in Ca2+ current density was largely reversed by chronic in vivo treatment with sulpiride, a dopamine D2 receptor antagonist. 4. Prolonging the time in culture from 5 h to 8 days did not significantly alter the Ca2+ channel activity of P3 melanotrophs, whereas the high-threshold Ca2+ current in previously innervated (P14) melanotrophs stayed small for the first 24 h and then increased 3-fold during the subsequent 4-5 days. This increase required RNA and protein synthesis and was prevented by adding D2 agonists to the culture medium. 5. These results provide evidence for a postnatal suppression of high-threshold Ca2+ current expression in pituitary melanotrophs mediated by presynaptic dopamine neurons through D2 dopamine receptors.

Millisecond studies of calcium-dependent exocytosis in pituitary melanotrophs: comparison of the photolabile calcium chelators nitrophenyl-EGTA and DM-nitrophen

DM-nitrophen (DMN) is a photolabile calcium chelator that has been used extensively to study calcium-triggered exocytosis. Nitrophenyl-EGTA (NPE) is a recently synthesized photolabile calcium chelator that, unlike DMN, selectively binds calcium over magnesium. Here, we compare NPE and DMN for their effectiveness in raising cytosolic calcium ([Ca]i) to trigger exocytosis. The whole cell patch clamp technique was used to monitor membrane capacitance (Cm) and to load both calcium indicator dye and photolabile chelators into rat pituitary melanotrophs prior to flash photolysis. In cells dialysed with DMN, a transient increase in [Ca]i was observed immediately after continuity between the patch pipette and the cell cytosol was achieved. This 'loading transient' reflects the release of calcium from DMN during the binding of intracellular magnesium. No such transient was seen with NPE, consistent with the negligible binding of magnesium to this chelator. Following flash photolysis of DMN or NPE, [Ca]i increased, triggering both a rapid exocytic burst and slower sustained phases of exocytosis. When flashes of the same intensity were compared, the photolysis of NPE resulted in smaller increases in [Ca]i and slower exocytic bursts than that of DMN. These findings are in accordance with the properties of the two compounds [Ellis-Davies G.C.R., Kaplan J.H. Nitrophenyl-EGTA, a photolabile chelator that selectively binds Ca2+ with high affinity and releases it rapidly upon photolysis. Proc Natl Acad Sci USA 1994; 91: 187-191] and the calcium dependency of the exocytic burst [Thomas P., Wong J.G., Lee A.K., Almers W. A low affinity Ca2+ receptor controls the final steps in peptide secretion from pituitary melanotrophs. Neuron 1993; 11: 93-104]. Although NPE is somewhat less effective than DMN in raising [Ca]i, this chelator promises to be a useful and interesting tool for the time-resolved study of calcium-dependent exocytosis in the presence of physiological concentrations of magnesium.

Selective inhibition of high voltage-activated L-type and Q-type Ca2+ currents by serotonin in rat melanotrophs

1. Whole-cell Ca2+ currents (ICa) from cultured rat melanotrophs were identified by their sensitivity to Ca2+ channel blockers, and their modulation by serotonin (5-HT) was studied. All cells displayed high voltage-activated (HVA; > -30 mV) Ca2+ currents. A low voltage-activated (LVA; > -60 mV) Ca2+ current was detected in 92% of the cells. 2. The whole-cell ICa was insensitive to omega-conotoxin GVIA (0.5-1 microM) indicating the absence of N-type Ca2+ channels. 3. At a holding potential (Vh) of -70 mV, the L-type channel blocker nifedipine reduced ICa in a dose-dependent manner with a half-maximal effective concentration (IC50) of 28 nM. The L-type current represented 39% of the total ICa. 4. omega-Agatoxin IVA (omega-Aga IVA) produced a biphasic dose-dependent inhibition of ICa, with IC50 values of 0.4 and 91 nM, indicating the presence of P-type and Q-type Ca2+ channels, which accounted respectively for 16 and 45% of the total ICa. The P-type current was also blocked by synthetic funnel-web spider toxin (sFTX 3.3; 1-10 microM) and was present only in a subpopulation (60-70%) of cells. 5. All cells possessed a Ca2+ current which was resistant to nifedipine (10 microM) and omega-Aga IVA (50 nM). This current was not affected by Ni2+ (40 microM) but was abolished by a low concentration of Cd2+ (10 microM) and by omega-conotoxin MVIIC (1 microM) indicating that it was a Q-type Ca2+ current. 6. 5-HT (10 microM) inhibited the whole-cell ICa in 70% of the cells tested (n = 120) by activating 5-HT1A and 5-HT2C receptors. 5-HT produced either a kinetic slowing of the activation phase (37% of the cells) or a scaling down (14% of the cells) of ICa. In the majority of cells (49%) both types of inhibition were found to coexist. 7. The effects of 5-HT were voltage dependent, rendered irreversible when GTP-gamma-S (30 microM) was present in the pipette solution and abolished by pretreatment of the cells with pertussis toxin (PTX; 150 ng ml-1, 18 h). 8. Low concentrations of omega-Aga IVA (20 nM), which blocked mainly P-type channels, did not reduce the effect of 5-HT on ICa. The scaling down effect of 5-HT on ICa was eliminated in the presence of nifedipine (10 microM) and the kinetic slowing effect of 5-HT persisted after blockade of L- and P-type channels but was abolished by omega-conotoxin MVIIC (1 microM). 9. We conclude that rat melanotrophs possess functional L-, P- and Q-type Ca2+ channels and that 5-HT inhibits selectively L-type and Q-type Ca2+ currents with different modalities. These effects are voltage dependent and mediated by a PTX-sensitive G-protein.

Dopamine receptors and brain function

In the central nervous system (CNS), dopamine is involved in the control of locomotion, cognition, affect and neuroendocrine secretion. These actions of dopamine are mediated by five different receptor subtypes, which are members of the large G-protein coupled receptor superfamily. The dopamine receptor subtypes are divided into two major subclasses: the D1-like and D2-like receptors, which typically couple to Gs and Gj mediated transduction systems. In the CNS, the various receptor subtypes display specific anatomical distributions, with D1-like receptors being mainly post-synaptic and D2-like receptors being both pre- and post-synaptic. D1 and D2 dopamine receptors, the most abundant subtypes in the CNS, appear to be expressed largely in distinct neurons. Substance P and dynorphin, which are expressed in D1 receptor-containing neurons, as well as pre-proenkephalin in D2 receptor-containing neurons, have been used as monitors of dopaminergic activity in the CNS. Expression of immediate early genes, in particular fos, has also been found to correlate with dopaminergic transmission. Dopamine released from the hypothalamus controls the synthesis and secretion of prolactin from the anterior pituitary via D2 dopamine receptors. As yet none of the dopamine receptor subtypes have been associated with the etiology of psychotic disorders, such as schizophrenia. However, the recent characterization of D3 and D4 receptors which are, interestingly, expressed in areas of the CNS mediating cognition and affect or showing increased affinity for certain neuroleptics, have renewed the interest and hope of finding effective neuroleptics devoid of side effects. Finally, the recent production of genetically-derived animals lacking several of these receptor genes should help elucidate which specific physiological paradigms the receptors mediate.

Pituitary PRL secretion induced by tetraethylammonium is inhibited by dopamine through D2 receptors

We have evaluated the inhibitory effect of dopamine on PRL secretion induced by blocking K+ channels. Tumor-derived GH4C1 cells and collagenase-dispersed normal anterior pituitary (AP) cells from young adult male rats were perifused with Krebs-Ringer Hepes medium. In both cell types blocking K+ channels with tetraethylammonium (TEA) induced PRL secretion but did not stimulate cyclic AMP generation. Blocking Na+ channels with 1 microM tetrodotoxin had no effect on basal or TEA-induced PRL secretion. Dopamine inhibited the TEA-induced rise in [Ca2+]i in GH4C1 cells expressing dopamine D2 short receptors. In normal AP cells, 1-100 nM dopamine blocked PRL secretion induced by 20 mM TEA in a log-linear concentration-dependent fashion, with a plateau at > 100 nM dopamine (IC50 30 nM). The D2 dopaminergic receptor agonist, quinpirole, at 100 nM completely blocked PRL secretion induced by 20 mM TEA. The D2 dopaminergic receptor antagonist, sulpiride, at 10 microM reversed the inhibitory effect of 10 microM dopamine on PRL secretion induced by 20 mM TEA. Pretreatment of cells with 100 ng/ml pertussis toxin (PTX) for 24 h prevented 100 nM dopamine inhibition of PRL secretion induced by 20 mM TEA. The data indicate that in both normal lactotroph cells and in tumor-derived cells expressing D2 receptors, PRL secretion stimulated by blocking K+ channels is inhibited by dopamine binding to D2 receptors on the plasma membrane. This inhibition involves interaction with PTX-sensitive Gi protein.

Blocking K+ channels with TEA induces plasmalemma depolarization, increased [Ca2+]i, and ACTH secretion in AtT-20 cells

Blocking K+ channels induces hormone secretion in various pituitary cell lines by a mechanism which is not completely delineated. In the present study, we employed the mouse pituitary tumor-derived AtT-20 cell as a model to evaluate this phenomenon. We correlated the effect of the K+ channel-blocker, tetraethylammonium (TEA), on K+ current and membrane potential utilizing whole cell recording, on cytosol Ca2+ ([Ca2+]i) concentration utilizing fura-2, and on ACTH secretion utilizing a perifusion system. TEA inhibited voltage-dependent K+ current and initiated membrane depolarization in a dose-dependent fashion. Divergences in the sensitivity to TEA between voltage-dependent K+ currents and membrane depolarization indicate that voltage-dependent K+ channels are not responsible for TEA-induced depolarization TEA (1-30 mM) also induced a concentration-dependent rise in [Ca2+]i concentration and ACTH secretion, both of which were inhibited by removing medium Ca2+. Our data indicate that TEA inhibits K+ currents and induces membrane depolarization; this opens Ca2+ channels in the plasmalemma, causing a rise in [Ca2+]i which initiates ACTH secretion. Alteration of K+ channel permeability by hormones or neurotransmitters may thus play an important regulatory role in controlling pituitary hormone secretion.

Dopamine D1 receptor modulates the voltage-gated sodium current in rat striatal neurones through a protein kinase A

1. Whole-cell recordings were made from striatal neurones obtained from neonatal rats and maintained in primary cultures. The effects of dopamine D1 receptor activation were studied on the voltage-gated sodium current. 2. Bath application of a specific D1 agonist, SKF38393 (1 microM), reduced the neuronal excitability recorded in current-clamp by increasing the threshold for generation of action potentials. 3. In voltage-clamp recordings, SKF38393 (1 microM) reversibly reduced the peak amplitude of the sodium current by 37.8 +/- 4.95%. This effect was reversed by the D1 antagonist SCH23390 and was blocked by the intracellular loading of GDP-beta-S (2 mM) suggesting GTP-binding protein involvement. 4. The D1 agonist reduced the peak amplitude of the sodium current without significantly affecting (i) the voltage dependence of the current-voltage relationship, (ii) the voltage dependence of the steady-state activation and inactivation, (iii) the kinetics of the time-dependent inactivation, and (iv) the kinetics of recovery from inactivation. 5. The peak amplitude of the sodium current was progressively reduced by intracellular loading of cyclic AMP-dependent protein kinase (100 U ml-1). 6. Diffusion of a specific peptide inhibitor of the cyclic AMP-dependent protein kinase (PKI; 10 microM) into the cytosol of neurones blocked the effect of the D1 agonist on the sodium current amplitude. 7. These results demonstrate that dopamine acting at the D1 receptor reduces the amplitude of the sodium current without modifying its voltage- and time-dependent properties. This effect involves activation of the cyclic AMP-dependent protein kinase and results in a depression of the striatal neuronal excitability by increasing the threshold for generation of action potentials.

Voltage dependence of G-protein-mediated inhibition of high-voltage-activated calcium channels in rat pituitary melanotropes

Dopamine D2 receptor stimulation inhibited high-threshold, slowly inactivating (L-type) barium currents of isolated, rat pituitary melanotropes in primary culture. The extent of inhibition depended on the concentration of LY 171555 applied. Current activation in the presence of LY 171555 was described by two time constants, a fast one, also observed under control conditions, and a slow one, induced by LY 171555. The slow time constant did not depend on the concentration of LY 171555. Guanosine-5'-O-(3-thiotriphosphate) (100 microM) induced a similar inhibition of the barium currents. Depolarizing prepulses more positive than -20 mV counteracted the inhibition induced by LY 171555 as well as guanosine-5'-O-(3-thiotriphosphate). The voltage dependence and time course of this disinhibition were obtained. The results suggest that the slow time course of activation during current inhibition reflects a voltage-dependent conversion of the channel from the inhibited state to an available state. This voltage dependence is probably an intrinsic property of the calcium channel. The voltage-dependent rate constants of a first-order kinetic model which describes the voltage-dependent inhibition and disinhibition were estimated.

Pharmacology of high-threshold calcium currents in GH4C1 pituitary cells and their regulation by activation of human D2 and D4 dopamine receptors

1. The objective of this study was to characterize the pharmacology of calcium currents in GH4C1 pituitary cells and determine whether activation of heterologously expressed human dopamine receptors can regulate their function. Human D2(short), D3 and D4.2 receptor cDNA's were separately transfected into GH4C1 cells and whole cell calcium currents were recorded by use of nystatin-perforated patch clamp techniques. 2. High-threshold calcium currents were antagonized in a biphasic manner by the dihydropyridine, nisoldipine. The half-maximally effective concentration for each site was 0.2 nM (pIC50 = 9.78 +/- 0.21, n = 4) and 339 nM (pIC50 = 6.47 +/- 0.12, n = 4). The component of current inhibited by 10 nM nisoldipine was also blocked by omega-conotoxin GVIA (30 +/- 9% at 30 nM, n = 6) or by omega-agatoxin IVA (34 +/- 7% at 100 nM, n = 4). 3. Activation of either D2 or D4 receptors by dopamine (10 microM) or quinpirole (0.1 to 10 microM) reduced the peak calcium current by ca. 20% in the majority of cells studied. No inhibition was observed in control or D3 transfected GH4C1 cell lines. 4. The mobilisation of intracellular calcium by thyrotropin releasing hormone in hD4-GH4C1 cells was also studied using Fura-2 AM microspectrofluorimetry. Thyrotropin releasing hormone caused a concentration-dependent increase in calcium mobilisation with an EC50 of 7 nM. D4 receptor activation had no effect upon either basal or hormone-induced [Ca2+]i transients. 5. These results demonstrate that GH4C1 pituitary cells have at least two types of dihydropyridine sensitive high-threshold calcium currents and that like D2 receptors, human D4 receptors can also regulate calcium channel function.

Depression of high-threshold calcium currents by activation of human D2 (short) dopamine receptors expressed in differentiated NG108-15 cells

1. This study examined the regulation of calcium currents in differentiated NG108-15 cells that had been stably transfected with cDNA encoding the short isoform of the human D2 dopamine receptor. Whole cell calcium currents were recorded by nystatin-perforated patch clamp recording. 2. Transient low-threshold calcium currents elicited by depolarizations from -100 mV to -20 mV were reversibly depressed by NiCl2 (84 +/- 8% at 30 microM; n = 3) and by omega-agatoxin IVA (15 +/- 5%; 100 nM, n = 7). These currents were unaffected by hD2 receptor activation. 3. High-threshold calcium currents elicited by depolarizations from -80 mV to 0 mV were partly blocked by omega-conotoxin GVIA (67 +/- 6% at 100 nM, n = 4) and by the subsequent addition of the dihydropyridine, nisoldipine (94 +/- 3% at 1 microM). Consistent with the presence of at least two distinct types of high-threshold calcium channels, nisoldipine alone (38 +/- 15% at 1 microM, n = 6) did not preclude the inhibition caused by omega-conotoxin GVIA (69 +/- 13% at 100 nM, n = 4). The residual current was completely blocked by 100 microM CdCl2 (98.8 +/- 0.4%, n = 7). 4. In hD2-transfected cells, but not untransfected cells, high-threshold currents were depressed by quinpirole (30 +/- 4% at 100 nM; n = 15) with a pEC50 of 8.61 +/- 0.22 (n = 5), as well as by (-)-noradrenaline (28 +/- 5% at 1 microM, n = 9). Responses to both agonists were selectively antagonized by S-(-)sulpiride (100 nM) but not by the alpha-adrenoceptor antagonist, phentolamine (1O microM). The depression caused by (-)-noradrenaline was positively correlated with that of quinpirole for each cell(r2 = 0.91, slope = 0.99).5. hD2-receptor-mediated inhibition of high-threshold calcium currents was abolished by pretreatment of cells with omega-conotoxin GVIA (100 nM; n = 4). However, a component of the high-threshold current was reversibly depressed by omega-conotoxin GVIA (67% to 45% depression after 10 min wash). This current was also depressed by hD2 receptor activation (59 +/- 9% depression in 100 nM quinpirole, n = 3),and was completely blocked by nisoldipine (95 +/- 2% at 1 MicroM).6. These data demonstrate that activation of hD2(short) dopamine receptors can regulate both wconotoxinGVIA, and dihydropyridine-sensitive high-threshold calcium currents in neuroblastoma cells.Morever, the ability of human D2 dopamine receptors to regulate more than one type of calcium current supports the notion that these receptors have a diverse functional role in the central nervous system.

Voltage-clamp analysis of the voltage-gated sodium current of the rat pituitary melanotroph

By using the whole-cell recording technique the Na+ current in cultured melanotrophs of the adult rat pituitary was studied. The Na+ current was eliminated by 1 microM TTX and its equilibrium potential confirmed that it was carried predominantly by Na+. The activation threshold was near -40 mV and half-maximal activation occurred at approximately -23 mV. The peak amplitude of 640 +/- 110 pA (n = 8) occurred near -10 mV. Steady-state half-inactivation occurred near -50 mV. Recovery from inactivation at -70 mV was biexponential: approximately half of the channels recovered with a time constant of 13 ms whereas the slower phase of recovery had a time constant of 430 ms. These properties of the Na+ current are discussed in relation to its role in cell firing and hormone secretion.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.