Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism

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.

Molecular mechanisms of pituitary endocrine cell calcium handling

Endocrine pituitary cells express numerous voltage-gated Na(+), Ca(2+), K(+), and Cl(-) channels and several ligand-gated channels, and they fire action potentials spontaneously. Depending on the cell type, this electrical activity can generate localized or global Ca(2+) signals, the latter reaching the threshold for stimulus-secretion coupling. These cells also express numerous G-protein-coupled receptors, which can stimulate or silence electrical activity and Ca(2+) influx through voltage-gated Ca(2+) channels and hormone release. Receptors positively coupled to the adenylyl cyclase signaling pathway stimulate electrical activity with cAMP, which activates hyperpolarization-activated cyclic nucleotide-regulated channels directly, or by cAMP-dependent kinase-mediated phosphorylation of K(+), Na(+), Ca(2+), and/or non-selective cation-conducting channels. Receptors that are negatively coupled to adenylyl cyclase signaling pathways inhibit spontaneous electrical activity and accompanied Ca(2+) transients predominantly through the activation of inwardly rectifying K(+) channels and the inhibition of voltage-gated Ca(2+) channels. The Ca(2+)-mobilizing receptors activate inositol trisphosphate-gated Ca(2+) channels in the endoplasmic reticulum, leading to Ca(2+) release in an oscillatory or non-oscillatory manner, depending on the cell type. This Ca(2+) release causes a cell type-specific modulation of electrical activity and intracellular Ca(2+) handling.

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Mechanisms of cytosolic Ca2+ suppression by prostaglandin E2 receptors in rat melanotrophs

We have previously reported that voltage-dependent Ca2+ (VDC) channels of rat melanotrophs are inhibited by prostaglandin E2 (PGE2). In this study, mechanisms involved in the inhibitory actions of PGE2 receptors of rat melanotrophs were analysed using reverse transcriptase-polymerase chain reaction (RT-PCR), Ca2+-imaging and whole-cell, patch-clamp techniques with recently developed EP agonists, each of which is selective for the known four subclasses of EP receptors (EP1-4). PGE2 reversibly suppressed the cytosolic Ca2+ concentration ([Ca2+]i). The maximum reduction in [Ca2+]i by PGE2 was comparable to that by dopamine or to that by extracellular Ca2+ removal. RT-PCR analysis of all four EP receptors revealed that EP3 and EP4 receptor mRNAs were expressed in the intermediate lobe. The effects of PGE2 to suppress [Ca2+]i were mimicked by the selective EP3 agonist, ONO-AE-248, whereas three other EP agonists, ONO-DI-004 (EP1), ONO-AE1-259 (EP2) and ONO-AE1-329 (EP4), had little or no effect on [Ca2+]i. All four G-protein activated inward rectifying K+ (GIRK) channel mRNAs were identified in intermediate lobe tissues by RT-PCR. Dopamine concentration-dependently activated GIRK currents, whereas PGE2 did not activate GIRK currents, even at the concentration causing maximal inhibition of VDC channels. These results suggest that PGE2 acts on EP3 receptors to suppress Ca2+ entry of rat melanotrophs by selectively inhibiting VDC channels of these cells. We have compared the possible cellular and molecular mechanisms of inhibition by dopamine and PGE2.

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.

Odor processing in the frog olfactory system

In the frog, unitary electrophysiological recordings have been extensively used to investigate odor processing along the olfactory pathways. From the responses of primary second-order neurons, neuroreceptor and mitral cells, odor stimuli could be classified in qualitative groups, revealing that neuronal discriminative mechanisms are partly based on the structure of odor molecule. In the olfactory bulb, thanks both to the anatomical convergence of primary afferences and intrinsic network properties, mitral cells have been demonstrated to gain in odor discrimination and detection power abilities. GABAergic bulbar interneurons were found to be involved in the control of mitral cell excitability, adjusting response thresholds and duration and promoting a progressive increase of burst discharges with stimulus concentration. Otherwise, dopamine was observed to shunt off mitral cell spontaneous activity without altering their odor responsivity properties. Dopamine was demonstrated to act through D2 receptors. Matching anatomical and electrophysiological data, D2 receptors are assumed to be localized on mitral cells. The frog olfactory cortex neurons, silent at rest, could be segregated in two functional groups basing on their odor response properties. The first group shared most intensity coding properties with mitral cells while showing a lower discriminative power, similar to that of neuroreceptor cells. By contrast, the second group provided only minimal intensity coding and, basing on its high discrimination power, was assumed to be mainly devoted to odor discrimination. Thus, along the olfactory pathways, intensity and quality odor parameters which are simultaneously encoded by a neuroreceptor or mitral cell, become specified by two distinct populations in the cortex.

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.

Dopaminergic modulation of mitral cell activity in the frog olfactory bulb: a combined radioligand binding-electrophysiological study

Dopamine content in the amphibian olfactory bulb is supplied by interneurons scattered among mitral cells in the external plexiform/mitral cell layer. In mammals, dopamine has been found to be involved in various aspects of bulbar information processing by influencing mitral cell odour responsiveness. Dopamine action in the bulb depends directly on the localization of its receptor targets, found to be mainly of the D2 type in mammals. The present study assessed, in the frog, both the anatomical localization of D2-like, radioligand-labelled receptors of dopamine and the in vivo action of dopamine on unitary mitral cell activity in response to odours delivered over a wide range of concentrations. The [125I]iodosulpride-labelled D2 binding sites were visualized on frozen sagittal sections of frog brains by film radioautography. The sites were found to be restricted to the external plexiform/mitral cell layer; other layers of the olfactory bulb were devoid of specific labelling. Electrophysiological recordings of mitral unit activity revealed that dopamine or its agonist apomorphine induced a drastic reduction of spontaneous firing rate of mitral cells in most cases without altering odour intensity coding properties of these cells. Moreover, pre-treatment with the D2 antagonist eticlopride blocked the dopamine-induced reduction of mitral cell spontaneous activity. In the frog olfactory bulb, both anatomical localization of D2-like receptors and functional data on dopamine involvement in information processing differ from those reported in mammals. This suggests a phylogenetic evolution of dopamine action in the olfactory bulb. In the frog, anatomical data perfectly corroborate electrophysiological results, together strongly suggesting a direct action of dopamine on mitral cells. In a physiologically operating system, such an action would result in a global improvement of signal-to-noise ratio.

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.

Calcium oscillations in melanotrope cells of Xenopus laevis are differentially regulated by cAMP-dependent and cAMP-independent mechanisms

Intracellular Ca2+ oscillations play an important role in the induction of alpha-MSH release from pituitary melanotrope cells of Xenopus laevis. Oscillatory, secretory and adenylyl cyclase activities are all inhibited by dopamine, neuropeptide Y (NPY) and baclofen (a GABAB receptor agonist) and stimulated by sauvagine. In this study, we test the hypothesis that these neural messengers regulate the Ca2+ oscillations via a cAMP/protein kinase A (PKA)-dependent mechanism. To this end, video-imaging microscopy was applied to single Xenopus melanotropes loaded with the Ca2+ indicator Fura-2. The cAMP-dependent PKA inhibitor H89 blocked Ca2+ oscillations as well as the stimulatory actions of 8-Br-cAMP and sauvagine. Treatment of cells inhibited by baclofen with either 8-Br-cAMP or sauvagine led to a reappearance of Ca2+ oscillations. A similar result was found for cells inhibited by NPY. Neither 8-Br-cAMP nor sauvagine induced Ca2+ oscillations in cells inhibited by dopamine. Depolarizing dopamine-inhibited cells with high potassium also failed to induce oscillations, but combining 8-Br-cAMP with membrane depolarization induced oscillations. It is concluded that sauvagine, baclofen and NPY work primarily through a cAMP/PKA-pathway while dopamine inhibits Ca2+ oscillations in a dual fashion, namely via both a cAMP-dependent and a cAMP-independent mechanism, the latter probably involving membrane hyperpolarization.

Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells

The functions of the D4 receptor, a newly cloned D2-like receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-like receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine. These results indicate the presence in the neonatal cerebellum of a functional D4 receptor inhibiting an L-type calcium current, an action involving a Gi/Go protein but independent from adenylate cyclase inhibition.

Melanostatin (NPY) inhibited electrical activity in frog melanotrophs through modulation of K+, Na+ and Ca2+ currents

1. Melanostatin, a thirty-six amino acid peptide recently isolated from the frog brain due to its ability to inhibit alpha-melanocyte-stimulating hormone (alpha-MSH) release, is the amphibian counterpart of mammalian neuropeptide Y (NPY). The effect of synthetic melanostatin on the bioelectrical activity of cultured frog melanotrophs was studied in 124 cells by using the whole-cell patch-clamp technique. 2. In current-clamp experiments, melanostatin (1 microM) provoked a reversible hyperpolarization and a suppression of spontaneous action potentials. In some cells the hyperpolarizing response was absent, but an arrest of spike firing still occurred. 3. Melanostatin-induced hyperpolarization was associated with a decrease in membrane resistance. In voltage-clamp experiments, melanostatin induced an outward current at a constant command potential. This hyperpolarizing outward current appeared to be carried by potassium ions. 4. Cell dialysis with the non-hydrolysable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) sustained the outward current produced by melanostatin. Dopamine (1 microM), which generates a similar hyperpolarizing outward current in frog melanotrophs, was not capable of increasing the current provoked by melanostatin and sustained by GTP gamma S. 5. Melanostatin also modulated voltage-operated currents. The amplitude of voltage-activated potassium current was increased by 30%. 6. Melanostatin reduced the fast sodium current. This inhibitory effect was rather persistent compared to the other modulated currents. 7. Melanostatin markedly scaled down high voltage-activated N- and L-like calcium currents. The activation kinetics of these two calcium currents were not altered by the peptide. 8. Pretreatment of melanotrophs with pertussis toxin (1 microgram ml-1) blocked melanostatin-induced inhibition of N- and L-like calcium currents. 9. It is concluded that the NPY-related peptide melanostatin generates a very complex pattern of electrical responses in frog melanotrophs, including hyperpolarization and modulation of voltage-activated currents underlying action potentials. G proteins appear to mediate at least part of these effects.

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.

Analysis of inositol phosphate metabolism in melanotrope cells of Xenopus laevis in relation to background adaptation

The present study examined inositol phosphate metabolism in melanotrope cells of Xenopus laevis to determine if inositol phosphates are involved in regulating the biosynthetic or secretory activity of these cells. No correlation could be found between inositol phosphate metabolism and the secretory activity of the melanotrope cells. Therefore, we conclude that inositol phosphate production is not directly involved in the regulation of release of alpha-MSH from these cells. However, there were dramatic differences in the capacity of the melanotrope cells to produce inositol phosphates dependent on the state of background adaptation of the animals from which the melanotropes were derived; cells from white-adapted animals had a low capacity to produce inositol phosphates, whereas melanotropes from black-adapted animals had a high capacity in this regard. During adaptation of animals from a white to a black background, the capacity of the melanotrope cells to produce inositol phosphates was only very slowly acquired, reminiscent of the slow acquisition displayed by these cells to produce POMC during background adaptations. Likewise, during black to white background adaptation, the melanotrope cells very slowly lost the capacity to phosphorylate inositol, which correlates with the slow loss of the biosynthetic capacity of melanotrope cells during such adaptations. Altogether we conclude that inositol phospholipid metabolism is likely involved in the regulation of the biosynthetic processes of melanotrope cells of Xenopus laevis.

Voltage-dependent modulation of calcium current by GTP gamma S and dopamine in cultured frog pituitary melanotrophs

Dopamine (1 microM) reversibly scaled down barium current through high-voltage activated (HVA) calcium channels but had little effect on the time course of current activation in cultured frog melanotrophs. Intracellular perfusion with guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S; 100 microM) sustained the effect of dopamine. Moreover, GTP gamma S drastically slowed down the current activation kinetics. The latter effect was in part reversed by dopamine. A conditioning prepulse to +70 mV facilitated the current in GTP gamma S-dialyzed cells but not in cells exposed to dopamine. These results suggest the existence of a dual G protein-mediated mechanism for reducing HVA calcium current.

Involvement of non-selective cationic channels in the generation of pacemaker depolarizations and firing behaviour in cultured frog melanotrophs

The firing patterns of cultured frog melanotrophs were studied using the patch-clamp technique. In the cell-attached mode, unitary currents were frequently observed as well as biphasic waveforms which were attributed to action potentials 'leaking' through the patch membrane. An inwardly rectifying single-unit current was observed with pipette solutions containing either 100 mM K+ or 100 mM Na+. Under both conditions, these channels displayed an identical I/V relationship, yielding a unitary conductance of 110 pS. The channel opening time was extremely long (50-3000 ms) and single-channel currents showed typical relaxations, which triggered bursts of action currents. In the whole-cell configuration large (2-12 mV) fluctuations in the membrane voltage of current-clamped cells frequently occurred. The deflections appeared to result from single-channel currents. Depolarizing 'events' often led to the discharge of action potentials. Taken together, our data provide evidence for the existence of high-conductance cationic channels in frog pars intermedia cells. These channels may, at least in some cases, be responsible for the generation of pacemaker depolarizations, thereby regulating firing behaviour. It is concluded, that the current traversing a single channel can seriously affect the membrane potential and excitability of frog melanotrophs.