Action potentials in gland cells of rat pituitary pars intermedia: inhibition by dopamine, an inhibitor of MSH secretion

Comparison of rat anterior and intermediate pituitary in tissue culture: corticotropin (ACTH) and beta-endorphin

The forms of immunoreactive beta-endorphin-sized material in extracts of anterior and intermediate-posterior pituitary from the rat examined by the ion exchange chromatography method of Zakarian & Smyth. The anterior pituitary primarily contained material that co-migrated with synthetic camel beta-endorphin(1-31), whereas the intermediate-posterior pituitary contained relatively little such material. The majority of immunoactive beta-endorphin-sized peptides in the intermediate pituitary eluted at lower concentrations of NaCl than did camel beta-endorphin. Conditions were developed for the stable, long-term tissue culture of dissociated intermediate-posterior pituitary cells. Extracts of cells maintained in tissue culture for 18 h or nine days had the same content of immunoreactive beta-endorphin, 16k fragment, ACTH(18-39) (or CLIP) and ACTH(17-24). Throughout the nine days in culture, characteristic cells that could be immunostained with antibodies to various regions of pro-ACTH/endorphin were present; during the time in culture, non-reactive background cells multiplied rapidly. The major proteolytic processing of pro-ACTH/endorphin remained characteristic of intermediate pituitary tissue throughout the nine days in tissue culture, and did not become similar to the simpler pattern of proteolytic processing found in the anterior pituitary.

Expression of tetrodotoxin-sensitive and resistant sodium channels by rat melanotrophs

Rat melanotrophs fire Na+ and Ca2(+)-dependent action potentials. Whereas the molecular identity of Ca2+ channels expressed by these cells is well documented, less is known about Na channels. We characterize the expression of seven sodium channel alpha-subunit and the beta1- and beta2-subunit mRNAs. The tetrodotoxin-resistant Nav1.8 and Nav1.9 alpha subunit mRNAs are detected in the newborn intermediate lobe and in cultured melanotrophs. Electrophysiological recordings further demonstrate the expression of both tetrodotoxin-sensitive and tetrodotoxin-resistant currents by dissociated melanotrophs. Moreover, activated sodium channels are able to elicit intracellular calcium waves, both in the absence or in the presence of tetrodotoxin. This work shows that rat melanotrophs express functional tetrodotoxin-resistant sodium channels, whose activation can lead to the generation of intracellular calcium waves.

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.

Postnatal decrease of sodium current density in rat pituitary melanotropes following the onset of dopaminergic innervation

Peptide secretion from rat melanotropes is tonically inhibited by a dopaminergic synaptic input that develops after birth and acts through D2 dopamine receptors. In this study, whole-cell Na(+) currents were recorded from melanotropes that were isolated from rat pituitary intermediate lobes at postnatal days 1-20 (P1-P20) and maintained in culture for 5-24 h. Coincident with the development of innervation, melanotropes exhibited a progressive decrease in peak Na(+) current density from P3 to P14. The decrease involved a 50% reduction in maximal Na(+) conductance with no detectable changes in channel gating. Subcutaneous injections of the D2 antagonist sulpiride, applied from P11 to P13, restored melanotrope Na(+) channel activity to pre-innervation levels. Thus, the activation of D2 receptors by the dopaminergic input reduces the functional expression of Na(+) channels in melanotropes.

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.

Tonic dopamine inhibition of L-type Ca2+ channel activity reduces alpha1D Ca2+ channel gene expression

Hormones and neurotransmitters have both short-term and long-term modulatory effects on the activity of voltage-gated Ca2+ channels. Although much is known about the signal transduction underlying short-term modulation, there is far less information on mechanisms that produce long-term effects. Here, the molecular basis of long-lasting suppression of Ca2+ channel current in pituitary melanotropes by chronic dopamine exposure is examined. Experiments involving in vivo and in vitro treatments with the dopaminergic drugs haloperidol, bromocriptine, and quinpirole show that D2 receptors persistently decrease alpha1D L-type Ca2+ channel mRNA and L-type Ca2+ channel current without altering channel gating properties. In contrast, another L-channel (alpha1C) mRNA and P/Q-channel (alpha1A) mRNA are unaffected. The downregulation of alpha1D mRNA does not require decreases in cAMP levels or P/Q-channel activity. However, it is mimicked and occluded by inhibition of L-type channels. Thus, interruption of the positive feedback between L-type Ca2+ channel activity and alpha1D gene expression can account for the long-lasting regulation of L-current produced by chronic activation of D2 dopamine receptors.

Tonic dopamine inhibition of L-type Ca2+ channel activity reduces alpha1D Ca2+ channel gene expression

Hormones and neurotransmitters have both short-term and long-term modulatory effects on the activity of voltage-gated Ca2+ channels. Although much is known about the signal transduction underlying short-term modulation, there is far less information on mechanisms that produce long-term effects. Here, the molecular basis of long-lasting suppression of Ca2+ channel current in pituitary melanotropes by chronic dopamine exposure is examined. Experiments involving in vivo and in vitro treatments with the dopaminergic drugs haloperidol, bromocriptine, and quinpirole show that D2 receptors persistently decrease alpha1D L-type Ca2+ channel mRNA and L-type Ca2+ channel current without altering channel gating properties. In contrast, another L-channel (alpha1C) mRNA and P/Q-channel (alpha1A) mRNA are unaffected. The downregulation of alpha1D mRNA does not require decreases in cAMP levels or P/Q-channel activity. However, it is mimicked and occluded by inhibition of L-type channels. Thus, interruption of the positive feedback between L-type Ca2+ channel activity and alpha1D gene expression can account for the long-lasting regulation of L-current produced by chronic activation of D2 dopamine receptors.

Dopamine D2 receptor effects on GABA(A) receptor expression may modify melanotrope peptide release

Stimulation of melanotrope dopamine D2 receptors decreases mitotic rate, calcium channel activity, and the biosynthesis of several proteins. This study demonstrates that D2 receptor activation also affects GABA(A) receptor beta2/beta3 subunit immunoreactivity. Following chronic treatment with haloperidol, a D2 receptor antagonist, GABA(A) receptor immunoreactivity increased, whereas it decreased after chronic treatment with bromocriptine, a dopamine D2 receptor agonist. Thus, these data indicate that D2 function regulates GABA(A) receptor expression in melanotropes, a mechanism by which peptide release may be modified.

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.

Characterization of functional GABAergic synapses formed between rat hypothalamic neurons and pituitary intermediate lobe cells in coculture: Ca2+ dependence of spontaneous IPSCs

Rat hypothalamic neurons and endocrine cells from the intermediate lobe of the pituitary were grown in dissociated coculture. Neurons positively stained with an antibody against glutamate decarboxylase established apparent contacts with the alpha-melanocyte-stimulating hormone-positive endocrine cells. These sites of contact were intensely labeled with an antibody against the synaptic protein synapsin I and displayed ultrastructural features characteristic of synapses. Using patch-clamp recordings, we have demonstrated that these contacts correspond to functional GABAergic synapses. The synaptic currents were blocked reversibly by bicuculline (5 microM) and SR95531 (5 microM), two competitive antagonists of the GABAA receptor. At a holding potential of -60 mV, spontaneously occurring IPSCs (s-IPSCs) had small amplitudes (10-100 pA), whereas electrically evoked IPSCs (ee-IPSCs) had amplitudes up to 1 nA. The rise times of both types of IPSCs were fast ( < or = 1 msec), and their decaying phases were fitted in most cases with a single exponential function (time constant 50 msec). The amplitude distribution of s-IPSCs did not reveal clear, equally spaced peaks and was little affected by tetrodotoxin, suggesting that most s-IPSCs were miniature IPSCs. Reduction of extracellular calcium concentration to 0.3 mM induced a marked decrease in s-IPSC frequency and revealed a single amplitude peak at 10 pA, suggesting that a single quantum of GABA activates 8-10 GABAA channels. Thus, our preparation might be an interesting model to study different aspects of synapse formation between a central neuron and its target as well as the fundamental mechanisms of synaptic transmission at central synapses.

Characterization of functional GABAergic synapses formed between rat hypothalamic neurons and pituitary intermediate lobe cells in coculture: Ca2+ dependence of spontaneous IPSCs

Rat hypothalamic neurons and endocrine cells from the intermediate lobe of the pituitary were grown in dissociated coculture. Neurons positively stained with an antibody against glutamate decarboxylase established apparent contacts with the alpha-melanocyte-stimulating hormone-positive endocrine cells. These sites of contact were intensely labeled with an antibody against the synaptic protein synapsin I and displayed ultrastructural features characteristic of synapses. Using patch-clamp recordings, we have demonstrated that these contacts correspond to functional GABAergic synapses. The synaptic currents were blocked reversibly by bicuculline (5 microM) and SR95531 (5 microM), two competitive antagonists of the GABAA receptor. At a holding potential of -60 mV, spontaneously occurring IPSCs (s-IPSCs) had small amplitudes (10-100 pA), whereas electrically evoked IPSCs (ee-IPSCs) had amplitudes up to 1 nA. The rise times of both types of IPSCs were fast ( < or = 1 msec), and their decaying phases were fitted in most cases with a single exponential function (time constant 50 msec). The amplitude distribution of s-IPSCs did not reveal clear, equally spaced peaks and was little affected by tetrodotoxin, suggesting that most s-IPSCs were miniature IPSCs. Reduction of extracellular calcium concentration to 0.3 mM induced a marked decrease in s-IPSC frequency and revealed a single amplitude peak at 10 pA, suggesting that a single quantum of GABA activates 8-10 GABAA channels. Thus, our preparation might be an interesting model to study different aspects of synapse formation between a central neuron and its target as well as the fundamental mechanisms of synaptic transmission at central synapses.

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.

Melanotrope dopamine D2 receptor isoform expression in the developing rat pituitary

This study measured melanotrope mRNA and protein expression for the dopamine D2 receptor, and its long isoform, in relation to the appearance of dopamine in axons of the postnatal rat pituitary intermediate lobe. At postnatal day 2, prior to the onset of dopaminergic innervation, D2 receptor (D2T) mRNA was expressed heterogeneously in a subpopulation of melanotropes which also expressed the long isoform (DL). The D2L mRNA appeared to be predominant during early postnatal development, since the D2T probe, which did not discriminate between the isoforms, and the D2L probe hybridized generally to the same cells, as demonstrated in serial sections. Immunohistochemical methods, using two different antisera for the D2T receptor, however, indicated a low level of protein in most melanotropes. Localization of D2L protein corresponded well to D2T receptor mRNA distribution. At day 10, representing a time when dopamine is present in axons throughout the lobe, both D2T receptor mRNA and protein were detected in a significantly larger population of melanotropes than those expressing D2L mRNA and protein. This suggests the appearance of detectable short isoform (D2S) mRNA in virtually all melanotropes and implicates dopamine as a possible signal for increasing D2S isoform mRNA expression.

GFAP expression induced by dopamine D(2) receptor agonists in the rat pituitary intermediate lobe

This study was conducted to determine if intermediate lobe glial-like cells are affected by compounds that regulate melanotrope biosynthetic activity via the dopamine D(2) receptor. Glial-like cells were stellate, and expressed glial fibrillary acidic protein (GFAP) and vimentin in cell bodies and processes as revealed by immunohistochemistry. Following bromocriptine and quinpirole treatments, the number of cell bodies and processes expressing vimentin did not change, whereas those expressing GFAP increased, although never to exceed the number of vimentin containing structures. The percent of cells and processes coexpressing GFAP and vimentin also increased. The GFAP response was reversible by haloperidol treatment following the agonist treatment. Haloperidol treatment alone did not change GFAP expression. Thus, following D(2) receptor agonist treatment, GFAP was induced in pre-existing vimentin-positive glial cells. Dopamine D(2) receptor mRNA and protein were detected in melanotropes, but not in cells expressing GFAP or vimentin. Although glial-like cells may express dopamine D(2) receptor mRNA and protein below the detection levels of our methods, the possibility of an indirect effect via dopamine D(2) receptor agonists acting on melanotropes needs to be considered.

Astrocytic GABA receptors

GABA receptors are distributed widely throughout the central nervous system on a variety of cell types. It has become increasingly clear that astrocytes, both in cell culture and tissue slices, express abundant GABAA receptors. In astrocytes, GABA activates Cl(-)-specific channels that are modulated by barbiturates and benzodiazepines; however, the neuronal inverse agonist methyl-4-ethyl-6, 7-dimethoxy-beta-carboline-3-carboxylate enhances the current in a subpopulation of astrocytes. The properties of astrocytic GABAA receptors, therefore, are remarkably similar to their neuronal counterparts, with only a few pharmacological exceptions. In stellate glial cells of the pituitary pars intermedia, GABA released from neuronal terminals activates postsynaptic potentials directly. The physiological significance of astrocytic GABAA-receptor activation remains unknown, but it may be involved in extracellular ion homeostasis and pH regulation. At present, there is considerably less evidence for the presence of GABAB receptors on astrocytes. The data that have emerged, however, indicate a prominent role for second-messenger regulation by this receptor.

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.

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.

Spontaneous calcium oscillations in Xenopus laevis melanotrope cells are mediated by omega-conotoxin sensitive calcium channels

The dynamics of intracellular Ca2+ signalling in single melanotrope cells of the pituitary gland of the amphibian Xenopus laevis have been studied by means of a digital imaging technique using the fluorescent dye Fura-2. When placed in vitro, the majority of the cells (77%) displayed spontaneous oscillatory changes in the free cytosolic Ca2+ concentration with a frequency of 1 +/- 0.25 (SD) min-1. The oscillations rapidly stopped when extracellular Ca2+ was reduced to nanomolar concentrations, revealing their complete dependence on Ca2+ influx. The fact that the Ca2+ oscillations were blocked by 1 microM omega-conotoxin, but not by nifedipine, at concentrations up to 50 microM, indicated that Ca2+ entered the cell via N-type rather than L-type voltage operated Ca2+ channels. Thapsigargin, a putative inhibitor of intracellular Ca(2+)-ATPase activity, elevated the baseline Ca2+ concentration but had no effect on the occurrence of the spontaneous oscillations. This suggests that intracellular Ca2+ pools are not involved in the mechanism underlying spontaneous Ca2+ oscillations. This is the first report showing spontaneous Ca2+ oscillations mediated by N-type Ca2+ channels in melanotrope cells.

Indications from Mn-quenching of Fura-2 fluorescence in melanotrophs that dopamine and baclofen close Ca channels that are spontaneously open but not those opened by high [K+]O; and that Cd preferentially blocks the latter

In unstimulated melanotrophs, which secrete spontaneously, Mn caused a progressive quenching of Fura-2 fluorescence (F360) which was: (a) unaffected by tetrodotoxin to suppress spontaneous Na-action potentials; (b) slowed by lowering temperature to 23 degrees C; and (c) arrested by the Ca channel blocker, Ni. Mn quenching slowed on lowering [K+]O from 5 to 2 mM to hyperpolarize (indicating Mn entry through voltage-dependent channels) and accelerated on raising [K+]O to 50 or 100 mM to strongly depolarize (indicating recruitment of high threshold channels). The secreto-inhibitor, dopamine, arrested spontaneous Mn quenching and so, too, did the GABAB agonist, baclofen; and these effects like those of the two agonists on secretion and [Ca2+]i were blocked by the specific D2 and GABAB antagonists, sulpiride and CGP 35348, respectively, and were lost following exposure to pertussis toxin. By contrast, neither dopamine nor baclofen prevented Mn quenching in response to high K, although this was arrested by Ni. A second Ca channel blocker, Cd, in concentrations that inhibited the response to high K, failed to inhibit spontaneous entry of Mn. This preferential effect offers an explanation for observations made with Cd that have been interpreted as contrary to the notion of Ca-regulated secretion in the melanotroph. The results we have obtained are interpreted to mean that in the melanotroph secreting spontaneously some voltage-dependent Ca channels are in the open state; that this open state is not dependent on any Na spiking activity; and that these channels are preferentially closed by dopamine and baclofen which are without effect on Ca channels opened by strongly depolarizing concentrations of K.

Dopamine D2 receptor stimulation differentially affects voltage-activated calcium channels in rat pituitary melanotropic cells

1. Whole-cell voltage clamp recordings were made from 141 rat pituitary melanotropic cells in short-term, serum-free, primary culture. The effects of the dopamine D2 receptor agonist, LY 171555, on sodium, potassium and barium currents were investigated. 2. Application of 1 microM-LY 171555 did not affect the inward sodium and outward potassium currents. 3. Application of LY 171555 reversibly inhibited barium currents, with the strongest inhibition on the early inward current. The effect was dose dependent (IC50 = 4 x 10(-8) M), maximal inhibition of the total current was 30% and the LY 171555-induced block (1 microM) was reversibly antagonized by (+/-)sulpiride (4 microM). 4. Using barium-selective saline solutions, different types of barium current (T, N, and two L components) were identified on the basis of their voltage-dependent kinetics. Their relative amplitudes differed between cells. 5. The T-type current activated at potentials positive to -60 mV, reaching peak amplitude between -20 and -10 mV. At -30 mV, this current was inhibited up to 30% by 1 microM-LY 171555. The time constants of activation (10-3 ms) and inactivation (50-20 ms) as well as the voltage dependence of inactivation (potential of half-maximal inactivation (H), -61 mV; slope factor (S), 4.9 mV) were not affected by LY 171555 application. 6. A rapidly inactivating (time constants 100-50 ms), high threshold current component was identified as an N-type current. This current activated at command potentials positive to -30 mV and reached a maximal amplitude at +10 mV. The steady-state inactivation was described by a single Boltzmann equation with H = -65 mV and S = 11.7 mV. Application of 1 microM-LY 171555 completely suppressed this current. 7. The slowly inactivating (time constants > 1500 ms), high-threshold, L-type current displayed the same voltage dependence of activation as the N current. The voltage dependence of inactivation was modelled by the sum of two Boltzmann equations (L1: H1 = -45 mV, S1 = 13.0 mV; L2:H2 = -11 mV, S2 = 6.0 mV), indicating the existence of two L channel populations. Neither time course, nor voltage dependence of inactivation were influenced by LY 171555. However, LY 171555 induced a slow-down in the time course of activation, which necessitated the use of two time constants to model the activation kinetics. One of these (approximately 2 ms) was also observed under control conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine inhibits voltage-activated calcium channel currents in rat pars intermedia pituitary cells

Several lines of evidence suggest that dopamine acts as a neurotransmitter that inhibits both hormone secretion and electrical activity in pituitary intermediate cells (melanotrophs). In this study we examined the effects of exogenously applied dopamine on voltage activated calcium currents recorded with the whole-cell mode of the patch-clamp technique from short-term primary cultures of melanotrophs. Two types of calcium currents were distinguished by their voltage dependence and kinetics of inactivation similar to the low voltage-activated currents (LVA; or T-type) and high voltage-activated currents (HVA; N&L-types) of calcium currents. Exogenously applied dopamine (2-20 microM) reversibly reduced both LVA and HVA types of calcium currents. Evidence for these results came from experiments in which LVA and HVA calcium currents were separated by stepping to different membrane potentials from a fixed holding potential (Vh) or by changing Vh. These results suggest that dopamine can regulate the entry of calcium into melanotrophs by acting on at least two different populations of calcium channels thereby affecting hormone secretion and electrical activity.

Patch-clamp analysis of voltage-gated currents in intermediate lobe cells from rat pituitary thin slices

Ionic currents of hypophyseal intermediate lobe cells were studied using a thin-slice preparation of the rat pituitary in conjunction with conventional and perforated whole-cell patch-clamp recording techniques. A majority (89%) of the cells studied generated Na+, Ca2+ and K+ currents upon depolarizing voltage steps and responded to bath application of gamma-aminobutyric acid (GABA; 20-50 microM) with inward currents (in symmetrical chloride, holding potential -80 mV). A small percentage of cells (11%) did not display inward membrane currents upon depolarization and was unresponsive to GABA. In the first type of cells, Ca2+ and K+ currents were further studied in isolation. Ca2+ tail currents showed a biphasic time course upon repolarization, with time constants and amplitudes of 2.07 +/- 0.29 ms, 123 +/- 22 pA (for the slowly deactivating component) and 0.14 +/- 0.06 ms, 437 +/- 33 pA (for the fast-deactivating component; means +/- SD of n = 4 cells). Slowly and fast-deactivating conductances were half-maximally activated at around -10 mV and +10 mV respectively. Depolarizing voltage steps elicited two types of K+ current, which were separated using a prepulse protocol. A fast-activating, transient component showed half-maximal steady-state inactivation between -65 mV and -45 mV depending on the divalent cation composition of the external solution. Its decay was fitted by single-exponential functions with time constants of 36 +/- 11 ms and 3.9 +/- 0.9 ms at -20 mV and +40 mV respectively (mean +/- SD; n = 4 cells). Whereas the peak current amplitudes of the transient K+ current component remained stable, the amplitude of the second, delayed component increased progressively throughout the course of whole-cell experiments. In cells recorded with the perforated whole-cell technique, bath application of dopamine (10 nM-1 microM) induced large hyperpolarizations from a spontaneous membrane potential of -40 mV, but did not consistently affect the amplitude of the voltage-gated K+ conductances. These data are compared to previous studies using other preparations of the intermediate lobe, and differences are discussed, thus helping to extend our knowledge of electrical excitability of hypophyseal cells.

Ca(2+)- and voltage-dependent inactivation of Ca2+ currents in rat intermediate pituitary

We used single electrode voltage-clamp methods to investigate the inactivation of Ca2+ currents in melanotrophs of the intermediate lobe of the pituitary. The low threshold transient current was inactivated by brief prepulses to potentials above -30 mV and inhibition remained complete as prepulse potential was increased from 0 to +70 mV. Both the high threshold transient and sustained currents, however, were inhibited to the greatest extent (60%) by prepulses to 0 mV. Prepulses to more positive potentials close to the Ca2+ reversal potential produced much less (15%) inactivation. Buffering intracellular Ca2+ by including BAPTA in the recording electrode or replacing extracellular Ca2+ with Ba2+ reduced the effect of prepulses. Slowing Ca2+ extrusion by reducing the Na+ gradient across the cell increased the duration of the effect of prepulses. We conclude that the low threshold, transient current is inactivated primarily by membrane voltage while both the high threshold currents are inhibited by elevation of intracellular Ca2+ although the two currents display different sensitivities to Ca2+ concentration. Inhibition of the high threshold transient current by the neurotransmitter dopamine, however, acts by a different mechanism not mediated by Ca(2+)-dependent inactivation.

Acetylcholine synthesis in a primary culture of porcine intermediate lobe cells

Abstract Previous pharmacological studies with the pituitary gland have suggested that acetylcholine (ACh) might be involved in the regulation of intermediate lobe (IL) function. Whether ACh is endogenous to the IL cells or provided from an extrinsic source is unclear. The present experiments tested the possibility that the endocrine cells of the IL may be a source of ACh by measuring certain cholinergic markers in a primary culture of dissociated porcine cells. The endogenous ACh content was readily measurable in both the freshly dissociated IL cells and in 2- or 4-day primary cultures. Choline acetyltransferase activity was also measurable in the freshly dissociated and cultured IL cells and was reduced by 53% in the presence of a specific inhibitor, napthylvinylpyridine (50 muM). IL cells incubated in the presence of [(14)C]choline (1 muM) were able to synthesize [(14)C]ACh and the accumulation of the new ACh was inhibited by hemicholinium-3 (30 muM), a competitive inhibitor of high affinity choline uptake at cholinergic nerve terminals. In conclusion, these results demonstrate that the endocrine cells of the IL are capable of synthesizing and storing ACh.

Dopamine-induced inhibition of action potentials in cultured frog pituitary melanotrophs is mediated through activation of potassium channels and inhibition of calcium and sodium channels

A patch-clamp study was conducted in order to investigate the effects of dopamine on the ionic currents in cultured frog melanotrophs. Brief applications of dopamine (1 microM) hyperpolarized the cell and inhibited the spontaneous action potentials. The hyperpolarization was accompanied by an increase in membrane conductance. Under voltage clamp, dopamine evoked a net outward current. The dopamine-induced outward current was negligible at the equilibrium potential for potassium ions. It was also observed that dopamine increased the intensity of a voltage-dependent outward potassium current monitored by constant depolarizing pulses. In addition, voltage-dependent L- and N-like calcium currents and sodium current were reduced. In the cell-attached configuration, two distinct channel types were activated and one channel type was blocked by dopamine exposure to the extrapatch membrane, which indicates the involvement of an intracellular factor in the signal transduction pathway. A higher conductance channel (100 pS) was characterized by a very low basal activity which rapidly increased upon dopamine application. A lower conductance channel (30 pS) displayed a basal activity with frequent opening events, and a delayed (30-40 s) increase of activity in response to dopamine. Both currents reversed at a deduced potential corresponding to the equilibrium potential for potassium ions. The channel type inhibited by dopamine had a low conductance of 15 pS. The inhibition of the electrical activity induced by dopamine was totally blocked by the D2 receptor antagonist S(-)-sulpiride (1 microM) but was not affected by the D1 receptor antagonist SKF-83566 (1 microM). It is concluded that dopamine activates potassium channels and inhibits calcium and sodium channels in frog melanotrophs. The results also indicate that stimulus-response coupling is mediated by intracellular messenger system(s).

Effects of BAY K 8644 on Ca-channel currents and electrical activity in mouse melanotrophs

The effects of BAY K 8644 on both evoked and spontaneous electrical activity were studied in mouse melanotrophs. The action potential in these cells consisted of both a Na and a Ca component. BAY K 8644 greatly increased the duration of the evoked action potential apparently by enhancing the Ca component since this prolongation remained when the Na component was blocked by tetrodotoxin but was diminished by the Ca-channel antagonists Co2+ or nimodipine. In addition to its effects on the evoked action potential, BAY K 8644 sometimes caused action potentials of long duration to occur in otherwise quiescent melanotrophs, even in the presence of tetrodotoxin. These action potentials appeared to be triggered by small, depolarizing prepotentials. Voltage-clamp experiments suggested the existence of 3 types of Ca-channel currents in these cells: one low-threshold current and two high-threshold currents, one of which rapidly inactivated and another which did not. BAY K 8644 caused a shift in the I-V relation for Ca-channel current to more negative values and reduced or abolished the hump in the initial part of the I-V curve produced by the low-threshold current suggesting that BAY K 8644 selectively affects high-threshold Ca current. The action of BAY K 8644 to initiate trains of long-lasting Ca spikes and thereby increase Ca influx, might explain its secretagogue effect on the unstimulated melanotroph.

A dopaminergic inhibitory postsynaptic potential mediated by an increased potassium conductance

Intracellular recordings from intact pituitary melanotrophs show that, in the same cell, inhibitory postsynaptic potentials resulting from either pituitary stalk stimulation or exogenous dopamine are abolished by D2 receptor antagonists, display identical conductance changes, are reversed in polarity at the same membrane potential and are sensitive to pertussis toxin pretreatment. The reversal potential of the inhibitory postsynaptic potential shows a 65 mV shift with a 10-fold change in external potassium concentration, which is close to that predicted by the Nernst equation. We conclude that activation of this synapse releases dopamine which acts on a D2 receptor to increase potassium conductance via a G-protein-mediated mechanism. This is the first characterization of an inhibitory dopaminergic synapse in the mammalian nervous system.

The firing patterns of rat melanotrophs recorded using the patch clamp technique

Cell-attached and whole-cell recordings were made from adult rat melanotrophs maintained in vitro by standard cell culture techniques. In cell-attached recordings the cells showed small biphasic currents which reflected spontaneous cell firing. Single channel currents often had distinct relaxations and depolarizing currents through single channels could trigger the discharge of an action potential in the cell; both observations are consistent with the high input resistance (1-10 G omega) measured in the whole-cell configuration. The discharge of action potentials occurring either spontaneously or by current injection was eliminated by tetrodotoxin or by removing Na from the external medium. A Na-dependent plateau depolarization which activated near the spike threshold was also seen. In cells exposed to tetrodotoxin and K-channel blocking agents it was possible to evoke a long-lasting (up to 20 s) action potential which was enhanced and reduced, respectively, by Ba and Cd and thus appeared to reflect currents through voltage-activated Ca channels. Small amplitude Ca-dependent depolarizations could also be evoked at membrane potentials as low as -40 mV. In cell-attached and whole-cell recordings 10 mM Ba caused the discharge of tetrodotoxin-insensitive action potentials prior to a maintained depolarization of the membrane. The low threshold for Ca-dependent depolarizations suggest that Ca influx might occur in these cells even at the resting potential. Additionally, both a Ca current and the current underlying the Na-dependent plateau depolarization may influence the rate of cell firing and in doing so further increase Ca influx through voltage-activated channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of calcium channels in rat pituitary intermediate lobe cells

The voltage-dependent inactivation of Ca currents was explored in dissociated intermediate lobe (IL) cells from the rat pituitary. On the basis of current-voltage relations two main inward currents could be identified in this cell, a transient current, (I-t), and a sustained current, (I-s). Inactivation was explored either by changing the holding potential and testing the change in the inward currents during a brief test pulse, or, by depolarizing the membrane and following the decay of the evoked inward current. Three current decay rates were identified, each with a characteristic dependence on membrane potential. The fastest decay rate (tau 1), was attributed to the inactivation of the I-t current and had a value of 57 ms at -40 mV, decreasing to 10 ms at -10 mV (extrapolated value of 6 ms at 0 mV). The other two decay rates, tau 2 and tau 3, decreased monotonically with depolarization of the membrane potential and reflected the inactivation of the I-s current with values of 1.8 and 20 s at 0 mV. I-s inactivation and reactivation was found to occur even in the normal resting potential range of this cell. These properties of the calcium channels can explain the voltage-dependent inactivation of secretion that has been observed previously in this and other secretory cells. In addition, they suggest that calcium currents, and hence secretion, may be modulated by external factors that cause small, but sustained, changes in the resting potential of the IL cell.

Growth hormone releasing factor evokes rhythmic hyperpolarizing currents in rat anterior pituitary cells

1. The effect of human pancreatic growth hormone releasing factor (hpGHRF) on the electrical activity of dissociated rat anterior pituitary cells in culture was studied, using both the cell-attached and whole-cell modes of the patch-clamp recording technique. 2. To avoid possible wash-out of the responses, extracellular records were made from cell-attached patches. Application of hpGHRF to the cells produced rhythmic inward currents through the patches, attributable to rhythmic hyperpolarizations of the cell membrane outside the patch. The amplitude of the current oscillations was 1-8 pA and the frequency 0.05-0.4 Hz. 3. Flooding the cells with K+ ions from a small pipette containing 50 mM or 100 mM-K+ resulted in a reversible attenuation or block of the rhythmic inward currents evoked by hpGHRF, indicating that changes in K+ conductance were involved in the responses. 4. Flooding the cells with a solution containing 10 mM-EGTA blocked these rhythmic inward currents reversibly, suggesting the involvement of Ca2+ in the responses. In addition, responses were blocked by adding Co2+ (5-10 mM) to the bathing medium. The presence of tetrodotoxin (3 microM) had no effect, ruling out the participation of voltage-gated Na+ channels. 5. With whole-cell recording, the resting potential (-41.46 +/- 7.78 mV) and input resistance (5.34 +/- 3.73 G omega) of anterior pituitary cells in culture were found to be similar to those previously reported for pituitary cells and chromaffin cells with the same recording method. 6. In whole-cell experiments, application of hpGHRF (shortly prior to intracellular penetration) evoked rhythmic outward currents, associated with conductance increases, when the cells were clamped at their resting potential. The persistence of these currents in the 'voltage-clamped' cell indicated that the rhythmicity was not related to voltage-dependent phenomena. The currents disappeared within 4 min after breaking into the cell, presumably because of 'washout' of cell constituents into the pipette. 7. The reversal potential (-60 mV) of the hpGHRF-induced currents was negative to the resting potential of the cells (-41 mV), further indicating that hpGHRF would evoke rhythmic hyperpolarizations in unclamped cells, possibly due to periodic increases in K+ conductance. 8. The possible relation of these rhythmic currents to hpGHRF-induced secretion of growth hormone is discussed.

A patch clamp study of gamma-aminobutyric acid (GABA)-induced macroscopic currents in rat melanotrophs in cell culture

1. The macroscopic currents induced in cultured rat melanotrophs by exogenous gamma-aminobutyric acid (GABA) were analysed using the patch clamp recording technique. 2. Using various concentrations of intra- and extracellular chloride it was demonstrated that the conductance activated by GABA was chloride selective. Since these currents were blocked with bicuculline and enhanced with chlordiazepoxide the involvement of GABAA receptors similar to those in the CNS is indicated. 3. When chloride was symmetrically distributed across the membrane the voltage/current relationship was linear; pronounced rectification of GABA mediated currents was evident when there was an asymmetrical distribution of chloride. 4. With concentrations of GABA greater than 10 microM a fading of the current was seen during prolonged (5-10 s) applications. This effect appeared to be due to a decline of conductance rather than a shift of the chloride equilibrium potential. 5. Values for the Hill coefficient derived from dose-response curves suggested that the binding of 2 molecules of GABA to the receptor is required for the activation of the chloride channel. 6. There was no indication of a direct, GABAB receptor-mediated change of conductance.

Sodium and potassium currents involved in action potential propagation in normal bovine lactotrophs

1. The properties of whole-cell and single-channel Na+ and K+ currents in immunocytochemically identified bovine lactotrophs were studied using the patch-clamp technique. 2. In the whole-cell, current-clamp mode, cells had membrane potentials of -94.7 +/- 6.7 mV and input resistances of 2-17 G omega. Current-induced action potentials were recorded with a threshold around -35 mV and amplitude of 40-65 mV. Repetitive firing was not sustained at frequencies greater than 1-2 Hz without total inactivation. 3. Under voltage clamp, action potentials were shown to be composed of an inward TTX-sensitive Na+ current and an outward K+ current that was abolished by internal Cs+. 4. The isolated Na+ current had a threshold for activation around -35 mV and rapidly inactivated to a steady state during a test voltage pulse. Inactivation was strongly voltage-dependent, with the Na+ current being half-inactivated at -20 mV. 5. Recovery from inactivation was voltage dependent and at a holding potential of -60 mV, 50% reactivation was achieved after 420 ms. The implications of this long reactivation time on sustained action potential frequency are discussed. 6. Single Na+ channel activity was examined with the outside-out patch configuration and yielded single-channel conductances of 22.5 pS. Reconstruction of the voltage and time dependence of single-channel currents provided an accurate picture of the whole-cell Na+ current. 7. Whole-cell outward current carried by K+ in the absence of Na+ and Ca2+ had a large conductance, was slowly activated and demonstrated no inactivation. A second, more rapidly activating Ca2+-dependent K+ current could also be demonstrated. 8. Ensemble analysis of whole-cell K+ currents in the absence of Ca2+ showed underlying single-channel amplitudes of 1.2 pA at +10 mV, with the lactotroph having about 350 active channels at this potential. 9. Recordings of single K+ channels also demonstrated two classes of channel: a small (50 pS) voltage-activated channel and a higher-conductance (100 pS) Ca2+- sensitive channel. 10. Prolactin secretion was shown to be TTX-insensitive but sensitive to membrane potential, demonstrated as increased release following increased external K+ but not Na+ concentration.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological responses to dopamine of rat hypophysial cells in lactotroph-enriched primary cultures

1. Cells from 14-day-old and lactating female rat pituitary glands were dissociated, separated and enriched on a continuous gradient of bovine serum albumin at unit gravity. They were maintained for at least 6 days in culture before perifusion and electrophysiological experiments were performed. 2. Immunofluorescent staining of the resulting gradient fractions (numbered F2 to F9) from both groups of animals indicated that the majority of lactotrophs were located in the light fractions (F3-F4). However, a second population of lactotrophs was observed in the heavy fractions (F7-F9) isolated from lactating females. 3. Basal secretion rates of prolactin were in the order of 2-40 ng 2 min-1 10(6) cells-1 and were inhibited by dopamine in a dose-dependent manner. 4. According to their electrophysiological properties, cells from 14-day-old females (first group) were categorized as follows: (1) inexcitable cells, which displayed a low resting potential of about -35 mV (39% of cells tested, n = 118); and (2) excitable cells, which displayed either triggered or spontaneous action potentials and resting membrane potentials higher than -50 mV (61% of cells tested, n = 185). 5. In the light fraction from lactating females (second group), the majority of the cells were excitable (70%) and showed high resting membrane potentials (-50 to -55 mV) and 15% of these cells displayed spontaneous action potentials. 6. Heavy fractions (third group) contained a high percentage of non-spontaneous but excitable cells (80% of the cells tested, n = 65). These cells were able to elicit action potentials after the cessation of hyperpolarizing current pulses ('off' potentials). 7. Action potentials were insensitive to the sodium channel blocker, tetrodotoxin (TTX; 5 x 10(-6) M) but were reversibly blocked by calcium channel blockers such as cobalt, manganese and cadmium (10 mM). 8. In excitable cells from the three groups, dopamine (10(-7) M) induced a hyperpolarizing response due to an increase of the membrane conductance. During this response, action potentials were inhibited. It was shown that this was not a direct effect of dopamine. The reversal potential of the dopamine-induced response in these cells was found to be at -100 mV. This value was shifted to more positive potentials (-50 mV) when high-potassium medium was used (56 mM). 9. In non-excitable cells (first group), dopamine (10(-7) M) induced a hyperpolarizing response due to a decrease of the membrane conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical stimulation of neurointermediate lobes of mice elicits calcium-dependent output of melanocyte-stimulating hormone

Current pulses applied to isolated neurointermediate lobes of mice increased output of melanocyte-stimulating hormone. This response was dependent on extracellular calcium over a wide range of stimulus intensities and thus appears to be a true secretory response from the melanotrophs. Since substantial responses persisted in the presence of tetrodotoxin, much of the effect seems to be independent of Na spiking.

Calcium channel currents in pars intermedia cells of the rat pituitary gland. Kinetic properties and washout during intracellular dialysis

Ca channel currents in primary cultured pars intermedia cells were studied using whole-cell recording with patch pipettes. Experiments were carried out at 18-21 degrees C in cells internally dialyzed with K-free, EGTA-containing solutions and in the presence of 10 mM Ca or 10 mM Ba in the external solution. Ca and Ba currents depended on the activity of two main populations of channels, SD and FD. With Ca as the charge carrier, these two populations differed in their closing time constants at -80 mV (SD, 1.8 ms; FD, 110 microseconds), apparent activation levels (SD, -40 mV; FD, -5 mV), half-maximal activation levels (SD, +5 to +10 mV; FD, +20 to +25 mV), half-times of activation at +20 mV (SD, 2.5-3.5 ms; FD, 1.0-1.3 ms), and time courses of inactivation (SD, fast; FD, slow). Functional FD channels were almost completely lost within 20-25 min of breaking into a cell, whereas SD channels retained most of their functional activity. In addition, the conductance-voltage curve for FD channels shifted approximately 15 mV toward more negative membrane potentials within 11-14 min under whole-cell recording. At that time, 60-70% of the FD channel maximum conductance was lost. However, the conductance-voltage curve for SD channels shifted less than 5 mV within 25 min. The addition of 3 mM MgATP and 40 microM GTP to the internal solution slowed down the loss of FD channels and prevented the shift in their activation curve. It was also found that the amplitude of the current carried by FD channels tends to increase as a function of the age of the culture, with no obvious changes in the kinetic properties of the channels or in SD channel activity.

Characterization of calcium and sodium currents in porcine pars intermedia cells

The whole-cell configuration of the patch-clamp technique was applied to porcine pars intermedia cells. A tetrodotoxin-sensitive sodium current was recorded. Three types of calcium current were observed. Depolarizations from a holding potential of 100 mV elicited a transient current (ICaT), whereas depolarizations from a holding potential of -40 mV evoked a sustained current (ICaS). A third current (ICaN; N for neither) was activated by strong depolarization to +10 or +20 mV from holding potentials of - 100 mV. Increasing internal Ca2+ significantly reduced the amplitude of ICaS.

GABAA and GABAB receptors on porcine pars intermedia cells in primary culture: functional role in modulating peptide release

A primary culture of porcine pars intermedia cells with particularly high yields has been developed. The cells, grown in monolayers, secrete the pro-opiomelanocortin-derived peptide alpha-melanocyte-stimulating hormone over several weeks. The patch-clamp technique has been used to demonstrate the presence of gamma-aminobutyrateA (GABAA) receptors on the cells. GABA or the selective GABAA receptor agonist isoguvacine produced a depolarizing increase in chloride conductance that desensitized rapidly. The response was antagonized by bicuculline and by the aminopyridazine derivative of GABA (SR 95103), a novel GABAA receptor antagonist. The effects of specific agonists for each receptor were tested on peptide release from cells maintained in a perfusion system. Isoguvacine (10 microM) potentiated Ba2+-evoked release of alpha-melanocyte-stimulating hormone, whereas (-)-baclofen (50 microM) decreased both basal and stimulated hormone release. This negative effect on peptide secretion was reproduced when GABA (50 microM) was perfused in the presence of bicuculline (10 microM) to block GABAA receptor activation. The possible mechanisms underlying these GABAA and GABAB effects on stimulus-secretion coupling in this neuroendocrine model are discussed.

Novel synaptic responses mediated by dopamine and gamma-aminobutyric acid in neuroendocrine cells of the intermediate pituitary

Neuroendocrine cells of the pituitary pars intermedia contain pro-opiomelanocortin and secrete mainly alpha-melanocyte-stimulating hormone and, in lesser amounts, corticotropin and endorphin. We have recorded intracellularly from rat pars intermedia cells in vitro and analysed the synaptic inputs to these cells. Electrical stimulation of the pituitary stalk evoked a biphasic postsynaptic response in pars intermedia cells. An initial inhibitory postsynaptic potential was most likely mediated by gamma-aminobutyric acid (GABA) because there was a conductance increase, and it was blocked by bicuculline, a GABA antagonist. A more prolonged hyperpolarization was also observed which could last 10-30 s and was associated with a decreased whole cell conductance. This late hyperpolarization was blocked by the dopamine antagonists, chlorpromazine and domperidone, and was therefore most likely dopamine-mediated.

Long-term haloperidol treatment elevates beta-endorphin levels in the intermediate pituitary but not in rat brain

Long-term treatment of rats with haloperidol, a dopamine receptor antagonist, produced a dose-dependent increase in immunoreactive beta-endorphin (i beta-END) concentrations in the neurointermediate lobe of the pituitary (NIL). In contrast, chronic haloperidol treatment had no significant effect on i beta-END levels in the hypothalamus, the midbrain or in discrete, microdissected brain nuclei even when administered at a dose (5 mg/kg) ten-fold higher than that which elevated i beta-END levels in the NIL. Chronic treatment with bromocriptine, a dopamine receptor agonist, had the opposite effect on the NIL, lowering i beta-END levels to approximately one-third of control values, but it did not affect hypothalamic i beta-END concentrations. These data are consistent with prior evidence that the synthesis of beta-END by IL melanotrophs is reciprocally modulated by dopaminergic ligands. The results indicate, however, that beta-END-releasing neurons are not similarly regulated.

Characterization of voltage-sensitive calcium channels in a clonal pituitary cell line

We have pharmacologically characterized voltage sensitive calcium channels (VSCCs) in GH3 cells, an anterior pituitary clonal cell line known to secrete prolactin and growth hormone. Raising the medium K+ concentration from 5 to 50 mM caused an immediate increase in net 45Ca2+ uptake which remained apparent over a 15 minute time course. 45Ca2+ uptake was maximally stimulated nearly 10-fold over basal levels. This K+-induced stimulation of Ca2+ uptake was not prevented by 10-5M tetrodotoxin or by replacing sodium with choline in the assay medium. Ca2+ uptake was, however, inhibited by several VSCC antagonists: nitrendipine, D-600, diltiazem and Cd2+. Further, the novel dihydropyridine VSCC agonists, BAY K8644 and CGP 28392, enhanced 50 mM K+-stimulated 45Ca2+ uptake and these effects were blocked by nitrendipine.

The hypothalamohypophyseal system in vitro: electrophysiology of the pars intermedia and evidence for both excitatory and inhibitory inputs

The purpose of this study was to better assess the function of catecholamine-containing nerve terminals in the pituitary pars intermedia lobe. Hypothalamohypophyseal explants, which included the intact mediobasal hypothalamus (MBH), median eminence, infundibular stalk and the neurointermediate lobe, were obtained from 2-3-week-old male and female albino rats. The tissue was placed in a perfusion chamber and maintained under physiological conditions for up to 12 h. A set of bipolar stimulating electrodes was positioned on the surface of the median eminence, infundibular stalk or the rostroventral arcuate nucleus of the MBH. A microelectrode recorded electrical activity in the pars intermedia gland. Two types of spontaneous action potentials were found; fast 2-4 ms duration neural fiber type spikes and slower 7-10 ms duration spikes probably derived from non-neural endocrine cells. Single-pulse electrical stimulation at all 3 sites evoked both kinds of potentials, while trains of stimuli (0.1-20 Hz) decreased or completely inhibited the basal firing rate of the slower ones. Application of the neuroleptic. L-sulpiride (0.01, 0.1 or 1.0 mumol), to the perfusion medium increased the spontaneous endocrine cell activity and blocked the stimulus-induced inhibition in the explants but had no effect on the activity in isolated pituitaries. Dopamine (0.1 mumol), which is known to inhibit the secretion of pro-opiomelanocortin peptides, reversibly suppressed the spontaneous endocrine cell potentials. These observations support a hypothesis for the presence of a functional tuberohypophyseal dopamine inhibitory system and a possible, but as yet unidentifiable, excitatory system in the pars intermedia. Thus, hypothalamohypophyseal explants can be used to elucidate specific information on this type of neuroendocrine axis.

Pharmacological and ionic features of gamma-aminobutyric acid receptors influencing electrical properties of melanotrophs isolated from the rat pars intermedia

Characteristics of the gamma-aminobutyric acid receptors on melanotrophs of the rat pars intermedia were studied by intracellular recording. Muscimol and 3-amino-1-propanesulfonic acid, but not baclofen or glycine, mimicked the depolarization and conductance increase produced by gamma-aminobutyric acid on the melanotrophs. These effects appeared to be due to an increase in chloride ion conductance since the null potentials for all three drugs were the same and were affected by changes in external or internal chloride ion concentration but not by changes in the concentrations of other ions present in the recording solution or by the addition of the calcium-channel blocker cobalt. Bicuculline abolished the effect of muscimol. Picrotoxin reduced the effect of gamma-aminobutyric acid; so too did furosemide. Muscimol mimicked the ability of gamma-aminobutyric acid to reduce the depolarization produced by excess potassium and this effect was also blocked by bicuculline. Rat melanotrophs thus appear to possess gamma-aminobutyric acid receptor-ionophore complexes similar to the classical sort found in neurons in the mammalian central nervous system. Furthermore, the parallels between the electrical responses observed and secretory effects previously noted, reinforce the view that electrical activity may participate in stimulus-secretion coupling in melanotrophs.