Constitutive expression of functional GABA(B) receptors in mIL-tsA58 cells requires both GABA(B(1)) and GABA(B(2)) genes.

Studies of gamma-aminobutyric acid (GABA)(B) receptor function in heterologous cell systems have suggested that expression of two distinct seven transmembrane G-protein coupled receptor subunits is necessary for receptor activation and signal transduction. Some results suggest that both receptor proteins must be inserted into the plasma membrane to create heterodimers; however, it is possible that subunit monomers or homodimers are functional in cells which constitutively express GABA(B) receptors. A new pituitary intermediate lobe melanotrope cell clone (mIL tsA58) has been isolated which constitutively expresses GABA(B), D(2) and corticotrophin releasing factor receptors. Here, we report on characterization of the GABA(B) receptors. Solution hybridization-nuclease protection assays reveal the presence of GABA(B(1)) and GABA(B(2)) transcripts. Western blots show GABA(B(1a)) and one of two GABA(B(2)) proteins. Addition of the GABA(B) agonist baclofen to cultured mIL-tsA58 (mIL) cells inhibits high voltage activated Ca(2+) channels, as measured by agonist-induced inhibition of the K(+)-depolarization-stimulated increase in Ca(2+) influx. CGP55845, a GABA(B) antagonist, blocks the response to baclofen. Knockdown of either GABA(B(1)) or GABA(B(2)) subunits with selective antisense oligodeoxynucleotides reduced GABA(B) protein levels and completely abolished the GABA(B) receptor response in the mIL cells. Taken together, these results indicate that functionally active GABA(B) receptors in mIL cells require the constitutive expression of both GABA(B) genes. This is a physiologic validation of results from recombinant overexpression in naive cells and shows that the mIL cell line is a useful model for studying GABA(B) receptor expression, regulation and function.

The anticonvulsant, antihyperalgesic agent gabapentin is an agonist at brain gamma-aminobutyric acid type B receptors negatively coupled to voltage-dependent calcium channels.

Gabapentin (Neurontin, Pfizer Global R & D) is a novel anticonvulsant, antihyperalgesic, and antinociceptive agent with a poorly understood mechanism of action. In this study, we show that gabapentin (EC50 2 microM) inhibited up to 70 to 80% of the total K+-evoked Ca2+ influx via voltage-dependent calcium channels (VD-CCs) in a mouse pituitary intermediate melanotrope clonal mIL-tsA58 (mIL) cell line. mIL cells endogenously express only gamma-aminobutyric acid type B (GABA(B)) gb1a-gb2 receptors. Moreover, activity of the agonist gabapentin was dose dependently and completely blocked with the GABA(B) antagonist CGP55845 and was nearly identical to the prototypic GABA(B) agonist baclofen in both extent and potency. Antisense knockdown of gb1a also completely blocked gabapentin activity, while gb1b antisense and control oligonucleotides had no effect, indicating that gabapentin inhibition of membrane Ca2+ mobilization in mIL cells was dependent on a functional GABA(B) (gb1a-gb2) heterodimer receptor. In addition, during combined whole cell recording and multiphoton Ca2+ imaging in hippocampal neurons in situ, gabapentin significantly inhibited in a dose-dependent manner subthreshold soma depolarizations and Ca2+ responses evoked by somatic current injection. Furthermore, gabapentin almost completely blocked Ca2+ action potentials and Ca2+ responses elicited by suprathreshold current injection. However, larger current injection overcame this inhibition of Ca2+ action potentials suggesting that gabapentin did not predominantly affect L-type Ca2+ channels. The depressant effect of gabapentin on Ca2+ responses was coupled to the activation of neuronal GABA(B) receptors since they were blocked by CGP55845, and baclofen produced similar effects. Thus gabapentin activation of neuronal GABA(B) gb1a-gb2 receptors negatively coupled to VD-CCs can be a potentially important therapeutic mechanism of action of gabapentin that may be linked to inhibition of neurotransmitter release in some systems.

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

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

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

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

Global precedence in visual search? Not so fast: evidence instead for an oblique effect.

The evidence from an earlier report of global precedence in visual search is reexamined. Two new experiments are reported. The results of the first experiment indicate that the confusability of oblique orientations (a class-2 oblique effect) rather than global precedence was responsible for the earlier results. The results of the second experiment show that the effect critically depends on the presence of heterogeneous distractors rather than on differences in raw processing speed for different spatial scales. The possible role of symmetry is discussed.

Evaluation, narcotics and behavioral treatment influences on pain ratings in chronic pain patients.

Changes in self-reported pain ratings were assessed in 95 chronic pain patients from data collected at three times: pretreatment evaluation, initial days of treatment and final days of treatment. These data were collected separately for regular, sporadic and nonusers of narcotic medication. Each patient completed a four-week interdisciplinary behaviorally based noninvasive treatment program. There was an average decrease of 7% in self-reported pain ratings between evaluation and the onset of treatment for the three groups. An additional decrease of 21%, 16% and 10% for the sporadic, nonusers and regular users of narcotics respectively was noted during treatment. Statistical analysis revealed a significant decrease in pain ratings across assessment phases but not between groups. Sporadic users of narcotics showed a pattern more similar to nonusers than to the regular users.

Mechanisms of action of inhibitors of prolactin secretion in GH3 pituitary cells. I. Ca2+-dependent inhibition of amino acid incorporation.

Post-transcriptional protein synthesis by GH3 cloned pituitary cells, which secrete prolactin and growth hormone, is dependent on Ca2+. The effects of antagonists of prolactin secretion were examined on overall protein synthesis in GH3 cells as a function of cellular Ca2+ depletion and restoration at varying concentrations of extracellular Ca2+. Leucine incorporation by Ca2+-depleted cells during short incubations was reduced by 80-90%. Trifluoperazine at micromolar concentrations inhibited leucine incorporation in a Ca2+-dependent manner. The extent of inhibition varied with extracellular Ca2+ concentration and was fully reversed at higher Ca2+ concentrations. Similar patterns of inhibition of leucine incorporation were observed with nifedipine, verapamil, calmidazolium, chlorpromazine, bromocriptine, ergotamine, and the (+)- and (-)-isomers of butaclamol, but dopamine, apomorphine, and chlorpromazine sulfoxide were not inhibitory. Muscarinic agonists decreased incorporation in a Ca2+-dependent manner, but lesser degrees of inhibition were obtained. Inhibitions were observed for a broad spectrum of polypeptide species, could not be explained by effects on Mg2+ availability or amino acid uptake, and were rapidly and fully reversed by Ca2+. Production of prolactin and growth hormone was decreased by secretory inhibitors to the same extent and with the same Ca2+ concentration dependence as was observed for inhibition of amino acid incorporation. It is proposed that these substances inhibit protein synthesis in GH3 cells through alterations in intracellular Ca2+ metabolism rather than through mechanisms mediated by calmodulin or dopamine receptors.

Mechanisms of action of inhibitors of prolactin secretion in GH3 pituitary cells. II. Blockade of voltage-dependent Ca2+ channels.

Pharmacologic agents reported to inhibit prolactin secretion by GH3 pituitary cells were observed to inhibit protein synthesis by these cells in a Ca2+-dependent manner. The possibility that these substances exert their effects on protein synthesis by restricting intracellular Ca2+ availability was explored. Trifluoperazine and chlorpromazine at concentrations that inhibit amino acid incorporation reduced uptake of 45Ca2+ by intact cells approximately 30% under nondepolarizing conditions. Increased extracellular K+ (30 mM), which depolarizes the membrane and opens the voltage-dependent Ca2+ channel of GH3 cells, produced a 2-fold increase in 45Ca2+ uptake; phenothiazines fully suppressed this effect of K+. Nifedipine, verapamil, ergotamine, bromocriptine, (+)- and (-)-butaclamol, and calmidazolium were also effective in inhibiting 45Ca2+ uptake under depolarizing and nondepolarizing conditions. The membrane potential of either depolarized or nondepolarized cells, as determined by [3H]tetraphenylphosphonium+ distribution, was not affected significantly by secretory inhibitors. Increased extracellular K+ altered protein synthesis by GH3 cells in a biphasic manner. Amino acid incorporation by cells incubated at low extracellular Ca2+ concentrations was stimulated by K+, whereas incorporation by cells in high Ca2+ medium was inhibited by K+. Trifluoperazine, chlorpromazine, nifedipine, and bromocriptine suppressed both effects of K+ on protein synthesis. It is proposed that these antagonists of secretion inhibit protein synthesis by GH3 cells through blockade of voltage-dependent Ca2+ channels.