Effectiveness of GABAB antagonists in inhibiting baclofen‐induced reductions in cytosolic free Ca concentration in isolated melanotrophs of rat

Shibuya, Izumi; Kongsamut, Sathapana; Douglas, W. W.

doi:10.1111/j.1476-5381.1992.tb09074.x

Cited by 25 publications

(8 citation statements)

References 30 publications

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“…This effect of baclofen is in agreement with receptors from Xenopus or rat melanotropes in which authors describe that baclofen not only decreases intracellular calcium levels [24, 25, 36]but that this decrease is transient in the presence of the drug [36]. Our results with baclofen on calcium levels in anterior pituitary cells differ from those of Horváth et al [37]or Lorsignol et al [38]who did not find any effect of the GABA B agonist on this parameter.…”

Section: Discussionsupporting

confidence: 68%

“…These recep12,- tors were described in gonadotropes, corticotropes and not in thyrotropes or somatotropes [20, 23], and we have found evidence of their presence in lactotropes [7]. GABA B receptors have also been described in melanotropes [24, 25]. In previous works we showed that GABA B receptor activation inhibited pituitary secretion [7, 8].…”

Section: Discussionmentioning

confidence: 89%

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GABAB Receptors in Anterior Pituitary Cells

et al. 2001

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The activation of pituitary GABA_B receptors by the specific agonist baclofen inhibits pituitary hormone secretion in vitro. Here we studied the mechanism of action of GABA_B receptors in rat adenohypophysis. Anterior pituitary cells were obtained by trypsinization and were either plated for hormonal studies and cAMP determination or incubated in FURA 2AM for calcium measurements. Baclofen (BACL: 1·10^–5 M) significantly inhibited basal and thyrotropic releasing hormone (TRH)-stimulated (1·10^–7 M) PRL secretion in anterior pituitary cells from proestrous rats. In the presence of pertussis toxin (PTX: 150 ng/ml, 20 h), which leads to the uncoupling of the G_i/o-protein from the receptor, both effects of BACL were abolished while the effect of dopamine (DA: 1·10^–8 M), used as an inhibitory control, was reduced from 70 to 25%. PTX also reversed BACL-induced inhibition of gonadotropin-releasing hormone (GnRH)-elicited luteinizing hormone (LH) secretion in anterior pituitary cells from 15-day-old female rats. In addition, though working in a pituitary mixed cell population, in which only some cell types possess GABA_B receptors, BACL (1·10^–5 M) attenuated the forskolin-induced (0.5 µM) increase in cAMP. This effect was prevented by co-incubation with the antagonist 2 hydroxysaclofen and by preincubation with PTX. BACL (5· 10^–5 M) and DA (5·10^–7 M) inhibited basal intracellular calcium concentrations ([Ca²⁺]_i) in pituitary cells and the effect of the latter was significantly stronger. The effect of BACL on [Ca²⁺]_i was abolished after preincubation with PTX. In the presence of the potassium channel blocking agents barium (200 µM and 1 mM) and tetraethylammonium (10 mM), BACL was still able to inhibit [Ca²⁺]_i. Blockade of voltage-sensitive calcium channels (VSCC) with either verapamil (5·10^–6 M) or nifedipine (1·10^–6 M) completely abolished the effect of BACL on [Ca²⁺]_i. In the presence of 12.5 mM potassium concentration baclofen significantly inhibited [Ca²⁺]_i. In conclusion, our results describe the negative coupling of adenohypophyseal GABA_B receptors to VSCC through PTX-sensitive G-proteins. These characteristics suggest a resemblance of these receptors to the typical presynaptic GABA_B sites described in the central nervous system.

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Section: Discussionsupporting

confidence: 68%

Section: Discussionmentioning

confidence: 89%

GABAB Receptors in Anterior Pituitary Cells

et al. 2001

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“…Alternately, AMPA receptor channel subunits have been shown to exist in two forms designated flip and flop. Homomeric and heteromeric assemblies of these flip and flop subunits [41], or het eromeric assemblies of AMPA and kainate receptor channel subunits [42,43], have been shown to exhibit different pharmacological and functional properties. Thus, the particular characteristics of the AMPA/kainate receptor in melanotropes may reflect this molecular het erogeneity.…”

Section: Discussionmentioning

confidence: 99%

Glutamate Receptor Agonists Modulate [Ca2+]i in Isolated Rat Melanotropes

Giovannucci¹,

Stuenkel²

1995

Neuroendocrinology

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Although glutamate is the predominant excitatory amino acid in the vertebrate central nervous system (CNS) where it affects a variety of physiological processes and pathophysiological states, the role that glutamate receptors may play outside the CNS has not been clearly established. In the present study, the effects of N-methyl-D-aspartate (NMDA), α-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) kainate, and metabotropic glutamate receptor agonists and antagonists were investigated on neuroendocrine melanotropes of the rat pars intermedia using single-cell dual-wavelength microfluorometry and the Ca²⁺-sensitive probe, fura-2, to monitor changes in [Ca²⁺]_i. Glutamate induced a rapid, concentration-dependent rise in [Ca²⁺]_i with an EC₅₀ of 24 µM that was Mg²⁺-sensitive and dependent on the presence of extracellular Ca²⁺. NMDA increased [Ca²⁺]_i in a glycine-dependent manner with an EC50 of 83 µM that was blocked by 1 µM MK-801 and 1 mM Mg²⁺. The non-NMDA receptor agonists kainate, AMPA, and quisqualate increased [Ca²⁺]_i with an EC50 of 124, 5 and 8 µM, respectively. Responses to kainic acid were blocked by 10 µM CNQX and were shown to be sensitive to Mg²⁺ and dihydropyridine. AMPA stimulation was the most potent, and glutamate stimulation was the most efficacious at mediating increases in [Ca²⁺]_i. The metabotropic receptor-specific agonist, trans-ACPD, failed to induce a change in [Ca²⁺]_i. The glutamate-induced Ca²⁺ influx was about half of that elicited by a 50 mM K⁺-induced membrane depolarization and activation of voltage-sensitive Ca²⁺ channels. These results demonstrate the presence of glutamate receptors on rat melanotropes and suggest that glutamate receptors in the intermediate lobe of the pituitary may provide the excitatory counterbalance to the well-described secretoinhibiting input via dopamine and γ-aminobutyric acid receptors.

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“…It is known that spontaneous secretion in these cells is inhibited by the secretoinhibitory transmitter, dopamine, contained in hypothalamic neurons that directly innervate melanotrophs (2). Measurements of cytosolic Ca 2ϩ concentrations ([Ca 2ϩ ] i ) with the fluorescent Ca 2ϩ indicator, fura-2, and the rate of manganese quenching of fura-2 in melanotrophs revealed that spontaneous Ca 2ϩ entry through the plasma membrane accounts for the spontaneous secretion (3)(4)(5). Although CRF, ␤-adrenergic agonists, and glutamate have been shown to stimulate melanotroph secretion in rats (6 -8), whether they are present in the rat PI under physiological conditions is still unknown.…”

Section: Elanotrophs Of the Pituitary Pars Intermedia (Pi)mentioning

confidence: 99%

“…[Ca 2ϩ ] i measurement and cell perifusion were performed as described previously (3,20). In brief, a small portion of the cell suspension (containing ϳ100 -300 cells) was transferred to a temperature-controlled chamber (PDMI-2, Medical Systems Corp., Greenvale, NY) of small volume (150 l).…”

Section: [Ca 2ϩ ] I Measurementmentioning

confidence: 99%

Pituitary Adenylate Cyclase-Activating Polypeptide Potentiation of Ca2+ Entry via Protein Kinase C and A Pathways in Melanotrophs of the Pituitary Pars Intermedia of Rats*

et al. 1997

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Pituitary adenylate cyclase-activating polypeptide (PACAP) has been reported to stimulate melanotroph secretion, and PACAP-like immunoreactivity and expression of PACAP type I receptor messenger RNA have been identified in the pituitary pars intermedia (PI). The present study showed that PACAP messenger RNA is also expressed in the PI. To examine the mechanism of PACAP action in the PI, cytosolic Ca2+ concentrations ([Ca2+]i) and ionic currents were measured in acutely dissociated rat melanotrophs. In about 40% of the melanotrophs studied, PACAP induced an increase in [Ca2+]i, which was suppressed by extracellular Ca2+ removal; extracellular Na+ replacement; the blocker of L-type Ca2+ channels, nicardipine; or the secreto-inhibitory neurotransmitter, dopamine. The PACAP-induced [Ca2+]i increase was mimicked by activators of protein kinase A (PKA) and protein kinase C (PKC), Sp-diastereomer of cAMP and 1-oleoyl-2-acetyl-sn-glycerol, and was reduced by inhibitors of PKA and PKC, Rp-diastereomer of cAMP and staurosporine. Patch-clamp analysis revealed that PACAP caused inward currents with a reversal potential of -0.8 mV and facilitated voltage-dependent Ba2+ currents. It further revealed that PACAP-induced inward currents were mimicked by 1-oleoyl-2-acetyl-sn-glycerol and inhibited by staurosporine, and that Sp-diastereomer of cAMP facilitated Ba2+ currents. These results suggest that PACAP potentiates Ca2+ entry mechanisms of rat melanotrophs by activation of nonselective cation channels via PKC and facilitation of voltage-dependent Ca2+ channels via PKA.

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Effectiveness of GABA_B antagonists in inhibiting baclofen‐induced reductions in cytosolic free Ca concentration in isolated melanotrophs of rat

Abstract: 1The purpose of the present experiments was to assess the activities of GABAB receptor antagonists in mammalian isolated melanotrophs.2 Cytosolic free Ca concentration ([Ca2+]

Cited by 25 publications

References 30 publications

GABA<sub>B</sub> Receptors in Anterior Pituitary Cells

GABA<sub>B</sub> Receptors in Anterior Pituitary Cells

Glutamate Receptor Agonists Modulate [Ca<sup>2+</sup>]<sub>i</sub> in Isolated Rat Melanotropes

Pituitary Adenylate Cyclase-Activating Polypeptide Potentiation of Ca2+ Entry via Protein Kinase C and A Pathways in Melanotrophs of the Pituitary Pars Intermedia of Rats*

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