Electrophysiological characterization of non-NMDA glutamate receptors on cultured intermediate lobe cells of the rat pituitary

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.

AMPA receptor activation leads to neurite retraction in tangentially migrating neurons in the intermediate zone of the embryonic rat neocortex

In rat (König et al. [1998] 28th Annual Meeting of the Society of Neuroscience, Los Angeles. 24:314.6) and mouse (Métin et al. [2000] J. Neurosci. 20:696-708), neurons migrating tangentially in the intermediate zone (IZ) of the neocortical anlage express functional AMPA receptors permeable to calcium. The role of these receptors is as yet unknown. We exposed organotypic cultures of rat telencephalon (embryonic day 15) to AMPA receptor agonists or antagonists, and analyzed the effects of these treatments on cells in the IZ labeled with antibodies against the isoforms a, b and c of microtubule associated protein 2 (MAP2) and the polysialylated neural cell adhesion molecule (PSA-NCAM). The presence of functional AMPA receptors permeable to calcium was checked by cobalt-loading. After exposure to AMPA alone for at least 6 hr, we observed a significant increase in the number of rounded, MAP2 positive cells in the IZ close to the migratory front. When AMPA was combined with cyclothiazide, the increase was already significant after 3 hr. These effects were dose-dependent and could be partially or totally blocked by DNQX or GYKI 53655 respectively, that suggests that they are mediated by AMPA receptors. Paracrine AMPA receptor activation might participate, together with other signals, in guiding the migratory stream, or provide stop signals for migrating cells.

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

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.