Neurotransmitter receptors as signaling platforms in anterior pituitary cells

Abstract

The functions of anterior pituitary cells are controlled by two major groups of hypothalamic and intrapituitary ligands: one exclusively acts on G protein-coupled receptors and the other activates both G protein-coupled receptors and ligand-gated receptor channels. The second group of ligands operates as neurotransmitters in neuronal cells and their receptors are termed as neurotransmitter receptors. Most information about pituitary neurotransmitter receptors was obtained from secretory studies, RT-PCR analyses of mRNA expression and immunohistochemical and biochemical analyses, all of which were performed using a mixed population of pituitary cells. However, recent electrophysiological and imaging experiments have characterized γ-aminobutyric acid-, acetylcholine-, and ATP-activated receptors and channels in single pituitary cell types, expanding this picture and revealing surprising differences in their expression between subtypes of secretory cells and between native and immortalized pituitary cells. The main focus of this review is on the electrophysiological and pharmacological properties of these receptors and their roles in calcium signaling and calcium-controlled hormone secretion.

Keywords: Action potentials; Calcium signaling; G protein-coupled receptors; Hormone secretion; Ligand-gated receptor channels; Neurotransmitters; Pituitary.

Figure 1

Characterization of neurotransmitters and their receptors in pituitary gland. Ligand-gated receptor channel (LGRC) agonists (GABA, ACh, GLU and ATP) are synthesized in hypothalamus are transported by the hypophysial portal blood flow to anterior pituitary cells, and/or synthesized and released by anterior pituitary cells. Evidence confirming functional expression of LGRCs in different subtypes of pituitary cells was obtained after their identification in a mixed population of pituitary cells using neuropeptides (10 nM DA, 1 nM CRH, 2 nM GnRH, 100 nM TRH and 100 nM SST) capable of modulating or stimulating specifically the electrical activity pattern in secretory pituitary cells. The secretory anterior pituitary cells other than lactotrophs, as well as folliculostellate cells, also synthetize glutamate, but functional iGLUR have not yet been electrophysiologically detected in any type of pituitary cells. Gray area indicates V_m values from −100 mV to +50mV (Y-axis) and duration of recordings: melanotrophs, 30 s; corticotrophs, 80 s; gonadotrophs and somatotrophs, 40 s; thyrotrophs, 75 s and lactotrophs, 95s (X-axis). Traces shown in this and following figures are derived from data shown in (Zemkova et al., 2006, Zemkova et al., 2008, Zemkova et al., 2013) and (Zemkova et al., 2016).

Figure 2

Comparison of currents in native (top panels) and immortalized (bottom panels) pituitary gonadotrophs. Both cell types express calcium-mobilizing GnRH and mACh receptors and immortalized gonadotrophs also express ATP-activated calcium-mobilizing P2Y receptors. However, activation of these receptors causes oscillatory calcium release in native gonadotrophs and non-ocillatory in immortalized gonadotrophs monitored as outward Ca²⁺-controlled K⁺ currents. Native gonadotrophs also express ionotropic ligand-gated receptor channels for ACh (shown as inward current stimulated simultaneously with oscillatory otward Ca²⁺-controlled K⁺ current), ATP and GABA, whereas immortalized gonadotrophs express only GABA_A receptor channels. In bottom panels, GnRH-, ATP- and GABA-induced responses are from αT3-1 cells and ACh-induced response is from LβT2 cells. Ligand-induced currents were recorded using perforated patch clamp from cells held at membrane potential of −40 mV (responses mediated by GPCR) or −60 mV (responses mediated by LGRC).

Figure 3

Excitatory effect of GABA in rat pituitary gonadotrophs (left) and lactotrophs (right). A, GABA induces a rapid increase in [Ca²⁺]_i in single cells. B, GABA initiates of AP firing in quiescent cells (left) and modulates the electrical activity pattern in spontaneously active cells (right). C, gramicidin-perforated patch-clamp recording of GABA-induced currents in cells held at different holding potentials (HP). In both cell types, reversal potentials for GABA-induced chloride currents (E_rev) was higher (less negative) as compared with resting membrane potential (V_m) of these cells. In these experimentrs, the type of pituitary cell was identified using TRH/DA (lactotrophs) and GnRH (gonadotrops).

Figure 4

Example records of GABA- and ATP-induced current or the lack of currents in identified secretory rat pituitary cells. Electrophysiological recording of functional P2XRs in somatotrophs is not available.