Modeling of membrane excitability in gonadotropin-releasing hormone-secreting hypothalamic neurons regulated by Ca2+-mobilizing and adenylyl cyclase-coupled receptors

Abstract

Gonadotropin-releasing hormone (GnRH) secretion from native and immortalized hypothalamic neurons is regulated by endogenous Ca(2+)-mobilizing and adenylyl cyclase (AC)-coupled receptors. Activation of both receptor types leads to an increase in action potential firing frequency and a rise in the intracellular Ca(2+) concentration ([Ca(2+)](i)) and neuropeptide secretion. The stimulatory action of Ca(2+)-mobilizing agonists on voltage-gated Ca(2+) influx is determined by depletion of the intracellular Ca(2+) pool, whereas AC agonist-stimulated Ca(2+) influx occurs independently of stored Ca(2+) and is controlled by cAMP, possibly through cyclic nucleotide-gated channels. Here, experimental records from immortalized GnRH-secreting neurons are simulated with a mathematical model to determine the requirements for generating complex membrane potential (V(m)) and [Ca(2+)](i) responses to Ca(2+)-mobilizing and AC agonists. Included in the model are three pacemaker currents: a store-operated Ca(2+) current (I(SOC)), an SK-type Ca(2+)-activated K(+) current (I(SK)), and an inward current that is modulated by cAMP and [Ca(2+)](i) (I(d)). Spontaneous electrical activity and Ca(2+) signaling in the model are predominantly controlled by I(d), which is activated by cAMP and inhibited by high [Ca(2+)](i). Depletion of the intracellular Ca(2+) pool mimics the receptor-induced activation of I(SOC) and I(SK), leading to an increase in the firing frequency and Ca(2+) influx after a transient cessation of electrical activity. However, increasing the activity of I(d) simulates the experimental response to forskolin-induced activation of AC. Analysis of the behaviors of I(SOC), I(d), and I(SK) in the model reveals the complexity in the interplay of these currents that is necessary to fully account for the experimental results.

Fig. 1.

Schematic diagram of the key elements of the model, showing the separation of both the cytosolic and ER pools into shell and bulk compartments and the three pacemaker currents,I_SK,I_SOC, andI_d. I_SOC is thought to be activated via a direct coupling to shell IP₃receptors (see Materials and Methods for details), whereasI_d is activated by cAMP. A rise in shell [Ca²⁺]_i activatesI_SK and inactivatesI_d. An animated version of this diagram may be viewed at http://mrb.niddk.nih.gov/alebeau/gt1.html.

Fig. 2.

Responses of GT1 neurons and model to GnRH stimulation. Simultaneous V_m(A) and [Ca²⁺]_i(B) responses to 100 nm GnRH in GT1 neurons. C–G, Model GnRH response, simulated by increasing ER membrane permeability to Ca²⁺ 25-fold. In this and following figures,heavy and light lines denote bulk and shell compartments, respectively. C,V_m response. D, [Ca²⁺]_i and [Ca²⁺]_er (E) response. F, I_SK andI_SOC (G, light line) and I_d (G, heavy line) membrane current responses. Calibration inA applies to A and B, and in C applies toC–G.

Fig. 3.

Responses of GT1 neurons and model to addition of GnRH during blockade of SK channels by apamin. SimultaneousV_m (A) and [Ca²⁺]_i (B) responses to 100 nm GnRH in GT1 neurons during constant perfusion with 100 nm apamin.C–G, Model simulations of GnRH plus apamin response. Apamin was simulated by setting the conductance ofI_SK to zero. GnRH was simulated as in Figure2. Calibration in A applies to all traces.

Fig. 4.

Responses of GT1 neurons and model to addition of endoplasmic reticulum Ca²⁺-ATPase pump blocker thapsigargin (Tg). Simultaneous V_m(A) and [Ca²⁺]_i(B) responses to 5 μm Tg in GT1 neurons. C–G, Model simulations of GnRH plus Tg response. Tg was simulated by setting the SERCA pump rates to zero. Calibration in A applies to A andB, and in C applies toC–G.

Fig. 5.

Responses of GT1 neurons and model to GnRH stimulation during [Ca²⁺]_i buffering with BAPTA. Simultaneous V_m(A) and [Ca²⁺]_i(B) responses to 100 nm GnRH in GT1 neurons with [Ca²⁺]_i clamped to ∼200 nm. C–G, Model simulations of GnRH plus BAPTA response. BAPTA was simulated by setting the fraction of free cytosolic Ca²⁺(f_cyt) to 1 × 10⁻⁵. Calibration in A applies toA and B, and in C applies to C–G.

Fig. 6.

Responses of GT1 neurons and model to activation of adenylyl cyclase by forskolin. SimultaneousV_m (A) and [Ca²⁺]_i (B) responses to 10 μm forskolin in GT1 neurons.C–G, Model simulations of forskolin response. Forskolin was simulated by a threefold increase in the conductance of I_d. Calibration inA applies to A and B, and in C applies toC–G.