Probing Neuro-Endocrine Interactions Through Remote Magnetothermal Adrenal Stimulation

Abstract

Exposure to stressful or traumatic stimuli may alter hypothalamic-pituitary-adrenal (HPA) axis and sympathoadrenal-medullary (SAM) reactivity. This altered reactivity may be a component or cause of mental illnesses. Dissecting these mechanisms requires tools to reliably probe HPA and SAM function, particularly the adrenal component, with temporal precision. We previously demonstrated magnetic nanoparticle (MNP) technology to remotely trigger adrenal hormone release by activating thermally sensitive ion channels. Here, we applied adrenal magnetothermal stimulation to probe stress-induced HPA axis and SAM changes. MNP and control nanoparticles were injected into the adrenal glands of outbred rats subjected to a tone-shock conditioning/extinction/recall paradigm. We measured MNP-triggered adrenal release before and after conditioning through physiologic (heart rate) and serum (epinephrine, corticosterone) markers. Aversive conditioning altered adrenal function, reducing corticosterone and blunting heart rate increases post-conditioning. MNP-based organ stimulation provides a novel approach to probing the function of SAM, HPA, and other neuro-endocrine axes and could help elucidate changes across stress and disease models.

Keywords: adrenal gland; corticosterone; epinephrine; hormones; magnetic nanoparticles; neuromodulation.

FIGURE 1

Experimental design. (A) When an alternating magnetic field (AMF) is applied, the magnetic nanoparticles dissipate heat that causes the opening of heat-sensitive calcium-permeable ion channels (TRPV1 receptors). The calcium influx causes E and CORT release from adrenal cells. (B) Schematic diagram of the procedures that were performed in order. 1. Adrenal MNP injections. 2. Heart rate monitoring before and during magnetothermal stimulation. 3. Aversive conditioning and extinction. 4. Serum collections during magnetothermal stimulation for hormone analyses. The experimental timeline was as follows: After adrenal MNP injections, heart rate monitoring took place during MNP stimulation for 2 days (days 1 and 2) before the 3-day behavioral testing (days 3–5). Five days later, blood was collected for serum hormone (E and CORT) measurements (days 10 and 11). (C–F) Adrenal injection specifications and histology. (C) The specific loss power (SLP) of MNPs at a frequency of 624 kHz was estimated for two amplitudes (13 and 14 kA/m) that were measured within the coil. (D) Finite element modeling of temperature increases at 2 small-volume MNP injection sites within the adrenal gland at 20-s, 40-s, and 60-s of AMF applied. Scale bar = 1 mm. (E) Representative mosaic of an adrenal gland section with two MNP injections (two black holes). Scale bar = 500 μm. (F) A map of MNP and WNP (control) injection sites in the adrenal gland across all animals (n = 16).

FIGURE 2

Summary chart of animal exclusions by analysis. A total of 28 rats were used in this study. Of these 28 animals, a number of animals were excluded from each of the heart rate, serum, and behavioral analyses for the reasons described in the diagram.

FIGURE 3

Magnetothermal adrenal stimulation effects on heart rate and hormone levels. (A,B) Heart rate measurement during magnetothermal stimulation on (A) days 1 and 2 before conditioning and (B) on day 4 after conditioning. (C,D) Serum hormone analysis. (C) Corticosterone and (D) epinephrine were measured before (pre) and after (post) magnetothermal stimulation. (E,F) Behavioral effects of magnetothermal adrenal stimulation. (E) Percent freezing per trial across the habituation, conditioning, extinction, and recall phases of the behavioral paradigm (controls, n = 13; active, n = 10). (F) Percent freezing averaged across the last three trials of extinction (Trials 18–20) for control and active MNP rats.