The circumventricular organs form a potential neural pathway for lactate sensitivity: implications for panic disorder

Abstract

Patients with panic disorder experience panic attacks after intravenous sodium lactate infusions by an as yet unexplained mechanism. Lactate elicits a panic-like response in rats with chronic dysfunction of GABA neurotransmission in the dorsomedial hypothalamus (DMH). The circumventricular organs, organum vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), are potential sites that could detect increases in plasma lactate levels and activate the DMH. To test this, we obtained baseline heart rate (HR) and blood pressure (BP) responses to lactate infusions in rats fit with femoral arterial and venous catheters. Next, unilateral chronic injection cannulae connected to an Alzet infusion pump filled with the GABA synthesis inhibitor L-allylglycine (L-AG) were implanted into the DMH. Another chronic injection cannula was implanted into the region of the OVLT, SFO, or an adjacent control site, the median preoptic area (MePOA). These rats were tested once again with lactate infusions after injection of either artificial cerebrospinal fluid (ACSF) or tetrodotoxin (TTX) into the CVO sites. Injecting TTX into the OVLT completely blocked the lactate-induced response, whereas TTX injections into the SFO or MePOA did not. Also, direct injections of lactate (100 or 500 nl) into the OVLT elicited robust anxiety-like responses in these rats. These results suggest that the OVLT may be the primary site that detects lactate infusions, activating an anxiety-like response in a compromised DMH, and provide the first neuroanatomical basis for lactate response in panic disorder.

Fig. 1.

Representative photomicrograph of histological sections (magnification, 8×) showing (A) the site of Alzet pump cannula implantation (marked byarrow) in the DMH. The sites were marked by injecting 50% India ink, and only data from the animals with proper cannulae placement were used in the analysis. The injection site is impregnated with India ink, and some tissue necrosis at the site can be seen. However, B shows a higher magnification (32×) photomicrograph of the DMH (area marked by the black rectangle in A) on the side of the cannula implantation, demonstrating that the implantation of the Alzet pumps did not result in the destruction of cells in the majority of the DMH, thereby decreasing GAD activity. Further, the same degree of cannula damage was seen in the DMH of rats implanted with d-AG pumps, suggesting that the possible tissue damage was not causing the lactate response. DMH, Dorsomedial hypothalamus;F, fornix; mt, mamillothalamic tract;VMH, ventromedial hypothalamus; 3V, third ventricle.

Fig. 2.

Changes in (A) heart rate (HR) and (B) blood pressure (BP) after intravenous infusions of sodium lactate (10 ml/kg of 0.5N solution) in rats that were implanted unilaterally with the GABA synthesis inhibitor l-allylglycine-infusing (l-AG) Alzet minipumps into the dorsomedial hypothalamus (DMH). On postpump day 6/7, rats were injected, in random order, either with tetrodotoxin (TTX; 1 pmol in 100 nl) or with vehicle (ACSF) into the organum vasculosum stria terminalis (OVLT) before intravenous lactate infusions. Data are presented as mean ± SEM. Symbols show significant difference from baseline (*) and at 4 d (#) after l-AG pump in the DMH; p < 0.05, repeated measures ANOVA with Fisher’s LSD test.

Fig. 3.

Changes (mean ± SEM) in (A) heart rate (HR), (B) blood pressure (BP), (C) number of cage crossings, and (D) rearings after direct injections of either sodium lactate (100 and 500 nl of 0.5N solution) or saline (100 or 500 nl) into the OVLT of rats that had l-AG Alzet infusion pumps implanted into the DMH for 5 d. Injection of lactate directly into the OVLT elicited a robust panic-like response. Symbols show significant difference from saline control (*) and 100 nl of lactate (#) by repeated measures ANOVA with Fisher’s LSD;p < 0.05.

Fig. 4.

Representative photomicrograph of histological sections (magnification, 8×) showing the site of microinjection cannula implantation (marked by arrow) in the OVLT. The sites were marked by injecting 100 nl of 50% India ink.ACA, Anterior commissure, anterior; HDB, horizontal limb of the diagonal band; MS, medial septal nucleus; OVLT, organum vasculosum lamina terminalis;VDB, vertical limb of the diagonal band;2N, optic nerve.

Fig. 5.

Photomicrograph of a histological section (magnification, 8×) showing the site of microinjection cannula implantation (marked by arrow) in the mePOA. The sites were marked by injecting 100 nl of 50% India ink. AC, Anterior commissure; F, fornix; MPA, medial preoptic area.

Fig. 6.

Changes (mean ± SEM) in (A) heart rate (HR), (B) blood pressure (BP), (C) number of cage crossings, and (D) rearings after direct injections of either sodium lactate (100 and 500 nl of 0.5N solution) or saline (100 or 500 nl) into the SFO of rats that had l-AG Alzet infusion pumps implanted into the DMH for 5 d. Direct injection of lactate into the SFO in these rats also elicited a panic-like response that was less robust than the response elicited by injecting lactate into the OVLT. Symbols show significant difference from saline control (*) and 100 nl of lactate (#) by repeated measures ANOVA with Fisher’s LSD;p < 0.05.

Fig. 7.

Photomicrograph of a histological section (magnification, 8×) showing the site of microinjection cannula implantation (marked by arrow) in the SFO. The sites were marked by injecting 100 nl of 50% India ink. cc, Corpus callosum; PT, paratenial thalamic nucleus;PVA, paraventricular thalamic nucleus, anterior;SFO, subfornical organ; Sm, stria medullaris thalami; VHC, ventral hippocampal commissure.

Fig. 8.

Simplified schematic representation of the connections of the circumventricular organs and the DMH that may be involved in the lactate response. Many other connections of the DMH are not included for the sake of simplicity. It is suggested that the OVLT acts as the primary sensory site for peripheral lactate input and in turn activates the DMH to elicit a panic response. AII, Angiotensin II; BP, blood pressure; CG, central gray; DMH, dorsomedial hypothalamus;EAA, excitatory amino acids; GABA, γ-aminobutyric acid; HR, heart rate;IML, intermediolateral column of the spinal cord;LC, locus ceruleus; LH, lateral hypothalamus; mePOA, medial preoptic area;NTS, nucleus of tractus solitarius; OVLT, organum vasculosum lamina terminalis; PVN, paraventricular nucleus of the hypothalamus; RR, respiratory rate; SFO, subfornical organ.