Plasma non-esterified docosahexaenoic acid is the major pool supplying the brain

Abstract

Despite being critical for normal brain function, the pools that supply docosahexaenoic acid (DHA) to the brain are not agreed upon. Using multiple kinetic models in free-living adult rats, we first demonstrate that DHA uptake from the plasma non-esterified fatty acid (NEFA) pool predicts brain uptake of DHA upon oral administration, which enters the plasma NEFA pool as well as multiple plasma esterified pools. The rate of DHA loss by the brain is similar to the uptake from the plasma NEFA pool. Furthermore, upon acute iv administration, although more radiolabeled lysophosphatidylcholine (LPC)-DHA enters the brain than NEFA-DHA, this is due to the longer plasma half-life and exposure to the brain. Direct comparison of the uptake rate of LPC-DHA and NEFA-DHA demonstrates that uptake of NEFA-DHA into the brain is 10-fold greater than LPC-DHA. In conclusion, plasma NEFA-DHA is the major plasma pool supplying the brain.

Figure 1. NEFA-DHA uptake by the brain.

(a) Schematic diagram of kinetic modeling following continuous intravenous (iv) infusion. (b) Plasma lipid radioactivity upon continuous iv infusion of 86 μCi NEFA-¹⁴C-DHA over 5 minutes (n = 6). Radioactivity from other plasma lipid pools was below detection, 23 pCi. (c) Schematic diagram of kinetic modeling following NEFA-¹⁴C-DHA gavage. (d) Plasma lipid radioactivity upon NEFA-¹⁴C-DHA oral gavage over 4 hours (n = 4) and the percent composition of total radiolabeled lipid pools in the plasma post-gavage of NEFA-¹⁴C-DHA. (e) Brain radioactivity of total phospholipids from rats gavaged or continuously iv infused with NEFA-¹⁴C-DHA. (f) The uptake incorporation coefficient of NEFA-¹⁴C-DHA (k^*) into brain total phospholipids from rat gavage or continuous iv infusion. (g) The plasma concentration of NEFA-DHA prior to NEFA-¹⁴C-DHA gavage or continuous iv infusion. (h) The net incorporation rate of DHA (J_in) to brain total phospholipids from rats gavaged or iv infused with NEFA-¹⁴C-DHA. (i) Schematic diagram of kinetic modeling following intracerebroventricular (icv) infusion. (j) Brain loss curve of esterified ¹⁴C-DHA from brain total phospholipids including lysophosphatidylcholine over 128 days (n = 3–4 independent samples per time point). (k) The loss of DHA from brain total phospholipids (rate of loss, J_out) from rats infused icv with NEFA-¹⁴C-DHA which was not significantly different from the J_in of rats continuously iv infused with NEFA-¹⁴C-DHA. (l) Calculated predictive k^* from the NEFA-¹⁴C-DHA icv infusion which was not significantly different from k^* from NEFA-¹⁴C-DHA iv infusion. All data are expressed as mean ± SEM; whereas percent plasma radioactivity data are expressed as mean. ^*P < 0.05 and ^**P < 0.01 indicate significant difference from gavage using Students’ t-test. AUC, area under the curve; CE, cholesteryl ester; DAG, diacylglycerol; LPC, lysophosphatidylcholine; NEFA, non-esterified fatty acid; PL, phospholipids; TAG, triacylglycerol.

Figure 2. Plasma LPC-DHA contribution to brain total DHA uptake via bolus iv infusion kinetic modeling.

(a) Schematic diagram of iv infusion kinetic modeling. (b,c) Plasma lipid radioactivity from NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA bolus iv infusion (13 μCi) followed by 2-hour plasma sampling (n = 4 each). (d,e) Percent composition of various radiolabeled lipid pools in the plasma at each time point following an acute iv bolus infusion of NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (f) Percent composition of various radiolabeled lipid pools in the plasma over 2-hour period following an acute iv bolus infusion of NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (g) Plasma radiotracer exposure from various lipid pools after bolus iv infusion of NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (h) Brain radioactivity of total phospholipids from rats iv infused with bolus NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (i) The uptake incorporation coefficient (k^*) into brain total phospholipids from rats iv infused with bolus NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (j) The baseline plasma concentration of total NEFA-DHA and LPC-DHA prior to NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA bolus iv infusion. LC/MS/MS spectra of NEFA-DHA and LPC-DHA. (k) The net incorporation rate of DHA (J_in) into brain total phospholipids from rats iv infused with bolus NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. Percent plasma radioactivity data are expressed as mean. Remaining data are expressed as mean ± SEM. ^*P < 0.05, ^**P < 0.01 and ^***P < 0.001 indicate significant difference from NEFA-¹⁴C-DHA infused rats. ⁺P < 0.01 indicates significant difference between plasma LPC-DHA and NEFA-DHA concentration. ^#P < 0.001 indicates significant difference between radiolabeled LPC pool from LPC-¹⁴C-DHA infused rats and radiolabeled NEFA-DHA pool from NEFA-¹⁴C-DHA infused rats. Students’ t-test was used to determine significant difference. See Fig. 2 for abbreviations.

Figure 3. Plasma LPC-DHA and NEFA-DHA contribution to brain total DHA uptake via continuous iv infusion kinetic model.

(a) Schematic diagram of continuous iv infusion kinetic modeling. (b,c) Plasma curve from continuous iv infusion of low-dose (10 μCi) NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA over 5 minutes (n = 4 each). (d) Brain radioactivity of total phospholipids from rats iv infused with NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (e) The uptake incorporation coefficient (k^*) into brain total phospholipids from rats continuously iv infused with NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. (f) The baseline plasma concentration of total NEFA-DHA and LPC-DHA prior to NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA continuous iv infusion. (g) The net incorporation rate of DHA (J_in) into brain total phospholipids from rats continuously iv infused with NEFA-¹⁴C-DHA or LPC-¹⁴C-DHA. All data are expressed as mean ± SEM. ^*P < 0.05, ^**P < 0.01 and ^***P < 0.001 indicate significant difference from NEFA-¹⁴C-DHA infused rats using Students’ t-test. ⁺P < 0.01 indicates significant difference between plasma LPC-DHA and NEFA-DHA concentration. See Fig. 2 for abbreviations.