The plasma membrane monoamine transporter (PMAT) is selective for 5-HT compared to noradrenaline in the nucleus tractus solitarius (NTS) of anesthetized male rats.

PS HOSFORD1, J MILLAR2, AG RAMAGE1. 1University College Londonon, London, UK, 2The London Queen Mary's School of Medicine and Dentistry, London, UK

Monoamine transmitters are known to be removed from their site of action by two types of transporters; uptake1, (a high –affinity, low-capacity transporter) and uptake2, (a low-affinity, high-capacity transporter; 1). It is now known that each monoamine has its own high –affinity, low-capacity transporter, SERT, NET and DAT (2). Recently uptake2, which is inhibited by corticosterone, has been identified as the organic cation transporter 3 (OCT3; 3) while another low –affinity, high-capacity transporter, insensitive to corticosterone, has been identified as PMAT (3). Both these transporters are blocked by decynium-22 (4). In vitro experiments have shown PMAT is 70% more effective in transporting 5-HT than NE (5). The present experiments were carried out to examine this reported difference by measuring the effects (i.v.) of low and high affinity uptake blockers on the decay rate of microinjected 5-HT (0.1 mM) and NE (1 mM) in the nucleus tractus solitarius (NTS), using fast-cyclic voltammetry (6) to measure amine levels.

Experiments were performed on male Sprague Dawley rats (200-300g) anaesthetized with isoflurane (5% in 100% O2) and maintained with α-chloralose (100 mg kg-1). A multi-barrelled microelectrode incorporating a carbon fibre was positioned stereotaxically in the NTS. One barrel was used to inject 5‑HT or NE at a constant pressure (2.5 psi for 1-3s) until it reached a concentration of ~1µM. The time taken for the monoamine to decay to 80% of its maximal value was measured as T80. Microinjections were repeated at 5 min intervals. This timing allowed the evoked signal to completely decay before the next application. At least 3 stable applications of 5-HT or noradrenaline were required before administration (i.v.) of test drugs (1mg kg-1 unless stated) or vehicle (saline or DMSO). Changes were compared to vehicle using a one-way ANOVA and the means compared with Fisher’s LSD test. P<0.05 was considered significant. All values are expressed as mean ± s.e.mean.

5-HT clearance: Decynium-22 (600 μg kg-1; n = 5) significantly increased the time taken for 5-HT clearance. T80 increased by 76 ± 13% compared to DMSO (n = 7), which caused a change of 2 ± 1.6%. Citalopram (n = 4) also increased T80 but only by 37 ± 5% compared to saline (n = 6), which caused a change of -3 ± 3%. Desipramine (n = 4) or corticosterone (10mg kg‑1; n=5) had no effect on T80.

Noradrenaline (NE) clearance: Decynium-22 ( 600μg kg-1 , n = 5) and citalopram (n = 4) had no significant effect on noradrenaline, causing a change of 0 ± 10 %, and 2 ± 3% respectively in T80 compared with time-matched saline (n = 6) and DMSO (n = 4) , which were 1 ± 1.2% and 7 ± 4% respectively. Desipramine (n = 5) did increase T80 significantly by 83 ± 17% compared to saline.

The data indicates that PMAT not OCT3 is involved in the removal of 5‑HT but not that of NE. Further the data implies that uptake2 (OCT3) is not operational in the NTS. Whether this applies to other areas of the brain remains to be determined.

PSH is in receipt of BBSRC studentship

(1) Iversen LL. (1967) Cambridge University Press.

(2) Amara SG & Kuhar MJ.(1993) Annu Rev Neurosci. 16:73–93

(3) Gründemann D et al.(1998) Nature Neurosci 1:349-351

(4) Hayer-Zillgen et al H (2002) Br J. Pharmacol 136:829-836

(5) Duan H, Wang J (2010). J PET 335:743-753

(6) Millar J. (1997) Methods Mol Biol. 72:251-66