Spatial and temporal patterns of serotonin release in the rat’s lumbar spinal cord following electrical stimulation of the nucleus raphe magnus

Abstract

The monoamine neurotransmitter serotonin is released from spinal terminals of nucleus raphe magnus (NRM) neurons and important in sensory and motor control, but its pattern of release has remained unclear. Serotonin was measured by the high-resolution method of fast cyclic voltammetry (2 Hz) with carbon-fiber microelectrodes in lumbar segments (L3-L6) of halothane-anesthetized rats during electrical stimulation of the NRM. Because sites of serotonin release are often histologically remote from membrane transporters and receptors, rapid emergence into aggregate extracellular space was expected. Increased monoamine oxidation currents were found in 94% of trials of 50-Hz, 20-s NRM stimulation across all laminae. The estimated peak serotonin concentration averaged 37.8 nM (maximum 287 nM), and was greater in dorsal and ventral laminae (I-III and VIII-IX) than in intermediate laminae (IV-VI). When measured near NRM-evoked changes, basal monoamine levels (relative to dorsal white matter) were highest in intermediate laminae, while changes in norepinephrine level produced by locus ceruleus (LC) stimulation were lowest in laminae II/III and VII. The NRM-evoked monoamine peak was linearly proportional to stimulus frequency (10-100 Hz). The peak often occurred before the stimulus ended (mean 15.6 s at 50 Hz, range 4-35 s) regardless of frequency, suggesting that release per impulse was constant during the rise but fell later. The latency from stimulus onset to electrochemical signal detection (mean 4.2 s, range 1-23 s) was inversely correlated with peak amplitude and directly correlated with time-to-peak. Quantitative modeling suggested that shorter latencies mostly reflected the time below detection threshold (5-10 nM), so that extrasynaptic serotonin was significantly elevated well within 1 s. Longer latencies (>5 s), which were confined to intermediate laminae, appeared mainly to be due to diffusion from distant sources. In conclusion, except possibly in intermediate laminae, serotonergic volume transmission is a significant mode of spinal control by the NRM.Keywords spinal cord; raphe; serotonin; monoamines; fast cyclic voltammetry; volume transmission The monoamine neurotransmitter serotonin is an important modulator of sensory and motor pathways in the spinal cord. It is released from axon terminals whose cell bodies reside mostly in the hindbrain raphe nuclei (Törk, 1990;Jones and Light, 1992;Kwiat and Basbaum, 1992), especially the nucleus raphe magnus (NRM). One of its major effects in the dorsal horn is to depress ascending nociceptive transmission (Furst, 1999;Millan, 2002). In the intermediate laminae and ventral horn it enhances locomotor rhythms and modulates various reflex pathways (Schmidt and Jordan, 2000;Hochman et al., 2001). Although numerous functional and anatomical studies of the spinal serotonergic system and its brainstem origins have appeared, measurements of spinal serotonin release on temporal and spatial scales relevant to its extracellular dynamics have not been previously reported. A...

Serotonin Concentrations in the Lumbosacral Spinal Cord of the Adult Rat Following Microinjection or Dorsal Surface Application

“…2) compared with in vitro (Fig. 1) (Hentall et al 2006;Noga et al 2004;Phillips and Stamford 1999) and oftentimes the oxidation current developed a shoulder during the course of the experiment, as described previously (Phillips and Stamford 1999). The reduction peaks also tended to be broader and were slightly displaced (cf.…”

“…CFMEs were constructed using established procedures (Armstrong-James and Millar 1984;Hentall et al 2003Hentall et al , 2006Noga et al 2004) and electrically pretreated as described previously (Hentall et al 2006). Arrays containing two to four CFMEs were made by cementing the electrodes together at fixed relative positions using fast-drying epoxy (J-B Kwik; J-B Weld, Sulfur Springs, TX).…”

“…Thus FCV can provide the high spatial and temporal resolution needed to study the spread of 5-HT in the spinal cord (Hentall et al 2006;Noga et al 2004) after exogenous application.…”

“…Based on our prior work mapping brain stem-evoked monoamine release within specific laminae (Hentall et al 2003(Hentall et al , 2006, we formed the hypothesis that extracellularly released monoamines do not spread much beyond a few hundred microns. Here we test this hypothesis by measuring the spatiotemporal spread of 5-HT in the lumbosacral spinal cord of the adult rat using two different drug application methods-intraspinal microinjection and dorsal surface application.…”

Midbrain Raphe Stimulation Improves Behavioral and Anatomical Recovery from Fluid-Percussion Brain Injury

“…39 Because firing in midbrain raphe neurons is related strongly to general arousal, 40 a 12-h daily hiatus was imposed. Larger effects on recovery could perhaps have been produced with different stimulation parameters or electrode configurations, as well as with other start times and numbers of days of the treatment.…”