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

Abstract

Brumley MR, Hentall ID, Pinzon A, Kadam BH, Blythe A, Sanchez FJ, Taberner AM, Noga BR. Serotonin concentrations in the lumbosacral spinal cord of the adult rat following microinjection or dorsal surface application. J Neurophysiol 98: 1440 -1450, 2007. First published July 18, 2007 doi:10.1152/jn.00309.2007. Application of neuroactive substances, including monoamines, is common in studies examining the spinal mechanisms of sensation and behavior. However, affected regions and time courses of transmitter activity are uncertain. We measured the spatial and temporal distribution of serotonin [5-hydroxytryptamine (5-HT)] in the lumbosacral spinal cord of halothane-anesthetized adult rats, following its intraspinal microinjection or surface application. Carbon fiber microelectrodes (CFMEs) were positioned at various locations in the spinal cord and oxidation currents corresponding to extracellular 5-HT were measured by fast cyclic voltammetry. Intraspinal microinjection of 5-HT (100 M, 1-3 l) produced responses that were most pronounced at CFMEs positioned Յ800 m from the drug micropipette: 5-HT concentration was significantly higher (1.43 vs. Ͻ0.28% of initial concentration) and response latency was shorter (67.1 vs. 598.2 s) compared with more distantly positioned CFMEs. Treatment with the selective 5-HT reuptake inhibitor clomipramine only slightly affected the spread of microinjected 5-HT. Surface application over several segments led to a transient rise in concentration that was usually apparent within 30 s and was dramatically attenuated with increasing depth: 0.25% of initial concentration (1 mM) within 400 m of the dorsal surface and Ͻ0.001% between 1,170 and 2,000 m. This initial response to superfusion was sometimes followed by a gradual increase to a new concentration plateau. In sum, compared with bath application, microinjection can deliver about tenfold higher transmitter concentrations, but to much more restricted areas of the spinal cord.

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“…Previous experiments using topical application of drugs suggest that these doses are within the expected effective range (Brumley et al 2007;Chau et al 2002). The most pronounced effect of SB-269970 was prolongation of extension in the hindlimbs that severely impaired locomotion.…”

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“…First, both are chemically distinct. Second, 5-HT is a natural neuromodulator that would be cleared faster than strychnine through 5-HT-transporters such as SERT (Brumley et al, 2007). Even if we cannot directly compare 5-HT and strychnine diffusion into the spinal cord, our data indicated that strychnine diffusion was also limited.…”

“…Recent observations by Brumley et al (2007) revealed that when 5-HT is applied on the dorsal surface of the rat spinal cord only a small fraction of the applied concentration could be detected in deep layers. A direct comparison The experiment was the same as in Figs.…”

“…The concentration of strychnine was chosen to be high (500 M), due to the pia matter presenting a significant barrier for drug penetration (Brumley et al, 2007). Brumley et al (2007) showed that for 5-HT, if applied on the dorsal surface of rat spinal cord, only <0.8% of the applied concentration can be detected within 0.4 mm from the dorsal surface and the concentration becomes even more diluted in deeper layers (<0.001% at ventral spinal cord, 1.17-2 mm from the dorsal surface).…”

“…Brumley et al (2007) showed that for 5-HT, if applied on the dorsal surface of rat spinal cord, only <0.8% of the applied concentration can be detected within 0.4 mm from the dorsal surface and the concentration becomes even more diluted in deeper layers (<0.001% at ventral spinal cord, 1.17-2 mm from the dorsal surface). However, this value is measured in the presents of 5- HT clearing mechanisms such as SERT (a 5-HT transporter).…”