CONTROLLED SUPERFUSION OF THE RAT SPINAL-CORD FOR STUDYING NONSYNAPTIC TRANSMISSION - AN AUTORADIOGRAPHIC ANALYSIS

Recently, evidence has been raised that long-term changes in the centr al nervous system are mediated by extrasynaptic spread of neuropeptide s ('volume transmission'). To study the effects of volume transmission in the spinal cord we developed the technique of controlled superfusi on of the rat cord dorsum. This paper presents quantitative data about the spread, local spinal tissue concentration and redistribution of ( 2-[I-125]iodohistidyl)neurokinin A, applied for 15, 30 or 60 min to th e spinal cord dorsum in concentrations of 0.05 or 50 mu M (10 mu l). A nalysis of autoradiograms of sagittal and transverse spinal cord secti ons was done by computer-assisted densitometry. Under all experimental conditions, the spread of radiolabel into the superfused spinal cord segments reached Rexed's laminae V and VI; maximal spread (1.6 +/- 0.3 mm) was measured after superfusion for 30 min. The amount of radiolab el decreased in ventral direction as a function of distance. Highest t issue concentrations of neurokinin A (NKA) were obtained within the su perficial spinal cord up to a depth of 0.5 mm and ranged from 700 to 2 000 pmol/g following superfusions for 15 or 30 min with 50 mu M NKA. T hus, these tissue concentrations were 25-70 times lower than the conce ntration of NKA in the superfusate. Since pool content was not exchang ed, the radioactivity within the spinal cord was lower after superfusi on periods of 60 min than after 15 or 30 min. Detection of radiolabel in blood and urine suggests that capillary clearance is relevant and l imits the accumulation of the peptide within the spinal cord tissue an d the spread into deeper laminae. The controlled superfusion of the ra t cord dorsum is a useful method to mimick the spinal release of endog enous neuropeptides such as NKA during intense noxious stimulation, an d it can be employed for versatile investigations of the effects of ne uroactive molecules on the processing of sensory information in the in tact spinal network.