Elevated nerve growth factor secreted by bladder smooth muscle may be associated with noradrenergic hyperinnervation of the bladder and hyperactive voiding in spontaneously hypertensive rats (SHR) and rats with bladder outlet obstruction. The present study was undertaken to determine if changes occur in efferent and afferent pathways supplying the SHR bladder similar to those in rats with bladder outlet obstruction. Fluoro-Gold (FG) retrograde tracing studies were conducted to examine the postganglionic efferent limb (major pelvic ganglion; MPG) and sensory afferent limb (L1, L2, L6, and S1 dorsal root ganglion; DRG) of the micturition reflex pathway of the SHR and Wistar-Kyoto (WKY) normotensive rat. A significant increase in cross sectional area profiles for labeled neurons in the MPG was observed in SHRs (830.5 +/- 9.0 microns2) as compared to WKYs (736.3 +/- 16.6 microns2). Neuronal cell areas in L2 (1,010.9 +/- 18.6 microns2) and S1 (1,024.6 +/- 28.3 microns2) of SHRs were significantly larger than those of WKYs (L2, 865.3 +/- 12.6 microns2, S1, 778.3 +/- 11.2 microns2). There was an increase in number of labeled cells in L6 within SHRs over WKYs. These results provide evidence that both efferent and afferent changes in neuronal innervation of the bladder occur in SHRs. The SHR strain may represent a genetic model to study changes in micturition reflex pathways that result from alterations in neuronal morphology such as those that occur with urethral outlet obstruction.