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Visceromotor and Spinal Neuronal Responses to Colorectal Distension in Rats With Aldosterone Onto the Amygdala

¹Department of Physiology, University of Oklahoma Health Sciences Center, 73190; and ²Oklahoma Foundation for Digestive Research, Basic Science Laboratories, Veterans Affairs Medical Center, Oklahoma City, Oklahoma 73104

Submitted 10 January 2003; accepted in final form 8 March 2003

Stereotaxic delivery of corticosterone onto the amygdala produces colorectal hypersensitivity through activation of lumbosacral spinal neurons. Since corticosterone activates both the mineralocorticoid (MR) and glucocorticoid (GR) receptors, the aim of this study was to determine the importance of MRs in the regulation of colorectal hypersensitivity through the use of aldosterone that preferentially binds to MRs. Fischer-344 rats received either aldosterone (n = 18)- or cholesterol (control, n = 18)-containing micropellets bilaterally placed stereotaxically on the dorsal margin of the amygdala. After 1 wk, colorectal sensitivity to distension (30 mmHg) was measured in a subgroup of rats (n = 8/group). In other rats (n = 10/group), extracellular potentials of single L₆–S₁ spinal neurons in response to colorectal distension (CRD; 10–80 mmHg) were recorded. In aldosterone-implanted rats, CRD produced a greater visceromotor behavioral response compared with cholesterol controls (19 ± 0.5 vs. 11.5 ± 2.7; P < 0.01). A total of 68/182 (37%) and 56/167 (34%) of spinal neurons responded to noxious CRD in aldosterone-implanted and control groups, respectively. A total of 36/42 (86%) neurons excited by CRD had spontaneous activity in aldosterone-implanted groups compared with control (19/33, 58%, P < 0.01). Neurons with low thresholds for excitatory responses to CRD were seen more frequently in aldosterone-implanted rats than those in the control group (35/39 vs. 18/31, P < 0.05). Maximal excitatory responses of neurons to CRD in aldosterone-implanted rats were significantly greater (23.9 ± 2.2 vs. 16.4 ± 2.0 imp/s, P < 0.05), and the durations were longer (34.3 ± 2.7 vs. 24.9 ± 1.4 s, P < 0.05) than those in control group. Finally, a greater number of neurons had wide dynamic range responses to somatic stimulation in aldosterone-treated rats compared with cholesterol controls. Our findings suggest that, in the amygdala, MR receptor–mediated mechanisms are likely involved in descending pathways onto lumbosacral spinal neurons that induce colorectal hypersensitivity to luminal distension.

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