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Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

L. Sundin^1,*, J. Turesson¹ and E. W. Taylor²

¹ Department of Zoology, Göteborg University, Box 463, S-40530 Gothenburg, Sweden
² School of Biosciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

View larger version (108K): [in a new window]   Fig. 1. Micrographs demonstrating the termination of the vagal projections into the medulla marked by the axonal transport of the tracer Fast Blue from an injection site in the nodose ganglion. (A) Transverse section (TS) taken 0.6 mm rostral to obex, showing the vagal sensory (Xs) and motor (Xm) areas, dorso-lateral to the fourth ventricle (4V). (B) TS taken 0.12 mm caudal to obex, showing commissural sensory fibres. (C) TS taken 0.08 mm caudal to obex, showing commissural fibres beneath the central canal (CC) of motor origin. (D) TS taken 0.3 mm rostral of obex to show the nucleus ambiguus (NA), situated ventro-laterally to the Xm. (E) Nissl-stained section from the same animal as in A, 580 µm rostral to obex. White scale bars represent 50 µm and the black scale bar represents 1 mm. Note that the Xs and Xm areas are clearly separate.

View larger version (25K): [in a new window]   Fig. 2. The coordinates of the vagal sensory (Xs) and motor (Xm) columns in the brainstem of the sculpin derived from the results of the neural tract tracing. The areas are plotted and drawn in both the horizontal and the sagittal planes. The dorsal rostrocaudal surface of the medulla, its midline and obex are all set to zero.

View larger version (30K): [in a new window]   Fig. 3. A schematic drawing of the medulla showing the responses to glutamate injection at each coordinate. Numbers in brackets denote the total number of animals injected at that coordinate. The other numbers above and beneath the symbols at a specific coordinate represent the number of animals that have shown that particular response. Filled symbols depict a decrease, whereas open symbols depict an increase. Note the differences in distribution of the pressor and depressor responses, as well as the excitatory ventilatory responses. Heart rate (fH), ventilation frequency (fV), ventilation amplitude (VAMP).

View larger version (28K): [in a new window]   Fig. 4. Responses to an injection of 40 nl glutamate (10-2moll-1) 1.5 mm rostral of obex and 0.5 mm lateral to midline at 0.2 mm depth. Note the specific increase in the ventilation amplitude (VAMP) only (divided vertical line denotes time of injection). Heart rate (fH), ventilation frequency (fV), ventral aortic blood pressure (PVA).

View larger version (28K): [in a new window]   Fig. 5. Responses to an injection of 80 nl glutamate 0.5 mm rostral of obex and 0.5 mm lateral to midline at 0.2 mm depth, showing both a ventilatory and a definitive depressor response with a transient bradycardia. Heart rate (FH), ventilation frequency (fV), ventral aortic blood pressure (PVA).

View larger version (29K): [in a new window]   Fig. 6. Responses to successive injections of 40 nl glutamate 1.0 mm rostral to obex and 0.5 mm lateral to midline at 0.1 mm and then at 0.3 mm depths showing a pressor response. Note the marked changes in the ventilatory responses arising from advancing the pipette just 0.2 mm.

View larger version (17K): [in a new window]   Fig. 7. Changes in ventral aortic blood pressure (PVA) and heart frequency (fH) arising from sequential injections of glutamate, vehicle (NaCl) and N-methyl-L-aspartate (NMDA) receptor antagonist MK-801 at 1.0 mm rostral of obex and 0.3 mm lateral to the midline. The arrows indicate the time of injection, and the numbers (in mm) indicate the depth of the injections. Note the rapid and the marked bradycardia. Also observe the absence of responses to NaCl and the preciseness of the MK-801 blockade. Injection volume ranged between 40 nl and 100 nl.

View larger version (23K): [in a new window]   Fig. 8. Changes in mean (±S.E.M.) heart rate (fH) following medullary injections of glutamate (40-100 nl) in five animals. A marked bradycardia was blocked by prior application of MK-801 in the same sites. * indicates significant changes from pre-injection values at P<0.01, and # indicates a significant difference in the same time point before and after injection of MK-801 at P<0.01.

View larger version (19K): [in a new window]   Fig. 9. Changes in mean (±S.E.M.) heart rate (fH) following medullary injections of glutamate (40-100 nl) in five animals. A marked bradycardia was blocked by prior systemic injection of atropine. * indicates significant changes from pre-injection values at P<0.0001, and # indicates a significant difference in the same time point before and after injection of atropine at P<0.001.