CARDIOVASCULAR AND SINGLE-UNIT RESPONSES ELICITED BY STIMULATION OF THE ISLANDS OF CALLEJA AND BY CHANGES IN ARTERIAL-PRESSURE

Recent experiments in this laboratory have investigated the distribution of the nuclear protein Fos in the rat brain after unilateral electrical stimulation of the aortic depressor nerve and have revealed intense ipsilateral staining in the islands of Calleja. To test the hypothesis that the islands of Calleja may have a role in cardiovascular control, we tested the effect on arterial pressure and heart rate of microinjection of L-glutamate into the islands of Calleja of the artificially ventilated, urethane anesthetized rat. Microinjection of glutamate into the islands of Calleja resulted in a decrease in arterial pressure (24.5 +/- 1.5 mmHg) and heart rate (16.6 +/- 1.6 bpm). These responses were eliminated by i.v. injection of propranolol or by transection of the spinal cord at the C-1 level. These results are the first demonstration that the islands of Calleja are involved in mediating cardiovascular reflexes primarily through an influence on sympathetic outflow to the heart and vessels. To determine the effects of increases or decreases of arterial pressure on single unit activity in the islands of Calleja, responses of single units in the islands to i.v injection of phenylephrine or sodium nitroprusside were recorded. Of 87 units recorded from in the islands of Calleja, 40 (46%) responded to phenylephrine and 27 (68%) decreased their firing frequency. Of these 27, 16 (59.3%) increased their firing frequency to administration of nitroprusside. In addition, in 11 (31%) of 35 units recorded from in the islands, glutamate microinjection in the nucleus tractus solitarius decreased the firing frequency in 9 (82%) units. Of these 9 units, 7 also decreased their firing frequency to i.v. phenylephrine and increased their firing frequency to i.v. nitroprusside. These results indicate that neurons in the islands of Calleja influence sympathetic control of arterial pressure and heart rate and that single units in the islands respond to changes in arterial pressure.