Rostrocaudal Localization of Cardiovascular Responses Induced by Intrathecal Administration of Apomorphine in Conscious, Freely Moving Rats

Abstract

In conscious freely moving rats, administration of apomorphine (179 nmol), a dopamine receptor agonist, into the intrathecal (i.t.) space decreased mean aortic blood pressure (MBP) and heart rate (HR). Both the magnitude and the time of appearance of the response varied according to the spinal level of administration. The largest and immediately appearing effect was observed after the injection at the upper thoracic site, whereas the magnitude of the responses was smaller with an immediate or slightly delayed (0.5-1.5 min) onset at lower thoracic and midcervical levels. More caudal responses appeared to be due to spreading of the drug along the spinal axis (onset in 1-2 min after administration). Behavioral responses (stereotyped movements) were observed within 2-3 min after administration and were nearly the same whatever the site of administration. These results corroborate, as do those provided by i.t. injections of tritiated apomorphine, the spinal origin of cardiovascular effects of i.t. apomorphine. Furthermore, spinal transection at the T5-T7 level did not change the magnitude and duration of decreases in MBP and HR elicited by i.t. apomorphine injected at the T2-T4 level. Moreover, this procedure enhanced responses to i.t. administration at the T9-T10 level. In conclusion, these results favor the existence of a spinal site of action for the cardiovascular effects of apomorphine. Furthermore, they indicate that spinal transection is accompanied by development of a hypersensitive phenomenon (of a mechanism to be determined).