1.
Isoprenaline, 3.5-20 ng, injected intracerebroventricularly in atropinized mice under
pentobarbitone anaesthesia produced a dose-dependent
tachycardia. 2. Pretreatment with either
reserpine or
pempidine blocked nervously-mediated
tachycardia as shown by marked reduction of that due to stimulation of the spinal outflow in pithed mice. After pretreatment with these drugs, intracerebroventricular
isoprenaline caused
tachycardia of a similar degree and time course to that in mice not so pretreated. 3. Pretreatment with either
reserpine or
pempidine caused supersensitivity to the
tachycardia due to intravenous
isoprenaline. 4. When allowance was made for this supersensitivity in the effect of intracerebroventricular
isoprenaline in pretreated mice, a small dose-dependent residual effect remained that could be attributed to leakage of
isoprenaline into the peripheral circulation. 5. This was confirmed by the appearance of a late-developing
tachycardia on intracerebroventricular injection of
isoprenaline in spinal mice. 6. It is therefore concluded that the
tachycardia caused by intracerebroventricular
isoprenaline in mice is, at least initially, of central origin.