The effects of arginine-8 vasopressin on blood vessels supplying the cerebral cortex

A videocamera and a dissecting microscope have been used to record the effects of arginine-8 vasopressin (AVP) upon pial blood vessels in anaesthetised rats. Topical application of AVP caused a contraction of pial arteries, but had no measureable effect upon the diameter of veins. The smallest concentration of AVP that was effective in contracting arteries was 10(-7) mU/microliter. Stronger solutions (10(-5) to 2.0 mU/microliter) produced approximately the same (45%) reduction of external diameter. Contraction was maximal 0.25-1.0 min after application of the hormone, had almost recovered (10% contraction) after 10 min, and showed complete recovery by 30 min. Concentrations of AVP that were greater than 10(-3) mU/microliter produced tachyphylaxis, so that a second application of AVP 30 min later had considerably less effect. Concentrations less than 10(-3) mU/microliter produced no detectable tachyphylaxis. These results suggest that blood flow to the normal cerebral cortex may be partly under tonic control by the local concentration of AVP.

The generation of cortical slow potentials in the rat anaesthetised with urethane and their modification by nicotine

This paper reports the generation of cortical slow potentials (SP), similar to the human contingent negative variation, in rats anaesthetised with urethane. Rats previously implanted with silver/silver chloride electrodes for recording slow potentials from the frontal cortex were anaesthetised with urethane, and given extensive associative conditioning in which a 100 msec, 70 dB tone (S1) preceded electric tail shock (S2). The S1-S2 interval was initially set at 0.5 sec, and increased over a number of training sessions to 3 sec. Negative slow potential responses developed during the interval between S1 and S2. Once the slow potential response was fully developed, it persisted without habituation during subsequent sessions. Withholding tail shock (S2) resulted in a rapid extinction of the slow potential response to S1 alone. In further experiments, rats (n = 8) that generated steady slow potentials under urethane, were injected with saline (n = 4) or nicotine (0.4 mg/kg, s.c.; n = 4), and their responses measured 0-10, 15-25 and 30-40 min after administration. While saline did not affect the magnitude of the slow potential responses compared with pre-saline control values, nicotine (0.4 mg/kg) caused significant decreases in the magnitude of slow potentials during all the measurement intervals. These results demonstrate that cortical slow potentials can be generated in rats anaesthetised with urethane, and that these slow potentials are sensitive to drugs such as nicotine. The slow potential responses measured in the rat anaesthetised with urethane thus have similar characteristics to those generated in the conscious rat. Such a preparation may therefore prove useful in carrying out experiments on slow potentials that were previously difficult or impossible in conscious animals.

The effects of arginine-8 vasopressin on the cerebral cortex of rats anaesthetised with urethane

The effects of arginine-8 vasopressin (AVP) on the spontaneous electrical activity of the cerebral cortex were investigated in rats anaesthetised with urethane. Direct application of a relatively large concentration of AVP (0.5-2.0 mU AVP/microliter; 1.25-5.00 ng AVP/microliter) to the pial surface of a small area of the parietal cortex produced a complete cessation of spontaneous neural activity, which was often preceded by a short-lasting increase in frequency of discharge. There was also a decline in the number of large waves in the local electrocorticogram. These changes occurred after 1-4 min. A slow recovery began 10-30 min later, but was sometimes still incomplete after more than an hour. Only the first application of AVP produced the local effects described above. A second application, 2 hr later, was without effect. Direct application to a small area of one hemisphere did not alter the electrical activity of the contralateral cortex. Smaller concentrations of AVP (0.005-0.200 mU AVP/microliter; 12.5 pg-0.5 ng AVP/microliter) were variable in their actions. Occasionally, no effect at all was seen, while sometimes the cells stopped firing completely. However, the most usual consequence was a reduction in discharge frequency, the magnitude of which appeared to be related to the concentration of AVP applied. A second application of a small concentration of AVP usually failed to reduce the rate of spontaneous discharge.

The effects of atropine on event-related slow potentials in the rat

Negative slow potentials were recorded from the cortex of male rats in a passive food reward paradigm. These event-related slow potentials developed during the 3 s interval between a warning auditory cue (S1) and automatic food presentation (S2). The effects of the muscarinic antagonist atropine were examined on the slow potential responses. Atropine (0.1-0.4 mg/kg s.c.) produced a dose-related reduction in the magnitude of the slow potential responses when compared with pre-drug saline control values. The quaternary analogue of atropine, methylatropine (0.4 mg/kg s.c.), which does not enter the brain readily, had no effect on the slow potential responses. These results suggest that a central muscarinic-cholinergic component may be involved in the generation of the slow potential responses in the rat.

The development of tolerance to the depressant effect of arginine-8 vasopressin on the spontaneous motor activity of rats

Arginine-8 vasopressin (1.0 IU/kg, s.c.) reduced the spontaneous motor activity of rats with no previous experience of the drug. Daily administration of arginine vasopressin (1.0 IU/kg, s.c., once daily) to rats resulted in the gradual development of tolerance to the depressant effect of the drug on spontaneous activity. In further experiments, rats with no previous experience of the activity box, but chronically treated with arginine vasopressin (1.0 IU/kg, s.c., 3 times daily), also displayed tolerance to the depressant effect of the drug. The development of tolerance was therefore dependent on repeated exposure to vasopressin and not on repeated experience of the activity box.