mise à jour du
22 juillet 2004
Behavioural Brain Res
1988; 28; 309-313
Effect of isolation on dopaminergic
agonist-induced penile erections
and stretching and yawning in male rats
Francesca Ferrari and Valeria Mangiafico
Institute of Pharmacology, Università di Modena, Modena, Italy


Introduction : It is well-known that environment plays a crucial part in determining animal behaviour; there is ample evidence that social isolation, easily reproducible in laboratory experiments, produces neurochemical, anatomical and physiological alterations in rodents. Isolated rats have been found to differ from those reared in a community in their sensitivity to the pharmacological effects of drugs as determined by behavioural parameters; moreover, marked changes in biogenic amine dynamics have been demonstrated in their brains and are probably connected with the observed behavioural modifications.
The aim of the present study was to verify some behavioural responses of isolated rats to dopaminergic (DA) autoreceptorial doses of DA agonists known to induce, in a dose-related manner, various modifications in the behaviour of rodents. The behavioural effects are: (1) hypomotifity, penile erections (PE) and stretching and yawning (SY), all attributable to stimulation of DA D2 receptors and/or presynaptic receptors (autoreceptors) which cause a reduction in firing in DA cells and inhibition of DA release; (2) stereotypies and hypermotility involving DA postsynaptic receptors.
In our experiments the drugs used as pharmacological tools for the investigation of PE and SY in isolated rats were n-N-propylnorapomorphine (NPA) which is typically active at low doses at DA-autoreceptors and B-HT 920, an azepine derivative regarded as a relatively selective agonist at DA-autoreceptors. [...]
Discussion :
Disruption in the normal behaviour of rodents after isolation has been widely documented as has their altered sensitivity to the action of various pharmacological drugs. Our present findings indicate that isolation, though neither extreme (olfactory and auditory communication was maintained with other animals in the same room) nor lengthy, is able to modify some behavioural responses to autoreceptorial doses of DA-agonists. While a reduction in PE could be attributed to mechanisms different from those specific to DA-systems, more intriguing is the simultaneous reduction in SY.
Impaired spontaneous sexual behaviour has already been reported after isolation, and modified sex steroids have been detected in male and female isolated rats. As Brain and Benton point out, hormones may alter regional concentrations of CNS neurotransmitters as well as the activities of enzymes involved in their metabolism. Also, evidence of changes in catecholaminergic pathways hasbeen described by several authors.
The hypothesis that reduced B-HT 920- and NPA-induced PE and SY in rats might be ascribed to some modification in DA-systems; is supported by the reports of increased stereotypies induced by amphetamine and amphetamine-like drugs and enhanced tailpinch-induced oral behaviour in isolated animals. In fact, it has been shown that both behaviours are linked to DA system and since, as previously mentioned, PE, SY and stereotypies have been proposed as signs of activation of different DA receptor subtypes, their modification might be interpreted as being due to altered sensitivity of the respective receptors. This hypothesis is further supported by the studies of Wilmot et al. who report changes in the motor activity response to low doses of apomorphine, suggestive of reduced sensitivity of DA autoreceptors. However, since the effect of any drug is dependent not only on the specific site of action but also on other factors, such as metabolism, transport and disposition, the possibility that isolation may affect these activities as well cannot be excluded.
The above findings have broader implications: since DA is one of the neurotransmitters mainly involved in the control and modulation of sexual behaviour, at least in rats (all DA-agonists within a certain dosage range not only stimulate PE in rats but also have a potent effect on the mating of males and lisuride, which acts on DA receptors, elicits homosexual mounting), the question arises as to whether expressions of sexual behaviour other than PE are similarly influenced by differential housing. If studies currently being undertaken in our Institute prove this to be the case, then isolation and behavioural signs viewed together may afford valuable insight into the neurochémical mechanisms underlying certain forms of animal behaviour and the effect of drugs.