Sex-linked behavioural differences in mice expressing a human insulin transgene in the medial habenula

Phenotypic studies on dopamine receptor subtype and associated signal transduction mutants: insights and challenges from 10 years at the psychopharmacology-molecular biology interface

BACKGROUND

Mutants with targeted gene deletion ('knockout') or insertion (transgenic) of D1, D2, D3, D4 and D5 dopamine (DA) receptor subtypes are complemented by an increasing variety of double knockout and transgenic-'knockout' models, together with knockout of critical components of DA receptor signalling cascades such as G alpha(olf)[G gamma7], adenylyl cyclase type 5, PKA [RIIbeta] and DARPP-32. However, it is increasingly recognised that these molecular techniques have a number of inherent limitations. Furthermore, there are poorly understood methodological factors that contribute to inconsistent phenotypic findings between laboratories.

OBJECTIVE

This review seeks to document the impact of DA receptor subtype and related transduction mutants on our understanding of the behavioural roles of these entities, primarily at the level of unconditioned psychomotor behaviour.

METHODS

It includes ethologically based and orofacial movement studies in our own laboratories, since these are the only studies to systematically compare each of the D1, D2, D3, D4 and D5 receptor and DARPP-32 signal transduction 'knockouts'.

DISCUSSION

There is a particular emphasis on identifying methodological factors that might influence phenotypic effects and account for inconsistencies. The findings are offered empirically to (1) specify the extent of phenotypic diversity among individual DA receptor subtypes and transduction components and (2) indicate relationships between D1, D2, D3, D4 and D5 receptor subtype proteins, associated G alpha(i)/G alpha(s)/G alpha(olf)[G gamma7]-adenylyl cyclase type 5-PKA [RIIbeta]-DARPP-32 signalling cascades and behaviour. The findings are also offered heuristically as a base for such phenotypic comparisons at additional levels of behaviour so that a yet more complete phenotypic profile might emerge.

Topographical Assessment of Ethological and Dopamine Receptor Agonist-Induced Behavioral Phenotype in Mutants with Congenic DARPP-32 'Knockout'

Congenic (10 backcrosses into C57BL/6J) mutants with targeted gene deletion of DARPP-32, a neuronal phosphoprotein regarded as an essential mediator of the biological effects of dopamine (DA), were assessed phenotypically using an ethologically based approach that resolves all topographies of behavior in the mouse repertoire. Over initial exploration, female, but not male, DARPP-32 mutants evidenced increased locomotion and decreased grooming, while a decrease in rearing seated was evident in mutants of both genders; continuing assessment over several hours did not reveal additional phenotypic effects. Following challenge with the nonselective DA receptor agonist apomorphine, low doses were associated with reduced levels of sniffing, grooming, total rearing, and rearing seated in DARPP-32 mutants relative to wildtypes; this would suggest some role for DARPP-32 in mediating the biological effects of presynaptic D(2)-like autoreceptor or inhibitory postsynaptic D(2)-like receptor activation. Following challenge with higher doses, while stereotyped sniffing and locomotion with chewing was largely unaltered, the additional murine response of Straub tail was essentially abolished in DARPP-32 mutants, indicating some specific involvement of DARPP-32 in mediating this topography of behavior; additionally, there were overall reductions in levels of sniffing, total rearing, rearing seated, and grooming in DARPP-32 mutants that were unrelated to the dose of apomorphine administered, indicating broader topographical effects following the stress of the injection procedure relative to more naturalistic conditions. The developmental absence of DARPP-32 following targeted gene deletion appears to be associated with compensatory processes that maintain certain topographies of spontaneous and agonist-induced DAergic function, while other topographies remain impaired.

Mouse strains differ under a simple schedule of operant learning

Recent advances in understanding the composition of the human and mouse genomes have paved the way to a more detailed understanding of the influence of genes on behavior, particularly learning and memory. One problem with many learning paradigms is the great length of training time required to generate a stable baseline. Our goal for the current studies was to develop a method of rapidly assessing learning and to use it to compare various strains of mice. The acquisition of a simple nose-poke was determined in operant chambers with two nose-poke holes illuminated: a single nose poke in one hole resulted in the presentation of 0.01 ml evaporated milk; responses in the other hole did not result in dipper presentation. All mice of the B6JxImJ F1, C57BL/6J, 129X1/SvJ and C3H/HeJ mice emitted 50 or more correct operant responses, whereas fewer than 50% of 129X1/SvJ and 75-90% of mice of Balb/cByJ, DBA/2J and the outbred CD-1 mice emitted 50 or more correct operant responses. On average, C57BL/6J emitted 50 operative responses in less than 30 min, whereas DBA/2J mice required nearly 1 h to complete 50 operative responses. Other strains performed at intermediate levels. There was no apparent relationship between operant activity and locomotor activity that may have influenced response acquisition. These findings are consistent with those reported using other learning paradigms and provide a rapid method of learning assessment.

Sex specific behavioural alterations in Mas-deficient mice

Male mice lacking the Mas protooncogene have been shown to exhibit an increased anxiety in the Elevated Plus Maze Task and sustained long-term potentiation in the hippocampus without effect on spatial learning in the Morris Water Maze Task. Here, we report behavioural studies in female mice lacking the Mas protooncogene. As for the males, we analysed the learning and anxiety behaviour using both behavioural tasks. With the exception of a trend to a better performance in the Morris Water Maze no differences were found in both tests between control and Mas-deficient females. This implicates that the lack of Mas protein influences spatial learning and anxiety in a sex-specific manner.

Human insulin gene insertion in mice. Effects on the sleep-wake cycle?

Recently, insulin synthesis and the presence of an insulin receptor have been demonstrated in the brain. Intracerebroventricular infusion of insulin causes a selective increase in the amount of slow-wave sleep. In the present study, the sleep-wake cycle of transgenic mice, with or without habenular neuronal expression of the human insulin gene, was studied to investigate the possible role of brain insulin as a sleep modulator. Slow-wave sleep duration was increased in those mice expressing human insulin in the habenula. However, it is possible that this effect was not due to expression of the insulin transgene, but to the genetic background of one of the parental strains (CBA) used for insertion of the transgene. Users of transgenic mice should be aware of this possibility and be cautious in interpreting results when hybrid embryos are used as transgene recipients.