B.M. Spruijt

Place Learning and Hippocampal Synaptic Plasticity in Streptozotocin-Induced Diabetic Rats

Moderate impairment of learning and memory has been recognized as a complication of diabetes. The present study examined behavioral and electrophysiological measures of cerebral function in streptozotocin (STZ)-induced diabetic rats. Behavioral testing consisted of a spatial learning task in a water maze. Electrophysiological testing consisted of in vitro assessment of hippocampal long-term potentiation (LTP), an activity-dependent form of synaptic plasticity, which is believed to be related to the cellular mechanisms of learning and memory. Two experiments were performed: the first with severely hyperglycemic rats and the second with moderately hyperglycemic rats. Rats were tested in the water maze 11 weeks after induction of diabetes. Next, LTP was measured in vitro in trained animals. Both spatial learning and LTP expression in the CA1 field of the hippocampus were impaired in severely hyperglycemic rats as compared with nondiabetic controls. In contrast, spatial learning and hippocampal LTP were unaffected in moderately hyperglycemic rats. The association of alterations in hippocampal LTP with specific learning impairments has previously been reported in conditions other than diabetes. Our findings suggest that changes in LTP-like forms of synaptic plasticity in the hippocampus, and possibly in other cerebral structures, are involved in learning deficits in STZ-induced diabetes. The beneficial effect of moderate glycemic control on both place learning and hippocampal LTP supports the significance of the relation between these two parameters and indicates that the development of the observed deficits may be related to the level of glycemic control.

Water maze learning and hippocampal synaptic plasticity in streptozotocin diabetic rats: effects of insulin treatment

Streptozotocin-diabetic rats express deficits in water maze learning and hippocampal synaptic plasticity. The present study examined whether these deficits could be prevented and/or reversed with insulin treatment. In addition, the water maze learning deficit in diabetic rats was further characterized. Insulin treatment was commenced at the onset of diabetes in a prevention experiment, and 10 weeks after diabetes induction in a reversal experiment. After 10 weeks of treatment, insulin-treated diabetic rats, untreated diabetic rats and non-diabetic controls were tested in a spatial version of the Morris water maze. Next, hippocampal long-term potentiation (LTP) was measured in vitro. To further characterize the effects of diabetes on water maze learning, a separate group of rats was pre-trained in a non-spatial version of the maze, prior to exposure to the spatial version. Both water maze learning and hippocampal LTP were impaired in diabetic rats. Insulin treatment commenced at the onset of diabetes prevented these impairments. In the reversal experiment, insulin treatment failed to reverse established deficits in maze learning and restored LTP only partially. Non-spatial pre-training abolished the performance deficit of diabetic rats in the spatial version of the maze. It is concluded that insulin treatment may prevent but not reverse deficits in water maze learning and LTP in streptozotocin-diabetic rats. The pre-training experiment suggests that the performance deficit of diabetic rats in the spatial version of the water maze is related to difficulties in learning the procedures of the maze rather than to impairments of spatial learning.

Isolation during the play period in infancy decreases adult social interactions in rats

The effects of 1 or 2 weeks of social isolation immediately after weaning on social activity in adulthood were investigated in rats. In addition, it was studied whether these effects were influenced by social experiences of the cagemate when rehoused after the isolation period. Isolation during weeks 4 and 5 of age caused a reduction of social activity as compared to non-isolated controls. Previous social experiences of the cagemate (isolated or non-isolated) did not affect this decreased social activity. Isolation during week 4 of age resulted in similar effects, but the reduced social activity was not present when the rats were rehoused with non-isolated rats. Isolation during week 5 of age did not influence social activity patterns in adulthood. These findings support the idea of a sensitive period in infancy for subsequent social behavior in rats. It is suggested that especially deprivation of acquiring play behavior underlies the social disturbances in adulthood.

Influence of environmental factors on social play behavior of juvenile rats

The effects of light level and familiarity to the testing environment on social behaviors related and unrelated to play were investigated in juvenile rats accustomed to dim light conditions. Pinning, a measure characteristic for social play in rats, was completely suppressed under intense light conditions. Following/chasing and boxing/wrestling, social behaviors related to play, were also decreased under intense light. Of the measures of social behavior not related to play, contact behavior was decreased under intense light whereas social exploration was hardly affected. Levels of social exploration and following/chasing gradually declined during the 15-min test period. Frequency of contact behavior decreased, whereas duration increased with time. Under dim light conditions, unfamiliarity to the test cage suppressed pinning and boxing/wrestling but not the other social behaviors in the first part of the test period. These findings show that social behavior in juvenile rats, as in adult rats, can be influenced by light level and familiarity to the test cage. Social behaviors related and unrelated to play seem to be differentially influenced by environmental stimuli.

Effects of morphine on different aspects of social play in juvenile rats

To clarify the influence of opioids on social play, the effects of morphine on playful and non-playful social behavior in juvenile rats was investigated under different conditions. Environmental variables employed were different (dim and intense) levels of illumination during testing, familiarity to the test cage, and different periods of social isolation prior to testing. Under dim light conditions, morphine markedly increased playful social behavior, such as pinning, boxing/wrestling and following/chasing, whereas non-playful social behavior such as social exploration and contact behavior was hardly affected. This effect of morphine was independent of duration of previous isolation and dose-dependent, with a maximal effect at 1.0 mg/kg. The mechanism of this effect is interpreted as an action on the rewarding aspects of play. A dose of 0.1 mg/kg of morphine abolished the initial suppression of play induced by unfamiliarity to the test cage, without influencing total levels of play. This may be an effect of morphine on the integration of sensory stimuli. Under intense light conditions, where playful behavior was completely suppressed, morphine itself hardly affected such behavior, but decreased some aspects of non-playful social behavior. These results suggest that in juvenile rats playful and non-playful forms of social behavior are differentially regulated. In addition, opioid systems may be involved at different levels in the regulation of social play.

Increased GABAergic input to ventral tegmental area dopaminergic neurons associated with decreased cocaine reinforcement in μ-opioid receptor knockout mice

There is general agreement that dopaminergic neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens and prefrontal cortex play a key role in drug reinforcement. The activity of these neurons is strongly modulated by the inhibitory and excitatory input they receive. Activation of mu-opioid receptors, located on GABAergic neurons in the VTA, causes hyperpolarization of these GABAergic neurons, thereby causing a disinhibition of VTA dopaminergic neurons. This effect of mu-opioid receptors upon GABA neurotransmission is a likely mechanism for mu-opioid receptor modulation of drug reinforcement. We studied mu-opioid receptor signaling in relation to cocaine reinforcement in wild-type and mu-opioid receptor knockout mice using a cocaine self-administration paradigm and in vitro electrophysiology. Cocaine self-administration was reduced in mu-opioid receptor knockout mice, suggesting a critical role of mu-opioid receptors in cocaine reinforcement. The frequency of spontaneous inhibitory post-synaptic currents onto dopaminergic neurons in the ventral tegmental area was increased in mu-opioid receptor knockout mice compared with wild-type controls, while the frequency of spontaneous excitatory post-synaptic currents was unaltered. The reduced cocaine self-administration and increased GABAergic input to VTA dopaminergic neurons in mu-opioid receptor knockout mice supports the notion that suppression of GABAergic input onto dopaminergic neurons in the VTA contributes to mu-opioid receptor modulation of cocaine reinforcement.

Olfactory bulbectomy temporarily impairs Morris maze performance: An ACTH(4–9) analog accellerates return of function

Rats with a bilateral lesion of the olfactory bulb are permanently anosmic. However, this lesion also produces nonspecific behavioral effects that recover over time. In this study olfactory bulb-lesioned animals are given a spatial orientation task--the Morris maze--which supposedly relies on visual and not olfactory cues. In exp. 1 this assumption was verified by subjecting animals with peripherally induced anosmia to the Morris maze (olfactory neurons in the nasal mucosae were destroyed by flushing the nose with ZnSO4). Anosmia did not affect the acquisition rate of the animals. In exp. 2 anosmia was produced by a central lesion to the bulbus olfactorius. Two weeks after lesioning the Morris maze performance is severely impaired. Interestingly, chronic administration (10 micrograms/48 h/rat, during these 14 days, SC) of the ACTH(4-9) analog ORG 2766 diminished the impairment in performance. In exp. 3 olfactory bulb-lesioned animals were allowed 6 wk to recover before Morris maze testing began, to investigate if spontaneous recovery of performance occurred. No difference was seen in the acquisition performance of lesioned animals when compared to sham animals at this timepoint. The effect of the peptide is discussed in the context of an acceleration of the recovery of nonspecific consequences of brain lesioning.

Prenatal exposure to morphine affects juvenile play behavior and adult social behavior in rats

The effects of morphine exposure in utero on play behavior and social behavior were investigated in a longitudinal study. Wistar rat dams were SC injected daily with saline (control) or 10 mg/kg morphine from day 8 to day 21 of gestation. Play behavior of the offspring was measured at 3 and 4 weeks of age and social behavior at 3 months of age. Pinning, a measure for play behavior and social grooming of the morphine-treated offspring were significantly elevated compared to saline controls, especially on day 21. The onset-latency of pinning behavior was not changed. Furthermore, prenatal morphine treatment resulted in more social approach and less social avoidance behavior in adulthood, whereas changes in general locomotor activity were not observed. The results are discussed in relation to the effects of in utero exposure of morphine on the development of incentive aspects of play and social behavior.

Morphine treatment during juvenile isolation increases social activity and opioid peptides release in the adult rat

The consequences of juvenile isolation and morphine treatment on general activity, social activity and endogenous opioid release during a social interaction test were investigated in the adult rat. Rats were either isolated or socially housed during weeks 4 and 5 of age and treated daily during this isolation period subcutaneously with either saline or morphine. Directly after a social interaction test at 10 weeks of age, rats were injected with [3H]-diprenorphine and subsequently prepared for in vivo autoradiography. The autoradiographic technique was used to visualise neuroanatomical changes in opioid receptor occupancy, probably reflecting changes in opioid peptide release, as a result of social activity. Juvenile isolation increased general activity during the social interaction test, an effect which was accompanied by a reduction of opioid receptor occupancy in many brain areas, suggesting an increased opioid peptide release as a consequence of socially-induced general activity. Morphine treatment in isolated rats caused an increase in adult social activity and enhanced opioid peptide release in some cortical regions and the ventral tegmental area as compared to saline treated rats. Both social activity and opioid receptor occupancy were unaffected by morphine treatment in non-isolated rats. The present study underscores the role of opioid systems in adult social behaviors as a consequence of juvenile isolation. The results suggest a relationship between social activity and opioid peptide release during social contact. Increased social activity seems to be accompanied by elevated opioid peptide release in distinct brain areas after morphine treatment during juvenile isolation.

Prolonged animal observation by use of digitized videodisplays

Quantitative assessment of animal behavior requires not only much time and good training, but is always dependent upon the interpretation of the observer. Therefore we employed a commonly-used microcomputer to analyze, without intervention of a human observer, the picture obtained via videocamera, a digitizer and disk-drive. The program, written in BASIC and listed in Appendix I, generates pictures of the monitored animal in the graphics of the computer. These pictures can be analyzed immediately or stored on disk for further analysis.