A further characterization of alpha-2 adrenoceptor involvement in the rough-and-tumble play of juvenile rats.

In order to further understand the extent to which alpha-2 adrenoceptors are involved in play behavior, a more detailed behavioral assessment was made of the effects of the alpha-2 agonist clonidine and the alpha-2 antagonist RX821002 on the rough-and-tumble play of juvenile rats. Clonidine reduced overall levels of playfulness, as indicated by fewer attacks directed to the nape, a lower probability of rotating completely to supine in response to nape attacks and fewer pins. In contrast, RX821002 increased playfulness, as indicated by more attacks directed to the nape, a higher probability of rotating completely to supine in response to nape attacks and more pins. Testing rats in an unfamiliar environment did not affect the ability of RX821002 to increase play. Abruptly turning on a bright light during an ongoing play session also did not affect the ability of RX821002 to increase play or the ability of clonidine to reduce play. These data suggest that alpha-2 noradrenergic compounds do not affect play by altering levels of distractibility. However, the extent to which RX821002 and clonidine influenced overall activity during a play bout was compromised by turning on a bright light. While this pattern of results is consistent with a presynaptic site of action for alpha-2 involvement in modulating overall activity during a play bout, the same cannot be concluded about any putative explanatory mechanism for alpha-2 involvement in play behavior

Rough-and-tumble play behavior in Fischer-344 and buffalo rats: effects of social isolation.

Play behavior was assessed in two inbred strains of rats. Rats of the Fischer-344 strain (F344) were found to be less playful than Buffalo rats after social isolation, as evident from fewer pins and fewer playful attacks to the nape. When tested in same-strain pairings, overall defense of the nape did not differ between strains, although there were strain differences in the specific pattern of defense. When tested in cross-strain pairings, F344 rats were less likely to direct nape attacks toward a Buffalo play partner, and were also less likely to defend their nape when attacked by Buffalo rats. Although different levels of pinning and nape attacks in the two strains were dependent on the amount of isolation prior to the play period, differences in nape defense were not dependent on prior isolation. This pattern of results suggests that the neural mechanisms for playful attack differ from those underlying playful defense. These data also suggest that the F344 strain could be useful in better understanding the neural and genetic bases of mammalian playfulness.

D2 dopamine receptor involvement in the rough-and-tumble play behavior of juvenile rats.

By using dorsal contacts and pinning to quantify play behavior in juvenile rats, it was found that the D2 agonist, quinpirole, reduced both measures of play at doses greater than 0.05 mg/kg. Eticlopride, a D2 antagonist, also reduced both measures of play and blocked the reduction caused by quinpirole. The effect of quinpirole on play was largely unaffected by concurrent administration of either a D1 agonist (SKF 38393) or a D1 antagonist (SCH 23390), suggesting that D1 and D2 receptors are functionally independent with respect to play behavior. Quinpirole also reduced overall activity, suggesting that the effects on play may not be selective to neural circuitry responsible for play behavior. Although low doses of quinpirole (0.001--0.03 mg/kg) had a tendency to increase pinning, this effect was not very robust. These data suggest that D2 dopamine receptors may not have a major role in the control of play behavior in juvenile rats.

Effects of MK-801 on rough-and-tumble play in juvenile rats.

Effects of the noncompetitive N-methyl-D-aspartate (NMDA) channel blocker, MK-801, on play behavior and activity were assessed in juvenile rats. A low dose (0.025 mg/kg) of MK-801 significantly increased both pinning and dorsal contacts, two indices of play behavior in the rat. Higher doses (> or = 0.1 mg/kg) significantly reduced both measures of play, with play being virtually abolished at 0.2 mg/kg. Simultaneous measurements of activity indicated that the reductions of play at the higher doses were accompanied by significant increases in horizontal activity. There was also a dose-related reduction in rearing, which was significant at all doses. When juvenile rats were tested individually for activity in an open field, MK-801 increased horizontal activity at 0.2 mg/kg and decreased rearing at all doses. So while the effects of MK-801 on play are biphasic, the effects of this compound on locomotor activity, per se, are linear. These data suggest that the effects of NMDA channel blockade on overall activity and play are mediated through independent neural systems.

Effects of neonatal decortication on the social play of juvenile rats.

The effects of radical neonatal decortication on the social play of juvenile rats, as well as the effects of neonatal ablation of frontal or parietal cortex, were examined in this series of experiments. When total decorticates were tested in like-lesioned pairs, the frequency of pinning was reduced by about 50% and their average pin durations were shorter. Nevertheless, the play of decorticates appeared essentially normal in general appearance, and did not differ from controls in a measure of overall play vigor using an electronic activity platform. Further, there were no differences in pin frequencies when controls and decorticates were paired together in cross-lesion testing. Separate tests of play solicitation behaviors did not detect any differences between controls and decorticates suggesting that play motivation was essentially intact after decortication. No deficits in pinning resulted from frontal ablations; however, pin durations were shorter in like-lesion testing. In cross-lesion testing, there was an increase in dorsal contacts and a trend toward shortening of pin durations. Parietal aspirations resulted in a 65% reduction in pin frequency, without substantially altering dorsal contacts. Anesthetization of the anterior surface of the animal's back with xylocaine reduced pinning in controls but eliminated pinning in parietals. Although the results generally indicate little participation of the neocortex in the instigation of rough-and-tumble play, the reliable numerical changes that were observed may be explained by apparent motor changes as well as reduced somatosensory sensitivity.

Effects of alpha-2 adrenoceptor antagonists on rough-and-tumble play in juvenile rats: evidence for a site of action independent of non-adrenoceptor imidazoline binding sites.

The pharmacological specificity of alpha-2 adrenoceptor involvement in the modulation of rough-and-tumble play behavior was assessed in juvenile rats. The alpha-2 adrenoceptor antagonists idazoxan and RX821002 both increased the frequency of pinning in individually housed rats that were given a brief opportunity to play. Dorsal contacts, a measure of play solicitation, were not consistently affected by these compounds. Since RX821002 shows little affinity for non-adrenoceptor imidazoline binding sites, it is likely that the facilitation of play following administration of these two compounds is due to blockade of alpha-2 receptors. The effect of RX821002 and idazoxan is unlikely to be an artifact associated with using rats that are reared in isolation, as RX821002 also increased pinning, as well as dorsal contacts, in group-housed rats that were isolated for a short period (4h) before the play session. The alpha-1 adrenoceptor antagonist prazosin, which also binds to alpha-2B receptors, reduced the frequency of both pinning and dorsal contacts. There was a strong trend for St 587, a centrally active alpha-1 agonist, to attenuate the effect of prazosin on play. While this leaves open the possibility that prazosin may be reducing play through alpha-1 blockade, antagonist activity at alpha-2B receptors cannot be ruled out. From these data, we conclude that the facilitation of play following idazoxan and RX821002 is likely due to blockade of alpha-2A adrenoceptors. These findings add further support for a specific role of alpha-adrenoceptors in the modulation of playfulness in the juvenile rat.

Evidence for enhanced synaptic excitation in transplanted neostriatal neurons.

Fetal neostriatal tissue was transplanted into either the neostriatum or substantia nigra of adult rats. One to 6 months after transplantation, coronal brain slices were taken through the rostrocaudal extent of the transplants and neurons were characterized electrophysiologically using an in vitro slice preparation. When compared to control neurons taken from intact adult neostriata, transplanted neostriatal neurons (TSNs) had higher input resistances and longer time constants. All other passive and active membrane properties assessed were comparable between transplanted and control neostriatal neurons. Regardless of the transplantation site, local extracellular stimulation outside the graft elicited high-amplitude, long-duration depolarizing synaptic potentials that typically triggered bursts of action potentials. These synaptic potentials contrast with lower amplitude, shorter duration synaptic potentials consistently elicited in control neostriatal neurons. The depolarizing synaptic potentials evoked in the TSNs appeared to be mediated by a combined activation of N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptors. Both the broad-spectrum excitatory amino acid antagonist kynurenic acid and the specific non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione significantly reduced postsynaptic potentials elicited in TSNs. The specific NMDA antagonist 2-amino-5-phosphonovalerate had less effect on the amplitude but markedly reduced the duration of the synaptic potentials. The duration and amplitude of the bursts were augmented by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline methiodide, indicating that inhibition occurred in TSNs. TSNs were also more sensitive than control neurons to direct application of glutamate or NMDA. These findings demonstrate that TSNs express altered electrophysiological properties. The pharmacological analysis indicates that depolarizing postsynaptic potentials were mediated by activation of excitatory amino acid receptors, suggesting either innervation of the graft by host fibers which contain excitatory amino acids or development of novel local excitatory interactions intrinsic to the graft. Furthermore, the occurrence of high-amplitude, long-duration depolarizing synaptic potentials in TSNs, regardless of the site of transplantation, suggests that grafted neostriatal neurons become hyperexcitable to synaptic input.

The energetic costs of rough-and-tumble play in the juvenile rat.

The metabolic costs of rough-and-tumble play behavior were studied in juvenile rats. Using indirect calorimetry, it was determined that energy expenditure during play is increased by 66-104% over the resting metabolic rate, indicating that play accounts for between 2% and 3% of the total daily energy budget of the rat. In a subsequent experiment, food intake and body weight were monitored for 3 weeks in rats allowed to play for 1 hr/day and in rats not allowed to play. While the body weights of the two groups did not differ significantly from each other, those rats allowed to play ate 7% more over the 3-week period than did those rats not given an opportunity to play. These data are consistent with previous reports describing the energetic costs of mammalian play, with play accounting for less than 10% of the daily energy budget in three species tested so far. These data are also consistent with viewing play as a type of exercise and may lead to a better understanding of putative benefits of this behavior.

Enhanced responses to NMDA receptor activation in the developing cat caudate nucleus.

An in vitro slice preparation was used to assess the effects of N-methyl-D-aspartate (NMDA) receptor activation in the developing cat caudate nucleus. Removal of Mg2+ from the bathing medium, in the presence of 10 microM bicuculline, increased the amplitude and duration of the excitatory postsynaptic potential induced by local extracellular stimulation at all ages tested. In neurons younger than 35 days of age, removal of Mg2+ in the presence of bicuculline produced an increase in excitatory postsynaptic potential amplitude and duration as well as bursts of action potentials when local extracellular stimulation was applied. The effects of Mg2+ removal were reversibly attenuated by the specific NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid. These findings are important because they demonstrate that NMDA receptor-mediated responses can be induced in developing caudate neurons by local extracellular stimulation and these responses are enhanced in early postnatal periods at ages when motor control is being established.

Idazoxan increases rough-and-tumble play, activity and exploration in juvenile rats.

The effects of idazoxan, an alpha-2 noradrenergic antagonist, on play and open field behavior were assessed in juvenile rats. Play was assessed in two separate paradigms. Initially, juvenile rats were housed individually and given a daily 5 min opportunity to play with a responsive partner. Idazoxan (1-8 mg/kg) increased pinning, an indicator variable of play, but did not affect the frequency of dorsal contacts, an index of play solicitation. When rats were tested in a separate test for play solicitation using an unresponsive play partner, idazoxan increased all three measures of play solicitation. Idazoxan increased activity and exploration when rats were tested in an open field, suggesting that the effects of idazoxan on play may be due to an increase in behavioral arousal and/or attention. These data are consistent with a modulatory role for norepinephrine in the control of behavior.

Influence of different concentrations of ethanol on energy expenditure, substrate utilisation, and activity in the rat.

The acute (one hour) effects of intraperitoneal injections of four concentrations (10%, 30%, 45% and 60%) of a single dose (0.5 g/kg) of ethanol were investigated in unanesthetised rats in an open-circuit calorimeter. Ethanol increased energy expenditure, with the greatest effect being produced by the two lowest concentrations. In contrast, ethanol decreased respiratory quotient, with the greatest effect being produced by the two highest concentrations. The decreased respiratory quotients indicate that ethanol promotes an exclusive reliance on lipids as a source of energy, and further causes lipids to be catabolised for the synthesis of glucose. The peak metabolic effects were produced at a dose that did not significantly affect motor activity, which indicates that the metabolic effects are not secondary to changes in activity. These data support the view that ethanol's effects on energy expenditure and substrate utilisation are mediated by distinct mechanisms. Moreover, since the different metabolic effects were produced by the same ethanol dose, they cannot be due to ethanol's energy content. Thus, ethanol concentration is a major modulator of its effects on energy expenditure and substrate utilisation quite apart from effects due to dose, motor activity or its energy content. This suggests the need to consider the effects of ethanol concentration when analysing ethanol's other pharmacological effects.

Effects of norepinephrine infused in the paraventricular hypothalamus on energy expenditure in the rat.

The metabolic effects of norepinephrine (NE), when infused into the paraventricular nucleus of the hypothalamus (PVN), were examined using indirect calorimetry. In two separate experiments, it was found that NE infused into the PVN reduced energy expenditure in freely moving rats. While NE also reduced motor activity, these reductions were not statistically significant. Reductions in voluntary motor activity were not necessary for a reduction in energy expenditure, as NE still reduced energy expenditure in rats that were lightly sedated. Clonidine, but not L-phenylephrine, mimicked the hypometabolic effect of NE, suggesting an action at alpha 2 receptors. Infusions of NE were also found to increase blood glucose shortly after infusion, although the specificity of this effect is questionable. Taken together, these data suggest that activation of noradrenergic neurons within the PVN results in a metabolic shift towards energy conservation.

Effects of ethanol and tertiary butanol on blood glucose levels and body temperature of rats.

The mechanisms of ethanol's hyperglycemic and hypothermic effects were investigated by comparing the effects of ethanol with those of tertiary butanol. Tertiary butanol is an intoxicant like ethanol, but unlike ethanol it is only minimally metabolized. Consequently, tertiary butanol does not produce appreciable amounts of active metabolites or energy. Tertiary butanol exerts its neural effects primarily by directly altering the physico-chemical properties of nerve cell membranes. It was found that ethanol and tertiary butanol produce hyperglycemic and hypothermic effects whose magnitude and time course are nearly identical. These data suggest that the hyperglycemic and hypothermic effects of ethanol represent a primary physico-chemical effect on nerve cell membranes and are not secondary to its energy content or metabolites.

Effects of ethanol and tertiary-butanol on energy expenditure and substrate utilization in the rat.

The acute effects of ethanol and tertiary-butanol, an alcohol which is not metabolized via the alcohol dehydrogenase pathway, on whole body metabolism were studied using indirect calorimetry. Ethanol, but not t-butanol, increased energy expenditure in food-deprived rats. Both ethanol and t-butanol reduced respiratory quotient (RQ), an index of overall body energy substrate utilization. The lowered RQ indicates an increased dependence upon lipids as an energy source. Taken together, these data suggest that ethanol, probably within a narrow dose range, can enhance energy expenditure in the rat, either via a metabolite (e.g., acetaldehyde) or through a consequence of its oxidation. The increase in lipid mobilization seen after acute treatment with ethanol, on the other hand, appears to be independent of its oxidation.

Hypothalamic modulation of thermogenesis and energy substrate utilization.

The metabolic effects of electrical stimulation of the hypothalamus were investigated using indirect calorimetry. Stimulation of either the ventromedial hypothalamic nucleus (VMH) or the lateral hypothalamic area (LH) increased both respiratory quotient (RQ) and energy expenditure (EE) in 23 lightly anesthetized rats. The use of a muscle relaxant to reduce motor activity and a regression analysis on the residual activity showed that the metabolic changes were independent of motor activity following LH stimulation. The increased RQ indicates that stimulation increased the dependence on carbohydrates as an energy substrate. The increased EE indicates that the LH modulates EE by mediating thermogenesis. The interpretation of the metabolic changes in RQ and EE following VMH stimulation is complicated by the fact that there were significant relationships between residual activity and metabolic changes in the sedated rats with VMH electrodes. Together, these data suggest that the hypothalamus regulates body weight by controlling energy expenditure, as well as energy intake. At the same time, hypothalamic activity influences which substrate the rat uses for energy.

Sensory modulation of juvenile play in rats.

A series of experiments was conducted to determine the extent to which somatosensory stimulation is necessary for the elaboration of juvenile play in rats. Anesthetization of the dorsal body surface of juvenile rats with xylocaine reduced the frequency of pinning, an indicator variable for play, by 35% to 70%, while motivation to play, as measured by dorsal contacts, an index of play solicitation, remained largely intact. These data suggest that dorsal body surface anesthetization impairs the ability of juvenile rats to perceive and/or respond to playful gestures. When untreated animals were paired with xylocaine-treated animals, the xylocaine-treated animals consistently pinned the untreated pups more than vice versa, further suggesting that somatosensation may be involved in the establishment and/or maintenance of play dominance relations. A preliminary examination assessing potential involvement of other modalities in the play of rats was also conducted, with the data suggesting a possible role for audition in the play of this species.

Juvenile play in the rat: thalamic and brain stem involvement.

Discrete electrolytic lesions of the parafascicular area of the thalamus (PFA) or posterior thalamic area (TP) reduced the frequency of pinning, an indicator variable for rough-and-tumble play in juvenile rats, while largely sparing motivation to play. Similar sized lesions placed within the ventrobasal thalamic area (VBT) had minimal effects on both pinning and measures of play solicitation. Lesions placed within the ventrolateral aspect of the brain stem also markedly reduced pinning, while having no effect on indices of play motivation. Although tests designed to assess the extent to which the observed play deficits may be attributable to a loss of somatic acuity proved inconclusive, the overall pattern of results were not inconsistent with positing a role for these neural areas in relaying somatic stimuli relating to playful intent.

Dorsomedial diencephalic involvement in the juvenile play of rats.

Lesions of either the dorsomedial thalamus (DMT) or the parafascicular region of the thalamus (PFA) reduced rough-and-tumble social play in juvenile rats, as measured by frequency of pinning. Pinning was reduced by 33% in pups with DMT lesions and by 73% in pups with PFA lesions. Although PFA lesions had minimal effects on average pin durations, DMT pups had pins that were, on average, 105% longer than those of controls. Lesions of the PFA, but not DMT, also reduced play solicitation behaviors. Pups with PFA lesions were also insensitive to the play modulating effects of both naloxone and morphine. DMT pups were sensitive to the facilitatory effects of morphine, but naloxone was without effect in these animals. Control studies designed to evaluate general behavioral competence showed that the observed lesion effects were relatively specific to play.

Energy balance and play in juvenile rats.

These experiments systematically analyzed the relationship between energy balance and play in the juvenile rat. As expected, depriving pups of food for 24 hours or more resulted in a reliable reduction in levels of play, with a single meal being sufficient to return play to baseline levels. Consumption of saccharin did not reverse deprivation-induced reductions of play. It is proposed that the food-induced restoration of play in food-deprived pups is perhaps reflective of normal satiety and may be useful in screening putative "satiety agents." Naloxone, cholecystokinin and bombesin, all putative satiety agents, were tested for their ability to reverse deprivation-induced reduction of play. Only bombesin was marginally effective in increasing play of food deprived rats.

Endophinergic modulation of acceptability of putative reinforcers.

A series of experiments was conducted in order to gain additional insight into how endogenous opioids may modulate taste reactivity and, thus, hedonic processes. Using a wide range of saccharin concentrations ranging from mildly preferred to aversive, it was demonstrated that naloxone reduced preference for saccharin over water. This reduction was not dependent upon concentration of saccharin and resulted in a downward displacement of the preference/aversion curve. Naltrexone was shown to result in a greater decrease in intake in animals drinking a mildly aversive quinine solution, as compared to animals drinking tap water. In conclusion, endogenous opioids may serve to broaden the range of food-related stimuli which are avidly accepted, perhaps by inhibiting any aversive component associated with ingestion.