Serotonergic modulation of 3,4-methylenedioxymethamphetamine (MDMA)-elicited reduction of response rate but not rewarding threshold in accumbal self-stimulation.

In a fixed interval 5-s rate-frequency function paradigm with rats, 3,4-methylenedioxymethamphetamine (MDMA; 0.5, 2 and 4 mg/kg) dose-dependently decreased response rate for nucleus accumbens self-stimulation while both D-amphetamine (0.3 and 1 mg/kg) and cocaine (5 and 15 mg /kg) increased response rates. The highest doses of MDMA caused a cessation of responding in many of the rats tested, but in those rats that continued to respond a significant reduction in frequency threshold for self-stimulation was seen. Cocaine and amphetamine dose-dependently reduced frequency threshold in all rats tested. The non-specific serotonin antagonist, methysergide (5 mg/kg), reversed the inhibitory effects of MDMA on response rates and caused all rats to respond following MDMA (4 mg/kg). Methysergide did not affect MDMA's threshold-lowering properties and when administered alone methysergide had not effect on self-stimulation. These results suggest serotonergic involvement in the performance but not reinforcement-modulating effect of MDMA in the self-stimulation paradigm.

Reward summation and the effects of dopamine D1 and D2 agonists and antagonists on fixed-interval responding for brain stimulation.

The effects of dopamine D1 and D2 agonists and antagonists on fixed-interval (FI) self-stimulation were investigated using a reward-summation model, trading off frequency with train duration. The D1 antagonist SCH 23390 (0.005-0.02 mg/kg) decreased FI self-stimulation and the inhibition was reversed by increasing stimulation frequency. Moreover, amphetamine (0.5 mg/kg) reversed the inhibition by a low dose of SCH 23390 (0.005 mg/kg) but not after a higher dose inhibition could not be dissociated from a performance deficit. There was no significant interaction between low doses of spiperone and SCH 23390 when coadministered that could not be predicted from their effects when given individually. Self-stimulation was inhibited by the D1 agonist SKF 38393 (5 mg/kg). When coadministered with amphetamine, SKF 38393 partially blocked amphetamine's facilitation. The D2 agonist bromocriptine (10 mg/kg) produced an extraordinary enhancement of performance that was also evident after a lower dose (5 mg/kg) when it was combined with amphetamine. This enhancement of performance showed little extinction when stimulation was no longer available, suggesting it was a novel form of stereotypy. These results support the concept that D1 dopamine receptors play a critical role in modulating the reinforcing consequences of lateral hypothalamic stimulation. The involvement of D2 receptors on reinforcement processes remains contentious due to their effects on performance and insensitivity of responding to coincide with changes in reinforcement magnitude.

The questionable wisdom of a low-fat diet and cholesterol reduction.

The prevalent wisdom that a low-fat diet and cholesterol reduction are essential to good cardiovascular health is coming under increased scrutiny. An examination of the foundations of this view suggests that in many respects it was ill-conceived from the outset and, with the accumulation of new evidence, it is becoming progressively less tenable. Cross-sectional, longitudinal and cross-cultural investigations have variously suggested that the relationship between dietary fat intake and death from heart disease is positive, negative and random. These data are incompatible with the view that dietary fat intake has any causal role in cardiovascular health. Although hypercholesterolemia is associated with increased liability to death from heart disease, it is as frequently associated with increased overall life expectancy as with decreased life expectancy. These findings are incompatible with labelling hypercholesterolemia an overall health hazard. Moreover, it is questionable if the cardiovascular liability associated with hypercholesterolemia is either causal or reversible. The complex relationships between diet, serum cholesterol, atherosclerosis and mortality and their interactions with genetic and environmental factors suggest that the effects of simple dietary prescriptions are unlikely to be predictable, let alone beneficial. These cautions are borne out by numerous studies which have shown that multifactorial primary intervention to lower cholesterol levels is as likely to increase death from cardiovascular causes as to decrease it. Importantly, the only significant overall effect of cholesterol-lowering intervention that has ever been shown is increased mortality. The stress and helplessness associated with misapprehensions as to the dangers of dietary fat and the asceticism inherent in the war on cholesterol have considerable implications for health practices. Recent research in behavioral immunology suggests that stress and helplessness are likely to compromise immunity and promote ill-health.

Contrasting effects of dopaminergic blockade on MDMA and d-amphetamine conditioned taste aversions.

A series of experiments examined the role of dopamine in the conditioned taste aversion (CTA) produced by 3,4-methylenedioxymethamphetamine (MDMA) and d-amphetamine in rats. The CTA induced by MDMA (1.0 mg/kg) was unaffected by the D1 dopamine receptor antagonist SCH23390 (0.3 or 0.6 mg/kg), the D2 receptor antagonist raclopride (0.3 or 0.6 mg/kg), SCH23390 and raclopride combined (both 0.3 or 0.6 mg/kg), or the D1/D2 receptor antagonist haloperidol (0.4 mg/kg). In contrast, the CTA produced by d-amphetamine (0.5 mg/kg) was attenuated by SCH23390 and raclopride combined (both 0.3 mg/kg) as well as haloperidol (0.4 mg/kg), but not by SCH23390 (0.3 or 0.6 mg/kg) or raclopride (0.3 or 0.6 mg/kg) alone. These results suggest that dopamine plays different roles in MDMA and amphetamine CTAs, and that the D1 and D2 receptors independently mediate the aversive effect of amphetamine in CTA.

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.

Glucagon and the paraventricular hypothalamus: modulation of energy balance.

The effects of glucagon injections (25, 100 and 200 ng) into the paraventricular nucleus of the hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. High doses of glucagon (100 and 200 ng) produced small and short-lasting increases in energy expenditure. The independence of these changes from changes in locomotor activity suggests that the thermogenesis represents a primary modulation and is not secondary to increased locomotion. All three doses of glucagon produced long-lasting and dose-related increases in respiratory quotient which were unrelated to any changes in locomotor activity. As with the changes in energy expenditure, this dissociation indicates that the effects are not secondary to changes in locomotor activity. These data constitute the first evidence that glucagon in the PVN modulates the metabolic parameters central to energy balance. In separate experiments, the three doses of glucagon increased blood glucose concentration over a one hour period, but they did not affect food and water intake and body weight over 24 h. These findings suggest that glucagon normally acts on the PVN in conditions of increased body fat to initiate autonomic mechanisms which increase glycemic levels, thermogenesis and carbohydrate utilization. These data constitute the first direct evidence for the involvement of the PVN in the regulation of energy balance by glucagon. The main effect of glucagon in the PVN is anabolic in that it increases dependence on carbohydrates as an energy substrate which results in a sparing of fat reserves.

Somatostatin and the paraventricular hypothalamus: modulation of energy balance.

The effects of somatostatin injections (0.1, 1 and 5 micrograms) into the paraventricular nucleus of the hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. The 0.1 and 1 microgram doses produced large and long-lasting decreases in respiratory quotient, which indicates the preferential utilization of fats as an energy substrate. The 5 micrograms dose produced a brief decrease in energy expenditure. Locomotor activity was not affected by any treatment which indicates that the effects on respiratory quotient and energy expenditure are not secondary to changes in activity. These findings demonstrate that somatostatin in the PVN inhibits thermogenesis and induces the preferential utilization of fats while sparing carbohydrate reserves. However, it is significant that the effects on energy expenditure and energy substrate utilisation occurred at different doses. These data constitute the first evidence that somatostatin in the PVN produces a primary modulation of the metabolic parameters central to energy balance. In separate experiments, all three doses of somatostatin increased blood glucose concentration over a one hour period, and the 5 micrograms dose decreased body weight over a 24 h period. Food and water intake were not affected by the somatostatin injections. Taken together, these findings are interpreted in a model in which somatostatin is a signal to the PVN of increased body fat. This mobilizes sympathetic mechanisms which increase fat utilization and blood glucose levels.

Metabolic modulation by amino acid stimulation of the paraventricular nucleus of the hypothalamus.

The role of the paraventricular nucleus of the hypothalamus (PVN) in the regulation of energy expenditure and energy substrate utilization was investigated after the injection of the excitatory amino acid D,L-homocysteic acid (DLH) or its vehicle. Male Wistar rats with chronic PVN cannulae were tested for 1 h with no food available in an open-circuit calorimeter. Whereas low (0.5 nmol), excitatory doses of DLH increased energy expenditure, the thermogenic effect became smaller and then vanished as the DLH dose was increased to inhibitory levels (7 and 50 nmol). None of these doses affected motor activity, indicating a primary thermogenic effect. The highest dose (100 nmol) increased energy expenditure, but this appeared to be secondary to increased locomotor activity. The increased locomotor activity produced by the highest dose of DLH constitutes the first demonstration of an activity effect induced by stimulating the PVN. However, this effect likely reflects the activation of neighboring areas. Only the 50 nmol dose of DLH increased respiratory quotient, indicating a shift toward the preferential utilization of carbohydrates as an energy substrate. These data complement our findings with neuropeptide Y and insulin in showing that different doses of the same substance injected into the PVN may produce qualitatively different effects. Furthermore, the present study demonstrates that exciting PVN neurons activates catabolic forces, whereas inhibiting them activates anabolic forces.

Comparison of conditioned taste aversions produced by MDMA and d-amphetamine.

Many drugs of abuse such as d-amphetamine support the development of taste aversion in a conditioned taste aversion paradigm. However, it has yet to be established whether methylenedioxymethamphetamine (MDMA), an amphetamine-like stimulant, has this property. A direct comparison was made between MDMA and d-amphetamine over a dose range of 0.125-2.0 mg/kg (SC). Two pairings of either drug with saccharin produced dose-related taste aversions to saccharin that were retained for at least three successive testing trials. The minimally effective dose was 1 mg/kg for MDMA and 0.5 mg/kg for d-amphetamine. The relative potency of MDMA to amphetamine was 4.5, similar to that previously reported for drug discrimination and self-stimulation.

Reward summation and the effects of pimozide, clonidine, and amphetamine on fixed-interval responding for brain stimulation.

Two models of reward summation were examined in 16 rats lever pressing for intracranial stimulation under fixed-interval (FI) reinforcement. The first model examined rate-frequency functions and the second model traded off frequency and train duration. The second model was selected to assess the effects of three drugs on reward summation. Both clonidine and pimozide inhibited FI self-stimulation, but pimozide's effect could not be distinguished from a performance deficit. Two amphetamine isomers facilitated self-stimulation in a manner suggesting enhanced reinforcement. The dextro isomer was four times more effective than the levo isomer to facilitate self-stimulation. This study shows that the combination of the FI schedule with a reward-summation model is well suited for evaluating the effects of drugs on self-stimulation. The advantages of this model are that inter-reinforcement intervals are separated, which minimizes priming and stimulation aftereffects, and more responding does not increase stimulation availability, thus eliminating rate-dependency effects.

Parametric manipulations and fixed-interval self-stimulation.

Three experiments investigated hypothalamic self-stimulation under a fixed-interval (FI) reinforcement schedule. An FI 20-s schedule was chosen to reduce stimulation density in order to minimize the influence of priming effects or stimulation aftereffects that can affect responding under other schedules of reinforcement. The first experiment showed that the influence of train duration is greatest at levels up to 1 s and thereafter level off over a wide range of train durations (1-32 s). The second experiment showed that altering frequency, current, or pulse width produced almost identical changes in FI responding. These findings show that the neutral network subserving hypothalamic self-stimulation simply integrates the amount of charge over time. It is relatively insensitive to the combination of stimulation parameters that make up a given waveform. In the third experiment, the chronaxies from the strength-duration curves indicate the neural substrate supporting self-stimulation has a great current-integrating capacity. Together, these experiments show that varying the amount of brain stimulation produce large and consistent changes in a number of FI response measures. These measures effectively describe different attributes of FI performance and include response rate, the postreinforcement pause, interresponse times of short duration and the temporal distribution of responses within the interval.

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.

Metabolic effects of galanin injections into the paraventricular nucleus of the hypothalamus.

The metabolic effects of single injections of galanin into the paraventricular nucleus of the hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. In the dose range of 0.03-0.3 nmol, galanin produced a very short-latency (approximately 2 minutes) and short-lasting (approximately 15 minutes) reduction in energy expenditure. Since the same doses had no effect on respiratory quotient or locomotor activity, the metabolic effect is not secondary to changes in energy substrate utilization or locomotor activity. This antithermogenic effect complements the eating stimulatory action of PVN galanin, and together these phenomena suggest a role for galanin as an anabolic neuropeptide. The similarity of galanin's effects to those of norepinephrine, with which it coexists in PVN nerve endings, further suggests the involvement of this amine and the PVN alpha2-noradrenergic system in galanin's mechanism of action.

Cocaine facilitation of prefrontal cortex self-stimulation: a microstructural and pharmacological analysis.

A novel self-stimulation methodology involving a fixed-interval (FI-5 s) schedule of reinforcement, microanalysis and threshold evaluation was used to investigate the effects of cocaine on rats lever pressing for electrical stimulation of the prefrontal cortex. Cocaine (15 mg/kg) increased medial prefrontal cortex (MPC) self-stimulation rates under FI-5 by a mean of 269% and reduced current thresholds for self-stimulation. A similar facilitation was evident with self-stimulation of the sulcal prefrontal cortex. Microanalysis showed that cocaine decreased inter-response times and post-reinforcement pauses, increased responding in the second and third quartiles of the inter-reinforcement interval (IRI) and decreased responding in the fourth IRI quartile. Schedule control of responding was still evident following cocaine despite the profound facilitation of response rates. Increased response rates were seen up to 48 h following a single dose of cocaine, suggesting sensitization of the PFC reinforcement substrate. The acute effects of cocaine on MPC self-stimulation were completely reversed by the dopamine (DA) D1 antagonist SCH 23390 0.02 mg/kg) and the D2 antagonist raclopride (0.3 mg/kg) but not by naloxone (0.5 mg/kg). These results are consistent with previous studies demonstrating the PFC as part of the neural substrate mediating cocaine reward. Further, these results implicate DA receptors in the reinforcing properties of both cocaine and MPC self-stimulation.

Prefrontal cortex self-stimulation and energy balance.

The relation between sulcal prefrontal cortex (SPC) and medial prefrontal cortex (MPC) self-stimulation and energy balance was investigated in rats. SPC but not MPC self-stimulation induced feeding but not the gnawing of wooden blocks. SPC but not MPC self-stimulation enhanced weight gain over several weeks of exposure to stimulation. Food deprivation (48 hr but not 24 hr) increased SPC self-stimulation rates under a 5-s fixed-interval reinforcement schedule and decreased current thresholds for SPC self-stimulation. MPC self-stimulation was unaffected by food deprivation. Insulin (4 U/kg) and 2-deoxy-D-glucose (300 mg/kg) inhibited both SPC and MPC self-stimulation, probably through interfering with performance. Satiety induced by prolonged intake of a sweetened solution or deprivation-induced feeding moderately facilitated SPC self-stimulation. Overall, it appears that SPC but not MPC self-stimulation modulates, and is modulated by, energy balance.

Insulin and the paraventricular hypothalamus: modulation of energy balance.

The effects of insulin injections (0.1, 1, 10 and 40 mU) into the paraventricular hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. The 0.1 and 1 mU doses had no effects on either respiratory quotient or energy expenditure. The 10 mU dose increased respiratory quotient which indicates increased dependency on carbohydrates as an energy substrate. The same dose had no effects on thermogenesis. In contrast, the 40 mU dose decreased respiratory quotient which indicates increased dependency on fats as an energy substrate. The higher dose also increased thermogenesis. Since neither dose significantly affected locomotor activity, the metabolic data are not confounded with activity effects. These data indicate that insulin in the PVN produces a primary modulation of the metabolic parameters central to maintaining energy balance. In separate experiments, the 4 doses of insulin reduced food intake and body weight over a 24 h period. They also produced a dose-related increase in blood glucose concentration over a one hour period. Taken together, these findings are interpreted in a model in which insulin in the PVN acts as a signal indicating increased body fat. This increases thermogenesis, fat utilization and glycemic levels, and inhibits feeding. The net effect of this integrated metabolic-behavioural response is a regulatory reduction in body fat.

Prefrontal cortex alpha 2 adrenoceptors and energy balance.

The sulcal prefrontal cortex (SPC) influences thermogenesis, energy substrate utilization and feeding behaviour. The present study examined the role of SPC alpha noradrenergic receptors in these effects. Fifty nmol norepinephrine (NE) injected into the SPC produced a large and long-lasting increase in respiratory quotient (RQ), indicating enhanced carbohydrate utilization and fat synthesis. This dose also reduced energy expenditure without corresponding decreases in locomotor activity, suggesting an inhibition of thermogenesis. Neither a lower dose of NE (25 nmol) injected into the SPC, nor injections of NE (50 nmol) into a variety of sites adjacent to the SPC affected energy balance. The alpha 2 agonist clonidine (20 nmol) injected into the SPC produced similar effects to 50 nmol NE, with a large increase in RQ and a decrease in thermogenesis. Forty nmol clonidine, however, decreased RQ and reduced both energy expenditure and activity. The alpha 1 agonist L-phenylephrine (20 and 40 nmol) injected into the SPC had no clear effect on energy balance. Finally, it was shown that clonidine or NE injected into the SPC promotes food intake. These results implicate alpha 2 adrenoceptors in the sulcal prefrontal cortex in the control of food intake, thermogenesis and metabolic substrate utilization.

Metabolic effects obtained from excitatory amino acid stimulation of the sulcal prefrontal cortex.

Indirect calorimetry was used to assess metabolic changes in rats following injections of the excitatory amino acid D,L-homocysteic acid (DLH) into the sulcal or medial prefrontal cortex. Injection of 7 nmol of DLH into the sulcal prefrontal cortex (SPC) increased respiratory quotient (RQ), indicating increased utilization of carbohydrate as an energy substrate. This treatment also decreased energy expenditure in the absence of related changes in motor activity, suggesting an inhibition of thermogenesis. A larger dose of DLH (50 nmol) injected into the SPC caused opposite effects, with a significant decrease in RQ and increased energy expenditure and motor activity. Rectal temperature was also increased by 20 or 50 nmol DLH but decreased with 7 nmol DLH. The anatomical specificity of these effects was indicated in that equivalent injections DLH into the medial prefrontal cortex did not affect energy balance. From this and related evidence it is concluded that SPC neurons exert a potent influence upon thermogenesis and metabolic substrate utilization that is bidirectional according to the magnitude of the excitatory stimulation that is applied.

Stressor-like effects of FG-7142 on medial prefrontal cortex self-stimulation.

The effects of uncontrollable stress and the anxiogenic beta-carboline FG-7142 were compared on rats bar-pressing for electrical stimulation of the medial prefrontal cortex (MPC). One minute of restraint stress produced an immediate 20% increase in response rates. A significant facilitation was also evident 24 and 48 h later. Similarly, FG-7142 (10 mg/kg) produced an immediate 20% elevation of self-stimulation rates and a significant facilitation 24 and 48 h following administration. Lower (3 mg/kg) and higher (20 mg/kg) doses of FG-7142 caused immediate decreases in MPC self-stimulation and no significant long-term effects. These results agree with previous findings that FG-7142, administered at certain doses, may mimic the effects of exposure to uncontrollable stress.

Metabolic effects of neuropeptide Y injections into the paraventricular nucleus of the hypothalamus.

The metabolic effects of single injections of neuropeptide Y (NPY) into the paraventricular hypothalamus were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. Over the dose range of 10-156 pmol NPY had large effects on respiratory quotient (RQ) while having no effect on energy expenditure or locomotor activity. The effects of NPY on RQ were unusual both in respect to their dose-response and time-dose-response characteristics. The lowest dose (10 pmol) produced a very low latency reduction in RQ which indicates a decreased utilization of carbohydrates as an energy substrate. The next higher dose (20 pmol) had no effect, whereas the next three doses (39, 78 and 156 pmol) produced increases in RQ which indicate an increased utilization of carbohydrates as an energy substrate. Surprisingly, the latencies of the increased RQs were dose-dependent over the range of 30 min to 20 h with the highest dose producing the longest latency effect. The finding of a positive relation of dose to response latency over a time range of from a few minutes to 20 h is unprecedented and appears to represent a neuromodulatory effect of NPY that acts in concert with its neurotransmitter effects. These data highlight the central role of NPY in modulating energy substrate utilization and indicate the importance of elucidating time-dose-response relationships when investigating the effects of NPY.