Stephen G. Holtzman

Long-lasting changes in stress-induced corticosterone response and anxiety-like behaviors as a consequence of neonatal maternal separation in Long-Evans rats.

Early neonatal environmental factors appear to have powerful and long-lasting influences on an organisms physiology and behavior. Long-Evans male rats separated from their dam for 3 h daily over the first 2 weeks of life (maternally separated, MS rats) when tested as adults exhibit exaggerated behavioral and neuroendocrine responses to stress compared to 15-min separated (handled, H) animals. The purpose of this study was to compare male and female adult rats that were MS, H or were undisturbed (nonhandled, NH) as neonates in anxiety-like behaviors, in the elevated plus-maze, and in response to startle-inducing auditory stimuli. We confirmed that MS males oversecrete corticosterone (CORT; 2.5-5 times) in response to mild handling stress. MS males and females were less likely to explore open arms of the plus-maze. MS males exhibited 35% higher startle amplitudes compared to controls. Furthermore, MS males were more likely to emit ultrasonic vocalizations in response to startle than were H controls. However, MS and control females did not differ in auditory startle response or in startle-induced ultrasonic vocalizations. Therefore, experiencing maternal separation results in a long-lasting increase in anxiety-like behaviors that occurs in a sex-dependent manner.

Suppression of deprivation-induced food and water intake in rats and mice by naloxone.

Naloxone, an opiate antagonist, was administered to male and female rats and male mice after periods of food or water deprivation ranging from 12 to 48 hr. Naloxone (0.01-10 mg/kg) reduced postdeprivational water intake in most groups of rats and mice in a dose-related manner. Naloxone suppression of water consumption appeared to be independent of sexual differences in rats, and phase of the diurnal cycle, and length of the deprivation interval in both rats and mice. Postdeprivational food intake in male rats and mice was also reduced by naloxone in a dose-dependent fashion. This naloxone effect was less pronounced than actions observed with water intake, and tended to diminish with lengthening food deprivation periods. In general, mice appeared to be less sensitive than rats to naloxone suppression of food and water intake. Naloxone appears to markedly reduce appetitive behavior, particularly water intake, following deprivation in both rats and mice. The fact that low doses of naloxone can elicit these effects suggests that the drug is acting at specific tissue sites, possibly endorphine recpetors.

Suppression of appetitive behavior in the rat by naloxone: lack of effect of prior morphine dependence.

Abstract Naloxone (0.3–10 mg/kg) produced a dose-related suppression of eating and drinking in rats that had been deprived of food for 48 hr or water for 24 hr. The suppression of water intake by naloxone was unaltered in rats that had been physically dependent upon morphine one week earlier and which were tolerant to the analgesic effect of morphine at the time naloxone was tested. These results confirm the ability of naloxone to suppress appetitive behavior in the rat but do not resolve the issue of whether or not this effect of naloxone is the consequence of an interaction with an endogenous opioid system.

Morphine training dose: A determinant of stimulus generalization to narcotic antagonists in the rat

Rats were trained to discriminate between saline and either 1.75 or 5.6 mg/kg of morphine in a two-choice discrete-trial avoidance paradigm. Both groups of rats generalized completely to higher doses of morphine as well as to profadol and pentazocine, analgesics with narcotic antagonist properties. The dose of each test drug required to elicit drug-appropriate responding was about one-half log-unit higher in the rats trained with 5.6 mg/kg of morphine than in the rats trained with 1.75 mg/kg. Rats trained with the lower dose of morphine also generalized completely to the narcotic antagonist nalbuphine and to the non-opioid drug d-amphetamine, and generalized partially to the narcotic antagonist cyclazocine. In contrast, rats trained with the higher dose of morphine showed only partial generalization to nalbuphine and virtually none to cyclazocine and d-amphetamine. The degree of stimulus generalization to the narcotic antagonists and to d-amphetamine in the two groups of rats corresponds well with the known similarities and differences between the syndromes of subjective effects engendered by these drugs and morphine in man. These results indicate that systematic variation of the morphine training dose can facilitate characterization of the discriminative stimulus properties of narcotic antagonist analgesics and enhance the value of drug discrimination procedures as a model for predicting the subjective effects of these drugs.

D1 and D2 dopamine receptor antagonists block caffeine-induced stimulation of locomotor activity in rats.

The mechanism of action for the behavioral stimulant effects of caffeine has been extensively studied, but results have been ambiguous and inconsistent. The present study examined the role of dopamine in caffeine-induced stimulation of locomotor activity in rats. d-Amphetamine was also tested for comparison. Locomotor activity of male Sprague-Dawley rats (300-350 g) was measured using two-channel electronic activity monitors. Activity counts were recorded for 30 min following a 30-min pretreatment with either caffeine (3.0-100 mg/kg, IP) or d-amphetamine (0.1-3.0 mg/kg, IP) alone and in combination with the D1 dopamine antagonist SCH23390 (0.01 and 0.003 mg/kg, SC) or the D2 dopamine antagonists sulpiride (30 mg/kg, SC) or eticlopride (0.03 mg/kg, SC). Caffeine and d-amphetamine dose dependently increased locomotor activity. This effect of both caffeine and d-amphetamine was blocked by SCH23390 as well as by eticlopride. Sulpiride blocked the stimulatory effects of caffeine but not d-amphetamine. These results suggest that the locomotor stimulant effect of caffeine, like that of d-amphetamine, is mediated through dopaminergic systems; both D1 and D2 receptors appear to be involved.

Opiate antagonists: central sites of action in suppressing water intake of the rat

The pure opiate antagonists, naloxone and naltrexone (0.1-10 mg/kg), dose-dependently suppressed water intake of 24 h water-deprived rats upon subcutaneous administration; their quaternary derivatives, methyl-naloxone and methyl-naltrexone, which are impermeable to the blood-brain barrier, failed to affect drinking. Upon intracerebroventricular administration, both quaternary analogs attenuated drinking at a dose of only 10 microgram. These results demonstrate that the antidipsogenic effects of opiate antagonists are primarily mediated at sites within the central nervous system.

Complete, reversible, drug-specific tolerance to stimulation of locomotor activity by caffeine

The development of tolerance to caffeine-induced stimulation of locomotor activity was evaluated in rats maintained chronically on average daily doses of 160 mg/kg or more of caffeine by the method of scheduled access to drinking water containing the drug. Dose-response curves were determined for caffeine (6.25-100 mg/kg) and d-amphetamine (0.39-6.4 mg/kg) during chronic drug treatment. In addition, the caffeine curve was redetermined 2-3 weeks after removal of the drug from the drinking water. A control group that had scheduled access to drug-free tap water was also tested. Caffeine produced dose-related increases in the locomotor activity of the controls but failed to modify locomotor activity of the chronic caffeine group. In contrast, d-amphetamine increased locomotor activity of both groups comparably. Spontaneous locomotor of the chronic caffeine group was reduced significantly for 4 days after drug-free tap water was substituted for the caffeine solution. The return of spontaneous locomotor activity to baseline values was associated with restored sensitivity to caffeine-induced stimulation of locomotor activity. Thus, chronic administration of caffeine to rats results in the development of tolerance to caffeine-induced stimulation of locomotor activity that is virtually complete, pharmacologically specific, and fully reversible when drug treatment is stopped. Decreases in spontaneous locomotor activity after abrupt termination of chronic caffeine administration follow a time course consistent with a drug withdrawal syndrome.

Suppression of drinking by naloxone in the rat: A further characterization

The effects of naloxone, an opiate antagonist, were examined on drinking induced by various dipsogenic stimuli. In rats deprived of water for 24 h, naloxone (0.1-10 mg/kg) produced a dose-related suppression of drinking immediately following water presentation but did not alter the latency to begin drinking. Naloxone also produced a dose-related suppression of water consumption induced by isoproterenol and angiotensin II, agents simulating conditions of extracellular dehydration. Naltrexone, a congener of naloxone, was more potent than naloxone in reducing isoproterenol-induced water intake. Schedule-induced polydipsia, which occurs in the absence of body fluid deficits, was not altered by either naloxone or naltrexone at doses attenuating drinking induced by the other methods. These data suggest that the suppressant effects of naloxone on water consumption are not a manifestation of an increased latency to drink or an impairment in the motor components of drinking activity. Furthermore, narcotic antagonists appear to attenuate regulatory, but not adjunctive drinking.

Tolerance to behavioral effects of caffeine in rats

Tolerance develops to three behavioral effects of caffeine in rats treated daily with the drug: stimulation of locomotor activity, rate-decreasing effect on food-reinforced operant responding, and discriminative stimulus effects. Tolerance induced by caffeine to stimulation of locomotor activity: (1) develops rapidly, being nearly maximal 24 hr after the start of daily treatment with 35-40 mg/kg in four divided oral doses; (2) is insurmountable (i.e., complete) so that rats are unresponsive, even to high doses of caffeine, and dose-response curves are displaced downward and flattened; (3) is pharmacologically specific, extending to other methylxanthines but not to nonxanthine psychomotor stimulants. Tolerance to the other two behavioral effects of caffeine: (1) appears to develop gradually; (2) is surmountable so that dose-response curves are displaced to the right; (3) extends to nonxanthine psychomotor stimulants, methylphenidate, in the case of discriminative effects. Thus, at least two distinct types of tolerance to behavioral effects of caffeine in the rat can be identified. The potency of an adenosine analog, R(-)-PIA, in depressing locomotor activity does not increase during or 24 hr after termination of chronic daily treatment with caffeine when rats are completely tolerant to caffeine-induced stimulation of locomotor activity. These results do not support the view that enhanced functional sensitivity of central adenosine systems (e.g., up-regulation of receptors) is the mechanism of tolerance to the stimulant effect of caffeine on locomotor activity.

Evidence that opiate receptors mediate suppression of hypertonic saline-induced drinking in the mouse by narcotic antagonists

Abstract The effects of naloxone, its dextro -stereisomer, and five other narcotic antagonists were determined on water intake induced by intracellular dehydration in the mouse. The intraperitoneal administration of a 2M sodium chloride solution served as the model for intracellular dehydration. 1 -Naloxone (0.01-10 mg/kg) reduced drinking in a dose-dependent fashion with an ED50 of 0.55 mg/kg. In contrast, d -naloxone failed to suppress water consumption at doses up to 10 mg/kg. The other narcotic antagonists tested --- naltrexone, diprenorphine, levallorphan, oxilorphan, and nalorphine --- also produced dose-dependent decreases in water consumption. The order of potency of these narcotic antagonists in suppressing water intake was highly correlated with their orders of potency in other procedures involving the opiate receptor. The stereoselectivity and order of potency suggest that the suppressant effects of the narcotic antagonists on drinking induced by hypertonic saline administration in the mouse are mediated through an opiate receptor-dependent mechanism.