Search PubMed⌕ Search

Biomedical subjects

S Schenk

Publications and source records attributed to S Schenk.

At least 91 records · Page 5Linked to original sources
PubMed1990

Preexposure sensitizes rats to the rewarding effects of cocaine.

During a preexposure period rats were injected once daily with either cocaine HCI (10 mg/kg, IP) or the saline vehicle for 12 consecutive days. Rats that were chronically exposed to cocaine during the pretreatment phase were more responsive to the motor activating effects of a subsequent injection of cocaine than were rats chronically treated with saline. In self-administration testing, saline-pretreated groups did not exhibit a significant preference for a lever producing a cocaine infusion relative to an inactive lever, suggesting that the doses tested (0.225 and 0.45 mg/kg/infusion) were subthreshold for cocaine reward. In contrast, subjects preexposed to cocaine had a higher rate of reinforced responses and exhibited a preference for a lever that resulted in a cocaine infusion. It was unlikely that the higher response rate was due to an elevation in nonspecific activity since inactive lever responding remained low and relatively invariant over the 9 days of testing. Thus the enhanced responding in the cocaine-preexposed rats suggests that the reinforcing effectiveness of the drug had increased. These data indicate that sensitivity to cocaine's behavioral effects can be enhanced and that predisposing factors to cocaine abuse can be manipulated.

Animals↗
PubMed1989

Self-administration of phenylpropanolamine (PPA) by rats previously trained to self-administer amphetamine.

Central nervous system stimulation, similar to that observed for amphetamine, has been attributed to phenylpropanolamine (PPA). However, formal tests, using evaluation of locomotion or of self-administration, fail to reveal that PPA is a stimulant. Self-administration studies have trained rats to self-administer cocaine and then have attempted to switch rats to PPA with no success of transfer. The present study further examined the reinforcing properties of PPA (0.08, 0.16, and 0.32 mg/infusion, IV) in rats that were initially trained to self-administer d-amphetamine (0.005-0.08 mg/infusion) in a two-lever paradigm. Self-administration of amphetamine was a function of dose with 0.01 mg/infusion producing reliably high responding on the active lever. The proportion of active/inactive lever presses remained constant (0.60-0.80) across the amphetamine dose range. PPA was dose-dependently self-administered during the first hour of each session with rats responding at approximately 70% on the active lever. In contrast, responding on the active lever dropped off to approximately 48% when saline was substituted for amphetamine. These data document that rats with prior exposure to amphetamine will self-administer PPA during the initial portion of a three-hour test.

Amphetamine↗
PubMed1988

Differentiation of the substrates for analgesia and vocalizations elicited by midbrain stimulation in rats: refractory period estimates.

Stimulation of periaqueductal gray sites resulted in both increased latencies to escape heated water with a tail-flick and audible vocalizations. To determine whether these two responses were the result of stimulation of the same substrates, refractory periods were estimated by delivering paired-pulse trains of stimulation. Stimulation consisted of 10 or 20 s trains of single pulses or pulse pairs. The pulse pair frequency threshold for both behaviors was determined for intra-pulse (C-T) intervals of 0.4-10 ms. The ratio of single to double pulse frequency thresholds provided an indication of relative effectiveness of the paired-pulse stimulation. For analgesia, the paired-pulse effectiveness gradually increased between C-T intervals of 2.0 and 5.0 ms, after which asymptotic effectiveness values were obtained. Thus, the estimated refractory period for analgesia was 2.0-5.0 ms. The refractory period estimate for vocalizations was shorter. Effectiveness values increased with a C-T interval of 1.2 ms and reached asymptote with a C-T interval of 2.0 ms. These results suggest that different fibers with overlapping spatial distributions contribute to analgesia and vocalizations produced by midbrain stimulation.

Animals↗
PubMed1988

Housing conditions fail to affect the intravenous self-administration of amphetamine.

Rats were housed either in isolation or in groups of 4 for 6 weeks following weaning (21 days). After this housing period, some of the rats were tested for the acquisition of intravenous self-administration of amphetamine (0.004-0.25 mg/kg/infusion) and others were tested for the locomotor activating effects of amphetamine (0-1.0 mg/kg, IP). In the self-administration tests, both the isolated and grouped rats readily acquired the operant to obtain drug infusions and exhibited dose-dependent behavior. These results are in direct contrast to those we have obtained concerning the influence of the environmental manipulation on cocaine self-administration. In those tests, only isolated rats self-administered cocaine. The results of the locomotor tests indicated that whereas the isolated rats were consistently more active, the dose/response curves for the effects of amphetamine on activity were parallel for the rats reared under the different housing conditions. Thus the environment has specific effects on behavior which may be a reflection of specific neurochemical effects of the manipulation.

Amphetamine↗
PubMed1987

Cocaine self-administration in rats influenced by environmental conditions: implications for the etiology of drug abuse.

The present study investigated the possibility of environmental factors as an explanation for between-subject differences in cocaine self-administration. Weaning rats (21 days) were housed in isolated or aggregated conditions for 6 weeks and were tested for intravenous cocaine self-administration (0.1-1.0 mg/kg/infusion). Rats housed in groups failed to reliably self-administer this drug whereas isolated rats readily acquired an operant to receive infusions of cocaine. These data suggest that environmental factors play a major role in determining individual differences in the propensity to self-administer cocaine and that, as such, they should be considered more seriously by those interested in the basis and treatment of drug abuse.

Animals↗
PubMed1987

Isolation housing decreases the effectiveness of morphine in the conditioned taste aversion paradigm.

Male Long Evans rats were obtained at 21 days of age and were housed in either an aggregated (four per double cage) or isolated (one per single cage) condition for 6 weeks. They were then placed on a fluid deprivation schedule that allowed them access to fluids for 20 min daily. This schedule was maintained for the remainder of the experiment. Following habituation, sensitivity to morphine-induced conditioned taste aversion (CTA) was compared in the differentially housed rats. On the 1st day and every 5 days thereafter the rats were presented with a 0.1% solution of sodium saccharin for the 20-min drinking period, followed immediately by an injection of morphine (0, 2.5, 5.0, 10.0, or 20.0 mg/kg). On intervening days they received water as the fluid. No drugs were given on these days. There was no difference in baseline saccharin consumption as a function of housing condition. In comparison with the isolated rats, the grouped animals were more sensitive to the CTA-inducing properties of low doses of morphine. These data strengthen the already existing evidence for the influence of the early housing environment on drug sensitivity and provide additional support for the conclusion that variability in response to a number of drugs of abuse can be reduced by environmental means. Possible mechanisms for the differences between isolation and aggregation housed rats are discussed.

Animals↗
PubMed1986

Differential effects of isolation housing on the conditioned place preference produced by cocaine and amphetamine.

Rats were obtained at 21 days of age and were housed either in isolation or in groups of 4 for 6 weeks. They were then tested for their sensitivity to cocaine HCl (0.31, 0.62, 1.25 or 2.5 mg/kg) or d-amphetamine SO4 (0.031, 0.062, 0.125, 0.25 or 0.5 mg/kg) using a modified place preference paradigm. The isolated rats were insensitive to cocaine in this paradigm whereas the group-housed animals showed peak effects at the lowest dose of this drug. In contrast, there was no difference in sensitivity to amphetamine as a function of housing conditions. These data strengthen the notion that the effects of the early environment on drug sensitivity in the adult are specific to certain classes of drugs. Further, these data lend support to the notion that the effects of cocaine and amphetamine in the place preference paradigm are mediated by different neural systems.

Amphetamine↗
PubMed1985

The substrates for self-stimulation of the lateral hypothalamus and medial prefrontal cortex: a comparison of strength-duration characteristics.

The directly activated substrates for self-stimulation of the lateral hypothalamus (LH) and medial prefrontal cortex (MPFC) were described by comparing their strength-duration characteristics. The current required to maintain a half-maximal rate of lever pressing was traded off against the pulse duration while all other stimulation parameters were kept constant. In this manner, cathodal strength-duration curves were obtained at four LH and eight MPFC sites; anodal curves were obtained at two of the LH and six of the MPFC sites. In general, the cathodal LH curves had lower rheobases than the cathodal MPFC curves and continued to descend after the MPFC curves had levelled off. At short pulse durations, the anodal curves lay above the cathodal curves, a finding more pronounced in the LH data. The two sets of curves converged at the longer pulse durations. The differences in the strength-duration curves are consistent with the notion that different directly stimulated neurons are responsible for the rewarding effects of LH and MPFC stimulation. Anatomical and physiological properties that could account for these differences are discussed.

Animals↗
PubMed1985

Spatio-temporal integration in the substrate for self-stimulation of the prefrontal cortex.

The number of stimulation pulses required to maintain a half maximal rate of self-stimulation of the prefrontal cortex (PFC) was determined for various currents. Over a restricted range, the effects of decreasing the stimulation frequency could be compensated for by increasing the current. This finding cannot easily be reconciled with the hypothesis that the rewarding impact of PFC stimulation is unaffected by increments in current. The minimum current that would support self-stimulation of the PFC at high frequencies was larger than has been reported at medial forebrain bundle sites.

Animals↗
PubMed1985

Chronic naltrexone treatment increases the heroin-produced facilitation of self-stimulation.

The facilitatory effects of heroin HCl (0.25 mg/kg, SC) on self-stimulation (SS) of the lateral hypothalamus before and after chronic treatment of naltrexone (10 mg/kg, SC, for 20 days) or vehicle were compared. The group that received chronic naltrexone had a larger heroin-induced facilitation of SS than the group that received vehicle. These data suggest that the sensitivity to the facilitatory effect of heroin on SS may be related to the amount of opiate receptor binding which is increased following chronic antagonist treatment. However, neither acute nor chronic treatment with naltrexone produced any significant changes in SS thresholds, suggesting that the directly stimulated substrate for the rewarding effect of brain stimulation is unlikely to be endorphinergic but is apparently modulated by the endogenous opioid system.

Animals↗
PubMed1985

An examination of heroin conditioning in preferred and nonpreferred environments and in differentially housed mature and immature rats.

The study addressed two issues. First, we examined the effectiveness of heroin as a conditioning agent in a preferred environment using a place preference paradigm. Four daily injections of 80 micrograms/kg (SC) of heroin HCl were paired with environments that rats initially found to be either preferred or non-preferred. In subsequent tests, only those that had experienced the drug effects in the non-preferred environment increased the percentage of time spent in that environment. Rats conditioned in the test chamber that was initially preferred failed to increase the amount of time spent in that chamber post-conditioning. These results suggest that the conditioned place preference paradigm does not simply assess the rewarding consequence of heroin injections. We also examined the effects of grouped and isolation housing conditions on the heroin-produced conditioned place preference. Rats were housed under these conditions either immediately post weaning or at 120 days of age. There was a difference between the magnitude of the place preference produced by 20 micrograms/kg heroin in the isolated but not in the group housed rats. When isolated at weaning the rats were less sensitive to the drug than were rats isolated at maturity. These data are discussed with particular reference to the development of the endogenous opioid system.

Aging↗
PubMed1984

Lower order cities and national urbanization policies: China and India.

The authors "examine the urbanization history and policies of China and India with a special emphasis on the lower order cities. [They] consider the proposition that, as the developing countries continue to urbanize at a rapid pace, the lower order cities can play a potentially effective role in guiding future urbanization and in securing a balanced economic and spatial development. Through an examination of the urbanization records, policies, and performance of the lower order cities in these two countries [the authors] discuss the 'top-down' versus the 'bottom-up' approaches to urbanization strategy and national development.... Differences between the urbanization policies of China and India and the transferability of the Chinese experience to other contexts [are also discussed]."

Asia↗
PubMed1983

Isolation versus grouped housing in rats: differential effects of low doses of heroin in the place preference paradigm.

Male Long Evans rats were reared from weaning (21-23 days) either in isolation or in groups of four for 40 days. Animals were then individually introduced to a testing apparatus consisting of two distinct chambers. A modified place preference paradigm was used consisting of 3 phases: (1) An habituation phase (4 days) during which rats were allowed free access to the entire test apparatus for 15 min. periods daily; (2) A conditioning phase (4 days) during which rats were confined to their non-preferred side for 15 minutes each day immediately following subcutaneous injection of 0, 20, 40 and 80 micrograms/kg of heroin HCl; (3) A test phase (1 day) during which rats were again allowed free access to the testing chamber following injection of vehicle. The difference in time spent on the conditioned side during habituation and test periods was determined. The group-reared rats showed similar effects for all doses of heroin whereas the same magnitude of drug effect was attained only at the highest dose used in the isolated rats. This differential sensitivity to heroin in the place preference paradigm is discussed in terms of the modification of behavioral effects of opiates by environmental influences.

Animals↗
PubMed1982

The substrates for lateral hypothalamic and medial pre-frontal cortex self-stimulation have different refractory periods and show poor spatial summation.

Refractory periods of the substrates for lateral hypothalamic (LH) and medial pre-frontal cortex (MPFC) self-stimulation were behaviorally estimated. The beginning of recovery from refractoriness was estimated as the time at which recovery was 20% complete. In all 7 rats, this estimate differed substantially across sites, averaging 0.66 msec and 1.59 msec for the LH and MPFC substrates, respectively. The recovery of excitability approached asymptote later in the MPFC substrate (3.5 msec) than in the LH substrate (1.5 msec). These findings are consistent with the view that different fibers subserve the reinforcing consequences of LH and MPFC stimulation. This notion is strengthened by the observation that the rewarding effects of stimulation summated poorly when stimulating pulses were concurrently delivered to these two sites.

Animals↗
PubMed1981

A within-subject comparison of the effects of morphine on lateral hypothalamic and central gray self-stimulation.

The effects of chronic administration of morphine (20 mg/kg) on self-stimulation (SS) of the central gray and lateral hypothalamus were investigated in a within-subject design. The magnitude and time course of the drug-produced changes in SS at the two placements were similar within subjects but varied substantially across subjects. These results are interpreted in the light of evidence pertaining to the anatomical linkage of the substrates for the rewarding effects of central gray and lateral hypothalamic stimulation. The facilitation of SS may be due to a drug-produced sensitization of reward-related neurons. If so, morphine acts either beyond the point of convergence of the two substrates or at an earlier stage in each substrate. The across-subject variability is attributed to individual differences in sensitivity to the effects of the drug. The importance of controlling for this subject variable is stressed.

Animals↗
PubMed1981

[Influence of oxytocin and prostaglandin E2 on icterus neonatorum (author's transl)].

The aim of this progressive study is to clarify whether the pain-in-labour drugs Oxytocin and Prostaglandin E2 do have an influence on postpartum serumbilirubin concentrations of the neonates. Two groups of neonates in which labour was induced with Oxytocin or Prostaglandin E2 were compared with a control group without pain-in-labour stimulants. The three groups were comparable in respect of obstetrical anamnesis and risk factors. The serumbilirubin concentration of the neonates was controlled at very frequent intervals during the first 72 hours. In the three groups, no difference could be found in the serumbilirubin values during the 72 hours' duration of the study.

Apgar Score↗