Rats as laboratory animals

There is a growing body of literature indicating that drug effects are influenced by the
context in which they are taken, and that neuroadaptations resulting from chronic drug
use are similarly context dependent. Contingent tolerance to amphetamine-induced
hypophagia is mediated by the learned suppression of stereotyped behaviors, and is an
example of a drug-environment interaction. This form of behavioral tolerance depends
upon instrumental learning, by which rats learn a strategy to suppress drug-induced
stereotypies that interfere with feeding. Considerable progress has been made in
understanding contingent tolerance at the behavioral level; little is known about the
neural mechanisms underlying contingent tolerance. Therefore, the purpose of this study
was to delineate neural circuitry involved in contingent tolerance. The differential
activation of neurons expressing the immediate early gene c-fos was analyzed throughout
the brains of amphetamine-tolerant and non-tolerant rats, using the Before-After
paradigm; the amphetamine tolerant group received injections of amphetamine (2.0
mg/kg) before access to milk, after-amphetamine and after-saline groups (i.e., nontolerant)
received injections of amphetamine after access to milk, and the saline group received saline at both time points. The experimental design permitted us to identify
structures uniquely involved in tolerance from those associated with drinking milk,
having a history of amphetamine, or receiving an injection of amphetamine on the final
test. The unique finding reported here is that when amphetamine is given in an
environment containing food, patterns of c-fos are very different than when the drug is
given in an environment without food. Results showed that amphetamine-tolerant
animals had significant increases in c-fos in a set of interconnected structures throughout
the brain, as compared with non-tolerant and saline rats. These data supported the
hypothesis that structures associated with the dorsal striatum mediate the response
selection of feeding and the inhibition of stereotypies, while the ventral striatum, via
instrumental learning, reinforces the selection and inhibition of competing motor
behaviors. Results also support the idea that the mechanisms of tolerance involve several
neural subsystems that function to modulate motor, motivational, and reward-based
learning. Specifically, the learned suppression of stereotypies involves the tolerance
"Response Selection," "Reinforcement," and "Instrumental Learning" subsystems.

The current study uses a diabetic rat model to measure the effects of pulsatile
versus continuous insulin delivery on IRS-2 in the liver. The purpose is to determine if
pulsatile, compared with continuous, insulin delivery leads to reduced insulin resistance
in diabetic rats. Insulin signaling in the liver is mediated primarily through IRS-2 and
tissue responsiveness to insulin may be detected by monitoring the lRS-2 signaling
pathway. Western blots were performed to measure IRS-2 protein levels for each
delivery condition and treatment day. Results revealed that overall, the pulsatile insulin
delivery method showed a significant increase in IRS-2 levels over the continuous insulin
delivery method by treatment day 5. These findings imply that the pulsatile delivery
method, over a period of time, triggers more insulin receptor action. Conversely, the
results of the continuous delivery system show a decrease in IRS-2 levels as the number
of doses of insulin increased.

In this thesis I studied propensity for behavioral sensitization to nicotine in
the LRHR phenotype and associated plasticity in the hippocampal mossy fiber
morphology. I also investigated therapeutic effects of bupropion and a
cannabinoid receptor antagonist on behavioral and morphological indices in
adolescence and adulthood. Male rats were classified into high responders (HR)
and low responders (LR) based on their locomotor response to a novel
environment. LRHR animals underwent behavioral sensitization to nicotine and
after one week of abstinence were challenged with a low dose of nicotine. HRs
expressed behavioral sensitization to nicotine and showed an increase in
hippocampal mossy fiber terminal field size. AM251 administration during
abstinence reversed behavioral sensitization in HRs and bupropion only
attenuated the locomotor response to na"ive nicotine exposure. Therapeutic
agents had differential effects on mossy fiber morphology dependent on
phenotype and age.

The effects of cocaine exposure and maternal deprivation on subsequent voluntary ingestion of cocaine and amphetamine was investigated in 7-day-old rat pups in order to further our understanding on the development of drug addiction. Maternally deprived and non-deprived pups were pre-exposed to a cocaine solution masked with 5% orange Tang solution. Four hours later, experimental pups were tested for subsequent cocaine self-administration (SA) (Exp. 1) or amphetamine SA, (Exp. 2), following a second deprivation period. Control pups were not deprived during this interval. Pups in both experiments were assessed for dose self-administered and for general activity. Results indicate that cocaine pre-exposure increased cocaine and amphetamine SA, and activity significantly increased after pre-exposure and testing sessions. Lastly, sensitization of the motor effects of cocaine was observed in pups pre-exposed to cocaine. This study provides a potential drug SA animal model not yet investigated in developing animals.

The effects of perinatal hypothyroidism and perinatal choline supplementation on radial arm maze performance were assessed. Animals were made hypothyroid by the administration of propylthiouracil (PTU) to the mash of dams from prenatal day 15 until postnatal day 15. Choline chloride was administered prenatally in drinking water and postnatally through injections to the pups until day 24. Hypothyroid animals made significantly more reference and working memory errors than did euthyroid animals. Perinatal choline supplementation reduced the number of working memory errors, but not reference memory errors, in both euthyroid and hypothyroid animals.

The effects of amphetamine on the milk intake, body weight, and behavioral activity of bottle- and cannula-fed rats was investigated in a before/after paradigm. Dose response determinations were conducted before (DR 1) and after (DR 2) chronic treatment (45 days) with amphetamine (8.0 mg/kg) given either before or after milk tests, to determine whether tolerance developed to the anorexic effect of the drug. Control groups were given saline during the chronic phase. Both cannula- and bottle-fed animals lost a significant amount of weight. Cannula-fed animals drank significantly more than bottle-fed animals throughout the chronic phase. Statistical analysis revealed significant increases in intake from DR 1 to DR 2 for the after and saline cannula-fed groups, and the before, after, and saline bottle-fed groups. All groups showed an increase in intake from DR 1 to DR 2, though the increase of the before cannula-fed group failed to reach statistical significance.

The purpose of this experiment was to determine if tolerance to cocaine-induced hypophagia involves learning to suppress stereotyped movements that interfere with feeding. Milk intake and motor activity were measured in rats fed via bottles or intraoral cannulas. On dose-response 1, the bottle group was more hypophagic than the cannula group at the 8, 16, and 32 mg/kg doses. After 60d of chronic cocaine (16 mg/kg), only the bottle-fed group showed tolerance, indicated by a shift to the right on dose-response 2. Tolerance was accompanied by a decrease in activity, while activity in the cannula-fed groups given 16 or 32 mg/kg showed sensitization. These results suggest that moderate doses of cocaine interfere with feeding primarily by producing incompatible behaviors. Tolerance involves learning to inhibit these behaviors in order to feed.

Projections of the nucleus reticularis pontis oralis (RPO), the pedunculopontine tegmental nucleus (PPT) and the median raphe nucleus (MR) were examined using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). The RPO projected strongly throughout the reticular core, to the PPT and to the intralaminar thalamic nuclei. Light projections were observed in the posterior hypothalamus but not the supramammillary nucleus. The pattern of labeling suggests that the RPO is arranged topographically in longitudinal columns. The PPT projected to the RPO, MR, medial thalamic and intralaminar nuclei, the supramammillary nucleus and septum. The MR was found to project heavily to the supramammillary nucleus, intralaminar thalamic nuclei, the septum, the hippocampus and several cortical areas. The results are discussed in terms of the modulation of hippocampal EEG and a model is introduced emphasizing interactions among ascending hippocampal EEG synchronizing and desynchronizing systems.

Mammals, unlike lower vertebrates, cannot normally regenerate injured central nervous system neurons. Although rat retinal ganglion cells (RGCs), following optic nerve crush, will undergo an initial period of sprouting, axon outgrowth is limited and subsequently aborted. This study examined how extensive the changes in fast transported proteins (FTPs) were during the early response to RGC damage and whether these changes were comparable to those known to occur in lower vertebrate RGCs. Changes in mRNA for several known proteins were also analyzed. It was found that, within 2 days, axotomized rat RGCs initiated a program of cell growth, involving the differential synthesis and transport of a broad range of FTPs. This response is very similar to that of lower vertebrates and indicates that rat RGCs are capable of initiating the metabolic responses necessary for regeneration to begin. This response, however, was not sustained beyond 5 days axotomy.

An experiment was conducted to examine whether cannula-fed rats could learn to suppress amphetamine-induced head movements when milk infusion was contingent upon suppression of these stereotyped movements. To test this hypothesis, rats in two cannula-fed conditions, Cannula/No Lick and Cannula/Lick, received injections of amphetamine (2.0 mg/kg) for 42 consecutive days. Pilot data had suggested that suppression of these movements was unlikely to occur unless some other stereotyped behavior (e.g., licking) could be made, hence the availability of a drinking tube in one of the conditions. Although neither group recovered to baseline intake levels, milk intakes measured over 6 consecutive weeks revealed that both groups had recovered from the initial hypophagic effect of amphetamine and had learned to suppress stereotyped head movements in order to receive milk infusion. There was no significant difference between the intakes of the two groups. These findings suggest that instrumental learning may be an appropriate model to describe the development of tolerance to amphetamine-induced stereotyped movements. They also imply that the channeling of one form of stereotyped head movement (e.g., head scanning) into another form (e.g., licking) is not necessary for tolerance to occur.