Conditioned response

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.

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.

Infant rats display differences in duration of loss-of-righting (LOR) in response to an hypnotic dose of morphine sulfate. These differences in LOR duration are predictive of the rats' preference for drinking morphine solutions as adults. Infants tested at 16 days of age were designated Short-, Medium-, or Long-sleep based upon a 2.5 mg/kg dose of morphine sulfate administered intraperitoneally. Infants displaying long durations of LOR (long-sleep) subsequently display a marked preference for morphine solutions when tested as adults. Conversely, infants that displayed little or no LOR (short-sleep) did not consume as much of the morphine solutions in subsequent testing. This effect was consistent across the animals tested and appears to be independent of the screening dose. These findings demonstrate that adult differences in preference for drinking morphine solutions can be predicted in infancy.

To investigate dissociated (state dependent) learning
of a classically conditioned heart rate response, 24 rats
were trained either normally or under cortical spreading
depression (CSD). Testing in extinction for experimentals
was carried out over two days with Ss experiencing a reversal
of state (i.e., depressed or non-depressed) in the first
extinction and then returned to the learning state on the
second extinction. Results indicated that all groups learned
in a comparable fashion. Experimental groups demonstrated no
retention of the learned response on the first extinction,
but extinguished normally on the second extinction when Ss
were returned to the cortical state present during training.
These results are consistent with an explanation of symmetrical
dissociation. Control ~s which experienced the same
state over all days demonstrated a predicted lack of dissociation.
The theory of subcortical state specific retrieval
was proposed to account for these data as well as results of
studies reporting dissociation with other agents to relate
dissociation phenomena to a single underlying process.