Sex differences in relation to conditioned fear-induced enhancement of morphine analgesia.

A number of studies have reported that both the immediate and proactive effects of exposure to a shock stressor are less pronounced in female than in male rats. A separate area of research has demonstrated that female rats are less sensitive to the analgesic effects of morphine than males. Experiments from our laboratory, as well as others, have found that exposure to a context associated with shock (i.e., conditioned fear context) at the time of morphine administration, enhances the analgesic effects of morphine. Since previous studies have exclusively employed male rats, the purpose of Experiment 1 was to determine if a sex difference exists to this context conditioned fear-induced enhancement of morphine-induced analgesia. The findings of Experiment 1 showed that females do not appear to exhibit conditioned fear-induced enhancement of morphine analgesia as compared to males. Experiment 2 demonstrated that females exhibited higher levels of conditioned fear-induced enhancement of morphine analgesia during diestrus I than estrous. Experiment 3 demonstrated that females exhibited lower levels of conditioned analgesia compared to males, while both groups exhibited similar freezing levels. The findings of the present experiments suggest that the sex difference observed in Experiment 1 may be due to differences in conditioned analgesia.

Dishabituation of the Tritonia escape swim.

When repeatedly elicited, the oscillatory escape swim of the marine mollusc Tritonia diomedea undergoes habituation of the number of cycles per swim. Although similar in most respects to habituation observed in vertebrates and other invertebrates, one key feature, dishabituation, has been surprisingly difficult to demonstrate. Here we evaluate the hypothesis that this is due to interference from short-term sensitization, which is manifested as a reduction in swim onset latency, that occurs simultaneously during habituation training. Robust dishabituation was obtained using a multisession habituation protocol designed to allow this sensitization to dissipate before the dishabituatory stimulus was applied. These results extend the similarity of habituation in Tritonia to that described in other species, strengthening the usefulness of this preparation as a model system for studies of the cellular basis of habituation.

Prepulse inhibition of the Tritonia escape swim.

Presenting a weak stimulus just before a strong, startle stimulus reduces the amplitude of the ensuing startle response in humans and other vertebrates. This phenomenon, termed "prepulse inhibition" (PPI), appears to function to reduce distraction while processing sensory input. To date, no detailed neural mechanism has been described for PPI. Here we demonstrate PPI in the marine mollusk Tritonia diomedea, which has a nervous system highly suitable for cellular analyses. We found that a 100 msec vibrotactile prepulse prevented the animal's escape swim response to a closely following 1 sec tail shock. This inhibition was highly transient, with a significant effect lasting just 2.5 sec. These findings indicate that the Tritonia escape swim response undergoes a form of PPI phenomenologically similar to that observed in vertebrates. Further tests showed that the vibrotactile stimulus had no inhibitory effect if applied after tail shock, while the animal was preparing to swim, but it acted to terminate swims once they were actively under way. As a first step toward a cellular analysis of PPI, we recorded from neurons of the swim circuit in a semi-intact preparation and found that the vibrotactile stimulus used in the behavioral experiments also prevented the tail shock-elicited swim motor program. These results represent the first explicit demonstration of PPI in an invertebrate and establish Tritonia as a model system for analyzing its physiological basis.

Sensitization of the Tritonia escape swim.

When repeatedly elicited, the oscillatory escape swim of the marine mollusc Tritonia diomedea undergoes habituation of the number of cycles per swim. Previous work has shown that this habituation is accompanied by sensitization of another feature of the behavior: latency to swim onset. Here we focused on the behavioral features of sensitization itself. Test swims elicited 5 min after a strong sensitizing head stimulus differed in several ways from control swims: sensitized animals had shorter latencies for gill and rhinophore withdrawal, a shorter latency for swim onset, a lower threshold for swim initiation, and an increased number of cycles per swim. Sensitized animals did not, however, swim any faster (no change in cycle period). A separate experiment found that swim onset latency also sensitized when Tritonia came into contact with one of their natural predators, the seastar Pycnopodia helianthoides, demonstrating the ecological relevance of this form of nonassociative learning. These results define the set of behavioral changes to be explained by cellular studies of sensitization in Tritonia.

Enhancement of morphine analgesia in rats following removal from contextual conditioned fear cues.

1. Previous studies have shown that morphine analgesia is enhanced when analgesia testing is conducted in an environment that has been previously paired with shock, but not in a novel or neutral environment. 2. Two experiments were conducted to assess if enhanced morphine analgesia could be demonstrated in a neutral context if rats were first exposed to conditioned fear cues. This was done by pre-exposing rats to a context previously paired with shock and testing for enhanced morphine analgesia in a neutral context immediately following removal from the conditioned fear context. To determine if conditioned analgesia contributed to the enhanced morphine analgesia, rats were tested for analgesic responsiveness immediately following removal from conditioned fear cues, prior to morphine administration. 3. In Experiment 1, although conditioned analgesia was not observed, a small enhancement of morphine analgesia was demonstrated in an neutral context in rats pre-exposed to conditioned fear cues, compared to non-conditioned controls. 4. In Experiment 2, which employed more sensitive test procedures, a strong enhancement of morphine analgesia was observed in a neutral context only in those rats that demonstrated conditioned analgesia.

Pavlovian aversive context conditioning using carbon dioxide as the unconditional stimulus.

Four experiments were conducted to examine the utility of carbon dioxide (CO2) as an aversive unconditioned stimulus (US) in a Pavlovian context conditioning paradigm. Experiment 1 demonstrated that rats exposed to CO2 in a distinctive context showed elevated levels of freezing relative to controls. Experiment 2 replicated this basic effect with a modified conditioning procedure and additionally demonstrated conditioned analgesia. Experiment 3 demonstrated a positive monotonic relationship between US duration and resistance to extinction of freezing behavior as well as conditioned analgesia. Experiment 4 demonstrated extinction and an extinction-related phenomenon, renewal. These studies clearly demonstrate the utility of CO2 as a Pavlovian US.