FUTURE WORK

Further work would begin with testing o f the above hypothesis to determine if anxiogenic and anxiolytic agents could modify the net efflux in the predicted ways. First, experiments would be carried out to test if the effects o f yohimbine and FG7142, on the noradrenergic response to novelty, could be reversed by pretreatment with anxiolytic agents. From experiments so far, it would be predicted that pretreatment with flurazepam before exposure to a novel environment would reverse the actions o f both FG7142 and yohimbine. That is, flurazepam would attenuate the reduction in net efflux caused by treatment with yohimbine while, conversely, it would attenuate the increase in net efflux caused by FG7142. This prediction is amenable to investigation using the procedures described in the previous chapters o f this thesis. If flurazepam reversed the effects o f both yohimbine and FG7142 on noradrenaline efflux further compounds could

be tested to build a profile o f results using established anxiolytic and anxiogenic agents

{e.g. buspirone, flurazepam, Pentylenetetrazole).

In addition to investigating the effects o f anxiolytic and anxiogenic agents, the effects o f various a2-adrenoceptor antagonists should be examined. This would determine whether these agents show the same neurochemical profile as yohimbine: that is a decrease in net efflux. This could have important implications on mechanisms underlying anxiety especially if other a2-adrenoceptor antagonists showed a different profile from the anxiogenic agent, yohimbine. It would suggest that either the a2~

adrenoceptor was not involved in anxiety, or that yohimbine showed a specific binding profile at the a2-adrenoceptor subtypes. In fact yohimbine has highest affinity at the ^2A’ ^ 2B ^2C -adrenoceptor subtypes with less affinity for the « 2 0 subtype (Bylund

et al., 1991). The effect of the atypical anxiolytic agent, buspirone, which is metabolized to an Œ2-adrenoceptor antagonist 1-(2-pyrimidinyl)-piperazine (1-PP), has previously been investigated in the novel environment. Dailey & colleagues (1996) found that, buspirone and diazepam did not alter the net efflux o f noradrenaline during exposure to novelty, although buspirone increased the underlying spontaneous efflux o f noradrenaline. This would agree with the predicted response o f anxiolytic agents on noradrenaline efflux during exposure to a novel environment: that is they alter the net efflux o f noradrenaline to the level required for an optimum coping response. In this case, the optimum net efflux was the same as the response achieved without the anxiolytic agent (i.e. with the stress alone). This suggests that the animal is producing an adequate coping response without the intervention o f drugs, probably due to the nature o f the environmental stress. Since buspirone and diazepam modified the underlying efflux o f noradrenaline in different ways, but both caused the same response to stress, this suggests that the stress response {i.e. net efflux) is independent o f the baseline. A key feature o f this work is that studies o f neurochemical changes are carried out under the same conditions as those used to evaluate drug effects on behaviour in animal models. In this respect, the experiments can be extended to include additional

procedures used to study the effects o f anxiolytic and anxiogenic drugs on behaviour. These include the social interaction test and the plus maze.

A long-term aim would be to compare the behaviour o f individual animals during tests o f anxiety with their neurochemical response to these stressors, i.e. to study behaviour and neurochemistry in the same subjects. However, additional refinement o f the microdialysis technique, with shorter sampling times, would be preferred. This could give some insight into the correlation of behaviour, if any, with changes in the efflux o f noradrenaline. As has been shown already, the relationship between behaviour and neurochemistry within a group o f individuals is not always the same across different subject groups (Stanford & Salmon, 1992).

Finally, the efflux o f dopamine under different test conditions within the separate shell and core zones o f the nucleus accumbens should be investigated. First, it would be interesting to compare the response to flurazepam in the shell and core region o f the nucleus accumbens to confirm that the lack o f effect o f this drug in the present study was, as speculated, due to the position o f the microdialysis probe within the core zone. If a regional variation in response to flurazepam is found then it would also be valuable to determine whether the same distinctive pattern o f responses are found during stress. Again, these studies should concentrate on the types o f stressful stimuli used in animal models o f anxiety.

Neurochemical studies using anxiolytic and anxiogenic agents in combination with the types o f stress used in animal models o f anxiety would allow more direct comparisons with behavioural studies in these models. This may give insight into the role o f the noradrenergic system in stress and anxiety.

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