Sherrel G. Howard - US grants

Previous work on this project demonstrated that Phencyclidine (PCP): (1) has a biphasic dose response curve with respect to dopamine (DA) release, and (2) uncouples the cholinergic system in the caudate nucleus, an effect that is probably secondary to the DA response, and (3) that behaviorally active metabolites have no effect on the cholinergic system, but do effect DA efflux. On the basis of these neurochemical and behavioral findings, we are hypothesizing that PCP has two effects on the presynaptic DA terminal and that the two responses are the result of two different actions of PCP. One effect is specific (at low doses of PCP) releasing DA and the second, non-specific (at high doses) inhibiting release of DA. In this proposal, we will attempt to separate the biphasic effects of PCP using congeners which have been demonstrated to bind to PCP receptors and to elicit PCP-like behavior. We are proposing to use Ketamine (KET) and N-ethylphenylcyclohexylamine (PCE) as PCP agonists to separate the increase and decrease in DA efflux by PCP. PCE has a higher affinity for the receptor, whereas KET has low binding properties. The dose response of these PCP congeners will be examined in neurochemical, behavioral and voltammetric protocols in an effort to separate the two DA efflux responses. If this biphasic response is resolvable into two components, each component can be characterized pharmacologically in terms of agonist and antagonist. The protocols that would be developed from this study could then be used to evaluate potential antagonists, from which a specific antagonist could be identified and synthesized. The project is divided into four components: 1) separating the biphasic response of DA efflux to PCP; 2 and 3) Characterizing the biphasic effect with pharmacological antagonists, selected on the basis of their site action on the DA neuron; D) determining the effect of prenatal exposure to PCP on postnatal neurochemical development.

The primary objectives are to analyze the dynamic aspects of dopaminergic and cholinergic neurotransmitter activity (synthesis and release), the effects of drugs on this activity during development, and the interaction between the dopaminergic and cholinergic systems in the neostriatum of the developing rat pup. The specific aims are: 1) to characterize the developmental neurochemistry of the striatal dopamine system in the rat, with respect to neurotransmitter synthesis and release, and to determine at what point in the course of development the dopaminergic system is functionally active; 2) to determine the mechanism by which amphetamine alters the release of dopamine in the developing rat pup; and 3) to examine the effect of the dopaminergic system on acetylcholine synthesis and release in the neostriatum of the developing rat pup. Drugs which are used clinically in the treatment of hyperkinetic children or Attention Deficit Disorder in children, such as amphetamine, decrease the release of striatal dopamine at early stages of development in the rat, when compared with the adult response. Using pharmacological manipulations and in vivo electrochemical techniques, we will determine the mechanism by which this paradoxical alteration in release, by amphtamine, occurs at this time in development. It has been shown that the dopaminergic and cholinergic systems interact in the neostriatum. However, it is not known whether these systems interact during development and, if so, the degree of this interaction. In the experiments proposed here, we will determine the influence of dopamine release on the cholinergic system in the neostriatum of the developing rat pup. These studies will provide information as to the development of synthesis and release in the dopaminergic and cholinergic system, the effects of drugs on this activity during development, and the degree of interaction between the dopaminergic and cholinergic systems in the neostriatum during development.