Dopaminergic behaviors and signal transduction mediated through adenylate cyclase and phospholipase C pathways

Abstract

We determined the relative effects of chemical receptor inactivation on dopaminergic signaling through adenylate cyclase and phospholipase C pathways and evaluated the behavioral implications of such receptor manipulations. Groups of rats were given intraperitoneal injections of 10 mg/kg N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), a reagent that differentially inactivates neurotransmitter receptors. Control and treated animals were used to assess dopaminergic-mediated behaviors or brain tissues were prepared from the animals and used to assay D₁-like receptor binding and agonist-stimulated second messenger formation. EEDQ decreased by 75% the number of D₁-like binding sites and completely abolished dopamine-stimulated cyclic AMP formation in striatal membranes. Conversely, dopamine-stimulated phosphoinositide hydrolysis was insensitive to inactivation by EEDQ as examined over different durations of EEDQ treatment, in different brain regions, or with different concentrations of the D₁-like receptor agonist SKF38393. EEDQ-pretreated animals lost their stereotypic response to apomorphine but showed increased vacuous jaw movements in response to apomorphine or SKF38393. Basal catalepsy was increased and SCH23390 was unable to further enhance catalepsy beyond the basal levels in the lesioned animals. In naive animals, SCH23390 catalepsy was reversed by apomorphine, and apomorphine stereotypy was reversed by SCH23390. Taken together, the present results imply that the dopamine-sensitive phospholipase C system mediates a subset of dopaminergic behaviors, notably vacuous jaw movements, in contrast to stereotypy and catalepsy which appear to be respectively mediated through stimulation and inhibition of the adenylate cyclase-coupled dopaminergic system.

Introduction

Dopamine mediates diverse behaviors in animals and humans. The physiological actions of dopamine are mediated through at least five subtypes of dopamine receptors, most of which are coupled to the stimulation or inhibition of adenylate cyclase (AC) and consequent modulation of cellular cyclic AMP levels. More recently, substantial evidence has accumulated in support of dopamine receptor coupling to phospholipase C (PLC) leading to the stimulation of phosphoinositide hydrolysis and generation of the second messengers diacylglycerol and inositol phosphate (Felder et al., 1989, Undie and Friedman, 1990, Undie and Friedman, 1992, Martin and Waszczak, 1993, Undie et al., 1994). These dopamine-sensitive AC and PLC systems show differences in regional brain distribution (Undie and Friedman, 1990) and in agonist and antagonist sensitivity (Undie and Friedman, 1992, Undie et al., 1994), suggesting possibly differential roles in dopaminergic physiology and pharmacology. The present study was undertaken as part of our broader efforts at understanding the relative contributions of each dopamine-sensitive signaling cascade to the spectrum of dopaminergic functions and dysfunctions in the brain.

Aminergic receptors can be inactivated to various degrees by the peptide coupling agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Inactivation of brain dopamine D₁ or D₂ receptors by EEDQ has been associated with decreased receptor binding and changes in various behavioral and biochemical functions in rodents (Meller et al., 1985, Hess et al., 1987, Braestrup and Andersen, 1987, Arnt et al., 1988, Goodale et al., 1988). Not all neurotransmitter receptors are functionally sensitive to EEDQ inactivation, and even within a class of receptors subpopulations of insensitive states have been identified (Rosengarten et al., 1989, Cox and Waszczak, 1990, Rosengarten et al., 1993, Salles et al., 1993, Ogren et al., 1994, Eglen et al., 1994). The primary target of EEDQ reactivity is the carboxylate function which may be present in several amino acid residues of receptor proteins (Garvey et al., 1972, Signorini et al., 1989). Additional factors that may determine the selective susceptibility of receptor proteins to inactivation by EEDQ would include accessibility of EEDQ to the relevant aminoacyl residues on the receptor, and the significance of the susceptible amino acid residues to receptor binding of cognate ligands. Thus, among the D₁-like dopamine receptors defined by binding to the specific radioligand [³H]SCH23390, an EEDQ-insensitive subpopulation has been identified (Meller et al., 1985, Rosengarten et al., 1993). Constituting some 20–30% of D₁-like binding sites in the rat striatum, these sites have been implicated in dopamine agonist mediation of repetitive or vacuous jaw movements (VJM) in rodents (Rosengarten et al., 1993).

Several studies have indicated that EEDQ treatment causes not only a marked reduction in SCH23390 bindings sites, but also in the ability of dopamine D₁-like agonists to stimulate the production of cyclic AMP (Hess et al., 1987, Arnt et al., 1988). Thus, the retention of a dopamine D₁-like agonist-elicited behavior in rat brains having compromised dopamine agonist stimulation of cyclic AMP formation, suggests that the postreceptor signaling mechanism involved in mediation of VJM may not be cyclic AMP. To address this hypothesis, we have assessed the elicitation of VJM, catalepsy, and stereotypic behaviors by dopaminergic agents, and measured the numbers and affinities of dopamine D₁-like receptors, and D₁-like receptor coupling to second messenger systems in naive and EEDQ-treated rats. Consistent with our preliminary findings previously reported (Undie and Cseresnyes, 1996), the present results strongly implicate the dopamine-sensitive PLC-coupled signaling cascade in the dopaminergic mediation of vacuous jaw movements in the rat. The findings further suggest that stereotypy and catalepsy may be opposite behaviors mediated through stimulation and inhibition, respectively, of the dopamine-sensitive AC-mediated signaling system.