Differential recruitment of N-, P- and Q-type voltage-operated calcium channels in striatal dopamine release evoked by ‘regular’ and ‘burst’ firing

Abstract

This study used the peptides ω-conotoxin GVIA, ω-agatoxin IVA and ω-conotoxin MVIIC, singly and in combination, to investigate the relative involvement of N-, P- and Q-type voltage-operated calcium channels in the control of striatal dopamine release. Electrically stimulated dopamine release was measured by fast cyclic voltammetry at carbon fibre microelectrodes in rat striatal slices. The contribution of these channel subtypes was compared in dorsolateral and medial neostriatum for ‘regular’ (discrete) and ‘burst’ stimulation modalities. In dorsolateral neostriatum, a role for N-, P- and Q-type channels was demonstrated for discrete stimulations, whilst at least one other unidentified channel was also involved in dopamine release on ‘burst’ stimulations. Similarly, in the medial axis of the neostriatum, N-, P- and Q-type channels were involved in dopamine release for discrete stimulations, and N-, Q- and at least one other channel type for ‘burst’ stimulations. However, blockade of P-type channels had no effect on dopamine release for ‘burst’ stimulations in the medial axis. In both regions and stimulation paradigms, N-type channels played a greater role than P/Q-type channels. In the medial axis of the neostriatum there was a smaller contribution by N- and P-type channels and the unidentified component, but a greater Q-type contribution to DA release. ‘Burst’ stimulations induced a lesser involvement of N- and P-type channels than discrete stimulations, and a greater role of the unidentified component. In summary, this study suggests that there is heterogeneity in the distribution of functional voltage-operated calcium channel subtypes in the neostriatum, and differences in subtype recruitment for different firing patterns.

Introduction

Chemical neurotransmission occurs by exocytosis of synaptic vesicles, triggered by a rise in cytosolic calcium, to expel transmitters into the extracellular compartment [17]. Under physiological conditions in neurones, this is achieved by entry of extracellular calcium through membrane-bound voltage-operated calcium channels (VOCCs), activated when an action potential invades a neurotransmitter release site.

VOCCs are subdivided based on their biophysical and pharmacological properties [2]. At present L-, N-, P-, Q-, R- and T-type channels have all been characterised, and O-type channels [1] have also been proposed.

T-type channels are activated at potentials between −65 and −50 mV, show voltage-dependent inactivation during maintained depolarisation and deactivate relatively slowly upon repolarisation [31]. Conversely, L-, N-, P-, Q- and R-type channels are all activated at approximately −20 mV. They differ in their pharmacology and their kinetics of inactivation during both maintained depolarisation and deactivation after repolarisation: R-type channels are the fastest to deactivate, followed in order by Q-, N-, L- and P-type channels.

In the past decade or so, the principal advances in the pharmacology of VOCC subtypes have come through the purification and characterisation of peptide toxins from invertebrate venoms. In particular, peptides from the predatory marine snails, Conus geographus and Conus magus (conotoxins) and the funnel web spider, Agelenopsis aperta (agatoxins) have shown specificity for VOCC subtypes [21].

Previous studies have addressed the involvement of voltage-operated calcium channel subtypes in dopamine (DA) release in the neostriatum (CPu). Typically, DA release was evoked by high concentrations of potassium [14], [18] or long electrical trains [8], [14]. Some of these studies also relied on measurement of [³H]-DA [8], [32]. With the use of real-time electrochemistry, endogenous DA can be measured on-line for relatively subtle electrical stimulations, and recordings can be carried out in discrete anatomical subregions of the CPu [27], [28].

The different high-voltage-activated VOCC subtypes differ in their deactivation kinetics, but the necessity for multiple subtypes controlling neurotransmitter release is rarely addressed. A possible role for this diversity could be to allow processing of patterned firing activity. Dopaminergic neurones are known to exhibit patterns of ‘regular’ and high frequency ‘burst’ firing [22] whereby action potentials are generated either singly or in bursts of up to 20 [9]. Thus, in this study, the effect of VOCC antagonists on DA efflux for discrete single-pulse electrical stimulations and 20-pulse high frequency trains were compared. Single-pulse stimulations were considered to equate to ‘regular’ neuronal firing, whereas 20-pulse stimulations approximate ‘bursts’ of electrical activity.

Several groups have also described regional heterogeneity of striatal DA release [5], [16], [23], where DA release on ‘burst’ stimulation is smallest in dorsolateral CPu, and greatest along the medial axis of the CPu. In part, it is thought that these differences may be a reflection of the different dopaminergic afferents to the two regions: unlike the dorsolateral quadrant of the striatum whose input is almost solely from the A9 cell group, the medial axis of the neostriatum is also innervated by the ventral tegmental area (A10) [10]. This study investigated the effects of VOCC antagonists on DA release in these two striatal regions to test whether these differences could be implemented through the use of different VOCC subtypes.