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Keywords:

  • addictive behaviour;
  • cocaine reward;
  • dopamine uptake;
  • local anaesthesia;
  • rats;
  • sodium channels

Abstract

Increasing evidence suggests that dopamine (DA) mechanisms alone cannot fully explain the psychoemotional and behavioural effects of cocaine, including its ability to induce drug-taking behaviour. Although it is known that cocaine, after intravenous administration or smoking, may reach brain levels high enough to inhibit Na+ transport, the role of this action remains unclear. To examine the contribution of local anaesthetic and DA mechanisms to changes in striatal and accumbal neuronal activity induced by cocaine, single-unit recording was combined with iontophoresis in awake, unrestrained rats. Most spontaneously active and glutamate-stimulated neurons were highly sensitive to brief cocaine applications (0–40 nA); cocaine-induced inhibitions occurred at small ejection currents (0–5 nA), were dose-dependent, highly stable during repeated applications and strongly dependent on basal activity rates. These neuronal responses remained almost unchanged after systemic administration of either a selective D1 antagonist (SCH-23390, 0.2 mg/kg) or a combination of SCH-23390 (1 mg/kg) and eticlopride (1 mg/kg), a D2 antagonist. Whereas SCH-23390 alone had a weak attenuating effect, no effect and even a slight enhancement of responses to cocaine occurred in fast-firing glutamate (GLU)-stimulated units after the combined blockade of D1 and D2 receptors. Responses to cocaine were mimicked by iontophoretic procaine (0–40 nA), a short-acting local anaesthetic with minimal effect on DA uptake. Procaine-induced inhibitions occurred at the same low currents, had a similar time-course, and were also strongly dependent on basal discharge rate. Our data support the existence of a DA-independent mechanism for the action of cocaine involving a direct interaction with Na+ channels. Although further studies are required to clarify this mechanism and its interaction with other pharmacological and behavioural variables, a direct interaction with Na+ channels may contribute to changes in neuronal activity induced by self-injected cocaine, thereby playing a role in mediating the psychoemotional and behavioural effects of this drug.