Dopamine receptor blockade by neuroleptic drugs in Aplysia neurones

Targeting the dopamine D2 receptor in schizophrenia

After a 12-year search for the antipsychotic receptor, the binding site was discovered and labelled by [3H]haloperidol in 1975. Of the various neurotransmitters, dopamine was the most potent in inhibiting the binding of [3H]haloperidol, indicating that the antipsychotic receptor was a dopamine receptor, now named the dopamine D2 receptor, a major targeting site in schizophrenia. All antipsychotic drugs, including traditional and newer antipsychotics, either bind to D2 in direct relation to their clinical potencies or hinder normal dopamine neurotransmission, as in the case of partial dopamine agonists. In fact, the antipsychotic concentrations found in the plasma water of treated patients closely match the predicted therapeutic absolute concentrations, adjusted for the 60-75% D2 occupancy needed for clinical efficacy. Antipsychotics that elicit low or no Parkinsonism or prolactinaemia are loosely attached to D2 and rapidly dissociate from D2, whereas those eliciting Parkinsonism stay tightly attached to D2 for many hours. Because animal models of psychosis (amfetamine sensitisation, brain lesions) all show a marked elevation in the number of high-affinity states of D2, the antipsychotics are thought to specifically target these D2High states in psychosis in general and schizophrenia in particular.

Effects of haloperidol and phentolamine on the crustacean cardiac ganglion

Haloperidol (a dopamine D2 blocker in vertebrates) and phentolamine (an alpha-adrenergic blocker) alter the pattern of bursting by the isolated cardiac ganglion of the lobster when perfused at concentrations of 10(-6)-10(-5) mol/l. Both drugs decrease the frequency of bursting and increase burst duration. They are most effective in slowing the ganglion when applied selectively to the anterior ganglionic trunk, the same region of the ganglion where dopamine (DA) and 5-hydroxytryptamine (5HT) are most effective in speeding up bursting. When exogenous monoamine transmitters are applied in the presence of 3x10(-6) mol/l haloperidol, the effect of 5HT, but not of DA, is significantly reduced. At the same concentration, phentolamine does not suppress the actions of DA, 5HT or noradrenaline (NA). Both haloperidol and phentolamine significantly alter the properties of endogenous burst-organizing potentials (driver potentials) generated by motorneurons in the ganglion. It is possible that the effects of these drugs on bursting reflect alteration of endogenous electrical properties of the constituent neurons, rather than receptor antagonism.

The effect of dopamine receptor blockade on motor behavior in Aplysia californica

The mammalian D1- and D2-like receptor blockers SCH-23390 and raclopride were used to block receptors in Aplysia californica, and the effect on reflexes and escape behavior was examined. Four groups of 20 young adults were each injected with SCH-23390, raclopride, SCH-23390+raclopride, or seawater. The drug (0.0125 mg/g of body weight) was injected 2 mm anterior to the parapodia. After the injection of either SCH-23390 or SCH-23390+raclopride, there was a significant increase in parapodia opening (P<.001), siphon withdrawal (P<.05), and galloping following tail pinch (P<.01) compared to raclopride-injected or control animals. The data showed that blockade of receptors by SCH-23390, but not raclopride, produced significant changes in motor behavior in A. californica.

Pre- and postsynaptic effects of dopamine antagonists on dopaminergic synaptic transmission in Helix pomatia

The rate of transmitter mobilization in identified dopaminergic synapses was decreased by the dopamine antagonists pimozide, chlorpromazine, haloperidol, cis-flupenthixol, curare, clozapine and high concentrations of ergometrine. The depolarizing postsynaptic potential was inhibited by pimozide, chlorpromazine, haloperidol, cis-flupenthixol, curare, clozapine, (+)-butaclamol and high concentrations of ergometrine. The hyperpolarizing synaptic potential was inhibited by naloxone, methysergide, (+)-butaclamol, haloperidol, 6-hydroxydopamine and low concentrations of ergometrine, while pimozide, cis-flupenthixol, trans-flupenthixol, curare, clozapine, promethazine, chlorpromazine and (-)-butaclamol had no clear effect. The presynaptic receptors involved in modulation of the mobilization rate showed similarities with the dopamine receptors mediating depolarizations. The dopamine antagonists changed dynamics of synaptic transmission.

Postsynaptic pharmacology of psychoactive substances in Aplysia californica

The effects of antidepressants and neuroleptics on postsynaptic iontophoretic responses to acetylcholine and dopamine have been examined in identified neurons of the central nervous system of Aplysia californica. The results demonstrate an antidopaminergic action of neuroleptics in accordance with in vivo and in vitro results in higher animals. Some of the antidepressant drugs tested also caused a blockade of dopamine hyperpolarizing responses. The incidence of anticholinergic activity of both antidepressants and neuroleptics also correlates well with other systems. The results furthermore suggest that the psychoactive drugs tested behave as antagonists for the neurotransmitter postsynaptic receptor sites.

Separate serotonin and dopamine receptors modulate the duration of post-tetanic potentiation at an Aplysia synapse without affecting other aspects of synaptic transmission

We have studied the effect of the biogenic amines, serotonin and dopamine, on post-tetanic potentiation (PTP) at an identified synapse in the abdominal ganglion of Aplysia californica. We found that: (1) 10(-7) M perfused serotonin doubles the rate constant of decay of PTP. The effect is specific in that neither the size of the non-potentiated (isolated) EPSP nor the amplitude of PTP is affected. As reported previously, higher doses of serotonin will also increase the amplitude of PTP and decrease the size of the isolated EPSP; (2) 5 X 10(-7) M dopamine in the perfusate increases the rate constant of decay of PTP by about 50%. The effect is also specific in that neither PTP amplitude nor the size of the isolated EPSP is affected; (3) SQ10,631, a serotonin antagonist, blocks the effect of perfused serotonin on PTP decay rate. It does not antagonize the dopamine effect. SQ10,631 also slows the endogenous decay of PTP in some preparations which exhibit an unusually fast PTP decay rate, suggesting a naturally occurring source of serotonin within the ganglion capable of affecting the rate constant of PTP decay; (4) (+)-butaclamol, a dopamine antagonist, blocks the effect of dopamine on the rate constant of PTP decay, whereas (-)-butaclamol has little effect. Butaclamol does not block the effect of serotonin on the rate constant of PTP decay; (5) phosphodiesterase inhibitors potentiate the effect of serotonin on the rate constant of PTP decay, and cyclic AMP analogues mimic the effect of the biogenic amines, suggesting that the aminergic modulation of the rate of decay of PTP is coupled with activation of adenylate cyclase and accumulation of cyclic AMP; and (6) the evidence presented is consistent with the hypothesis that serotonin and dopamine are capable of specifically modifying the rate of change in the efficacy of transmitter release which underlies PTP. It also suggests that the two biogenic amines operate separately and in parallel via presynaptic receptor mechanisms.

Dopamine, serotonin and related compounds: presynaptic effects on synaptic depression, frequency facilitation, and post-tetanic potentiation at a synapse in Aplysia californica

Dopamine, serotonin and related compounds (referred to collectively as biogenic amines) were found to modify transmission at the presumably cholinergic synapse made by an axon in the right visceropleural connective onto cell R15 of the abdominal ganglion of Aplysia californica. (1) With chronic application, dopamine hyperpolarizes R15, and serotonin depolarizes R15. Both actions upon the membrane potential desensitize in 10 min. All the actions described below were studied with chronic perfusion of the biogenic amines after desensitization of this postsynaptic action. (2) The biogenic amines drastically reduce the size of the EPSP evoked at the synapse under investigation; but they do not alter the ACh potential evoked in the soma of R15. (3) The biogenic amines reduce the amplitude of synaptic depression. The relationship between the effects of the amines on the size of an isolated EPSP and on synaptic depression differed from this relationship as affected by post-tetanic potentiation (PTP) or by changes in the Ca2+-Mg2+ balance. (4) The biogenic amines increase frequency facilitation, when the latter is defined as the ratio of the facilitated to the isolated EPSP. However, the absolute magnitude of the facilitated EPSP is always reduced at long times after introduction of the agent; shortly after introduction of the biogenic amines the absolute magnitude of the facilitated EPSP is unaffected in most preparations.