Central antiemetic actions of pimozide and haloperidol in the dog

Neuroactive substances in the dorsal vagal complex of the medulla oblongata: nucleus of the tractus solitarius, area postrema, and dorsal motor nucleus of the vagus

The distributions of classical and putative neurotransmitters within somata and fibres of the dorsal vagal complex are reviewed. The occurrence within the dorsal medulla oblongata of receptors specific for some of these substances is examined, and possible functional correlations of the specific neurochemicals with respect to their distribution within the dorsal vagal complex are discussed. Many of the known transmitters and putative transmitters are represented in the dorsal vagal complex, particularly within various subnuclei of the nucleus of the solitary tract, the main vagal afferent nucleus. In a few cases, some of these have been examined in detail, particularly with respect to their possible mediation of cardiovascular or gastrointestinal functions. For example, the catecholamines, substance P and angiotensin II in the nucleus of the solitary tract have all been strongly implicated as playing a role in the central control of cardiovascular function. Other neurotransmitters or putative transmitters may be involved as well, but probably to a lesser extent. Similarly, the roles in the dorsal vagal complex of dopamine, the endorphins and cholecystokinin in control of the gut have been studied in some detail. Future investigations of the distributions of and electrophysiological parameters of neurotransmitters at the cellular level should provide much needed clues to advance our knowledge of the correlations between anatomical distributions of specific neurochemicals and physiological functions mediated by them.

Characteristics of the dopamine receptors involved in the anorectic effects of apomorphine in mice

In food-deprived mice apomorphine injected SC induced a brief (15-30 min) dose-dependent (30-150 micrograms/kg) reduction in food intake. This effect occurred in naive mice as well as in mice habituated to a food deprivation procedure. The anorectic effect of apomorphine (150 micrograms/kg SC) was antagonized by sulpiride (ID50 = 8.6 mg/kg) and by haloperidol (ID50 = 66 micrograms/kg) but domperidone was ineffective (250 micrograms/kg). Mice submitted to a semi-chronic (6 d) blockade of dopamine receptors by haloperidol or injected intracerebroventricularly with 125 micrograms 6-hydroxydopamine 21 d before testing failed to develop a hypersensitivity to the anorectic effect of apomorphine (60 micrograms/kg). Although a single apomorphine injection (5 mg/kg) induced tolerance to the hypothermic effect of a second apomorphine injection of 150 micrograms/kg, it did not modify the anorectic effect. Repeated apomorphine injection (5 x 5 mg/kg) resulted in a slight but significant reduction in apomorphine-induced anorexia. A similarly significant reduction was not observed in mice submitted to repeated injections of dexamphetamine (5 x 5 mg/kg).

Gastric relaxation and vomiting by apomorphine, morphine and fentanyl in the conscious dog

Apomorphine (0.03 mg . kg-1 s.c.) and morphine (0.5 mg . kg-1 s.c.) produced gastric relaxation and vomiting in the conscious dog. Domperidone (0.1-1 mg . kg-1 i.v.), haloperidol (0.1 mg . kg-1 i.v.) and pimozide (0.025 mg . kg-1 i.v.) selectively blocked the gastric relaxation as well as the vomiting caused by apomorphine. Naloxone (0.07 mg . kg-1 i.v.) selectively blocked the gastric relaxation and the vomiting caused by morphine; after naloxone, morphine produced a delayed gastric relaxation in 2 experiments out of 7. These results suggest that for gastric relaxation as well as for vomiting, apomorphine and morphine act on different receptors. Fentanyl elicited marked gastric relaxation, blocked by naloxone, but did not elicit vomiting. After fentanyl, morphine and apomorphine no longer produced gastric relaxation and vomiting. This observation shows that the known blocking effect of fentanyl at the vomiting center does not affect its gastric relaxing effect.