Benzodiazepines decrease norepinephrine release from rat pineal nerves by acting on peripheral type binding sites.

In 3 000g-supernatants of rat pineal homogenates a single population of benzodiazepine (BZP) binding sites with dissociation constant= 97-102 nM and maximal number of sites= 6.5-9 pmoles/mg protein was detected by employing 3H-flunitrazepam (FNZP) as a radioligand. The following order of affinity for several BZP was found (Ki, nM): Ro 5-4864 (8), FNZP (99), clonazepam (7,900) Ro 15-1788 (10,000). Two weeks after bilateral superior cervical ganglionectomy (SCGx) a 18-28% reduction of site number without significant changes in affinity of 3H-FNZP binding was detectable in rat pineal glands. In pineal explants priorly incubated with 3H-norepinephrine, exposure to 0.1-10 microM of Ro 5-4864 or diazepam decreased significantly transmitter release elicited by 80 mM K+, whereas clonazepam did not affect it significantly. At 10 microM-concentrations, Ro 5-4864, diazepam or clonazepam increased pineal melatonin content of explants incubated for 6 h with the drug. In pineal explants of rats subjected to SCGx 14 days earlier, only 10 microM of Ro 5-4864 increased melatonin content significantly to about half of the percent increase detected in innervated glands. These results suggest that BZP decrease transmitter release from pineal sympathetic nerves by acting on peripheral BZP binding sites, an effect which is about 2 orders of magnitude greater than the postsynaptic stimulation of pineal melatonin synthesis.