THE INHIBITION OF PROESTROUS LH SURGE AND OVULATION IN RATS BEARING LESIONS OF THE DORSAL RAPHE NUCLEUS IS MEDIATED BY THE LOCUS COERULEUS

Lesions of the dorsal raphe nucleus (DRn) were shown to block the proestrous surge of LH and the shed of ova normally found the following morning. The possibility that this effect could be mediated by the locus coeruleus (LC) was studied in the present work in view of the fact that stimulation of the LC produced similar responses on LH release and that systemic injection of the .beta.-adrenergic blocking agent, propranolol, partially prevented the effect of DRn lesions. The blocking effect of DRn lesions on proestrous LH surge and ovulation was reproduced by the injection of either methysergide (1 .mu.g on the day of proestrus), a serotonin (5-HT) blocking agent, or p-chlorophenylalanine (0.5 .mu.g on diestrus 1 and diestrus 2), an inhibitor of 5-HT synthesis, into the LC or normal cycling rats. On the contrary, the injection of 5-HT (1 .mu.g on the day of proestrus) into the LC of rats bearing DRn lesions restored the normal LH surge and ovulation. It is concluded that serotonergic afferents into the LC from the DRn tonically inhibit the activity of LC neurons and that the interference of this inflow results in an activation of neurons and thereby in inhibition of LH release.