Electrophysiological identification of a pathway from the septal area to the medial amygdala: sensitivity to estrogen and luteinizing hormone-releasing hormone.

Medial amygdala neurons responsive to electrical stimulation of the medial septal area were electrophysiologically identified in ovariectomized, urethane-anesthetized female rats. Peristimulus time histograms were collected and used to define the orthodromic response. The action of iontophoretically applied luteinizing hormone-releasing hormone (LHRH) and an LHRH fragment, Ac-LHRH5-10, on the activity of orthodromically responsive neurons was tested. Of a total of 187 neurons recorded, 119 were identified as orthodromically responsive. Three types of orthodromic responses were observed: excitatory, inhibitory, and complex. Priming the animals with 5 micrograms estradiol benzoate (EB) 48 hr prior to recording had no effect on the overall number of neurons responding to septal area stimulation, but EB priming did significantly reduce the percentage of orthodromically excited neurons. The firing rate of the majority of amygdala neurons responsive to septal area stimulation was not affected by iontophoretically applied LHRH (59 of 76) or LHRH fragment (41 of 65). In some cases, application of LHRH (10 of 76) or Ac-LHRH5-10 (12 of 55) produced a change in neuronal firing that was similar in direction to the orthodromically evoked response. When applied during the collection of peristimulus time histograms, both peptides were also able to modulate the orthodromically evoked response (five of 18 cells tested with LHRH and three of 14 cells tested with Ac-LHRH5-10). The results demonstrate a large projection from the septal area to the amygdala, one component of which is altered by estrogen priming.(ABSTRACT TRUNCATED AT 250 WORDS)