interspersed among non-LHRH neurons, making the possibility of successfully 
locating the cells and then recording from them remote. 
Means have been devised for monitoring LHRH secretory activity. The most 
direct method involves cannulation of the tiny portal blood vessels that carry 
LHRH released at the median eminence of the hypothalamus to the anterior 
pituitary. With this approach, during an LH surge the output of LHRH increases 
(Sarkar et al. 1976). However, the necessity for anesthesia in performing these 
studies precludes accurate quantitative assessment of changes in LHRH that 
occur in the awake state. In sheep, portal blood collection is possible in the 
awake animal, and measurements of LHRH reveal increases in LHRH release 
that accompany LH pulses in the basal state, with dramatic increases during an 
LH surge (Moenter et al. 1990). Yet the methods employed in sheep cannot be 
easily adapted to smaller mammals. An alternative has been the use of push- 
pull cannulae placed either in the median eminence or in the anterior pituitary. 
LHRH measured by push-pull techniques shows pulsatility during baseline 
states and moderate, but clear, increases in LHRH output during an LH surge 
(Levine and Ramirez 1982; Park and Ramirez 1989). Unfortunately, damage 
to the axons that are under investigation invariably accompanies the collection 
of samples. Moreover, although data from push-pull experiments convincingly 
link increases in LHRH secretion with increases in LH secretion, they tell little 
about the stimulation of LHRH neurons at a cellular level. Measurement of 
changes in IEG expression provided the first direct evidence that cellular LHRH 
activity accompanied LH surges (Hoffman et al. 1990; Lee etal. 1990a, 1990b). 
LHRH neurons normally do not express c-Fos. However, c-Fos expression is 
induced within 45 minutes following electrochemical stimulation and during 
induced or spontaneous LH surges (Hoffman et al. 1990; Lee et al. 1990a, 
1990b). An analysis of the location of stimulated LHRH neurons expressing 
c-Fos during the peak of an LH surge identifies a subpopulation of LHRH 
neurons in the preoptic area in the vicinity of the organum vasculosum of the 
lamina terminals (OVLT), below the anterior commissure, which extends into 
the anterior hypothalamus (figure 1). Interestingly, the LHRH neurons located 
above the anterior commissure and rostral to the OVLT remain “quiet” during 
an LH surge. Yet these same cells will express c-Fos after electrochemical 
stimulation. During an LH surge, the activated LHRH population may have 
been targeted by selective innervation of LHRH neurons, which, when 
contacted by catecholamine or neurotensin axons (Hoffman 1985), are 
distributed in a pattern similar to that observed for c-Fos expression in LHRH 
neurons during a surge (Hoffman et al. 1990; Lee et al. 1990a, 1990b). 
In the authors’ studies, female rats were cannulated on the morning of the 
proestrus LH surge, and blood samples were monitored throughout the day at 
30- to 60-minute intervals until the time of sacrifice. These were assayed by 
radioimmunoassay for LH. Each animal then was anesthetized and perfused 
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