SIMULTANEOUS MEASUREMENT OF GONADOTROPIN-RELEASING-HORMONE IN THE 3RDVENTRICULAR CEREBROSPINAL-FLUID AND HYPOPHYSEAL PORTAL BLOOD OF THE EWE

GnRH is present in the hypophyseal portal blood and cerebrospinal flui d (CSF) of several species investigated, including sheep, but the prec ise relationship between these two compartments of GnRH is unknown. In the present study, ovariectomized steroid-treated ewes were surgicall y prepared for the simultaneous collection of portal blood and third v entricular CSF. Ten-minute samples were collected for pulse analysis a fter progesterone removal and hourly for comparisons during the estrad iol-induced LH surge. The time of onset of the pol-tal (15.3 +/- 0.5 h after estradiol) and CSF (15.9 +/- 0.2 h) GnRH surges was similar and occurred coincidentally with the LH surge (15.6 +/- 0.4 h). The perio d of the surge during which GnRH concentrations exceeded half-maximal levels (portal, 7.3 +/- 1.5 h; CSF, 7.3 +/- 0.3 h) was the same and ou tlasted the corresponding LH surge period (3.3 +/- 0.3 h). LH pulses s tarted and peaked later than the corresponding portal GnRH pulses (ons et difference, 10 +/- 1 min; peak difference, 16 +/- 1 min; P < 0.01 f or both), but the times of pulse onset and peak were not significantly different from those of concomitant CSF GnRH pulses (onset difference , 8 +/- 6 min; peak difference, 8 +/- 4 min). Although the times of pu lse onset and peak did not differ between the portal and CSF GnRH comp artments (onset difference, 4 +/- 6 min; peak difference, 6 +/- 2 min) , CSF GnRH pulses were longer than their portal counterparts (CSF, 38 +/- 3 min; portal, 15 +/- 1 min; P < 0.01). The amplitude of jugular L H pulses was strongly correlated (r(2) = 0.85) with portal GnRH pulse amplitude, but not with that of CSF GnRH pulses (r(2) = 0.45); there w as no correlation between portal and CSF GnRH pulse amplitudes (r(2) = 0.25). These data show that third ventricular CSF GnRH reliably relat es neurosecretory events occurring within the hypophyseal portal syste m at the time of the preovulatory LH surge, but is not as precise as p ortal GnRH in marking a LH pulse.