Forebrain Structures which Mediate the Effects of Stress on Prolactin and Growth Hormone Secretion in the Rat*

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Abstract
A major aim of this study was to determine whether telencephalic structures are essential for stress-induced stimulation of PRL secretion. Another aim was to determine whether the basal telencephalon, left in connection with the diencephalon, is sufficient to support the normal inhibitory effects of stress on GH secretion. A final aim was to assess whether the medial forebrain bundle (MFB) mediates the effects of stress on PRL secretion. In the first experiment, male rats were subjected to removal of the telencephalon, partial telencephalon removal (sparing basal structures such as parts of the diagonal band, preoptic area, bed nucleus of stria terminalis, and nucleus accumbens septi), or sham telencephalon removal. Twenty-four hours after surgery, nonstress plasma PRL concentrations were similar in the three groups. Whereas a 10-min leg restraint-blood withdrawal stress stimulated PRL secretion in sham-operated controls and rats in which the basal telencephalon was spared, the PRL response was abolished in rats with complete telencephalon removal. Complete telencephalon removal led to elevated nonstress plasma GH levels, and partial ablation led to decreased nonstress plasma GH concentrations. Normal stress-induced suppression of plasma GH levels was blocked with either total or partial telencephalon removal. Total removal, however, was associated with an increment in GH levels in response to stress. These results suggest that most of the telencephalon is not necessary for PRL responses to the stressor used; only basal telencephalic structures are essential in this regard. The telencephalon is required for the control of GH secretion under nonstress conditions and for the stress-induced suppression of GH secretion. However, reversed GH responses occur in rats with total, but not partial, telencephalon removal. In the second experiment, MFB lesions blocked PRL responses to stress in rats with partial telencephalon removal, indicating that the MFB and its projections to the basal telencephalon are essential for these responses.