This study used a pharmacological approach to evaluate the consequences of the metabolic perturbations of neurotransmitters on brain development. Pregnant rats received p‐chlorophenylalanine (pCPA), an inhibitor of serotonin (5‐hydroxytryptamine, 5‐HT) synthesis, or saline (control) from the 11th day of gestation once or daily up to the 15th, 17th and 20th day, followed by processing of the forebrain and/or nasal cranium of foetal males and females for high‐performance liquid chromatography of monoamines, radioimmunoassay of gonadotropin‐releasing hormone (GnRH) and quantitative and semiquantitative immunocytochemistry for GnRH. The pCPA treatment resulted in a 50–70% depletion of 5‐HT in the nasal crania and forebrains at any studied age. Radioimmunoassay showed no change in GnRH content in 5‐HT deficient foetuses at E16 compared to controls, being higher in both cases in the rostral forebrain than in the hypothalamus. In controls at E21, the GnRH content in the hypothalamus exceeded that in the rostral forebrain, whereas in the 5‐HT deficient group the opposite was found. These data suggest that 5‐HT provided a stimulating effect on GnRH neurone migration, and this was confirmed by quantification of GnRH‐immunoreactive neurones in the forebrain along the trajectory of their migration. At E18 and E21, the fractions of GnRH neurones in the rostral part of the trajectory in pCPA‐treated foetuses were greater than those in control foetuses but the opposite was true for the caudal part of the trajectory. Moreover, 5‐HT appeared to control the proliferation of the precursor cells of GnRH neurones and their differentiation, as derived from the observations of the increased number of GnRH neurones in the forebrain of foetuses of both sexes, as well as the region‐specific decreased neuronal size and content of GnRH in 5‐HT‐deficient females. Thus, 5‐HT appears to contribute to the regulation of the origin, differentiation and migration of GnRH neurones.

This study has determined in rats the ontogenetic schedule of the onset of pituitary prolactin (PRL) synthesis and release as well as of the establishment of the dopamine (DA) inhibitory control of PRL secretion. RIA recognized PRL traces in the pituitary at the 18th embryonic day (E18), although a clearly detectable amount of this hormone was first measured at E20, suggesting the onset of PRL synthesis. The PRL level in the pituitary increased significantly by E22, in females to a higher extent than in males. Decapitation of fetuses did not cause any change in the PRL plasma level in males showing no PRL release from the pituitary until term. Conversely, there was a slight but significant fall of plasma PRL in decapitated females, suggesting PRL release from the pituitary. An inhibition of DA receptors on lactotropes of fetuses resulted in an increased level of plasma PRL at E20, but not at E18, while the pituitary content of PRL remained unchanged. The same treatment at E22 caused a significant increase of the PRL concentration in plasma and a concomitant fall in the pituitary that could be prevented by preliminary encephalectomy. These data show that the tuberoinfundibular DA system begins to inhibit PRL release from lactotropes between E20 and E22, completely arresting PRL release from the pituitary in males but not in females.