John O. Willoughby

Hypothalamic Interconnections of Somatostatin and Growth Hormone Releasing Factor Neurons

Combined immunohistochemical labelling for neurons containing growth hormone (GH) releasing factor (GRF) or somatostatin and single labelling immunohistochemistry combined with Fluorogold retrograde transport labelling were used to examine whether somatostatin or GRF neurons might be reciprocally innervated. Occasional somatostatin-immunoreactive neurons in the periventricular preoptic area were found to be closely approached by GRF-immunoreactive fibres, providing possible evidence of scant innervation of somatostatin neurons by GRF cells. In contrast, many GRF-immunoreactive neurons in the arcuate nucleus appeared to have somatostatin-immunoreactive fibres closely applied to their perikarya suggesting that GRF neurons might be innervated by somatostatin cells. Combined retrograde tracing and fluorescence immunohistochemistry revealed few somatostatin-immunoreactive neurons doubly labelled following injections of Fluorogold in the basal hypothalamus. Occasional GRF-immunoreactive neurons in the basal hypothalamus were doubly labelled following PO/AHA injections of Fluorogold. Numerous somatostatin-immunoractive perikarya were observed in the periventricular arcuate region in colchicine-pretreated animals. We conclude that GH-regulating neurons do not have strong reciprocal innervation. The innervation of GRF neurons by somatostatin fibres may be derived from local somatostatin neurons.

Activation of GABA receptors in the hypothalamus stimulates secretion of growth hormone and prolactin

Localized intracerebral microinjections of GABA, muscimol, picrotoxin and bicuculline were made in the anterior and basal hypothalamus to determine possible sites of action of GABA in the regulation of prolactin and growth hormone (GH) secretion. Studies were carried out in unanesthetized male rats with chronic indwelling atrial cannulae and intracerebral guide cannulae which permitted stress free blood sampling and intrahypothalamic injections, respectively. Preoptic/anterior hypothalamic area. (PO/AHA) injection of muscimol (0.16 nmol) stimulated both prolactin and GH secretion. GABA (1600 nmol) stimulated prolactin. Bicuculline (0.016 and 0.16 nmol) inhibited GH secretion. Medial basal hypothalamic (MBH) injection of muscimol (0.1 and 1.0 nmol) and GABA (1000 nmol) stimulated prolactin but had no effect on GH secretion. Picrotoxin and bicuculline did not stimulate GH. These findings indicate that activation of PO/AHA GABAergic receptors facilitates secretion of GH and prolactin and activation of MBH GABAergic receptors stimulates secretion of prolactin. It is proposed that GABA inhibits somatostatin neurons in the PO/AHA to facilitate GH and inhibits tuberoinfundibular dopamine or GABA neurons in the MBH to stimulate prolactin.

Evidence That the Regulation of Growth Hormone Secretion Is Mediated Predominantly by a Growth Hormone Releasing Factor

Spontaneous pulsatile growth hormone (GH) secretion and stress-induced suppression of GH was examined in chronically cannulated male rats with electrolytic lesions of the periventricular preoptic anterior hypothalamic area (PO/AHA) where somatostatin neurons innervating the median eminence are known to be located. Median eminence somatostatin was depleted in lesioned animals by 85%. Bursts of GH secretion occurred more frequently than in sham-lesioned animals (1.9 +/- 0.2 vs. 2.6 +/- 0.2 h, respectively, p less than 0.025), however, average concentrations of GH were reduced by lesions (118.4 +/- 11.6 vs 192.3 +/- 28.4 ng/ml, p less than 0.025). Suppression of GH by stress was unaffected by PO/AHA lesions. It is concluded that somatostatin plays only minor roles in the generation of GH troughs during rhythmic GH secretion, and in the suppression of GH during stress. Inhibition of GH releasing factor secretion, therefore, is presumed to be the likely method by which GH suppression is achieved in these physiological situations.

Activation of opioid receptors in the mediobasal hypothalamus stimulates prolactin secretion in the conscious rat.

In an attempt to localize the opioid receptor(s) (mu, delta and kappa) involved in opioid‐stimulated prolactin release in the conscious male rat, opioid agonists were microinjected into the mediobasal hypothalamus and prolactin levels measured before and after injection.

Intrahypothalamic Actions of Somatostatin and Growth Hormone Releasing Factor on Growth Hormone Secretion

Possible effects of GRF on somatostatin neurons and of somatostatin on GRF neurons were examined by measuring the effects of localised intracerebral injections of these peptides on growth hormone (GH) secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of somatostatin or GRF in the preoptic anterior hypothalamic area (PO/AHA) and medial basal hypothalamus. Injections of 0.1 and 1 nmol of GRF in medial basal hypothalamus or 10 nmol somatostatin in the PO/AHA, respectively, had stimulatory or inhibitory effects on GH, which were assumed to be due to diffusion of the peptide from the injection site to the median eminence and pituitary gland. Injection of lower doses of somatostatin around GRF neurons in the medial basal hypothalamus were without significant effect on secretion of GH, but 0.1 nmol somatostatin in the PO/AHA resulted in an increase in GH concentrations from 128 +/- 61 to 524 +/- 103 ng/ml, p less than 0.02. Injections of GRF in lower doses amongst somatostatin neurons in the PO/AH or amongst GRF neurons in the medial basal hypothalamus were both without effect on GH secretion. We conclude that somatostatin may stimulate GH secretion by an effect on or close to somatostatin neurons in the PO/AHA. Somatostatin, though present in terminals on GRF neurons, is without effect at this site in our model. Furthermore, we have been unable to demonstrate any significant intrahypothalamic effect of GRF on GH regulation.

Activation of hypothalamic gamma-aminobutyric Acid receptors resets the pendulum of the growth hormone clock.

Plasma levels of growth hormone in male rats exhibit an ultradian rhythm having a periodicity of 3 to 4 h, bursts of growth hormone being separated by troughs when growth hormone secretion ceases. To determine if neurons in the vicinity of somatostatin neurons in the preoptic/anterior hypothalamic area participate in the generation of the rhythm, we temporarily inhibited this region with injections of the γ‐aminobutyric acid agonist, muscimol, in unanaesthetized rats previously prepared with a venous catheter and stereotaxically implanted intracerebral guide tubes. The injection resulted in an immediate burst of growth hormone release, followed by a trough period and another growth hormone burst, 3.4 ± 0.1 h later. The induction of a trough and synchronization of growth hormone bursts was not reproduced by a burst of intravenously injected exogenous growth hormone at least as large as the first burst initiated by muscimol. These findings indicate that, in the short term, the ultradian rhythm is independent of growth hormone feedback and provide the first evidence that structures in the anterior hypothalamus receiving a γ‐aminobutyric acid input are an important component of the neural generator of the ultradian rhythm of growth hormone.

Intrahypothalamic Mu-, not Delta- or Kappa-Opioid Receptor Activation Causes Growth Hormone Secretion

The possible effects of opioid receptor agonists on growth hormone (GH)‐releasing factor or somatostatin neurons were examined by measuring the effects of localized intracerebral injections of mu‐, delta‐ and kappa‐selective agonists on GH secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of opioid agonists in the preoptic anterior hypothalamic area and medial basal hypothalamus.