Anxiety-relieving mechanism of growth hormone discerned in a new study

NEW DELHI: Researchers have detailed the mechanisms through which growth hormones, which help build bones and muscles, also help relieve anxiety.

The researchers at the University of São Paulo (USP), Brazil, found that male mice displayed higher levels of anxiety when their neurons lacked growth hormone receptors, or agents mediating chemicals acting on and the physiological response of neurons.

Specifically, these receptors were found to be lacking in a group of somatostatin-expressing neurons, which inhibit an anxiety response by releasing the protein constituent somatostatin.

While there is growing evidence around the role of hormones in regulating neurological processes, including their influence on one's vulnerability to neuropsychiatric disorders, the researchers said the regulatory mechanism in neurons associated with such disorders had not yet been discovered.

"We demonstrated that (growth hormone) changes the synapses (or interconnections of neurons) and structurally alters the neurons that secrete somatostatin," said José Donato Júnior, a professor at the university's Biomedical Sciences Institute and last author of the study published in the Journal of Neuroscience.

Further, the scientists found that the lack of growth hormones in somatostatin-releasing neurons in both male and female mice decreased "fear memory", characteristic of post-traumatic stress disorder. This meant a reduced capacity to form fear memory, they said.

The findings offering a "possible chemical explanation for neuropsychiatric disorders" could contribute towards developing a new class of anxiety-relieving, or anxiolytic, drugs, they said.

Located at the base of the brain, the master gland of the endocrine system, the pituitary gland, is responsible for releasing the growth hormone in the bloodstream, promoting tissue growth throughout the body.

For the study, the researchers conducted experiments with mice, "well-established" for analysing anxious behaviour and fear memory, which are elements of post-traumatic stress. This helped them explore the role of growth hormone as the mice made decisions.

The study also showed that anxiety, post-traumatic stress and fear memory are different facets of the same neuronal circuit, the researchers said.

While Donato defined anxiety as "excessive fear or distrust", he said that fear memory related to an adverse past event that produced a brain alteration, which triggers an exacerbated response whenever the subject is exposed to a similar stimulus.

The response may range from weeping to tremors and even paralysis, he said.

"All this happens in the same neuron population, which expresses the (growth hormone) receptor. In our experiment, fear memory was reduced in mice when we switched the GH receptor off. This means the capacity to form fear memory is impaired. It may be the case that GH contributes to the development of post-traumatic stress," he said.

Donato said that the team's next steps include investigating the role of these growth hormones during pregnancy, when the chances of post-partum depression rise.