Zoology/Biology session

The hypothalamus plays a vital role in regulating the autonomic nervous system and the pituitary gland, thereby contributing to the homeostatic balance. Recently the definition of homeostasis has been extended to the balance of social interactions, in which the hypothalamus has a significant role. In the execution of these various, however, synchronized tasks, numerous hypothalamic nuclei are involved. Such nuclei are the dorsomedial- (DMH) which is of key importance in controlling thermoregulation and energy expenditure, and the ventromedial hypothalamic nucleus (VMH) which manages aggressive and sexual behaviour.

The aim was to map the inputs of the DMH and VMH with an emphasis on the projections from inhibitory neurons since the finetuning of the neuronal mechanisms is primarily achieved via inhibitions. During the research, the focus was on the neural circuits contributing to the social homeostasis. Since there is no such study that compares the DMH and VMH in this aspect, the results of this research can fill this gap.

For this, a retrograde neural tracer, which travels from the axons towards the soma, the cholera toxin B subunit was injected into the target area in male C57/B6 mice. For the identification of the inhibitory inputs, a mouse strain was used in which the vesicular GABA transporter expressing cells produce a green fluorescent protein. Thereby the fluorescence of the GABAergic neurons is detectable beside the CTB which is marked by differently coloured fluorescence. The double-marked neurons were counted with machine learning-based analysing software.

During the comparison of the cells projecting to the DMH and VMH, many similarities and differences were found. Both areas receive significant GABAergic inputs from the lateral septum (LS) and the medial preoptic area (MPOA) which take part in the regulation of social behaviour. Whereas from the LS exclusively inhibitory neurons project, from the MPOA excitatory cells also provide inputs to these nuclei. Moreover, both the VMH and DMH get strong inhibitory input from the medial nucleus of the amygdala whilst the basolateral amygdala only projects to the VMH by excitatory neurons. The posterior intralaminar thalamic nucleus (PIL), which takes part in the transmission of social impulses, innervates the DMH with excitatory neurons whereas the VMH is connected to the more lateral peripeduncular nucleus (PP) that plays a key role in the reproductive behaviour. The parabrachial nucleus (PBN) and the nucleus tractus solitary (NTS) are also sensory relay stations sending excitatory innervation in a similar quantity towards both nuclei.

From the functions of the unravelled brain regions, we can draw such conclusions that the sum of the various excitatory and inhibitory inputs from the regions controlling social behaviour enables the hypothalamic nuclei to establish a harmonic relationship between the physiological and social balance through a complicated and synchronized network.