Functional hypothalamic asymmetry and introduction to a novel estrogen/estrous phase-dependent regulatory mechanism in mitochondrial energy levels in the female rat hypothalamus

Abstract

The hypothalamus plays a key role in the central regulation of various homeostatic systems and related functions, such as energy metabolism, reproduction and sleep-wake behavior. Our research group has investigated the neuronal mechanisms underlying the hypothalamic regulation of gonadotropin hormone-releasing hormone (GnRH) secretion/release and consequential pituitary luteinizing hormone- (LH) surge. Those studies have made it clear that the cyclic nature of female reproductive physiology is the consequence of fluctuating synaptic reorganization in the neuroendocrine hypothalamus. The latter synaptic events, also known as morphological synaptic plasticity, determine the actual number of stimulatory and inhibitory synapses in the hypothalamus, thus continuously imposing a limit to the functional intensity of the two basic types (excitation-inhibition) of neuronal functions. Today, it is generally accepted that the aforementioned synaptic plasticity is responsible for the final shaping of the patterns detectable in hypothalamic functions, with special regard to the regulation of GnRH-release (but also including a number of other hypothalamus-driven mechanisms, e.g., the food-intake, etc.). The brain is an organ with symmetric tissue organization; in the mature organism paired brain areas usually have distinct physiological functions. Experimental results raising the possibility of functional asymmetry in the neuroendocrine hypothalamus, particularly in the hypothalamus-gonad axis, have emerged long ago. Based on these findings, our research group reopened this question examining the mitochondrial metabolic activity throughout the estrous cycle differentially in the two halves of the rat hypothalamus. The reason for applying mitochondrial metabolic measurements is supported by the fact that synapse generation and neuronal functions, especially neurotransmission, are highly energy dependent. Therefore, the actual ATP level and the regulation of it must be a suitable indicator for all those neuronal functions and dynamic plastic events, which occur during the phases of the estrous cycle. Considering the aforementioned data, we proposed two hypotheses: 1) The regulation of hypothalamic cellular energy levels is asymmetric, and 2) NTPDase3, as an ATP-hydrolyzing enzyme, plays a role in the regulation of hypothalamic mitochondrial ATP-levels. Given that the cyclic activity of the female hypothalamus periodically enters the state of high energy (ATP) need, our first working hypothesis states that if functional hypothalamic asymmetry existed, it should be detectable at some point of the reproductive cycle on the level of a general parameter of neuronal metabolism, the mitochondrial respiration. Earlier, our research group was the first to identify type 3 ecto nucleoside triphosphate diphosphohydrolase 3 (NTPDase3) in the CNS and map its distribution in the rat brain. Particularly high expression levels of NTPDase3 were found in the mitochondria of stimulatory neurons, but not in other cell types of the hypothalamus. Based on these findings, our second working hypothesis states that if NTPDase3 is present in mitochondria, experimental inhibition of its ATP-hydrolyzing activity should significantly decrease ADPdependent state3 mitochondrial respiration (St3), and the enzyme’s expression and/or activity should be estrogen- (E2) dependent. Experimental support of our second working hypothesis would make NTPDase3 a likely candidate for the regulation of mitochondrial energy levels in hypothalamic stimulatory neurons.