Electrophysiological Properties of Melanin-Concentrating Hormone and Orexin Neurons in Adolescent Rats

Abstract

Orexin and melanin-concentrating hormone (MCH) neurons have complementary roles in various physiological functions including energy balance and the sleep/wake cycle. in vitro electrophysiological studies investigating these cells typically use post-weaning rodents, corresponding to adolescence. However, it is unclear whether these neurons are functionally mature at this period and whether these studies can be generalized to adult cells. Therefore, we examined the electrophysiological properties of orexin and MCH neurons in brain slices from post-weaning rats and found that MCH neurons undergo an age-dependent reduction in excitability, but not orexin neurons. Specifically, MCH neurons displayed an age-dependent hyperpolarization of the resting membrane potential (RMP), depolarizing shift of the threshold, and decrease in excitatory transmission, which reach the adult level by 7 weeks of age. In contrast, basic properties of orexin neurons were stable from 4 weeks to 14 weeks of age. Furthermore, a robust short-term facilitation of excitatory synapses was found in MCH neurons, which showed age-dependent changes during the post-weaning period. On the other hand, a strong short-term depression was observed in orexin neurons, which was similar throughout the same period. These differences in synaptic responses and age dependence likely differentially affect the network activity within the lateral hypothalamus where these cells co-exist. In summary, our study suggests that orexin neurons are electrophysiologically mature before adolescence whereas MCH neurons continue to develop until late adolescence. These changes in MCH neurons may contribute to growth spurts or consolidation of adult sleep patterns associated with adolescence. Furthermore, these results highlight the importance of considering the age of animals in studies involving MCH neurons.

Neurokinin B-Expressing Neurons of the Central Extended Amygdala Mediate Inhibitory Synaptic Input onto Melanin-Concentrating Hormone Neuron Subpopulations

“…Furthermore, a majority of the cells exhibited consistently hyperpolarized resting membrane potentials (RMP; mean 6 SEM, À76.94 6 1.23 mV; Fig. 1F), broadly consistent with the distinctive properties of rodent LHA MCH neurons in hypothalamic slices, as previously described (van den Pol et al, 2004;Burdakov et al, 2005;Linehan and Hirasawa, 2018). In whole-cell current-clamp configuration, we puff applied a selective NK3R agonist senktide (100 or 500 nM) in close proximity to LHA MCH neuron cell bodies in the presence of blockers of fast synaptic transmission, kynurenic acid (1 mM) and picrotoxin (100 mM).…”

“…In terms of functional diversity of LHA MCH neurons, we examined the possibility that transcriptionally distinct subpopulations exhibit divergent electrical phenotypes. Although we found that the overall electrophysiological properties of LHA MCH neurons align with previous descriptions of rodent LHA MCH neurons in hypothalamic slices (van den Pol et al, 2004;Burdakov et al, 2005;Linehan and Hirasawa, 2018), they are also quite heterogeneous. Cluster analysis, based on passive and active intrinsic membrane properties, revealed two broad categories, referred to as E-types A and B.…”

Neuroanatomical, electrophysiological, and morphological characterization of melanin-concentrating hormone cells coexpressing cocaine- and amphetamine-regulated transcript

“…Finally, as the excitability of MCH cells may be higher in the juvenile period (Linehan and Hirasawa, 2018), we determined if the excitability of MCH cells were related to age. The change in firing frequency of female MCH/CART+ cells increased with age ( R 2 = 0.22, p = 0.016) but no other active membrane properties were age-dependent ( Supplemental Table 1 ).…”

“…The excitability of rat MCH cells may decrease after the juvenile period (> 7 weeks of age) (Linehan and Hirasawa, 2018). We recorded from a wide age range of animals, but as only 3% (2/69) of our male cells and 21% (12/57) of female cells were from mice under 7 weeks old, it was difficult for us to assess changes in excitability from the juvenile period.…”

“…However, we did detect a sample of glutamatergic events in male CART-negative cells that occur at a higher frequency with greater amplitude and area, and we can speculate that this could promote increased dendritic growth in these cells. MCH cells receive higher glutamatergic input during development (Li and Van Den Pol, 2009) or in the juvenile period (Linehan and Hirasawa, 2018), following sleep deprivation (Briggs et al, 2018), or high fat feeding (Linehan et al, 2020). Furthermore, hypothalamic cells can also increase dendritic branching following restraint stress (Grafe et al, 2019), thus behavioural experiences may shape the branching pattern and contribute to the plasticity of MCH cells even in adulthood.…”