Probes for INS

Major depressive disorder (MDD) patients display a common but often variable set of symptoms making successful, sustained treatment difficult to achieve. Separate depressive symptoms may be encoded by differential changes in distinct circuits in the brain, yet how discrete circuits underlie behavioral subsets of depression and how they adapt in response to stress has not been addressed. We identify two discrete circuits of parvalbumin-positive (PV) neurons in the ventral pallidum (VP) projecting to either the lateral habenula or ventral tegmental area contributing to depression. We find that these populations undergo different electrophysiological adaptations in response to social defeat stress, which are normalized by antidepressant treatment. Furthermore, manipulation of each population mediates either social withdrawal or behavioral despair, but not both. We propose that distinct components of the VP PV circuit can subserve related, yet separate depressive-like phenotypes in mice, which could ultimately provide a platform for symptom-specific treatments of depression.

The lateral habenula (LHb) has a key role in integrating a variety of neural circuits associated with reward and aversive behaviors. There is limited information about how the different cell types and neuronal circuits within the LHb coordinate physiological and motivational states. Here, we report a cell type in the medial division of the LHb (LHbM) in male rats that is distinguished by: (1) a molecular signature for GABAergic neurotransmission (Slc32a1/VGAT) and estrogen receptor (Esr1/ERα) expression, at both mRNA and protein levels, as well as the mRNA for vesicular glutamate transporter Slc17a6/VGLUT2, which we term the GABAergic estrogen-receptive neuron (GERN); (2) its axonal projection patterns, identified by in vivo juxtacellular labeling, to both local LHb and to midbrain modulatory systems; and (3) its somatic expression of receptors for vasopressin, serotonin and dopamine, and mRNA for orexin receptor 2. This cell type is anatomically located to receive afferents from midbrain reward (dopamine and serotonin) and hypothalamic water and energy homeostasis (vasopressin and orexin) circuits. These afferents shared the expression of estrogen synthase (aromatase) and VGLUT2, both in their somata and axon terminals. We demonstrate dynamic changes in LHbM VGAT+ cell density, dependent upon gonadal functional status, that closely correlate with motivational behavior in response to predator and forced swim stressors. The findings suggest that the homeostasis and reward-related glutamatergic convergent projecting pathways to LHbMC employ a localized neurosteroid signaling mechanism via axonal expression of aromatase, to act as a switch for GERN excitation/inhibition output prevalence, influencing depressive or motivated behavior.

The rat nucleus incertus (NI) contains GABA/peptide-projection neurons responsive to orexin (hypocretin)/orexin receptor-2 (OX2) signalling. Melanin-concentrating hormone (MCH) and orexin neurons often innervate and influence common target areas. Therefore, we assessed the relationship between these hypothalamic peptidergic systems and rat NI, by investigating the presence of an MCH innervation and MCH receptor-1 (MCH1) expression, and neurophysiological and behavioural effects of MCH c.f. orexin-A (OXA), within the NI. We identified lateral hypothalamus (LH), perifornical and sub-zona incerta MCH neurons that innervate NI, and characterised the rostrocaudal distribution of MCH-containing fibres in NI. Single-cell RT-PCR detected MCH1 and OX2 mRNA in NI, and multiplex, fluorescent in situ hybridisation revealed distinct co-expression patterns of MCH1 and OX2 mRNA in NI neurons expressing vesicular GABA transporter (vGAT) mRNA. Patch-clamp recordings revealed 34% of NI neurons tested were hyperpolarised by MCH (1 μM), representing a distinct population from OXA-sensitive NI neurons (35%). Intra-NI OXA infusion (600 pmol) in satiated rats during the light/inactive phase produced increased locomotor activity and food (standard chow) intake, whereas intra-NI MCH infusion (600 pmol) produced only a trend for decreased locomotor activity and no effect on food intake. Furthermore, in satiated or pre-fasted rats tested during the dark/active phase, intra-NI infusion of MCH did not alter the elevated locomotor activity or higher food intake observed. However, quantification of neuropeptide-immunostaining revealed differential diurnal fluctuations in orexin and MCH trafficking to NI. Our findings identify MCH and orexin inputs onto divergent NI populations which may differentially influence arousal and motivated behaviours.

Abstract

Dravet syndrome (DS) is a form of epilepsy with a high incidence of sudden unexpected death in epilepsy (SUDEP). Respiratory failure is a leading cause of SUDEP, and DS patients' frequently exhibit disordered breathing. Despite this, mechanisms underlying respiratory dysfunction in DS are unknown. We found that mice expressing a DS-associated Scn1a missense mutation (A1783V) conditionally in inhibitory neurons (Slc32a1cre/+::Scn1aA1783V fl/+; defined as Scn1aΔE26) exhibit spontaneous seizures, die prematurely and present a respiratory phenotype including hypoventilation, apnea, and a diminished ventilatory response to CO2. At the cellular level in the retrotrapezoid nucleus (RTN), we found inhibitory neurons expressing the Scn1a A1783V variant are less excitable, whereas glutamatergic chemosensitive RTN neurons, which are a key source of the CO2/H+-dependent drive to breathe, are hyper-excitable in slices from Scn1aΔE26 mice. These results show loss of Scn1a function can disrupt respiratory control at the cellular and whole animal levels.