On innate behaviors : focus on parental behavior and aggression

The neural substrates orchestrating a number of social behaviors, including parental behavior and aggression, are known to exist in the hypothalamus. Through the control of the pituitary gland, the hypothalamus regulates the release of a number of hormones necessary for the physiological control of bodily functions and the expression of appropriate behaviors. In recent years the neuroscience community has invested large resources in identifying, through molecular markers, subsets of neurons whose activity impacts behavioral expression. This approach, however, has several weaknesses, among which is the assumption that a neuron’s function and output adhere to generalized principles. Consequently, such investigations often fail to identify the intricate organization of neural networks, which adapt the neural code in order to tune a system’s output to the behavior it modulates. The aim of this thesis is to expand on basic neurophysiological concepts regarding the complex organization within and among neural groups. Here we addressed the principles of how a set of neurons self-tune their activity through the use of their own neurotransmitter, intra- and inter-network connectivity designs and spike rate coding of neurotransmitter release. Following this interrogation of neural network properties, we attempted to link the activity of these neural nodes to behavioral output, where we identified two distinct subsets of neurons driving parental behavior and aggression respectively. In paper I, we performed a study on the properties of autoregulation in a neural network, and identified the ionic mechanisms through which the tuberoinfundibular dopamine (TIDA) neurons control their own activity via the use of their own neurotransmitter, dopamine (DA). In paper II, we encountered an unexpected species difference in baseline activity and oscillation frequency between rat and mouse TIDA neurons. Following an in-depth investigation, we attributed this difference to the presence vs complete absence of electrical coupling in the rat and mouse TIDA cells respectively. This generated the question of how different modes of TIDA neuron activity impact DA release at their terminals, which was addressed in paper III where, using fast-scan cyclic voltammetry, we performed the first investigation coupling patterns of electrophysiological activity to DA release in the TIDA system. In paper IV we addressed the possibility that this discrepancy in TIDA neuron activity has a behavioral impact. Following a step-by-step breakdown of the lactotropic axis in the male rat and mouse, we ultimately provided a link between TIDA neuron activity and the suppression vs expression of paternal behavior in the two species. The final part of this thesis includes two studies focusing on aggressive behavior. In paper V, we performed a functional interrogation of a subset of ventral premammillary (PMv) neurons involved in intermale aggression, while in paper VI we identified that the very same neurons are activated by maternal hormones and modulate the expression of maternal aggression in lactating female mice. Overall, the work presented in this thesis provides a step forward in our understanding of neural function and on the neural substrates underlying social behavior

Dopamine Autoreceptor Regulation of a Hypothalamic Dopaminergic Network

Acknowledgments The authors thank Drs. Gilberto Fisone, Jessica Ausborn, Abdel El Manira, Gilad Silberberg, and members of the C.B. laboratory for advice, as well as Paul Williams for expert help with the graphical abstract. This study was supported by a Starting Investigator Grant from the ERC (ENDOSWITCH 261286), the Swedish Research Council (2010-3250), Novo Nordisk Fonden, and the Strategic Research Programme in Diabetes at Karolinska Institutet.Peer reviewedPublisher PD

Experience-dependent plasticity in an innate social behavior is mediated by hypothalamic LTP

All animals can perform certain survival behaviors without prior experience, suggesting a “hard wiring” of underlying neural circuits. Experience, however, can alter the expression of innate behaviors. Where in the brain and how such plasticity occurs remains largely unknown. Previous studies have established the phenomenon of “aggression training,” in which the repeated experience of winning successive aggressive encounters across multiple days leads to increased aggressiveness. Here, we show that this procedure also leads to long-term potentiation (LTP) at an excitatory synapse, derived from the posteromedial part of the amygdalohippocampal area (AHiPM), onto estrogen receptor 1-expressing (Esr1⁺) neurons in the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl). We demonstrate further that the optogenetic induction of such LTP in vivo facilitates, while optogenetic long-term depression (LTD) diminishes, the behavioral effect of aggression training, implying a causal role for potentiation at AHiPM→VMHvl^(Esr1) synapses in mediating the effect of this training. Interestingly, ∼25% of inbred C57BL/6 mice fail to respond to aggression training. We show that these individual differences are correlated both with lower levels of testosterone, relative to mice that respond to such training, and with a failure to exhibit LTP after aggression training. Administration of exogenous testosterone to such nonaggressive mice restores both behavioral and physiological plasticity. Together, these findings reveal that LTP at a hypothalamic circuit node mediates a form of experience-dependent plasticity in an innate social behavior, and a potential hormone-dependent basis for individual differences in such plasticity among genetically identical mice

Experience-dependent plasticity in an innate social behavior is mediated by hypothalamic LTP

All animals can perform certain survival behaviors without prior experience, suggesting a “hard wiring” of underlying neural circuits. Experience, however, can alter the expression of innate behaviors. Where in the brain and how such plasticity occurs remains largely unknown. Previous studies have established the phenomenon of “aggression training,” in which the repeated experience of winning successive aggressive encounters across multiple days leads to increased aggressiveness. Here, we show that this procedure also leads to long-term potentiation (LTP) at an excitatory synapse, derived from the posteromedial part of the amygdalohippocampal area (AHiPM), onto estrogen receptor 1-expressing (Esr1⁺) neurons in the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl). We demonstrate further that the optogenetic induction of such LTP in vivo facilitates, while optogenetic long-term depression (LTD) diminishes, the behavioral effect of aggression training, implying a causal role for potentiation at AHiPM→VMHvl^(Esr1) synapses in mediating the effect of this training. Interestingly, ∼25% of inbred C57BL/6 mice fail to respond to aggression training. We show that these individual differences are correlated both with lower levels of testosterone, relative to mice that respond to such training, and with a failure to exhibit LTP after aggression training. Administration of exogenous testosterone to such nonaggressive mice restores both behavioral and physiological plasticity. Together, these findings reveal that LTP at a hypothalamic circuit node mediates a form of experience-dependent plasticity in an innate social behavior, and a potential hormone-dependent basis for individual differences in such plasticity among genetically identical mice

Dopamine release dynamics in the tuberoinfundibular dopamine system

The relationship between neuronal impulse activity and neurotransmitter release remains elusive. This issue is especially poorly understood in the neuroendocrine system, with its particular demands on periodically voluminous release of neurohormones at the interface of axon terminals and vasculature. A shortage of techniques with sufficient temporal resolution has hindered real-time monitoring of the secretion of the peptides that dominate among the neurohormones. The lactotropic axis provides an important exception in neurochemical identity, however, as pituitary prolactin secretion is primarily under monoaminergic control, via tuberoinfundibular dopamine (TIDA) neurons projecting to the median eminence (ME). Here, we combined electrical or optogenetic stimulation and fast-scan cyclic voltammetry to address dopamine release dynamics in the male mouse TIDA system. Imposing different discharge frequencies during brief (3 s) stimulation of TIDA terminals in the ME revealed that dopamine output is maximal at 10 Hz, which was found to parallel the TIDA neuron action potential frequency distribution during phasic discharge. Over more sustained stimulation periods (150 s), maximal output occurred at 5 Hz, similar to the average action potential firing frequency of tonically active TIDA neurons. Application of the dopamine transporter blocker, methylphenidate, significantly increased dopamine levels in the ME, supporting a functional role of the transporter at the neurons' terminals. Lastly, TIDA neuron stimulation at the cell body yielded perisomatic release of dopamine, which may contribute to an ultrafast negative feedback mechanism to constrain TIDA electrical activity. Together, these data shed light on how spiking patterns in the neuroendocrine system translate to vesicular release toward the pituitary and identify how dopamine dynamics are controlled in the TIDA system at different cellular compartments

Dopamine release dynamics in the tuberoinfundibular dopamine system

The relationship between neuronal impulse activity and neurotransmitter release remains elusive. This issue is especially poorly understood in the neuroendocrine system, with its particular demands on periodically voluminous release of neurohormones at the interface of axon terminals and vasculature. A shortage of techniques with sufficient temporal resolution has hindered real-time monitoring of the secretion of the peptides that dominate among the neurohormones. The lactotropic axis provides an important exception in neurochemical identity, however, as pituitary prolactin secretion is primarily under monoaminergic control, via tuberoinfundibular dopamine (TIDA) neurons projecting to the median eminence (ME). Here, we combined electrical or optogenetic stimulation and fast-scan cyclic voltammetry to address dopamine release dynamics in the male mouse TIDA system. Imposing different discharge frequencies during brief (3 s) stimulation of TIDA terminals in the ME revealed that dopamine output is maximal at 10 Hz, which was found to parallel the TIDA neuron action potential frequency distribution during phasic discharge. Over more sustained stimulation periods (150 s), maximal output occurred at 5 Hz, similar to the average action potential firing frequency of tonically active TIDA neurons. Application of the dopamine transporter blocker, methylphenidate, significantly increased dopamine levels in the ME, supporting a functional role of the transporter at the neurons' terminals. Lastly, TIDA neuron stimulation at the cell body yielded perisomatic release of dopamine, which may contribute to an ultrafast negative feedback mechanism to constrain TIDA electrical activity. Together, these data shed light on how spiking patterns in the neuroendocrine system translate to vesicular release toward the pituitary and identify how dopamine dynamics are controlled in the TIDA system at different cellular compartments

Stimulus-specific hypothalamic encoding of a persistent defensive state

Persistent neural activity in cortical, hippocampal, and motor networks has been described as mediating working memory for transiently encountered stimuli. Internal emotional states, such as fear, also persist following exposure to an inciting stimulus, but it is unclear whether slow neural dynamics are involved in this process. Neurons in the dorsomedial and central subdivisions of the ventromedial hypothalamus (VMHdm/c) that express the nuclear receptor protein NR5A1 (also known as SF1) are necessary for defensive responses to predators in mice. Optogenetic activation of these neurons, referred to here as VMHdm^(SF1) neurons, elicits defensive behaviours that outlast stimulation, which suggests the induction of a persistent internal state of fear or anxiety. Here we show that in response to naturalistic threatening stimuli, VMHdm^(SF1) neurons in mice exhibit activity that lasts for many tens of seconds. This persistent activity was correlated with, and required for, persistent defensive behaviour in an open-field assay, and depended on neurotransmitter release from VMHdm^(SF1) neurons. Stimulation and calcium imaging in acute slices showed that there is local excitatory connectivity between VMHdm^(SF1) neurons. Microendoscopic calcium imaging of VMHdm^(SF1) neurons revealed that persistent activity at the population level reflects heterogeneous dynamics among individual cells. Unexpectedly, distinct but overlapping VMHdm^(SF1) subpopulations were persistently activated by different modalities of threatening stimulus. Computational modelling suggests that neither recurrent excitation nor slow-acting neuromodulators alone can account for persistent activity that maintains stimulus identity. Our results show that stimulus-specific slow neural dynamics in the hypothalamus, on a time scale orders of magnitude longer than that of working memory in the cortex, contribute to a persistent emotional state

Polysynaptic inhibition between striatal cholinergic interneurons shapes their network activity patterns in a dopamine-dependent manner

Striatal activity is dynamically modulated by acetylcholine and dopamine, both of which are essential for basal ganglia function. Synchronized pauses in the activity of striatal cholinergic interneurons (ChINs) are correlated with elevated activity of midbrain dopaminergic neurons, whereas synchronous firing of ChINs induces local release of dopamine. The mechanisms underlying ChIN synchronization and its interplay with dopamine release are not fully understood. Here we show that polysynaptic inhibition between ChINs is a robust network motif and instrumental in shaping the network activity of ChINs. Action potentials in ChINs evoke large inhibitory responses in multiple neighboring ChINs, strong enough to suppress their tonic activity. Using a combination of optogenetics and chemogenetics we show the involvement of striatal tyrosine hydroxylase-expressing interneurons in mediating this inhibition. Inhibition between ChINs is attenuated by dopaminergic midbrain afferents acting presynaptically on D2 receptors. Our results present a novel form of interaction between striatal dopamine and acetylcholine dynamics

Stimulus-specific hypothalamic encoding of a persistent defensive state

Persistent neural activity in cortical, hippocampal, and motor networks has been described as mediating working memory for transiently encountered stimuli. Internal emotional states, such as fear, also persist following exposure to an inciting stimulus, but it is unclear whether slow neural dynamics are involved in this process. Neurons in the dorsomedial and central subdivisions of the ventromedial hypothalamus (VMHdm/c) that express the nuclear receptor protein NR5A1 (also known as SF1) are necessary for defensive responses to predators in mice. Optogenetic activation of these neurons, referred to here as VMHdm^(SF1) neurons, elicits defensive behaviours that outlast stimulation, which suggests the induction of a persistent internal state of fear or anxiety. Here we show that in response to naturalistic threatening stimuli, VMHdm^(SF1) neurons in mice exhibit activity that lasts for many tens of seconds. This persistent activity was correlated with, and required for, persistent defensive behaviour in an open-field assay, and depended on neurotransmitter release from VMHdm^(SF1) neurons. Stimulation and calcium imaging in acute slices showed that there is local excitatory connectivity between VMHdm^(SF1) neurons. Microendoscopic calcium imaging of VMHdm^(SF1) neurons revealed that persistent activity at the population level reflects heterogeneous dynamics among individual cells. Unexpectedly, distinct but overlapping VMHdm^(SF1) subpopulations were persistently activated by different modalities of threatening stimulus. Computational modelling suggests that neither recurrent excitation nor slow-acting neuromodulators alone can account for persistent activity that maintains stimulus identity. Our results show that stimulus-specific slow neural dynamics in the hypothalamus, on a time scale orders of magnitude longer than that of working memory in the cortex, contribute to a persistent emotional state

A Neuro-hormonal Circuit for Paternal Behavior Controlled by a Hypothalamic Network Oscillation

Parental behavior is pervasive throughout the animal kingdom and essential for species survival. However, the relative contribution of the father to offspring care differs markedly across animals, even between related species. The mechanisms that organize and control paternal behavior remain poorly understood. Using Sprague-Dawley rats and C57BL/6 mice, two species at opposite ends of the paternal spectrum, we identified that distinct electrical oscillation patterns in neuroendocrine dopamine neurons link to a chain of low dopamine release, high circulating prolactin, prolactin receptor-dependent activation of medial preoptic area galanin neurons, and paternal care behavior in male mice. In rats, the same parameters exhibit inverse profiles. Optogenetic manipulation of these rhythms in mice dramatically shifted serum prolactin and paternal behavior, whereas injecting prolactin into non-paternal rat sires triggered expression of parental care. These findings identify a frequency-tuned brain-endocrine-brain circuit that can act as a gain control system determining a species’ parental strategy