Dorsal raphe dopamine neurons represent the experience of social isolation

The degree to which DRN DA neurons modulate behavior is predicted by social rank.

“Finally, the degree to which these neurons modulate behavior is predicted by social rank, together supporting a role for DRN dopamine neurons in mediating a loneliness-like state.”

DRN DA neurons show increased activity upon social contact following isolation, as revealed by in vivo calcium imaging.

“DRN DA neurons show increased activity upon social contact following isolation, revealed by in vivo calcium imaging.”

Optogenetic activation of DRN DA neurons causes place avoidance.

“Optogenetic activation of DRN DA neurons increases social preference but causes place avoidance.”

DRN DA neurons are necessary for promoting rebound sociability following an acute period of isolation.

“Furthermore, these neurons are necessary for promoting rebound sociability following an acute period of isolation.”

DRN DA neurons show synaptic changes following acute social isolation.

“Here, we identify a functional role for DA neurons in the dorsal raphe nucleus (DRN), in which we observe synaptic changes following acute social isolation.”

Optogenetic activation of DRN DA neurons increases social preference.

“Optogenetic activation of DRN DA neurons increases social preference but causes place avoidance.”

There was particularly dense terminal expression within the dlBNST and the lateral part of the CeA.

“We observed particularly dense terminal expression within the dorsolateral BNST (dlBNST) (Figure 3E) and the lateral part of the CeA (Figure 3F) and, therefore, tested the effects of optical activation of DRN DA neurons on these regions.”

In 3/23 dlBNST neurons and 4/24 CeA neurons, IPSC responses with a long and variable latency were observed.

“However, in 3/23 dlBNST neurons and 4/24 CeA neurons we observed IPSC responses with a long and variable latency (Figures 4G and 4H), suggesting that terminal stimulation can activate GABAergic neurons to elicit a polysynaptic IPSC.”

In TH::Cre mice, 77.7% of eYFP+ neurons co-labeled with TH using immunohistochemistry.

“In TH::Cre mice we found that 77.7% of eYFP+ neurons co-labeled with TH using immunohistochemistry (Figures S2B, S2D, and S2F).”

Optical stimulation of DRN DA neurons elicited DA release in both the dlBNST and the CeA.

“Optical stimulation of DRN DA neurons elicited DA release in both the dlBNST (Figure 3G) and the CeA (Figure 3H).”

Optical inhibition in NpHR-expressing mice resulted in a significant decrease in social preference following isolation.

“While eYFP-expressing mice showed the typical trend toward an increase in social preference following isolation, optical inhibition in NpHR-expressing mice resulted in a significant decrease in social preference following isolation (Figures 6D and 6E).”

There was no significant difference in the proportion of neurons responding with an EPSC in TH::Cre and DAT::Cre mice.

“There was no significant difference in the proportion of neurons responding with an EPSC in TH::Cre and DAT::Cre mice, so these data were pooled (Figures S4E–S4G).”

The peak-evoked DA release was greater in the dlBNST than the CeA at 30 Hz and 50 Hz.

“The peak-evoked DA release was greater in the dlBNST than the CeA at 30 Hz and 50 Hz (Figures S3D–S3F), suggesting possible differences in the dynamics of DA release and reuptake in these two regions.”

The rectification index (RI) of the AMPAR current was significantly greater in socially isolated mice.

“We found that the rectification index (RI) of the AMPAR current was significantly greater in socially isolated mice (Figure 1D), suggesting an increase in GluR2-lacking AMPARs, which exhibit a characteristic inwardly rectifying current at positive potentials (Bellone and Lüscher, 2006, Liu and Zukin, 2007).”

In socially isolated mice, there was a significant increase in fluorescence signal in response to first contact with a juvenile mouse compared with group-housed mice.

“Strikingly, in socially isolated mice, we observed a significant increase in the fluorescence signal in response to first contact with the juvenile mouse, compared with group-housed mice (Figures 2C–2F; Movie S1).”

eYFP expression in DRN DA somata resulted in terminal expression in several regions including mPFC, BNST, lateral hypothalamus, CeA, entorhinal cortex, and basolateral amygdala.

“Consistent with previous reports, eYFP expression in DRN DA somata resulted in terminal expression in several regions including the medial prefrontal cortex (mPFC), bed nucleus of the stria terminalis (BNST), lateral hypothalamus, central amygdala (CeA), entorhinal cortex, and basolateral amygdala (Hasue and Shammah-Lagnado, 2002, Meloni et al., 2006, Swanson, 1982, Yoshida et al., 1989).”

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• cites Meloni et al., 2006 sourceNot checked yet

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• cites Swanson, 1982 sourceNot checked yet

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There were no significant effects of optical inhibition on behavioral measures of arousal or anxiety-related behavior.

“Furthermore, we did not observe significant effects of optical inhibition on behavioral measures of arousal or anxiety-related behavior (Figures S6B–S6J).”

The paired-pulse ratio (PPR) in the presence of spermine was greater in socially isolated mice.

“Indeed, we found that the paired-pulse ratio (PPR) in the presence of spermine was greater in socially isolated mice (Figures S1F and S1G).”

The EPSCs persisted in the presence of TTX and 4AP.

“Furthermore, the EPSCs persisted in the presence of tetradotoxin (TTX) and 4-aminopyridine (4AP), suggesting that they represent monosynaptic glutamate release from DRN DA terminals (Petreanu et al., 2007).”

In DAT::IRES-Cre mice, 79.8% of eYFP+ neurons were colabeled with TH.

“For comparison, we also examined the DRN of DAT::IRES-Cre mice and found a similar proportion (79.8%) of eYFP+ neurons were colabeled with TH (Figure S2C, S2E, and S2G).”

There was almost no overlap between TH+ neurons and VGAT.

“With Cre-dependent expression of eYFP in the DRN, a subset of TH+ neurons co-expressed VGLUT2 (Figures 4I and 4J), yet there was almost no overlap with VGAT (Figures 4K and 4L).”

Optical stimulation of DRN DA neurons did not support ICSS in TH::Cre or DAT::Cre mice.

“However, we found that optical stimulation of DRN DA neurons did not support ICSS in TH::Cre (Figures 5E and 5F) or DAT::Cre mice (Figure S5E).”

In RTPA, there was avoidance of the light-paired side of the chamber relative to the unstimulated side in ChR2-, but not eYFP-expressing, TH::Cre and DAT::Cre mice.

“Here, we observed avoidance of the light-paired side of the chamber relative to the unstimulated side in ChR2-, but not eYFP-expressing, TH::Cre and DAT::Cre mice (Figures S5F–S5H).”

In isolated mice, activity in response to initial social contact was significantly greater than in response to initial interaction with a novel object.

“Furthermore, in isolated mice, the activity in response to initial social contact was significantly greater than in response to initial interaction with a novel object (Figures 2C–2F).”

The increase in social zone time was replicated in DAT::Cre mice.

“We found that in TH::Cre mice expressing ChR2, but not eYFP, optical stimulation resulted in a significant increase in the proportion of time spent in the social zone (Figures 5A–5D), a result we replicated in DAT::Cre mice (Figures S5C and S5D).”

There was no significant difference in decay time constant of the NMDAR current between group-housed and socially-isolated mice.

“Contrastingly, we did not detect a significant difference in decay time constant of the NMDAR current between group-housed and socially-isolated mice (Figure S1H).”

Transfer into a new cage as a group had no detectable effect on AMPAR/NMDAR ratio.

“We found that transfer into a new cage as a group had no detectable effect on AMPAR/NMDAR ratio, but social isolation in a new cage also increased the AMPAR/NMDAR ratio (Figure 1C).”

Mice expressing NpHR spent a significantly lower proportion of time in the social zone compared with eYFP-expressing mice after 24 hr isolation.

“This revealed that mice expressing NpHR spent a significantly lower proportion of time in the social zone, compared with eYFP-expressing mice (Figure 6C).”

A subset of TH+ neurons co-expressed VGLUT2.

“With Cre-dependent expression of eYFP in the DRN, a subset of TH+ neurons co-expressed VGLUT2 (Figures 4I and 4J), yet there was almost no overlap with VGAT (Figures 4K and 4L).”

There was no difference in AMPAR/NMDAR ratio between group-housed and socially isolated mice in VTA DA neurons.

“However, we did not detect a difference in AMPAR/NMDAR ratio between group-housed and socially isolated mice in VTA DA neurons (Figure 1A).”

NASPM reduced the amplitude of the evoked AMPAR current recorded in DRN DA neurons from socially isolated, but not group-housed, mice.

“This reduced the amplitude of the evoked AMPAR current recorded in DRN DA neurons from socially isolated, but not group-housed, mice (Figure 1E).”

eYFP expression did not overlap with 5-HT+ (serotonergic) neurons in either TH::Cre or DAT::Cre mice.

“Importantly, eYFP expression did not overlap with 5-HT+ (serotonergic) neurons in either TH::Cre or DAT::Cre mice (Figures S2B–S2G).”

Social isolation in a new cage increased the AMPAR/NMDAR ratio.

“We found that transfer into a new cage as a group had no detectable effect on AMPAR/NMDAR ratio, but social isolation in a new cage also increased the AMPAR/NMDAR ratio (Figure 1C).”

In TH::Cre mice expressing ChR2, but not eYFP, optical stimulation resulted in a significant increase in the proportion of time spent in the social zone.

“We found that in TH::Cre mice expressing ChR2, but not eYFP, optical stimulation resulted in a significant increase in the proportion of time spent in the social zone (Figures 5A–5D), a result we replicated in DAT::Cre mice (Figures S5C and S5D).”

In CPP, ChR2-expressing mice showed significant avoidance of the stimulation-associated zone, relative to eYFP-expressing mice.

“During the test session, ChR2-expressing mice showed significant avoidance of the stimulation-associated zone, relative to eYFP-expressing mice (Figures 5H–5J).”

Optical stimulation of DAergic terminals did not elicit a short-latency GABA_A-mediated IPSC in the dlBNST or CeA.

“In contrast, optical stimulation of DAergic terminals did not elicit a short-latency GABAA-mediated inhibitory postsynaptic current (IPSC) in the dlBNST or CeA (Figures 4C and 4D).”

Photostimulation of ChR2-expressing DA terminals elicited a short-latency fast AMPAR-mediated EPSC in 25/30 neurons recorded in the dlBNST and 17/21 neurons in the CeA.

“Photostimulation of ChR2-expressing DA terminals elicited a short-latency fast AMPAR-mediated excitatory postsynaptic current (EPSC) in 25/30 neurons recorded in the dlBNST and 17/21 neurons in the CeA (Figure 4C-F).”

DRN DA neurons in mice socially isolated for 24 hr exhibited a significantly greater AMPAR/NMDAR ratio than group-housed, naive mice.

“Strikingly, DRN DA neurons in mice that were socially isolated for 24 hr exhibited a significantly greater AMPAR/NMDAR ratio than group-housed, naive mice (Figure 1C).”

Relative dominance and change in social preference were positively correlated, such that optical stimulation of DRN DA neurons appeared to be more effective at promoting social preference in dominant mice.

“Intriguingly, we found that relative dominance and change in social preference were positively correlated, such that optical stimulation of DRN DA neurons appeared to be more effective at promoting social preference in dominant mice (Figures 7C, 7D, and S7C).”

• AssumptionRests on an assumption: The subjective experience of social interaction differs between members of a social group depending on their social rank.

In response to eight pulses of 30 Hz stimulation delivered every 5 s, DA transients were consistently recorded in the dlBNST.

“In response to eight pulses of 30 Hz stimulation, delivered every 5 s, DA transients were consistently recorded in the dlBNST.”

In group-housed mice, there are limited changes in DRN activity related to initial juvenile contact.

“in group-housed mice, we observed limited changes in DRN activity related to initial juvenile contact (Figure 2)”

There was a significant negative correlation for NpHR-expressing mice, indicating that photoinhibition produced a greater reduction in social preference following isolation in more dominant mice.

“Conversely, we observed a significant negative correlation (Figures 7G, 7H, S7E, and S7F), indicating that photoinhibition produced a greater reduction in social preference following isolation in more dominant mice.”

• AssumptionRests on an assumption: The subjective experience of social interaction differs between members of a social group depending on their social rank.

Glutamate can be released with optical stimulation of the DRN DA neurons.

“we also demonstrate that glutamate can be released with optical stimulation of the DRN DA neurons”

Following social isolation, photoinhibition causes a reduction in social preference.

“following social isolation, photoinhibition caused a reduction in social preference (Figure 6).”

Acute social isolation induces synaptic plasticity at synapses onto DRN DA neurons.

“Our finding that acute social isolation induces synaptic plasticity at synapses onto DRN DA neurons (Figure 1)”

Activating DRN DA neurons promotes social preference in group-housed mice.

“With photoactivation, we revealed that activating DRN DA neurons promoted social preference in group-housed mice (Figure 5).”

Photoinhibition of DRN DA neurons does not alter social behavior in group-housed mice.

“photoinhibition of these neurons did not alter social behavior (Figure 6).”

There was a negative correlation between relative dominance and preference for the stimulation zone in the RTPA assay, such that more dominant mice displayed greater avoidance of the light-paired side.

“Here, we observed a negative correlation between relative dominance and preference for the stimulation zone, such that more dominant mice displayed greater avoidance of the light-paired side of the chamber (Figures 7E, 7F, and S7D).”

• AssumptionRests on an assumption: The subjective experience of social interaction differs between members of a social group depending on their social rank.

In the absence of a social target, mice avoid photoactivation of DRN DA neurons.

“However, in the absence of a social target, mice avoided photoactivation of DRN DA neurons”

Manipulations of DRN DA activity were not as effective in promoting behavioral adaptations in subordinate animals.

“Therefore, this may be one reason why manipulations of DRN DA activity were not as effective in promoting behavioral adaptations in subordinate animals.”

In socially isolated mice, there is a significantly greater increase in DRN DA activity during initial contact with a social target compared with group-housed mice.

“Consistently, in socially isolated mice, we observed a significantly greater increase in DRN DA activity during initial contact with a social target (Figure 2), compared with group-housed mice.”

Dominant mice are more sensitive to the behavioral effects of manipulating DRN DA activity.

“we observed a relationship in which dominant mice were more sensitive to the behavioral effects of manipulating DRN DA activity (Figure 7).”

Activation of DRN DA neurons plays a causal role in driving social behavior.

“This suggests that activation of DRN DA neurons plays a causal role in driving social behavior.”

DRN DA activity is required for motivating sociability in response to the negative state of isolation.

“This supports the hypothesis that DRN DA activity is required for motivating sociability in response to the negative state of isolation.”

The negative state of isolation can trigger the motivation to seek and engage in social contact.

“Therefore, the negative state of isolation can trigger the motivation to seek and engage in social contact (Baumeister and Leary, 1995, Maner et al., 2007, Williams and Sommer, 1997), perhaps as an evolutionarily conserved mechanism to maintain social connections (Buss, 1990).”

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• cites Maner et al., 2007 sourceNot checked yet

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• cites Williams and Sommer, 1997 sourceNot checked yet

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The absence of social contact triggers a strong desire to seek social interaction.

“Conversely, the absence of social contact also triggers a strong desire to seek social interaction.”

The increase in RI suggests an increase in GluR2-lacking AMPARs.

“We found that the rectification index (RI) of the AMPAR current was significantly greater in socially isolated mice (Figure 1D), suggesting an increase in GluR2-lacking AMPARs, which exhibit a characteristic inwardly rectifying current at positive potentials (Bellone and Lüscher, 2006, Liu and Zukin, 2007).”

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• cites Liu and Zukin, 2007Not checked yet

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The EPSCs represent monosynaptic glutamate release from DRN DA terminals.

“Furthermore, the EPSCs persisted in the presence of tetradotoxin (TTX) and 4-aminopyridine (4AP), suggesting that they represent monosynaptic glutamate release from DRN DA terminals (Petreanu et al., 2007).”

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Social isolation is aversive to rodents.

“Social isolation is also aversive to rodents.”

Prior social environment contributes to subsequent isolation-induced synaptic strength.

“This suggests that prior social environment contributes to subsequent isolation-induced synaptic strength.”

DRN and VTA DA neurons may be functionally distinct.

“Amid the sparse existing knowledge of the functional role of DRN DA neurons, optical stimulation of these neurons does not support intra-cranial self-stimulation (ICSS) (McDevitt et al., 2014), in contrast to the VTA (Witten et al., 2011), suggesting that DRN and VTA DA neurons may be functionally distinct.”

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Following social isolation, the presence of a social stimulus is associated with a significant increase in DRN DA activity in vivo.

“This suggests that, following social isolation, the presence of a social stimulus is associated with a significant increase in DRN DA activity in vivo.”

Social isolation induces a relative increase in GluR2-lacking AMPARs at glutamatergic synapses onto DRN DA neurons.

“Taken together, this suggests that social isolation induces a relative increase in GluR2-lacking AMPARs at glutamatergic synapses onto DRN DA neurons (Figure 1F).”

Even an acute period of isolation in rodents increases motivation to seek out and engage with conspecifics.

“Even an acute period of isolation in rodents increases motivation to seek out and engage with conspecifics (Niesink and van Ree, 1982, Panksepp and Beatty, 1980).”

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Inhibition of DRN DA neurons prevents the typical restoration of social contact following a period of isolation.

“Collectively, these data suggest that inhibition of DRN DA neurons prevents the typical restoration of social contact following a period of isolation.”

Optical stimulation of ChR2-expressing DRN DA neurons is sufficient to trigger rapid DA and glutamate release in two major downstream targets.

“Taken together, this demonstrates that optical stimulation of ChR2-expressing DRN DA neurons is sufficient to trigger rapid DA and glutamate release in two major downstream targets.”

Terminal stimulation can activate GABAergic neurons to elicit a polysynaptic IPSC.

“However, in 3/23 dlBNST neurons and 4/24 CeA neurons we observed IPSC responses with a long and variable latency (Figures 4G and 4H), suggesting that terminal stimulation can activate GABAergic neurons to elicit a polysynaptic IPSC.”

Optical activation of DRN DA neurons produces an aversive state.

“This suggests that, in stark contrast to VTA DA neurons, optical activation of DRN DA neurons produces an aversive state.”

Under 'sated' group-housed conditions, this type of motivation may not be playing a major role.

“Therefore, under “sated” group-housed conditions, this type of motivation may not be playing a major role.”

DRN DA neurons are not merely indicating the removal of sensory stimuli by social isolation but actually representing the subjective experience of a loneliness-like state.

“Importantly, this also suggests that DRN DA neurons are not merely indicating the removal of sensory stimuli by social isolation but actually representing the subjective experience of a loneliness-like state.”

Socially isolated and subordinate monkeys show similar D2/3 receptor availability.

“the observation that socially isolated and subordinate monkeys show similar D2/3 receptor availability (Morgan et al., 2002)”

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In humans, perceived social isolation predicts a poor outcome in numerous physical and mental health-related measures, entirely independent of the level of objective isolation.

“In humans, perceived social isolation predicts a poor outcome in numerous physical and mental health-related measures, entirely independent of the level of objective isolation (Adam et al., 2006, Cacioppo et al., 2006, Hawkley et al., 2006, Wen et al., 2006).”

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DRN DA neurons have a relative bias in their projections to the BNST and CeA.

“The relative bias of the DRN DA neurons in their projections to the BNST and CeA (Hasue and Shammah-Lagnado, 2002, Meloni et al., 2006)”

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Both the BNST and CeA may be important in mediating the observed effects of DRN DA stimulation on behavior.

“it is likely that both of these regions may be important in mediating the observed effects of DRN DA stimulation on behavior.”

In their representation of a loneliness-like state, changes in DRN DA activity may only exert a significant effect on the behavior of individuals who are engaged in positively valued social interactions.

“Therefore, in their representation of a loneliness-like state, changes in DRN DA activity may only exert a significant effect on the behavior of individuals who are engaged in positively valued social interactions.”

24 hr of social isolation results in a blunting of long-term potentiation (LTP) in the BNST.

“Indeed, 24 hr of social isolation results in a blunting of long-term potentiation (LTP) in the BNST (Conrad et al., 2011).”

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In monkeys, the attainment of a dominant social rank increases striatal D2/3 receptor availability.

“In monkeys, the attainment of a dominant social rank increases striatal D2/3 receptor availability (Morgan et al., 2002)”

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Stimulation of DRN DA neurons is aversive.

“suggesting that stimulation is aversive.”

Dominant mice showed a greater degree of behavioral modulation upon optogenetic manipulation of DRN DA neurons.

“Thus, in all cases, dominant mice showed a greater degree of behavioral modulation upon optogenetic manipulation of DRN DA neurons.”

• AssumptionRests on an assumption: The subjective experience of social interaction differs between members of a social group depending on their social rank.

The characteristics observed in DRN DA neurons bear remarkable similarities to the hypotheses generated from human psychology describing the 'need to belong'.

“The characteristics we have observed in DRN DA neurons bear remarkable similarities to the hypotheses generated from human psychology in describing the “need to belong” (Baumeister and Leary, 1995).”

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More lonely individuals show increased attention toward social cues.

“more lonely individuals show increased attention toward social cues (Gardner et al., 2005, Pickett et al., 2004).”

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The activity within DRN DA neurons may only be necessary in situations in which the motivation for social contact is high, such as that experienced after social isolation.

“This indicates that the activity within these neurons may only be necessary in situations in which the motivation for social contact is high, such as that experienced after social isolation.”

Subordinate animals may be in a loneliness-like state, even while group-housed.

“supports the notion that subordinate animals may be in a loneliness-like state, even while group-housed.”

DRN DA neurons are part of a distinct circuit from the VTA population.

“represents an important distinction from the VTA population and suggests these DA neurons are part of a distinct circuit.”

In a state of loneliness, wherein an individual's basic need for social connection is unmet, processing of socially relevant information should be prioritized.

“in a state of loneliness, wherein an individual’s basic need for social connection is unmet (Peplau, 1978), processing of socially relevant information should be prioritized (Baumeister and Leary, 1995).”

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Our data support a novel role for a relatively unstudied population of DA neurons in representing the experience of social isolation.

“Our data present an additional element for consideration in the control of social behavior, and support a novel role for a relatively unstudied population of DA neurons in representing the experience of social isolation.”

In humans, socially excluded individuals display an enhanced memory for social events.

“in humans, socially excluded individuals display an enhanced memory for social events (Gardner et al., 2000)”

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