Vasopressin, oxytocin, and social odor recognition

vasopressin in the olfactory bulb

• Vasopressin neurons are present in the MOB

• These neurons do not project outside the bulb, suggesting a local role in processing social odours.

• VP can inhibit mitral cell activity, the main output neurons of the MOB — acting like a “filter” for familiar social cues.

How does social odour recognition work

• VP is essential for recognizing familiar conspecifics (e.g., other rats).

• Blocking V1a receptors disrupts social recognition, shown by loss of preference for novel individuals.

• This effect is specific to social odours

What are the 2 Mechanisms of Filtering Social Odours

• First exposure to a new animal activates mitral cells and possibly “primes” vasopressin release.

• Second exposure triggers dendritic release of VP, which inhibits mitral cell response, reducing investigation time — essentially, the brain remembers the social odour.

Role of the Anterior Olfactory Nucleus (AON)

• higher processing of social olfactory information.

• VP neurons in the AON respond to social and predator odours

• Egr1 gene expression (a marker of neural activation) is used to map this response.

Summary Diagram:

1. First Encounter:
   Novel social odour → MOB mitral cells active → initial social recognition

2. Priming:
   AON feedback or prolonged exposure → VP vesicle mobilization

3. Second Encounter:
   Familiar odour → VP release → mitral cell inhibition → reduced investigation

A very brief summary of vasopressin

1. Vasopressin acts locally within the olfactory bulb and AON to filter and facilitate social memory

2. This helps the brain quickly recognize familiar individuals by suppressing redundant sensory input during repeated encounters.

What are the 4 key structures of the olfactory system

Key Structure’s :

• MOB (Main Olfactory Bulb): Processes general odour information.

• AOB (Accessory Olfactory Bulb): Processes pheromones via the vomeronasal organ (VNO).

• AON (Anterior Olfactory Nucleus): Higher-order processing and feedback to MOB.

• Mitral cells: Output neurons of the MOB that relay odour information.

What are social odours

• Carried by urine, body scent, or pheromones.

• Processed first in MOB/AOB,

• then sent to AON, amygdala, and hypothalamus.

How does Vasopressin (VP) in the Olfactory Bulb work

1. VP-expressing neurons are present in MOB

2. These neurons lack V1a receptors meaning..

3. VP neurons in the MOB are real and functionally active, but do not project beyond the bulb — their function is local.

How does Vasopressin act as a Local Modulator

1. VP inhibits mitral cell activity, both:

• Spontaneous

• Odour-evoked

2. This suggests VP acts as a filtering agent, suppressing familiar odour signals to reduce unnecessary processing.

What is The Social Recognition Paradigam

Behavioural Test:

• Rats exposed to a familiar and a novel juvenile.

• Normal rats show high preference for novel (i.e., they remember the familiar one).

Manipulations:

1. V1a Antagonist: Blocks VP receptors – disrupts recognition.

2. siRNA for V1a: Silences receptor expression – prevents recognition.

3. Diphtheria toxin: Destroys VP cells in MOB – abolishes social memory.

Key Conclusion: VP signalling in MOB is essential for recognising individual conspecifics.

What were the Habituation/Dishabituation Tests

• Repeated exposure to same juvenile = reduced investigation (habituation).

• New juvenile = increased investigation (dishabituation).

• V1a siRNA prevents this for social odours, but not for neutral odours, proving specificity.

Role of the Anterior Olfactory Nucleus (AON) (6)

1. AON receives input from MOB and contributes to social odour memory.

2. Egr1 expression (immediate early gene) used to measure neural activation.

Stimuli and AON Activation:

3. Social odour: Activates AON pars lateralis and dorsalis.

4. Predator odours: Activate different areas.

5. Non-social odours : Minimal AON activation.

6. AON is selectively engaged in social odour processing, distinguishing social from non-social inputs.

What were the Two-Step Model of Social Recognition

First Exposure (“Priming”)

• Novel conspecific activates mitral cells in MOB.

• Weak or delayed VP release; VP neurons become “primed” (mobilize vesicles for release).

Second Exposure (“Filtering”)

• Familiar odour triggers dendritic VP release.

• VP inhibits mitral cells activation , reducing response.

• Leads to decreased interest/investigation time — the animal remembers the other individual.

What are the 4 key concepts to understand

• Local vs. Projection-based modulation: VP acts within MOB, not through long-range circuits.

• V1a Receptor: Required for social memory; disruption eliminates recognition.

• Dendritic release: VP released from dendrites (not axons) to locally suppress neural activity.

• Neural plasticity: Repeated exposure alters MOB responsiveness to support memory.

What are the 3 main points about the lecture

• Vasopressin neurons are found in the (MOB), (AOB) and (AON) of the rat

• Vasopressin cells in the AON appear to be involved in processing social odour cues

• Vasopressin in the MOB appears to be a retrograde signal that filters activation of mitral cells