Localizing Lesion-Induced Food Aversion

To identify the specific basal forebrain site(s) where neural damage leads to enhanced food aversion, resolving ambiguity from previous studies with large, overlapping lesions in the lateral hypothalamus (LH), globus pallidus (GP), or ventral pallidum/substantia innominata (VP/SI).

Subjects: $52$ male Sprague-Dawley rats.
Lesions: Small, bilateral excitotoxin injections (quinolinic acid: $10 ext{ extmu g}$ in $1 ext{ extmu l}$ , or ibotenic acid: $15 ext{ extmu g}$ in $1 ext{ extmu l}$ ) were made into the GP, VP/SI, or LH. Sham injections served as controls.
Behavioral Testing: Affective taste reactions to sucrose ( $1 ext{ M}$ ) were elicited via chronic oral cannulae and videotaped.
Taste Reactivity Scoring: Aversive responses (gapes, chin-rubbing, head-shaking, forelimb flails) were tallied.
Lesion Mapping:
- Conventional neuron counting procedure.
- Modified 'fractionator' procedure for precise identification of lesion location, size, and boundaries, using a >55-70 ext{%} neuron depletion threshold for aversion. A 'subtraction analysis' identified the crucial site (damaged in every rat showing enhanced aversion).

Aphagia and Adipsia:
- GP lesions did not produce significant aphagia or weight loss.
- LH lesions caused aphagia (avg. $4.9$ days) and weight loss (avg. $57 ext{ g}$ , 14 ext{%} decrease from controls).
- VP/SI lesions caused aphagia (avg. $5$ days) and greater weight loss (avg. $64 ext{ g}$ , 21 ext{%} decrease from controls).
Enhanced Aversion:
- LH lesion group ( $n=8$ ) did not show enhanced aversive reactions; they showed a trend towards suppression of both aversive and hedonic responses.
- VP/SI lesion group ( $n=8$ ) showed a significant increase in aversive responses to sucrose ( $13.5 ext{ responses}$ vs. $0.00$ in controls; Mann-Whitney U test, P < 0.001).
Crucial Aversion Site Identification:
- Enhanced aversive reactions occurred only following bilateral neuron loss (>70 ext{%}) from the caudal ventromedial VP/SI alone.
- This shared site had a lateral diameter of $1.0 ext{ mm}$ , a dorsoventral diameter of $0.5 ext{ mm}$ , and a rostrocaudal diameter of $1.0 ext{ mm}$ .
- Damage restricted to the LH never produced enhanced aversion, even when it caused aphagia.
- The crucial region for aversion is located ventral and medial to the globus pallidus and dorsal and lateral to the lateral hypothalamus.

There is a single crucial anatomical site for lesion-induced enhanced food aversion, located in the ventromedial corner of the ventral pallidum/substantia innominata (vmVP/SI), critically outside the lateral hypothalamus.
The ventromedial VP/SI showed necessary (damaged in all aversion-showing rats) and sufficient (damage consistently produced aversion) neuron loss (>70 ext{%}) for enhanced aversion.
Previous findings of aversion from LH or GP lesions were likely due to unintended encroachment on this vmVP/SI region.
Several basal forebrain subsystems (ventral striatopallidal pathway, extended amygdala system, magnocellular corticopetal group, taste-substantia innominata system) overlap this site, making specific mechanistic attribution challenging.