Relationship between behavioral recovery from unilateral 6-OHDA lesion of the substantia nigra and changes in the tuberomammillary-striatal projection as measured by HRP-labelling

The tuberomammillary nucleus (TM) is the only known neuronal cell group in the brain which contains histamine. There is evidence for a reciprocal rela tionship between the effects of histamine and dopamine on behavior. A unila teral lesion of dopamine cells in the Substantia Nigra (SN) results in asym metrical behavior, and recovery from this asymmetry was previously found to be associated with asymmetrical changes in the nigro-striatal projection a s determined by anatomical tracing technique. In view of the apparent oppos ing interaction between the TM and the SN, we examined whether the tuberoma mmillary-striatal projection also shows changes after a unilateral lesion o f the SN. Rats had 6-OHDA injected into one substantia nigra and were teste d for behavioral asymmetry over a 15 day period. Horseradish peroxidase (HR P) was then deposited in the caudate-putamen (CPU) ipsilateral to the lesio n. Controls devoid of a lesion had the tracer deposited in either the right or left CPU. More HRP-labeled cells were found in the anterior part of TM ipsilateral to the lesion in the animals which failed to recover from lesio n-induced asymmetry in turning behavior, as compared to normal controls or animals which showed behavioral recovery. The labeled cells in the anterior part of the ipsilateral TM were smaller in animals examined 15 days after the lesion than in the control animals. The results indicate an increase in HRP uptake or transport in the anterior part of the tuberomammillary-stria tal projection predominantly ipsilateral to the lesion in animals which fai led to recover from lesion induced asymmetry. Previously, we had found an o pposite effect in the nigro-striatal projection, namely an increase in HRP uptake or transport only in animals which recovered from their lesion induc ed asymmetry. The results are discussed in terms of the evidence for a reci procal relationship between the TM- and SN-striatal systems.