Relationship between Behavioral Recovery from Unilateral 6-OHDA Lesion of the Substania 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 relationship between the effects of histamine and dopamine on behavior. A unilateral lesion of dopamine cells in the Substantia Nigra (SN) results in asymmetrical behavior, and recovery from this asymmetry was previously found to be associated with asymmetrical changes in the nigro-striatal projection as determined by anatomical tracing technique. In view of the apparent opposing interaction between the TM and the SN, we examined whether the tuberomammillary-striatal projection also shows changes after a unilateral lesion of the SN. Rats had 6-OHDA injected into one substantia nigra and were tested for behavioral asymmetry over a 15 day period. Horseradish peroxidase (HRP) was then deposited in the caudate-putamen (CPU) ipsilateral to the lesion. Controls devoid of a lesion had the tracer deposited in either the right of 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 lesion-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 lesions than in the control animals. The results indicate an increase in HRP uptake or transport in the anterior part of the tuberomammillary-striatal projection predominantly ipsilateral to the lesion in animals which failed to recover from lesion induced asymmetry. Previously, we had found an opposite effect in the nigro-striatal projection, namely an increase in HRP uptake or transport only in animals which recovered from their lesion induced asymmetry. The results are discussed in terms of the evidence for a reciprocal relationship between the TM- and SN-striatal systems.