The TPc, the avian substantia nigra: pharmacology and behavior.

The nucleus tegmenti pedunculo-pontinus, pars compacta (TPc) may be the avian analogue of the mammalian substantia nigra (SN). The analogy is suggested by both comparative neuroanatomical and neurohistochemical observations. To test the proposed analogy certain drugs (agonists or antagonists of putative transmitters that modulate the dopaminergic and GABAergic systems in rat) were injected into the TPc of the pigeon and the behavior effects observed. Muscimol (a GABA agonist) injected into the caudal TPc induced contralateral rotation and postural asymmetries. Pretreatment with subcutaneous injections of apomorphine hydrochloride enhanced and haloperidol suppressed the rotatory response, while the postural asymmetries were not altered by either drug. Muscimol injected into the rostral TPc induced contralateral rotation, marked ataxia, and postural asymmetries, particularly the head and neck, legs and wings. Whereas apomorphine (subcutaneous injection) was without effect on the rotatory response to muscimol, haloperidol suppressed the rotatory response. Neither drug effected the postural asymmetry. Following drug injections into either the pigeon TPc or the rat SN the behaviors induced in both bird and rat suggest that the TPc and SN are analogous.