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In 1-week-old domestic chicks, the connectivity of the lobus parolfactorius (LPO), part of the avian basal ganglia, was investigated using Phaseolus vulgaris leucoagglutinin and horseradish peroxidase for anterograde and retrograde pathway tracing, respectively. Tyrosine hydroxylase immunocytochemistry was applied in combination with Phaseolus lectin to assess the overlap between LPO efferents and diencephalic and mesencephalic catecholamine centres. Anterograde projections from LPO were detected in the hyperstriatum, neostriatum, and paleostriatum. Intranuclear connections were also apparent within the LPO. Descending LPO efferents innervated the lateral mammillary and intramedial nuclei and the dorsomedial thalamic complex. Fibres from LPO were observed in the tectal gray, substantia nigra, area ventralis tegmentalis of Tsai, and the adjacent nucleus mesencephalicus profundus. Further caudally, projections from LPO reached the nucleus papillioformis, locus coeruleus, and subcoeruleus ventralis. LPO efferents were coextensive with tyrosine hydroxylase-positive cells in the nuclei mamillaris lateralis and intramedialis of the hypothalamus, area ventralis tegmentalis, substantia nigra, locus coeruleus, and subcoeruleus ventralis of mesencephalic and pontine tegmentum. Close contacts between LPO fibres and catecholamine cells were visible in the nigra and the area ventralis tegmentalis. Retrograde labelling from LPO was found in the archistriatum, dorsomedial thalamic complex, nuclei lateralis anterior and superficialis parvicellularis thalami, substantia nigra, central gray, area ventralis tegmentalis of Tsai, and locus coeruleus and in cells dorsal to the decussation of brachium conjunctivum. Reciprocal connections were verified between the LPO and the following areas: dorsomedial thalamic complex, central gray, substantia nigra, area ventralis of Tsai, and locus coeruleus.

The visual hyperstriatum of the 100-g chick has been investigated for responses evoked by electrical stimuli to the optic papilla. The visual hyperstriatum is not restricted to the anterior forebrain; posterior regions are also responsive, but for maximum response in amplitude the stimulating electrodes need to be re-positioned and, for reliable detection, a 10-fold gain in amplification is required.

The thalamic nuclei in birds are known to receive afferents from the retina and project to the hyperstriatum. However, the efferent connections of one of these, the anterior lateral nucleus (LA) have been a subject of controversy for some time. Using horseradish peroxidase tracing techniques we have shown that, in the chick, the efferents of the thalamus (including the LA) project to the visual hyperstriatum. In the case of the LA this is to the right hyperstriatum only and to posterior regions not previously regarded as having a visual function.

Repeated stimulation can lead to a decrement in the amplitude of a response, or its disappearance. This is known as habituation. Responses recorded in the reticular formation of the brain to peripherally applied stimuli tend to habituate. Decerebration considerably reduces the occurrence of this habituation and naloxone reverses the effect. In decerebrate animals in which such responses rarely habituate, habituation can be induced by administration of D-Ala2,Met5-enkephalinamide and then reversed by a subsequent administration of naloxone. This would suggest that opiates are involved in habituation of responses recorded in the reticular formation of the frog.

The thalamic input to the barrelfield of rats with a solitary mystacial vibrissa (C3) intact was examined using succinic dehydrogenase techniques. Rats were lesioned on postnatal day (PD) 1 and brains examined at either 7, 12, 21 or 28 days later. The resultant segmentation in the hemisphere contralateral to the lesioned muzzle consisted of fused rows (A to E). At all ages row C differed from the other rows in that the center of the row was enlarged and separated from the row on either side of it. Thus the presence of a single vibrissa is sufficient to organize the thalamic input to its barrel.

The distribution of [3H]naloxone binding sites was investigated in the brains of one-day-old domestic chicks using quantitative receptor autoradiography. Among the forebrain regions, the greatest levels of opiate binding were found in hyperstriatum dorsale, hyperstriatum ventrale, hyperstriatum intercalatum supremum and neostriatum. Intermediate levels were found throughout the paleostriatal regions, septum, thalamus, archistriatum, hyperstriatum accessorium and area parahippocampalis whilst in hippocampus and ectostriatum the density of [3H]naloxone binding sites was low. In the hindbrain, high levels of opiate binding were found in optic tectum whereas in cerebellum the density of binding sites was barely above background. The greatest densities of opiate binding appear to coincide with regions found to be involved in sensory processing and memory storage.