An investigation of the neural mechanisms controlling the colour change responses of the dogfish, Scyliorhinus canicula L. by mesencephalic and diencephalic lesions

Abstract

Small, central and bilateral electrolytic lesions and cuts were placed in the mesencephalon and diencephalon of the dogfish ( Scyliorhinus canicula ) to locate the areas involved in the inhibitory control of release of melanophore-stimulating hormone from the neurointermediate lobe of the pituitary. A mid-line hypothalamic tract was demonstrated from the region of the nucleus medius hypothalamicus (n. m. h.) to the neurointermediate lobe; section of this tract led to darkening of the fish. This system is thought to be aminergic. Ablations of the pre-optic nucleus or ‘classical’ neurosecretory tracts anterior to this system were without effect on colour change as were lesions in the more lateral regions of the hypothalamus involving the nuclei lobi inferiores (n. l. i.). In the posterior thalamus bilateral areas immediately anterior to the recessus posterior were shown to be involved in the mechanisms controlling melanophore-stimulating hormone release. Damage to the optic system, including optic nerves, ascending tractus opticus or tectum also produced a darkening response. The neuronal connexions from the optic system to the thalamus or hypothalamus were not located though the bilateral thalamic region may link anteriorly with the n. m. h. Ultrastructural observations of the n. m. h. and n. l. i. demonstrated that both nuclei are composed of neurons containing dense-cored vesicles of 90 nm diameter, thought to be carriers of amines. Somatic-synapses, containing similar vesicles, were present on the cells of both nuclei; thus there may be another aminergic component, associated with the n. m. h., in the colour change mechanisms.