1. A unique feature of cephalopods is their skin, which contains thousands of
chromatophore organs under direct neural control from the chromatophore lobes of
the brain. Control of skin colour changes are described in this thesis in terms of the
morphology, electrical activity, and pharmacology, of the Posterior Chromatophore
Lobe (PCL) neurons, in the cephalopods Alloteuthis sub ulat a, Loligo vulgaris,
Eledone cirrhosa, and Octopus vulgaris. The morphology and electrophysiology of
the Lateral Basal Lobe (LBL) neurons and the influence of this lobe on PCL
neurons are also investigated.
2. The chromatophore lobe neurons are inaccessible in vivo and thus a brain slice
preparation was developed to allow access to the motoneurons in the PCL. This
allowed a study of the morphology, electrical activity, and pharmacology of these
neurons to be carried out.
3. An artificial sea water solution was developed to provide a suitable bathing
medium for the brain during slicing and recording procedures. The brain slice
could be maintained for up to eight hours using this medium, and stable
intracellular recordings could be obtained.
4. Over 500 neurons in PCL slices, from 146 animals, were impaled with
intracellular recording electrodes and of these, 115 were subsequently filled with
the fluorescent dye Lucifer Yellow to reveal their morphologies. It was found that
PCL neurons are regularly arranged, with the largest cell somata being on the
periphery of the lobe. On the basis of differences in the morphologies and locations of these cells, they could be divided into four types, referred to in this
thesis as cell type I, II, III and IV.
5. The intracellular recordings revealed four different types of spontaneous electrical
activity: tonic, irregular bursting, regular bursting, and excitatory postsynaptic
potentials (EPSPs). Regular bursting activity was only observed in cell type II.
EPSP activity was observed in all the cell types but was most predominant in cell
type III.
6. Evidence of both dye and electrical coupling between PCL neurons was obtained.
The possible implications of this coupling are discussed.
7. In the light of the existing morphological data, the afferent input from the Lateral
Basal Lobe (LBL) was established physiologically by electrical stimulation of the
LBL tract connecting with the PCL. The effect on the PCL neurons was excitatory
or inhibitory. It is proposed that one LBL cell influences many PCL motoneurons
via inhibitory and excitatory synapses. Patterning on the skin could therefore be
achieved by control of the frequency of activity displayed in PCL motoneurons,
which is regulated by the LBL neurons.
8. Extracellular field potentials revealed the extent and nature of the activity in
groups of PCL neurons in response to LBL tract stimulation. These responses were
largest in the neuropil area, becoming weaker towards the periphery of the lobe.
Field potential responses showed that the synaptic connections between the LBL
and PCL neurons occurred at the point where the LBL tract enters the PCL
neuropil.
9. Application of acetylcholine (ACh) and carbachol to tissue slices, caused tonically
active PCL motoneurons to stop firing, while 5-HT caused an increase in the
frequency of tonic activity and the number of EPSPs. The application of antagonist
drugs of putative neurotransmitters revealed that L-glutamate, 5-HT and ACh
receptors are present on the postsynaptic membrane of PCL cells.
10. These results demonstrate that the brain slice is a useful and versatile preparation
for the study of the electrical properties and connections of cells in the cephalopod
brain and provide the first physiological clues about how PCL neurons may be
controlled as they change the colour of cephalopod skin.