Sub-cellular distribution of pharmacologically active substances in guinea pig brain

Role of substance P as a sensory transmitter in spinal cord and sympathetic ganglia

The hypothesis that substance P (SP) might be a transmitter of primary sensory neurons was first proposed by Lembeck in 1953. A large amount of evidence supporting this hypothesis has recently accumulated, particularly since the elucidation of the chemical structure of SP by Leeman and her colleagues in 1971, which made a number of new approaches possible (e.g. radioimmunoassay for SP, immunohistochemistry and electrophysiological tests of SP action on central and peripheral neurons). SP is concentrated in certain primary afferent terminals in the spinal cord, is released therefrom when the dorsal roots are electrically stimulated, and exerts a powerful excitant action on spinal neurons. It is therefore likely that SP produces excitatory postsynaptic potentials (EPSPs) in spinal neurons, although the characteristics of SP-mediated EPSPs, i.e. their time course, ionic mechanisms, etc., remain to be revealed. Recent electrophysiological and neurochemical studies on the prevertebral ganglia of the guinea-pig strongly suggest that SP is released from axon collaterals of visceral primary afferent neurons in the ganglia and serves as a transmitter that generates non-cholinergic slow EPSPs in principal cells. There is evidence that this SP-mediated synaptic transmission in the sympathetic ganglia is under the influence of enkephalinergic presynaptic inhibition. Some preliminary experiments on the interaction between SP and enkephalins in the spinal cord are described.

THE EXCITATION AND DEPRESSION OF MAMMALIAN CORTICAL NEURONES BY AMINO ACIDS

Amino acids related to L-glutamic and gamma-amino-n-butyric acid have been administered electrophoretically, and by pressure ejection, into the extraneuronal environment of single neurones in the pericruciate cortex of cats anaesthetized with allobarbitone or allobarbitone-urethane. Acidic amino acids related to glutamic acid, particularly N-methyl-D-aspartic acid, excited cortical neurones. Neutral amino acids related to gamma-amino-n-butyric acid, particularly 3-amino-1-propanesulphonic acid, depressed cortical neurones. Some of the depressants blocked the antidromic invasion of Betz cells by pyramidal volleys. There are no essential differences between the sensitivities of cortical and spinal neurones towards locally administered amino acids. A transmitter function of such amino acids within the mammalian central nervous system is considered unlikely.

Substance P in rat brain synaptosomes

1. Rat brain synaptosomes were incubated under different conditions to study the release of substance P (SP). 2. Potassium ions and electrical field stimulation induced a loss of SP from synaptosomes. The release of SP by potassium in high concentrations (23.8 mM) was shown to be calcium dependent. 3. Substance P was retained in synaptosomes during incubation in 0.32 M sucrose at +4 degrees C up to 120 min. During incubation at 30 degrees C the SP content fell initially (30 min) but was gradually restored (120 min). 4. If these pre-incubated synaptosomes were reincubated for 45 min at 30 degrees C in potassium free Krebs-Ringer-phosphate buffer a further rise in their SP content occurred which was taken as indication that SP is being synthesized in synaptosomes. 5. The newly synthesized SP is presumably stored by binding to phosphatidyl serine until a sudden release is initiated by depolarization.

BOUND γ-AMINOBUTYRIC AND OTHER AMINO ACIDS IN BRAIN

Previous work has shown that when brain is homogenized in saline medium, part of the γ-aminobutyric acid (GABA) in the tissue is found in free solution and part is bound or occluded in particulate material. In the present work it is shown that the GABA content of solid brain or anaerobic brain suspensions increases with time and the increase is all in the free form. In suspensions in the presence of oxygen and glucose no increase in GABA occurs. A lower proportion of the GABA is found in bound form if the suspension is prepared in the cold rather than at room or body temperature. The amount found in bound form is sharply dependent on pH. The optimum is about pH 7. If brain is homogenized in plain sucrose solutions, or if potassium is substituted for sodium in saline solution, the amount of bound GABA found is half that found in isosmotic sodium chloride. The normal amount remains bound if 10 mM sodium ion or 5 mM calcium ion is present in the sucrose solution. Glutamic acid, glutamine, alanine, and aspartic and acetylaspartic acids are also partly bound in brain. The amounts of these amino acids as well as of GABA found in brain frozen in situ are considerably lower than in brain which has been excised rapidly after death but without freezing.