Temperature is an important factor for nervous activity. In physiological conditions we expect the temperature-regulating mechanisms of the homeothermic animal to prevent large temperature changes. However, in addition to the well-documented changes in brain temperature during feeding and sleeping (Abrams & Hammel, 1964), we have recently observed other, surprisingly large variations of brain temperature in behaving rats (Moser, Mathiesen & Andersen, 1993). There are also indications that brain temperature may change appreciably in man, both during exercise and after cooling of the facial skin (Nielsen, 1988). Furthermore, drugs and anaesthesia may cause the temperature of the brain to drop appreciably.

In the present short survey, we review the temperature changes that can be recorded from the brain of freely moving rats and the consequences such changes have on field and unitary potentials.

Field potentials and brain temperature in freely moving rats

In rats swimming in a Morris water maze (Morris, 1984) at 18°C (Moser et al., 1993), we noticed quite large and fast changes of simultaneously recorded hippocampal field potentials (Fig. 20.1 A, B). There was an increased latency of both the field excitatory postsynaptic potential (f-EPSP) and the population spike. Surprisingly, the latter increased greatly in amplitude. The slope of the f-EPSP, measured at its maximum, was reduced. All field potential changes depended on the temperature of the water, but were still quite large at the usually employed temperature of 25°C.