  Temperature is one of the most relevant parameters for the regulation of
intracellular processes. Measuring localized subcellular temperature gradients
is fundamental for a deeper understanding of cell function, such as the genesis
of action potentials, and cell metabolism. Here, we detect for the first time
temperature variations (1{\deg}C) associated with potentiation and depletion of
neuronal firing, exploiting a nanoscale thermometer based on optically detected
magnetic resonance in nanodiamonds. Our results provide a tool for assessing
neuronal spiking activity under physiological and pathological conditions and,
conjugated with the high sensitivity of this technique (in perspective
sensitive to < 0.1{\deg}C variations), pave the way to a systematic study of
the generation of localized temperature gradients. Furthermore, they prompt
further studies explaining in detail the physiological mechanism originating
this effect.
