Credit: Confocal image of mature dentate granule neurons born during development (green) and adulthood (red). Dividing progenitor cells of the developing and adult dentate gyrus of the same mouse were labelled by retroviral transduction with green (GFP) or red fluorescent proteins (mRFP1), respectively. Image courtesy of V. C. Piatti and N. A. Morgenstern, Fundación Instituto Leloir, Buenos Aires, Argentina.

New neurons are generated in the hippocampal dentate gyrus throughout life. Although immature neurons born in adulthood differ in their functional properties from neurons born perinatally, it is unknown whether this distinction continues to exist upon maturation of the cells. Two recent studies set out to answer this question, finding both similarities and differences between old and young mature neurons.

In a series of experiments in mice, Laplagne et al. used double retroviral labelling so that neural progenitor cells born perinatally were labelled with a green fluorescent marker, whereas cells born in adulthood were labelled red. When the mice were 19 weeks old and all labelled neurons had matured, electrophysiological recordings revealed that evoked excitatory postsynaptic currents in red-labelled neurons were comparable to those in neighbouring green-labelled neurons. These currents were blocked by glutamate receptor antagonists, indicating that old and young mature granule cells received similar glutamatergic input from the entorhinal cortex. Likewise, inhibitory postsynaptic currents did not differ between neurons of different ages, suggesting similar GABA (γ-aminobutyric acid)-mediated afferent connectivity with interneurons.

Finally, the authors showed that firing behaviour in response to an excitatory stimulus was comparable in mature neurons born in adulthood and those born during development, indicating that excitatory input was integrated in a similar way in both types of neurons. Taken together, these results indicate that mature adult-born and perinatally born mouse hippocampal neurons have similar inhibitory/excitatory input balance and firing behaviours and in these respects form a homogeneous population.

By contrast, a second paper showed that, in rats, mature hippocampal neurons of different ages respond differently to certain behavioural conditions. Ramirez-Amaya et al. used high-resolution confocal microscopy to detect granule cells immunolabelled for ARC, an immediate-early gene product which is produced in some neurons in response to exploration and is involved in long-term synaptic plasticity. The proportion of cells producing ARC after spatial exploration was higher in adult-born neurons than in perinatally born ones, indicating that neurons born in adulthood show enhanced plasticity when mature.

Keeping in mind that the two studies used different species and housing conditions, the data provide evidence that adult-born and perinatally born hippocampal neurons are integrated into similar neural networks, but might have different roles in spatial memory formation.