Dissociation of hypertension and fixed interval responding in two separate strains of genetically hypertensive rat

The behaviour of spontaneously hypertensive rats (SHR) may model attention-deficit hyperactivity disorder. For example, SHR are hyperactive in an open field and show high terminal rates of responding on certain fixed-interval schedules. Open field behaviour has been dissociated from the accompanying spontaneous hypertension but fixed interval responding has not. We compared the fixed interval responding of two unrelated strains of genetically hypertensive rat, the SHR (n=6) and the New Zealand genetically hypertensive rat (GH, n=5), with their normotensive control strains, the Wistar Kyoto (WKY, n=6) and Wistar (n=5), respectively. Both hypertensive rat strains showed increased terminal lever-pressing rates on a multiple fixed-interval schedule (FI-EXT) compared to controls. In order to investigate the association of hypertension and the behavioural characteristics in question, an F-2 hybrid strain was obtained by cross-breeding GH and Wistar rats. When these F-2 hybrids (n=33) were tested on the FI-EXT schedule, terminal lever-pressing rate was not correlated with blood pressure. The independent segregation of these phenotypical characteristics in the hybrids suggests independent genetic control. By contrast, other behavioural characteristics, including high lever-pressing rates during the extinction component and a tendency to emit responses in bursts, did cosegregate with terminal lever-pressing rates. Taken together, these findings suggest that the genetic loci for high blood pressure and responding on the FI-EXT schedule in these two unrelated rat strains are close but distinct.