Central and baroreflex control of heart period during the wake-sleep cycle in spontaneously hypertensive rats

We investigated whether the relative contribution of the baroreflex and central commands to the control of heart period differs between spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive rats (WKY) during physiological behavior. Rats were instrumented with an arterial catheter and with electrodes for discriminating wakefulness, nonrapid eye movement sleep (NREMS), and rapid eye movement sleep (REMS). The cross-correlation function (CCF) between spontaneous fluctuations of heart period and mean arterial pressure was computed at frequencies <0.2 Hz. The baroreflex determines a positive correlation between heart period and previous pressure values. This pattern was observed in the CCF during quiet wakefulness (QW) and NREMS, and in QW, it was accompanied by a pronounced negative correlation between heart period and subsequent pressure values. The relative baroreflex contribution to the control of heart period, estimated from the positive peak value of the CCF, was lower in SHR than in WKY during QW but not during NREMS. During REMS, the CCF showed a negative correlation between heart period and both previous and subsequent pressure values, reflecting the prevalence of central autonomic commands. The relative contribution of central commands to the control of heart period, estimated from the negative peak value of the CCF, was lower in SHR than in WKY during REMS. These results suggest that during QW and REMS, the control of heart period exerted by the baroreflex and central commands, respectively, is less effective in SHR than in WKY. This difference is not apparent in a behavioral state of autonomic stability such as NREMS.

Sleep-dependent changes in cerebral oxygen consumption in newborn lambs

During rapid-eye-movement (REM) sleep in adult subjects, the cerebral metabolic rate of oxygen consumption (CMRO(2)) is as high as that during wakefulness. We investigated whether CMRO(2) during active sleep is already at the waking level in newborn life, to support the role of active sleep as a state of endogenous brain activation during early postnatal development. Newborn lambs, 2-5 days old (n = 6), were instrumented with electrodes for sleep-state scoring, catheters for blood sample withdrawal and pressure monitoring, and a transit-time ultrasonic blood-flow probe around the superior sagittal sinus. At the age of 19 +/- 3 days, blood samples were obtained simultaneously from the carotid artery and the superior sagittal sinus during uninterrupted epochs of wakefulness, quiet sleep, and active sleep. The arteriovenous difference in blood oxygen concentration was multiplied by cerebral blood flow to determine CMRO(2). CMRO(2) during active sleep (47 +/- 5 micromol min(-1)) was similar to the value in wakefulness (44 +/- 6 micromol min(-1)) and significantly higher than in quiet sleep (39 +/- 5 micromol min(-1), P < 0.05). These data show that active sleep provides newborn lambs with brain activity at a level similar to that in wakefulness in terms of cerebral oxygen metabolism. The high CMRO(2) during active sleep supports its functional role during early postnatal development, when time spent in active sleep is at a lifetime maximum, albeit constituting a metabolic challenge for newborns, because of the impairment of systemic and cerebral vascular regulation in this sleep state.

Subchronic rolipram delivery activates hippocampal CREB and arc, enhances retention and slows down extinction of conditioned fear

Rolipram, a type IV-specific phosphodiesterase inhibitor, is known to improve memory under various learning tasks. Moreover, Rolipram treatments have been shown to increase expression and phosphorylation of a key factor for hippocampal memory consolidation, the cAMP-dependent response element-binding protein, CREB. However, the exact correlation between hippocampal CREB phosphorylation and memory improvement induced by Rolipram has not yet been determined in a CREB-dependent type of hippocampal-related learning in normogenic, intact rodents. Here, we report that subchronic Rolipram delivery by using osmotic minipumps increased the basal rat hippocampal expression and phosphorylation of CREB, as well as the expression of the cAMP-dependent, memory-related protein, Arc. In parallel, the same treatment improved memory consolidation of conditioned fear. Furthermore, the increase of CREB phosphorylation and Arc expression consequent to the learning experience was enhanced in Rolipram-treated rats, compared to controls. By evaluating the time course of memory extinction over 10 days after the initial learning test, we also observed significant slowing down of the memory extinction rate in Rolipram-treated rats. This effect could be attributed to CREB phosphorylation and memory having been initially higher, as osmotic minipumps stopped to release Rolipram the first day after the initial learning test. Our data define the conditions through which the pharmacological manipulation of hippocampal CREB expression and activation result in memory amelioration in normogenic, intact animals. These results are relevant for the study of molecular correlates of memory, and may also be important in view of the efforts to design new pharmacological treatments, targeting the CREB pathway and leading to enhancement of learning and memory, even in the absence of patent neuropathology.

Dysregulation of memory-related proteins in the hippocampus of aged rats and their relation with cognitive impairment

In the present experiments, we used conditioned fear to study whether changes in expression or functional state of proteins known to be involved in hippocampal learning could suggest correlation with age-related memory deficits. We focused on both alterations constitutively present in the hippocampus of aged rats and alterations related to different learning responses. Our results point at the dysregulation of the phosphorylation state of CREB in the hippocampus of aged rats as a primary biochemical correlate of their impaired memory. Other proteins, known to be important for various steps of memory formation and consolidation and linked to CREB, are to some extent altered in their constitutive expression or in the response to learning in the aged hippocampus. In particular, phosphorylated CREB and Arc, a protein functionally related to CREB in memory consolidation, are both present at constitutively higher levels in the hippocampus of aged rats, but they are not susceptible to the learning-related up-regulation occurring in young adults. Two other CREB-regulated proteins involved in memory consolidation, the neurotrophin BDNF and the transcription factor C/EBPbeta, are expressed at similar levels in the hippocampus of young-adult and aged rats, but their response to conditioned fear learning appears dysregulated by aging. Calcineurin, a protein phosphatase having CREB among its substrates and whose expression negatively correlates with learning, is more expressed in the hippocampus of aged rats. However, while calcineurin expression decreases in the hippocampus of young adults after learning, no changes are observed in the hippocampus of aged, learning-impaired rats.

Sleep-related brain activation does not increase the permeability of the blood-brain barrier to glucose

We compared blood-brain barrier (BBB) permeability to glucose between quiet wakefulness and rapid-eye-movement (REM) sleep to assess whether changes in BBB permeability play a role in coupling glucose supply to the physiologic metabolic needs of the brain. Male Sprague-Dawley rats were prepared with electrodes for wake-sleep state scoring and with arterial and venous catheters. Using the single-pass, dual-label indicator method, unidirectional glucose extraction by the brain and cerebral blood flow (CBF) were simultaneously measured during states of quiet wakefulness (n=12) or REM sleep (n=7). The product of BBB surface area and permeability to glucose (PS product) was computed in each state. During REM sleep, CBF significantly exceeded that during quiet wakefulness in all regions but the cerebellum, whereas the difference in the PS product between quiet wakefulness and REM sleep was not statistically significant in any brain region. In the brain as a whole, CBF significantly increased 29% from quiet wakefulness to REM sleep, while a nonsignificant 0.8% increase occurred in the PS product. During REM sleep, the increase in CBF indicates a higher rate of brain glucose consumption than in quiet wakefulness, given the tight flow-metabolism coupling in the brain. Therefore, these data show that modulation of BBB permeability to glucose is not a mechanism that provides 'energy on demand' during the physiologic brain activation characterising REM sleep.