Predicting extinction phenotype to optimize fear reduction

Fear conditioning is widely employed to study dysregulations of the fear system. The repeated presentation of a conditioned stimulus in the absence of a reinforcer leads to a decrease in fear responding-a phenomenon known as extinction. From a translational perspective, identifying whether an individual might respond well to extinction prior to intervention could prove important to treatment outcomes. Here, we test the hypothesis that CO₂ reactivity predicts extinction phenotype in rats, and that variability in CO₂ reactivity as well as extinction long-term memory (LTM) significantly predicts orexin activity in the lateral hypothalamus (LH). Our results validate a rat model of CO₂ reactivity and show that subcomponents of behavioral reactivity following acute CO₂ exposure explain a significant portion of the variance in extinction LTM. Furthermore, we show evidence that variability in CO₂ reactivity is also significantly predictive of orexin activity in the LH, and that orexin activity, in turn, significantly accounts for LTM variance. Our findings open the possibility that we may be able to use CO₂ reactivity as a screening tool to determine if individuals are good candidates for an extinction/exposure-based approach.

Vascular recovery following hemorrhage in the dogfish shark Squalus acanthias

Cardiovascular regulatory systems were examined in the dogfish shark after hemorrhage of 1% of body weight. An immediate 45% decrease in mean dorsal aortic pressure and delayed increases in plasma epinephrine concentration and plasma norepinephrine concentration were noted. During the recovery period following hemorrhage, the catecholamine levels peaked (epinephrine 203% of control, norepinephrine 148% of control) and then returned toward control values. Hematocrit, however, continued to decline as blood pressure recovered. Phentolamine pretreatment, which in itself caused a 55% decrease in dorsal aortic pressure, did not prevent the recovery of blood pressure after hemorrhage. This study indicates that 1) hemorrhage causes an increase in circulatory catecholamine concentration, possibly by a baroreceptor reflex; 2) volume recruitment, as indicated by the drop in hematocrit, is a major mechanism contributing to the recovery of dorsal aortic pressure after hypotension; and 3) alpha-adrenergic receptors are not necessary for the recovery of dorsal aortic pressure after hemorrhage.

Effect of ganglionic blockade on catecholamine secretion in exercised dogfish

A brief bout of vigorous exercise results in significant increases in plasma epinephrine (E) and norepinephrine (NE) in the dogfish, Squalus acanthias. Since the presence of a functioning sympathetic nervous system in dogfish is in doubt, experiments were undertaken to show whether or not exercise-induced catecholamine (CA) secretion is under autonomic neurogenic control. Changes in plasma E and NE in a control group of exercised fish were compared with changes in fish exercised while under the influence of ganglionic blockade. Ganglionic blockade was induced in dogfish by hexamethonium infusion before exercise. CA secretion in response to a subsequent bout of exercise was significantly reduced without impairment of the ability of the fish to exercise. The pattern of systemic arterial pressure response to exercise and recovery (initial decrease during exercise followed by a prompt recovery to control level) was not significantly altered by ganglionic blockade. It is concluded that in dogfish some fraction of CA secretion capacity is possibly or potentially under neurogenically related control. Apparently the fraction of CA secretion under such control is not essential for performing exercise. The pattern of CA secretion accompanying the events of exercise and recovery in dogfish suggests that CA may play a more important role in recovery from exercise than in its performance.

Dual mechanism for catecholamine secretion in the dogfish shark Squalus acanthias

Both 1,1-dimethyl-4-phenylpiperazinium iodide, a ganglionic stimulating drug (DMPP), and potassium ion (K+) cause a pressor response when injected into Squalus acanthias, an elasmobranch. The pressor responses are due to increased secretion of epinephrine and norepinephrine. The pressor response to DMPP can be blocked by prior infusion of hexamethonium, a ganglionic blocking drug. However, ganglionic blockade does not inhibit the pressor response to K+. Plasma catecholamine concentrations do not increase significantly in response to challenge with DMPP after hexamethonium infusion, but exceedingly high levels of plasma catecholamines quickly appear after K+ injection following hexamethonium infusion. It is concluded that there are at least two mechanisms controlling catecholamine secretion in the dogfish, one of which involves the ganglion cells that are intimately associated with chromaffin cells in the chromophil bodies that are so characteristic of this species and elasmobranchs in general.

Dogfish pressor response to potassium blocked by magnesium and phentolamine

In vivo infusion of MgCl2 blocks the dogfish pressor response to K+. This action of Mg2+ was contrasted to phentolamine in in vivo and in vitro experiments. Mg2+ blocks the spontaneous release of catecholamines from dogfish chromaffin tissue but does not alter the norepinephrine-induced contraction of the isolated dogfish artery. In vivo infusion of Mg2+ causes a significant decrease in resting catecholamine levels and diminishes the catecholamine release caused by K+ challenge. Both Mg2+ and phentolamine block the pressor action of K+, Mg2+ by preventing the K+-induced release of catecholamines and phentolamine by preventing the circulating catecholamines from interacting with alpha-adrenergic receptor sites.

Catecholamine release and blood pressure changes induced by exercise in dogfish

Plasma norepinephrine (NE), epinephrine (E), and potassium (K+) were measured before, during a 3-min bout of exercise, and at intervals after exercise in Squalus acanthias. The dorsal aortic pressure response following 1 min of exercise was observed in another series of experiments. Plasma E, NE, and K+ increased significantly and progressively during the exercise period and for 2 min after exercise. Plasma E increased significantly during the 1st min; NE during the 2nd min; but K+ did not increase significantly until the 3rd min of exercise. Blood pressure decreased significantly during 1 min of exercise but increased over control level within 4 min after the end of exercise coincident with the peak of plasma E and NE concentrations. Plasma NE, E, and blood pressure decreased slowly in parallel fashion during the 37-min postexercise period of observation. Increased plasma K+ from skeletal muscle is probably not the primary stimulus for early catecholamine release in exercising dogfish, but increased plasma K+ does contribute to releasing and maintaining plasma NE and E levels in the postexercise period. This results in a sustained pressor effect that promotes blood flow and aids in metabolic recovery.

Systemic arterial pressor responses induced by potassium in dogfish, Squalus acanthias

Intravascular injection of small doses of potassium (0.025-0.5 meq) into dogfish results in dose-related dorsal aortic pressor responses. The responses are blocked by phentolamine, an alpha-adrenergic blocking agent. Assays of plasma catecholamines before and after injection of potassium (K+) showed that plasma levels of epinephrine (E) and norepinephrine (NE) had increased significantly (E, 314%; NE, 233%) 1 min after injection. The pressor responses were initiated 40-90 s after K+ injection at which time plasma E and NE levels were already significantly elevated. Experiments on isolated dogfish arterial strips showed that phentolamine cannot block the increase in vascular smooth muscle tension that occurs after direct exposure to small increases (0.003 meq/ml) in K+ concentration. Because phentolamine effectively blocks the pressor response to even higher doses of K+ in vivo, it is thought that, in addition to any direct vasoconstrictor effect or K+, the adrenergic stimulation provided by catecholamine release is required to produce the observed pressor response.