The modulation of memory processes by vasotocin, the evolutionarily oldest neurosecretory principle

Biochemistry of vasopressin fragments

Vasopressin (VP) undergoes a step-wise aminopeptidase conversion process in the brain, leading to accumulation of several metabolites. Some of these metabolites, in particular [pGlu4,Cyt6]VP 4-9 and 4-8, show behavioral effects comparable to VP, but are more potent and selective than VP. Most data favor the existence of a separate receptor for the VP metabolites distinct of the classical VP and oxytocin receptors, although its identity has remained obscure thus far. The characterization of this receptor is a major challenge to understand how the brain VP system generates and regulates divers central functions.

The behavioral neurobiology of learning and memory: a conceptual reorientation

Research on the neurobiology of learning and memory has been guided by two major theories: (i) memory as a psychological process and (ii) memory as a change in synaptic neural connectivity. It is not widely recognised that not only are these theories different but, moreover, they are fundamentally incompatible. Confusion concerning basic concepts in the learning and memory field in mammals has lead to the creation of an extensive but often inconclusive experimental literature. However, one important conclusion suggested by recent work in this field is that experience-dependent changes in neural connectivity occur in many different brain systems. Particular brain structures, such as the hippocampus, do not play any uniquely important role in experience-dependent behavior. Research in learning and memory can be best pursued on the basis of biological studies of animal behavior and a cellular approach to brain function.

Influence of an analog of vasopressin on the reaction of command neurons of defensive behavior of the edible snail during the stimulation of nerves

An increase was found at the level of the command neurons of defensive behavior in the amplitude of summary excitatory postsynaptic potential (EPSP) recorded in the command neurons in response to nerve stimulation; as were an increase in the probability of the occurrence of an action potential in the reaction, and under certain conditions, a substantial decrease in the rate of habituation during rhythmic (0.1 Hz) nerve stimulation in the presence of a peptide. This last effect was observed when two groups of neurons, the control and those to which the peptide was presented prior to the first stimulation series in the experiment, i.e., without the preceding development of habituation, were compared. The decrease in the rate of habituation affected both the amplitude of the summary EPSP and the probability of the occurrence of an action potential in the reaction. All these changes in the presence of the analog may underlie the increase in spike reactions of the command neurons of defensive behavior, and may thus underlie the initiation or an increase in defensive behavioral reactions. The effects obtained were not long-lived, and took place only in the presence of the peptide in the extracellular milieu.

Penile erection and yawning induced by oxytocin and related peptides: structure-activity relationship

The potency of several oxytocin-related peptides in inducing penile erection and yawning after injection into a lateral ventricle of male rats was compared. Substitution of two amino acids in the oxytocin molecule or deletion of the C-terminal glycinamide as in des-GlyNH2-oxytocin [oxytocin(1-8)] reduced oxytocin potency in inducing both effects, the rank order being: oxytocin greater than [Thr4,Gly7]-oxytocin congruent to isotocin [( Ser4,Ile8]-oxytocin) greater than vasopressin [( Phe3,Arg8]-oxytocin) greater than des-GlyNH2-oxytocin. Oxytocin's ability to induce penile erection and yawning was abolished by permanent opening of the disulfide bridge by reduction and carboxymethylation. Oxytocin(1-6) and oxytocin(7-9) were also inactive. Penile erection and yawning induced by oxytocin-related peptides were antagonized in a dose-dependent manner by nonapeptide antagonists with a rank order of potency that follows their antioxytocic activity (d[(CH2)5Tyr(Me)Orn8]-vasotocin congruent to [Pen1,Phe(Me)2,Thr4,Orn8]-oxytocin greater than d[(CH2)5Tyr(Me)Arg8]-vasopression). Carboxymethylated oxytocin, oxytocin(1-6), and oxytocin(7-9) were devoid of antagonistic activity. The present results suggest that central oxytocin receptors mediating the expression of penile erection and yawning are structurally related to those present in the uterus and in the mammary gland.

Influence of protease inhibitors on the antidepressant activity of oxytocin

Both in the behavioral despair test and in the learned helplessness test, the antidepressant-like activity of oxytocin (0.500 mg/Kg i.p.) was prevented by the administration of a protease inhibitor (EACA, 400 mg/Kg i.p.; pepstatin, 0.1 mg/Kg i.p.). On the other hand, the linear tripeptide tail of oxytocin, MIF-1, was inactive in the behavioral despair test, and less active than oxytocin in the learned helplessness test. We conclude that the antidepressant activity of oxytocin is due to its cleavage to some smaller fragment(s) different from MIF-1.

Modulation of the immune response by emotional stress

The influence of mild, emotional stress was investigated for its effect on the immune system by subjecting rats to the one-trial-learning passive avoidance test. The reactivity of the immune system was tested by determining the proliferative response after mitogenic stimulation in vitro as well as the capacity to generate a primary antibody response in vivo after immunization with sheep red blood cells. Our results demonstrate that exposure of rats to a single electric footshock (learning trial) or habituation to the passive avoidance apparatus, induces an increase of the immune response in vitro and in vivo. Thus, emotional stimuli seem to facilitate immunological responsiveness. However, when the animal is confronted with a conflict situation, as tested by the retention of the avoidance response after a single learning trial, the initially enhanced reactivity of the immune system decreases. It is concluded that the immune system is capable of reacting specifically and immediately to distinct psychological stimuli.

Effects of peripherally injected pituitary peptides on recognition memory in pigeons

The effects of peripheral injection of arginine vasopressin (AVP), oxytocin (OT), arginine vasotocin (AVT), adrenocorticotrophic hormone (ACTH4-10) and alpha-melanocyte-stimulating hormone (alpha-MSH) were studied, using pigeon subjects and a pair comparison task, with and without intervening delays between stimuli presentation. The highest dose (20 micrograms/kg) of AVT impaired performance by disrupting input, but not storage. General cognitive ability was unimpaired, as were perceptual mechanisms. None of the other peptides affected recognition memory, in that forgetting curves were unchanged when compared with control. The results are discussed in terms of species-specific roles for these peptides.

A critique of the vasopressin-memory hypothesis

During the past 20 years, evidence has accumulated to suggest that the neuropeptide vasopressin (VP) enhances memory by acting on central mechanisms, and that oxytocin (OT) has amnestic effects. In this review, the evidence for the memory hypothesis with respect to VP is considered and alternative interpretations evaluated. A critical approach has been adopted; negative findings, design considerations and problems with the various hypotheses are given prominence. It is concluded that the memory hypothesis fails to provide an adequate account, and some alternative theories and suggestions are discussed. It is speculated that the peptide may affect behaviour by two distinct mechanisms: peripheral action may involve reinforcement mechanisms, but its central role may be to modulate arousal level, especially in stressful situations.

Oxytocin is a precursor of potent behaviourally active neuropeptides

An oxytocin fragment which accumulated during the incubation of oxytocin with brain synaptic membranes was chemically characterized as the hexapeptide pGlu-Asn-Cys(Cys)-Pro-Leu-Gly-NH2 [( pGlu4, Cyt6]OXT-(4-9]. This peptide was approximately a hundred times more potent than oxytocin in attenuating memory consolidation as tested in a passive avoidance test situation; the dose-response relationship was bell-shaped. The des-glycinamide derivative [pGlu4, Cyt6]OXT-(4-8) was nearly as active, but showed a linear dose-response relationship. The data indicate that oxytoxin can act as precursor for potent behaviourally active neuropeptides.

A major metabolite of arginine vasopressin in the brain is a highly potent neuropeptide

A peptide that accumulated as the major product during the proteolysis of arginine vasopressin by rat brain synaptic membranes was isolated and its structure was shown to be the hexapeptide pGlu-Asn-Cys(Cys)-Pro-Arg-Gly-NH2. When administered intracerebroventricularly in extremely low doses, this vasopressin fragment and its desglycinamide derivative facilitated memory consolidation in a passive avoidance situation. These vasopressin metabolites, which are devoid of pressor activity, constitute highly potent neuropeptides with selective effects on memory and related processes; they are activated via proteolytic processing of vasopressin by brain peptidases.

Neuropeptides, mental performance and aging

The presence of peptidergic neuronal networks in the brain and the modulating action of neuropeptides on brain functions as evidenced by their behavioral influence in particular support the concept that the brain like the peripheral endocrine glands is an endocrine target organ which is as sensitive to treatment with neuropeptides as the peripheral glands are to pituitary hormones. Animal and human data are reviewed showing that neuropeptides related to ACTH/MSH affect motivational and attentional processes and that those related to vasopressin are involved in memory processes. Since these functions decline during aging it is postulated that a decreased bioavailability of neuropeptides in brain of elderly people is associated with specific disturbances in mental performance. Thus, the decreased mental ability of the aged may be restored by treatment with neuropeptides particularly those with little, if any, peripheral, endocrine activity, like the ACTH neuropeptide Org 2766 and the vasopressin neuropeptide DGAVP.