Abolition of conditioned heart-rate responses in rabbits following central administration of [N-MePhe3, D-Pro4] morphiceptin

Morphine promotes autonomic learning

Morphine (0.01-10 mg/kg) promoted rapid autonomic learning of discriminative, Pavlovian conditioned heart rate decelerations to tone signals in male and female rabbits, and the higher doses (1-10 mg/kg) promoted decelerative heart rate orienting reflexes to novel tones. Morphine dose dependently reduced heart rate acceleration to signaled shock but had no effect on heart rate acceleration to unsignaled shock. Morphine did not impair retention of cardiac conditioned reflexes, and its U-shaped dose effect, increasing conditioned heart rate discrimination early in training, reappeared in extinction. The authors propose that morphine promotes autonomic learning of preparatory, compensatory reflexes to signaled stressors that reduce their stressful effects. This action may mimic the normal, adaptive function of an endogenous messenger released by the Pavlovian contingency.

Evidence for an involvement of the mu-type of opioid receptor in the modulation of learning

Pavlovian conditioning of the rabbit's nictitating membrane response was used to determine whether the retardation of learning produced by enkephalin derivatives was mediated through an action on mu- or delta-opioid receptors. Intraventricular injection of the highly selective mu-agonist, [D-Ala2, MePhe4, Gly-ol5] enkephalin (DAMGO) significantly retarded (2 nmol) or completely blocked (20 nmol) the acquisition of conditioned responses. The retarded or blocked acquisition of conditioned responses produced by DAMGO could still be detected when the rabbits were tested two days after cessation of drug injections, suggesting that DAMGO had affected learning and not simply performance of conditioned responses. Intraventricular injection of a comparable dose (20 nmol) of the highly selective delta-agonist, [D-Pen2,5] enkephalin (DPDPE) did not retard learning. A higher dose of DPDPE (200 nmol), which would be expected to act at both mu- and delta-receptors, did produce a retardation of acquisition that was comparable to the effects of a 2-nmol dose of DAMGO. However, unlike DAMGO, the retardation of learning produced by this dose of DPDPE appeared to be due, at least in part, to a reduction in the aversive properties of the unconditioned stimulus. The retardation of learning produced by 2 nmol of DAMGO and 200 nmol of DPDPE was due to an action on an opioid receptor because this effect was blocked by a subcutaneous injection of naloxone (2.5 mumol/kg), a specific opioid antagonist. The stereoselective effects of the mu-opioid agonist levorphanol further confirmed that the retarded learning was mediated through actions at the mu-opioid receptor. Autoradiographic studies indicated that a 2-nmol, intraventricular dose of DAMGO achieved contents greater than 1 pmol/mg in tissue lying within 2 mm of the lateral ventricle, third ventricle and cerebral aqueduct. These data suggest that enkephalins produce a retardation of learning through an effect on the mu-type of opioid receptor and that at least one locus of their actions is produced at periventricular sites lying within 2 mm of the ventricles.(ABSTRACT TRUNCATED AT 250 WORDS)

Acquisition of classical conditioning without cerebellar cortex

The left cerebellar cortex was surgically aspirated in rabbits who were then subsequently trained for classical conditioning of the nictitating membrane. All rabbits were trained sequentially on both eyes. Rabbits with the lesion confined to the cerebellar cortex were able to learn with the eye ipsilateral to the lesion although it took many times longer than reported for either naive rabbits or for rabbits first trained on the unlesioned, contralateral side. Rabbits with lesions that included the cerebellar cortex and the cerebellar interpositus nucleus did not learn with the eye ipsilateral to the lesion. Learning with the eye contralateral to either type of lesion was always very rapid. It is now clear on the basis of this and previous studies that cerebellar cortex, unlike the cerebellar interpositus nucleus, is not essential for acquisition or relearning/retention of classical conditioning. However, cerebellar cortex normally plays an important role since acquisition of classical eyeblink conditioning is prolonged and of poor quality in its absence.

Sex-dependent effects of neonatally administered morphiceptin and D-ala-D-leu-enkephalin on maze learning in rats

Newborn rats were injected (ip) with morphiceptin [72.7 micrograms/kg (a mu-type opioid receptor agonist)], D-alanine2-D-leucine5-enkephalin [79.4 micrograms/kg (a delta-receptor agonist)], or saline for 7 days after birth. Testing on a complex maze on Day 25 revealed a significant sex-dependent facilitation of performance by the opioid peptides. Peptide-treated females performed better than the males on the first day of training as measured by errors. Opioid treatment increased mortality, as three times as many peptide-treated animals died in comparison to the saline control group. Peptide treatment did not affect locomotor activity measured in an open field. Weight at Day 24 was also affected by the peptide treatment, females and males injected with the opioids being lighter and heavier, respectively, than the control group.

Effect of bilateral lesions of the dentate and interpositus cerebellar nuclei on conditioning of heart-rate and nictitating membrane/eyelid responses in the rabbit

It has been shown that unilateral lesions of the medial dentate/lateral interpositus nuclear region of the cerebellum abolish the learned nictitating membrane (NM)/eyelid response of the eye ipsilateral to the lesion. The present study examined the effects of bilateral cerebellar lesions on acquisition of heart-rate conditioning (often viewed as a measure of 'conditioned fear') and both its short-and long-term effects on NM/eyelid learning and relearning. The results demonstrate that cerebellar lesions that completely and permanently abolish acquisition or retention of the somatic response (NM/eyelid) bilaterally have no effect on heart-rate conditioning. The neuronal circuits necessary for learning of the heart-rate response and for learning of the adaptive somatic response are thus in significant part different. Results are tentatively interpreted within the context of a two-process theory of aversive learning: an initial phase indexed by conditioned autonomic and 'non-specific' responses such as heart-rate and a subsequent phase of learning the specific adaptive responses.