The effects of 8-OH-DPAT and (+/-)-pindolol on isolation-induced ultrasonic vocalizations in 3-, 10-, and 14-day-old rats.

It has been established that administration of 5-HT1A agonists attenuates the rate of isolation-induced ultrasonic vocalizations (USV) in 10-day-old rat pups. In this study we extended these findings by examining the effects of administration of the serotonergic 1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and mixed 5-HT1A antagonist/beta adrenergic antagonist, (+/-)-pindolol, in 3-, 10-, and 14-day-old rat pups in order to assess the effect of these drugs from a developmental perspective. At all three ages, 8-OH-DPAT significantly reduced the rate of isolation-induced USV. While only the highest dose (1.0 mg/kg) of 8-OH-DPAT administered to the 10- and 14-day-olds significantly reduced the rate of vocalization, both the 0.1- and 1.0-mg/kg doses significantly attenuated the vocalization rate in the 3-day-olds. Pindolol administration did not alter the rate of USV at any age nor did it block the quieting effect that generally occurs when an anesthetized littermate is placed with the isolated pup. We conclude that 8-OH-DPAT is effective as early as 3 days of age in the quieting of isolation-induced USV and that the regional age-dependent development of 5-HT1A receptors and projections are important factors in the observed differential sensitivity to 8-OH-DPAT administration during development.

Isolation alters striatal met-enkephalin immunoreactivity in rat pups.

Isolated preweanling rats emit ultrasonic vocalizations. Mu- and delta-opioid agonists quiet isolated pups; naltrexone, an opioid receptor blocker, prevents this quieting. A littermate companion is as effective as morphine in quieting vocalizations, and naltrexone also blocks companion quieting. We have now quantified methionine enkephalin (Met-ENK) immunoreactivity in the brains of 10-day-old Wistar rat pups taken directly from the home cage or kept either alone or with a companion for a brief or prolonged period. Met-ENK is an endogenous ligand that binds to the mu- and delta-opioid receptors. Striatal peptide levels were higher when pups were with a companion than when they were kept alone; the peptide level of pups in the home cage did not differ from either. Comparisons of pups in the brief (5 min) and prolonged (60 min) separation conditions showed significantly higher peptide levels following a brief period out of the nest than at the end of an hour. In hypothalamus, hippocampus, and frontal cortex neither social condition nor duration of separation significantly altered peptide quantity. Larger amounts of Met-ENK in pups provided with a companion could reflect an increase in posttranslational cleavage of the precursor molecule leading to stimulation of receptors that act to diminish USV. Reduced levels following 60 min out of the home cage might reflect depletion of the peptide following an initial release during the period when the pup's vocal response is most vociferous.

The isolation and companion comfort responses of 7- and 3-day-old rat pups are modulated by drugs active at the opioid receptor.

Rat pups emit ultrasonic vocalizations (USVs) when isolated in a novel environment. In 10-day-olds, USV has been shown to be reduced by either the administration of 0.125 mg/kg morphine (MOR) or the presence of a littermate; these effects were both reversed by naltrexone (NLX), an opioid receptor blocker. The present study reports that the same dose of MOR produced NLX-antagonized quieting without sedation in 7- and 3-day-old pups; higher doses of MOR decreased USV but produced motor deficits as well. The 0.125 mg/kg dose of MOR is less effective in reducing USV in 3- and 7-day-olds; calling rates declined by no more than 42%, compared with 65% at 10 days of age. The presence of a companion also lowered the USV of 3- and 7-day-olds by a lesser amount (55-57%) than the 67% seen in 10-day-olds or the 90% decline when pups are 2 weeks old. This suggests that age-related changes in the opioid system may be relevant to the increased salience of a social companion that comes with maturity.

U50,488 increases ultrasonic vocalizations in 3-, 10-, and 18-day-old rat pups in isolation and the home cage.

Separation-induced calling in the young of many species can be modulated by the opioid system. Morphine reduces ultrasonic vocalizations (USVs) produced by isolated rat pups, an effect blocked by naltrexone. Central administration of the mu and delta opiate receptor agonists DAMGO and DPDPE reduce USV; kappa receptor agonist U50,488 increases them. We now find that peripheral U50,488 not only boosted calling rates in isolated 3-, 10-, and 18-day-old rat pups, but also induced calling in pups of these ages tested in the home cage with their littermates, where USVs are seldom heard in nature. U50,488 lowered rectal temperature, although temperature loss and USV were not correlated within drug treatment groups.

Ultrasonic vocalizations are elicited from rat pups in the home cage by pentylenetetrazol and U50,488, but not naltrexone.

Although isolated rat pups emit ultrasonic vocalizations (USVs), those kept warm and undisturbed in the home cage with their littermates seldom do. Drugs were administered to 10-day-old pups in the home cage to determine whether pharmacological agents can elicit USV in this familiar environment. Ten-day-old Wistar rats were injected with U50,488, a highly selective kappa opioid agonist; pentylenetetrazol (PTZ), an anxiogenic drug that binds at the GABA-benzodiazepine receptor complex; or naltrexone (NLX), an opiate receptor blocker, and then were returned to their littermates in the home cage. U50,488 increased USV and activity levels, lowered body temperature, and disrupted contact with littermates. PTZ raised activity levels but had a smaller effect on vocalization rates and did not alter temperature or contact with littermates. Behavioral measures and body temperature were unchanged by NLX.

Effect of a social companion on the ultrasonic vocalizations and contact responses of 3-day-old rat pups.

Rat pups that are isolated in a novel environment emit ultrasonic calls. Vocalization in 3-day-old pups has been thought to be predominantly under thermal control (Allin & Banks, 1971). By the 2nd week of life, ultrasonic vocalizations are reduced when pups are tested in the company of a single anesthetized littermate (Carden & Hofer, 1990a; Hofer & Shair, 1978). In the present study, it was demonstrated that the vocalization rate of 3-day-old pups is also decreased when an anesthetized littermate is present in the isolation chamber. To determine whether this quieting was a function of the body heat of the companion, in a 2nd experiment, the axillary temperature of the companion was lowered until it was the same as the ambient temperature of the test chamber, 22 degrees C. In the presence of a cool companion, ultrasonic vocalizations were also reduced. For the last experiment, a textured plastic surrogate was substituted for the littermate. Calls were not diminished in the presence of the surrogate.

Differential effects of specific opioid receptor agonists on rat pup isolation calls.

The possibility was investigated that specific opioid receptor types might selectively alter the production of isolation-induced ultrasonic vocalizations. Intracisternal injections of mu, delta and kappa opioid receptor agonists were administered to isolated 10-day-old rat pups. The mu receptor agonist [D-Ala2-NMe-Phe4-Gly-ol]-enkephalin (DAMGO) and delta receptor agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) both reduced the rate of isolation-induced ultrasonic calling in the absence of sedation. The kappa receptor agonist U50,488 had the opposite effect, significantly raising the rate of vocalization. Fourteen-day-old pups, with a larger delta receptor population, showed a greater sensitivity to DPDPE than was seen in the younger animals.

Isolation-induced vocalization in Wistar rat pups is not increased by naltrexone.

Rat pups, when removed from dam and littermates and isolated in an unfamiliar milieu, emit a characteristic ultrasonic vocalization that can be quieted by the administration of a nonsedating dose of morphine. Wistar pups, aged 7, 10, 12, 14, or 16 days, were tested after receiving intraperitoneal injections of the opiate antagonist naltrexone (0.0, 0.5, 1.0, or 5.0 mg/kg). The rate of isolation-induced ultrasonic vocalizations was unaffected by naltrexone at any dose, and there were no significant naltrexone-related changes on other behavioral measures. The complexity of the opioid system is discussed, as it may be involved in the vocal reaction to isolation.

The effects of opioid and benzodiazepine antagonists on dam-induced reductions in rat pup isolation distress.

In 10-day-old pups isolated in a novel environment, the presence of the dam reduced the rate of ultrasonic vocalization by 93%. The opiate antagonist naltrexone caused a dose-related increase in the rate of vocalization that was significant only at 5.0 mg/kg. This suggests that the dam exerts comforting effects through endogenous opioid mechanisms. Although the benzodiazepine agonist chlordiazepoxide is as effective as morphine in the quieting of isolation distress, its antagonist, Ro 15-1788, at doses up to 20 mg/kg did not alter the lowered rate of vocalization associated with the presence of the dam. Instead, it facilitated quiet contact with the companion. This raises the possibility that the presence of a social companion precludes the release of an endogenous anxiogen that binds to the benzodiazepine receptor.

Socially mediated reduction of isolation distress in rat pups is blocked by naltrexone but not by Ro 15-1788.

The presence of a single anesthetized littermate significantly reduced the rate of ultrasonic vocalization by 10-day-old pups isolated in a novel environment. Naltrexone (1.0 mg/kg) returned the vocalization rate to the level of pups tested alone and disrupted the maintenance of body contact between the test pup and a companion. This suggests that the companion exerts comforting effects through endogenous opioid mechanisms. Although chlordiazepoxide is as effective as morphine in the quieting of isolation distress, the benzodiazepine (BDZ) antagonist Ro 15-1788 (5, 10, or 20 mg/kg) was ineffective in blocking the comfort effect and facilitated quiet contact with the companion. In isolated pups, Ro 15-1788 caused a significant, but not a dose-related, decrease in vocalization, a possible indication of the displacement of an endogenous anxiogenic ligand at the BDZ receptor complex.

Diazepam's impact on self-stimulation but not stimulation-escape suggests hedonic modulation.

In rats that self-administered lateral hypothalamic (LH) stimulation through chronically implanted electrodes, ip diazepam (DZ) increased rates and decreased thresholds of self-stimulation (SS) in a dose-related manner. Stimulation-escape (SE), however, was refractory to the drug. There was a complete dichotomy in electrode placements along the anterior/posterior plane. Every pure-reward electrode location was posterior to every reward-escape electrode. DZ-sensitive SS appears to be mediated by a reward substrate common to both pure-reward and reward-escape rats, whereas SE is supported by an aversive system unaffected by DZ and stimulated only in those rats with anterior placements. The lack of control over SE suggests that the drug's effect on stimulation-induced conduct is to increase reward rather than to decrease aversion. This hypothesis is discussed in the context of DZ's interactions with drugs of abuse.

Independence of benzodiazepine and opiate action in the suppression of isolation distress in rat pups.

To determine whether benzodiazepines (BDZs) quiet isolation distress in 10-day-old rat pups by causing a release of endogenous opioids, a blockade of the effects of chlordiazepoxide (CDP) by the opiate antagonist naltrexone (NLX) was sought. Nonsedating doses of morphine (MOR) (0.125 mg/kg) and CDP (2.0 mg/kg) were equally effective in reducing ultrasonic vocalizations and other indices of isolation distress. Appropriate blocking agents NLX, (0.5 mg/kg) against MOR and Ro 15-1788 (4.0 mg/kg) against CDP returned distress measures to levels of saline-treated rat pups. However, NLX failed to reverse the quieting effects of CDP. If CDP potentiates endogenous opioid release, then NLX should block the CDP effect. A higher dose of CDP did not reveal a release of endogenous opioids, and a higher dose of NLX did not antagonize CDP. The quieting effects of BDZs on isolation distress do not appear to be mediated by the opiate system.

Diazepam modulates lateral hypothalamic self-stimulation but not stimulation-escape in rats.

Rats electrically stimulated via chronically implanted lateral hypothalamic (LH) electrodes were assessed with and without diazepam (DZ), for thresholds of stimulation-bound feeding (SBF) and for barpressing rates to administer and to escape from the same current, Six pure-reward rats, who self-stimulated but did not escape LH stimulation, exhibited SBF. Their electrode tips lay in medical forebrain bundle (MFB) and zona inserta along the entire rostral-caudal extent of the ventromedial nucleus of the hypothalamus (VMH). Six reward-escape rats, who self-stimulated and escaped from LH stimulation, did not (with one histologically deviant exception) show SBF. Reward-escape electrode tips were anterior to all the pure-reward placements. They lay in MFB rostral to the VMH up to the level of the bed nucleus of the stria terminalis (with the deviant electrode tip located on the zona inserta/ventral thalamic border). After i.p. injections of DZ, self-stimulation (SS) rates increased for both groups of animals and SBF thresholds decreased. Stimulation-escape (SE) rates, however, remained unchanged by the drug. The results are consistent with the existence of dual substrates: a DZ-sensitive reward system, present in both groups of animals, and a simultaneously stimulated, drug-resistant aversion system which is powerfully engaged in reward-escape animals only.