Electrophysiological support in favor of multiple opiate receptors in the caudate and the central gray of the rat

Periaqueductal gray afferents synapse onto dopamine and GABA neurons in the rat ventral tegmental area

The midbrain central gray (periaqueductal gray; PAG) mediates defensive behaviors and is implicated in the rewarding effects of opiate drugs. Projections from the PAG to the ventral tegmental area (VTA) suggest that this region might also regulate behaviors involving motivation and cognition. However, studies have not yet examined the morphological features of PAG axons in the VTA or whether they synapse onto dopamine (DA) or GABA neurons. In this study, we injected anterograde tracers into the rat PAG and used immunoperoxidase to visualize the projections to the VTA. Immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA was then used to identify the phenotype of innervated cells. Electron microscopic examination of the VTA revealed axons labeled anterogradely from the PAG, including myelinated and unmyelinated fibers and axon varicosities, some of which formed identifiable synapses. Approximately 55% of these synaptic contacts were of the symmetric (presumably inhibitory) type; the rest were asymmetric (presumably excitatory). These findings are consistent with the presence of both GABA and glutamate projection neurons in the PAG. Some PAG axons contained dense-cored vesicles indicating the presence of neuropeptides in addition to classical neurotransmitters. PAG projections synapsed onto both DA and GABA cells with no obvious selectivity, providing the first anatomical evidence for these direct connections. The results suggest a diverse nature of PAG physiological actions on midbrain neurons. Moreover, as both the VTA and PAG are implicated in the reinforcing actions of opiates, our findings provide a potential substrate for some of the rewarding effects of these drugs.

Naloxone-induced modulation of feline aggression elicited from midbrain periaqueductal gray

In the present study, peripheral administration of naloxone hydrochloride (IP) was employed to identify the role of endogenous opioid peptides in the regulation of two forms of aggressive behavior in the cat--affective defense and quiet biting attack behavior. These forms of aggressive behavior were elicited by electrical stimulation of dorsal and ventral aspects of the midbrain periaqueductal gray, respectively, utilizing monopolar electrodes. Following the establishment of stable baseline thresholds for affective defense and quiet biting attack behavior, naloxone (0.5, 1.0, 4.0 and 7.0 mg/kg) and saline (vehicle control) were administered peripherally (IP). The response thresholds were tested 5-30, 30-60, 60-90, 180-210 and 1440-1470 min following naloxone administration. These results indicated that a dose level of 7.0 mg/kg of naloxone had a profound facilitatory effect on affective defense behavior. Response threshold values returned to prenaloxone baseline levels at 1440-1470 min postinjection. Administration of lower doses of naloxone (1.0 and 4.0 mg/kg) also resulted in a significant facilitation of this response but of shorter durations. Neither the lowest dose of naloxone (0.5 mg/kg) nor saline (vehicle control) were effective in modifying the threshold for affective defense behavior. In contrast, when tested for its effects upon quiet biting attack, the maximum dose utilized in this study (7.0 mg/kg) tended to suppress this response although the overall effect was not significant. The selective dose-dependent facilitatory effects of naloxone upon affective defense behavior in the cat suggests that the opioid peptide system plays a significant (inhibitory) role in the regulation of this response.